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JAMES H. OTTO • ALBERT TOWLE

MODERN BIOLOGY

HOLT, RINEHART AND WINSTON, INC., NEW YORK

ABOUT THE AUTHORS

james h. otto is head of the Science Department
at George Washington High School, Indianapolis, Indiana
albert towle is head of the Science Department
at James Lick High School, San Jose, California

Cover photograph: Mitosis in onion root tip
(Walter Dawn)

Unit and chapter opening photographs: 1. Model
of DNA molecule (Walter Dawn); 2. Dro¬
sophila chromosomes (Walter Dawn); 3. Pro¬
tozoans (Walter Dawn); 4. Sections of woody
stem (USDA); 5. Spongin fibers (Walter
Dawn); 6. Cross section of bone (Walter
Dawn); 7. Human blood with Trypanosoma
gambiensi (Walter Dawn); 8. Drop of pond
water (Bausch and Lomb, Inc., Rochester,
N.Y.)

12345 039 1817161514

by Holt, Rinehart and Winston, Inc.

Printed in the United States of America

03-047900-2

LIBRARY

UNIVERSITY OF ALBERTA

PREFACE

Biology today is vastly different from biology ten years ago. In the field of cellular
biology, the electron microscope has yielded discoveries long suspected but in¬
capable of positive proof. Improved techniques in biochemistry have revealed new
vistas at the molecular level. Man’s understanding of genetics, microbiology, and
ecology has reached new heights. As a result of this increased body of knowledge,
it is possible to encompass only the basic principles during one school year of study.

The authors of MODERN BIOLOGY have always felt that the learning
process should involve a mastery of certain fundamental biological concepts at the
beginning of the program. From these initial understandings, the student should
then progress from the cell to protists, to plants and animals, and to man. Cul¬
minating his biological knowledge, the interrelationships of living things and their
environmental adaptations should bring about a clearer comprehension of the sig¬
nificance of life and its importance in human welfare. Accordingly, this revision
of MODERN BIOLOGY contains the most recently available knowledge, but the
authors have avoided selecting any one or two of these areas as deserving of pri¬
mary emphasis at the expense of other equally important areas.

The authors have preserved the approach and methodology that has evolved
through the years so successfully in secondary school classrooms. These features
have been tested, tried, and proved effective by thousands of teachers in our na¬
tion’s schools. Many professional biologists who are making significant discoveries
today in the research laboratories and who learned from earlier editions of this text
are evidence of the motivational value of such a course.

In this complete revision some material from previous editions has been
telescoped to produce a pattern more consistent with current trends in the various
science curricula. Thus, conservation is somewhat reduced in scope as is the sub¬
ject of disease. Both these topics are generally covered in the elementary and junior
high school science programs and do not now require emphasis they enjoyed in pre¬
vious editions of this book.

The book begins with molecular and cellular biology from which it logi¬
cally moves into reproduction and genetics. From there it moves to evolution and
hence into classification, thus building a basic structure of biological principles on
which to build further concepts and facts — as much as circumstances, interest, and

2 3S1 Ti *

vi PREFACE

curricula requirements permit. This orderly arrangement has advantages that no
other program today possesses — flexibility. The program is adapted to either a
concentrated course in certain areas of biology or a survey course which hits the
high spots.

The greatly expanded, thoroughly modern unit on genetics is a feature of
this revision. Also modern in scope is the unit on molecular and cellular biology,
which includes now the treatment of cell reproduction formerly discussed much
later in the text. The ecology unit is completely new and a more intensive discus¬
sion of the subject is presented, together with the treatment of conservation, since
the two go hand in hand and one can hardly be mentioned at the secondary school
level without the other.

The language of science has again been an important consideration, as in
previous editions of this book, and the style of writing has been kept as informal
as possible. The average ninth and tenth grade student is seldom a reader who can
digest material much beyond his grade level. Therefore, the authors have tried to
keep the readability commensurate with the age of the student. Difficult words
are pronounced phonetically and all new words or terms are printed in boldface
italics and are defined the first time they appear.

The present authors are deeply indebted to the late Truman J. Moon, whose
successful texts Biology for Beginners and Biology were the predecessors of this
book. Mrs. Elizabeth H. Crider, teacher of biology in George Washington High
School, Indianapolis, Indiana has helped the senior author in the preparation of
the manuscript for Units Two and Six and he herewith gratefully acknowledges her
assistance. Mrs. Mildred I. Ross, Science Librarian at the George Washington
High School in Indianapolis, has again prepared the bibliographies at the ends of
each unit, and in this revision periodicals as well as books are included. The photo¬
graphs for this revision were obtained by Frances L. Orkin.

Each unit has been read not only by a high school teacher who has had long
classroom experience, but also by a specialist in that particular field of biology at
the university level. These biologists have given generously of their time and have
offered invaluable suggestions and criticisms. The authors express their deep ap¬
preciation and are grateful to each person for his active interest in the project.

The university professors are as follows: Dr. Donald R. Griffin of Harvard
University; Dr. Donald Heyneman of the University of California; Dr. David J.
Merrell of the University of Minnesota; Dr. Peter M. Ray of the University of
Michigan; Dr. Philip Siekevitz of the Rockefeller Institute; Dr. William R. Sistrom
of the University of Oregon; Dr. Richard C. Wolf of the University of Wisconsin.

The high school teachers are: Lyle D. Anderson of West High School,
Waterloo, Iowa; William Berman of Tilden High School, Brooklyn, New York;
Richard K. Hayes of Pilgrim High School, Warwick, Rhode Island; James F.
Snyder, Walt Whitman High School, Huntington Station, New York.

CONTENTS

UNIT ONE THE

Chapter 1 :
Chapter 2:
Chapter 3:
Chapter 4:
Chapter 5 :
Chapter 6:
Chapter 7 :
Chapter 8:

NATURE OF LIFE

The Science of Life

The Living Condition

The Chemical Basis of Life

The Structural Basis of Life

The Cell and Its Environment

Cell Nutrition

Cell Metabolism

Cell Growth and Reproduction

UNIT TWO THE

Chapter 9:
Chapter 10:
Chapter 11:
Chapter 12:
Chapter 13:
Chapter 14:

CONTINUITY OF LIFE

Principles of Heredity
The Genetic Material
Genes in Human Populations
Applied Genetics
Organic Variation
The Diversity of Life

UNIT THREE MICROBIOLOGY

Chapter 15
Chapter 16
Chapter 17
Chapter 18
Chapter 19
Chapter 20

The Viruses

Bacteria and Related Organisms

Infectious Disease

The Protozoans

The Fungi

The Algae

UNIT FOUR MULTICELLULAR PLANTS

Chapter 21
Chapter 22
Chapter 23
Chapter 24

Mosses and Ferns
The Seed Plants
Root Structure and Function
Stem Structure and Function

103

115

116
131
155
170
182
199

211

212

222

238

254

265

277

293

294
303
313
325

viii CONTENTS

Chapter 25: Leaf Structure and Function 341

Chapter 26: Reproduction in Flowering Plants 352

UNIT FIVE BIOLOGY OF THE INVERTEBRATES 373

Chapter 27: Sponges and Coelenterates 374

Chapter 28: The Worms 385

Chapter 29: Mollusks and Echinoderms 399

Chapter 30: The Arthropods 410

Chapter 31: Insects — A Representative Study 423

Chapter 32: Insect Diversity 432

UNIT SIX BIOLOGY OF THE VERTEBRATES 451

Chapter 33: Introduction to the Vertebrates 452

Chapter 34: The Fishes 459

Chapter 35: The Amphibians 472

Chapter 36: The Reptiles 489

Chapter 37: The Birds 506

Chapter 38: The Mammals 522

UNIT SEVEN THE BIOLOGY OF MAN 541

Chapter 39: The History of Man 542

Chapter 40: The Body Framework 551

Chapter 41: Nutrition 565

Chapter 42: Transport and Excretion 581

Chapter 43: Respiration and Energy Exchange 600

Chapter 44: Body Controls 612

Chapter 45: Alcohol, Narcotics, and Tobacco 630

Chapter 46: Body Regulators 639

Chapter 47: Reproduction and Development 648

UNIT EIGHT ECOLOGICAL RELATIONSHIPS 659

Chapter 48: Introduction to Ecology 660

Chapter 49: The Habitat 672

Chapter 50: Periodic Changes in the Environment 685

Chapter 51: Biogeography 699

Chapter 52: Soil and Water Conservation 712

Chapter 53: Forest and Wildlife Conservation 723

Appendix 743

Glossary 755

Index 774

UNIT ONE

THE NATURE
OF LIFE

What is life? What is the elusive condition we refer to as the living state? All
living things have chemical similarities. They have structural characteristics which
set them apart from nonliving materials. They grow by organizing more of their
own substance. They reproduce and perpetuate life, age after age. They constantly
require energy to maintain their many chemical activities. In your study of biolog\
you will start with an unknown — the living state — and will explore it in many
ways. You may never explain life, but your investigations will bring you to a closer
understanding of the marvelous living condition.

CHAPTER 1

THE

SCIENCE
OF LIFE

Biology in a golden age. What is biol¬
ogy? The word comes from the Greek
bios, which means “life,” and logos,
which means “study of,” or “science
of.” Biology is the knowledge about
living things that has come to us from
previous generations and to which we
are adding the contributions of biolo¬
gists of our time. Like all the other
sciences, biology is a method of investi¬
gating events and problems we need to
solve. It arises from curiosity about life
and our need to survive and to improve
our position in the living world.

Biology, like the other sciences, is
pushing forward at an unbelievable rate.
It is probably safe to say that we have
gained more biological knowledge in the
past 20 years than in the previous 20
centuries! This is truly a golden age in
biolog}-. Why this sudden explosion of

knowledge in our time? Let us exam¬
ine the position of biology today and
see if we can discover what lies behind
the many achievements of recent years.

Science is not limited by national
boundaries. With the coming of the
Renaissance, science began to develop
on a worldwide basis. Slowly at first,
but with increasing pace, scientists
pushed back the frontier of ignorance,
superstition, and prejudiced thinking
that had stifled progress in medieval
times. A Belgian medical student, An¬
dreas Vesalius, rebelled against medieval
medicine and established the scientific
study of anatomy in the 16th century.
In the 17th century, William Harvey, an
English physician, disputed the ancient
belief that blood ebbed and flowed in
the veins of the body like the tides of the
sea and proposed that it circulated
through arteries and veins. Later in
this same century, Marcello Malpighi
(mdZ-pee-gee), an Italian scientist, saw
vessels in the lung of a frog and proved
Harvey’s theory of circulation. Mal¬
pighi had a microscope; Harvey did
not. In the 18th century, Edward Jen-
ner, an English doctor, performed the
first vaccination when he immunized a
small boy against smallpox. Louis Pas¬
teur of France established the science of
bacteriology in the 19th century. A few
years later Robert Koch (kohk) of Ger¬
many gave the world a method of in¬
vestigating infectious diseases and tech¬
niques for culturing bacteria in the lab¬
oratory. A century ago Gregor Men¬
del (men-d’l), an Austrian monk, con¬
ducted his famous experiments with gar¬
den peas and discovered laws that still
serve as the basis for much of the mod¬
ern science of heredity. A few years
later, Paul Ehrlich, a German doctor,
discovered an arsenic compound that
would kill the syphilis organism in the

CHAPTER 1 THE SCIENCE OF LIFE 3

1-1 William Harvey is best known for his
classic investigation of circulation. He also
published work on embryology and physiol¬
ogy. (National Portrait Gallery, London)

human blood stream. This ushered in
the age of chemotherapy in medicine.
In 1929 an English biologist, Alexander
Fleming, discovered penicillin, the first
of a long line of antibiotics.

These are but a few of the impor¬
tant milestones in the progress of biolog¬
ical science through the recent centuries.
A contribution of one scientist becomes
the property of scientists to follow. In
this way scientific knowledge is com¬
pounded year after year and decade
after decade.

Through the years scientists have
established a method of investigation.
When science succeeded in throwing
off the yoke of superstition and preju¬
dice, a new system of objective thought
and investigation developed. You may
call it the scientific method. Actually
it is nothing more than unrestrained
common sense. Man is naturally curi¬
ous. He has always sought explanations

for the events and phenomena he could
not understand. In earlier times he
was satisfied with unreasonable, un¬
proved answers derived from nothing
more than superstition. A person’s life
was supposedly influenced by the posi¬
tion of heavenly bodies at the time of
his birth. Did anyone ever prove this?
Did anyone even try? The mud in a
pond was supposedly transformed into
eels, fish, or frogs. The air from a
marsh was thought to cause malaria.
The name of this disease means, liter¬
ally, “bad air.”

Several centuries were necessary to
gain for the scientist the freedom he re¬
quires — freedom to investigate; free¬
dom to prove and disprove; freedom to
base his conclusions on observed facts.

Science has progressed at a remark¬
able rate in areas related to biology.
Such related areas include chemistry,
physics, earth science, oceanography,
and space science. At one time defi¬
nite lines could be drawn between the
major areas of science. Today no such
separations exist. All scientific knowl¬
edge is interrelated. As you study the
organization of the many substances in¬
volved in the living condition, you may
be surprised to find that life can be fi¬
nally reduced to a biochemical state.
Growth is chemical. Response is chem¬
ical. Heredity is chemical. In the last
analysis everything about life is chemi¬
cal because it involves matter and
changes in matter. Life also involves en¬
ergy changes. The forces that govern
the changes in the matter of our earth
have a vital influence on all of life.
How can we separate physics from biol¬
ogy? Satellites sent aloft are sending
back information that is changing all our
concepts of the shape of the earth. The
earth is not round but somewhat pear-
shaped. It is flattened at the poles and

4 UNIT 1 THE NATURE OF LIFE

1-2 The effect of acetic acid on bread mold. The left half of the loaf was
treated with vinegar; the right half was not treated. Is this effect chemical or
biological? (Fleischmann Laboratories, Standard Brands, Inc.)

bulges at the equator. Its surface has
great raised areas and depressions far
more extensive than mountain ranges
and valleys. Its crust varies in thickness.

Do all these factors have a profound
influence on life and the distribution of
living organisms over the face of the
earth? Only time will tell. This is a
completely new area for investigation.
Oceanographers are exploring the ocean
depths and plotting great currents that
circulate water throughout the expanse
of the seas, fust how do these currents
influence the distribution of life in the
seas? Many generations of biologists
will investigate problems such as these.
Science is progressing rapidly on all
fronts. Advances in all of these fields
are bound to have a dynamic influence
on our study of life.

Progress in biology is directly re¬
lated to the tools available for research.
Biolog}’ took a giant step forward in the
18th and 19th centuries with the inven¬

tion and perfection of the light micro¬
scope such as you have in your high
school laboratory. Without magnifica¬
tion, the biologist could examine only
the gross structures of an organism — a
leaf or stem, a heart or a kidney. But
organs such as these are composed of
much smaller parts. Without lenses to
see cells and tissues, many significant
discoveries of the past centuries would
have been impossible. Without a mi¬
croscope Louis Pasteur could never have
given his germ theory to the scientists
of the world. The whole area of micro¬
biology, vital in biology today, could
never have developed.

The modern light microscope re¬
veals much of the structure of a cell.
But beyond the limit of visibility with
even the finest light microscope are cell
structures that have provided clues for
the dramatic discoveries of the past few
years. This remarkable tool of the mod¬
ern biologist is, as you undoubtedly

CHAPTER 1 THE SCIENCE OF LIFE 5

know, the electron microscope. We
will describe this important biological
tool later in this chapter.

As a result of the new techniques
in microscopy and the advances in
chemistry outlined on page 3, biolo¬
gists in the last twenty years have con¬
centrated more on the cellular and mo¬
lecular levels of life. While in the past
biologists were more concerned with the
great diversity of living things, in recent
years they have become more interested
in their unity. Cells and certain mole¬
cules are common to all organisms. As
you study biology it will become obvi¬
ous to you that certain principles govern
everything that is alive — that living
things are as remarkable for their simi¬
larities as they are for their differences.

The scientist has begun to recog¬
nize his own limitations. In medieval
times science was considered absolute
authority. Scientific “truths handed
down from early times in the body of
knowledge were not to be questioned,
much less disputed or disproved.

1-3 The biologist of today is often con¬
cerned with the chemical aspects of life.
(Parke, Davis & Co.)

Many were those who were ridiculed or
persecuted or forced to flee because they
dared to disagree with the established
authority. Even a generation ago sci¬
ence was considered much more exact
than it is today. Fortunately, we no
longer consider any scientific explana¬
tion a final answer. Any concept must
be subject to change and revision in the
light of new discoveries.

The public has come to accept and
support modern science. Two hundred
years ago, townspeople threw rocks at
Edward Jenner when he vaccinated a
boy against smallpox and probably
saved his life. One hundred years ago,
people ridiculed Louis Pasteur when he
tried to convince them that invisible mi¬
crobes could cause infectious disease.
Today both of these great scientists of
the past would undoubtedly have re¬
ceived Nobel prizes for their outstand¬
ing contributions.

A century of research and triumph in
our time. The conquest of polio is an
excellent example of a gigantic program
of biological research extending over a
period of more than 100 years. Scien¬
tists of many nations made contribu¬
tions that led step by step to the even¬
tual victory. The story reads like a de¬
tective story. Each discovery opened a
door for further investigation. The
conquest of polio is especially interest¬
ing to us because it represents a tremen¬
dous undertaking, supported by the
American public through 3,000 chapters
of The National Foundation for Infan¬
tile Paralysis, and the contributions of
millions of citizens through the March
of Dimes campaign in honor of a Pres¬
ident of the United States, Franklin D.
Roosevelt, himself a victim of polio.

Polio is short for poliomyelitis
(po/z-lee-oh-my-e-Zy-tis ) , the medical
name of the disease. It is a virus infec-

6 UNIT 1 THE NATURE OF LIFE

tion of nerve cells in the brain and spi¬
nal cord. Depending on the seriousness
of the infection, the cells may be dam¬
aged or destroyed. In spinal polio, tem¬
porary or permanent paralysis of any
body part may occur, although the
limbs are most frequently involved. In
bulbar polio the infection centers in the
medulla, a region at the base of the
brain sometimes called the spinal bulb.
In bulbar polio paralysis of breathing
muscles may occur. However, in 60
percent of the cases of polio, there is no
permanent damage. Permanent paraly¬
sis occurs in only 14 percent of the cases.

Progress in the investigation of po¬
lio was slow; one reason was the fact
that there were many mistaken ideas
about the disease. We might begin
with the familiar name, “infantile paral¬
ysis,” which gives two false impressions.
Polio is not primarily a disease of in¬
fants — it may strike at any age, and the
infection does not always result in paral¬
ysis. There was also a mistaken idea
that polio was a disease of the bones
and muscles rather than of nerve cells
in the spinal cord.

Milestones in the conquest of polio.
In 1840 the little son of a German vil¬
lager was stricken with paralytic polio.
Both of his legs were limp and useless.
His mother brought him to Dr. Jacob
Heine (hy- ne), a bone specialist in
Cannstadt, Germany. Dr. Heine exam¬
ined his young patient thoroughly and
came to an important medical conclu¬
sion. Neither the bones nor the mus¬
cles of the boy’s legs were diseased.
The problem centered in the nerves
that controlled the leg muscles. This
was the first association of polio with
the central nervous system. Dr. Heine
wrote a book describing his observa¬
tions, the earliest published study of the
disease.

The first known polio epidemic oc¬
curred in Stockholm in 1887. During
this epidemic Dr. Medin of the Univer¬
sity of Stockholm made extensive stud¬
ies of polio cases and published the first
report on the early symptoms of the in¬
fection.

Another terrible epidemic of polio
struck the Scandinavian countries in
1905. This epidemic attracted world¬
wide attention and resulted in a detailed
study of the spread of the infection by
Dr. Wickman, another early investi¬
gator. Dr. Wickman advanced the the¬
ory that polio was spread by human car¬
riers.

Three years later, in 1908 and 1909,
Dr. Karl Landsteiner and Dr. Erwin
Popper conducted studies of polio in
Vienna. They succeeded in producing
polio infections in monkeys and first
demonstrated that the infection is
caused by a virus that attacks the cells
of nerves in the spinal cord.

Forty years later, in 1949, another
milestone was passed when Dr. John F.
Enders of Harvard University and two
of his associates, Dr. Thomas H. Weller
and Dr. Frederick C. Robbins, succeed¬
ed in growing polio virus in tissue cul¬
tures in test tubes. They grew the virus
in a variety of animal tissues, but found
that monkey kidney tissue was most sat¬
isfactory. For this significant advance
the Nobel prize in medicine and physi-
ologv was awarded in 1954.

The discovery of gamma globulin.
One of the goals of all who worked with
polio was the production of an immune
product that could be used to transfer
immunity by injection into the blood
stream. This major breakthrough in
the conquest of polio came in 1951 and
1952. A part of the human blood
known as gamma globulin was found to
contain antibodies against the polio vi-

CHAPTER 1 THE SCIENCE OF LIFE 7

ms. Blood banks were available by this
time to furnish an abundant supply of
gamma globulin from adult donors.

A gigantic test was devised to deter¬
mine the effectiveness of gamma globu¬
lin. Injections were given to 55,000
children. By comparing the incidence
of polio among these protected children
with children who were not immunized,
it was definitely established that gam¬
ma globulin was effective. However,
the battle against polio was not yet won.
While gamma globulin establishes im¬
munity immediately, making it valuable
in protecting a child during an epidem¬
ic or at a time of known exposure, the
goal was not yet reached. Injected
gamma globulin gives only temporary
or passive immunity, lasting no more
than eight weeks.

The long-awaited polio vaccine. Our
scene now shifts to the University of
Pittsburgh and the laboratory of Dr.
Jonas E. Salk. Dr. Salk found, as had
Enders, that polio virus could be grown
most satisfactorily in cultures of monkey
kidney tissue. After a short period of
rapid growth, the virus was removed
and made harmless by treatment with
formaldehyde. Before it was used in
field tests, the Salk vaccine was checked
carefully for effectiveness and safety.

Finally the anxiously awaited tests
to determine the effectiveness of the
vaccine were conducted under the di¬
rection of the National Foundation. In
1954, 1,830,000 school children were
used as subjects. Only 440,000 were
given the vaccine. The others were giv¬
en a sterile solution that looked like
the vaccine but had no medical value.
All children and their injections were
coded. Neither the children nor the
doctors who gave the injections knew
which children received vaccine and
which ones served as controls.

1-4 Dr. Jonas Salk shown injecting one of
the 440,000 school children with the trial vac¬
cine that had been developed after many
years of research. Another large group of
children, acting as controls, were given
sterile solution. (National Foundation —
March of Dimes)

Children in both groups were fol¬
lowed up closely for several years for
occurrence of polio. The results were
extremely gratifying. The vaccinated
group had 80 to 90 percent fewer cases
of polio than the unvaccinated group.
In other words, the vaccine was found
to be 80 to 90 percent effective.

Today Salk vaccine is given in three
doses. The second injection is given
two to four weeks after the first and the
third injection, not before seven months
after the second.

A chapter yet to finish. While the use
of Salk vaccine has reduced the inci¬
dence of polio by as much as 90 percent,
there are still paralytic cases, even
among vaccinated people. There are
also people who are immune to the ef-

8 UNIT 1 THE NATURE OF LIFE

fects of virus on the nervous system but
harbor the virus in their intestinal tracts
and are thus dangerous carriers of the
disease.

These problems are being elimi¬
nated with the use of an oral virus vac¬
cine developed by Dr. Albert Sabin of
the University of Cincinnati. The Sa¬
bin vaccine is composed of live but
weakened polio viruses. Since it is tak¬
en by mouth, it reaches the digestive
system, where the virus is harbored in a
carrier. Alreadv tested in several mil¬
lion people, the Sabin vaccine is becom¬
ing another effective weapon in the war
on polio.

Scientific methods. Throughout the
history of the conquest of polio there
are many examples of the use of a scien¬
tific method. A scientific method is a
logical and orderly procedure of investi¬
gation. Such a systematic method of
inquiry distinguishes scientific study
from curious dabbling and hit-or-miss
efforts to solve a problem. However,
scientific methods are not magic formu¬
las that always lead to a successful solu¬
tion of a problem. On the contrary, the
best planned and executed scientific ex¬
periment can and often does end in fail¬
ure. But eventual success mav come

from numerous failures as the scientist
analyzes each result and continues his
investigation in a new direction. Often
this new direction vields an even more

important discovery than the scientist
had expected originally.

Various methods are used in scien¬
tific study, as indicated by the nature of
the problem. We shall consider two
methods, both of which will be used in
your course in biology.

The research method. The scientist
plans an experiment and outlines his
own procedure in the research method.
It is the means bv which new knowl¬

edge is acquired and new concepts are
established. You will have opportuni¬
ties to use the research method in manv
phases of your biology course. An ex¬
periment may be performed by the class,
by a group, or by an individual in an
area of special interest.

The steps a research scientist fol¬
lows in investigating a problem are logi¬
cal and orderly:

Defining the problem. Scientific
research requires first of all an inquisi¬
tive mind and the ability to recognize a
problem. In the conquest of polio, for
example, Dr. Heine recognized the fact
that the disease attacked the nervous
system rather than the muscles. Land-
steiner and Popper determined that po¬
lio is caused by a virus. In this way the
nature of the disease was defined, so
that further research could attempt to
conquer it.

1-5 Joseph D. Locker won a scholarship in
the Westinghouse Science Talent Search
with his investigation of the effects of in¬
cubation temperature on chick embryos.
(Westinghouse Electric Corporation)

CHAPTER 1 THE SCIENCE OF LIFE 9

How does a root absorb water from
the soil? How does light influence the
growth of a stem and cause it to bend
toward the light? How does a nerve
stimulate a muscle and cause it to con¬
tract? What controls the rhythmic con¬
tractions of a beating heart? All of
these questions can be answered by
means of well-planned experiments.
Problems arise continually in the study
of science, and new problems grow out
of solutions. In this way successful re¬
search leads to new research.

Collecting information relating to
the problem. The scientist does not set
out to prove everything for himself. If
this were true, science could not pro¬
gress beyond the achievement of a sin¬
gle lifetime. Before resorting to experi¬
mentation, the research investigator
makes use of significant data and infor¬
mation relating to the problem. This
avoids duplication of effort and repeti¬
tion of work already done. Thus, an
extensive library of research papers, sci¬
entific journals, and reference books is a
vital part of a research center. Your
textbook and laboratory guide as well as
supplementary readings will serve as
sources of information in solving bio¬
logical problems in your course.

Returning to our polio story, this
phase of the research method was em¬
ployed when Medin and Wickman stud¬
ied the spread of polio epidemics in
Sweden. From their collection of data
thev were able to conclude that human
carriers spread the disease.

Formulating a hypothesis. When
a problem has been investigated to the
fringe of knowledge without solution,
it becomes necessary to proceed further
by means of experimentation. At this
point the researcher uses creative think¬
ing and imagination in determining a
tentative solution or outcome. This

possible result is called a hypothesis. It
might also be called a scientific hunch,
or an educated guess. However, while
the hypothesis may seem to be a reason¬
able solution or result in the light of
known facts, it cannot be accepted un¬
til proved. Thus, the research worker
must be not only imaginative in estab¬
lishing a hypothesis, but also open-
minded in discarding it if necessary.

In the research on polio, the inves¬
tigators’ hypothesis was that if the polio
virus could be isolated and cultured, a
vaccine might be produced that would
provide immunity to the disease. En-
ders’ work with monkey kidney tissue
paved the way for the testing of this
hypothesis.

Experimenting to test the hypothe¬
sis. The scientist must set up an experi¬
ment in which the hypothesis will be
either proved or disproved. All factors
must be removed or accounted for ex¬
cept the one to be tested. We refer to
this as the variable , or experimental fac¬
tor. In other words the researcher must
limit his experiment to the testing of
but one condition — that involved in
the hypothesis. Frequently an experi¬
ment is conducted in duplicate, with all
factors the same in the second experi¬
ment except for the experimental fac¬
tor. This second, or control , experi¬
ment demonstrates the importance of
the missing experimental factor.

The trials of both gamma globulin
and the Salk vaccine are excellent exam¬
ples of controlled experimentation in
research. The only variable in the Salk
test was the vaccine. Remember that
over a half million children serving as
controls were given injections of sterile
solution.

Observing the experiment. What
does the experiment prove? What does
it disprove? At this point the scientist

10 UNIT 1 THE NATURE OF LIFE

must use critical observation. What
are the results in relation to the hypoth¬
esis? In the research on polio, the re¬
sults bore out the hypothesis that a vac¬
cine could produce immunity. Often
the experiment that does not work as
planned yields results even more impor¬
tant than those expected.

Organizing and recording data
from an experiment. Every phase of
the experiment — the way it was
planned and set up, the conditions un¬
der which it was conducted, significant
observations during its progress, and the
results — must be recorded accurately.
These records may be in the form of
notes, drawings, tables, graphs, or cal¬
culations. In modern research, data are
often processed by means of computers.
In the research on polio the results of
the Salk test would have been of no
value had the investigators not kept
careful records of the results.

Drawing conclusions. Scientific
data are valuable only when they are
put to use. This is accomplished by
drawing valid conclusions from an ex¬
periment. Such conclusions must be
based entirely on facts proved in the ex¬
periment. Often a conclusion leads to
the discovery of a principle or the un¬
derstanding of a concept that can be
applied to other situations. This may
result in the solution of a problem not
involved in the original experiment.
The Sabin oral vaccine is, for example,
an outgrowth of the research done for
the Salk vaccine.

Accurate reporting of research
methods, results, and conclusions. Re¬
sults of scientific research are frequently
published in papers and journals and be¬
come valuable contributions to scien¬
tific literature. This is a recognized ob¬
ligation of research scientists. Through
the literature, scientists the world over

are kept informed of significant devel¬
opments in their particular fields. This
cooperative exchange of information
saves effort, time, and money and
greatly accelerates scientific progress.
Conducting a controlled experiment.
We can illustrate the steps followed in
the research method in a simple con¬
trolled experiment that you can conduct
in the laboratory. The experiment will
involve the growth and development of
bean seedlings and will relate to one en¬
vironmental factor, light. We may de¬
fine the problem as follows: Is light
necessary for the normal growth and
development of a bean seedling?

Having defined the problem it
would be well to examine various ref¬
erences in the library for information
concerning the relation of light to plant
growth and nutrition. Much has been
written on the subject. However, you
may not find a specific answer to your
problem regarding bean seedlings.

At this point you formulate the hy¬
pothesis, or tentative answer to your
question, and assume that light is nec¬
essary for the normal growth and devel¬
opment of bean seedlings. This hy¬
pothesis must now be proved or dis¬
proved by experimentation. The logi¬
cal experiment would involve the germi¬
nation and growth of two sets of bean
seedlings — one set in a dark place and
the other in full light.

Two beans are planted in each of
six, three-inch pots filled with loose,
sandy soil. Three of the pots are
marked experimental and are placed in
a dark cupboard or stockroom. The
other three pots are marked control and
are set on a window shelf or other loca¬
tion where they receive full light. The
temperature should be as nearly uniform
as possible in the two locations. All
the pots must be watered regularly and

CHAPTER 1 THE SCIENCE OF LIFE 11

uniformly throughout the experiment,
which lasts about four weeks.

As the experiment progresses, accu¬
rate observations of the condition of
each seedling must be made each day.
The time of germination of each plant
should be recorded. Other data to be
obtained each day include the length
and diameter of the stems, the number
and size of the leaves, and the color of
the plants. These data should be re¬
corded in a table.

The experiment should show strik¬
ing differences in the two sets of plants.
Those grown in the light should have
sturdy stems and large, healthy, green
leaves, while those grown in the dark
should have longer, more spindly stems
and small, yellow leaves. These results
would provide sufficient evidence to
prove the hypothesis: Light is neces¬
sary for the normal growth and develop¬
ment of bean seedlings.

In a controlled experiment it is nec¬
essary to consider all possible factors,
but to vary only one. In this experi¬
ment, light was the only factor that
varied in the experimental and control
plants. If any of the other factors had
varied, the results would not have been
valid. If, for example, the seeds in the
dark set had been planted in clay rather
than loam, one could not say whether
the poor growth was due to lack of light
or to poor soil. The reason for using
several seedlings in each set was to per¬
mit an average of the results. Other¬
wise the difference in growth rate and
condition of the plants could have been
due to a defective seed or a weak plant.

While we may conclude from this
experiment that light is necessary for
the normal growth and development of
bean seedlings, this statement immedi¬
ately raises several more questions.
How much light is required? We know

that light consists of radiations of vari¬
ous wavelengths which appear as the col¬
ors of the spectrum. Does a plant re¬
quire red, yellow, green, blue, and violet
rays equally, or are certain colors ab¬
sorbed more than others? We also
know that plants are normally subjected
to periods of light and darkness. Is a
dark period important? Why is light
necessary for the growth of green plants,
such as beans? These are but a few of
the questions that grow out of a basic
experiment involving light and the
growth of plants. The proof of one hy¬
pothesis leads to many more questions
and more experiments.

The technical method. While research
is a vital part of science, a far greater
number of people are engaged in the al¬
lied area of technology. The ratio of
technicians to research scientists has
been estimated to be 20 to one.

The technician is not seeking to
prove or disprove a hypothesis or seek¬
ing new knowledge. Rather, he uses es¬
tablished procedures to make accurate
checks and verify results. The techni¬
cal method involves several steps that
mav be summarized as follows:

1. Follow an outlined procedure with¬
out variation. The results are valid
and reliable only if the procedure was
followed accurately.

2. Make accurate observations. An er¬
ror or an oversight might make the
entire procedure worthless.

3. Record and report all findings.
Again, the technician must be ex¬
tremely accurate in recording all re¬
sults of a procedure.

The research and technical meth¬
ods are frequently combined in scien¬
tific investigation. For example, re¬
search scientists investigated the rela¬
tionship of weakened or killed polio vi¬
rus to immunity. This is a research

12 UNIT 1 THE NATURE OF LIFE

problem. However, in testing the vac¬
cine for effectiveness, they followed a
standard procedure and applied the
technical method.

The technical method is widely
used in all branches of science. Tech¬
nicians follow outlined procedures in
checking the bacterial content of water,
milk, and other foods, in identifying
various bacteria, and in checking the
strength of antibiotics and other drugs
used in medicine. Your biology course
will include many outlined technical
procedures as, for example, when you
prepare subjects for microscopic study,
extract pigments from leaves, observe a
muscle contraction, or dissect an animal
to study its internal organs.

Pure and applied science. We often
make a distinction between pure sci¬
ence y or basic research, and applied sci¬
ence, largely from the standpoint of the
nature and purpose of the work. In
pure science research is conducted for
the sake of knowledge itself. Applied
science makes practical use of this
knowledge. For example, much re¬
search has been conducted in recent
years on the effects of radiation on liv¬
ing matter. It remained for applied
science to make use of the knowledge
gained from the research in destroying
cancerous tissue by means of radiation.
A biologist looks at life. There are
many ways in which a biologist examines
living organisms. First there is the mat¬
ter of structure. The body of a sponge
has little resemblance to that of an earth¬
worm, and the structure of an insect
is totally different from that of a fish or
a frog. There is just as great diversity
in the structures of a mushroom, a moss,
a fern, and a seed plant. Anatomy is
the area of biology devoted to the study
of the structure of organisms. Knowl¬
edge of structure is in turn basic to an

understanding of the functions of living
things. This area of biology is known
as physiology. Anatomy and physiol¬
ogy — form and function — are closely
related. Either would lack meaning
without the other.

No organism lives as an isolated
unit apart from its surroundings. Con¬
ditions in the environment determine
whether the organism lives or perishes.
What are these conditions? Some, like
water, light, and temperature are physi¬
cal. Equally important are the other
plants and animals that share the en¬
vironment, some supplying needs and
others threatening survival. Thus, a
living society is a dynamic, interacting
system. In the study of ecology ( i-kahl -
uh-je), the biologist determines the in¬
terrelationships of organisms and their
surroundings.

For more than two centuries biolo¬
gists have been grouping and classifying
the organisms of the earth — plant and
animal, large and small, simple and com¬
plex — into systematic groups. This
has been an enormous scientific prob¬
lem in the area of biology known as
taxonomy.

What sort of inner control causes
a cow to produce more cows rather than
horses or sheep, and why does a cow of
a particular breed produce calves with
the particular characteristics of its
breed? You know, of course, that this
is true, but do you know why? The an¬
swer to this question lies in an under¬
standing of genetics, one of the most
fascinating areas in the entire field of
modern biology.

As you can see, biology is a com¬
plex science composed of numerous spe¬
cialized branches, each examining life
in a different way. A table giving many
of these branches may be found in the
Appendix.

CHAPTER 1 THE SCIENCE OF LIFE 13

The biologist’s principal tool — the mi¬
croscope. It is difficult to say who ac¬
tually invented the microscope. Since
a single lens such as a magnifying glass
is a simple microscope, we would have
to go back to the Middle Ages for its
inventor, for lenses capable of magnify¬
ing 10 to 20 times were ground during
this period. However, we usually credit
the compound microscope, using several
lenses, and its first biological use to An¬
ton Van Leeuwenhoek ( fizy-ven-hook ) ,
a Dutch lens grinder. Van Leeuwen¬
hoek lived in Delft, Holland, 300 years
ago (1632-1723). Microscope construc¬
tion became his hobby. Altogether he
is said to have made 247 different micro¬
scopes, each designed to examine a
specific material.

One of Van Leeuwenhoek’s early
microscopes consisted of a tube for hold¬
ing a small fish and a frame in which a
magnifying lens was mounted. By hold¬
ing the lens close to his eye, he could
see blood surging through vessels in the
tail of the fish. Other crude micro¬
scopes consisted of tubes with a lens
mounted in each end. Thus, he used a
lens to magnify the enlarged image of
another lens. With a microscope built
to examine pond water, he saw teeming
microscopic animals, which he de¬
scribed as “cavorting beasties.” With
various perfections Van Leeuwenhoek’s
microscopes could enlarge materials 40
to 270 times. Thus, a Dutch lens
grinder, pursuing a hobby, set the stage
for one of the most important fields of
today — the field of microbiology.

The modern compound microscope.
The compound microscope has been
improved steadily since the time of Van
Leeuwenhoek. Improved lenses pro¬
viding greater magnifications and preci¬
sion mechanical parts are incorporated
in the microscopes we use today. An

1-6 Top, a very early microscope; middle, a
modern compound optical microscope; bot¬
tom, an electron microscope. (Bettmann
Archive; Bausch and Lomb, Inc.; RCA)

14 UNIT 1 THE NATURE OF LIFE

1-7 Top, photomicrograph of pigeon blood
taken in 1871; middle, onion root cel is with
a modern compound optical microscope;
bottom, an electron micrograph of parts of
two onion root cells. (Armed Forces Institute
of Pathology; General Biological Supply
House, Inc.; K. R. Porter — Scientific Ameri¬
can)

instrument such as those in your high
school laboratory is shown in Fig. 1-6.
This light, or optical, microscope con¬
tains several sets of magnifying lenses.
One set is in the eyepiece, or ocular,
through which the observer views the
magnified materials. Other lens sys¬
tems are contained in objectives. Mi¬
croscopes have from one to four objec¬
tives. These, in combination with eye¬
pieces of varying magnifications, give
different degrees of enlargement. Light
is received by a mirror and is directed
through the lens systems to the observ¬
er s eye.

A standard microscope, used in
most high school laboratories, usually
provides a low power magnification of
100 times and a high power magnifica¬
tion of 430 or 440 times. These magni¬
fications reveal cells and many micro¬
scopic plants and animals. Bacteria
and other extremely small organisms, as
well as smaller cell structures, require
greater magnification. This is provided
in a special objective that provides an
enlargement of 1,000 to 1,300 times, de¬
pending on the eyepiece used with it.
Such bacteriological, medical, and re¬
search microscopes are high-precision
instruments with extremely sensitive op¬
tical systems and mechanical parts.
Limitations of the light microscope.
You may wonder why, in this age of
precision instruments for nearly every
use, it is not possible to build a light
microscope with a magnification much
greater than 1,300 times. It is true that
we could produce the lenses and the
mechanical parts for such a microscope.
However, the problem lies in the prop¬
erties of light itself.

Two factors must be considered in
the use of a microscope. One is mag¬
nification, or the enlargement of an
image. The other is resolution, or the

CHAPTER 1 THE SCIENCE OF LIFE 15

providing of a visible image in which
details can be seen. As light rays pass
through material on a microscope stage,
they reflect from surfaces of the mate¬
rial. This produces an image. The
light then passes through the lenses of
the objective and the eyepiece. Each
lens bends the light rays, thus spreading
them apart. This forms the magnified
image that reaches the observer’s eye.
The greater the magnification, the more
the rays are bent and spread. This re¬
duces the amount of light that reaches
the observer’s eye and decreases the res¬
olution. You will notice the difference
in the brightness of the field and the
resolution under low power (100X) as
compared with high power (430 or
440X) . There is a limit to which light
rays may be spread and still produce an
image with sufficient resolution to be
visible. This is a magnification of
about 1,500 to 2,000 times.

You might compare this problem
with the magnification of a newspaper
picture. Without magnification, the
picture is clear and the details are sharp.
If you view it through a reading glass,
you see that the picture is made up of
small dots. Now, if you use stronger
and stronger lenses, the dots appear far¬

ther and farther apart. Finally, so few
are visible that the picture is lost.

The electron microscope. An entirely
new principle of magnification allowed
the biologist to explore a realm of ultra-
microscopic particles that the light mi¬
croscope had never revealed. This was
the electron microscope. This remark¬
able instrument substitutes streams of
electrons for light.

Figure 1-8 shows a comparison of
a light and the most recent electron
microscope. We shall follow the elec¬
tron beams from the energy source to
the view plate on which the highly mag¬
nified image appears. A very fine tung¬
sten wire is heated by an electric current
to a temperature of 2,000° C. This pro¬
duces a cloud of electrons that boil off
around the wire filament. The elec¬
trons are then accelerated into a vacu¬
um chamber by an electron gun. The
chamber through which the electrons
stream must be nearly free of air mole¬
cules with which the electrons would
collide. As the electrons enter the vac¬
uum chamber, they pass through two
electromagnets. The material to be ex¬
amined is mounted on fine copper wire
mesh. The electron beam passes
through the material on the wire mesh

SOURCE OF ILLUMINATION
Electrons

CONDENSER LENS
Magnetic

SPECIMEN STAGE

OBJECTIVE LENS
Magnetic

PROJECTOR LENS
Magnetic

IMAGE

Light

Glass

1-8 The electron micro¬
scope compared with a
modern light micro¬
scope. (RCA)

ELECTRON MICROSCOPE

LIGHT MICROSCOPE

16 UNIT 1 THE NATURE OF LIFE

and enters the electromagnets. Both of
these bend the electron beams and pro¬
duce high magnifications. The beams
are invisible and are dangerous radia¬
tions if contacted by the operator.
However, when they strike a phospho-
rous-coated view plate enclosed in lead¬
ed glass for safety, they produce a visible
image. Frequently the image on the
view plate is photographed for more de¬
tailed study. Photographic plates are
inserted below the view plate. The
electrons penetrate the photographically
sensitive substance on the plate. This
photographs the magnified image just as
you would take a picture with a camera.

Prints made from the plate negatives
may be enlarged to make extremely fine
details visible to the eve. But no details
will appear in the enlargement that the
electron microscope did not record.

With one electromagnet the in¬
strument magnifies from 1,410 to 32,000
times. Thus, the lowest magnification
of the electron microscope overlaps the
highest magnification of the light mi¬
croscope. By changing the electromag¬
net it is possible to reach a magnifica¬
tion of 100,000 to 200,000 times. If
this image is enlarged five times photo¬
graphically, an object in a print appears
1,000,000 times its actual size.

IN CONCLUSION

Biology has probably progressed more rapidly in the last 20 years than it did in
the previous 20 centuries. This progress is due largely to the fact that the
electron microscope and biochemical procedures have brought the science to
the molecular level, at which all living things have much in common.

Biology, like all of science, is an organized body of knowledge gamed
from experimentation and objective observation, considered to be true and re¬
liable but subject to change and revision in the light of new discoveries. Biol¬
ogy is also a method of inquiry and problem solving involving several proce¬
dures for investigation. The conquest of polio affords a dramatic example of
the success of scientific methods.

The controlled experiment narrows a procedure to the testing of a single
variable or experimental factor involved in the hypothesis. The experiment is
accompanied by a control, in which all factors except the experimental factor

are duplicated.

The biologist looks at life from many different standpoints: structure,
function, interrelationships with the environment, classification, and heredi¬
tary mechanisms are but some examples of approaches in biology.

In the next chapter we shall attempt a general definition of the phenome¬
non of life - what makes living things different from nonliving things?

BIOLOGICALLY SPEAKING

anatomy
applied science
control
ecology
experimental

genetics
hypothesis
physiology
pure science

research method
taxonomy
technical method
variable

CHAPTER 1 THE SCIENCE OF LIFE 17

QUESTIONS FOR REVIEW

1. Give several examples of scientific achievements that illustrate the inter¬
national aspects of science.

2. List examples of the influence of progress in areas related to biology to
advances in the biological sciences.

3. Describe the contributions of Heine, Medin, Wickman, Landsteiner, Salk,
and Sabin in the conquest of polio.

4. How was a scientific control involved in the testing of gamma globulin?

5. How is the Sabin polio vaccine different from the Salk vaccine?

6. Describe the steps of the research method.

7. What is a hypothesis?

8. We often think of research as centering in the laboratory. Why is a
library equally important?

9. What is the purpose of a control in scientific experimentation?

10. Why is the reporting of methods, results, and conclusions an important
part of scientific research?

11. Give several examples of application of the technical method.

12. Distinguish between pure and applied science.

13. Using a tree as an example, explain how a biologist might examine it from
the standpoint of anatomy, physiology, ecology, taxonomy, and genetics.

14. Describe one of Van Leeuwenhoek’s early miscroscopes.

15. Locate the lens systems in a modern compound microscope.

16. Distinguish between magnification and resolution of a light microscope.

17. What energy source is substituted for light in an electron microscope?

18. How is the enlarged image received in an electron microscope?

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the integral relationship of academic freedom and scientific pur¬
suits.

2. In what ways is science incompatible with superstition?

3. Discuss the ways in which progress in biology has paralleled the perfec¬
tion of the microscope.

4. Why is it important that a scientist be willing to recognize his own limita¬
tions as well as those of science generally?

5. What stages in the conquest of polio illustrate scientific cooperation on a

worldwide basis?

6. Outline a controlled experiment designed to test a single experimental
factor.

7. Compare the research and technical methods from the standpoint of pur¬
pose and procedure.

8. Discuss the principles of an electron microscope that overcome the limita¬
tions of the light microscope.

CHAPTER 2

THE

LIVING

CONDITION

The living and the nonliving. What
is life? How did it originate? As a first
step in investigating the living condi¬
tion, we shall make some basic compari¬
sons of living and nonliving things. Fig.
2-1 shows a natural setting in a shallow
pool at the edge of a woods. A frog is
sitting on a rock while a dragonfly hov¬
ers overhead. A bluejay, perched on
the branch of a shrub, is watching the
squirrel on the ground. Several mush¬
rooms have pushed through the moist
soil. Several kinds of aquatic plants
are growing in the shallow water of the
pool. It is no problem to separate the
living things from the nonliving in this
scene. The plants and animals are or¬
ganisms. That is, they are complete
and entire living things, composed of
living substances and performing life
activities. The soil, the water, and the

rock are nonliving. Their substances
are different and they lack the life ac¬
tivities of the organisms.

Isn’t it true, however, that the ma¬
terials composing the organisms came,
directly or indirectly, from the soil, air,
and water. It is also true that when the
organisms die, they will decompose and
return to nonliving substances. Thus,
living substances and nonliving materi¬
als of the earth have a close relationship.
To distinguish one from the other, we
must consider differences in origin,
chemical composition, structure, and
function. Let us take a closer look at
living and nonliving things.

Life is self-perpetuating. Life arises
from life. Biologists call this the prin¬
ciple of biogenesis (by-oh-jen-i-sis) .
The bluejay shown in Fig. 2-1 began its
life as a fertilized egg from which it de¬
veloped and hatched. A tiny mass of
living substance no larger than a pin¬
head developed into a new organism.
However, this substance did not origi¬
nate in the egg. It was contributed by
the parent birds. In this manner the
life of the parents is perpetuated in the
offspring. The same can be said of the
squirrel, the frog, and the dragonfly.
Similarly, the original substance of the
grass came from seed that was formed
from materials of the parent plants.

You might compare this perpetua¬
tion of life to the lighting of a new fire
from one which is burning. If a fire
goes out, the flame is lost and is not re¬
kindled. This can be compared with
the death of an organism. But if new
supplies of fuel are lighted from burn¬
ing fires, the flames can be preserved
endlessly. Could it be, then, that the
original life of the earth still exists in all
of the forms in which we find living or¬
ganisms today? Certainly, living organ¬
isms have not arisen suddenly from non-

CHAPTER 2 THE LIVING CONDITION 19

2-1 Name the living and nonliving things shown in the drawing. Explain why
you classify them in these two groups.

living materials. Life comes only from
life. Would it surprise you to learn
that this concept of biogenesis is a
rather recent concept?

The myth of spontaneous generation.
From ancient times until less than a
century ago, people generally believed
that certain nonliving or dead materials
could be transformed into living organ¬
isms. We refer to this belief as abio-
genesis ( ay-by-oh-jen-i-sis ) , or spontane¬
ous generation.

The ancients were familiar with the
hatching of birds from eggs and with
the birth of larger animals such as mam¬
mals. However, they knew little or
nothing about the growth and develop¬
ment of smaller animals such as insects
and worms. Animals of this sort were
linked with stories of spontaneous gen¬
eration.

The Roman poet Vergil outlined a
recipe for producing insects from mud.
It was generally believed that decaying

meat changed to maggots, which later
became flies. One of the most astound¬
ing accounts of spontaneous generation
came from Jean Van Helmont about
three centuries ago. Van Helmont out¬
lined a method of producing mice from
grains of wheat and human sweat. Ac¬
cording to his directions a dirty shirt
placed in a container with grains of
wheat would produce mice in 21 days.
Supposedly the mice were formed from
the fermenting wheat, while the human
sweat in the dirty shirt provided the
“active principle” necessary for the
process. Other accounts of spontane¬
ous generation are equally interesting
and amazing. Frogs and fish were
thought to be generated in clouds during
a thunderstorm and fall to the earth
with rain. Honeybees supposedly came
from the decaying carcasses of animals
such as horses. Actually, these insects
were flies that resemble honeybees and
they came from maggots that hatched

20 UNIT 1 THE NATURE OF LIFE

2-2 This drawing of the legendary Wak-Wak
tree of the South Pacific, which bore human
fruit, appeared in a Turkish history book
published in 1730. (Bettmann Archive)

from eggs laid in the carcass. However,
for centuries, no one observed this egg
laying, and the theory remained un¬
challenged. According to another su¬
perstition, geese were formed from bar¬
nacles attached to wrecked ships.

Redi’s blow to spontaneous generation.
From ancient times until late in the
17th century, the best minds accepted
the theory of spontaneous generation
without question. No one had ever
proved that spontaneous generation
could occur, but at that time demon¬
strated facts were not necessary for
drawing conclusions. However, Fran¬
cesco Redi (ray-dee), a 17th century
Italian scientist, demanded more than
theorv and supposition for his views on
spontaneous generation. Can decaying

flesh form flies? Redi said no. Redi
claimed that flies came from eggs laid
by flies and that the decaying meat pro¬
vided nothing more than nourishment
for the maggots.

To prove his theory Redi conduct¬
ed an experiment in the year 1668
which would be considered reliable even
by the standards of modern science.
According to his own detailed account
of the experiment, Redi placed some
pieces of snake, some fish, some “eels of
Arno,” and a slice of milk-fed veal in
each of four clean jars. He then pre¬
pared a duplicate set of four jars. One
set of jars was left open. The other,
which we would designate as a control
today, was covered and securely sealed
with wax. Flies were soon attracted to
the open jars, which they entered to lay
eggs. Within a short time maggots
appeared in all of the open jars. Several
weeks later Redi opened the sealed jars
and found putrefied meat but no mag¬
gots. On the basis of evidence from the
experiment, Redi concluded that flies
originate from flies and not from decay¬
ing meat by spontaneous generation.

Had he stopped at this point, how¬
ever, his critics would have argued that
air was necessary as an “active princi¬
ple” in spontaneous generation. It was
true that the sealed jars did not admit
air. Thus, Redi’s first experiment in¬
volved not one, but two variable factors
— air and flies.

In a second experiment Redi pre¬
pared four jars with the same materials
as before and covered each with a fine
cloth he called “fine Naples veil.” Air
passed through the cloth freely, but
flies could not enter the jars. As the
meat decayed, flies gathered on the
cloth covers and laid eggs, but no mag¬
gots appeared in the decaying meat. In
this second experiment Redi supplied

CHAPTER 2 THE LIVING CONDITION 21

2-3 Redi’s controlled experiment showed the relationship between flies and the
occurrence of maggots in rotten meat.

the convincing proof that flies come
only from flies. Did Redi’s work dis¬
prove spontaneous generation? You
might think so, but this theory was to
remain in general acceptance for more
than two centuries after Redi. True,

Redi had proved that flies come only
from pre-existing flies and this might ap¬
ply to other insects, but he had not
proved that worms and other lowly an¬
imals do not originate by spontaneous
generation.

22 UNIT 1 THE NATURE OF LIFE

Microorganisms and abiogenesis. With
the development of the microscope,
18th century biologists found various
broths and sugar solutions to be swarm¬
ing with microorganisms. This present¬
ed a new problem in disproving spon¬
taneous generation. Where had these
organisms come from if not from the
broths? Two schools of thought devel¬
oped to explain the origin of microor¬
ganisms. One group supported spon¬
taneous generation as the only possible

answer. The other insisted that bacte¬
ria and other organisms could come
only from pre-existing organisms. Re¬
member that biologists in this day had
no idea that bacteria grow and repro¬
duce and that they are abundant every¬
where.

One of the supporters of the theory
of spontaneous generation of micro¬
organisms was the English scientist
John Needham (1713-1781). Need¬
ham prepared mutton broth in corked

fl H fi n

Not boiled Boiled Vi min. Boiled 1 min. Boiled IV2 min. Boiled 2 min.

2-4 Spallanzani’s hay-infusion experiment showed the flaws in the hypothesis
of spontaneous generation.

CHAPTER 2 THE LIVING CONDITION 23

flasks which he boiled for a few min¬
utes. After a few days he examined the
broth with a microscope and found it
to be teeming with microorganisms.
When he repeated the experiment with
various meat and vegetable broths, the
results were the same. Needham ar¬
gued that boiling had destroyed all life
in the broth and, since the flasks were
closed with stoppers, the organisms
present after standing must have been
formed by spontaneous generation.

Lazzaro Spallanzani (1729-1799),
an Italian scientist, led the opposition
in the attack on Needham’s conclu¬
sions. He boiled seeds in water for 30
minutes. Such a mixture of organic
material and water is called an infusion.
Spallanzani’s infusions were prepared in
loosely stoppered flasks that air could
enter. Microorganisms appeared in
them as they had in Needham’s broth.
This led him to further investigation in
which he used a series of five flasks con¬
taining seed infusion. One flask was
left open. Four were sealed and boiled;
one for a half minute, one for one min¬
ute, one for one and a half minutes, and
one for two minutes. After two days
Spallanzani examined all of the infu¬
sions with a microscope. He found the
infusion in the open flask to be teeming
with bacteria. The four sealed flasks
also contained bacteria, but in much
smaller numbers. However, the num¬
ber of bacteria in the sealed flasks de¬
creased as the boiling time was in¬
creased. This was a significant observa¬
tion. If boiling the infusion for two
minutes reduced the number of bacteria

to the lowest in the series, would not a
longer boiling time prevent any organ¬
isms from growing? To test this as¬
sumption Spallanzani prepared addi¬
tional infusion and boiled it in sealed
flasks for 30 minutes. No microorgan-

2-5 Spallanzani (standing) also conducted
experiments with birds. (Bettmann Archive)

isms appeared in these flasks as long as
thev were sealed from the air. How-
ever, if the flasks were opened, micro¬
organisms appeared in the infusion
within a few days. To Spallanzani this
was proof that the microorganisms
came from the air, not from the infu¬
sion by spontaneous generation. But
this did not satisfy the opposition who
now claimed that the boiling had
spoiled the "active principle” in the air
necessary for spontaneous generation.
Thus the argument continued for an¬
other 50 years.

Pasteur’s decisive defeat of spontaneous
generation. It remained for Louis Pas¬
teur (1822-1895), a French chemist, to
deal a final and convincing blow to the
theory of spontaneous generation. As
a young man Pasteur was engaged in
studies of fermentation and the chemi¬
cal changes that occur as sugars are con¬
verted to alcohol. He had observed the
many yeasts and other microorganisms
present in fermenting fruit juices and

24 UNIT 1 THE NATURE OF LIFE

sugar beet juice and found that the
number of organisms increased as fer¬
mentation progressed. Pasteur was
convinced that the microorganisms as¬
sociated with fermentation came from
the air and that dust particles, grape
skins, and all other materials exposed to
the air harbored great numbers of bac¬
teria, yeasts, and other minute organ¬
isms. In testing this hypothesis Pasteur
carried out several experiments which,
while simple, were so well planned and
convincing that they could not be re¬
futed by even the strongest supporters
of the theory of spontaneous genera¬
tion.

In his first series of experiments, he
used a variety of liquids which would
support bacteria and other microorgan¬
isms if exposed to the air. These in¬
cluded an infusion of yeast in water,
brewer’s yeast, sugar and water, and su¬
gar beet juice. Each of these liquids
was sealed in a long-necked flask and
boiled for several minutes. He then
took the sterile flasks to a variety of
places in which the air contained dif¬

ferent amounts of dust. Flasks opened
along dusty roads were quickly contam¬
inated as evidenced by abundant
growth of microorganisms within a few
days. However, flasks opened on hills
and mountains showed much less
growth of microorganisms. These re¬
sults bore out his theory that bacteria
and other organisms were present in the
air with dust particles.

It now remained for Pasteur to
prove his theory under the controlled
conditions of his laboratory. This ex¬
periment involved his famous swan¬
necked flasks. Again he prepared a
liquid containing sugar and yeast and
poured it into a long-necked flask. He
then heated the neck of the flask and
bent it into an S-shaped curve resem¬
bling a swan’s neck. After preparing
the flask, he boiled the liquid for several
minutes. Air forced out of the flask
during the boiling returned through the
neck as the liquid cooled. Throughout
the experiment air moved through the
open neck. Water and dust particles,
however, settled in the trap formed by

2-6 Pasteur's experiment with swan-necked flasks convinced most biologists
that spontaneous generation does not occur.

CHAPTER 2 THE LIVING CONDITION 25

the bent neck. Even though the sugar Living things have a unique chemical
solution was in direct contact with the organization. Only in living organisms
outside air, no organisms appeared in do we find the organized activity of a
the liquid in the flask. Pasteur found complex system of substances that we
that such a flask would remain sterile refer to as life. We often speak collec¬
tor more than a year. But if the flask tively of this system of substances as
was tipped to allow the liquid to flow protoplasm. And we would be correct
into the bent neck and contact the in saying that only living organisms

trapped dust, microorganisms appeared
in the flask in great numbers within a
few days.

Thus, Pasteur’s simple experiment
with his swan-necked flasks refuted all
of the arguments his opponents could
advance. Boiling had not destroyed the
property of the liquid in the flask to
support microorganisms, nor had the ex¬
periment excluded the “active princi¬
ple” of the air thought to be necessary
for spontaneous generation. Both the
liquid and the air in the flask were suit¬
able for the growth of bacteria, molds,
and other organisms if they were intro¬
duced from an outside source.

So it was that Louis Pasteur substi¬
tuted a proved and valid concept, bio¬
genesis, for a theory which had been ac¬
cepted for centuries without any real
evidence. Life comes only from life.
Experimental evidence has borne this
out from the time of Pasteur to the
present day. However, we cannot as¬
sume that abiogenesis has never oc¬
curred, nor that it could never happen
again. How did life begin in the first
place? Will a biologist some day as¬
semble just the right nonliving materi¬
als in a test tube and organize living
matter? Is a virus particle a form of
matter which fluctuates between the
nonliving and the living condition?
While we accept biogenesis as a funda¬
mental property of the living condition,
we must keep these exceptions in mind
and explore them further at a more ap¬
propriate time.

organize protoplasm.

At one time biologists thought that
protoplasm was actually a living sub¬
stance. Today we know that it is not
a substance in the sense that water,
salts, sugars, and acids are substances,
nor is it living. Furthermore, none of
the substances composing protoplasm
are living. However, when certain pro¬
teins, carbohydrates, fats, and other sub¬
stances are organized into a system by
a living organism, a state of chemical
activity we call a living condition is
established.

2-7 From this experiment, performed in the
17th century, van Helmont concluded that
the living matter of the tree came entirely
from water. Of what substance was he un¬
aware?

26 UNIT 1 THE NATURE OF LIFE

If you analyze the protoplasm of
numerous plants and animals you will
find basic similarities in the substances
present. In this respect protoplasm is
a unifying characteristic of all organ¬
isms. But while there are similarities,
there are almost limitless variations in
protoplasm. It differs in every kind of
organism. It even varies in individuals.
The materials in your protoplasm are
not exactly like those of any other per¬
son, unless you have an identical twin.
Furthermore, these materials are con¬
stantly changing. You can see, then,
why protoplasm is an indefinite and
elusive word. It is still a good biologi¬
cal term, however, as long as you use it
properly — not as a definite substance
and not as a living material, but as a
complex, continually changing system
of substances which establishes the liv¬
ing condition.

Living things have a constant energy
requirement. All chemical activities
require energy. Since life is basically
chemical activity, it requires a constant
source of energy. As you will learn, al¬
most all of the energy used by living
things comes ultimately from the sun.
Both plants and animals obtain energy
more directly by the breakdown of com¬
plex chemical substances we call foods.
The energy released in this way is in
turn used to support other chemical and
physical processes. Life continues only
as long as these energy transformations
occur.

Living things have a cellular organiza¬
tion. If you conducted a microscopic
survey of a large number of organisms
and parts of organisms, you would dis¬
cover that regardless of size and com¬
plexity, they are all composed of cells —
one cell, a few cells, or billions of cells.
Cellular make-up is peculiar to organ¬
isms. Because cells enter into the

make-up of every organism, they have
been referred to as the common denom¬
inator of life. As you study diverse or¬
ganisms from bacteria to seed plants
and ameba to man, you will deal con¬
tinually with cells. Some will be rela¬
tively simple in organization and others
will be highly specialized. But the vari¬
ous substances associated with the liv¬
ing condition are always organized in
these basic structural units.

Cells may remain for some time
after the death of an organism. Thus,
we cannot say that the presence of cells
is evidence of a living condition. How¬
ever, we can say that cells are a product
of living organisms and that they are
never organized in nonliving materials.

If cells are basic units of organisms,
then what about the viruses? Are they
organisms? Are they living? When
you deal with the composition of viruses
in Chapter 15, you will discover that
virus particles are not cells. They are
subcellular, or below the level of cells in
composition. However, they have cer¬
tain chemical properties that are found
only in living cells. Perhaps the viruses
are a link between living organisms and
nonliving substances. We shall explore
this possibility more fully in Chapter
15.

Living organisms are capable of growth.
At least for a time, living organisms
grow by enlargement. So do crystals
and icicles and they are, of course, non¬
living. Is it correct, then, to refer to
growth as a characteristic of the living
condition? First we must determine
just what we mean by growth.

An icicle hanging from a roof may
increase in size as water trickles over its
surface and freezes before dripping off.
This kind of enlargement is growth by
external addition. The same would ap¬
ply to a crystal growing in a solution.

CHAPTER 2 THE LIVING CONDITION 27

The growth of a living organism is
entirely different. A tree does not
grow by taking more of its own sub¬
stance from the soil and atmosphere
and adding it to the material already
present. Nor can you say that you grow
by adding food to your body or your
body is an accumulation of the foods
you have eaten.

There are many complex chemical
processes involved in the growth of an
organism. During these processes large
and complicated molecules are formed.
As a result of these biochemical activi¬
ties, substances quite unlike those com¬
posing nonliving materials are incorpo¬
rated in the make-up of living organ¬
isms. Thus, living things do not accu¬
mulate their substance — they organize
it, or assimilate it.

Perhaps you question the reference
to growth as a characteristic of the liv¬
ing condition. Plants and animals may
continue to live long after growth has
apparently ceased. However, the sub¬
stances composing organisms are only
temporary. Replacement must occur
continually. Thus, growth and main¬
tenance without enlargement continue
throughout life.

Living things have a definite form and
size range. You can describe a black
bear, a jack rabbit, a rainbow trout, a
sugar maple tree, or a Douglas fir with
reasonable accuracy. With some varia¬
tion, your description will fit all others
of the same kind. Furthermore, you
can predict the approximate size each
will attain at maturity. On what do
you base these descriptions and predic¬
tions of size? No doubt you would be
amazed to see a rainbow trout weighing
200 pounds or a maple tree towering
300 feet in a forest. Some fish reach
this size, and so do some trees. How¬
ever, you expect a plant or animal to re¬

semble its parents both in form and in
size at maturity. Have you wondered
why? Perhaps you would say that size
and form are determined by heredity.

It might be more accurate to say that
they are determined by substances
called genes. Genes cause the rainbow
trout to develop a particular form of
fins, a certain body form, and a distinc¬
tive coloration. All fish have genes, but
only rainbow trout have the particular
gene structure and combinations which,
when transmitted to offspring, regulate
the development of young rainbow
trout.

Like produces like — we are all fa¬
miliar with this concept. But only in
recent years have we understood how
genes operate in heredity. You will
find out much more about the composi¬
tion and regulatory functions of genes
in your study of genetics.

Living organisms have a life span. A
rock you pick up may be a million years
old. And it may remain in its present
condition another million years or more.
The substance composing the rock is
quite a contrast to the substances com¬
posing an organism. Life is activity.
When conditions are no longer favor¬
able for this activity, life ceases. Thus,
all organisms have a definite period of
existence which we refer to as the life
span. We may divide this period of
existence into periods or stages, as fol¬
lows: 1. beginning, or origin; 2. growth;
3. maturity; 4. decline; and 5. death.

A period of rapid growth follows
the formation of an organism. This
growth period may last a few minutes,
a few weeks, several months, or many
years, depending on the organism. As
the mass of the plant or animal in¬
creases during its period of growth, the
rate of growth decreases. Finally ma¬
turity is reached. During this period

28 UNIT 1 THE NATURE OF LIFE

growth is reduced to repair and replace¬
ment of vital substances. Eventually
the organism reaches a point at which
repair or replacement of damaged or
broken-down materials is impossible.
This marks the period of decline, or
senility, which is followed inevitably by
death.

How long may a plant or animal
survive? Here we find great variations.
Furthermore, we find that the life spans
of organisms are for the most part regu¬
lated by factors or conditions prede¬
termined. That is, barring disease or
accidental death, the life span of any
particular plant or animal is about the
same as all others of its kind. The pe¬
tunia, marigold, and zinnia plants of
your flower garden grow, reproduce, and
die in a single season. On the other
hand, a white oak tree may live 500
years. A “big tree” ( Sequoia gigantea)
of California would still be young at
this age. These remarkable trees may
live several thousand years.

Certain insects live but a few
weeks. Five years is old for some fish.
Five to 10 years is the normal life span
of a chicken. Horses may reach an age
of 30 or more. The average life span of
man is 68 to 70 years, but may be ex¬
tended to 100 years or more in a few
generations.

We mav sav that a definite life

J J

span distinguishes living organisms
from nonliving materials. Is a life span
a positive limitation of the living con¬
dition? If an organism can grow and
maintain its substances part of its life,
why can’t it live indefinitely? Perhaps
it could, if certain changes could be
avoided. In your study of biology, you
will deal with various one-celled organ¬
isms. Among these are bacteria. An
individual bacterium may be formed
and mature in half an hour or less. At

this point it splits into two bacteria,
both of which are immature and capa¬
ble of growth. Thus, a bacterium never
dies of old age as long as conditions are
favorable for growth and reproduction.
The same applies to the ameba and oth¬
er one-celled animals. Perhaps you
have already reasoned that an ameba
you see today is part of the first ameba
which ever lived.

Continued growth becomes a prob¬
lem when organisms become larger and
many-celled. When a certain size is
reached, growth ceases and decline is
inevitable. Would it be possible, then,
to remove part of a larger organism
from its own mass which spells eventual
doom and allow it to live indefinitely?
This was actually done some years ago
at the Rockefeller Institute for Medical
Research. On January 17, 1912, Dr.
Alexis Carrel removed a small piece of
tissue from the heart of a chick just
hatching. He placed this mass of
throbbing heart muscle in a solution of
chicken blood plasma (the fluid part of
blood) and put it in a chamber. The
warmth and humidity in the chamber
duplicated the conditions in the chick¬
en’s body. For some time the heart tis¬
sue continued to beat normally. Grad¬
ually, however, the beat slowed down
and showed evidence of stopping en¬
tirely. It appeared that waste products
formed in the active tissue were accu¬
mulating and would soon poison it. At
this point the tissue was washed with
salt solutions and beating resumed nor¬
mally. Regular replacement of plasma
removed this problem and the tissue be¬
gan to grow. As the bulk of the tissue
increased, a new problem arose. Tissue
deep in the mass could no longer re¬
ceive nourishment. This problem was
corrected bv dividing the mass. A rou¬
tine for care of the tissue culture was

CHAPTER 2 THE LIVING CONDITION 29

2-8 A piece of the original chicken heart muscle growing in the apparatus de¬
veloped by Dr. Carrel. (Rockefeller Institute)

established in which new plasma was
added and the mass was divided every
48 hours. Is the heart tissue still living
after more than 50 years? It could be,
had not the experiment ended in the
late 1940’s. As a part of a normal chick¬
en heart, subject to growth and matu¬
rity, it would have died in five to 10 years
or less. Thus, we might conclude that
organisms have definite life spans, even
though their substances have the ca¬
pacity for indefinite life.

Living things have the capacity to re¬
produce. Since the life of a plant or
animal is limited in duration, reproduc¬
tion is necessary to perpetuate life. Re¬
production takes many forms in the liv¬
ing world. It mav be the division of a
cell, or the formation of a special cell
such as an egg. It may even consist of
the removal of a part of a parent organ¬

ism which is capable of independent
growth, such as the cutting of a stem or
a root of a plant. The same principle
is always involved, however — a mass is
divided or a small portion of a mass is
separated from the parent. A seed con¬
tains a small amount of substance of
the parent plants. A human being de¬
velops from a mass of living material no
larger than a pinhead, contributed by
both parents and capable of living and
growing a lifetime.

Living things are capable of response.
Only living protoplasm has the capabil¬
ity of responding to external conditions.
We refer to this interaction of a living
system and its environment as irritabil¬
ity. The environmental stimulus may
be a light factor, temperature, water,
sound, pressure, the presence of chemi¬
cal substances or a source of food, or a

30 UNIT 1 THE NATURE OF LIFE

threat to survival. The response , or re¬
action of the organism to the stimulus,
varies with its capability. This capabil¬
ity is determined by the structural and
physiological organization of the organ¬
ism. For example, a plant response
may be a growth reaction, as when a
root pushes through the soil toward a
water supply, or when a stem grows un¬
evenly and bends toward the light on a
window sill.

Animals are capable of reacting in
more complex ways or on higher levels.
Sight, hearing, taste, touch, and smell
are responses to environmental condi¬
tions. More complicated responses in¬
clude fleeing from an enemy, defending
oneself in time of danger, hunting for
food, and seeking a place in which to
build a nest. Nonliving substances may
be influenced by environmental condi¬
tions, as when water freezes and be¬
comes ice or changes to steam at the
boiling point. But these changes in
form are not responses. Only living or¬
ganisms are capable of responding to a
stimulus.

Living things have a critical relation¬
ship with the environment. All organ¬
isms face a constant struggle for life.
Part of this struggle centers around re¬
quirements for maintaining the living
condition. Another part involves a
struggle with other forms of life to sur¬
vive in a highly competitive biological
society.

Environmental factors such as
light, moisture, oxygen supply, tempera¬
ture, air currents, soil conditions, and
variations in the earth’s surface have a
direct influence on living organisms.
Environmental conditions differ in vari¬
ous localities. As these conditions vary,
plant and animal life in the region var¬
ies. Desert plants and animals cannot
survive in a moist woods. Nor can prai¬

rie life survive in marshes. From the
arctic wastelands to the tropics and
from mountains to valleys, there are cer¬
tain kinds of organisms that find each
environment ideal.

The environment must supply
plant life with materials with which to
organize the complex chemical sub¬
stances required as food. Animals in
turn eat the plants. Thus, plants be¬
come part of the necessary environment
of animal life.

Even if the environment is proper,
a plant or animal must compete with
other living things. Sometimes the
struggle is competition for the needs of
life. Other times it is a struggle with
natural enemies. A small tree growing
from the forest floor must compete with
many other plants for a place to grow.
A few survive, while many perish. The
robin is a constant threat to the worm
and caterpillar. But hawks, crows, and
cats are a constant threat to robins.
The struggle for existence is a problem
to all living things.

Variation and adaptation. Conditions
in an environment change from time to
time. Sometimes these changes are
sudden, as in the case of a severe
drought, a destructive storm, or a devas¬
tating fire. Other changes may occur
much more slowly and over a long pe¬
riod of time. These include climatic
changes, changes in soil, or the gradual
erosion of hills and mountains. If an
organism is not entirely suited to its en¬
vironment, it can no longer satisfac¬
torily compete with other living things.
One of three things must happen: 1. it
must migrate to more suitable surround¬
ings; 2. adaptations must occur; or 3. it
will perish as a species.

Animals capable of movement may
leave unfavorable surroundings and seek
an environment in which they can meet

CHAPTER 2 THE LIVING CONDITION 31

their needs. These migrations may be
seasonal or more permanent, depending
on the nature of the environmental
changes. Plants for the most part lack
this motility and must survive or perish
in the place where they are growing.
However, even the nonmotile plants
produce seeds and fruits or other repro¬
ductive structures which may be distrib¬
uted far from the parent plant. If even
a few seeds chance to fall in favorable
places, the species survives.

Another characteristic that allows
organisms to survive changing condi¬
tions is variation. This means that no
two offspring are exactly alike, nor are
they exactly like their parents. Count¬
less variations occur which do not affect
survival. Some are harmful and may
even hasten death. Occasionally, how¬
ever, a variation occurs which gives an
organism a better chance to survive. If
the plants or animals having these fa¬
vorable chance variations interbreed,
the species may gradually become bet¬
ter suited to its environment. We
speak of this process as adaptation.

We shall consider one example of
adaptation in the deer family. The
white-tailed deer ranges over most of
North America, Mexico, and Central
America. In the North, it is a slender,

long-legged animal. It can run at top
speed through a dense forest and hurdle
logs five feet or more off the ground.
In Florida a tiny deer known as the Key
deer lives in the marshes. This “toy”
variety of the white-tailed deer weighs
50 pounds or less. It can easily hide in
a clump of marsh grass. The Key deer
could not survive in the northern forest.
Wolves and other flesh-eating animals
would have exterminated it years ago.
But neither could the northern white¬
tailed deer find shelter in the marshes
of Florida.

In referring to adaptation we often
speak of an organism as modifying to
fit its environment. This does not hap¬
pen. Plants and animals do not change
in order to survive. They survive be¬
cause of change. The northern variety
of the white-tailed deer did not develop
long legs in order to run fast. It runs
fast because it has long legs. This hap¬
pens to be a favorable variation for the
northern deer. Long legs make it better
adapted to its total environment by
enormously improving its chances for
survival.

Thus, whether variations are favor¬
able or unfavorable, we may add the
possibility of variation to our list of
characteristics of the living condition.

IN CONCLUSION

What, after all, is the living condition? Is it origin by biogenesis? Is it the
organization of the complex materials composing protoplasm? Is it cellular
organization and cell activity? Is it growth, reproduction, and control by
genes? Is it capability of response and a critical relationship with the environ¬
ment? . ... • jrn

The fact is that life is all of these and more. A definition of lire is aim-

cult to make. Biologists have compared the problem of defining life with
shadow boxing. You strike out with what seems to be a specific property of
the living condition onlv to find that it does not always apply, or that it is only
a partial definition. Perhaps we can summarize by saying that life is energy

32 UNIT 1 THE NATURE OF LIFE

transformations occurring in chemical changes involving the various materials
present in a cell.

It is logical, then, that our next approach to the study of life should con¬
cern chemistrv, from the most basic forms of matter to the complex chemical
products of life activities.

BIOLOGICALLY SPEAKING

adaptation life span spontaneous generation

biogenesis organism variation

irritability protoplasm

QUESTIONS FOR REVIEW

1. What is the biological concept of an organism?

2. Summarize the meaning of biogenesis.

3. Give examples of myths founded on belief in spontaneous generation.

4. Describe Redi’s experiment to disprove spontaneous generation.

5. What evidence in Needham’s experiments led him to the false conclusion
that microorganisms developed in broth by spontaneous generation?

6. How did Spallanzani refute Needham’s conclusion?

7. Describe the flasks Pasteur used in his experiments to disprove sponta¬
neous generation.

8. Give a definition of protoplasm in line with modern biological concepts.

9. In what way is growth by assimilation different from growth of nonliving
material?

10. Explain how growth of an organism is regulated internally.

11. List five stages in the life span of an organism.

12. In what respect is the cell the common denominator of life?

13. Define irritability.

14. List several environmental conditions that have a direct influence on liv¬
ing organisms.

15. List three possible consequences of an organism not being suited to its
environment.

16. Explain the relationship of variations to adaptations.

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss how Redi’s experiments complv with modern scientific practice.

2. Explain how Pasteur accounted for all variable factors in his experiments
disproving spontaneous generation of microorganisms.

3. Discuss the biological principles demonstrated in Dr. Alexis Carrel’s fa¬
mous experiment with chick heart tissue.

4. Discuss various ways in which living organisms respond to external stimuli.

5. In what ways is the struggle for existence a problem to all living things?

CHAPTER 3

THE

CHEMICAL
BASIS
OF LIFE

What is matter? We may define mat¬
ter, simply, as anything which occupies
space and has mass. Or you might
think of matter as any form of sub¬
stance — a liquid, a solid, or a gas. Can
you think of any form of matter that
would not fit this definition? We ordi¬
narily think of solids and liquids as oc¬
cupying space and having mass. But
what about gases, such as oxygen, nitro¬
gen, hydrogen, and carbon dioxide? Do
they occupy space and have mass? Air
is a mixture of gases. Does an inflated
tire weigh more than a flat tire? If so,
air has mass. And what about hydro¬
gen? A hydrogen-filled balloon rises
through the air. Does hydrogen weigh
even less than air? Light as it is, hy¬

drogen still has mass and therefore has
weight.

Matter may be changed from one
form to another without altering its
chemical composition. Such a change
is referred to as a physical change. This
occurs when water changes from a liq¬
uid to ice at the freezing point and to
steam at the boiling point. It may also
change from a liquid to gaseous water
vapor by evaporation. In these changes
the chemical structure of the water is
not altered.

When a chemical change occurs,
matter changes chemical composition.
A chemical change takes place when
wood burns. The substances compos¬
ing the wood are transformed into vari¬
ous gases which enter the atmosphere
and solid materials which remain as ash.
Thus, the substances which composed
the wood are in new chemical forms as
a result of a chemical change. How
was the wood formed in the first place?
This required many chemical changes
which occurred in the cells of a living
tree.

Energy and energy changes. Energy is
associated with matter, but it is not a
form of matter. We may define it as
the capacity for doing work or causing
a change, or a force which acts on mat¬
ter or has the potentiality for acting on
matter. Notice that energy may be an
active force, and that it may also exist
with the possibility of release as an ac¬
tive force.

We speak of energy representing
capacity but not actively released as
potential energy. When it is actively
expressed, as in motion, we refer to it
as kinetic (ki-net-ik). During an en¬
ergy change, potential energy may be¬
come kinetic. Similarly, kinetic energy
may be converted to potential energy.
For example, a boulder resting at the

34 UNIT 1 THE NATURE OF LIFE

3-1 The potential energy of water at a high
level changes to kinetic energy as it falls
over the water wheel. (Ewing Galloway)

top of a hill has potential energy be¬
cause of its position. As it rolls down
the hill, this energy becomes kinetic, or
energy of motion.

Energy of position is one form of
potential energy. Chemical energy is
another form. You will discover how
this energy exists in the organization of
chemical compounds and how it may be
released and converted to kinetic en¬
ergy during chemical changes. Kinetic
energy may occur as electric current,
heat, visible light, and invisible radia¬
tions including ultraviolet rays, X rays,
gamma rays, and cosmic rays.

Elements, the alphabet of matter. All
of the words of our language are formed
from 26 letters in various combinations.
When you think of the size of an un¬
abridged dictionary, you realize what an
enormous number of combinations

these letters can form. In somewhat
the same way all matter in the world is
composed of 92 natural, basic sub¬
stances called elements, in various
chemical combinations. In addition to
these 92 natural elements, 11 more have
been produced in atomic research lab¬
oratories. Thus the total number of
known elements at the present time is
103. Only about 30 elements, however,
are well known, and less than half this
number form more than 99 percent of
the substances of the earth and atmos¬
phere. We might think of the natural
elements as a 92-letter chemical alpha¬
bet with most of the words formed from
no more than 30 of these letters.

Elements are composed of ex¬
tremely small units of matter called
atoms. Even the largest atoms are less
than one 50-millionth of an inch in di¬
ameter. The smallest atoms, those of
hydrogen, have been estimated to be
less than one 250-millionth of an inch
in diameter. When we say that atoms
are basic units of matter, we mean that
in ordinary chemical reactions, a sub¬
stance is reduced no further than its
individual atoms. Similarly, you can
reduce a word to its letters, but you can¬
not split its letters.

We designate one atom of an ele¬
ment by means of a symbol. The sym¬
bol is the first letter or first two letters
of the name of the element. For exam¬
ple, C stands for one atom of carbon.
H represents one atom of hydrogen,
while O is one atom of oxygen. Some
chemical symbols refer to the Latin
name of an element which is no longer
used. For example, Na is sodium, Fe
is iron, and Ag is silver. Chemical sym¬
bols are used in referring to elements
and to the substances formed from
combinations of elements. They are
the chemist’s shorthand.

CHAPTER 3 THE CHEMICAL BASIS OF LIFE 35

The structure of an atom. Can you
think of anything smaller than the
smallest atom? You probably can be¬
cause you live in an age in which we
speak of atomic particles. But this has
not always been true. For centuries
even the best informed chemists thought
that atoms were indivisible forms of
matter — the smallest particles that
could exist. Today, however, we know
that atoms are composed of still smaller
particles. These same particles make
up all atoms. The difference between
oxygen, hydrogen, sulfur, iron, gold,
uranium, and all other elements is not
the kind of particles composing their
atoms but the number and arrangement
of these particles.

An atom is composed of two and,
in most cases, three basic particles.
The central mass of an atom is referred
to as its nucleus. The nucleus con¬
tains one or more particles with posi¬
tive electrical charges which we refer to
as protons. Varying numbers of new
trons are also present in the nucleus.
These particles weigh about the same
as protons, but have no electric charges.
The atomic mass of an element is the
sum of its protons and neutrons. The
atomic number refers to the number of

protons in the nucleus of that atom, re¬
gardless of the number of neutrons pres¬
ent. No two elements have the same
number of protons.

Extremely small, relatively weight¬
less particles move around the nucleus
at a high rate of speed. These are elec¬
trons. Each electron bears a negative
charge. With certain exceptions, an
atom has the same number of electrons
as protons. Thus, the negative electron
charges balance the positive proton
charges in the nucleus. Such an atom
is electrically neutral.

Hydrogen to uranium — the series of
natural elements. If you had a pile of
protons and a pile of electrons and
started to construct 92 elements from
these particles, where would you begin?
Wouldn’t you start with one proton and
one electron? This is the structure of
a hydrogen atom, with the atomic num¬
ber 1. Add another proton and a sec¬
ond electron and you have an atom with
the atomic number 2. This is helium.
Lithium has the atomic number 3. As
atomic numbers are increased, different
elements are produced. At the end of
this series of natural elements is ura¬
nium, with 92 protons and therefore the
atomic number 92.

Oxygen

Helium

Carbon

3-2 Diagrammatic representations of atoms. Protons are shown as white cir¬
cles; neutrons as black circles. Electron orbits are shown as streaks around the
nucleus.

36 UNIT 1 THE NATURE OF LIFE

Now we must consider atomic
masses — the sum of the protons and
neutrons. For example, oxygen has the
atomic number 8, but an atomic mass
of 16 in most cases. How many neu¬
trons are there in an oxygen atom?
Similarly, carbon atoms have the atomic
number 6, and most have an atomic
mass of 12. What does this indicate?
Most atoms of uranium, with the atomic
number 92, have an atomic mass of 238.
How many neutrons are present?

What are isotopes? The atomic num¬
ber of an element never varies. In
other words, the number of protons is
constant. However, the number of neu¬
trons may vary in different atoms of the
same element. This variation in neu¬
trons results in differences in the atomic
masses of atoms. Such different forms
of an element are referred to as isotopes.
Various isotopes may exist naturally, or
they may be produced artificially.

Hydrogen is an example of an ele¬
ment with three isotopes. Ordinary
hydrogen has a single proton and there¬
fore an atomic mass of 1. However, an¬
other hydrogen isotope, known as deu¬
terium, has a proton and a neutron
composing its nucleus and thus weighs
twice as much as ordinary hydrogen.
A third isotope of hydrogen, known as
tritium, has been produced artificially.
This isotope has a proton and two neu¬
trons, giving it three times the weight
of ordinary hydrogen. The atomic
weight of hydrogen is actually not 1 but
1.0079. In other words, we determine
the atomic weight by averaging the
weight of all of the natural isotopes of
an element. The actual atomic weight
of hydrogen shows you that very few
atoms in a mixture of natural hydrogen
isotopes are deuterium, the heavier form
of the element. All atomic weights to¬
day are based on a comparison of the

Electron
Proton
Neutron

DEUTERIUM

Electron
Proton

Neutron

TRITIUM

3-3 Diagrams representing the isotopes of
hydrogen.

most common isotope of carbon, with a
mass of exactly 12. The common hy¬
drogen atom has one-twelfth the mass
of the common carbon atom, and thus
is said to have a mass of one.

Many other elements have isotopes,
some natural and some artificial. Oxy¬
gen has three natural isotopes and three
that have been formed artificially. The
same is true of carbon. There are four
natural isotopes of iron and four arti¬
ficial forms. Iodine has a single natural
isotope, but 17 other isotopes of iodine
have been produced artificially.

Several natural elements have un¬
stable nuclei that undergo gradual dis¬
integration and emit radiations. These
unstable isotopes are classed as radio¬
active. Elements with naturally occur¬
ring radioactive isotopes are uranium,
radium, and thorium. Their radiations
may be alpha particles, which are he¬
lium nuclei (two protons and two neu¬
trons) traveling at a speed of 10,000 to
20,000 miles per second. Beta particles
are electrons which travel at a speed of

CHAPTER 3 THE CHEMICAL BASIS OF LIFE 37

60,000 to 160,000 miles per second.
Gamma rays, another form of radiation,
are high-energy X rays. These are the
most penetrating radiations emitted
from the nuclei of radioactive elements.

In addition to the natural radio¬
active elements, many other elements
have radioactive isotopes which have
been produced artificially. Among
these are oxygen, calcium, cobalt, and
strontium. These radioactive isotopes
are produced by bombarding atomic
nuclei with neutron “bullets” in atomic
reactors. Radioactive materials are very
important in scientific research as tracer
elements which can be identified in
chemical reactions. Others, including
radioactive phosphorus, cobalt, and

iodine have important medical uses
both in the diagnosis and in the treat¬
ment of various diseases.

The combining properties of elements.
The particles in the nucleus determine
the mass of an atom. The number and
arrangement of electrons determine its
chemical activity. As the number of
protons in the nucleus increases, so
does the number of electrons speeding
around this central mass. Do electrons
orbit around the nucleus in definite
patterns, or do they travel in erratic
paths? Scientists are not sure. Some
have referred to an electron cloud or
haze around the nucleus. Others have
likened moving electrons to bees swarm¬
ing around a hive. However, even if

3-4 The Geiger counter shown is being applied to the thyroid gland of a patient
who has been fed radioactive iodine. The areas of concentration of iodine are
recorded at the right. (Oak Ridge Operations Office)

38 UNIT 1 THE NATURE OF LIFE

electron paths are erratic and indefinite,
there is some order in their arrange¬
ment, or configuration.

Electrons move at different dis¬
tances from the nucleus of an atom.
The greater the distance, the higher the
energy level. These energy levels are
also referred to as shells. Furthermore,
chemists have found evidence that vari¬
ous shells can accommodate only lim¬
ited numbers of electrons. The first
energy level, or K shell, lies nearest to
the nucleus. This shell can hold but
2 electrons. The second energy level,
or L shell, may contain 8 electrons.
The third energy level, or M shell, also
usually holds 8 electrons. Beyond the
third level, the electron configuration
of atoms becomes more and more com¬
plex. Energy levels continue to a sev¬
enth shell.

The chemical activity of an atom
is determined by the number of elec¬
trons in its highest energy level, or outer
shell. If the shell is filled, the atom is
said to be stable and has little or no
chemical activity. For example, he¬
lium has only 2 electrons. These fill
the K shell. Helium is therefore a
stable element. The next stable num¬
ber is 10 (2 electrons in the K shell and
8 in the L shell). The element with
this atomic number is neon, another
stable element. The third energy level
is filled with 8 electrons when the
atomic number 18 is reached (2 in the
K shell, 8 in the L shell, and 8 in the
M shell). Argon, another stable ele¬
ment, has this atomic number.

However, most atoms have elec¬
trons missing in their outer shell. This
produces chemical activity, since an
atom tends to become more stable by
filling its outer electron shell. This
chemical activity will result in one of
two things:

1. Atoms may share electrons with other
atoms, or

2. Atoms may transfer electrons from
one atom to another.

Whether an atom is an electron
sharer or an electron borrower or lender
determines the nature of the substance
the combined elements will produce.
The nature of covalent bonds. When
atoms join and fill their outer shells by
sharing pairs of electrons, they reach a
more stable state. The force which
holds the atoms together is known as a
bond. When the force results from the
sharing of pairs of electrons, it is known
as a covalent bond.

Certain atoms, including gases
such as hydrogen, oxygen, nitrogen,
chlorine, and others share a pair or pairs
of electrons with each other. This pro¬
duces a molecule of a single element.
A molecule is the smallest portion of
a substance that keeps the properties
shown by the substance in large quan¬
tity. Thus these gases do not retain
their properties when they are in the
form of single atoms, but only when
they are in combinations of two. We
refer to such combinations as diatomic
molecules and designate them as H2,
02, N2, or Cl2.

Unlike atoms also form molecules
by covalent bonding. Water is a good
example. Oxygen, with the atomic
number 8, has two electrons missing in
its L shell. It therefore has a tendency
to gain two electrons to complete this
shell. Hydrogen, you will recall, has a
single electron. Thus, an oxygen atom
may share electrons with two hydrogen
atoms and produce a molecule of water.
This molecule is a unit of a compound.
Unlike elements are bonded together
and the elements lose their properties.
Water is unlike either hydrogen or
oxygen.

CHAPTER 3 THE CHEMICAL BASIS OF LIFE 39

Carbon atom

Oxygen atom

Hydrogen atom

Carbon dioxide
molecule

Oxygen

molecule

Water molecule

3-5 Diagrammatic representations of mole¬
cules of oxygen, carbon dioxide, and water.

The chemical shorthand for a com¬
pound is called a formula. As you prob¬
ably know, HoO is the formula for wa¬
ter. The water molecule may also be
written as H— O— H, using dashes to in¬
dicate covalent bonds. This method
of shorthand is often preferred by chem¬
ists, as it shows the actual arrangement
of elements in the compound.

The formation of ionic bonds. We
need look no further than table salt —
sodium chloride — for an example of
the transfer of electrons from one atom
to another. In this compound, sodium
is an electron lender and chlorine is the
electron borrower. Now, let’s examine
this matter of electron transfer more
closely. A sodium atom is normally
balanced electrically. The number of

electrons is the same as the number of
protons. This is also true of a chlorine
atom. If a sodium atom transfers one
electron, it loses a negative charge.
This loss results in an electrically unbal¬
anced atom with a positive charge.
Similarly, the addition of an electron
to a chlorine atom gives it a negative

charge. We refer to these charged
atoms as ions. The opposite electrical
charges of the two ions lock them to¬
gether with a force we refer to as an
ionic bond. The resulting ionic com¬
pound is sodium chloride. Many other
compounds are formed by ionic bond¬
ing. In all of them the electron trans¬
fer resulting in the compound involves
a metal and a nonmetal. The units of
these compounds are not molecules, as
in covalent bonding, but are referred to
as ion pairs.

Properties of compounds. Relatively
few elements are found free in nature.
Chemical reactions have bonded most
of their atoms with other atoms to form
compounds, either molecular or ionic.

Compounds are products of chem¬
ical 'changes which are accompanied by
energy changes. We will use carbon
dioxide as an example of a common
molecular compound and show how it
is formed by covalent bonding. As an
illustration we will use the familiar char¬
coal grill. To start the chemical re¬
action it is necessary to light the char¬
coal. Once the charcoal is lighted it
will glow for hours and continue giving
off energy as heat and light. The chem¬
ist would show the change that occurs
by the following equation :

C + 02 - > C02

In a chemical equation the sub¬
stances on the left of the arrow, the
reactants , are undergoing chemical
change. In the equation above the re¬
actants are carbon, C (the charcoal)
and oxygen, 02. What was the source
of the oxygen that entered into the re¬
action? How could you prove this?
The substances on the right of the ar¬
row, the products , are the result of the
chemical change. Notice that none of
the atoms of the reactants are lost dur-

40 UNIT 1 THE NATURE OF LIFE

ing the change. All are present in some
form in the products. Since carbon
dioxide is a colorless and odorless gas,
you are probably not aware that it is
being formed when charcoal burns.
This simple chemical change illustrates
still another characteristic of the forma¬
tion of a molecule by covalent bonding
— energy is usually given off, as it was
in this reaction.

Now to return to the carbon diox¬
ide that escaped unnoticed from the
charcoal fire. It was unnoticed because
it is a colorless, odorless, tasteless gas.
Furthermore, it is heavier than air,
which indicates that it probably settled
to the ground. Its molecules contain
carbon, but do they resemble carbon in
any way? They also contain oxygen,
but the only resemblance between car¬
bon dioxide and oxygen is the fact that
both are colorless, odorless, tasteless
gases. In other words, both carbon and
oxygen atoms lost their identity in a
molecule of carbon dioxide. Each
compound has its own physical and
chemical properties.

If you examine the formula for car¬
bon dioxide, you will find still another
property of a compound. A molecule
of carbon dioxide is formed only by the
chemical bonding of two oxygen atoms
and one carbon atom. Suppose a sin¬
gle oxygen atom is joined to a carbon
atom. This produces the deadly gas
carbon monoxide, with the chemical
formula CO. The properties of this
compound are quite different from those
of carbon dioxide.

How would you separate the atoms
in a molecule of carbon dioxide? This
would require a chemical change, just
as the formation of the molecule requires
a chemical change.

We can summarize the properties
of a compound as follows:

1. The elements forming a compound
are joined by chemical bonds, either
covalent or ionic.

2. A chemical change is necessary to
separate the elements combined in a
compound.

3. The elements forming a compound
combine in definite proportions.

4. A compound has its own physical
and chemical properties which are
different from those of its constitu¬
ents.

5. The formation and decomposition of
a compound involve energy changes.

Properties of mixtures. Elements or
elements and compounds may be asso¬
ciated in a mixture, with characteristics
quite different from those of a com¬
pound. No chemical change is involved
in forming a mixture. No bonding of
atoms occurs. No new molecular or
ionic substance results. We can best
describe a mixture by comparing its
properties with those of a compound, as
follows :

1. The substances forming a mixture
are associated physically or mechan¬
ically, rather than chemically.

2. The substances in a mixture can be
separated by physical means.

3. The proportions of substances in a
mixture are variable rather than defi¬
nite.

4. The mixture has the properties of
the various substances forming it.

5. Energy changes are not involved in
a chemical action in forming a mix¬
ture.

Air is a mixture of gases. The follow¬
ing gases compose air at sea level in
these approximate percentages by vol¬
ume: nitrogen, 78 percent; oxygen, 21
percent; rare gases including argon, he¬
lium, neon, krypton, and xenon, 0.94
percent; carbon dioxide, 0.04 percent.
Water vapor is also present in air and

CHAPTER 3 THE CHEMICAL BASIS OF LIFE 41

3-6 Crystals of sodium chloride, common ta¬
ble salt. When the crystal dissolves, the re¬
sulting solution contains dissociated sodium
ions and chloride ions. (Bell Telephone
Laboratories)

varies from a small amount to 2 percent
or more.

If air were not a mixture of gases,
there would be no life on the earth as
we know it. We breathe air into our
lungs, dissolve part of the oxygen into
the blood, and exhale the nitrogen and
other gases. Aquatic animals absorb
oxygen dissolved in water. They can¬
not make use of the oxygen combined
with hydrogen that forms the water
itself.

Solutions and suspensions. When a
mixture of two or more substances is
homogeneous, we refer to it as a solu¬
tion. The dissolving medium of a so¬
lution is the solvent. The dissolved
substance is the solute. A solution is
formed when molecules or ions of a sol¬
ute are evenly dispersed through mole¬
cules of a solvent.

When you add sugar to water, the
crystals of sugar dissolve in the water.
Molecules of sugar are dispersed evenly
among the molecules of water. You
can hasten this process by stirring. In
this solution, water is the solvent and
sugar is the solute. Sugar water may

contain varying amounts of sugar.
When a given volume of water is hold¬
ing all the sugar it can and sugar is set¬
tling out, we say the solution is satu¬
rated. Molecular solutions may in¬
volve liquids and gases, two liquids, or
a liquid and a solid, as in the example
of sugar and water. In all solutions,
solute molecules are evenly dispersed
through solvent molecules.

Ionic compounds such as sodium
chloride are generally very soluble in
water. The ion pairs separate, leaving
the rigid crystal structure, and spread at
random throughout the solution. This
process is called dissociation. It may be
represented by the following equation:

NaCI -» Na+ + Cl-

Ionic solutions such as sodium
chloride and water conduct electricity.
We refer to them as electrolytes.
Molecular solutions such as sugar wa¬
ter are nonelectrolytes.

Ions of various compounds are very
important in the chemical activities of
living organisms. They are present in
cells, in the body fluids of animals, and
in the solutions present in plants. Ions
are involved in absorption, in the trans¬
mission of nerve impulses, in muscle
contraction, and in a great many body
activities which we shall discuss later.

The most common ions in organ¬
isms include sodium ions (Na+), chlo¬
ride ions (Cl-), calcium ions (Ca++),
potassium ions (K+), hvdrogen ions
(H+), hydroxide ions (OH-), magne¬
sium ions (Mg++), sulfate ions (S04=),
phosphate ions (P04=), nitrate ions
(N03-), carbonate ions (C03=), and
bicarbonate ions (HC03-).

A substance composed of particles
that are larger than ions or molecules
mav form a mixture we refer to as a
suspension. We can form a suspension

42 UNIT 1 THE NATURE OF LIFE

by stirring starch in water. The parti¬
cles may remain dispersed through the
water for a time, but will eventually
settle to the bottom. They do this be¬
cause the force of gravity is greater than
the force that holds them in suspension.
Thus, the size of the dispersed particles
determines whether a substance will be
dissolved in a solution or suspended.
Generally speaking, particles large
enough to form suspensions can be seen
with an optical microscope.

Many substances are composed of
particles that are between the size of
small molecules which form solutions
and large particles which settle out in
suspensions. These may be very large
molecules or groups of smaller mole¬
cules. These substances mix with water
in colloidal suspensions. In describing
colloids {kahl- oids) we speak of dis¬
persed particles rather than solute and
dispersing medium rather than solvent.
However, particles in a colloid remain
suspended as do molecules or ions in a
solution.

Protoplasm — a colloidal system. In
protoplasm large molecules or groups of
molecules are dispersed in water, form¬
ing a colloidal suspension. The dis¬
persed molecules have an interesting re¬
lationship to water molecules in col¬
loidal protoplasm. Without the water
content changing, a colloid may change
from a fluid, or sol , to a semisolid, or
gel. In the sol phase the dispersed
molecules are distributed uniformly
through the water molecules. The gel
phase results when the dispersed mole¬
cules join and produce a spongy net¬
work. This physical change traps the
water in pools in the molecular net and
changes the colloid from a sol to a
gel. When the molecules separate and
return to a dispersed state, the colloid
changes from a gel to a sol. We refer

to such a change from sol to gel or gel
to sol as a phase reversal. This rever¬
sal is a common occurrence in proto¬
plasm.

External conditions such as temper¬
ature may cause phase reversals in col¬
loids. Gelatin, for example, forms a gel
when it is cool, but changes to a sol
when it is heated. As the temperature
is lowered after heating, it changes back
to the gel state. Gelatin is an example
of a reversible colloid. On the other
hand, egg albumen, which is a sol at
room temperature, changes to a semi¬
solid when it is heated. A fried egg can¬
not be changed back to a sol by cooling
it. Egg albumen is thus an irreversible
colloid.

Elements, compounds, and organisms.
Earlier in this chapter, we referred to
the 92 natural elements which compose

ELEMENTS ESSENTIAL TO MAN

Element

Amount in Body
{154-pound man)

Zinc

Copper

Fluorine

Minute Traces

Silicon

Iodine

.00006 lb.

Manganese .00451b.

Iron

.006 lb.

Magnesium .077 lb.

Chlorine

.231b.

Sodium

.231b.

Sulfur

.351b.

Potassium

.54 lb.

Phosphorus 1.541b.

Calcium

2.31 lb.

Nitrogen

4.62 lb.

Hydrogen

15.4 lb.

Carbon

27.72 lb.

Oxygen

100.1 lb.

CHAPTER 3 THE CHEMICAL BASIS OF LIFE 43

all of the substances of the earth. Now
that we are ready to limit our discus¬
sion of chemistry to biology, we can cut
this number of elements to about 18.
These are the elements most common
in compounds which form living organ¬
isms and supply the substances neces¬
sary for their vital chemical activities.
Eighteen elements present in the body
of a 154-pound man are listed in the
table on page 42.

With certain variations these ele¬
ments might be listed for any organisms.
Notice that those present in quantity are
among the most abundant elements of
the earth. How would the life of our
earth be altered if certain of the rare
elements were necessary to produce an
organism or maintain its existence?

With the exception of atmospheric
oxygen, used by most organisms in res¬
piration, all of these essential elements
come from compounds present in the
soil and atmosphere. We class these
compounds as inorganic to distinguish
them from the more complex organic
products of the chemical processes of
living organisms.

Water is an inorganic compound
so important to organisms that it is a
primary factor in determining where
living things can survive. We will re¬
fer to water relations in many phases of
our study of biology.

We can summarize some of the
ways in which living organisms depend
on water as follows:

1. Water is the chemical source of hy¬
drogen and provides some of the
necessary oxygen.

2. It is the medium in which materials
are dispersed in the organization of
protoplasm.

3. It is the medium in which soluble
materials are absorbed from the en¬
vironment.

4. It is the medium of transport of
foods, minerals, and other vital sub¬
stances in living systems.

5. Water provides the environment for
aquatic organisms.

6. Water pressure in plant tissues pro¬
vides the firmness that supports the
plant.

Carbon dioxide is an inorganic
compound that is the source of carbon
as well as oxygen. Carbon is a key ele¬
ment in the organization of all organic
compounds. Thus, we can say that car¬
bon dioxide is, directly or indirectly,
essential for life.

The other essential elements come
from mineral compounds. These may
be in the form of soil minerals, minerals
dissolved in water, or salts present in sea
water.

Perhaps it has occurred to you that
we, as living organisms, do not directly
utilize carbon dioxide and many of the
essential minerals. We could not pro¬
duce our bodies from water, carbon
dioxide, and minerals. There must be
a link between inorganic compounds
and the organic substances we require
as foods. The green plants of the earth
provide this vital chemical link. You
will deal with many phases of this bio¬
chemical relationship in your study of
biology.

Carbon — the key to organic com¬
pounds. We might divide all com¬
pounds into two basic chemical groups
— inorganic and organic compounds.
All of the organic compounds are car¬
bon compounds. At one time, chem¬
ists thought that all organic compounds
were products of the chemical processes
of organisms. This accounts for the
name organic. Today, however, we
know that this is not the case. In fact,
many of the organic compounds we use
today are synthetic products of chemical

44 UNIT 1 THE NATURE OF LIFE

industries. However, we can say that
all organic molecules are formed around
carbon atoms. Organic chemistry
might thus be spoken of more accurately
as carbon chemistry.

Carbon has several chemical prop¬
erties which make it the key element in
organic compounds. One is the nature
of its atomic structure. A carbon atom
has four electrons in its outer shell,
which is the L shell. Since a completed
L shell consists of 8 electrons, carbon
atoms tend to complete their outer
shells by sharing their 4 available elec¬
trons with those of other atoms. Car¬
bon atoms thus link together by cova¬
lent bonds in chains or rings. These car¬
bon groups form the framework to which
atoms of other elements attach to form
large and complex molecules. This
property of carbon makes possible the
formation of an enormous number of
organic molecules. The structural for¬
mulas of several organic molecules are
shown in Fig. 3-7. Notice that the car¬
bon atoms form chains in some of the
molecules and rings in others. The
covalent bonds are shown as lines join¬

ing atoms. When the carbon atoms
share two electrons instead of one, a
double bond, shown by two lines, is
formed. A triple bond, shown by three
lines, is formed when the carbon atoms
share three electrons. Notice that each
carbon atom has a total of four single
bonds linking it to another carbon atom
or to an atom of another element.

The organization of organic mole¬
cules by living organisms is known as
biosynthesis. Just how the build-up of
atoms and smaller molecules to the in¬
tricate pattern of organic molecules oc¬
curs is a matter of great interest to
biochemists today. Somewhere in this
maze of cell chemistry must lie the key
to life. We shall continue this search
in the exploration of organic com¬
pounds.

The nature of carbohydrates. A carbo¬
hydrate is an organic compound con¬
taining carbon, hydrogen, and oxygen.
The hydrogen and oxygen are present in
a ratio of 2:1, as in water (H20).

Sugars are the most abundant car¬
bohydrates. While they all contain the
same elements, the number and arrange-

GLUCOSE

FRUCTOSE

GALACTOSE

CHO

CH2OH

CHO

H-

- C - OH

1 _

H0-

- C - H

H-

- C - OH

H-

- C - OH

H-

- C - OH

H-

- C - OH

_l_

CH2OH

3-7 Structural formulas of molecules of three simple sugars. Note that all three
have the same number of carbon, hydrogen, and oxygen atoms. Their proper¬
ties are different because of the arrangement of atoms in the molecule.

CHAPTER 3 THE CHEMICAL BASIS OF LIFE 45

ment of atoms in sugar molecules pro¬
duces a great many different com¬
pounds. Many sugar molecules are con¬
structed around a chain of six carbon
atoms (Fig. 3-7). These six-carbon
sugars, known as simple sugars, or
monosaccharides ( mon-oh-sczk-uh-ryds ) ,
have the chemical formula C(;H1206.
Variations in the arrangement of these
atoms results in several different sugars
with the same chemical formula. Glu¬
cose is a most important sugar in organ¬
isms. It is abundant in plants as well
as in animals. We also refer to it as
dextrose and blood sugar. Fructose , or
fruit sugar, and galactose are other
monosaccharides.

Certain plants combine two six-
carbon sugar molecules, remove one
molecule of water, and form a more
complex double sugar, or disaccharide
(dy-sczfe-uh-ryd). These sugars have
the formula C12H22011. Two glucose
molecules are joined in maltose , or malt
sugar (Fig. 3-8). A molecule of glu¬
cose may combine with a molecule of
fructose to form sucrose , or cane sugar.
This is our common table sugar, pro¬
duced by the sugar cane and the sugar
beet. A molecule of glucose joined to

GLUCOSE

CH2OH

CH20H

a molecule of galactose produces lac¬
tose, or milk sugar.

The starches are complex carbohy¬
drates composed of glucose units in
chains (Fig. 3-9). They are classed as
polysaccharides. Each glucose unit in
a polysaccharide consists of a six-carbon
sugar molecule from which a molecule
of water has been removed. The chem¬
ical formula for a polysaccharide is fre¬
quently shown as (C6H10O5)n. The
letter n indicates a chain of the basic
saccharide units.

Glycogen (gly- ko-jen) is a polysac¬
charide composed of 12 to 18 glucose
units in a chain. It is produced in the
liver as a carbohydrate storage product.
When the need arises, the liver recon¬
verts glycogen to glucose bv adding a
molecule of water to each unit. The
glucose is then delivered to the tissues
as blood sugar.

Starches such as corn starch and
potato starch are even more complex
polysaccharides. These molecules are
composed of a chain of 24 to 26 glucose
units. Cellulose molecules consist of
as many as 2,000 glucose units. This
complex carbohydrate is formed in plant
cells and deposited in cell walls. You

MALTOSE + WATER

CH20H h2o ch2oh

H 0

H OH H OH H OH H OH

3-8 Two glucose molecules combine to form one molecule of maitose, splitting
off one molecule of water in the process. Here glucose molecules are shown
as ring structures rather than chains, as in Fig. 3-7.

46 UNIT 1 THE NATURE OF LIFE

STARCH

3-9 A diagram showing how many simple sugar molecules may combine in a
starch molecule. Only a small part of the starch molecule is represented.

are familiar with cellulose as paper,
wood, cotton, hemp, linen, and other
common substances.

For the most part carbohydrates
are the energy sources in living organ¬
isms. Not only in plants, which organ¬
ize them, but also in animals, which
consume them, carbohydrates are vital
fuel nutrients.

Fats, oils, and related substances. Fats ,
including oils and waxes, make up a
second class of organic compounds.
They are composed of carbon, hydro¬
gen, and oxygen and in this respect are
similar to carbohydrates.

Fat molecules are built up from
combinations of simpler units known as
glycerol (glycerin) and fatty acids. Gly¬

cerol molecules contain a chain of three
carbon atoms and have the formula
C3H8Og. Fatty acids vary in chemical
make-up and contain a chain of from
two to more than 20 carbon atoms. In
forming fats three fatty acid molecules
are joined to one glycerol molecule (Fig.
3-10). Three water molecules are
formed as a by-product.

During fat digestion, water is com¬
bined with fat molecules and the fat is
broken down to fatty acids and glycerol.
We refer to this process as hydrolysis.

The conversion of vegetable oils to
animal fats is an interesting process.
These oils, liquid at room temperature,
have double bonds joining some of their
carbon atoms. Chemists refer to them

GLYCEROL + FATTY- ACID

H H H

I I I

H — C OH + H — 0— C — C — C — H

FAT + WATER
H H H

C— H + H2O

H — C- OH + H

H H

H C C H

0 H H

H — C - 0 - C - C — C— H + H2O

H 'Ml

3-10 A molecule of glycerol combines with three fatty acid molecules to form
a fat molecule, splitting off three molecules of water.

CHAPTER 3 THE CHEMICAL BASIS OF LIFE 47

as unsaturated because their molecules
are not holding all of the hydrogen
atoms of which they are capable. When
one of each of these double carbon
bonds is taken over by a hydrogen atom,
the molecule is changed to one of a fat
which is solid at room temperatures.
Such fats are spoken of as saturated.
The process in which they are formed is
hydrogenation.

Waxes are related to fats but are
more complex. Other relatives of fats
and oils are the sterols , including ergos-
terol ( er-gcz/is-ter-ol ) , a substance in
foods and in the skin from which vita¬
min D is produced, and cholesterol , a
fatty material that forms in the body.
Proteins and amino acids. The most
abundant chemical compounds in living
cells are proteins. They are also the
most complex and variable. Protein
molecules contain carbon, hydrogen,
oxygen, and nitrogen. Sulfur is often
present and phosphorus and iron may
be included.

Protein molecules consist of com¬
plex groups of smaller units known as
amino acids. All amino acids are alike
in containing an amino group and a
carboxyl group. The amino group con¬
sists of two hydrogen atoms bonded to a
nitrogen atom (NH2), while the car¬
boxyl group contains an atom of carbon,
two atoms of oxygen, and a hydrogen
atom (COOH). More than 20 differ¬

ent amino acids are formed from these
two groups bonded with other atoms and
groups of atoms (Fig. 3-11 ).

Amino acids are arranged in one or
more chains in forming a protein mole¬
cule. The number of amino acids in a
protein chain may be from 300 to 3,000.
When you consider that there are more
than 20 different amino acids and that
proteins differ not only in the kind of
amino acids present but in their number
and arrangement in chains, you can see
that the number of diEerent kinds of
proteins is almost endless.

The protein content of diEerent
kinds of plants and animals varies
greatly. Members of a species have
many kinds of proteins in common. For
example, certain kinds of proteins are
found in all humans, and they are dif¬
ferent from those of other animals or of
plants. However, while we are like all
other humans in certain parts of our pro¬
tein make-up, each of us has his own in¬
dividual protein make-up. Each of our
cells is said to contain as many as 2,000
diEerent kinds of protein. Could it be,
then, that certain proteins make us hu¬
man while others make us individuals?
This is exactly the case, and later you
will discover how these proteins are or¬
ganized and why they make you a hu¬
man and an individual human.

When we consume plant and ani¬
mal proteins as food, we take in sub-

AMINO ACID
H 0

H H

AMINO ACID
H 0

A / c

H H

PART OF PROTEIN CHAIN

A/c
H A

H H

"A/
HA
A

H H

+ H20

3-11 Proteins are composed of amino acids linked up in various combinations.
Amino acids contain nitrogen, N, as well as hydrogen, oxygen, and carbon.

48 UNIT 1 THE NATURE OF LIFE

stances that are foreign to our make-up.
In other words the proteins we consume
could not be organized into our own
substance. However, during digestion
we break them down to amino acids.
Later, we rearrange the amino acids and
form new protein chains according to
our own individual formulas. Alto¬
gether, about 20 different amino acids
are found in our cells. Foods contain¬
ing all of the amino acids composing our
own proteins are said to be complete.
As you might expect, these are animal
proteins. On the other hand, certain
amino acids may be lacking in plant pro¬
teins. For this reason they are classed
as incomplete proteins.

The vital role of enzymes. So far we

have discussed various organic materials
which compose living matter. They are
the products of the chemical activity of
organisms. The chemical phases of the
living condition involve literally thou¬
sands of changes in which molecules re¬
act with other molecules. A living cell

3-12 Francis H. C. Crick (above) and
James D. Watson were awarded the Nobel
prize in 1962 for their workable model of
one of the nucleic acids, DNA. (Wide World
Photos)

3-13 Dr. Watson shown holding a regular
biology class for Harvard and Radcliffe stu¬
dents the day he was told of receiving the
Nobel prize. (Wide World Photos)

is thus a complex chemical system, con¬
stantly building up substances and tear¬
ing them down.

However, the manv chemical activi¬
ties involved in the living condition
could not occur without enzyme action.
An enzvme is an organic catalyst. A
catalyst enables a chemical reaction to
occur which would otherwise occur too
slowly or not at all. The catalyst is not
changed during the reaction and does
not enter into the products formed. For
this reason, they are not used up in a
chemical change. Without enzymes as
catalvsts, vou could not digest a meal,
release energv in your body, or form your
own bodv substances. In fact, without
enzvmes there would be no life.

All enzvmes are protein molecules.
The number of enzvmes present in a
cell mav be as manv as several thousand.
Each kind of enzyme has an effect only
on a certain kind of molecule. In this
respect enzvmes are specific. Thus, for
evenr chemical activitv of an organism,
there is a corresponding enzvme. Often

CHAPTER 3 THE CHEMICAL BASIS OF LIFE 49

a series of chemical changes is involved
in a chemical activity of a cell. This re¬
quires a group of enzymes we refer to as
an enzyme system.

In certain chemical reactions en¬
zymes require the assistance of nonpro¬
tein molecules known as coenzymes.
Coenzymes serve as electron donors or
acceptors in chemical reactions. Cer¬
tain chemical reactions which we shall
study later involve the removal or addi¬
tion of hvdrogen atoms from molecules.
In these reactions a coenzyme teams
with an enzyme in causing the reaction.
A coenzyme acceptor receives the hydro¬
gen as it is released, while a coenzvme
donor supplies the hydrogen as it is
needed.

The organization of nucleic acids. Life
is self-perpetuating. Living things have
a unique chemical organization. Liv¬
ing things are capable of growth. Living
things have a definite form and size
range. Do you remember these charac¬
teristics of the living condition we dis¬
cussed in Chapter 2? We presented
them merely as interesting observations.
But what factor or property of living
material accounts for them? This was
a puzzle to the biologist until a few years
ago when two scientists made a discov¬
ery which is among the most signifi¬
cant biological advances of all time.
What had been isolated pieces of a puz¬
zle suddenly formed a revealing and
thrilling picture of life when the missing
piece was finally supplied.

For many years biologists had looked
to the cell nucleus as the center of con¬
trol of all of its activities. Furthermore,
they were familiar with the dark, rod¬
shaped bodies known as chromosomes,
which are contained in the nucleus.
Other studies prior to World War II had
established that chromosomes contain
two substances, protein and nucleic acid.

3-14 A portion of the DNA molecule as con¬
ceived by Watson and Crick. (Sloan-Ketter-
ing Cancer Center)

The question was whether the control
of cell activity centered in the protein
or in the nucleic acid. Studies at the
Rockefeller Institute during World War
II provided evidence that nucleic acid
and not protein controlled all chemical
activities of an organism. What were
these nucleic acid molecules which
possessed such remarkable properties?
Their chemical composition was known,
but their physical structure — the way
their atoms are actually arranged — was
not.

In 1953 two young scientists work¬
ing in the Cavendish Laboratory at Cam¬
bridge University made a startling dis¬
covery. One member of this remark¬
able team is an American biologist,
James D. Watson. The other is a Brit¬
ish biophysicist, F. H. C. Crick. To¬
gether they worked out the physical

50 UNIT 1 THE NATURE OF LIFE

structure of probably the most complex
of all organic molecules, deoxyribonu¬
cleic acid, or DNA.

Watson and Crick described the
DNA molecule as a double helix, con¬
sisting of two strands wound around
each other and connected by cross pieces.
You might think of it as a ladder twisted
into a spiral or a corkscrew. While we
speak of DNA as a molecule, it is ac¬
tually a super molecule composed of
numerous smaller molecules.

If we return to the spiral ladder
comparison, the strands forming the
side pieces are composed of alternating
units of deoxyribose sugar, a five-carbon
sugar, and phosphates. The links be¬
tween these strands, or steps of the

ladder, are composed of pairs of nitro¬
gen-containing bases. These bases con¬
sist of four kinds of organic molecules,
belonging to two classes of compounds,
as follows:

The purines (pyur-ee ns) include
adenine (ad-e n-een) and guanine
(gwahn-ee n) molecules.

The pyrimidines (py-nm-i-deens)
include thymine and cytosine (syt- o-
seen) molecules.

Watson and Crick found that there
is a definite pattern in which these base
molecules link in pairs. Adenine is
joined to thymine, while guanine is
linked to cytosine. Notice that one
of each pair is a purine and the other
a pyrimidine. These base pairs are in

3-15 The first three diagrams show how a double helix might be formed by
twisting a flexible ladder. The last diagram shows the basic structure of the
DNA molecule according to the Watson-Crick model.

CHAPTER 3 THE CHEMICAL BASIS OF LIFE 51

3-16 An uncoiled portion of the DNA molecule, with its parts abbreviated as
letters.

turn fastened to sugar molecules in the
side pieces, or helical strands.

If you pulled two strands of a DNA
molecule apart by separating the base
pairs, you would have a series of units
consisting of a sugar unit, an attached
purine or pyrimidine, and a phosphate
group. This portion of the molecule is
called a nucleotide. Thus, you might
think of a DNA molecule as a double
row of nucleotides joined by their base
molecules.

An uncoiled DNA molecule with its
component parts abbreviated as letters
is shown in Fig. 3-16. Notice how the
nucleotide units fit together as the base
pairs are joined. Notice also that A
and T are always joined, as are G and C,
although their positions (right and left)
may vary. There is also variation in the
linear sequence of the base pairs. This
is important in the genetic function of
DNA, as you will see in Unit Two.

A second nucleic acid known as
ribonucleic acid, or RNA, is a near du¬
plicate of DNA. The sugar in this mole¬
cule is ribose rather than deoxyribose (it
contains one more oxygen atom), and
uracil, another pyrimidine, is substituted
for thymine in the composition of its
bases. RNA is a product of DNA and
serves as its agent in controlling certain
cell activities. We will return to these
nucleic acids in the discussion of the cell
and its chemical activities.

DNA — the key to life. DNA is unlike
any other substance in the world. Life
is a unique condition because DNA is a
unique molecule. In what ways is it
unique? Let us consider some of the
characteristics of this remarkable mole¬
cule.

A DNA molecule is capable of mak¬
ing an exact duplicate of itself by a
process known as replication. Can a
water molecule form another water mole-

52 UNIT 1 THE NATURE OF LIFE

cule, a sugar molecule organize another
sugar molecule, or a protein molecule
another protein molecule? We say that
life is self-perpetuating or that living
matter can form more living matter.
Actually, it is the DNA which is self-
perpetuating.

DNA, in the composition of its
nucleotides, bears the genetic code that
determines exactly what an organism will
become. We say that like produces
like. What we really are saying is that
like DNA produces like organisms.

DNA, together with its product
RNA, controls the organization of en¬
zymes. These in turn determine all of
the chemical activity of the cell. When
we say that living things have a unique
chemical organization, we are really say¬
ing that DNA molecules regulate all the
possible phases of cell biochemistry.

From time to time DNA molecules
make chemical mistakes during replica¬

tion. A different DNA molecule ap¬
pears and transmits its new structure to
other molecules as it replicates. A ge¬
netic variation in an organism results.
Occasionally the variation is beneficial to
the organism, or it may be unfavorable
or even fatal. If the organism repro¬
duces, the change is transmitted to the
offspring. Thus organisms do not re¬
main in a static form age after age.
There is always the possibility of im¬
provement of life through variation in
the structure of DNA molecules.

As you proceed through your course
in biology, DNA will come into the dis¬
cussion time after time. How do organ¬
isms grow? How do they reproduce?
How does a species maintain its iden¬
tity? Why do new organisms resemble
their parents in some respects and differ
in others? DNA provides the answers
to all of these questions. DNA is the
key to life.

IN CONCLUSION

The story of the chemical basis of life is an account of progressive changes in
the organization of matter — an elevation to higher and higher levels of com¬
plexity. Consider the basic particles that form the hydrogen atom and build
up through all possible proton and electron arrangements and numbers to
uranium, the 92nd element. Through all of this series of natural elements,
matter becomes more complex. Then consider the grouping of atoms in the
simpler inorganic compounds as covalent and ionic bonds raise matter to a
higher level of complexity. When organisms arrived on the scene at some
time in the distant past, an entire new form of compound came into being,
organized around carbon atoms by chemical changes in living organisms. Of
course, DNA and RNA came first. How and from what, we may only guess
and theorize. Perhaps we will have this answer some day. Perhaps we never
will.

We now proceed to cells, the basic units of life — the living products of
DNA.

CHAPTER 3 THE CHEMICAL BASIS OF LIFE 53

BIOLOGICALLY SPEAKING

atom

DNA

atomic mass

electron

atomic number

element

biosynthesis

energy

carbohydrate

enzyme

coenzyme

fat

colloid

hydrolysis

compound

inorganic

covalent bond

ion

ionic bond

nucleus

ionization

organic

isotope

protein

matter

proton

mixture

RNA

molecule

replication

neutron

solution

nucleotide

suspension

QUESTIONS FOR REVIEW

1. Distinguish between a physical change and a chemical change.

2. Give an example of conversion of potential energy to kinetic energy.

3. Describe the location of protons, neutrons, and electrons in an atom.

4. Distinguish between the atomic number and the atomic weight of an
element.

5. What are isotopes?

6. What part of an atom determines its chemical activity?

7. Distinguish between covalent bonding and ionic bonding in the forma¬
tion of a compound.

8. Give an example of a molecule which is not a compound; a compound
which is not molecular; a molecular compound.

9. Summarize the properties that distinguish a compound from a mixture.

10. Explain the relation of a solvent and a solute in a solution.

11. Distinguish between a molecular solution and an ionic solution.

12. What factor determines whether a substance will form a solution, a sus¬
pension, or a colloid?

13. Describe phase reversal in a colloid. How may external conditions cause
phase reversal?

14. Distinguish between a reversible and an irreversible colloid.

15. List the inorganic compounds which supply the essential elements to
organisms.

16. What is biosynthesis?

17. Distinguish between monosaccharides, disaccharides, and polysaccharides.

18. What two basic units compose fat molecules?

19. Explain the relationship of amino acids to protein molecules.

20. What is an enzyme?

21. Describe the units of a DNA molecule and their relation to one another.

22. Which groups in a DNA molecule are constant in its structure? Which
groups are variable?

54 UNIT 1 THE NATURE OF LIFE

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the increase in the complexity of matter through the series of
natural elements.

2. Discuss the key position of carbon in the organization of organic com¬
pounds.

3. Discuss the vital role of enzymes in biochemistry.

4. If you were to select one of the substances you have studied as “the living
substance/’ which one would be your choice? Justify your selection.

5. Why is replication an extremely important property of a DNA molecule?

6. Discuss the role of DNA in determining what an organism is to be and in
producing variations in organisms.

CHAPTER 4

THE

STRUCTURAL
RASIS OF LIFE

Having become familiar with the
various elements and compounds, both
inorganic and organic, found in organ¬
isms, we shall now consider how these
materials actually compose a plant or
animal. The cell is the structural unit
of life. All the materials in an organism
are organized into specific cell structures.
In this organization these materials cease
being mere chemical substances and be¬
come living matter.

A real knowledge of cellular biology
is essential in any area of biological
study. Nearly all the specialized branch¬
es of biology relate in some way to cells.
Physical scientists ushered in the atomic
age as a result of exhaustive studies of
molecules, atoms, alpha particles, beta

particles, and other invisible forms of
matter. Equally important biological
discoveries have come from our nation’s
laboratories where biologists are finding
out more and more about the basic unit
of life, the cell.

Three hundred years of cell explora¬
tion. Three hundred years ago, in the
year 1665, the British scientist Robert
Hooke (1635-1703) discovered cells.
His material was a piece of cork sliced
thin enough to let light pass through it
for his microscopic examination. To
his surprise he found that the cork con¬
sisted of a mass of tiny cavities. Each
cavity was enclosed by walls, reminding
him of cells in a honeycomb. Cell was
a logical name for these structures, which
he described in a report of his discovery
titled Micro gr aphid.

Hooke did not realize, however,
that the most important parts of these
cells were lacking. He saw only the
empty shells of cells that had once con¬
tained active, living materials. Having
made this discovery, it is surprising that
Hooke did not follow it up with an ex¬
amination of many other parts of plants
or animals in which he might have seen
cell content. But 170 years passed be¬
fore another scientist made a significant
discovery relating to cells.

In 1835 the French biologist Du-
jardin (doo-jar-dahn) viewed some liv¬
ing cells with a microscope and found
that they had content. Dujardin named
this material sarcode , a term later to be
changed to protoplasm. Three years
later the German botanist Matthias
Schleiden proposed, as a result of exten¬
sive studies, that all plants are composed
of cells. The following year Theodor
Schwann, a German zoologist, made a
similar statement regarding animal struc¬
ture. The work of these men, together
with the contributions of other 19th

56 UNIT 1 THE NATURE OF LIFE

4-1 Hooke’s drawings of thin sections of
cork; longitudinal (left) and cross section
(right). (Bettmann Archive)

century biologists, established the cell
theory, which states that:

1. The cell is the unit of structure and
function of all living things.

2. Cells come from pre-existing cells,
by cell reproduction.

For the next 100 years biologists
added greatly to our knowledge of cells
and their activities. Their work, how¬
ever, was confined by the magnification
limits of the light microscope. About
20 years ago a new era of understanding
of cell structure and physiology was
ushered in with the electron microscope.
New techniques provided additional
tools for analyzing the cell structures
that the electron microscope revealed.
High speed centrifuges separated cell
materials, and advanced methods in bio-
chemistrv determined their chemical

make-up and molecular structure. By
means of new techniques in microsur¬
gery, biologists actually removed cell
structures for isolated study. Radioac¬
tive isotopes, serving as tracer elements,
allowed biologists and biochemists to
follow the course of chemical reactions

through the machinery of the cell and
relate them to specific cell structures.
As a result, we have learned more about
cells in the past 10 years than biologists
were able to learn in the previous 290
years or more!

Today we are discussing cells in an
entirely new light. Life has taken on
new meaning. We believe we finally
have the answers to many questions
about the organization and activity of
living cells. However, reliable as our
evidence may seem, we must remember
that many of these concepts are still
based on hypotheses and not on facts.
We may revise our knowledge of cells
many times in the years to come, but
each new discovery will bring us nearer
to an understanding of the marvels of a
living cell.

Cells — the basic units of life. All the
substances composing an organism are
contained in its cells. Thus each cell
is a unit mass of protoplasm, or the in¬
dividual part of which the whole or¬
ganism is composed. The simplest or¬
ganisms consist of but one cell. Organ¬
isms above this level of organization
may be made up of thousands, millions,
or even billions of cells. It has been
estimated that the human body con¬
tains more than 50 thousand billion
cells. The size of the organism is de¬
termined not by the size of its cells but
by their number. Elephant cells are no
larger than ant cells. There are just
more of them. Large or small, simple
or complex, the cell is the unit of struc¬
ture and function of all living organisms.
Processes of a cell. All of the processes
of a living cell involve energy transfor¬
mations. The source of all of this life
energy is chemical activity within the
cell. It is difficult to separate one cell
activity from all the others which are
closely related and occurring simultane-

CHAPTER 4 THE STRUCTURAL BASIS OF LIFE 57

ously. Similarly, there are specialized
cell parts, each of which is concerned
with one or more of the vital chemical
reactions. Thus, in discussing the cell,
we cannot separate structure from func¬
tion. However, even an artificial sepa¬
ration of the total chemical activities
into phases or processes simplifies a dis¬
cussion of cell structures and their as¬
sociated functions. We shall summa¬
rize the processes of a cell as follows:

Nutrition. Food molecules are nec¬
essary to support the processes of the
cell, as well as to form its substances.
Some cells manufacture their own food
molecules. Others receive them from
the environment.

Digestion. Certain enzymes syn¬
thesized in the cell accelerate chemical
reactions and simplify food for cell use.

Absorption. Water, food mole¬
cules, ions, and other essential materials
are transported into the cell from its en¬
vironment.

Synthesis. Cells organize their own
specific proteins. Synthesis of these
complex molecules from amino acids is
controlled by DNA and RNA. The re¬
sult of these activities is growth and reg¬
ulation of all chemical activity of the
cell by enzyme action.

Respiration. Energy is released
in the cell when certain organic mole¬
cules, especially glucose, are split, or de¬
graded. This chemical energy is essen¬
tial in maintaining life.

Excretion. Various waste materials
are formed as by-products of cell activi¬
ties. These substances pass from the
cell to its environment.

Secretion. Certain cells synthesize
molecules which influence the activity
of other cells. Such cell secretions in¬
clude vitamins and hormones.

Movement. Some of the energy re¬
leased in a cell is used in movement.

This may occur as flowing of the cell
content, locomotion of the cell by means
of special structures, or cellular con¬
tractions, as in muscle cells.

Response. External conditions
such as heat, light, and physical contact
alter the activities of a cell and cause a
response.

Reproduction. A cell divides its
mass periodically. In this manner the
number of cells increases or new organ¬
isms are formed.

Parts of a cell. In classifying the parts
of a cell, we may divide them into specif¬
ic structures, as follows:

1. The nucleus , including the: a. nu¬
clear membrane, b. chromosomes,

c. nucleolus.

2. The cytoplasm (syt-o-plaz-m), in¬
cluding the: a. plasma membrane
(cell membrane), b. vacuolar mem¬
brane, c. endoplasmic reticulum,

d. ribosomes, e. mitochondria, f. Golgi
bodies, g. plastids, h. vacuoles.

3. The cell wall , including the: a. mid¬
dle lamella (intercellular layer),
b. primary wall, c. secondary wall.

Structure of the nucleus. We will be¬
gin our discussion of the parts of a cell
with the nucleus , the control center of
all cell activity. When a cdll is stained
for microscopic examination, the nu¬
cleus stands out as the most prominent
part. It is usually spherical or oval in
shape and often lies near the center of
the cell.

The nucleus is bounded by a thin
nuclear membrane which separates the
nuclear materials from other parts of the
cell. However, the nuclear membrane
is not a barrier. What appears to be a
skinlike covering under the light micro¬
scope shows as a porous structure under
the electron microscope. Nuclear ma¬
terials pass freely into the surrounding
cell substance.

58 UNIT 1 THE NATURE OF LIFE

4-2 Nutrient agar in sol and gel phases may
be used to illustrate the colloidal nature of
protoplasm. Above: at about 98u C the agar
in the sol phase flows like a liquid. Below:
at about 40° C the gel phase forms. (Rich¬
ard F. Trump)

The nucleus contains a viscous col¬
loid, rich in protein, which is often re¬
ferred to as nucleoplasm. It also con¬
tains one or more spherical bodies called
nucleoli. Embedded in the nucleo¬
plasm are numerous fine strands called
chromosomes. Chromosomes are com¬
posed of DNA joined to protein mole¬
cules to form nucleoproteins. DNA
molecules are the key components of
chromosomes. As you learned earlier,
these molecules act as the genetic code
of the organism. As a result they are the
most vital parts of the cell. Perhaps you
wonder how DNA, located in the nu¬
cleus, can control all of the activities of

the cell when they occur outside the
nucleus.

In recent years biologists have found
evidence that DNA synthesizes a near
duplicate molecule known as RNA. W e
referred to this nucleic acid in Chapter
3. Each RNA molecule bears a genetic
code identical to the DNA molecule that
organized it. RNA molecules pass
through the nuclear membrane into the
cytoplasm. Here they control the or¬
ganization of enzymes according to the
genetic code supplied them by DNA.
These enzymes in turn control the syn¬
thesis of specific proteins and all other
chemical activities of the cell.

Structure of the cytoplasm. We refer
to the cell substances outside the nu¬
cleus as cytoplasm. Under the light
microscope, cytoplasm appears as a semi¬
fluid material filling most of the cell. It
often flows through the cell in a manner
we call streaming. As it changes posi¬
tion in the cell, it may revert from sol to
gel and back to sol. The nucleus often
flows with the cytoplasm and changes
shape as it moves.

The exposed outer edge of the cyto¬
plasm forms a thin molecular layer
known as the plasma membrane , or cell
membrane. This thin boundary sepa¬
rates the cell from neighboring cells and
from the fluids that bathe it. But like
the nuclear membrane it is not a barrier,
for molecules pass through it. How¬
ever, the plasma membrane is the vital
regulator of this molecular traffic. It is
selective in allowing certain molecules
to pass through and rejecting others.

The plasma membrane forms where
the colloidal cytoplasm borders on
another substance, such as a fluid outside
the cell. We speak of this boundary as
an interface. Molecules rearrange along
the interface and form a thin, gelati¬
nous layer. You might compare the for-

CHAPTER 4 THE STRUCTURAL BASIS OF LIFE 59

\ # ' A

h°y$&r-

PRINCIPAL PARTS
OF A CELL

CELL WALL

Middle lamella
Cellulose layers

CYTOPLASM

Plasma membrane
Vacuolar membrane
Plastids
Mitochondria

NUCLEUS

Nuclear membrane
Nucleoplasm
Nucleolus
Chromosomes

PLANT CELL

ANIMAL CELL

4-3 Diagrams of typical plant and animal cells. Remember that cells are actu¬
ally three dimensional and that it is difficult to portray them accurately on a flat
surface.

mation of a plasma membrane with the
skim that forms on the surface of soup
or hot chocolate as it cools. Extremely
high magnifications by the electron mi¬
croscope have revealed that a plasma
membrane is not a simple, smooth, skin-
like structure, as it appears under the
light microscope. It is not a single
molecular layer but a layer several mole¬
cules thick. The inner and outer layers
of the membrane are composed of pro¬
tein molecules. Between these is a
double layer of fatlike molecules. The
membrane surface is intricately folded

with pouches extending inward and
blisters bulging outward. The infolded
pouches are tiny gateways through
which solid materials may pass through
the membrane, as you will discover in
the next chapter when we discuss the
cell and its environment. If a plasma
membrane is pierced or cut, cytoplasm
may ooze through the opening for a
short time. Soon, however, other mole¬
cules move into position and plug the
opening. This rapid repair occurs fre¬
quently as large particles pass through
the membrane into the cell.

60 UNIT 1 THE NATURE OF LIFE

A membrane similar to the surface
plasma membrane forms along an inter¬
face where cytoplasm borders on a cen¬
tral cavity within a cell. Because of its
location we refer to this membrane as a
vacuolar membrane.

The electron microscope has re¬
vealed another characteristic of cyto¬
plasm, unknown until recent years. Un¬
der the light microscope, cytoplasm ap¬
pears as a uniform, semifluid material
containing numerous granules. How¬
ever, a magnification of 40,000 times or
more shows a much more intricate struc¬
ture. Cytoplasm contains a complex
system of membranes which tend to he
parallel to one another. This network,
or endoplasmic reticulum , fastens to the
plasma membrane and nuclear mem¬
brane. Biologists believe that the endo¬
plasmic reticulum may be a system of
tubes through which materials pass from
the plasma membrane to the nucleus.
However, this is only a reasonable guess,
since the function of the endoplasmic
reticulum is not yet known.

Cytoplasmic organelles. Various or¬
ganized bodies are present in the cyto¬
plasm. We refer to these specialized
structures as organelles , which means
“little organs.” Certain of these cell
organelles are visible under the light
microscope. Others are so small that
they were not known to exist before we
had the electron microscope. Each of
these structures is associated with a
specific process or activity of the cell.
In our present investigation of the cell,
we are concerned primarily with their
structure, composition, and location. In
chapters to follow, we will explore their
chemical activity in greater detail.

Under extremely high magnification
with an electron microscope, tiny dense
granules may be seen attached to the
endoplasmic reticulum and lying be¬

tween its folds. These bodies are the
ribosomes , so named because of the
large amount of RNA they contain. In
addition to RNA received from the nu¬
cleus, ribosomes contain protein-synthe¬
sizing enzymes. Recent studies indicate
that they are the protein factories of the
cell. Thus, while they are among the
smallest of all cell structures, their func¬
tion is among the most vital.

The electron microscope also re¬
veals the detailed structure of rod-shaped
bodies in the cytoplasm. These are
the mitochondria ( myt-o-fed/m-dree-a ) .
They are known to be centers of cellular
respiration, during which energy is re¬
leased to support cell activities. The
mitochondria are the powerhouses of
the cell. A mitochondrion is enclosed
in two membranes, each consisting of
a layer of protein molecules and a layer
of fat molecules. The inner membrane
folds inward at various places forming
partial partitions within the mitochon¬
drion. This infolding increases the sur¬
face area of the membrane. We refer
to these partitions as cristae. Mitochon¬
dria are known to contain enzymes that
split organic molecules and transfer en¬
ergy to other compounds from which it
is released in the cell.

The function of certain cell struc¬
tures remains a mystery. Among these
cell unknowns are the Golgi bodies ( gol -
jee), or Golgi apparatus. They were
first seen in 1898 in nerve cells by the
Italian neurologist Camillo Golgi. The
electron microscope has revealed much
more of their structural detail than Gol¬
gi was able to determine, but this knowl¬
edge has provided little or no clue to
their function. Golgi bodies appear as
small groups of parallel membranes in
the cytoplasm near the nucleus. The
membranes form plates with channels
along their edges. The fact that they

CHAPTER 4 THE STRUCTURAL BASIS OF LIFE 61

Golgi bodies

Centriole

Vacuolar

membrane

Nucleus

Nucleolus

Chromosomes

Nuclear

membrane

4.4 a generalized cell in diagrammatic form. Most of the structures are shown
larger in proportion than they are in the actual cell. This diagram is based on
electron micrographs of many cells.

are most numerous in cells composing
glands may indicate that they secrete
special substances.

Other microscopic bodies present
in the cytoplasm are known as plastids.

Some plastids function as chemical fac¬
tories, while others serve as storehouses
of the cell. Plastids are found most fre¬
quently in the cells of plants and primi¬
tive organisms.

The most familiar plastid is the
chloroplast. These plastids contain
green pigments called chlorophylls.

We often refer to chloroplasts as carbo¬
hydrate factories for they contain the
enzymes involved in the organization of
carbohydrates. Chloroplasts place the
green plant in a key biological position,
as most living organisms depend on the
food-making activities of cells contain¬

ing chlorophylls. In addition to chloro-
phvlls, chloroplasts may contain pale-
yellow pigments called xanthophylls
(zcm-tho-fils ) and deep-yellow or reddish-
orange pigments known as carotenes.

Chloroplasts vary both in size and
in shape. In the cells of advanced
plants, such as leaf cells, they are often
disk-shaped. When viewed with a
light microscope, chlorophyll appears to
be distributed throughout the chloro¬
plast. The electron microscope reveals
that the chlorophyll is contained in lay¬
ers of disklike protein bodies known as
grana. The disks are held together by
fatlike molecules.

Other plastids produce red and
blue pigments in addition to yellow and
orange. These plastids, often referred
to as chromoplasts, are found in flower

Mitochondrion

Vacuole

Endoplasmic

reticulum

Cytoplasm

Pinocytotic
infolding
of membrane

Plasma

membrane

Ribosomes

UNIT 1 THE NATURE OF LIFE

petals and in the skins of fruits such as
the tomato, cherry, and pepper. In
some cells chromoplasts are chloro-
plasts that have lost their green chloro¬
phyll. This occurs in the ripening of
a banana or a tomato when the skin
changes from green to yellow or red.

Leucoplasts (Zoo-koh-plasts) are
usually colorless plastids which serve as
food storehouses in many plant cells.
These plastids contain enzymes neces¬
sary to link glucose molecules together
and form starch molecules. This mav
occur temporarily in chloroplasts, but
leucoplasts are the principal starch
storage centers. Plastids are the only
centers of starch storage in a cell. Be¬
cause starch is insoluble it never reaches
the nucleus or the cytoplasm outside
plastids. Leucoplasts may be found in
the cells of roots, stems, and in other
storage areas of plants. You can see
large numbers of them in the cells of
the white potato. It is interesting that
leucoplasts may develop chlorophyll and
change to chloroplasts when they are
exposed to light. This may occur in the
cells of the white potato.

Cell vacuoles. Plant cells frequently
contain one or more fluid-filled cavities
in the cytoplasm. Such cavities are
known as vacuoles. The bordering vac¬
uolar membrane , formed by the cyto¬
plasm, regulates the molecular traffic
between the vacuole and the cell sub¬
stances. A young cell often contains
several vacuoles which unite as the cell
increases in size and form a single, large
central vacuole. Pressure exerted by
the fluids in the central vacuole may
force the cytoplasm into a thin layer
around the edge of the cell.

The fluid in the vacuoles of plant
cells is largely water. Ions of mineral
compounds, molecules of sugars and
other soluble substances, and large

4-5 Electron micrograph of the nucleus in
detail. The nucleolus is the dark mass
within the nucleus. Note the apparent pores
in the nuclear membrane. The darker
spheres in the cytoplasm are mitochondria.
(Don W. Fawcett)

molecules of proteins and other organic
materials in colloidal suspension in the
water form the content of the vacuole
that we often refer to as cell sap. Other
vacuoles may contain food materials
and waste products. Still others serve
as water-eliminating organelles in many
one-celled organisms.

Vacuoles of plant cells also con¬
tain water-soluble pigments. Common
among vacuolar pigments are antho-
cyanins ( dn-thoh-sy-a-nins ) which have
shades of scarlet, crimson, blue, purple,
and violet. These pigments provide the
red shades of autumn leaves, the purple
of the turnip top, the red of the radish
and beet, and the petal coloration of
asters, geraniums, tulips, hyacinths, and
many other flowers. In the hydrangea,
variations in acidity change the antho-

CHAPTER 4 THE STRUCTURAL BASIS OF LIFE 63

cyanin from red to blue. The forma¬
tion of anthocyanins is determined by
many internal and external factors.
DNA regulates the production of en¬
zymes involved in the organization of
anthocyanin molecules. Thus, “roses
are red and violets are blue,” depend¬
ing on their DNA. In addition, the
accumulation of sugar seems to stimu¬
late the production of anthocyanins.
External factors include both tempera¬
ture and light — usually low tempera¬
ture and bright light.

The cell wall. Most plant cells are en¬
cased in an outer protective and support¬
ing structure, the cell wall. At first
glance a cell wall may appear as a sin¬
gle layer, but it is usually composed of
several distinct layers. Where two cells
lie against each other, each has formed
a portion of the wall separating them.

4-6 Electron micrograph showing: top, mito¬
chondria; center, endoplasmic reticulum;
bottom, nucleus. (G. E. Palade and Journal
of Cell Biology)

The layers of cell walls have been com¬
pared to the plastered walls separating
two rooms. The center of the wall is
like the layer of plaster board. This
portion of a plant cell wall is referred
to as the intercellular layer, or middle
lamella. It contains various pectin
(pek- tin) substances which are jellylike
in texture. Many fruits, including ap¬
ples, contain a large amount of pectin,
which is released during cooking and
forms a jelly as it cools.

Adjacent cells form thin primary
walls on both sides of the middle la¬
mella. We might compare these to the
first coat of plaster on each side of the
wall between two rooms. The primary
wall is composed of cellulose fibers and
pectin. Cellulose, you will recall, is a
carbohydrate similar to starch. Soft
plant structures such as leaf blades,
flower petals, and pulpy fruits have thin
primary walls.

Additional cellulose layers are
built up in secondary walls. These
walls are firm and rigid and remain long
after the cell is dead. We find thick
secondary walls in the cells of wood,
plant fibers, and nut shells.

In a primary wall cellulose fibers
are arranged in a network. In second¬
ary walls they are more or less parallel
and crisscrossed in layers. This ar¬
rangement of cellulose fibers, as in the
thin sheets composing plywood, gives
these walls great strength. Spaces be¬
tween the fibers contain pectin and
lignin, a complex organic compound.
Lignin, second only to cellulose in
abundance in wood, adds stiffness and
rigidity to cell walls.

Modern methods of cell investigation.
As we have been discussing the struc¬
ture and chemical composition of ex¬
tremely small cellular bodies such as
ribosomes and mitochondria, haven’t

64 UNIT 1 THE NATURE OF LIFE

4-7 Electron micrograph showing Golgi
bodies. (C. Bruni)

you wondered how biologists and bio¬
chemists have made these remarkable
discoveries? We can enlarge a cell
40,000 times or more with an electron
microscope and see formerly invisible
structures. But how could these mi¬
nute bodies be separated from other cell
structures for chemical analysis? In one
method cell materials are pulverized
with fine sand supplying the cutting
edges. The pulverized material is then
placed in a centrifuge tube and spun at
high speed. Various cell substances
and organized bodies have different
masses and densities — that is, they dif¬
fer in size and weight. During high
speed centrifugation, centrifugal force
separates them into different layers in
the tube. The most dense materials,
including intact cells and nuclei, oc¬
cupy the bottom layer in the tube.
Next might be a layer of plastids, then
a layer of mitochondria. Since the
ribosomes are among the smallest cell
structures, they would occupy a layer
near the surface, along with pieces of

the endoplasmic reticulum. Finally,
the layer nearest the top would contain
water and various cell fluids. The biol¬
ogist may then remove material from
any layer of the centrifuge tube and
find a concentration of the specific cell
materials he wishes to examine or
analyze.

Although centrifugation separates
cell structures, it does not show what
is going on in the intact, living cell.
Recent advances in a technique called
radioautography have aided the biolo¬
gist in tracing cell processes. Radio-
autography is actually a form of photog¬
raphy. In photography light forms an
image on a sensitive substance called
a photographic emulsion. You will re¬
member from Chapter 3 that certain
isotopes give off radiations spontane¬
ously. These radioactive isotopes form
an image on a photographic emulsion
just as light does.

In radioautography a substance
normally taken in by cells is labeled
with a radioactive isotope such as trit¬
ium. A slide bearing cells and the
labeled substance is then treated with a
photographic emulsion. After a time
the slide is examined under a micro¬
scope, and the labeled substance ap¬
pears as black dots. Thus if the sub¬
stance has been taken into the nuclei
of the cells, black dots will appear in
the nuclei. The number of black dots
indicates the amount taken up. You
can see that by this method it is pos¬
sible to determine what part of the cell
takes up the labeled substance, and
how much. For example, a substance
known to be used in the synthesis of
DNA may be labeled with tritium. If
black dots appear in the nucleus of cer¬
tain cells on the treated slide, it can be
assumed that DNA is being synthe¬
sized in those cells at that time. In

CHAPTER 4 THE STRUCTURAL BASIS OF LIFE 65

4-8 Left: electron micrograph of expanded cotton fiber showing cell wall layers.
Right: portion of cross-section of cotton fiber enlarged still further to show
fibrous texture of layers of the cell wall. (USDA, Southern Utilization Research
and Development Division)

this way radioautography has revealed
the location, timing, and extent of
many of the processes in the living cell.
The cellular level of organization. We
may consider the cell as the first level of
the organization of living things. Since
the cell is the unit of structure as well as
function of living organisms, a single
cell can constitute a complete organism.
This is the case in unicellular organisms
such as bacteria, yeasts, the ameba, and
manv other forms of life vou will studv.

J J J

Some unicellular organisms are grouped
together in colonies. In such colonial
organisms, the cells have no direct func¬
tional relationship to one another. More
complex plants and animals are com¬
posed of many specialized cells working
closely together and depending on one
another. Such division of labor among
cells is characteristic of multicellular or¬
ganisms.

Cells and the organization of tissues.
When we speak of a multicellular or¬
ganism, are we referring just to a large
number of cells? Is your body a mere
mass of billions of cells, all alike and
forming a tremendous cell colony? Of

course it isn’t! If it were, your ability
would be limited to the activity of a
single kind of cell. Your body and those
of complex plants and animals are the
result of cell specialization. This means
that there are many different kinds of
cells. Each is developed for a particular
kind of activity, which leads us to the
subject of tissues.

We define a tissue as a group of
structurally similar cells performing a
similar activity. This is the second
level of organization of living things.
In your body you have muscle tissue,
nerve tissue, bone tissue, cartilage, and
many others. Plants also have tissues.
When you name a tissue you think im¬
mediately of a certain kind of activity.
Your movement is the best movement
skeletal muscles can provide. Your
skeleton is the best framework bone
cells can provide. And your hearing,
seeing, tasting, and control of your body
are the marvelous activities of your
nerve cells.

Each cell in a tissue is a specialist.
However, while it may be structurally
and chemically organized to carry on a

66 UNIT 1 THE NATURE OF LIFE

specific process with great efficiency, it
must also perform all of the processes
necessary to maintain its own living con¬
dition. That is, it must receive food
molecules, respire, synthesize proteins
and other essential substances, and gen¬
erally maintain all basic life activities.
In this respect it is like all other cells.
But when a cell is part of a multicellular
organism, there can be division of labor.
A nerve cell can be a specialist in re¬
sponse because other cells supply its

4-9 Radioautograph of cells from a mouse
tumor. DNA, labeled with radioactive trit¬
ium, appears as black dots. Note that there
is a concentration of dots in the nuclear
material of the dividing cell (center). (Re-
nato Baserga)

food molecules, furnish it with oxygen,
and carry off its waste materials. Such
interdependence might be compared to
a complex society. The doctor can de¬
vote his time to medicine because he
can depend on the grocer, the carpen¬
ter, the machinist, and other specialists
for his nonmedical requirements.
These specialists in turn depend on the
doctor in matters of health. If each one
were required to do everything for him¬
self, he could never develop a special¬
ized skill. So it is with cells.

Tissues are grouped to form organs. In
higher plants and animals even a special¬
ized tissue cannot perform a life activity
to perfection. This requires several tis¬
sues functioning as a unit. We call such
a unit of biological organization an
organ. This brings us to the third level
of biological organization.

A hand is an organ. It is com¬
posed of skin, muscle, bone, tendons,
ligaments, blood, and nerves. Your
heart, stomach, liver, brain, and kidneys

Volvox

Yeast

4-10 Unicellular and colonial organisms.

CHAPTER 4 THE STRUCTURAL BASIS OF LIFE 67

ONION

Cell wall
Middle lamella

Oil droplets

WOOD

Leucoplasts
which contain starch

POTATO

TOMATO

Striations

Chromoplasts

EPITHELIUM

MUSCLE

Ciliated

epithelium

Concave surface
of red corpuscles

Epithelium from cheek lining

nuclei

granules

BLOOD

White corpuscles

Nerve ending

NERVE

4-11 Cells are grouped as tissues in the more complex plants and animals.
Note the great variety of forms that protoplasm may take.

68 UNIT 1 THE NATURE OF LIFE

are other organs. A plant stem is also
an organ. It has bark tissues, wood,
pith, and other tissues, all working to¬
gether. The stem supports the leaves,
flowers, and fruits, and moves materials
up and down between the roots and
leaves.

Organs may be grouped into systems.
In the higher forms of life, especially
among animals, several organs may co¬
operate as a functioning unit. This in¬
troduces the fourth level of biological
organization, the organ system. For ex¬
ample, many organs are involved in
converting a meal you eat to molecules
your cells may use. How many of these
organs can you name? To deliver these
molecules to cells throughout your
body, there must be an efficient trans¬

port system. Your heart, blood vessels,
and lymph vessels perform this vital
activity. In the higher animals an or¬
gan system is devoted to nearly every
one of the activities involved in the liv¬
ing condition.

We could continue to a fifth level
of biological organization, the organism.
An organism is a complete and entire
living thing. However, we find organ¬
isms at all levels of organization. Many
never go beyond the cellular level.
Others reach the tissue level but never
develop organs. Still others have organs
but lack well-defined systems. You will
become familiar with all levels of or¬
ganization as you survey the world of
plants and animals — in both their prim¬
itive and advanced forms.

IN CONCLUSION

We have discussed the structural organization of organisms from molecules to
cell structures. Each cell is a unit of life and in a sense a tiny living organism.
As living things become more complex, cells become specialists. Tissues, or¬
gans, and systems allow cell specialization to the utmost.

Just as the cell is the unit of structure of all plants and animals, so it is
the unit of their functions or chemical activities. We are now ready to explore
these functions in greater detail, starting with the vital relationships between

cells and their environments.

BIOLOGICALLY SPEAKING

cell

cell specialization

cell theory

cell wall

chromosome

colonial

cytoplasm

division of labor

endoplasmic reticulum

Golgi body

mitochondrion

multicellular

nuclear membrane

nucleoli

nucleus

organ

organ system

organelle

plasma membrane

plastid

ribosome

tissue

unicellular

vacuolar membrane

vacuole

CHAPTER 4 THE STRUCTURAL BASIS OF LIFE 69

QUESTIONS FOR REVIEW

1. In one sense Robert Hooke discovered cells; in another he did not. Ex¬
plain.

2. Describe the contributions of Dujardin, Schleiden, and Schwann in early
studies of cells.

3. Name the two principles of the cell theory.

4. List the processes of a cell.

5. The nuclear membrane confines certain structures but is not a barrier.
Why is this important?

6. Describe the formation and structure of a plasma membrane.

7. Describe the endoplasmic reticulum.

8. Locate the ribosomes in a cell and describe their function.

9. Locate and describe the mitochondria. What is their principal function?

10. Why do biologists believe Golgi bodies may function in secretion?

11. Why are chloroplasts sometimes referred to as cell factories?

12. What pigments other than chlorophyll may be present in chloroplasts?

13. Describe various contents of cell vacuoles.

14. Describe the composition of the wall of a woody plant cell.

15. What two general similarities do cells have in the formation of a tissue?

16. What is the biological meaning of the term organism ?

APPLYING PRINCIPLES AND CONCEPTS

1. In what respect is the cell the basic unit of life?

2. Do you believe that the mere presence in a test tube of all the vital sub¬
stances composing protoplasm would result in a living condition? Give
possible reasons supporting your opinion.

3. Discuss centrifugation as an important research tool in the study of cells.

4. Discuss radioautography as a research tool in the study of cell processes.

5. Discuss the role of the nucleus as the control center of a cell.

6. Discuss various levels of biological organization from cells to complex or¬
ganisms. In what respect is each level a higher station of development?

CHAPTER 5

THE CELL
AND ITS
ENVIRONMENT

Homeostasis — balance on a biological
tightwire. Have you ever watched a
circus performer walk a tightwire high
above the heads of a crowd of tense on¬
lookers? Every step requires precision
balance. If you watch the performer
closely you will notice that he leans
slightly to the left or to the right to ad¬
just his balance as he takes each pre¬
carious step. Balance and adjustment
to maintain balance — these are the
basic skills in tightwire walking.

In a sense every living organism
walks a biological tightwire. Living
things maintain an intricate balance in
the face of constantly changing condi¬
tions, both internal and external. Sur¬
vival depends on making adjustments to
these changing conditions. We refer to
the balance organisms maintain by these

self-regulating adjustments as homeosta¬
sis. Biologists also call this balance a
steady state.

Homeostasis occurs at all levels in
the organization of living things. It in¬
volves adjustment of the entire organ¬
ism. A gopher tortoise retreats to a
deep burrow through the hot summer
days in southeastern United States and
ventures out at night in search of food
under more favorable conditions. A
desert is a place of desolation under the
broiling sun but a place of great activ¬
ity at night when trap-door spiders,
scorpions, lizards, and sidewinder rattle¬
snakes come out of their daytime hid¬
ing. The wall-eyed pike seeks the cool
water of the depths of a lake when the
summer sun heats the surface water.
The frog buries in mud at the bottom
of a pond to escape the cold of winter
and the heat of summer. These sur¬
vival adjustments are self-regulating ho¬
meostatic devices. Cells and tissues of
the frog could not survive either tem¬
perature extreme. Much of your biol¬
ogy course will deal with adjustments of
organisms to provide optimum environ¬
mental conditions for survival.

In the higher animals we find con¬
tinuous homeostatic adjustments at the
organ and system level. Organs func¬
tion in close association with other or¬
gans in maintaining an optimum inter¬
nal environment. All organs depend on
heart action to supply their constant en¬
vironmental requirements with circulat¬
ing blood. A weakened heart, entering
a state of failure, usually becomes en¬
larged in order to continue its vital
function. A pair of kidneys normally
removes cell wastes from the blood
stream in the human system. If a dis¬
eased kidney ceases to function or must
be removed for one reason or another,
its mate enlarges, often to twice its nor-

CHAPTER 5 THE CELL AND ITS ENVIRONMENT 71

mal size, and doubles its blood filtering
capacity.

On the tissue level we find a pre¬
carious homeostatic regulation of the
body fluids that bathe cells and provide
the immediate internal environment.
Fluids that bathe cells must supply food
nutrients and oxygen and receive waste
products from cell activities. Salts reg¬
ulate the osmotic concentration of these
fluids. A critical acid-base balance must
be maintained. In such an optimum
environment the survival problems of
our cells are reduced to a minimum.
They may specialize in such functions
as sensitivity, movement, secretion, or
excretion. But with this high degree of
specialization, cells become more and
more dependent on ideal environmen¬
tal conditions.

Our lakes and streams, oceans and
seas support an enormous population of
one-celled and simple colonial organ¬
isms. Here we find single cells or groups
of cells adjusting to extreme environ¬
mental changes. Could we expect a
nerve cell or a muscle cell, removed
from your body, to survive in a jar of
pond water? Certainly not! Removed
from the closely regulated environment
of tissue fluid, these cells would perish
immediately.

To what extent can cells make ho¬
meostatic adjustments and maintain
their vital balance? As a first approach
to homeostasis, we will consider the
way in which the plasma membrane
regulates the flow of materials in and
out of the cell.

The molecular boundary of a cell.
You will remember from your study of
the cell in Chapter 4 that molecules of
cytoplasm rearrange on the exposed sur¬
face and form a membranous layer.
The life of the cell depends on the mo¬
lecular traffic through this all-important

layer. This colloidal boundary, or plas¬
ma membrane, consists of an outer laver
of protein molecules, a middle double
layer of fatlike molecules, and an inner
protein layer. Molecules must enter
continuously to supply the chemical ac¬
tivities of the cell substances. Other
molecules, the by-products of these ac¬
tivities, pass outward through the mem¬
brane.

Molecules and ions move against
the plasma membrane in a steady
stream. Some move rapidly; others,
more slowly — large molecules, small
molecules, and still smaller ions. Some
pass through the membrane quickly;
others enter at a slower rate; while still
others move through very slowly. Some
do not penetrate at all. What forces
regulate this molecular flow? Is it the
size of the molecules? This is partly
true, but some large molecules pass
through more rapidly than much small¬
er ions. Is it the structure or composi¬
tion of the membrane? This is also
part of the answer. Do conditions out¬
side the membrane or inside the cell
regulate the flow? These are also im¬
portant factors. Each one has an im¬
portant influence on the vital relations
between the cell and its environment.

The structure of this membrane is
of extreme importance in regulating mo¬
lecular movement through it. WTile
the membrane may appear skinlike un¬
der the microscope, it is apparently po¬
rous. Undoubtedly, there are numer¬
ous spaces between the various mole¬
cules composing it. This would explain
why very large molecules such as pro¬
teins do not pass through the mem¬
brane, while smaller ones penetrate
more freely.

Another requirement for passage of
most substances through the plasma
membrane is that they be soluble in

72 UNIT 1 THE NATURE OF LIFE

THE CAPACITY OF VARIOUS SUBSTANCES
TO PENETRATE THE PLASMA MEMBRANE

Rapid

Penetration

Slower

Penetration

Very Slow
Penetration

Little or No
Penetration

Gases

Oxygen

Carbon dioxide
Nitrogen

Water

Fat solvents

Alcohol

Ether

Chloroform

Monosaccharides

Glucose

Fructose

Galactose

Amino acids

Fatty acids

Glycerol

Disaccharides

Sucrose

Lactose

Maltose

Ions of

Mineral salts
Acids

Bases

Polysaccharides

Starches

Cellulose

Proteins

Lipids (fats)
Phospholipids

water. A cell is usually bathed in a
water solution containing dispersed
molecules and ions. A material which
forms a colloid in water cannot pass
through, nor can insoluble substances
such as starches, which do not disperse
among water molecules.

Permeability of membranes. If a sub¬
stance passes through the membrane,
we say that the membrane is permeable
to that substance. However, as we have
pointed out, a plasma membrane allows
some substances to penetrate freely,
while others pass through at varying
rates and still others are rejected en¬
tirely. This variation in the rate and
degree of penetration is a property of a

differentially permeable membrane, the
type that encloses a cell.

The rate of penetration of various
molecules is very important in absorp¬
tion, or the transport of substances
through the plasma membrane from the
environment into the cell. It is also vi¬
tal in excretion, or the transport of mol¬
ecules through the membrane from the
cell into the surrounding external fluids.

We can classify various substances
into groups, based on the rate and de¬
gree of their molecular penetration of a
plasma membrane (see table above).
What factors determine the rates and
degree of penetration of these and other
substances? Some are purely physical

5-1 This diagram shows how molecules of a substance collide with one another
in the process of diffusion.

CHAPTER 5 THE CELL AND ITS ENVIRONMENT 73

forces over which the cell has no con¬
trol. We refer to this passage as passive
transport. Others involve energy with¬
in the cell. This is active transport. In
addition, the penetration of some sub¬
stances is regulated by the structure of
the membrane itself.-
Diffusion, the spreading of a substance
by molecular motion. Let s visualize a
substance as a mass of quivering mole¬
cules, bumping into one another like a
crowd of people at a bargain counter.
Molecules move in straight lines until
they collide with other molecules,
bounce off, and collide again. The
force moving these molecules is internal
kinetic energy rather than an outside
force. Each molecule thus moves in¬
dependently of other molecules. This
molecular movement occurs in gases
and liquids and to a lesser extent in
solids. The force of collisions causes
molecules to spread apart from one
another and distribute equally in a given
area. We refer to this molecular dis¬
tribution as diffusion.

To illustrate diffusion of gaseous
molecules, let's imagine that you open
a bottle of ether in the front of a
closed room (Fig. 5-2). Ether mole¬
cules are concentrated inside the bottle.
As soon as you open the bottle, they
start diffusing into the air. Soon you
begin to smell ether across the room.
As diffusion continues, the ether odor
becomes stronger. More and more
ether molecules are mingling with the
molecules of gases of the air. Finally,
when all of the ether molecules are dis¬
tributed equally among the air mole¬
cules, diffusion stops. A state of equi¬
librium has been reached.

According to the laws of diffusion,
two things happened. Ether molecules
diffused from the bottle into the air,
and air molecules diffused into the ether

5-2 When a beaker of ether is uncovered in
a tightly closed room, ether molecules min¬
gle with molecules of the gases in the air.
Eventually, molecules of ether and mole¬
cules of other gases are evenly distributed
through all parts of the room.

bottle. Both movements were from a
region of greater molecular concentra¬
tion to one of lesser molecular concen¬
tration. This did not occur in a uni¬
form flow of molecules, but in a ran¬
dom spreading out resulting from mo¬
lecular collisions. Diffusion continued
until the concentration of air and ether
was equal in all areas of the room.
Other familiar examples of diffusion.
Solids and liquids, or liquids and other
liquids, may diffuse as readily as gases
if they normally mix and do not repel
each other. Drop a cube of sugar into
a glass of water. As the sugar dissolves
taste the water from time to time. You
will be able to taste the increase in the
number of sugar molecules as diffusion

74 UNIT 1 THE NATURE OF LIFE

occurs. Finally, the sugar will dissolve
completely without any stirring. Sugar
molecules diffused from a region of
greater concentration (the lump) into a
region of lesser concentration (the wa¬
ter). Meanwhile, water molecules dif¬
fused into the sugar. When equilibri¬
um was reached, all parts of the solu¬
tion tasted equally sweet.

You can watch diffusion occur by
dropping some crystals of potassium per¬
manganate into a jar of water. Deep
purple particles stream from the crystals
into the water. The color of the solu¬
tion deepens as diffusion progresses.
Finally, the crystals disappear and the
water solution is uniformly a deep pur¬
ple color.

External factors that influence diffusion
rates. In addition to molecular concen¬
tration, two other factors influence the
rate at which diffusion occurs. One of
these is temperature. The higher the
temperature the greater the speed of
molecular movement. Thus diffusion
occurs from an area of higher tempera¬
ture to one of lower temperature. Simi¬
larly, pressure accelerates molecular
movement, resulting in diffusion from
a region of higher pressure to one of
lower pressure. Thus differences in
molecular concentration, temperature,
and pressure affect diffusion. We refer
to the force resulting from these differ¬
ences as diffusion pressure.

Diffusion through a permeable mem¬
brane. How strong are the forces of
diffusion? To what extent does a mem¬
brane interrupt or alter the movement of
diffusing molecules? This, of course, de¬
pends on the nature of the membrane
as well as the diffusing substances.

First we shall consider the case of
diffusion through a permeable mem¬
brane. Fig. 5-4 shows a simple difhusion
apparatus that you can easily set up in
the laboratory. The lower bulb of a
thistle tube is filled with a molasses so¬
lution (commercial molasses may be
used). A piece of fine muslin is tied
tightly over the open end of the bulb.
The thistle tube is then fastened to a
clamp (a ring stand may be used) and
submerged in a jar of water.

Since the muslin is permeable both
to water molecules and to sugar mole¬
cules in the molasses, two things will
happen. Water molecules will diffuse
from the jar into the molasses in the
thistle tube. At the same time sugar
molecules will diffuse from the thistle
tube into the jar. You can observe this
molecular movement as the solution in
the jar deepens in color. Diffusion will
continue until both water molecules
and sugar molecules are equally distrib¬
uted on both sides of the muslin. At
this point a state of equilibrium is
reached. Or we can say that there is
no longer diffusion pressure because of

5-3 Three stages in the diffusion of a solid into a liquid.

CHAPTER 5 THE CELL AND ITS ENVIRONMENT 75

equal molecular distribution. What
would happen to a cell if its membrane
were permeable to this degree? It is
true that water and molecules of other
substances could enter readily. But
wouldn’t the cell’s own molecules dif¬
fuse into the environment just as read¬
ily? Logically, a plasma membrane
must be differentially permeable, allow¬
ing certain materials to enter but retain¬
ing the molecules of proteins and other
substances composing the vital struc¬
tures of the cell.

Diffusion through a differentially per¬
meable membrane. A variation of the
preceding experiment is shown in Fig.
5-5. The apparatus used is the same
except that a differentially permeable
membrane (sheep bladder may be
used) is substituted for the muslin. In
the classification of penetration rates of
various molecules (page 72), you will
recall that water molecules penetrate a
differentially permeable membrane rap¬
idly, while those of sucrose pass through
very slowly. The molasses in the thistle
tube is a concentrated sucrose solution.
According to the laws of diffusion, wa¬
ter molecules should move through the
membrane from the region of greater
concentration in the jar to the region of
lower concentration in the thistle tube.
This movement occurs freely. Howev¬
er, the membrane permits but little dif¬
fusion of the more concentrated sucrose
molecules in the thistle tube to the jar.
Consequently there is uneven molecu¬
lar diffusion. Over a period of time
(usually several hours) the water level
in the tube rises. However, as the solu¬
tion rises in the tube, another force is
involved. This is the force of gravity
tending to pull the column downward.
The weight of the solution in the thistle
tube creates a pressure against the inner
surface of the membrane. When this

5-4 Diffusion through a permeable mem¬
brane.

force is equal to the diffusion pressure
of the water molecules, diffusion ceases.
Osmosis — a diffusion of water. The
experiment we have just described in¬
volved the diffusion of water through a
differentially permeable membrane from
a region of greater concentration to a
region of lesser concentration. Biolo¬
gists refer to this water diffusion as os¬
mosis. In the definition of osmosis, con¬
centration refers only to water mole¬
cules, not to substances dissolved in
water.

As water diffuses into a cell by os¬
mosis, it builds up a pressure, known as
turgor (ter- ger) pressure. Since the
plant cell is encased in a wall, it does
not stretch or bulge as this internal pres¬
sure increases. Internal water pressure
forces the plasma membrane and cyto¬
plasm firmly against the wall, causing
the cell to become stiff, or turgid ( ter -
jid). When turgor pressure, which
would tend to force water molecules out

76 UNIT 1 THE NATURE OF LIFE

5-5 Diffusion through a differentially per¬
meable membrane.

of the cell, becomes as great as the dif¬
fusion pressure which is causing them
to diffuse into the cell, an equilibrium
is reached. Diffusion stops.

Thin-walled plant tissues such as
those composing leaves, flower petals,
and soft stems maintain their stiffness
bv cell turgidity. As long as there is
sufficient water supply in the cell envi¬
ronment, this internal turgor pressure is
maintained. Otherwise the plant wilts.
Water problems in animal cells. A
plant cell builds up turgor pressure be¬
cause its wall can withstand this inter¬
nal force. However, an animal cell
lacks this supporting structure and
would burst if turgor pressure were built
up. For this reason animal cells and
tissues must have water-eliminating
mechanisms.

Many one-celled organisms, living
in a fresh-water environment, are
equipped with special water-eliminating
organelles known as contractile vacu¬

oles. These tiny pumps work continu¬
ously, eliminating water through the
membrane as rapidly as it diffuses into
the cell. Were it not for these special
structures, the cells would soon burst.
Fish and other gill-breathing aquatic ani¬
mals take in large amounts of water with
the oxygen they absorb into the blood.
In these animals excess water is excreted
from the body in urine. Our kidneys,
sweat glands, and lungs eliminate excess
water and prevent our cells from receiv¬
ing too much water.

You can observe the effects of ex¬
cessive water absorption in animal cells
by putting one-celled animals in dis¬
tilled water. The diffusion pressure in
distilled water is greater than these or¬
ganisms can overcome by water-elimi¬
nating organelles. Soon the cells swell
and burst. The presence of minerals
and other soluble materials in pond wa¬
ter prevents this from happening in na¬
ture bv reducing the diffusion pressure.
Similarly, if you place a drop of blood
in distilled water, the cells swell and
burst almost immediately.

Plasmolysis, the loss of cell turgor. Re¬
member that a cell has no control over
the movement of water molecules
through its plasma membrane. The di¬
rection in which water diffuses is deter¬
mined by differences in the concentra¬
tion of water inside and outside the cell.
If a water solution outside the cell con¬
tains more dissolved substances and
therefore has a lower concentration of
water molecules than the solutions in¬
side the cell, water will diffuse from the
cell into its environment. This results
in loss of turgor pressure and shrinking
of the cell content. This condition is
called plasmolysis (plaz-mohZ-y-sis) .

You can observe this water loss in
several ways. Slice two pieces of pota¬
to, and place one in strong salt water

CHAPTER 5 THE CELL AND ITS ENVIRONMENT 77

5-6 The effects of absorption on red blood cells. Left: cells in a normal salt
solution; middle: in a strong solution; right: in a weak solution.

and the other in tap water. After 10 or
] 5 minutes, examine both slices. The
slice in salt water will be limp, or flac¬
cid, , indicating that the cells have lost
water and therefore turgor pressure.
You can watch this process under the
microscope as it occurs in a leaf of an
aquatic plant like waterweed, Elodea
(i-Zohd-ee-a) . Mount the leaf in water
and notice that the cells are turgid.
Then add one or two drops of strong
salt solution at the edge of the cover
glass. You will see the cell content
shrink from the wall and collapse in a
mass near the center. In animal cells,
such as blood cells, the entire cell
shrinks since there is no wall to give it
firmness. Temporary plasmolysis may
be corrected bv the intake of water.
But the cell will die if the condition
continues very long. Now you can un¬
derstand why salt kills grass and why
shipwrecked men die from drinking salt
water. Plasmolysis explains, also, why
too heavy an application of strong fer¬
tilizers to the soil may kill the roots they
contact.

Penetration of cell membranes by al¬
cohol and other solvents. Alcohol,
ether, and chloroform penetrate the
plasma membrane and enter a cell even
more rapidly than water. Biologists be¬
lieve that these solvents dissolve the fat
layer of the membrane and pass through

quickly. These substances interfere
with normal cell activities and have an
anesthetic effect on cells. The rapid
penetration of the membrane would ex¬
plain why they act so quickly. The cell
membrane is not permanently damaged,
as other fat molecules move into the
dissolved region.

Diffusion of ions through a plasma
membrane. Various mineral salts, acids,
and bases form ionic solutions in water.
These charged particles penetrate a
plasma membrane very slowly. Biolo¬
gists have made extensive studies of ion
absorption by a cell and still have many
unanswered questions. One possible
explanation suggests that the membrane
itself has an electrical charge and that
this charge is usually negative. This
would repel negatively charged ions,
since like charges repel each other.
Furthermore, the negative ions would
be attracted to positive ions present in
the solution outside the cell. This at¬
traction may be strong enough to pre¬
vent many ions from penetrating the
membrane. Nevertheless, some ions do
penetrate the plasma membrane and
are important in the chemical activities
of the cell.

The principle of active transport. We

know that many cells receive or excrete
molecules against a diffusion pressure
that would normally cause movement

78 UNIT 1 THE NATURE OF LIFE

in the opposite direction. For example,
a root cell may absorb mineral ions
from soil solutions that contain fewer
ions than the cell already contains. Ac¬
cording to the laws of diffusion, ions
should move from the cell into the soil
water. Similarly, cells of certain algae
living in the ocean absorb iodine com¬
pounds when the concentration of these
substances within the cells is already
much higher than that of sea water sur¬
rounding them.

It is obvious that some force other
than diffusion accounts for this move¬
ment through the membrane. Biolo¬
gists refer to this force as active trans¬
port. They know that cell energy is
used in transporting these substances
through the membrane, although they
are not certain just how the energy is

5-7 Since there are fewer sodium ions out¬
side the cell than inside, they must enter by
active transport. Since there are more po¬
tassium ions on the outside, they enter by
passive transport.

5-8 An electron micrograph showing the
process of pinocytosis. (S. L. Clark, Jr.)

used. They have proved that cell en¬
ergy is involved by measuring the oxy¬
gen intake and carbon dioxide release
during active transport. If the solute
concentration outside of the cell is
greater than that of the cell content,
solutes enter the cell by diffusion.
However, if the solute concentration
outside the cell is lower than that of the
cell, the oxygen intake and energy re¬
lease increases sharply. The cell is then
using its energy in solute absorption.
This is active transport, as opposed to
passive transport, or membrane penetra¬
tion by diffusion pressure alone.

Entry of large molecules into a cell. So
far we have discussed the penetration of
a plasma membrane by water molecules,
ions, and other materials which could
pass through pores in the membrane.
But what about larger molecules, such
as amino acids, lipids, and even larger
masses of material? These particles
cannot pass through membrane pores,
yet they are known to enter cells.

In the discussion of the structure of
a plasma membrane, do you remember

CHAPTER 5 THE CELL AND ITS ENVIRONMENT 79

the reference to numerous infolding
pockets? Biologists believe that large
molecules and other particles flow into
these indentations and are sealed off as
the membrane closes behind them. The

material is then enclosed in a membrane
within the cell as a vacuole. This en¬
gulfing process is referred to as pino-
cytosis (pi-noh-cy-to/i-sis) . It is another
important factor in cell absorption.

IN CONCLUSION

In order to survive, an organism must maintain a steady state in all its cells,
even though conditions in the environment are constantly changing and may
become unfavorable; This involves adjustment of the whole organism to ex¬
ternal environmental conditions as well as adjustments of organs, tissues, and
cells in its internal environment. We refer to these self-regulating devices as
homeostasis.

Whether the environment of a cell will supply its chemical requirements
for maintaining life activities or remove vital substances and destroy the cell
depends on many factors. One of these is diffusion, a form of passive trans¬
port over which the cell has no control. Water normally enters a cell by
osmosis, but may also leave the cell and lead to its destruction. But while
the cell cannot overcome diffusion pressure in water movement, it does over¬
come this force in active transport. When active transport occurs, the cell
uses its own energy to absorb minerals in amounts much greater than the con¬
centration of these materials in the solutions outside the cell.

What happens to the many molecules and ions that penetrate the mem¬
brane and enter the cell substances? Some are building blocks used in molec¬
ular construction. Others are fuel for the cell powerhouses. We summarize
the chemical activities involved in the growth, repair, and maintenance of the
cell under the general heading of metabolism. We shall explore this next.

BIOLOGICALLY SPEAKING

active transport
contractile vacuole
differentially permeable
diffusion

diffusion pressure

homeostasis

osmosis

passive transport

permeable
pinocytosis
plasmolysis
turgor pressure

QUESTIONS FOR REVIEW

1. Define homeostasis and explain why homeostatic adjustments are necessary
for the survival of an organism.

2. Give examples of homeostatic adjustments at various levels of organization
of living things.

3. Distinguish between a permeable and a differentially permeable membrane.

4. Explain diffusion in terms of molecular movement.

80 UNIT 1 THE NATURE OF LIFE

5. Under what condition is a state of equilibrium reached?

6. How are temperature and pressure related to the rate of diffusion?

7. What is diffusion pressure?

8. Define osmosis.

9. What is turgor?

10. Would a cell build up turgor pressure if its membrane were permeable to
all of the molecules it contacts? Explain.

11. Explain why animal cells do not build up turgor pressure.

12. Why do blood cells burst quickly if they are put in distilled water?

13. Describe several methods by which animal cells eliminate excess water.

14. Describe the cause of plasmolysis of a plant cell and the physical changes
that occur.

15. Why do alcohol, ether, and chloroform penetrate a cell membrane rapidly?

16. What force exceeds diffusion pressure during active transport of minerals
through a cell membrane?

17. Describe the entry of substances into a cell by pinocytosis.

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the homeostatic adjustments you have observed in organisms of
your region.

2. It was once thought that the size of the pores in a cell membrane deter¬
mined molecular penetration. Give evidence to show that this is not al¬
ways true.

3. Why is it necessary that a cell have a higher solute concentration than the
surrounding external solutions?

4. What might happen to the cells of a fresh-water plant if it were placed in
salt water? Why would fresh water destroy a salt-water plant?

5. How have biologists demonstrated that active transport involves cell energy?

CHAPTER 6

CELL

NUTRITION

The source of energy. Visualize a
wrecking crew tearing out the interior
of a building while a construction crew,
working in the same rooms, is picking
up the debris and building it up again.
Such destructive and constructive proc¬
esses occur simultaneously in a living
cell. Complex molecules are built up
in some processes and broken down in
others. Why all of this seemingly
wasteful activity? Cells must have en-
ergy every second of their existence.
This energy comes from the chemical
bonds of complex molecules. It is nec¬
essary to break these molecules down to
set the energy free. Where will the
next energv come from? From other
fuel molecules, of course. Does the cell
build up these molecules? Some cells
can, but many cannot. Thus you can
see that some cells are “self-fueling,”
while others depend on other cells.

These tearing down and building
up processes apply to substances that

form the cell as well as to fuel mole¬
cules. Cell structures are constantly
changing. Cell parts wear out and must
be replaced. Growth and repair is a
continuous process in a living cell.
Where does the cell get the molecules
necessary to form its own substances?
Again, some are products of the cell’s
own chemical activities, while others
are received from the environment.
Energy to maintain life activities. In
our study of cell processes we must con¬
sider not only cells but the larger struc¬
tures they form and, of course, entire
organisms. Let us imagine that we can
completely enclose an entire commu¬
nity, perhaps several square miles in
area, and prevent any organisms or ma¬
terials from entering or leaving the
closed environment. We will include
wooded areas, farms, open fields, and
ponds in the gigantic enclosure. Ani¬
mal life native to the region as well as
domestic animals will also be included,
along with the people who live there.
In other words, life would continue nor¬
mally, except that the whole region
would be isolated from the outside
world. Could this isolated society con¬
tinue? Plants and animals would live
and die. Their chemical remains would
supply the requirements of new genera¬
tions. Matter would be used and re¬
used, organized in complex substances
and broken down to simpler form, in a
continuous cycle year after year and
generation after generation.

Now suppose we covered this giant
enclosure and shut out all of the light.
Do vou think life could continue as be¬
fore? You have probably concluded
that the people, the animals, and even
the green plants would be doomed.
Life might continue until all the food
was consumed; then all would perish.
The matter could still cvcle, but what

82 UNIT 1 THE NATURE OF LIFE

about the energy necessary to build food
molecules and support life? Can the
heat energy released in living cells be
trapped and reused? Can the plants
and animals capture the energy used in
their own activities? No, energy does
not cycle. When we shut out the light,
we cut off the supply of new energy
which must enter the living world con¬
stantly.

What if we removed all the green
plants and left the animals and non¬
green plants, such as fungi, in full light?
Would life continue? No, the animals
would be as helpless as though there
were no light. They would have energy
all around, but none in the form they
could use to supply their cells. Doesn’t
this place the cells of green plants in a
unique position in the world of life?
Marvelous, indeed, must be the chemi¬
cal processes of plant cells that capture
sunlight and store chemical energy in
the molecules that fuel the cells of liv¬
ing things.

A marvel of plant life. To explore the
machinery of the cell and the process in
which fuel molecules are organized, we
can select almost any plant part as long
as it is green. We might choose a leaf
cell, a cell from a green stem, or a hair¬
like strand of an alga growing in a pond.
Any of the seaweeds could be used, for
while they may be brown or red in col¬
or, they also contain chlorophylls.

The process we are exploring is
known as photosynthesis. The name
nearly defines the process, for photo re¬
fers to light, while synthesis means the
building of a complex substance from
simpler substances. The simpler sub¬
stances are carbon dioxide and water ,
while the complex substance is glucose.
You may have already concluded that
light is necessary for photosynthesis and
that some of the light energy is some-

6-1 This micrograph of a chloroplast shows
a magnification of 15,000 times by an elec¬
tron microscope. Arrows point to the grana.
(Martin Co. — RIAS)

how captured in the sugar molecules.
But how is this accomplished? Cer¬
tainly, carbon dioxide and water will
not form glucose in a flask set in sun¬
light.

Chloroplasts, the machinery of photo¬
synthesis. In our discussion of the parts
of a cell, we referred to cytoplasmic in¬
clusions known as chloroplasts (klor- oh-
plasts). Do you remember that they
are made up of stacks of cylindrical disk¬
shaped protein bodies held together by
layers of fatlike molecules? These are
the grana.

CHAPTER 6 CELL NUTRITION 83

6-2 A model of the chlorophyll a molecule.
(Harvard University News Office)

While we often speak of chloro-
plasts as the machinery of photosynthe¬
sis, certain cells that carry on the proc¬
ess have no organized chloroplasts.
Such primitive cells are found in the
blue-green algae. However, the elec¬
tron microscope reveals that even these
cells have grana. They are dispersed
through the cytoplasm rather than con¬
fined to organized chloroplasts. Per¬
haps we should say, then, that grana
rather than chloroplasts constitute the
machinery of photosynthesis.
Chlorophyll, the agent of photosyn¬
thesis. If bean or corn seedlings are
grown in total darkness, the leaves will
be yellow rather than green because
light is necessary to produce the chloro¬
phylls. If such a plant is moved to a
light place, photosynthesis will not occur
until chlorophyll has formed in its

6-3 The structural formula of the chloro¬
phyll a molecule.

leaves. Chlorophyll, then, is essential
for photosynthesis.

Chlorophyll molecules are embed¬
ded partially in the protein disks of the
grana and partially in the fatlike layers
between them. Frequently molecules
of different forms of chlorophyll are
mixed in the grana. Four different
tvpes of chlorophyll are found in vari¬
ous plant cells. The most abundant
form, and the most important in photo¬
synthesis, is chlorophyll a. This bright
bluish-green pigment has molecules con¬
taining carbon, hvdrogen, oxygen, nitro¬
gen, and a single atom of magnesium
that lies in the center of the molecule.
The chemical formula for chlorophvll a
is C55H72OsN4Mg. A second yellowish-
green pigment is referred to as chlo¬
rophyll b. Its chemical formula,
C55H70O6N4Mg, shows that it differs

84 UNIT 1 THE NATURE OF LIFE

from chlorophyll a only in the number
of hydrogen and oxygen atoms present
in the molecule. Small amounts of
iron compounds are necessary for the
formation of chlorophylls, although iron
atoms do not enter into their molecular
structure.

Chloroplasts in the cells of seed
plants usually contain both of these
chlorophylls in the approximate ratio
of three parts of chlorophyll a to one
part of chlorophyll b. Certain other
plants, more primitive than seed plants,
contain other chlorophylls. Brown al¬
gae contain chlorophyll a and chloro¬
phyll c, while red algae contain chloro¬
phyll a and chlorophyll d. The purple
sulfur bacteria have a unique form of
chlorophyll, known as bacteriochloro-
phyll, contained in granules rather than
grana. These bacteria are exceptional
in that they are capable of photosyn¬
thesis.

Chlorophyll a is necessary for pho¬
tosynthesis, and yet it does not enter
into the process chemically. Remem¬
ber that we refer to such an accelerator
of a chemical reaction as a catalyst.
Other chlorophylls may assist the proc¬
ess by absorbing light energy and trans¬
ferring it to chlorophyll a. Other pig¬
ments in the chloroplasts, including
xanthophylls and carotenes, may also
transfer energy to chlorophyll a.

You can remove chlorophyll from
cells by heating a plant part such as a
leaf in alcohol or some other solvent.
You might even assume that you could
use this catalyst in the laboratory and
carrv on photosynthesis artificially.
Chloroplasts have been removed from
cells and, under these conditions, have
retained their photosvnthetic abilities.
But chlorophvll alone cannot produce
a carbohvdrate. Thus photosynthesis
seems to be limited to the processes of

a living cell, or at least to the grana of
the chloroplasts.

In addition to chlorophyll, chloro¬
plasts contain enzymes which are essen¬
tial to photosynthesis. Enzymes also
serve as catalysts and, together with
chlorophyll, cause the chemical reac¬
tions involved in the various steps of
photosynthesis to occur, without being
changed or used up in the processes.
The general nature of photosynthesis.
Biologists have had a general under¬
standing of photosynthesis for many
years. They assumed, however, that it
was a single chemical reaction in which
carbon dioxide and water were com¬
bined in a cell to form a glucose mole¬
cule. Oxygen was known to be a by¬
product of the process. Chlorophyll
has long been recognized as a catalytic
agent of the process, although its actual
role was not really understood until re¬
cent years. Biologists have known, too,
that light energy is absorbed during the
process and that, somehow, it is locked
in the bonds of glucose molecules. The
overall, simplified chemical equation
for photosynthesis is commonly written :

6 C02 + 6 H20 + light energy

C6H1206 + 6 02

This equation, while it accounts for
the materials required and the products
formed in the process, fails to summa¬
rize the true nature of photosynthesis.
Is photosynthesis a single reaction as
the equation indicates? Does carbon
dioxide react with water directly to form
glucose? Does the oxygen released as
a by-product come from the carbon di¬
oxide, the water, or both? How is light
energy transformed to chemical energy
in glucose molecules? How is chloro¬
phyll related to the process? These
questions biologists asked for years.
Only recently have they begun to solve

CHAPTER 6 CELL NUTRITION 85

the mystery of the most important
chemical process in the world.

Tools to explore the nature of photo¬
synthesis. By 1941 biologists and chem¬
ists had been provided with a new and
effective tool for chemical research. Iso¬
topes were available from atomic reac¬
tors in research centers. Among the
available isotopes were oxygen-18
(O18) and carbon-14 (C14), both heavy
forms of the elements. These could be
traced through chemical reactions as
tagged atoms, provided the reactions
could be stopped at a given instant so
that products formed could be isolated
and analyzed. The problem now was
to find a suitable plant which could be
killed and analyzed quickly. It would
be difficult to kill leaf cells instantly and
remove the products of photosynthesis
at various stages. But a tiny, fast-grow¬
ing alga, known to the biologist as Chlo-
rella (klo-rcZ-a), proved to be a perfect
subject. This microscopic one-celled

6-4 Chlorella, a tiny alga, has contributed
vastly to our knowledge of photosynthesis.
(Robert W. Krauss)

alga can be killed quickly for extraction
of the intermediate products which are
formed, then changed almost instantly
during photosynthesis. We owe much
to this tiny plant, for it yielded one of
the most significant biological discover¬
ies of our age — the answer to the rid¬
dle of photosynthesis. It is interesting,
too, that Chlorella is becoming impor¬
tant in two other areas. Tanks of this
alga may accompany a spaceman on a
journey to the moon or another planet.
Why would photosynthesis be an im¬
portant process in the closed environ¬
ment of a space capsule? Chlorella is
also being cultured experimentally in
large quantities, with a possible view to
using it as food for livestock or even for
man.

The secret of photosynthesis unlocked.
The first major discovery in the recent
investigations of photosynthesis con¬
cerned the fate of water molecules in the
process. Remember that biologists had
not been able to determine whether ox¬
ygen came from water or carbon dioxide.
Oxygen-18 provided the first answer.
When Chlorella cells were grown in
water containing heavy oxygen (H2018),
a remarkable discovery was made. The
oxygen streaming from the Chlorella
cells was heavy! Furthermore, glucose
extracted from the cells contained no
heavy oxygen. It was evident that,
somewhere in the process, water mole¬
cules were split. This process requires
a great amount of energy. It can be
done electrically by means of an elec¬
trolysis apparatus. But here were tiny
algae splitting water molecules in the
grana of their chloroplasts. Where
could this energy be coming from?
Since light is necessary for the process,
it was the logical energy source. Biolo¬
gists had known that light provides the
energy stored in glucose molecules, but

86 UNIT 1 THE NATURE OF LIFE

that light energy splits water molecules
was an entirely new concept. Chloro¬
phyll plays an important part in this
energy transfer. Thus, water molecules,
light energy, and chlorophyll molecules
are involved in the first stage, or light
reaction , of photosynthesis.

The splitting of water molecules in
the light reaction is only the first stage
in glucose production. What happens
to the carbon dioxide? Carbon-14 pro¬
vided this answer, at least in part. The
use of C1402 in controlled photosynthe¬
sis led to the discovery of the dark re¬
action, the second stage of the process.
During this stage carbon is fixed in a
series of chemical reactions, none of
which requires light. In the path of car¬
bon from inorganic carbon dioxide to
organic glucose, many steps are involved
and several intermediate products are
formed. During these changes carbon
atoms are bonded to form chains.
Many other atoms may be joined to
these chains to form an almost endless
number of organic compounds.

We may summarize the overall
chemical changes that occur during
both the light and dark reactions in
photosynthesis in the equation:

6 C02 4- 12 H,0 + light energy ->

C6H1206 + 6 H20 + 60 2

Heavy oxygen, as it was used in re¬
search in photosynthesis, is shown in
green. Notice that the six molecules of
oxygen (60,) released as a by-product
result from the splitting of water mole¬
cules. The oxygen in the glucose comes
from carbon dioxide. Notice, too, that
water enters into the reaction and that
it is a product as well. However, the
water used in the process has no rela¬
tion to the water produced.

While this equation represents the
overall photosynthetic reaction as we

6-5 A simple technique for finding the rela¬
tionship between light intensity and the rate
of photosynthesis. Simply count the bub¬
bles of oxygen that are released by sub¬
merged water plants under intense light.
(Richard F. Trump)

understand it today, we must examine
both the light and dark reactions more
thoroughly to appreciate what occurs.
The light reaction, or photo phase.
The first stage of photosynthesis, some¬
times referred to as the photo phase, oc¬
curs only in the light and requires chlo¬
rophyll a. Thus it is obvious that light
energy enters into reactions during the
photo phase. As we describe the vari¬
ous activities of the light reaction, it
might seem that it is a lengthy series of
processes. Actually, this phase of pho¬
tosynthesis occurs in a split second, per¬
haps in as short a time as 1/100,000 of
a second! But we can best describe the
various reactions that take place during
the splitting of water molecules by light
energy, or photolysis , as though they
were steps, even though they occur al¬
most simultaneously.

CHAPTER 6 CELL NUTRITION 87

6-6 This drawing summarizes the light reaction of photosynthesis.

Chlorophyll is energized. As chlo¬
rophyll molecules lying along the grana
of chloroplasts receive light energy, it is
believed that a sudden change occurs in
the molecules themselves. Electrons
move to shells farther away from their
nuclei, and consequently from lower to
higher energy levels. The chlorophyll
molecules are now energized, or excited.
Absorption of energy by chlorophyll
molecules transforms kinetic light en¬
ergy to potential chemical energy. Thus
chlorophyll functions as an energy car¬
rier in the photo phase. Energy re¬
mains in the chlorophyll as long as the
molecules remain in this excited condi¬
tion. When this energy is released the
electrons drop back to lower energy lev¬
els. The chlorophyll is no longer ener¬
gized, but may be illuminated again and
trap more light energy.

Water molecules are split. Energy
released from energized chlorophyll sup¬
plies the force necessary to pry the at¬
oms in water molecules apart. As we
stated earlier, hydrogen atoms are
joined to oxygen by very strong bonds
in a water molecule. Just how these
bonds are broken and stable water mol¬
ecules are split is still an unanswered
question. However, we do know that
the process requires a great amount of
energy, supplied by energized chloro¬
phyll.

Additional energy is trapped in
ATP. The photo phase of photosyn¬
thesis involves another important energy
transfer necessary to support later chem¬
ical reactions. Chloroplasts contain a
compound known as ADP (adenosine
diphosphate). In addition they con¬
tain phosphate groups which can be
added to ADP to form a higher energy
compound known as ATP (adenosine
[a-den-oh-seen] triphosphate). Part of
the energy released from energized chlo¬
rophyll is used in this process. Thus,
ATP is a second energy carrier in pho¬
tosynthesis. Energy remains in the mol¬
ecule as long as the extra phosphate
group is attached. Chemical removal
of the phosphate releases the energy
and reforms ADP. It then receives en¬
ergy and repeats the transfer process.

Hydrogen is trapped by TPN. As
hydrogen is released during the splitting
of water molecules in the light reaction,
it must be captured immediately to pre¬
vent it from escaping from the cell or
recombining with oxygen to form water.
This is accomplished by a coenzvme re¬
ferred to as TPN ( triphosphopyridine
nucleotide). We speak of TPN as a
hydrogen acceptor because it combines
readily with hydrogen to form TPNH2.
This acceptance is on a “loan basis,”
however, for the hydrogen is soon passed
to another compound.

88 UNIT 1 THE NATURE OF LIFE

ATP

ADP

6-7 This drawing summarizes the dark reaction of photosynthesis.

The oxygen released when water
molecules are split during the light reac¬
tion escapes from the cell as a by-prod¬
uct.

The dark reaction, or synthetic phase.
The transfer of energy from chlorophyll
to ATP during the light reaction
‘‘charges'’ the chloroplast for reactions
to follow. With chemical energy avail¬
able, these activities do not require light
and are thus referred to as the dark re¬
action. But this does not imply that
the dark reaction must occur in dark¬
ness. Actually, it occurs in the light
and accompanies the light reaction.

We can summarize the most im¬
portant result of the dark reaction as
the fixing of carbon in a carbohydrate.
This process occurs in several steps
which form a cycle. Compounds are
formed, broken down, and formed
again. In this chemical activity, carbon
atoms form chains to which atoms of
hvdrogen and oxygen may be joined by
chemical bonds to form carbohydrate
molecules. In this way the organic
chemical world originates. We can

summarize reactions in this stage of
photosynthesis as follows:

Carbon dioxide is fixed by RDP.
Chloroplasts contain a 5-carbon sugar
phosphate known as RDP ( ribulose di¬
phosphate). This compound is com¬
posed of a 5-carbon sugar molecule to
which two phosphate groups are at¬
tached. Within a fraction of a second
after carbon dioxide reaches a chloro¬
plast, it is fixed in a chemical compound
by combining with RDP. Thus RDP
serves as the highly important carbon
dioxide acceptor. The immediate prod¬
uct of this reaction is a 6-carbon sugar
which is very unstable and splits quickly
into two molecules of PGA ( phospho -
glyceric acid) . PGA is thus the first sta¬
ble product of photosynthesis.

PGA is converted to PG AL. With¬
in a fraction of a second PGA combines
with hydrogen, supplied by TPNH2.
The products of this reaction are PGAL
(phospho glycer aldehyde), also known
as triose phosphate, and water. This re¬
action requires a large amount of en¬
ergy, since PGA is a low energy com-

CHAPTER 6 CELL NUTRITION 89

pound while PGAL has a high energy
level. This energy is supplied from
ATP by the removal of a phosphate
group, converting it to ADP. PGAL
can be used directly by a cell and for
this reason might be considered to be
the principal product of photosynthesis.
While a cell does make immediate use
of PGAL, much of it is converted to
other products for transport from the
cell or for storage. We will summarize
several of these changes involved in the
destiny of PGAL.

The destiny of PGAL. Since mole¬
cules of PGAL contain 3-carbon chains,
PGAL may be used directly by a plant
cell as a nutrient. In fact, plants nour¬
ished artificially with PGAL can survive
without photosynthesis or any outside
source of organic nutrients. However,
a cell produces far more PGAL during
photosynthesis than it needs, and much
of the PGAL is changed to other prod¬
ucts in further chemical reactions.

One essential product of PGAL is
additional RDP to refuel the cell for
further photosynthesis. You will recall
that this 5-carbon sugar phosphate was
present in the chloroplast and served as
the carbon dioxide acceptor. You can
see now how chemical reactions of the
dark phase of photosynthesis constitute
cycles. RDP was necessary to form
PGAL. Part of the PGAL is converted
back to RDP to prepare the chloroplasts
for the next round.

Other molecules of PGAL are con¬
verted to glucose. This is accomplished
bv the combination of two molecules
of PGAL, the removal of the phosphate
group from each molecule, and the sub¬
stitution of a hydrogen atom for each
phosphate group. The formula for
PGAL is C3H50*3-(P). The attached
phosphate is indicated in the parenthe¬
ses. You can see how the combination

of two PGAL molecules and the substi¬
tution of hydrogen for each would pro¬
duce a glucose molecule (C6H12Oe).

Fructose, another monosaccharide
with the same chemical formula as glu¬
cose, may also be produced from PGAL.
Sucrose, a disaccharide with the chemi¬
cal formula C12H22011, may then be
formed by the combination of a fruc¬
tose molecule and a glucose molecule
and the removal of a molecule of water.
Two glucose molecules may be com¬
bined and one water molecule removed
to form maltose, another familiar di-
saccharide. Hexose sugar molecules
(C6H1206) may also be joined in chains
to form starches and cellulose. Some
plants build PGAL molecules into vari¬
ous oils, such as corn oil, linseed oil,
castor oil, and olive oil. Thus you can
see that this important product of pho¬
tosynthesis is the basis for a wide variety
of products.

Conditions for photosynthesis. Since
light is the energy source for photosyn¬
thesis, you would expect light condi¬
tions to have a vital relationship to the
process. As you know, sunlight is com¬
posed of light rays of varying wave¬
lengths and energy. The various rays
making up sunlight — those producing
red, orange, yellow, green, blue, and vio¬
let colors — are what we see in a rain¬
bow. Together they are called the vis¬
ible spectrum. Red rays have the long¬
est wavelength and the least energy of
the visible radiations. Violet rays, at
the opposite end of the spectrum, have
the shortest wavelength and the highest
energy. Chloroplasts absorb these rays
in varying amounts. Furthermore,
plants vary in the rays they absorb.
Most land plants absorb their greatest
amount of energy from violet and blue
rays and a somewhat smaller amount of
energy from red and orange rays. While

90 UNIT 1 THE NATURE OF LIFE

100

\n 80
<D

4-»

c/>

*5 60

*4—

4->

_03

— . . . '

IPXI

10 20 30

Temperature °C

6-8 The effect of temperature on photosyn¬
thesis. The upper curve represents high
light, the lower low light.

some of the green and yellow light is
absorbed, much is reflected. Since only
reflected light rays are visible, this ac¬
counts for the green or greenish-yellow
color of chloroplasts.

Plants such as algae growing in the
ocean have different light problems.
Sea water absorbs most of the red and
violet rays, in addition to reducing the
total intensity of light. Much of the
energy for photosynthesis in plants of
the more shallow waters comes from
the blue, green, and yellow portions of
the spectrum. Deep-water algae, living
at depths of 50 to 200 feet or more, re¬
ceive most of their light energy from
green and blue rays.

Temperature also influences the
rate of photosynthesis, but not as much
as you might expect. There is, how¬
ever, a relationship between tempera¬
ture and the carbon dioxide supply and
the rate of photosynthesis. Plants
growing in the normal atmosphere, with
a carbon dioxide content of about 0.04
percent bv volume, carry on photosyn¬
thesis most rapidly at a temperature of
about 21° C. But if the carbon dioxide

content is raised to 1.25 percent, photo¬
synthesis occurs most rapidly at a tem¬
perature of about 30° C. Temperature
probably varies the activity of enzymes
involved in various steps of photosyn¬
thesis. However, temperature varia¬
tions within the normal range during a
plant growing season seem to have little
effect on enzyme action until tempera¬
tures exceed about 32° C. From this
point on, increase in temperature seems
to reduce the rate of photosynthesis.

Water supply is another factor that
influences the rate of photosynthesis.
Water shortage affects the entire physi¬
ology of the cell and therefore reduces
the rate of photosynthesis.

Biological significance of photosynthe¬
sis. Were it not for photosynthesis the
life of our earth would probably be lim¬
ited to a few bacteria. There would be
no forests or grasslands. Certainly there
would be no animal populations. What
makes the process so vital to life? It

100

■g 75

-O

o
c n
JD
ro

•— 50

H—

■4— '

ji3 25

400 Blue 500 Green 600 Orange 700
Violet Q . -=D> Red

Wavelength of light (in mu)

1 1

— A 1 ~l

ft i ft
fll t
« ii ft

I II l
v u ft

I w §

I~ 1

. I ■
tl 1

111 1

1 1

f l

■

i i

i i
i i
i i

1 K
i i
i i

i i

i
t
%

6-9 The absorption spectrum of chlorophyll
in alcohol. Note that most of the green light
is reflected.

CHAPTER 6 CELL NUTRITION 91

provides the chemical link between the
inorganic and the organic chemical
worlds. You might liken photosynthe¬
sis to the crossing of a chemical bridge,
with carbon dioxide and water on one
side and PGAL and glucose on the oth¬
er. Chlorophyll, coenzymes, and en¬
ergy form the bridge.

In discussing the food relations of
organisms, we refer to green plants as
autotrophs ( ot-oh-trohfs ) . This term
means “self-feeders” and refers to the
capability of these organisms to organ¬
ize organic molecules. Organisms lack¬
ing this capacity are classed as hetero-
trophs (het-e- ro-trohf), or “other feed¬
ers.” Animals and nongreen plants
such as fungi are among these nutri¬
tionally dependent organisms. While
many heterotrophs have remarkable
chemical abilities, none can produce a

molecule of glucose from inorganic ma¬
terials. In other words, they lack the
capacity for photosynthesis and must
rely on autotrophs for their basic chem¬
ical requirements.

Chemosynthesis. Our discussion of car¬
bohydrate synthesis would not be com¬
plete without a brief discussion of a
small but very important group of or¬
ganisms which do not rely on photo¬
synthesis. Certain bacteria organize car¬
bohydrates without using light energy
by a process known as chemosynthesis.
These bacteria have enzyme systems
which are capable of trapping energy
released during inorganic chemical re¬
actions. Certain of these bacteria are
able to add oxygen to hydrogen to form
water. Others change ammonia to ni¬
trites and nitrites to nitrates. Still oth¬
ers receive energy from reactions involv-

ORGANIC

INORGANIC

Starches

6-10 Photosynthesis may be likened to a chemical bridge between the inorganic
and the organic world.

92 UNIT 1 THE NATURE OF LIFE

ing iron compounds and sulfur com¬
pounds. Energy from these reactions is
used in synthesizing carbohydrates from
carbon dioxide. From carbohydrate
molecules, chemosynthetic bacteria can
form all of their fats, proteins, and nu¬
cleic acids. We consider bacteria lowly
organisms. But are these organisms
lowly? From the standpoint of cell

structure and specialization they are.
But from the standpoint of cell chem¬
istry, they are probably as well equipped
as any organisms in existence. We,
who rank among the most nutritionally
dependent of all forms of life, would
find life much simpler if we shared the
remarkable chemical capabilities of these
chemosynthetic bacteria.

IN CONCLUSION

Having studied cell nutrition we might conclude that the living world is pow¬
ered by sunshine. Organic compounds are the media in which solar energy
is trapped, stored, transported, and made available to living cells. Each day
the sun illuminates the vegetation of the earth, and new energy enters into the
processes that sustain life and build the structures of living organisms. There
is a wide gap between solar energy and energy release in the mitochondria of
every living cell. The green plant bridges this gap with its photosynthetic
abilities - a chemical capability that places it in a key position in the living

world.

Having explored the nutritional activities of living cells, we now shift our
study to further cell processes. Cells synthesize protein, they respire, they ex¬
crete wastes, and they secrete special substances. These are cell activities we
will explore in the next chapter.

BIOLOGICALLY SPEAKING

ADP

ATP

autotroph

carbon dioxide acceptor

chemosyn thesis
chlorophyll
dark reaction
energy carrier

heterotroph
hydrogen acceptor
light reaction
photosynthesis

QUESTIONS FOR REVIEW

1. Explain why living organisms require a continuous supply of new energy.

2. Explain the relationship of chlorophyll molecules to the grana of chloro

plasts.

3. List various forms of chlorophyll found in different plant cells. Which
form is necessary for photosynthesis?

4. Why is Chlorella an ideal organism to use in research in photosynthesis?

5. What is energized chlorophyll? What part does it play in photosynthesis?

6. Explain how we know that water molecules are split during the light re¬
action.

CHAPTER 6 CELL NUTRITION 93

7. Explain the importance of ADP and ATP in photosynthesis.

8. Explain the relationship of TPN and hydrogen in the dark reaction.

9. Outline the steps in the fixation of carbon dioxide in the dark reaction.

10. Describe the chemical reactions involved in the conversion of PGA to
PGAL.

11. List several organic compounds that may be formed from PGAL.

12. Explain why chloroplasts appear green.

13. In what respect is chemosyn thesis fundamentally different from photo¬
synthesis?

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the matter and energy relationships in an energy carrier such as
ATP.

2. Explain how RDP is involved in a cycle in the dark reaction of photo¬
synthesis.

3. Discuss the nutritional relationship between autotrophs and heterotrophs.

CHAPTER 7

CELL

METABOLISM

Cell metabolism. All the processes of a
cell that relate to the use of food are
included in metabolism. Certain of
these processes are building activities.
As you learned in the last chapter, the
building activity most significant to life
is photosynthesis, in which fuel mole¬
cules in the form of carbohydrates are
synthesized. Other building processes
include the synthesis of proteins. This
results in growth and repair of the cell
structures — its cytoplasm and mem¬
brane, cytoplasmic inclusions, and nu¬
clear structures. In all of these proc¬
esses, molecules are built up. Simple
molecules are used in organizing more
complex molecules. We refer to these
building activities, or the constructive
phase of metabolism, as anabolism.

Equally important is the destructive
phase of metabolism, known as catab¬
olism. In this phase large molecules
are broken down and chemical energy

is released, some as heat and some as
energy to support all of the chemical
activities of the cell. We may think of
this energy output in terms of muscle
activity as we move our bodies, or nerve
impulses as we think and reason and
receive impressions from our sense or¬
gans. However, all of this activity and
every other life-supporting process re¬
quires a constant energy supply from
fuel molecules split in the tiny “power¬
houses” of our cells.

Protein synthesis. While photosynthe¬
sis is limited to the cells of autotrophs,
this cannot be said of protein synthesis.
No living matter has ever been found
which does not contain protein. Every
cell organizes its own protein molecules.
Thus protein synthesis is a universal
phase of cell anabolism.

There is another striking difference
between protein synthesis and carbohy¬
drate synthesis. All cells capable of
photosynthesis organize the same
PGAL and glucose molecules in the
same series of chemical reactions. This
is not true of proteins. A cell builds
specific proteins which vary from spe¬
cies to species, individual to individual,
and, to some extent, in various kinds of
cells in the same organism. You may
have read accounts of attempts to trans¬
plant tissues or organs from one indi¬
vidual to another. These transplants
are almost always unsuccessful, except
between identical twins, because the
unique proteins of the recipient set up
a reaction to the donor’s quite different
proteins.

A cell thus expresses individuality
in protein synthesis. How is this syn¬
thesis regulated? What sort of cellular
code determines exactly what proteins
will be formed and how these molecules
are to be constructed? This is a fasci¬
nating story. However, before we ex-

CHAPTER 7 CELL METABOLISM 95

plore the “protein factories” of a cell,
we will take a closer look at cell pro¬
teins.

Functions and organization of proteins.

Proteins serve many functions in living
cells. Some are structural proteins.
These molecules remain in the cell and
form its various parts — its cytoplasm,
membranes, and the various cytoplasmic
inclusions. Enzymes are protein mol¬
ecules which are essential in all of the
cell’s chemical activities. We classify
these as intracellular when they remain
in the cell. Other enzymes are extra¬
cellular. These enzymes are secreted
from the cell and act as catalysts in
chemical reactions in other regions of
the organism. Digestive enzymes are
extracellular. Still other proteins are
hormones , which regulate specific activi¬
ties often far removed from the cells
that produce them. Other proteins
form pigments in plant cells, the hemo¬
globin in red corpuscles, and the pro¬
teins composing blood serum. These are
but a few of the protein substances syn¬
thesized in cells. We can summarize
the importance of proteins by stating
that they are indispensable for life.

Various proteins are composed of
different numbers and arrangements of
approximately 20 subunits, or amino
acids. The amino acids are linked to¬
gether in a linear chain. Other groups
of atoms are attached to the amino acid
chains to form molecules of amazing
complexity. The size and complexity
of protein molecules is indicated in their
formulas, in which the number of at¬
oms may range from a few thousand to
several million in the extremely complex
and largest of all molecules.

The way in which amino acids are
arranged to form a specific protein is
determined by a chemical code within
the cell. We must return to the DNA

molecule in order to understand the na¬
ture of this synthesis.

Cell protein factories and instructions
from “headquarters.” We know that
the ribosomes are the centers of protein
synthesis. It is in these “factories that
amino acids are joined in the organiza¬
tion of protein molecules. However,
the exact structure of these proteins is
determined by DNA molecules in the
nucleus. How, then, is a pattern, or
“blueprint,” delivered from the nucleus
to the ribosome factories?

Biologists know that a molecule of
DNA bears a genetic code formed by its
nitrogen-containing bases. It would be
well to review the arrangement of these
bases, which we described in Chapter 3,
before discussing their importance in
protein synthesis. Four kinds of bases
are present in the nucleotides of DNA

7-1 Electron micrograph of part of a cell
from the pancreas of a frog. The nuclear
membrane is marked NM. The numerous
ribosomes appear as dense granules in the
cytoplasm. (K. R. Porter)

96 UNIT 1 THE NATURE OF LIFE

molecules. These are adenine, thy¬
mine, guanine, and cytosine, designated
bv the letters A, T, G, and C. The
bases are paired in the double-strand
DNA molecule as A-T or T-A and G-C
or C-G. RNA molecules contain all
of these bases except thymine, for which
uracil (yur-a-sil), another base, is sub¬
stituted. Thus, the bases in RNA may
be designated as A, U, G, and C.

The bases in a strand of DNA may
occur in any linear sequence. However,
biologists have found that it is not sin¬
gle bases but groups of three, or triplets,
that function in the control of protein
formation. In your study of protein
synthesis you will discover how these
triplet bases determine exactly what
amino acids will be attached in chains
to form specific proteins called for by
DNA in the nucleus. This is a fasci¬
nating discovery of recent years.

Let us assume that three sets of the
base triplets in a strand of DNA are ar¬
ranged as AGC, ACC, ATG. You will
find these in Diagram 1 in Fig. 7-2. If
this is the left helix of a double strand
of DNA, its mate, the right helix (not
shown), has matching but not duplicate
bases TCG, TGG, TAC.

We know that DNA in the nucleus
controls the synthesis of proteins on the
surface of ribosomes in the cytoplasm.
Yet, the DNA does not leave the nucleus
in the process. Somehow, DNA must
deliver its message to the ribosomes.
How is this information relayed to the
ribosomes? This involves a form of
RNA known as messenger RNA. If you
study Fig. 7-2 closely, you will see how
the messenger RNA receives its code in
the nucleus and determines just what
protein will be formed on the surface of
a ribosome.

Diagram 1 in Fig. 7-2 shows a sin¬
gle helix of DNA in the process of form¬

ing a strand of messenger RNA in the
nucleus. Three sets of base triplets are
shown in the DNA: AGC, ACC, ATG.
Corresponding bases in the messenger
RNA are UCG, UGG, UAC. Notice
that U (uracil) is substituted for T (thy¬
mine) in the RNA. You might compare
the base structure of DNA and messen¬
ger RNA to a positive print of a photo¬
graph and the negative. One is the re¬
verse, or the complement, of the other.

As molecules of messenger RNA are
formed, they may be stored in the nu¬
cleus for a time. Soon, however, they
leave the nucleus and move to the ribo¬
somes to deliver their important instruc¬
tions for protein synthesis. When mol¬
ecules of messenger RNA reach the
ribosomes, they attach to the ribosomal
surfaces and form patterns, or templates.
These are shown in Diagram 2 in Fig.
7-2. Ribosomes contain a second kind
of RNA, known as ribosomal RNA.
The function of this RNA in protein
synthesis is not definitely known. How¬
ever, it is believed to come from the nu¬
cleus and to combine with proteins in
the structure of the ribosome. There is
evidence, also, that ribosomal RNA
may function in the attachment of mes¬
senger RNA to the ribosome and that it
may regulate the enzymes involved in
activities of the ribosome.

The next question concerns de¬
livery of amino acids to the template
formed by the messenger RNA on the
ribosomes. Remember that these can¬
not be any of the 20 amino acids that
may be present in the cell, but certain
ones and in a certain arrangement, as
determined by DNA through its messen¬
ger RNA. This assembly of amino acids
involves a third kind of RNA known as
transfer RNA. Strands of transfer RNA
are believed to have at one end a partic¬
ular sequence of three bases in a triplet

CHAPTER 7 CELL METABOLISM 97

7-2 Protein synthesis. 1. A single strand of DNA forms a strand of messenger
RNA having complimentary bases. 2. Messenger RNA is deposited on a ri¬
bosome, where it acts as a template. 3. Transfer RNA strands bring specific
amino acid molecules to the ribosome. 4. As the amino acids are laid down on
the template, a protein is synthesized.

of nucleotides. These base triplets
might be coded AGC, ACC, AUG, etc.
Depending on the bases present in the
three nucleotides, a single transfer RNA
will attach only one specific amino acid
in the cytoplasm. Furthermore, a spe¬
cific enzyme seems to be involved.

Now let us return to the strand of
messenger RNA which has formed a

template on the ribosome. Notice in
Diagram 3 in Fig. 7-2 that specific
transfer RNA strands are bringing one
amino acid each to the template. The
first base triplets, UCG, will call for a
transfer RNA coded AGC. This strand
of transfer RNA will be bearing a spe¬
cific amino acid known as arginine
(czhr-je-neen ) . The second base triplet,

98 UNIT 1 THE NATURE OF LIFE

UGG, will receive the transfer RNA
containing the bases ACC, to which the
amino acid glycine is attached. The
third triplet bases, UAC, will receive
transfer RNA with the base code AUG.
The amino acid histidine (his- ti-deen)
will be attached to this strand of trans¬
fer RNA. Now, let’s line up the bases
in the three strands of transfer RNA we
have used. AGC, ACC, AUG - where
have you seen this sequence before?
With the exception of uracil in the
third triplet, this was the base sequence
in the nucleotides of the DNA in the
nucleus. DNA duplicates its code in
strands of transfer RNA by means of
messenger RNA. Amino acids are
joined in strands such as arginine to
glycine to histidine (and many others),
according to the base coding of DNA.

The assembled amino acids along
the ribosome are now joined by chemi¬
cal bonds. Energy for this process is be¬
lieved to come from the ribosomes.
The protein molecules are released and
the transfer RNA is freed, as shown in
Diagram 4 in Fig. 7-2. In some cells
transfer RNA may attach additional
amino acids and repeat the process. In
others they seem to function only once.

What happens to the proteins as
they are released from the ribosomes?
Some may be structural proteins, but
most are believed to be enzymes.
These enzymes regulate all of the chem¬
ical activities of the cell.

Cellular respiration. So far we have dis¬
cussed only synthetic processes of the
cell in which complex molecules are
formed and molecular energy is built up.
We now shift our discussion to the
breakdown of molecules and the release
of chemical energy. All of the activities
of a living cell require a continuous sup¬
ply of energy. This energy must be re¬
leased within the cell during the process

known as respiration. It constitutes a
vital part of the destructive or catabolic
phase of metabolism.

You are familiar with the degrad¬
ing of organic fuel molecules and the
transformation of chemical energy to
heat, light, and mechanical energy.
This happens when you burn gasoline
in your automobile engine, heat your
house with fuel oil, gas, or coal, or burn
a log in your fireplace. The energy re¬
leased during combustion of these or¬
ganic fuels is solar energy, present in
molecules since they were formed orig¬
inally by photosynthesis. The energy
is released as the fuel molecules are de¬
graded by oxidation. As you know, this
oxidation requires a high temperature.
Furthermore, the oxidation that occurs
is uncontrolled in that almost all of the
molecules are broken down. While
there is some similarity between the oxi¬
dation of fuels during combustion and
the degrading of fuel molecules in a
cell, there are also many differences.
The cell mitochondria, in which much
of respiration occurs, could not with¬
stand the high temperatures of fuel
combustion. Furthermore, oxidation in
a cell is controlled, so that it occurs in
small steps, each of which releases small
quantities of energy. How can organic
fuel molecules be degraded at the nor¬
mal temperature of an organism? Re¬
spiratory enzymes accomplish this and
control the process as well.

When you think of respiration in
your own body, you probably think of
gaseous exchanges during breathing.
While we are most aware of breathing
and the intake and exhalation of gases,
remember that this phase of the process
is a gaseous exchange between the
blood and the atmosphere. The actual
seat of respiration is the body cells and,
more specificallv, the mitochondria of

CHAPTER 7 CELL METABOLISM 99

these cells. It is here that the cellular
“fires” burn constantly.

The fuel for respiration. Any organic
molecules present in a cell may be a fuel
for respiration. These include carbohy¬
drates in the form of glucose molecules,
fatty acids and glycerol, amino acids,
and even vitamins and enzymes.

In Chapter 3 we discussed the
chemical bonds that hold atoms to¬
gether in a molecule. These bonds are
electrical forces and are referred to as
chemical energy. As long as they re¬
main, the molecule contains stored
chemical energy. When the molecule
is decomposed to atoms or simpler
molecules, this energy is set free. Or¬
ganic fuels used in respiration contain
energy that was once solar energy that
has been locked in molecules since they
were organized from simpler molecules
during photosynthesis. Thus the en¬
ergy liberated in respiration is chemi¬
cal bond energy which was once light
energy.

The energy released in a cell during
respiration results from oxidation of
glucose. Oxidation involves either the
addition of oxygen or the removal of
hydrogen from a molecule, with an ac¬
companying release of energy. Most of
the oxidation in a cell is brought about
by the removal of hydrogen.

Three products result from the oxi¬
dation of glucose during respiration:
1. fuel fragments; 2. hydrogen; and
3. energy. Fuel fragments include the
smaller molecules resulting from the
breakdown of glucose. If respiration is
complete, the fuel fragment is carbon
dioxide. Hydrogen removed from glu¬
cose molecules combines with a hydro¬
gen acceptor. Energy released from glu¬
cose is trapped in ATP in much the
same way as it was in the case of photo¬
synthesis.

Various forms of respiration. Chemical
changes involved in various forms of
respiration are extremely complex and
need not be described in detail, since we
are concerned primarily with energy re¬
lease and end products. We will de¬
scribe three forms of respiration in sim¬
ple steps.

If free atmospheric oxygen is the
hydrogen acceptor, we refer to the proc¬
ess as aerobic respiration (a-ro/i-bik) .
We can summarize this process in the
following steps:

1. Molecules of glucose (C6H12Oe) are
split into two molecules of a 3-car¬
bon compound (C3H603) by en¬
zyme action.

2. Other respiratory enzymes break
down these molecules to pyruvic acid

7-3 The chemical changes involved in aero¬
bic respiration are summarized in this dia¬
gram of a mitochondrion.

100 UNIT 1 THE NATURE OF LIFE

(py-roo-vik) (C3H403) and hydro¬
gen (H2).

3. Atmospheric oxygen serves as the hy¬
drogen acceptor, with the result that
water is formed.

4. Energy is released.

5. Pyruvic acid is completely changed
to carbon dioxide and water.

In a second kind of respiration, cer¬
tain bacteria release oxygen from chemi¬
cal compounds such as nitrates and use
this oxygen, rather than atmospheric
oxygen, as the hydrogen acceptor in the
oxidation of glucose. We refer to this
process as anaerobic respiration (an-a-
roh-bik) .

A third kind of glucose oxidation is
known as fermentation , which is also
a form of anaerobic respiration. In fer¬
mentation glucose is split into molecules
of pyruvic acid. Hydrogen recombines
with pyruvic acid or with some other or¬
ganic molecule formed from pyruvic
acid. In certain bacteria and animals
lactic acid (lak- tik) is a product of fer¬
mentation. Plants, veasts, and certain

bacteria form alcohol during fermenta¬
tion. In both kinds of fermentation,
carbon dioxide is formed as a fuel frag¬
ment. Since both lactic acid and al¬
cohol are high energy organic com¬
pounds, little energy is released during
fermentation.

Energy transport in a cell. What hap¬
pens to the energy released in the mito¬
chondria during respiration? Some of
it is given off as heat. In a warm¬
blooded animal this heat maintains a
constant body temperature. In cold¬
blooded animals and plants, however,
heat energy from respiration is of little
value. The remaining energy must sup¬
port cell activities. The question, then,
is how this energy can be trapped and
transported from the mitochondria to
all the substances and structures of the
cell.

We discussed ADP (adenosine
diphosphate) and ATP (adenosine tri¬
phosphate) as energy carriers in photo¬
synthesis. These compounds are also
involved in respiration. Energy re-

Pyruvic acid

Glucose

Hydrogen

7-4 The chemical changes involved in fermentation are summarized in this
diagram of a yeast cell.

CHAPTER 7 CELL METABOLISM 101

leased from the oxidation of glucose is
used in adding a phosphate group to a
molecule of ADP. This results in ATP,
a higher energy compound. ATP may
move to any part of the cell and supply
energy as it is needed. The removal of
one phosphate group from ATP releases
energy and reforms ADP, which can
then receive more energy. We may
show these changes, both of which re¬
quire enzymes, in a simple equation:

ADP + phosphate + energy ATP

Think of these compounds as a
portable bank. ADP is the receiving
window, while ATP is the paying win¬
dow. Energy is the money. Energy re¬
leased in the mitochondria is deposited

in the bank at the receiving window
when a phosphate group is added to
ADP. The energy is now in the bank
as bond energy in the high energy com¬
pound, ATP. Securely locked in these
molecules, the energy is transported to
all parts of the cell. As energy is
needed in cell activities, ATP molecules
are changed to ADP with energy re¬
lease. Thus ATP is the energy vault,
the energy transport system, and the en-
ergy-yielding substance of respiration.

Respiration is a continuous process
in every cell. Each cell functions as a
tiny power plant in liberating the en¬
ergy required for its many activities.
Deprived of this energy, even for an in¬
stant, the cell dies.

IN CONCLUSION

Cell metabolism involves both constructive processes that use energy and de¬
structive processes that release it. Carbohydrate and protein synthesis are
anabolic processes, while respiration is catabolic.

DNA holds the key to protein synthesis. Every cell organizes its own
protein molecules according to instructions received from the DNA molecule
by way of messenger RNA. There are as many kinds of proteins as there are
forms of DNA.

In respiration an organic fuel is oxidized, usually by the removal of hydro¬
gen, with a resulting release of energy. Respiration may be aerobic, anaerobic,
or in the form of fermentation. The energy released in respiration is trans¬
ported in the cell by ATP.

Having discussed the various ways the cell maintains itself and grows, we
shall in the next chapter study the way the cell reproduces. Our knowledge
of this process has been very much enhanced by discoveries made possible with
the electron microscope.

BIOLOGICALLY SPEAKING

fermentation
messenger RNA
metabolism
oxidation

aerobic respiration
anabolism

anaerobic respiration
catabolism

protein synthesis
respiration
ribosomal RNA
transfer RNA

102 UNIT 1 THE NATURE OF LIFE

QUESTIONS FOR REVIEW

1. Define metabolism by naming and describing its constructive and destruc¬
tive phases. . .

2. Give two differences between carbohydrate and protein synthesis.

3 List several kinds of protein materials formed in cells.

4. What is the structural difference between DNA and RNA?

5. Describe the function of messenger RNA in protein synthesis.

6. Describe the function of transfer RNA in protein synthesis.

7. What is the relationship between the base sequence in DNA and that
in transfer RNA? What is the significance of this relationship?

8. Define the process by which energy is released in cell respiration.

9. What three products result from cell respiration?

10. Describe briefly the process by which energy is transported in the cell.

APPLYING PRINCIPLES AND CONCEPTS

1 Discuss the chemical mechanism by which DNA controls protein synthesis.

2. Explain why proteins are specific in living organisms.

3. There are four kinds of bases in the DNA molecule and there are about
20 amino acids. Supposing the amino acid code consisted of two bases in¬
stead of three, how many amino acids could be coded?

4. Why is an energy transport system vital to a cell?

CHAPTER 8

CELL GROWTH
AND

REPRODUCTION

Growth of cells. One of the results of
protein synthesis in a cell is replacement
of worn-out structures. The rate of syn-

thesis, however, normally exceeds the
requirements of materials for repair and
replacement. Accumulation of these
additional materials results in growth of
the cell.

Is there a limit to the size a cell
may reach? Writers of science fiction
have constructed weird tales of cells
that did not stop growing. Giant blobs
of protoplasm move into cities and flow
down streets, engulfing terrorized peo¬
ple who cannot escape. While stories
such as these make fascinating reading,
you know that they could never hap¬
pen. Do you know why?

As a cell adds to its substance, one
of two things must happen. Either its
rate of synthesis must reduce or it must

divide its mass. Otherwise its mass
would lead to its own destruction.

Why cells divide. All of the materials
necessary to support the life-sustaining
processes of a cell must enter through
its enveloping membrane. Further¬
more, all of the waste products result¬
ing from chemical reactions within the
cell must pass through this membrane.
Thus, there is a critical relationship be¬
tween the volume of cell content and
the surface exposure of the membrane.
As a cell grows, its protoplasmic volume
increases, but its membrane surface
does not increase proportionally. Sup¬
plying the protoplasmic content be¬
comes an increasing problem.

What is the logical solution? Di¬
vision of the cell will reduce the amount
of cell substance and add a new mem¬
brane surface between two masses. The
cell is now two smaller cells, each re¬
juvenated by division of the original
mass.

The nature of cell division. Cell growth,
or increase in size, is normally followed
by cell division, or increase in number.
Since cell division involves the splitting
of a cell, biologists refer to the process
as fission (fish-in). Most cells divide
into two approximately equal parts, or
undergo binary fission.

We designate a cell that has under¬
gone growth and is ready to divide as a
mother cell. The division of a mother
cell results in two approximately equal
daughter cells. Fission generally in¬
volves two distinct phases. One is the
duplication of nuclear materials, as a
result of which each daughter cell re¬
ceives nuclear materials that are identi¬
cal to those of the mother cell. This
duplication of nuclear materials main¬
tains the characteristics of a cell in all
of its descendants. The first cell of your
body contained a specific DNA compo-

103

104 UNIT 1 THE NATURE OF LIFE

8-1 This photomicrograph
shows the chromosomes in
the nucleus of a cell from
the salivary gland of a fruit
fly. (Bausch and Lomb,
Inc.)

sition that was different from that of
any other human being. Now your body
is composed of several billion cells, all
with identical DNA composition be¬
cause of nuclear duplication each time
a cell divides. We refer to the events in
which nuclear materials are duplicated
as mitosis (my-toh-sis) .

The second phase of cell fission is
division, or cleavage , of the cytoplasm
into two approximately equal parts.
This phase of the process involves a me¬
chanical separation of cytoplasmic struc¬
tures of the mother cell by a membrane
or a wall. Obviously, mitotic division
of the nucleus must precede cleavage of
the cytoplasm.

Mitotic division. Before we discuss the
sequence of events occurring in mitosis,
it would be well to review certain of the
nuclear structures discussed in Chapter
4. The key components of the nucleus
are molecules of DNA. These complex
molecules bear the genetic code of the
cell as determined by the nature and se¬
quence of their bases (purines and py¬
rimidines) joined in the double helix of
the molecule. These groups of bases in
a strand of DNA which express genetic
traits are known as genes. DNA strands
are spiraled around protein molecules
to form chromosomes.

Generally, mitosis includes the self¬
duplication of DNA and other chro¬
mosomal materials and the equal dis¬
tribution of these materials to two
daughter nuclei. We frequently divide
the sequence of events during mitosis
into stages or phases, as follows: 1. in¬
terphase; 2. prophase; 3. metaphase;
4. anaphase; 5. telophase. These stages
are not identified by abrupt changes.
However, certain significant events oc¬
cur which will make it possible for you
to distinguish each stage.

Interphase. Following the formation of
a daughter cell and prior to a subsequent
division, there is a period of growth and
enlargement. Until recently biologists
thought the nucleus was inactive during
this period, referred to as the interphase.
This was a logical conclusion since
changes occurring in the nucleus at this
time are not microscopically visible.
Today, however, we know that the nu¬
cleus, in addition to directing the all-
important protein synthesis, is preparing
for a coming division during at least the
latter part of the interphase. DNA mol¬
ecules are doubling their structure and
reproducing genes and chromosomes. It
appears that the double strands of nu¬
cleotides forming the DNA molecules
separate, as you might split a ladder by

CHAPTER 8 CELL GROWTH AND REPRODUCTION 105

cutting through the middle of its rungs.
Each side piece and attached purines
and pyrimidines reconstructs its missing
portions. The result is identical mole¬
cules joined lengthwise. We refer to
this self-duplication of DNA as replica¬
tion. Replication of DNA results in
doubling of the genes and chromosomes.
Thus, by the close of interphase, the nu¬
cleus contains twin DNA, genes, and
chromosomes. All of this has occurred
without microscopic evidence of change.
Prophase. Early stages of the prophase
mark the first visible signs that a mitotic
division is beginning. In animal cells
perhaps the earliest event is the division
of the centriole , a cytoplasmic body
lying just outside the nucleus. The two
granular bodies resulting from this di¬
vision move apart as though each re¬
pelled the other. They finally occupy

positions on opposite sides of the nu¬
cleus. The cytoplasm around each cen¬
triole changes in form from sol to gel
and forms fibrils which radiate from the
centriole like rays from a star. These
cytoplasmic fibrils are appropriately
named asters. Additional fibrils form
between the centrioles, bowing in the
center to form a delicate structure of
threads known as the spindle. In plant
cells neither centrioles nor asters form.
Instead a spindle of protein fibers ex¬
tends from one end of the cell to the
other.

During prophase the chromosomes,
which were invisible in interphase, short¬
en and thicken and become clearly visi¬
ble with the light microscope. Close ex¬
amination will reveal that they are dis¬
tinctly double. We now term each part
of a double chromosome a chromatid.

Dissolving nuclear membrane
Chromatid

Chromosome

Centromere

Metaphase

Centri

Aster

Early prophase

Spindle

Late prophase

Anaphase

Cleavage furrow

Nuclear membrane reforming

Early telophase

Late telophase

8-2 The five phases of mitosis as they occur in animal cells. In what ways does
mitosis in plant cells differ from this?

106 UNIT 1 THE NATURE OF LIFE

A pair of chromatids is attached along
its length by a single granule, known as
a centromere.

Other changes accompany chromo¬
some shortening during mitosis. The
nuclear membrane dissolves, allowing
the nuclear substances to mix freely
with the cytoplasm. The nucleoli dis¬
integrate and disappear into the cyto¬
plasm. By the end of prophase, only
the joined chromatids and spindle fibers
and, in animal cells, the centrioles and
asters, are visible in the region of the
nucleus.

Metaphase. In describing the cell spin¬
dle we often refer to each end as a pole
and the point midway between the poles
as the equator. You might think of the
equatorial plate as a plane extending
across the spindle fibers at right angles
to the axis of the spindle. During meta¬
phase each centromere moves toward
the equatorial plate with its paired chro¬
matids trailing behind it. Each centro¬
mere attaches to a thread of the spindle.
Late in metaphase each centromere di¬
vides and the chromatids are separated
on the spindle fiber. We now refer to
them as chromosomes.

Anaphase. Immediately after division
of the centromere, the freed chromo¬
some pairs seem to repel each other.
Each moves toward the pole opposite

8-3 Mitosis in an onion root. (Walter Dawn)

the other. Chromosome migration
from equator to poles constitutes the
anaphase stage of mitosis. Each chro¬
mosome moves along a spindle fiber
and is apparently pulled toward the
pole. Just how and why this movement
occurs has not been explained. It is pos¬
sible that the pull results from shorten¬
ing of the spindle fiber by the removal
of protein molecules. During move¬
ment toward the poles, the centromeres
again lead with the chromosomes trail¬
ing behind. The anaphase ends with
the arrival of the chromosomes at the
poles and the formation of clusters.

8-4 Locate as many different
stages as you can of mitosis
as it occurs in an animal cell.
This is a photomicrograph of
stained cells from a white-
fish embryo. (General Bio¬
logical Supply House, Inc.)

CHAPTER 8 CELL GROWTH AND REPRODUCTION 107

Telophase. The final stage of mitosis is
marked by the reorganization of daugh¬
ter nuclei and the division of the cyto¬
plasm to form two daughter cells. Soon
after reaching their respective poles, the
chromosomes lengthen and gradually
disappear, leaving only a granular sub¬
stance in the nucleus. The spindle fi¬
bers and asters of animal cells disappear
as their substance reverts from gel to
sol. New nucleoli are organized and a
new membrane forms around each nu¬
cleus. Reorganization of the daughter
nuclei is accompanied by division of the
cell into two cells of approximately
equal size. This part of telophase dif¬
fers distinctly in plant and animal cells.

In plant cells cellulose molecules
form a wall across the cell in the region
of the equator. This primary wall, or
division plate , forms a common bound¬
ary between the daughter cells. In ani¬
mal cells division usually begins with
the appearance of an indentation, or
cleavage furrow , in the region of the
equator. The cleavage furrow deepens
and finally constricts the cell into two
parts. We mark the end of telophase
and the close of mitosis with the di¬
vision of the cell and the reorganization
of daughter nuclei. Both cells then en¬
ter a new interphase stage.

As a result of cytoplasmic cleavage,
the original mother cell has divided its
cytoplasmic structures, such as ribo¬
somes, endoplasmic reticulum, and
RNA molecules. Before another divi¬
sion begins, these and other essential
cell structures will be increased in num¬
ber, as directed by DNA in the nucleus.
Significance of mitosis. As a result of
mitosis, the daughter cells contain the
same gene and chromosome structure as
the mother cell that produced them.
Furthermore, the entire cytoplasmic
content of the mother cell is divided

nearly equally in the process. Thus the
daughter cells are both structurally and
functionally like the mother cell.

In a multicellular organism cell di¬
vision accomplishes cell replacement in
regions where tissues are damaged. It
also results in increase in the number of
cells and growth of the organism.

The rate of cell division varies in
different plant and animal tissues.
Generally, it is most rapid in the least
specialized tissues. For example, nerve
tissue divides very slowly, if at all. Em¬
bryonic tissue, on the other hand, is in
a continuous state of cell division. For
this reason, these tissues are frequently
used for the preparation of microscope
slides showing cells in various stages of
mitosis.

Cell division is stimulated by injury
to a tissue. For example, cells in your
skin undergo division slowly under nor¬
mal conditions. A wound stimulates
very rapid cell division and regeneration
of the damaged tissues.

Cell division is also stimulated by
separation of cells. Do you remember
the chicken heart muscle we described
in Chapter 2? This tissue lived more
than 30 years and remained active as
long as its mass was divided at regular
intervals. The same principle applies
to tissue cultures. When animal tissue,
such as that composing a kidney, is
ground or placed in a nutrient solution,
the cells undergo rapid division. This
would not have occurred in the intact
kidney. It would seem that the rate of
division is held in check by the presence
of other cells in a tissue.

Asexual reproduction. In one-celled or¬
ganisms cell division constitutes repro¬
duction of the entire organism. This
accounts for the rapid increase in the
number of protozoans, algae, and bac¬
teria in environments offering ideal con-

108 UNIT 1 THE NATURE OF LIFE

8-5 Vegetative reproduction in the potato.
(USDA)

ditions for growth. Protozoans may di¬
vide as often as twice a day. But this
rate is far exceeded by bacteria, which
may divide as often as every 20 or 30
minutes!

Budding is also a form of asexual
reproduction. In this process, nuclear
division follows the appearance of a
knob on the side of a cell. This pro¬
jection, containing its own nuclear ma¬
terial, enlarges and becomes an inde¬
pendent cell, finally separating from the
mother cell that produced it. Although
we often associate budding with yeast
cells, the process occurs in many other
organisms, including molds. Even
some of the lower animals such as the

sponges reproduce by budding. A mul¬
ticellular sponge develops from a bulge
on the side of a parent organism. After
a period of growth and enlargement, the
sponge bud separates from the parent
and becomes independent.

Vegetative reproduction is another
method of asexual reproduction in high¬
er plants. Did you ever plant potatoes?
You merely cut a tuber into pieces, be¬
ing careful to include one or two eyes
in each piece. When these pieces are
planted in soil, each eye will produce an
entire plant. The same process occurs
when you start a rose bush from a stem
cutting rooted in sand or grow a pussy¬
willow tree by putting a cut stem in a
jar of water.

Spore production is one of the
most widespread methods of asexual re¬
production. Spores are reproductive
cells formed by divisions of special spore
mother cells. Spores differ from cells
composing the body of an organism in
that they are discharged from the parent
organism. The spores of some or¬
ganisms are carried by the wind. If
they are produced by aquatic organisms,
they usually have whiplike structures
with which they swim from the parent
to a new location. Spores are usually
protected by a resistant wall and may
endure severe environmental conditions
such as drying or freezing. Under suit¬
able conditions a spore may germinate
and form an active cell directly. That
is, a spore does not fuse with another
spore in the production of a new organ¬
ism. While we usually associate spores
with plants and plantlike organisms,
you will discover in your study of micro¬
biology certain animal-like cells that
produce spores.

All forms of asexual reproduction
involve the direct production of a new
organism, either from a single cell or

CHAPTER 8 CELL GROWTH AND REPRODUCTION 109

8-6 Spore production in bread mold. (Hugh Spencer)

from a group of cells. Thus it is a
highly efficient process. Organisms ca¬
pable of asexual reproduction usually
have the capability of increasing their
numbers rapidly if conditions for
growth are favorable.

Sexual reproduction. Sexual reproduc¬
tion involves the fusion of two special
cells. We refer to these sex cells as
gametes (gah -meets). The fusion of
gametes is termed fertilization , or syn-
gamy (sinj-g a-me). We refer to the
fusion body formed by the union of two
gametes as a zygote (zy-goht).

In some organisms all gametes
formed are alike. These are called iso¬
gametes. We cannot designate them as
male or female gametes because of their
structural similaritv. We know that

there are functional differences, how¬
ever, because only certain isogametes at¬
tract each other and fuse during fertili¬

zation. Heterogametes , however, have
structural differences that permit us to
designate them as male or female sex
cells. We refer to a male gamete as a
sperm and a female gamete as an egg,
or ovum.

In your study of sexual reproduc¬
tion in various organisms, you will find
interesting differences in gamete pro¬
duction. You will find isogametes being
produced by different individuals of a
species, indicating maleness and female¬
ness even though the gametes are iden¬
tical in form. You will also find both
male and female heterogametes being
produced by a single individual. More
frequently, however, heterogametes are
formed by different individuals which
we designate as male and female par¬
ents.

Cell reproduction and chromosome
numbers. Each time a cell nucleus un-

110 UNIT 1 THE NATURE OF LIFE

dergoes a mitotic division, pairs of chro¬
mosomes, each composed of joined chro¬
matids, appear during prophase and
move to the equatorial plate during
metaphase. We refer to the chromo¬
somes of these pairs as homologous
(hoh-mdhZ-a-gus ) because they are
identical in form, gene composition, and
linear arrangement of genes. A homo-
logue is a single chromosome of a homol¬
ogous pair. Is the number of homolo¬
gous chromosome pairs constant in all
cells of an individual organism? Not
only is it constant for all cells of the or¬
ganism, but it is constant for all normal
individuals of a particular species . All
human body cells contain 23 pairs of
chromosomes, or 46 in all. This num¬
ber is constant in all human beings the
world over. Furthermore, the distribu¬
tion and arrangement of genes on these
chromosomes is always the same. When
a cell contains a full set of homologous
pairs of chromosomes, we say that it has
the diploid , or 2n, chromosome number.
Mitotic division maintains this chromo¬
some number in all cells, since genes
replicate and chromosomes split and
form chromatids which are equally dis¬
tributed in the process. Your life began
as a single diploid cell containing homol¬
ogous gene and chromosome pairs.
Perhaps you have already figured out
that one full set of genes and chromo¬
somes came from one parent and one
set from the other.

Chromosome number in gametes. If all
human body cells contain 23 pairs of
chromosomes, or 46 in all, would this
same number be found in an egg or a
sperm? If this were true, a fertilized
egg would contain 92 chromosomes and
all cells resulting from it by mitotic di¬
vision would contain this abnormal
chromosome number. This, of course,
could not occur. At some time in the

formation of eggs and sperm, the diploid
chromosome number must be reduced
to half. Wouldn’t the logical biological
answer be separation of the chromosome
pairs? This does occur in egg and sperm
formation. Each gamete contains but
one chromosome of a homologous pair.
We refer to this chromosome content as
the haploid, or n, number. It is impor¬
tant that you understand that the hap¬
loid is not merely half of the diploid
chromosome number. All human eggs
and sperm contain only 23 chromo¬
somes, but not just any 23. These gam¬
etes contain one full set of homologues
— one of every pair of chromosomes.
You might think of an egg, then, as
“half a person” even though it contains
one of every kind of gene necessary to
produce the new individual.

This raises an interesting biological
question. If an unfertilized egg were to
develop without fertilization by a sperm,
would not a haploid organism develop?
Did you know that this actually occurs
in certain organisms? We refer to it as
parthenogenesis. One of the best ex¬
amples is the male, or drone, bee.
Drones are produced from unfertilized
eggs laid by the queen at certain times
of the year. In nearly all organisms,
however, an egg does not develop until
fertilization has occurred. In this proc¬
ess each gamete contributes one full set
of chromosomes. This results in a zy¬
gote with the diploid chromosome
number.

Cells of the ovaries, which produce
eggs, and those of the testes, from
which sperm are formed, have the dip¬
loid chromosome number, like all other
body cells. The question, then, is how
the chromosome content is reduced to
the haploid number in the formation
of gametes. The science of genetics de¬
pends on this.

CHAPTER 8 CELL GROWTH AND REPRODUCTION 111

Meiosis, or reduction division. The
series of cell divisions involved in the
formation of eggs and sperm is called
meiosis (my-oh-sis), or reduction divi¬
sion. The process is shown in Fig. 8-7.
In order to simplify the diagrams, we
have shown only three pairs of homol¬

ogous chromosomes. As we describe
the formation of an egg, study the right
side of the diagram closely and note the
chromosome changes which occur.

The first sign of production of an
egg is enlargement of an oogonial cell
(oh-uh-go/i-ne-al) in the tissue of the

112 UNIT 1 THE NATURE OF LIFE

ovary. This cell, like all body cells, con¬
tains the diploid chromosome number.
This cell will mature into the primary
oocyte (ofi-uh-syt).

Early in the prophase, as in mitosis,
chromosomes appear and migrate to the
equatorial plate. You will recall that
this chromosome movement occurred in
mitosis and that the chromosomes ar¬
ranged at random along the equatorial
plate. In meiosis, however, the homo-
logues of each chromosome pair join
throughout their lengths at the equa¬
torial plate. We refer to this pairing of
homologous chromosomes as synapsis.

While the chromosome pairs are in
synapsis, they shorten and thicken and
coil about each other. Meanwhile, rep¬
lication is occurring and chromatids
appear, each “twin” joined by a centro¬
mere. This results in a tetrad, or group
of four chromatids. We may now con¬
sider the chromosome number to be
tetraploid (4n), since each chromatid
contains a full set of genes. The coil¬
ing of chromosomes and chromatids
about each other at this stage in meiosis
is very important in genetics, as you will
discover later. The enlarged primary
oocyte, meanwhile, has formed a spin¬
dle to which the centromeres of the
tetrad have attached. Paired chroma¬
tids now separate from the tetrad and
move toward opposite poles. This is
the anaphase stage. Note, however,
that paired chromatids move together,
whereas in the anaphase of mitosis they
separated.

Following migration of the paired
chromatids, the primary oocyte divides,
forming a secondary oocyte and the first
polar body. Notice that these cells con¬
tain but one of the original pair of
homologous chromosomes, each now
composed of joined chromatids. This
is the real reduction division.

The secondary oocyte soon divides
again, forming an ootid (oh-uh-tid) and
a second polar body. In this division
the chromatids separate. Both the ootid
and the second polar body now contain
the haploid (n) chromosome number.
Meanwhile, a similar division is occur¬
ring in the first polar body. Here the
joined chromatids separate and two ad¬
ditional second polar bodies, each with
haploid (n) chromosome number, are
formed.

The ootid matures into an egg.
The three polar bodies have no function
in reproduction and gradually disappear.

If you compare sperm production
with egg formation, you will notice that
the processes are similar. In the case
of sperm production, however, the pri¬
mary spermatocyte divides and forms
two functioning secondary spermato¬
cytes. These in turn form four sperma¬
tids, each of which matures into a func¬
tioning sperm.

Fertilization, and the restoration of the
diploid chromosome number. Notice
the chromosome content of the egg and
the sperm in Fig. 8-7. All contain the
haploid (n) chromosome number, hav¬
ing one homologue of each chromo¬
some pair. Any of the sperm may fer¬
tilize the egg and produce the zygote.
When this occurs, both cells contribute
a homologue of a chromosome pair and
the diploid (2n) number is re-estab¬
lished. Just how chromosomes are
paired in this chance union depends on
the specific chromosomal make-up of
the egg and sperm involved in fertiliza¬
tion. It is the chance distribution of
chromosomes during meiosis and the
chance combination of chromosomes in
fertilization that provides the basis for
genetic variations in offspring. We will
discuss many of these possibilities in the
study of genetics in Unit 2.

CHAPTER 8 CELL GROWTH AND REPRODUCTION 113

IN CONCLUSION

The synthesis of protoplasmic substances is a never-ending process in living
organisms. It began when life originated and has continued from that time
to this. Each new generation receives substances from the previous generation.

Synthesis and cell reproduction are inseparable. As cells divide larger
masses become smaller masses, each capable of further growth. As organisms
grow and reach maturity, cells or groups of cells separate from the parents and
establish new organisms. Certain reproductive processes preserve life without
genetic change. This applies to mitotic division of a mother cell to form iden¬
tical daughter cells, to a spore, which duplicates its parent in a new organism,
and to a bud in which body cells increase a population. But organisms, in addi¬
tion to increasing their cell numbers and perpetuating themselves in identical
offspring, are subject to change and, by chance, to improvement. This involves
new combinations of genes, made possible by sexual reproduction.

How does gametic union provide this possibility of change? Can we pre¬
dict the characteristics of offspring by examining parental traits? Can a trait
which neither parent possessed appear in an offspring? Questions like these
will be answered in our study of genetics in the unit to follow.

BIOLOGICALLY SPEAKING

anaphase

equatorial plate

parthenogenesis

asexual reproduction

fertilization

prophase

aster

gamete

reduction division

binary fission

haploid

replication

budding

heterogamete

sexual reproduction

centriole

homologous

sperm

centromere

homologue

spindle

chromatid

interphase

spore

cleavage

isogamete

synapsis

cleavage furrow

meiosis

telophase

daughter cell

metaphase

tetraploid

diploid

mitosis

vegetative reproduction

division plate

mother cell

zygote

QUESTIONS FOR REVIEW

1. What is binary fission?

2. What two phases constitute mitotic cell division?

3. Explain the relationship between genes and chromosomes.

4. Describe the events that occur during interphase.

5. Locate the centrioles of a dividing animal cell.

6. What are asters?

7. Of what importance is the spindle during mitosis?

8. Distinguish between a chromosome and a chromatid.

9. Summarize the events that occur during prophase.

10. Explain how joined chromatids are attached to the spindle fibers.

114 UNIT 1 THE NATURE OF LIFE

11. Summarize the events occurring during anaphase.

12. Explain the changes that occur in chromosomes during telophase.

13. How does division of the cytoplasm differ in plant and animal cells?

14. Describe several forms of asexual reproduction.

15. Distinguish between a gamete and a spore.

16. How are isogametes different from heterogametes?

17. In what ways are homologous chromosomes alike?

18. Distinguish between the diploid and haploid chromosome number.

19. What is the basic difference between mitosis and meiosis?

20. Explain how the tetraploid number occurs briefly during meiosis.

21. Explain why a single oogonial cell gives rise, indirectly, to a single egg,
while four sperm develop from a single spermatogonial cell.

22. When is the diploid chromosome number established in a zygote?

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the necessitv of cell division in perpetuating the life of a cell.

2. Discuss the relationship of cell division to growth and to reproduction?

3. Discuss the significance of mitosis as a reproductive process.

4. How are characteristics of an organism preserved in asexual reproductive
processes?

5. Is sexual reproduction a basis for variation or change in organisms?

RELATED READING

Books

Asimov, Isaac. The Genetic Code.

Orion Press, New York. 1963
Asimov, Isaac. A Short History of Bi¬
ology. Doubleday and Co., Garden
City, New York. 1964
Butler, j. A. V. Inside the Living Cell:
Some Secrets of Life. Basic Books,
Inc., New York. 1959
Cohn, Norman S. Elements of Cytol¬
ogy. Harcourt, Brace, and World,
Cleveland. 1964

Fox, Russell. The Science of Science.

Walker and Co., New York. 1964
Hoffman, Katherine B. and Lacey,
Archie L. Chemistry of Life.
Scholastic Book Service, Dayton,
Ohio. 1964

Hutchins, Carleen M. Life's Key —
DNA: A Biological Adventure into
the Unknown. Coward-McCann,
Inc., New York. 1961

Morowitz, Harold J. Life and the Phys¬
ical Sciences. Holt, Rinehart and
Winston, New York. 1963
Rowland, John. The Polio Man: The
Story of Jonas Salk. Roy Publish¬
ers, New York. 1960
Swanson, Carl P. The Cell. Prentice-
Hall, Inc., Englewood Cliffs, N. J.
1964

Articles

Brown, Harrison. “The Age of the So¬
lar System.” Scientific American.
April, 1957

Pfeiffer, John E. “Enzymes.” Scien¬
tific American. December, 1948
Scientific American Magazine. Cells.

Vol. 205, September, 1961
Wald, George. “Innovation in Biol¬
ogy.” Scientific American. Sep¬
tember, 1958

Wald, George. “The Origin of Life.”
Scientific American. Aug. 1954

UNIT TWO

THE

CONTINUITY
OF LIFE

When sexual reproduction occurs in an organism, both parents transmit chemical
instructions which control the development of the offspring. Each new organism
resembles its parents, yet differs in some respects and so provides the basis for the
mechanism of variation. What chemical controls exert this genetic influence?
The answer lies in a search of cells, their nuclei and their chromosomes and finally,
the nucleoproteins which compose them. Here we find the substance DNA and
a genetic code expressed as genes which control the development of every inherited
trait in an organism.

CHAPTER 9

PRINCIPLES
OF HEREDITY

Heredity and environment. Since the
day you were born, two kinds of influ¬
ences have been interacting to determine
your individual makeup. The first of
these is heredity. Heredity is the trans¬
mission of characteristics from parents
to offspring. These characteristics in¬
clude the color of your hair and eyes,
body build, facial features, and many
others. The development of these traits
is controlled by a chemical code trans¬
mitted to you through the reproductive
cells of your parents. This code is con¬
tained in the genes of which chromo¬
somes are composed. The branch of
biology that is concerned with the mech¬
anisms and substance of heredity is
therefore appropriately called genetics.

The second factor involved in your
development is environment. This in¬
cludes all the external forces that in¬
fluence the expression of your heredity.
It is difficult to determine where heredi¬
tary influences end and environmental

ones begin. For example, body size is
controlled by heredity. But it is also
determined partially bv diet and by the
type of activity in which you participate.
Similarly, the tanning of your skin is the
result of the interaction of sunlight and
an inherited ability to produce addi¬
tional pigment. If you lack this pig¬
ment, you will sunburn rather than tan,
but you will do neither if you are not
exposed to the sun. Your hereditv thus
determines what you may become, but
what kind of individual you do become
depends on the interaction of your
heredity and environment.

What kinds of characteristics are in¬
herited? In certain respects all mem¬
bers of a species are alike. For example,
man normally inherits the characteris¬
tics of the human race that make him
like other human beings. These species
characteristics include the abilitv to walk

erect, hands with fingers for grasping,
and a highly developed nervous system
with a brain superior to that of all other
organisms.

In addition to species characteristics
you have inherited certain individual
characteristics that make you different
from all other people. Many of these
characteristics are passed on from parent
to offspring. The result is that you may
resemble your parents to a certain de¬
gree, but be quite different from each
because you have inherited characteris¬
tics from both.

Mendel’s work with garden peas. In
1866, Gregor Mendel, an Austrian
monk, published the results of a master¬
ful piece of work on the laws of heredity.
He was not the first to experiment in
the field of inheritance, but his findings
were the first of anv scientific conse-

quence. His paper, representing years
of work with garden peas, was published
by the Natural History Society of Briinn,

116

CHAPTER 9 PRINCIPLES OF HEREDITY 117

9-1 Gregor Mendel in his garden experi¬
menting with garden peas. (Bettmann Ar¬
chive)

Austria. Mendel had been dead for 16
years when three other scientists dis¬
covered his work and began to make use
of his findings. It is, however, a great
tribute to Mendel that the laws he for¬
mulated from his experiments with gar¬
den peas stand today, practically un¬
changed, as the basis of the science of
genetics. It is also very remarkable that
his principles of inheritance were de¬
veloped without a knowledge of chro¬
mosomes and their behavior.

During his years as a teacher in a
high school in Briinn, Mendel kept a
small garden plot at the monastery
where he lived. He used several kinds
of plants for his experiments, but the
work for which he is remembered are the
experiments he conducted with the gar¬
den peas. Why did Mendel choose
garden peas for his experiment? First,
he had observed that they differed in
certain definite characteristics. Some

plants were short and bushy, while
others were tall and climbing. Some
produced yellow seeds, some green seeds;
some had colored seed coats and some
white. Mendel identified seven different
pairs of traits in which the plants dif¬
fered consistently.

In order to understand the second
reason why Mendel found garden peas
ideal for his experiments, you will need
a brief introduction to reproduction in
the seed plants. You are probably fa¬
miliar with the fact that the flower is
the reproductive structure in seed plants.
The flowers of pea plants bear male
structures called stamens, which pro¬
duce pollen grains which form sperm,
and female structures called pistils,
which contain egg cells at their base.
The transferring of pollen from stamens
to pistil, before fertilization occurs, is
called pollination. Pea plants normally
carry on self-pollination, which means
that pollen is transferred from stamens
to pistil on the same flower or another
flower of the same plant. Cross-pollina¬
tion involves flowers on two different
plants.

Mendel found that cross-pollina¬
tion could be performed easily in pea
plants. He could accomplish this by re¬
moving the stamens from a flower so
that self-pollination and self-fertilization
could not occur. He then transferred
flower pollen from one plant to the pistil
of a flower of another plant. This flower
was carefully protected from any pollen
grains which might be transferred to it
by wind or insects. Because of the pea’s
seven different characteristics, and be¬
cause cross-pollination was easy to per¬
form, Mendel had selected an ideal sub¬
ject for breeding experiments.

Mendel discovers the principle of dom¬
inance. Mendel allowed several gener¬
ations of peas to self-pollinate. He

118 UNIT 2 THE CONTINUITY OF LIFE

9-2 The garden pea: flower, young fruit, mature fruit with seeds.

found that the seven characteristics he
had identified were always handed down
from parent to offspring. Seeds from
tall plants produced other tall plants,
and yellow seeds produced plants with
yellow seeds. Mendel’s next step was to
see what would happen if he crossed
two plants with contrasting traits. Ac¬
cordingly he selected one tall parent and
one short one. He made hundreds of
crosses by transferring the pollen from
the tall plants to the pistils of the short
ones. When the seeds matured on the
short plants, he sowed them to find out
the results of his cross. Would the off¬
spring be short like one parent, tall like
the other, or of medium height with
characteristics of both? He discovered
that all the plants were tall, like the
plant from which he had taken the
pollen in making the cross.

His next step was to determine if it
made any difference which plant he used
for pollen and which he used to produce
the seeds. Accordingly he reversed the
process of pollination, using a short
plant for pollen and a tall one for seed
production. Mendel found that the re¬
sults were as before — all the offspring
were tall.

Mendel then experimented with
other characteristics. He limited his
study of each cross to a single character¬
istic involving only one trait at a time.
For example, he crossed plants con¬
trasted in just one trait, such as yellow
seeds and green seeds. He found that
all of the first generation of the cross
had yellow seeds. Similarly, he discov¬
ered that round-seeded varieties crossed
with plants with wrinkled seeds pro¬
duced a generation with round seeds.
He repeated these crosses until he had
tested the seven different characteristics.
Mendel was surprised to find that in all
seven crosses, one of the characteristics
present in the parent plant seemed to be
lost in the next generation. What
would happen if he permitted these off¬
spring to self-pollinate? This step in his
experiment was destined to make his¬
tory, since it led to the discovery of two
important laws of heredity.

Mendel’s conclusions relating to in¬
heritance of traits were based on data
accumulated from the study of a large
number of offspring. He kept accurate
record of all the crosses he made. In re¬
cording his generations of crosses, Men¬
del designated the parent plants used

CHAPTER 9 PRINCIPLES OF HEREDITY 119

in the first cross as P. He referred to
the generation resulting from this cross
as the first filial or F1 generation. By
allowing the tall plants to self-pollinate,
he produced a second filial or F2 genera¬
tion. The results of this self-pollination
were quite striking. Some of the plants
were tall while others were short. None
were in-between. Furthermore, three
fourths of the plants were tall, while one
fourth were short. The reappearance of
short plants in this generation was of
great significance to Mendel. The F1
plants had possessed a character for
shortness without showing it.

When he permitted other Fx gen¬
eration plants to self-pollinate, he had
the same results. When allowed to self-
pollinate, the yellow peas that had been
produced by crossing parent plants with

yellow seeds and those with green seeds
produced peas of which three fourths
were yellow, and one fourth green.
Mendel’s first laws of heredity. The
fact that tall peas crossed with short peas
produced an Fj generation of tall peas
and that short peas reappeared in the F2
generation led Mendel to reason that
something within the plant controlled
a characteristic such as height. He
called these unknown influences factors.
Today we call them genes. He reasoned
further that height in peas was con¬
trolled by a pair of factors, since some
peas were short while others were tall.
On this basis he formulated his first law
of heredity, the law of unit characters.
This law states that the various heredi¬
tary characteristics are controlled by fac¬
tors (genes), and that these factors occur

Trait

studied

Dominant <

Stem

length

Tall

Flower

position

Axial

- 1_

Seed coat|
color

Colored

Pod shape

Inflated

Pod color

Green

_| - 1_

9-3 Mendel’s seven pairs of contrasting traits in garden peas.

120 UNIT 2 THE CONTINUITY OF LIFE

in pairs. Of course Mendel did not
know about genes or chromosomes,
which makes even more remarkable the
fact that his law of unit characters
is now a basic principle in genetics.

Mendel reasoned further that the
tall plants of his Fx generation were
not like the pure tall parent plants.
These peas were carrying a concealed
factor for shortness that would reappear
in the next generation. This reasoning
led to the discovery of his second law of
heredity, the law of dominance. This
law states that one factor (gene) in a
pair may mask or prevent expression of
the other. Mendel gave the name
dominant to the characteristic, such as
tallness, that always appeared in the off¬
spring of a cross between parents with
contrasting characters. He referred to
the characteristic, such as shortness, that
did not appear in the Fx generation but
reappeared in the F2 generation as re¬
cessive.

In Mendel’s crosses one parent was
pure tall, having both genes for tallness.
The other was pure short, having both
genes for shortness. The members of
the Fr generation were all tall but were
hybrid, a term we use to designate the
offspring of a cross between two parents
that differ in one or more traits. The
members of this generation had one
gene for tallness and one for shortness,
but appeared tall because the gene for
tallness was dominant Over the one for
shortness.

If we let the letter T stand for tall,
a pure tall plant would be written TT,
indicating that both its genes for this
character were for tallness. The capital
T indicates that tallness is dominant
over the contrasting character, shortness.
In like manner, the small letter t stands
for short, and a pure short individual
would be designated as ft.

You learned in Chapter 8 that all
body cells contain the diploid number
of chromosomes. That is, chromosomes
are present in pairs in these cells. Sex
cells, on the other hand, contain the
haploid number of chromosomes, hav¬
ing only one member of each pair. Re¬
member that genes are located on chro¬
mosomes. Consequently, the egg cell
in the female organ of the pea plant and
the sperm formed by the pollen grain
have only one gene for each character.
When eggs or sperm are formed by a
pure tall pea plant, one sperm receives
one T and the other receives the other T.
In like manner the ft genes present in
all body cells of a pure short plant are
separated in reduction division to f and t
in the formation of eggs or sperm. Dur¬
ing fertilization, each parent thus con¬
tributes one member of each pair of
genes, and the diploid number is re¬
stored.

Mendel’s law of segregation. Mendel
based his third law of heredity, referred
to as the law of segregation , on this
reasoning. According to this law, a pair
of factors (genes) is segregated, or sep¬
arated, during the formation of gametes
(spores in lower plants) in reduction di¬
vision. That is, a gamete contains only
one gene of a pair, the other having
gone to another gamete. Furthermore,
the composition of one gene is not al¬
tered by the presence of another gene
in a pair. For example, a recessive gene
in a hybrid is not altered by the presence
of a dominant gene. If, in an offspring
of the hybrid, the recessive gene is paired
with another recessive gene, the recessive
character will reappear (Fig. 9-4).

Some genetic terms. The genes of any
organism can be designated by paired
symbols for any characteristic you are
studying. These symbols indicate the
genotype of the organism. The effect

CHAPTER 9 PRINCIPLES OF HEREDITY 121

9-4 Mendel’s law of segregation. Note that although the Ft generation in this
cross consists only of tall plants, the recessive gene responsible for shortness re¬
appears in the F2 generation and produces plants in the ratio of three fourths
tall to one fourth short. What kind of plants would result from a cross between
the two F2 plants shown at the bottom left? Give the probable ratio of tall
plants to short plants and the genetic make-up of each type.

122 UNIT 2 THE CONTINUITY OF LIFE

of genes in an individual is described as
its phenotype. It refers to the organ¬
ism’s size, color, structure, and other
characteristics. For example, in hybrid
tall peas, the genotype is Tt; its pheno¬
type is tall. If the paired genes for a
particular trait are identical, we call the
organism homozygous (hoh-moh-zy-
guhs) for that trait. An organism hav¬
ing different gene pairs is called hetero¬
zygous ( het-e-roh-zy-guhs ) . The alter¬
native forms of a gene are called alleles
(a-Zee-elz). Some genes may have as
many as 100 alternative alleles. In the
gene pair Tt, T is an allele of t ; t is an
allele of T. In the gene pair Yy, Y is an
allele of y; y is an allele of Y. But T is
not an allele of Y. The separation of
alleles during reduction division demon¬
strates Mendel’s law of segregation.
Method of diagramming Mendel’s
crosses. In the study of genetics we
use special charts resembling checker¬
boards to determine the possible results
of various crosses. This grid system is
called a Punnett square after R. C. Pun-
nett, who devised it. The gametes
formed by the female are shown across
the top of the grid. Gametes formed
by the male parent are shown along the
side. You can determine all possible
combinations of the gametes by filling
in the squares of the grid. Mendel’s
work with peas can be shown more
clearly by diagramming his crosses on
grids such as those that follow in this
chapter. In working the grids just
“cross-multiply” each gamete and fill
in the correct offspring.

The monohybrid cross. When one pair
of characteristics in an individual is
crossed, the individuals possessing mixed
genes are called monohybrids. A cross
between a homozygous tall (TT) and a
homozygous short (tt) pea plant is dia¬
grammed as follows:

RESULTS OF CROSSING
TT AND tt

Female — »

GENES

Male J,

All of the offspring

are

monohybrids,

with the genotype Tt. However, they
appear equally as tall as the tall parent
plant because the gene for tallness is
dominant to the gene for shortness.

If the heterozygous Tt plants are
permitted to self-pollinate, it is easy to
see that four combinations of genes may
occur, as shown in the following grid:

RESULTS OF CROSSING
Tt AND Tt

Female — »

GENES

Male l

The grid also shows how the genes T
and t from the heterozygous parents,
though they combine by chance, result
in offspring that are one-fourth pure
dominant (TT), one-half hybrid (Tt),
and one-fourth pure recessive (tt) . This
may be expressed as a ratio of 1:2:1,
which states that the expected ratio of
the genotypes is one-fourth pure tall,
one-half hybrid tall, and one-fourth pure
short. Another way to express the
result is by phenotypes, which would
be expected to occur in a ratio of 3:1 —
three tall plants to every one short plant.

CHAPTER 9 PRINCIPLES OF HEREDITY 123

RESULTS OF MENDEL’S MONOHYBRID CROSSES

Pi Cross

Fi Plants

Fe Plants

Ratio

Round X wrinkled
seeds

All round

5,474 round

1,850 wrinkled

2.96:1

Yellow X green
seeds

All yellow

6,022 yellow

2,001 green

3.01:1

Red flowers X
white flowers

All red

705 red

224 white

3.15:1

Inflated pods X
constricted pods

All inflated

882 inflated

299 constricted

2.95:1

Green pods X
yellow pods

All green

428 green

152 yellow

2.82:1

Axial flowers X
terminal flowers

All axial

651 axial

207 terminal

3:14:1

Long stem X
short stem

All long

787 long

277 short

2.84:1

The same scheme explains the
ratios resulting from other crosses. The
cross between a heterozygous tall (Tt)
and a homozygous tall (TT) pea plant
will give the ratios shown in the grid
that follows. The phenotype of all the
plants is tall.

RESULTS OF CROSSING

TT AND Tt

zygous tall (Tt) pea plant. The ratio
of the phenotype is one-half tall to one-
half short plants; the ratio of the geno¬
types is one-half Tt to one-half tt, as
shown in the following grid.

RESULTS OF CROSSING

Tt AND tt

Female —>
GENES

t t

Female — >
GENES

Male l

T t

Male |

Tt Tt

TT Tt

tt tt

TT Tt

The results that Mendel obtained with

We can also

determine the ex-

two generations of garden peas are
listed in the table at the top of the page.

pected results from the cross between a Dominant and recessive genes in guinea
homozygous short (tt) and a hetero- pigs. The same results Mendel obtained

124 UNIT 2 THE CONTINUITY OF LIFE

in crossing tall and short peas are shown
in the inheritance of coat color in guinea
pigs. In these animals the color black
is dominant to white.

Let’s see what happens when we
cross a homozygous black guinea pig
with a homozygous white one. All of
the Fx generation will be heterozygous
black. In order to determine whether
the animal is earning a recessive gene
for white, we can cross two of the off¬
spring. The expected ratio of the off¬
spring of this cross is one-fourth homo¬
zygous black, one-half heterozygous
black, and one-fourth homozygous white.
If only one animal shows the recessive
trait, we have demonstrated that the Fx
generation was heterozygous for white.
The expected phenotype of the cross is
three-fourths black and one-fourth white.
The grid for the cross between the two

heterozygous ( Bb ) offspring of the Fx
generation follows (see also Fig. 9-5):

RESULTS OF CROSSING
Bb AND Bb

Female — »

GENES

Male l

The same ratios occur after the
crossing of rough-coated and smooth-
coated guinea pigs. In the pair of al¬
leles governing coat texture, the gene
for rough coat is dominant over that for
smooth coat.

Pure black Hybrid black Hybrid black Pure white

9-5 The cross between a pure white guinea pig and a pure black one produces
hybrid black animals in the F} generation. What results are obtained when
these F, generation animals are crossed?

CHAPTER 9 PRINCIPLES OF HEREDITY 125

9-6 This Punnett square
shows the various types of
seeds that can result in the
F2 generation from a cross
between a pea plant with
round ( R ) and green (y) seeds
and one with wrinkled (r) and
yellow (F) seeds. The pos¬
sible female gene combina¬
tions are printed across the
top of the square while the
possible male gene combina¬
tions are printed down the
left side of the square. Dom¬
inant genes appear in black;
recessive ones in color.

Crosses involving two characters.
Crosses involving two characters be¬
come more complicated than simple
crosses in which only one pair of con¬
trasting characters is considered. The
same principles apply, but the possible
gene combinations are increased. When
two pairs of characteristics are involved,
the individuals possessing mixed genes
for both characters are called dihybrids.

If a pea with round green seeds
(two characters) is crossed with a pea
having wrinkled yellow seeds, all mem¬
bers of the Fj generation have round
and yellow seeds. The recessive charac¬
ters of green color and wrinkled seed
coat are overshadowed by the two dom¬
inant traits. In this cross R will stand
for* a gene for round seed coat, r for

wrinkled, Y for yellow color, y for green.
The Fx dihybrids would all have the
genotype RrYy,. a gene for round seed
coat (R) having come from one parent,
and a gene for wrinkled (r) having come
from the other. In like manner one
parent supplied a gene for yellow color
(Y), while the other supplied a gene for
green (y).

When the two dihybrid round yel¬
low peas are crossed, the situation be¬
comes more complicated. Each dihy¬
brid with the genotype RrYy may pro¬
duce four kinds of eggs or sperm. Dur¬
ing reduction division the pairs R and r
as well as Y and y must separate and go
into different cells. R may pair with Y
to form RY or R may pair with y, re¬
sulting in Ry. Similarly r may pair with

126 UNIT 2 THE CONTINUITY OF LIFE

Y to form rY or with y to form ry. The
nature of the offspring in such a cross
depends on which eggs and sperm hap¬
pen to unite during fertilization.

The possible offspring that may re¬
sult from such a cross and the ratio of
their occurrence may be diagrammed as
in the crossing of a single contrasting
character, except that space must be
provided for more possible crosses. Fig¬
ure 9-6 shows the result of such a cross.
One of the parents was pure round green
(RRyy), while the other was pure
wrinkled yellow (rrYY). You will note
that all the F1 generation are alike, being
dihybrid round yellow ( RrYy ). In the

F2 generation, however, four different
phenotypes have been produced, as fol¬
lows:

of the offspring have seeds that are
round and yellow (both dominant
traits) .

Hie, have seeds that are round and green
(one dominant and one recessive
trait) .

Hie, have seeds that are wrinkled and
yellow (the other dominant and
the other recessive trait).

He, has seeds that are wrinkled and
green (both recessive traits).
Notice that these four phenotypes
occur in an expected ratio of 9: 3: 3:1.

9-7 Incomplete dominance in four-o’clock flowers.

CHAPTER 9 PRINCIPLES OF HEREDITY 127

You will note, also, that the genotypes
show that yellow seeds may be either
pure yellow or hybrid yellow; also that
round seeds may be either pure round
or hybrid round. A recessive character
only shows when both genes for the re¬
cessive character are present. Both re¬
cessive characters appeared only once in
the 16 possibilities.

When heterozygous black, rough-
coated guinea pigs are crossed (genes
for black and rough are dominant),
similar results can be expected: He of
the offspring are black and rough, He
are black and smooth. He are white
and rough, and He is white and
smooth.

The law of independent assortment.

The dihybrid crosses you have studied
illustrate another of Mendel’s laws, the
law of independent assortment. Ac¬
cording to this law, the separation of
gene pairs on a given pair of chromo¬
somes and distribution of the genes to
gametes ( spores in lower plants) during
reduction division is entirely independ¬
ent of the distribution of other gene
pairs on other pairs of chromosomes.
This law applies only when genes are
on different chromosome pairs, since
it is chromosomes and not genes that
assort independently.

Incomplete dominance. Genes are not
always dominant or recessive. In some
characteristics, both alleles of a pair may
be expressed. This incomplete domi¬
nance , as it is called, may be illustrated
in crossing the flowers of four-o’clocks
and snapdragons. When pure red four-
o’clocks (rr) are crossed with pure
white (ww) varieties, all of the first gen¬
eration are pink (rw) . Neither red nor
white is completely dominant, so that
both colors are expressed in the hetero¬
zygous Fx offspring as pink. However,
when two of these heterozygous pink

(rw) flowers are crossed, the F2 genera¬
tion includes one-fourth red, one-half
pink, and one-fourth white individuals.
The fact that genes for red and white
actually did not mix in the pink off¬
spring is indicated in the fact that both
pure characteristics appear again in the
second generation (Fig. 9-7).

Similarly, the color of Shorthorn
cattle illustrates incomplete dominance.
A homozygous red animal mated with
a homozygous white animal produces a
blend of red and white called roan off¬
spring. When two roan animals are
mated, the expected ratio of the off¬
spring would be one-fourth red, one-
half roan, and one-fourth white, illus¬
trating again the 1:2:1 ratio (Fig. 9-8).
Ratios are based on averages. The ratios
obtained in breeding experiments repre¬
sent averages and not definite numbers
that will always appear. These ratios
are accurate only when large numbers
of individuals are considered. For ex¬
ample, two roan shorthorns bred four
times will not necessarily produce one
red calf, two roan ones, and a white
one. Two heterozygous black guinea
pigs will not always produce one homo¬
zygous black, two heterozygous blacks,
and one homozygous white. If only
four eggs and four sperm were involved
in the process, the ratio would work
out. But actually the eggs may be more
or less than four in number, and the
sperm usually number in the millions.
Thus it is a matter of chance as to how
the eggs and sperm will unite.

Chance ratios may be shown with
two coins. When you flip them, they
will light in these possible combina¬
tions: two heads, one head and one tail,
or two tails. There is twice the chance
of one head and one tail appearing as
two heads or two tails. One of the
reasons for the great accuracy of Men-

128 UNIT 2 THE CONTINUITY OF LIFE

Red (RR)

White (rr)

Roan (Rr)

Red (RR)

Roan (Rr)

White (rr) j

9-8 Incomplete dominance in Shorthorn cattle.

del’s work with the peas is due to the
fact that he used such large numbers
of plants.

Application of Mendel’s laws to other
organisms. Scientists have worked with
numerous traits of plants and animals,
and have shown that the truth of the

Mendelian laws is beyond question.
There is overwhelming evidence that
inheritance in human beings also fol¬
lows the Mendelian laws. In the chap¬
ters to follow we shall discuss several
examples of Mendelian genetics in
man.

IN CONCLUSION

Hereditv and environment are the two important forces that interact to control
our individual make-up. Hereditary traits are produced by genes, which are
contained in chromosomes. Although Mendel knew nothing of genes or even
chromosomes, his work before the turn of the century gave us the basis for our
modern understanding of genetics.

The genius of Gregor Mendel was his discovery that variations appeared
in an orderlv manner. Laws seemed to control the heredity of those peas —
laws so exact that he could predict the kind of peas the seeds would produce.
We have gone far beyond Mendel, but we have never revised our understand¬
ing of his laws.

As you continue your studv of genetics, you will learn more about chro¬
mosomes, gene structure, and the mechanics of gene action.

CHAPTER 9 PRINCIPLES OF HEREDITY 129

BIOLOGICALLY SPEAKING

allele
dihybrid
dominant trait
environment
Fx generation
F2 generation
genetics
genotype
heredity

heterozygous

homozygous

hybrid

incomplete dominance
individual charac¬
teristic

law of dominance
law of independent
assortment
law of segregation

law of unit
characters
monohybrid
P generation
phenotype
Punnett square
recessive trait
species charac¬
teristic

QUESTIONS FOR REVIEW

1. What is the relative importance of hereditary and environmental character¬
istics?

2. Why did Mendel choose garden peas for his experiments?

3. List seven pairs of contrasting traits Mendel found in garden peas. Which
of these traits are dominant?

4. Why did Mendel limit each cross to a single characteristic?

5. The gene for black coat color is dominant in guinea pigs. How is a homo¬
zygous black different from a heterozygous black, even though the guinea
pigs look alike?

6. When two hybrid animals are crossed, homozygous dominant, heterozy¬
gous dominant, and homozygous recessive individuals appear. Account
for this.

7. When two parents that are heterozygous for one character are crossed,
what ratio of offspring (F1 generation) are expected to show the dominant
character and what ratio the recessive character?

8. Explain the law of independent assortment.

9. In what way is incomplete dominance an exception to the law of domi¬
nance?

10. In breeding experiments, why do the ratios obtained represent averages
rather than definite numbers?

APPLYING PRINCIPLES AND CONCEPTS

1. Outline a possible cross to determine whether a black guinea pig is homo¬
zygous or heterozygous for the coat-color trait.

2. In guinea pigs, black coat color is due to a dominant gene, B, and white
is due to its recessive allele, b; short hair to a dominant gene, S, and long
hair to its recessive allele, s. The gene for rough coat, R, is dominant to
that for smooth, r. Cross a homozygous rough, short-haired black guinea
pig with a smooth, long-haired white one. What are the phenotypes of
the Fj and F2 generations?

130 UNIT 2 THE CONTINUITY OF LIFE

3. In snapdragons the inheritance of flower color and size of leaves are exam¬
ples of incomplete dominance. When red-flowered plants are crossed with
white ones, all the flowers are pink. Similarly, when plants with broad
leaves are crossed with plants having narrow leaves, the offspring have in¬
termediate leaves. Cross a homozygous red-flowered, broad-leaved plant
with a homozygous white-flowered, narrow-leaved plant. What kind of
offspring are produced in the generation? Now cross two of these plants
and find the phenotype ratio of the offspring. Explain the relationship of
the 9: 3:3:1 ratio to the one you obtained.

4. Why should a hybridizer know which traits in plants or animals with which
he is working are dominant or recessive?

5. Why does the law of independent assortment apply only to certain pairs
of genes?

CHAPTER 10

THE GENETIC
MATERIAL

The chromosome theory of inheritance.

The work of Gregor Mendel seems even
more remarkable when you consider
that he made his brilliant observations,
drew valid conclusions, and formulated
his laws without any knowledge of ge¬
netic materials. It was 20 years after
publication of his paper that the cell
nucleus was recognized as the center of
hereditary materials. Mendel knew
nothing about reduction division and
segregation of chromosomes during
meiosis. In fact he never heard of a
chromosome. Yet he formulated his
law of segregation on the basis of what
he observed in crossing garden peas.
Mendel described what happened in
various genetic crosses without any idea
of why they occurred.

A few years after Mendel pub¬
lished his studies of garden peas, bi¬
ologists began searching for evidence of
hereditary information in a cell. The
nature of sexual reproduction, in which

an egg and a sperm unite during fertili¬
zation, was known to biologists at this
time. Furthermore, it was evident that
each new individual produced by sexual
reproduction bore characteristics of
both parents. It was evident that these
characteristics were transmitted to the
offspring by means of certain factors
contained in the egg and sperm. Is the
entire content of the egg and the sperm
involved in this hereditary influence?
If this were true, the egg should exert
the greater influence since it is often
much larger than the sperm. Do the
egg and sperm contribute equally to the
genetic makeup of the new individual?
What properties do an egg and a sperm
have in common that might account for
this equal influence? The most obvi¬
ous similarity is the nucleus in each.
These cell structures are of approxi¬
mately equal size in an egg and a sperm.
Could the nucleus, then, contain the
genetic information? If this were true,
an egg or a sperm must contain only
half of the genetic material of the or¬
ganism. Otherwise, this material
would be doubled in fertilization.

Questions such as these were being
asked by biologists in the 1880’s. It was
during this time that August Weis-
mann, a German biologist, suggested
that reduction divisions occurred in the
formation of eggs and sperm and pre¬
dicted that biologists would soon dis¬
cover how the process occurred. His
prediction was soon fulfilled. About
one year later, Theodor Boveri, a Ger¬
man biologist, actually observed meiosis
in the cells of A scans (czs-ka-ris), a com¬
mon parasitic roundworm.

It is ironic that, during these years
of searching for proof of reduction di¬
vision, Mendel’s paper containing the
evidence for his law of segregation lay
forgotten in the library of the Natural

131

132 UNIT 2 THE CONTINUITY OF LIFE

10-1 These photomicrographs of human sperm and an ovum show the cells
greatly magnified. (L. B. Shettles)

History Society of Briinn, Austria.
Then in 1900 three biologists, working
independently in the area of cell repro¬
duction and heredity, were searching
scientific libraries for information re¬
lated to their problem. One was an
Austrian biologist, Von Tschermak; an¬
other, De Vries, a Dutch botanist; and
the third, Correns, a German botanist.
All three of these men found copies of
Mendel’s paper in the library in Briinn.
After 34 years of oversight, his work was
finally rediscovered and put to use. By
this time biologists were familiar with
chromosomes and believed that they
carried hereditary information. Men¬
del’s paper supplied evidence of their
function. This was the information
needed to start a landslide of investiga¬
tion and discovery' in genetics.

The gene hypothesis. In 1903, just
three years after the rediscovery of Men¬
del’s paper, Walter S. Sutton, a young
graduate student at Columbia University
in New York, presented a hypothesis of
great significance. In a research paper
published in the Biological Bulletin, he
proposed that hereditary particles, or
genes, are component parts of chromo¬
somes. This was the first reference to
the gene as the determiner of a genetic
characteristic.

In examining Mendel’s work, Sut¬
ton found a striking similarity between
the behavior of genetic traits of garden
peas he described and the behavior of
chromosomes during meiosis. Mendel
had proposed the segregation of genetic
traits in the formation of gametes. Sut¬
ton was familiar with the segregation of

CHAPTER 10 THE GENETIC MATERIAL 133

chromosomes during meiosis. This par¬
allel led Sutton to the conclusion that
chromosomes contained the material of
heredity.

Furthermore, Sutton was familiar
with Mendel’s reciprocal crosses in gar¬
den peas. You will recall that Mendel
reversed many of his crosses to see if
it made a difference in the offspring if
a parent plant was used to bear seeds
in one cross and to supply pollen in
another. In all these crosses the results
were the same. Both parents contrib¬
uted equally to the offspring. Sutton
knew that eggs and sperm are not alike
in size or in structure, but he found
likeness in their chromosomes. This
provided further evidence that chromo¬
somes bear the particles that determine
hereditarv characteristics. Furthermore,

J 7

he reasoned that these determiners, or
genes, must be situated in identical posi¬
tions on corresponding chromosomes,
a belief later found to be true.

Sutton reasoned further that chro¬
mosome pairs maintain their identity
with each division of a somatic, or body,
cell of an organism. In these divisions
segregation does not occur. All chromo¬
somes split lengthwise, thus providing
each daughter cell with identical genetic
material. This occurs in mitosis, a proc¬
ess of cell reproduction which you have
already studied.

Generally Sutton found three paral¬
lels between Mendel’s hereditary factors
and the behavior of chromosomes and
genes:

1. Chromosomes and genes occur in
pairs in the zygote and in all somatic
cells.

2. Chromosomes and genes segregate
during meiosis, and only one mem¬
ber of each pair normally enters a
gamete.

3. Chromosomes and genes maintain
their individuality during segregation.
Each pair segregates independently
of all other pairs. This confirmed
Mendel’s law of independent assort¬
ment of the characteristics he ob¬
served in garden peas.

How many genes on a chromosome?
Having established the gene hypothesis,
Sutton’s next question concerned the
number of genes on an individual chro¬
mosome. If, for example, an organism
had 10 pairs of chromosomes, would it
not have more than 10 pairs of genes?
Organisms certainly had many more in¬
heritable characteristics than pairs of

10-2 Walter S. Sutton first proposed that
hereditary particles are component parts of
chromosomes. (V. A. McKusick)

134 UNIT 2 THE CONTINUITY OF LIFE

10-3 The great American geneticist, Dr.
Thomas Hunt Morgan, did pioneer work on
the fruit fly which led to the explanation of
sex determination. (California Institute of
Technology)

chromosomes. Undoubtedly many
genes were located on each chromo¬
some. It seemed unlikely to Sutton
that genes segregated independently dur¬
ing meiosis. Rather, they must move
in sets on a chromosome. In this ex¬
planation, Sutton proposed gene link¬
age. What, then, of Mendel’s seven
pairs of contrasting traits such as tall
stem and short stem, round seeds and
wrinkled seeds, yellow seeds and green
seeds? Mendel based his law of inde¬
pendent assortment on the fact that
all of the pairs of traits he studied seg¬
regated and recombined independently.
Was a gene for each of these traits sit¬
uated on a different chromosome or
were certain genes linked on the same

chromosome? Sutton believed that the
genes involved in Mendel’s studies must
have been on different chromosomes
since they segregated independently, al¬
though he could not prove this. Today
we know that he was right in his as¬
sumption. The peas Mendel used had
seven pairs of chromosomes. By coin¬
cidence each of the contrasting traits
he studied was determined by genes on
different chromosome pairs.

While Sutton believed that gene
linkage must occur, he was never able
to establish proof. However, a few
vears later, another great contributor to
genetics found experimental evidence to
support Sutton’s brilliant deduction.
Discovery of sex chromosomes. Soon
after Sutton established that chromo¬
somes and genes are genetic materials,
Thomas Hunt Morgan and a group of
associates working at Columbia Univer¬
sity made a discovery in one tiny fruit
fly that was destined to make genetic
history. Thomas Hunt Morgan was
one of the pillars in the study of genet¬
ics. His genius was recognized when he
was awarded the Nobel prize in medi¬
cine and physiology in 1933 for his re¬
search in genetics. Among his asso¬
ciates, all considerably younger, were
three men who also made outstanding
contributions in genetic research: Cal¬
vin Bridges, A. H. Sturtevant, and H. J.
Muller. Dr. Muller was also awarded
the Nobel prize in 1946 for outstand¬
ing work we shall discuss later in this
chapter.

Dr. Morgan and his associates were
growing large numbers of fruit flies in
their studies of genetic traits at Colum¬
bia University. This small fly is com¬
mon around overripe fruit. You prob¬
ably think of it as a pest rather than
a valuable subject for research in genet¬
ics. Biologists refer to the fruit fly

CHAPTER 10 THE GENETIC MATERIAL 135

10-4 Left: a normal red-eyed female fruit fly; right: a normal red-eyed male.
Note that the female is larger than the male and that the posterior end of the
male is darker and blunter than that of the female. (Irwin I. Oster)

as Drosophila melanogaster (droh -sahf-
i-la me-lan-oh-gds-ter), but we shall
shorten its name to Drosophila here.
Several characteristics of Drosophila
make it an ideal subject for research in
genetics. It is easily raised in jars con¬
taining mashed bananas and other spe¬
cially prepared diets. The life cycle is
short, varying from about 10 to 15 days,
depending on the environmental tem¬
perature. Thus, many generations can
be observed in a short time. Further¬
more, the sexes of Drosophila are easily
distinguished. The male is usually
smaller than the female, has character¬
istic combs of dark bristles on its first
pair of legs, and has a black-tipped,
blunt posterior end. Genetic variations
in Drosophila include eye color, body
color, and wing structure. These are
but a few of the thousands of variations
in Drosophila with which geneticists are
familiar today.

Now, to return to the one fruit fly

that made genetic history. One day
Dr. Morgan and his associates were ex¬
amining a large number of Drosophila
and, to their surprise, found one fly
that had white eyes instead of the nor¬
mal red eyes. The fly was a male. This
unusual fly was mated with a normal
red-eyed female. All of the Fx gen¬
eration resulting from this mating had
normal red eyes. By applying Mendel’s
law of dominance, Morgan and his as¬
sociates concluded that red eyes are
dominant over white eyes in Drosophila.
The investigation was continued with
the mating of flies of the F1 generation
to produce an F2 generation. About
three quarters of these flies had red
eyes, and about one quarter had white
eyes. This, again, conformed to the
results Mendel had obtained in garden
peas that were hybrid, or heterozygous,
for one trait. At this point Morgan
made a significant discovery. All of
the white-eyed flies were males! This

136 UNIT 2 THE CONTINUITY OF LIFE

could not be the result of chance. There
was a definite association of eye color
and sex. Morgan had discovered a sex-
linked trait in Drosophila. Two prob¬
lems had now arisen — sex determina¬
tion in Drosophila and the relation of
a gene for white eyes to the process.
We shall now consider both of these
problems.

Sex determination. At the time Dr.
Morgan and his associates were experi¬
menting with Drosophila , no one had
examined the chromosomes of this fruit
fly. This was the next step for Morgan
and his associates in investigating the
white-eyed male. Examination of the
nuclei of somatic cells of Drosophila re¬
vealed four pairs of chromosomes. This
is the diploid chromosome number.
The cells of female flies contained four
kinds of chromosomes with identical
mates (Fig. 10-5). But there was a dif¬
ference in the chromosomes of the male.
Three pairs were like those of the female,
but one chromosome was different. It
did not match its mate. Instead of
being rod-shaped, it was bent like a
hook, or Y-shaped. The rod-shaped
chromosomes are designated as X chro-

•in!*

/r A

V Y XX

10-5 The sex chromosomes of the fruit fly,
Drosophila, are shown in color in this dia¬
gram. The female (right) has two straight
X chromosomes. The male (left) has one
straight X chromosome and one bent Y chro¬
mosome. In mating, combinations of X and
X chromosomes produce females, while com¬
binations of X and Y chromosomes produce
male offspring.

mosomes, while the hook-shaped mem¬
ber of the pair is named the Y chromo¬
some. These are the sex chromosomes.
The remaining three pairs in Drosophila ,
identical in both males and females, are
designated as autosomes.

Following Morgan’s fine work on
sex chromosomes in Drosophila , studies
were made of many other animals.
Similar chromosomes were found in
most animals and in many plants in
which there is a sexual difference. The
presence of two X chromosomes (XX)
produces a female organism, while a
single X chromosome paired with a Y
chromosome (XY) results in a male.

Further study of sex chromosomes
in many organisms has revealed a varia¬
tion in what we might term XY sex
determination. Some organisms have
no Y chromosome in males. In these
organisms XX produces a female, while
XO results in a male.

It is easy to diagram sex determina¬
tion by means of a grid, such as you
used in showing Mendelian crosses in¬
volving a single trait. As we describe
the formation of gametes and segrega¬
tion of sex chromosomes, then ferti¬
lization and recombination of chromo¬
some pairs, follow the changes shown
on page 137. In the segregation of sex
chromosomes in egg formation, the dip¬
loid number XX is reduced to the hap¬
loid number X. Thus, all eggs contain
a single X chromosome. However, in
the formation of sperm, the diploid
number XY is reduced to X and Y. In
other words half of the sperm contain
the X sex chromosome and half the
Y chromosome. The sex of the off¬
spring is determined by the chance
union of an egg and a sperm. If the
X-containing sperm fertilizes the egg,
the offspring is female. Union with a
Y-containing sperm results in a male.

CHAPTER 10 THE GENETIC MATERIAL 137

10-6 The inheritance of sex
in the fruit fly, Drosophila
melanogaster.

Male(X Y)

Female(XX)

PARENTS

OFFSPRING

SEX DETERMINATION

Female

CHROMOSOMES

Male

You may be interested in knowing
that sex determination in human beings
occurs in the same manner. Each of
your somatic cells contains 23 pairs of
chromosomes. Twenty-two of these
pairs are autosomes. The remaining
pair are sex chromosomes.

Sex linkage. Now that you are familiar
with sex chromosomes and their func¬
tion in the determination of sex, we
shall return to a discussion of Morgan’s
white-eyed male flies. The crosses he
made are diagrammed at the right and on
page 138. Follow each step as it is de¬
scribed. We shall designate the gene
for normal red eyes as R and use r to
indicate white eye color. Both of these
genes are situated on X chromosomes
and are therefore sex-linked. The Y
chromosome contains no functional

DNA and therefore does not act in
determining eye color in Drosophila.

The original white-eyed male that
Morgan discovered had a gene for white
eyes on its X chromosome. We may
.represent this individual as XrY. The
normal red-eyed female, with a gene for
normal red eyes, would be indicated as
XRXR. As a result of segregation, the
sperm formed by the white-eyed male
would be of two types, Xr and Y. All
of the eggs produced by the normal red -
eyed female would contain XR. All
members of the F1 generation would be
red-eyed, although they would be of
two genetic types. Eggs fertilized by
half of the sperm would contain XRXr
chromosomes and would be females,
heterozygous for eye color. The other
half would contain XRY. These would

INHERITANCE OF
SEX-LINKED CHARACTERISTICS

Female

SEX CHROMOSOMES

Male

XRXr

XRY

138 UNIT 2 THE CONTINUITY OF LIFE

be males with a single gene for red
eyes. All members of this generation
would be red-eyed, however, because of
dominance of the red eye gene.

In mating two flies of this genera¬
tion, the offspring shown in the grid at
the right would occur in the F2 genera¬
tion. Half of the eggs produced by a
female of this generation would contain
XR and half would contain Xr. Half of
the sperm would contain XR and half Y .
Thus, you can see that some of the
offspring would be XRXR and others
would be XRXr. All of these flies would
be red-eyed females. On the other hand

some of the males produced would be
XRY, with red eyes, while others would
be XrY, with white eyes. Actually, the
law of averages would probably result
in about a quarter of the offspring being
of each type. This would result in an
eye color ratio of three-quarters red¬
eyed and one-quarter white-eyed. Fur¬
thermore, all of the white-eyed offspring
would be males, as Morgan determined.

Will a white-eyed female appear if
mating is continued for another gen¬
eration? This could result if a heter¬
ozygous red-eyed female, XRXr, were
mated with a white-eyed male, XrY.

White-eyed male (X^Y) Red-eyed female(XRXR)

XRXr
9^

Red-eyed Red-eyed

Red-eyed male (XRY)

Red-eyed Red-eyed 10_7 Sex |jn^age jn pro.

Red-eyed female (XRXr) sophila.

'©

Red-eyed White-eyed

CHAPTER 10 THE GENETIC MATERIAL 139

10-8 Normal cross in
Drosophila between a red¬
eyed male and a vermil¬
ion-eyed female.

Red-eyed male (XRY) Vermilion-eyed female (XrXr)

RESULTS OF
CROSSING F1 HYBRIDS
RED-EYED MALE AND
RED-EYED FEMALE

Female XR Xr

SEX CHROMOSOMES
Male

X» XRXR XRXr

Y X»Y XrY

The discovery of sex linkage in
Drosophila by Morgan and his associates
introduced a new and important prin¬
ciple in genetics. Sex-linked traits are
by no means limited to Drosophila.
Did you know that color blindness oc¬
curs as much as 10 times more fre¬
quently in men than in women and
that a similar ratio is found among vic¬
tims of hemophilia, or “bleeder’s dis¬
ease”? Does this ratio of occurrence
give you a clue? We shall reserve the
v discussion of the inheritance of these
conditions for Chapter 11.

Other characteristics associated with
sex are not sex-linked. For example,
baldness is far more common in men
than in women. Yet it is not a sex-
linked trait. Conditions such as this
will also be discussed in Chapter 11.

Nondisjunction — abnormal segregation
of sex chromosomes. About ten years
after Morgan discovered sex-linked traits
in Drosophila , C. B. Bridges, one of his
former graduate students, made another
startling discovery. Bridges was work¬
ing with sex-linked genes that deter¬
mine eye color in Drosophila much as
he had in the earlier studies in which
he assisted Morgan. In these genes,
however, the alleles were red eyes and
vermilion eyes. Red eyes in Drosophila
are dark red, while vermilion eyes are
much brighter red. The gene for nor¬
mal red eyes is dominant over that for
vermilion. These genes normally be¬
have as other sex-linked traits. When
a vermilion-eyed female is mated with
a red-eyed male, half the offspring are
red-eyed females and half are vermilion-
eyed males, as below and in Fig. 10-8.

RESULTS OF CROSSING
VERMILION-EYED FEMALE
AND RED-EYED MALE

Female

SEX CHROMOSOMES

Male

XRXr

XrY

140 UNIT 2 THE CONTINUITY OF LIFE

Red-eyed male(XRY)

Vermilion-eyed female(XrXr)

Red-eyed,
usually dies

( XrXr Y J

viy

Vermilion-eyed

Red-eyed, Lacks X chromosome,
sterile dies

10-9 Nondisjunction

Drosophila.

In about one individual in 2,000,
however, a striking thing occurred. A
vermilion-eyed female mated with a red¬
eyed male produced a vermilion-eyed fe¬
male. If you examine the results of
such a cross as shown you will dis¬
cover that this is impossible under nor¬
mal conditions. In the grid and in Fig.
10-8 the dominant gene for red eye color
is represented as R and the recessive
gene for vermilion eye color as r. The
male fly with red eyes is represented as
XRY, while the vermilion-eyed female
is XrXr. Females in the Fj generation
must receive an XR chromosome from
the male parent and an Xr chromosome
from the female parent. How, then,
could a vermilion-eyed female result?
This would require two Xr chromo¬
somes, both of which are in the fe¬
male parent. Yet it did happen and
threatened to upset the whole theory
of sex-linked traits. To Bridges, this
one-in-two-thousand female fly with
vermilion eyes had great genetic signifi¬
cance. Surely, it could be accounted
for in terms of sex chromosomes and
sex-linked traits. The answer was to

be found in the cells of the fly. Bridges
examined cells from the fly’s body and
found a remarkable condition. This
vermilion-eyed female had two X chro¬
mosomes and a Y chromosome. We
would represent it genetically as XrXrY.
The extra X chromosome produced a
female, even in the presence of the Y
chromosome. The two recessive genes
located on these X chromosomes pro¬
duced vermilion eyes — an impossibility
in this mating under normal conditions.

Figure 10-9 shows how abnormal
chromosome segregation occurred to
produce this unusual situation. We
refer to the phenomenon as nondisjunc¬
tion. The red-eyed male parent pro¬
duced normal sperm, half containing
the XR chromosome and half contain¬
ing the Y chromosome. However, seg¬
regation of the XrXr of the female fly
did not occur when eggs were formed.
Thus, half of the eggs contained XrXr
chromosomes, as in the somatic cells,
while half contained no sex chromo¬
somes. Notice in Fig. 10-9 that four
kinds of offspring may be produced by
various egg and sperm combinations:

CHAPTER 10 THE GENETIC MATERIAL 141

V\ red-eyed females (XRXrXr), which
usually die

lA vermilion-eyed females (XrXrY)
lA red-eyed males (XR), which lack a
' chromosome and are sterile
lA flies (Y), with no X chromosomes
and therefore no eye-color genes,
which always die.

In discovering nondisjunction,
Bridges, far from disproving the chro¬
mosome theory of heredity, substan¬
tiated it further. His work left no doubt
that genes are located on chromosomes
and that the genes for eye color in
Drosophila are on the X chromosomes.

Nondisjunction may occur in var¬
ious autosomal chromosomes as well as
in sex chromosomes. Recent studies of
this phenomenon in human beings has
explained the heredity of various tragic
"conditions such as Mongolian idiocy.
We shall discuss this condition in Chap-
\ ter 11.

Gene linkage and crossing over. Keep
in mind that it is chromosomes and not
individual genes that segregate during
meiosis. Furthermore, a single chromo¬
some contains a large number of genes
joined together in a linear arrangement.
We refer to this condition as gene link -
age. If a chromosome has 50 genes
linked together, its mate will have the
alleles, or the 50 corresponding genes.
Thus an organism can have no more
pairs of genes sorting out independently
than it has pairs of chromosomes.

However, this linkage is not perfect.
Segments of chromosomes, bearing
many genes, may separate and exchange
with a corresponding segment of the
other member of a chromosome pair.
We refer to this phenomenon as cross¬
ing over. When do you think a homol¬
ogous pair of chromosomes would be
most likely to exchange segments?
When are they in closest contact? Do

you remember in meiosis when the
genes have replicated and each chro¬
mosome forms two chromatids? The
joined chromatids of a homologous pair
come together and form a tetrad, or
group of four. It is here that segments
of two chromatids (one of each homo-
logue) may exchange segments contain¬
ing varying numbers of genes. For ex¬
ample, let us represent three of the
many genes on a chromatid as A, B,
and C. Corresponding genes on a
chromatid of the other homologue are
a, b, and c. Now, we will assume that
a segment of one chromatid separates
between genes A and B. A similar sep¬
aration occurs between genes a and b of
another chromatid. These two chro¬
matids exchange segments and produce
a new gene linkage on two of the four
chromatids in the tetrad. These new
linkages are now a, B, and C and A, b ,
and c. Following separation of the
chromatids in meiosis, eggs or sperm
will receive these new gene combinations
not present in either parent. We have
described a single crossover. Double
and even triple crossovers are known to
occur.

C a

10-10 Crossing over. Note the final result,
which is a new grouping of genes on the
chromosomes.

142 UNIT 2 THE CONTINUITY OF LIFE

The genes shown on the chromo¬
some diagrams in Fig. 10-10 are widely
separated. Biologists have reasoned that
the greater the distance between two
genes on a chromosome, the more likely
a separation will occur between them.
The rate at which these separations oc¬
cur in specific chromosomes has been
used as an index in determining the dis¬
tance between two genes. This is one
of the ways in which geneticists have
been able to construct chromosome
maps locating the genes in relation to
each other. Such chromosome mapping
has been done for both Drosophila and
corn. Similar methods are being used
to locate specific genes on human chro¬
mosomes.

What is a gene? Our discussion so far
has concerned chromosomes and the
genetic traits that genes produce. But
what is a gene and how does it operate?
Why does one gene produce a short pea
plant while another produces a tall
plant? To answer these questions, we
must return to the DNA molecule. Re¬
member that chromosomes contain
DNA, and that DNA is able to replicate
itself every time mitosis occurs and two
new cells result from one. Remember
also that the DNA in every cell of the
body bears the genetic code, which de¬
termines the traits of an organism, and
that by means of messenger RNA, it
transmits this coded information and
thus controls all cellular activities. Thus
it is evident that a gene must be defined
in terms of the DNA molecule. Let us
review the structure of that molecule to
establish this relationship.

We noted in Chapter 3 that the
DNA molecule is in the form of a
twisted ladder, or double helix. The
side pieces are composed of alternating
phosphate and deoxyribose sugar units.
The steps of the ladder are formed by

pairs of bases. Recall that there are
four different bases — adenine, guanine,
thymine, and cytosine — and that these
bases may occur in any sequence as long
as they are properly paired. Now, since
the sequence of bases is the only var¬
iable part of the DNA molecule, it is
logical to conclude that in this sequence
lies the code that is able to control the
numerous different traits of an organ¬
ism. This is indeed the case.

In Chapter 6 we described the sep¬
aration of the two strands of the DNA
molecule, the formation of RNA from
a single strand of DNA, the movement
of this RNA to a ribosome where it
becomes a template, and the coding of
a protein molecule by the arrangement
of bases in the template. Thus, in
studying protein synthesis, we have al¬
ready pinpointed the genetic code. The
coded arrangement of bases ultimately
determines what proteins will be pro¬
duced. Since most proteins are en¬
zymes, and enzymes direct all cellular
activities, we have arrived at a logical
explanation for genetic control of the
organism.

In Chapter 6 we referred to the
current evidence that a triplet of bases
acts as a code for a single amino acid.
Thus the synthesis of a protein that
contains 100 amino acids would require
a portion of a DNA molecule with 100
base triplets, or 300 bases. Actually no
one has yet worked out the complete
coding sequence of bases for a single
protein. Also, there seem to be triplets
along the sequence that do not act as
code “words/' but might be called “non¬
sense syllables." They may be “punc¬
tuation marks" indicating the end of
a protein or part of a protein, although
this is not established.

Although we have described the
genetic code, we have not yet defined

CHAPTER 10 THE GENETIC MATERIAL 143

a gene. Actually biologists are in some
disagreement as to the definition of a
gene. Chromosomes contain strands of
DNA and other chromatin materials.
The DNA is spiraled around a protein
backbone. The nature of this binding
seems to determine which part of the
DNA molecule is active genetically.
The ends of the molecule are not active.
However, since the strands may con¬
tain hundreds of base pairs, many other
parts of the molecule are active genet¬
ically. In general a gene may be de¬
fined as that portion of a DNA mole¬
cule that is genetically active and pro¬
duces a trait — that is, it is a unit of
hereditary information. Biologists are
not agreed as to whether this unit should
be defined as that which codes a single
protein, more than a single protein, or
only a portion of it. At any rate, it is
becoming increasingly clear that the
triplets of nitrogen-containing bases code
amino acids. Also, it logically follows
that each chromosome pair in the cells

of a given organism contains different
coding sequences, making possible the
numerous genetic traits.

Transformation in pneumococcus —
proof of DNA. Perhaps you have won¬
dered how biologists have been able
to prove that DNA is the genetic mate¬
rial. One of the most convincing proofs
has been found in studies of pneumococ¬
cus, the bacterium that causes one kind
of pneumonia. It is interesting that
these studies began nearly 40 years ago
with the work of an investigator who
had never heard of DNA.

In 1928, Frederick Griffith, a Brit¬
ish bacteriologist, was working with two
strains of pneumococcus. The cells of
the two strains were similar — tiny
spheres usually joined in pairs or short
filaments. However, there was a sharp
distinction between the two strains.
One formed a slimy capsule that sur¬
rounded the cells. The other did not.
Griffith found that the noncapsulated
organisms did not cause pneumonia.

10-11 The left-hand photograph shows the capsulated, infectious strain of
Diplococcus pneumoniae , commonly called pneumococcus. The right-hand
photograph shows the noncapsulated, noninfectious strain. (Robert Austrian

and Journal of Experimental Medicine )

144 UNIT 2 THE CONTINUITY OF LIFE

Apparently the cells were destroyed by
white corpuscles when they were in¬
jected into a mouse or other test an¬
imal. However, mice that received in¬
jections of the capsule-forming strain
were dead or dying of pneumonia within
a short time. Apparently the capsule
prevented white corpuscles from de¬
stroying cells of this strain.

In one phase of his investigation,
Griffith inoculated mice with the two
strains of pneumococcus. In one in¬
jection he used living noncapsulated
organisms. In the other he used cap-
sulated organisms that had been killed
with heat. Much to his surprise the
mice soon died of pneumonia. When
he examined the blood of these mice,
he found pneumonia organisms with
capsules ! None of the capsulated or¬
ganisms he had injected were living.
Yet here were capsulated bacteria which,
when placed in cultures, continued to
grow and were therefore alive. Appar¬
ently the living noncapsulated cells had
been transformed into a capsule-produc¬
ing strain (Fig. 10-12, Phase I).

About 15 years later Oswald T.
Avery and his associates, C. M. Mac¬
Leod and M. McCarthy, began a search
for an explanation of Griffith’s results
at the Rockefeller Institute for Medical
Research in New York City. They grew
both noncapsulated (Type II) and cap¬
sulated (Type III) pneumonia organ¬
isms in laboratory cultures. The cap¬
sulated organisms formed slimy colonies
on the surface of the culture medium
that were easily distinguished from col¬
onies of the Type II organisms. Each
strain continued to produce its own
characteristic cells generation after gen¬
eration. They next prepared an extract
of heat-killed Type III capsulated cells
and mixed it with living Type II non¬
capsulated cells. When this extract

was cultured in a sterile medium, a
few Type III colonies appeared. These
newly transformed capsulated cells were
then isolated, cultured, and further¬
more, these formerly noninfectious Type
II cells now caused pneumonia when in¬
jected into mice. Some factor in the
extract made from dead capsulated or¬
ganisms had entered these cells and
produced a genetic change. Avery and
his associates were able to isolate and
purify this transforming substance. At
first it was thought to be a protein.
However, it proved to be a nucleic acid
or more specifically, DNA.

The discovery of the phenomenon
of transformation in pneumococcus is of
great genetic importance for at least two
reasons. It proves that the gene is DNA.
Furthermore, it demonstrates that genes
can be transferred from one organism to
another and continue to express their
genetic qualities.

Gene action. Having established that
genes determine hereditary traits and
that they are in fact portions of DNA
molecules, perhaps we should clarify
the way genes act in producing traits.
Remember that DNA acts through mes¬
senger RNA, which in turn encodes
amino acids to synthesize proteins.
What kinds of proteins? Most pro¬
teins are enzymes, which determine the
chemical activity of the cell. Thus,
RNA serves as the messenger in gene
action, but enzymes are the active
agents. You inherited a specific set of
genes, which are present in pairs on your
46 chromosomes. As a result of the
action of these many thousands of genes,
you also have many thousands of en¬
zymes controlling the chemical activi¬
ties of your cells.

But let us examine this situation
more closely. Every cell in your body
normally contains all of your genes.

CHAPTER 10 THE GENETIC MATERIAL 145

D Type II
Type III
(heat-killed )

Type II (noncapsulated)
pneumococcus

Type III (capsulated)
pneumococcus

C Type III organisms
killed by heat

Extract containing both
living Type II and
heat-killed Type III

10-12 The discovery of transformation in pneumococcus. Phase I — A. Griffith
found that Type II (noncapsulated) pneumococcus did not cause pneumonia
when injected into mice. B. When Type III (capsulated) pneumococcus was in¬
jected into mice, they contracted pneumonia and died. C. Type III organisms
were killed by heat. D. When both living Type II organisms and heat-killed
Type III organisms were injected, the mice died. Apparently some of the
Type II had been transformed into Type III. Phase II — A. Type II and Type III
pneumococcus were again cultured, and the Type III was heat-killed. B. An ex¬
tract containing both living Type II and heat-killed Type III was prepared.
C. When this extract was cultured on a sterile medium, some Type III colonies
appeared. It was later determined that DNA from the heat-killed Type III had
entered some of the noncapsulated pneumococcus cells and transformed them
into capsulated cells.

146 UNIT 2 THE CONTINUITY OF LIFE

This genetic makeup was established
at the beginning of your life when an
egg and a sperm combined their haploid
chromosome numbers to produce a
diploid zygote — your first somatic cell.
From that moment on, every gene has
replicated prior to a mitotic cell division.
Thus every cell in your body contains
your complete genetic code. If you
have blue eyes, every cell contains genes
for blue eyes. Yet only in the cells of
the iris of your eyes is this trait ex¬
pressed. Here the gene for eye color
produces enzymes which in turn reg¬
ulate pigmentation of the iris. The
cells of the iris also contain genes that
determine the color and texture of your
hair. But these genes act only on the
cells of hair roots.

Could it be, then, that genes act only
in a particular cell environment? They
may do so because of differences in the
chemical composition of the cytoplasm
of various cells. Undoubtedly, many
other unknown factors are involved.
But we may be sure that genes and cells
function in a close relationship in de¬
termining gene action. Blue-eyed peo¬
ple never have blue skin, and genes that
determine blood type never express hair
color or the shape of an ear!

Mutations — errors in the genetic code.
Normally genes replicate and chromo¬
somes segregate during meiosis with re¬
markable accuracy. This may occur
millions of times without an error. Vari¬
ations occur in offspring as new gene
combinations are established in sexual
reproduction. But the individual genes
remain unchanged.

From time to time, however, an or¬
ganism may appear with a characteristic
totally unlike any of those of either
parent. This new characteristic is trans¬
mitted to offspring. Thus we know that
it represents a sudden genetic change

rather than an environmental influence.
We refer to such a change as a mutation
and to the organism possessing it as a
mutant. Any change in the base coding
of a DNA molecule would alter or
destroy the trait associated with the
gene. This produces a gene mutation,
also referred to as a point mutation.
While less common than gene muta¬
tions, other mutations may occur as
chromosomal aberrations, or chromo¬
some mutations. These may be the re¬
sult of nondisjunction during segrega¬
tion in meiosis, loss of an entire chro¬
mosome or piece of a chromosome, or
possibly the recombination of chromo¬
some segments by crossing over.

If a mutation occurs in a body cell
of a plant or animal, the variation ap¬
pears in all of the tissue that descends
from the original mutant cell. This is
known as a somatic mutation. It is
not transmitted to offspring since re¬
productive cells are not involved. So¬
matic mutations may be preserved in
plants, however, if the variation is de¬
sirable, by vegetative propagation. So¬
matic mutations have been the source
of new plant varieties we shall discuss
in Chapter 12. If a mutation occurs in
a reproductive cell, it may be trans¬
mitted to offspring. Such germ muta¬
tions are of great genetic importance.
The nature of gene mutations. Minor
mutations may occur from time to time
with little or no visible effect on the or¬
ganism. These are the most common of
all mutations. However, a major muta¬
tion results in a drastic change in a char¬
acteristic. Minor mutations may be
either beneficial or harmful. Major mu¬
tations are nearly always harmful. Many
are lethal and result in death of the or¬
ganism. Many lethal characteristics
have been found in cattle. Among these
are the parrot-beak calf, with abnormal

CHAPTER 10 THE GENETIC MATERIAL 147

10-13 Lethal traits such as albinism in corn are due to gene mutations. These
seedlings will live only as long as stored food in the endosperm is available to
them. (A. M. Winchester)

jaw structure; short-spine calf with
shortened and fused vertebrae; and an
amputated calf lacking both lower jaws
and legs. Mutation of a gene that con¬
trols chlorophyll production in a plant
would be a lethal condition since a
plant that lacks chlorophyll cannot carry
on photosynthesis.

Mutations occur in all levels of
life. They are known to occur in
viruses and are frequent among bacteria.
They have been found in all plants and
animals that have been studied genet¬
ically. Human beings are also subject
to mutations. We shall discuss some
of these in Chapter 11.

A mutant gene is nearly always re¬
cessive to a normal allele. For this rea¬
son the mutation does not appear in a

heterozygous individual. The gene is
nevertheless passed to offspring and,
paired with another mutant gene from a
heterozygous parent, would produce the
characteristic.

The rate at which different muta¬
tions occur varies considerably, as some
genes seem to be more chemically stable
than others. Some mutate as often as
once in 2,000 cell divisions, while others
mutate only once in millions of cell di¬
visions. Sometimes genes mutate sev¬
eral times in rapid succession. They
often mutate and then mutate again
back to the original form. Of course,
the rate at which many genes mutate
is difficult to determine because the mu¬
tation has no apparent effect on the
organism.

148 UNIT 2 THE CONTINUITY OF LIFE

Causes of mutations. One of the most
frequent causes of mutations is exposure
to high-energy radiation. Cosmic rays
from outer space and radiation from
radioactive elements may cause natural
mutations. They may also be produced
experimentally by exposure to X rays,
gamma rays, beta particles, and ultra¬
violet light. Biologists have used arti¬
ficial sources of radiation to increase
both the number and the rate of muta¬
tions in organisms under experimental
conditions. Temperature increase has
also been used to increase the rate at
which mutations occur. Certain chem¬
icals have also been used to produce mu¬
tations. Among these are formalde¬
hyde, nitrous acid, peroxide, and mus¬
tard gas.

Radiation as a cause of mutations. The
first proof that radiation causes muta¬
tions came from research conducted by
Hermann J. Muller in 1927. Muller
was one of the graduate students who
worked with Morgan and Bridges at

10-14 Dr. Hermann J. Muller first showed
that radiations cause gene mutations and
won the Nobel prize in 1946 for his outstand¬
ing contribution to genetics. (Indiana Uni¬
versity)

Columbia University in the early studies
of inheritance in Drosophila. While at
the University of Texas, Muller con¬
ducted a series of experiments to es¬
tablish that radiation can cause muta¬
tions and that their rate can be in¬
creased with artificial radiation. As
in his earlier work, Muller used Dro¬
sophila in his investigations. His work,
for which he received the Nobel prize in
medicine and physiology in 1946, rep¬
resents one of the most significant ad¬
vances in genetics.

Prior to Mullers work, geneticists
had tried many methods of producing
mutations artificially without success.
They had experimented with tempera¬
ture changes, variation in light condi¬
tions, different diets, and other factors
on various animals, including Drosoph¬
ila. One thing these other investi¬
gators probably overlooked that Muller
considered was a condition under which
a gene mutated. Consider that a pair
of genes, normal for an organism and
lying close to each other in correspond¬
ing positions on corresponding chromo¬
somes, would be affected equally by
chemical changes in the cell or an en¬
vironmental condition such as temper¬
ature. Suddenly one gene of the pair
mutates while the other remains un¬
changed. What could change one gene
and not the other? The most likely
cause of such a pinpoint effect would
be high-energy radiation. It was to
prove this idea that Muller began his
series of experiments. Muller reasoned,
further, that lethal mutations would be
the most likely to result from radiation.

The following is a greatly simpli¬
fied summary of the brilliant work Mul¬
ler conducted. He selected a male
Drosophila with known sex-linked traits
and radiated it with a strong dose of
X ravs. This ffv was mated with a

j j

CHAPTER 10 THE GENETIC MATERIAL 149

Curly wings

White eyes

Mutations in
reproductive cel Is of

-p>

wild-type flies

Stubby wings

Yellow body color

Lethal condition

10-15 This drawing shows some of the various mutations that occur in the re¬
productive cells of wild-type Drosophila.

female with known sex-linked genes on
the X chromosomes. The female was
not radiated. In the Fx generation, half
of the flies were females, all containing
a radiated X chromosome from the male
and a normal X chromosome from the
female parent. The other half were
males, containing a normal X chromo¬
some from the female and a radiated

Y chromosome from the male parent.
When two of these particular flies were
crossed, four kinds of offspring should
have been accounted for in the F2 gen¬
eration. This mating should have re¬
sulted in about one-fourth females with
two normal X chromosomes, one-fourth
females with one normal X chromosome
and one radiated X chromosome, one-
fourth males with a normal X chromo¬
some and a radiated Y chromosome,
and one-fourth males with both X and

Y chromosomes radiated. However,
this last group of flies did not appear.
The radiated X chromosome bore a

lethal gene that prevented them from
developing. This did not occur in the
females that contained a radiated X
chromosome because of a dominant
gene on the other X chromosome.

Since Muller’s original work, he and
other geneticists have produced many
more mutations in Drosophila as well
as in other organisms. Artificial radia¬
tion has been found to increase the
rate at which mutations occur as much
as 150 times above the normal frequency.
A common mold with great genetic
significance. Much of the research in
genetics today is being conducted with
simple organisms, including molds and
bacteria. Experimental results may be
obtained quickly because of the short¬
life cycles of these organisms. Further¬
more, metabolic processes may be stud¬
ied readily, thus establishing evidence of
gene action in cell biochemistry.

One such organism which has been
the subject of extensive genetic studies

150 UNIT 2 THE CONTINUITY OF LIFE

10-16 The common baker’s mold Neuro-
spora is one of the most widely used or¬
ganisms in research concerning genetic
mutations. (David R. Stadler)

in recent years is the common baker’s
red mold, Neurospora crassa. This com¬
mon mold, a relative of the yeasts, may
be found growing on bread and other
food products. As the mold grows, it
forms a mass of white threads which
penetrate the medium. Within a short
time, stalks arise above the medium.
The tip of each stalk becomes a spore
case in which eight salmon-pink spores
are produced. Each spore may produce
a new mold plant.

While we would consider Neuro¬
spora a primitive organism from the
standpoint of evolution, it is far from
simple in its biochemical capacity. The
chemical abilities of this mold far sur¬
pass those of more highly evolved or¬
ganisms, including most animals. Neu¬
rospora can be grown on a simple me¬
dium containing only salts, sugars, and
biotin, which is a vitamin.

It is known now that Neurospora
can synthesize amino acids and produce

its own vitamins as well as proteins,
and form carbohydrates, fats, nucleic
acids, and other essential substances.
However, all of these processes require
specific enzymes. These, we know to¬
day, are related directly to DNA. The
fact that many enzymes function in a
series of metabolic processes has made
Neurospora an ideal subject for the
study of genetic mutations and the in¬
fluence of genes on enzyme production.
Altered genes and interrupted metabolic
process. Neurospora came into promi¬
nence in genetic studies when it was
used by two investigators at Stanford
University in important experiments in
the 1940’s. George W. Beadle and Ed¬
ward L. Tatum produced several muta¬
tions in Neurospora by exposing the
spores to X rays. After treatment the
spores were placed in the simple me¬
dium normally used to culture the mold.
Beadle and Tatum found that certain
of the treated spores could not grow in
the simple medium. Apparently a mu¬
tation had occurred that prevented these
spores from synthesizing certain sub¬
stances essential for growth. The ques¬
tion now was what substance or sub¬
stances could no longer be synthesized
and whether a single gene or several
genes had mutated in causing that par¬
ticular condition.

They next prepared a simple me¬
dium to which they added all vitamins
and essential amino acids. The spores
germinated and produced apparently
normal mold plants in this medium.
Spores produced by this mutant mold
supplied the investigators with material
to test the same mold in other nutrient
solutions.

At this point in the investigation,
Beadle and Tatum devised a method of
determining which one of 20 amino
acids the mutated Neurospora could no

CHAPTER 10 THE GENETIC MATERIAL 151

10-17 Dr. George Beadle (left) and Dr. Edward L. Tatum (right) were responsible
for producing mutations in Neurospora by exposing the spores to X rays.
(Left: University of Chicago; right: Rockefeller Institute)

longer synthesize. They prepared 20
tube cultures, each containing the sim¬
ple nutrient solution, vitamins, and a
different amino acid. Each culture was
inoculated with spores of the mutated
mold. Growth occurred in only one of
the 20 cultures. This was in the tube
containing the amino acid arginine.
This indicated that X rays had altered
or destroyed the gene or genes control¬
ling the synthesis of this essential amino
acid.

Biochemists had established earlier
that arginine is synthesized in a series
of chemical steps. This chemical se¬
quence begins with a prior substance
from which ornithine, a nitrogen-con¬
taining organic molecule, is formed. In
a second step, citrulline is produced

from ornithine. Arginine is in turn syn¬
thesized from citrulline.

The next question concerned the
relation of the damaged genes in the
mutated Neurospora to this series of
processes. Were all three steps nor¬
mally controlled by a single gene, or
was a different gene associated with
each step in the process? Beadle and
Tatum suggested that each step in¬
volved a different gene and that any or
all of these genes might be involved in
the mutation. We can diagram the
chemical steps leading to arginine and
the possible relation of genes to each
as shown below.

If any one or all three of these
genes had mutated, the mold could
grow if arginine were added to the cul-

gene A

gene B

gene C

prior

substance

■> ornithine

citrulline - » arginine

152 UNIT 2 THE CONTINUITY OF LIFE

Neurospora growing
in a simple
medium -F vitamins

No growth in simple
medium + vitamins

Simple medium,
vitamins, and all 20
amino acids added

No growth —
representing results
with 19 amino
acids (all but arginine)

Growth with simple
medium, vitamin,
and arginine

10-18 This is the technique used by Beadle and Tatum to detect a nutritional
mutant in Neurospora and to identify an amino acid which could not be syn¬
thesized.

ture. However, if only gene A had mu¬
tated, the mold could grow if ornithine,
citrulline, or arginine were added. If
gene B or both A and B mutated, their
normal functions could be bypassed by
adding citrulline or arginine. In the
event of a mutation in gene C, arginine
would have to be added, even if genes
A and B were functioning normally.
Beadle and Tatum tested these possi¬
bilities by adding the different com¬
pounds to the culture media of various
mutants. They found that their rea¬

soning had been correct that a different
gene was in fact involved in each step.

The work of Beadle and Tatum
with various mutated strains of Neuro¬
spora established the “one gene — one
enzyme” hypothesis of gene action. Ac¬
tually, it was an enzyme and not the
gene itself that was necessary for each
step in the series of reactions leading
to arginine. However, a gene was nec¬
essary for the synthesis of each enzyme.
Mutation destroyed the capability of a
gene to produce an enzyme, thus block-

CHAPTER 10 THE GENETIC MATERIAL 153

ing the synthesis of arginine. For their
brilliant work in investigating gene ac¬
tion, Beadle and Tatum were awarded
a share of the Nobel prize in medicine
and physiology in 1958.

Would it surprise you to learn that
a similar series of chemical changes
occurs in the cells of your liver? Here

ornithine is converted to citrulline,
which in turn is used in producing ar¬
ginine. Arginine is converted to orni¬
thine by reacting with NH3 + C02.
The urea also produced is excreted.
This series of changes occurs in what
biochemists refer to as the ornithine
cycle.

IN CONCLUSION

As you read of the achievements of great biologists in searching for the mate¬
rials of genetics you were, perhaps, aware of the narrowing of the search as
more and more discoveries were made. The spotlight has narrowed from cells
to nuclei to chromosomes to genes and, finally, to DNA and the bases that
provide its genetic code.

The years ahead promise even more exciting discoveries because men like
Sutton, Morgan, Bridges, Griffith, Muller, Beadle, and Tatum provided a firm
foundation for genetics in the future.

BIOLOGICALLY SPEAKING

genetic code
lethal mutation
mutant
mutation

autosome
chromosome map
crossing over
gene linkage

nondisjunction
sex chromosome
sex linkage
transformation

QUESTIONS FOR REVIEW

1. What logical conclusion led Weismann to propose that reduction division
occurred in the formation of eggs and sperm?

2. List three parallels Sutton found between Mendel’s hereditary factors and
the behavior of chromosomes and genes?

3. Explain the principle of gene linkage that Sutton proposed.

4. Account for the fact that all seven of Mendel’s contrasting traits sorted
out independently.

5. Describe the chromosome makeup of Drosophila.

6. Explain how sex chromosomes function in determining the sex of an or¬
ganism.

7. What is a sex-linked gene?

8. What characteristic did Morgan discover in the white-eyed Drosophila of
an F2 generation that led to his discovery of sex linkage?

9. Distinguish between sex chromosomes and autosomes.

10. Explain the mechanism of chromosomal nondisjunction.

154 UNIT 2 THE CONTINUITY OF LIFE

11. How does crossing over alter gene linkage?

12. What is a chromosome map?

13. What observation in Griffith’s work with pneumococcus led to the dis¬
covery of transformation?

14. Explain the relation of enzymes to gene action.

15. List several natural causes of mutations.

16. Distinguish between a somatic mutation and a germ mutation.

17. What energy source did Muller use to produce mutations in Drosophila
by radiation?

18. Describe three steps in the synthesis of arginine by Neurospora.

19. In what ways did X ray-induced mutations alter the Neurospora studied by
Beadle and Tatum?

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the significance of the gene hypothesis proposed by Sutton.

2. Review the work of Morgan and his associates in the discovery of sex
chromosomes and sex linkage.

3. Explain why nondisjunction is lethal in many offspring.

4. Discuss possible changes that might occur in a gene when it mutates in
terms of what you have learned about the genetic code.

5. Discuss two important genetic principles illustrated in transformation.

6. Discuss the importance of Muller’s selection of lethal sex-linked traits in
his studies of mutation in Drosophila.

7. How did the work of Beadle and Tatum with Neurospora support the
hypothesis of “one gene — one enzyme”?

8. Suppose that a mutant strain of Neurospora grew when arginine was
added to it. Outline the procedure you would follow to determine which
gene or genes had mutated.

CHAPTER II

GENES IN
HUMAN
POPULATIONS

The nature of human heredity. Un¬
doubtedly, as you have been reading the
many accounts of genetic investigations
in garden peas, guinea pigs, Drosophila,
pneumococcus, Neurospora, and other
organisms you have been thinking, do
these same laws and principles apply to
me? They do, of course, although we
probably know more about inheritance
in Drosophila than we do in humans.
There are several reasons for this. One
is the length of the human life span.
An investigator can study many gen¬
erations of Drosophila in a few months.
Or, in bacteria, he can observe many
generations in a week. But we do well
to see six human generations in a life¬
time. The number of individuals pre¬
sents another problem in studying hu¬
man genetics. A single mating in an¬

imals or a cross in plants may produce
hundreds of offspring. When you com¬
pare this rate of reproduction with the
limited size of human families, you can
see that the family represents a very
small sampling of genetic possibilities.

Perhaps the greatest problem in
studying human heredity is separating
the influence of heredity from that of
the environment. Look at a human
face. Perhaps a person has his father’s
ears and his mother’s nose. We might
account for his eye color and the color
of his hair in terms of genes, but don’t
cares and worries or satisfaction and
contentment leave their mark too? Sim¬
ilarly, genes may influence your height
and the general build of your body, but
so do your diet, your general health,
and the activity of your glands. We
know that glandular function is related
to emotional states and these in turn
reflect the conditions and events of your
environment. Thus, another set of fac¬
tors is introduced that complicates in¬
vestigations in human genetics. This
cannot be avoided since man is a think¬
ing, reasoning, responsive organism.

Another complicating factor in hu¬
man inheritance is the fact that most
people come from mixed ancestry. Few,
if any, human genetic traits are pure.
Each time a marriage occurs, the ge¬
netic backgrounds of two entirely dif¬
ferent families are combined in the off¬
spring.

With all of these complications,
however, geneticists have made extensive
investigations of human genetics, espe¬
cially in recent years. We know that
the almost limitless number of human
hereditary traits is produced by the ac¬
tion of an enormous number of genes
on the 46 chromosomes in every body
cell. When you consider the hundreds
of genetic traits expressed in Drosophila ,

155

156 UNIT 2 THE CONTINUITY OF LIFE

(

2 3

l i

l«

1 i

8 9,

K X

A *

6 A

& I *

1 4 1

•> x

h A

A A

ft K *

i «

19 20

21 22

14 15;

18! 19 20

21 22 y

11-1 Left: chromosomes of the human female; right: chromosomes of the hu¬
man male. The numbers are those used by geneticists to label chromosomes
for reference purposes. (Theodore T. Puck)

which has but four pairs of chromo¬
somes, you can get some idea of the
great variety of gene combinations we
might have with 23 chromosome pairs.
Is it any wonder, then, that among the
hundreds of millions of people living
in the world today there is no exact
duplicate of you? That is, of course,
unless you have an identical twin.
Population genetics. You will recall
that Mendel established his laws of
heredity by observing several genera¬
tions of garden peas. No single genera¬
tion could have supplied the data nec¬
essary to establish these principles.
When geneticists started to apply Men¬
del’s laws to human inheritance, the
problem of limited numbers arose.
Familv characteristics could be traced
from children to their parents and
grandparents. Aunts, uncles, and other
close relatives could be used. However,
all of these individuals represented a
small sampling. A family history might
extend a study, but except for certain

easily recognized traits, data concerning
ancestors were usually vague and unre¬
liable.

Has this thought ever occurred to
you? You have two parents, four grand¬
parents, eight great grandparents, six¬
teen great great grandparents, thirty-two
great great great grandparents, and so on
in numbers that double each generation.
Each of these marriages united two dif¬
ferent families. You may be surprised
when vou realize what a large segment
of the population you may claim as
distant relatives!

There is a better method of study¬
ing the frequency of genetic traits and
predicting how often they will appear
than attempting to trace them through
families. Rather than extending the
time to involve many generations, broad¬
en the study to include a much larger
sampling of a population at a given time.
In other words, determine how fre-
quentlv given genes appear in a popula¬
tion. From this information it is pos-

CHAPTER 11 GENES IN HUMAN POPULATIONS 157

sible to predict the probability of ap¬
pearance of any given genetic trait in off¬
spring. Population genetics, as we call
this study, is based on determinations of
gene frequencies.

Sampling a population. From time to
time you read the results of an opinion
poll in your newspaper. Based on a
sampling of the population, the poll
may predict the election of one candi¬
date for political office and the defeat
of another. It will even predict the
margin of votes for the winning candi¬
date. You may have been amazed at
the accuracy of these polls. How can
the results of the sampling of a rela¬
tively small percentage predict the opin¬
ions of the population of the entire na¬
tion? The accuracy of the prediction
depends on the selection of a represent¬
ative group of people in conducting
the poll. The national population in¬
cludes people of all ages, educational
backgrounds, professions, trades, busi¬
nesses, social backgrounds, races, cul¬
tural backgrounds, and economic levels.
Our sampling, then, must include peo¬
ple from all of these groups and, of
course, approximately equal numbers of
men and women.

Now, let us apply this method of
sampling to a genetic survey you might
conduct in your class. The trait we
will use will be the ability to taste
phenylthiocarb amide ( fenl-thy oh -kar-
ba-myde) [PTC] on a strip of test paper.
This substance is extremely bitter to
some individuals referred to as “tasters,”
but has no flavor to a “nontaster.” The
ability to taste PTC is due to a dom¬
inant gene T. Thus a taster may be
homozygous (TT) or heterozygous
(' Tt ). A nontaster must be a homo¬
zygous recessive (tt). Now, let us as¬
sume that 24 members of your class
ask 25 people to taste PTC papers.

This would provide results from a total
of 600 people. The poll should include
people of various ages and both sexes
to provide a representative sample. If
possible, include people of different
races and geographic and national back¬
grounds.

Let us assume that 430 people
tested are tasters and 170 are nontasters.
If we divide the number of tasters, 430,
by the total number of people surveyed,
600, we find the result to be 0.716, or
about 72 percent. The number of non¬
tasters would then be about 28 percent
of the people tested. This result would
be near the national average, which has
been determined to be 70 percent tast¬
ers and 30 percent nontasters.

Having determined the number and
the percentage of tasters and nontasters
in a sample of a population, it is a sim¬
ple matter to establish the probable
number of each in the total population.
Let us assume that the population of
your community is about 120,000, so
that each person tested in your survey
would represent 200 in the total popu¬
lation. Thus you would expect 86,000
people in the community to be tasters,
while 34,000 would be nontasters.

Another interesting inherited trait
which can be sampled in a population
easily is tongue rolling. Some people
can roll their tongues into a U-shape
while others cannot. Since the gene
for this characteristic is dominant, a
tongue roller may be homozygous (RR)
or heterozygous (Rr). Those who can¬
not roll their tongues are homozygous
recessives (rr). Since tongue rolling in¬
volves a single pair of genes with dom¬
inance, you would expect a survey of
a population for this trait to yield re¬
sults similar to the PTC taster study.
The gene pool. The geneticist refers to
all the genes present in a given popula-

158 UNIT 2 THE CONTINUITY OF LIFE

11-2 The ability to roll the tongue is genetically determined. The character¬
istic is dominant, so that a tongue roller may be either homozygous or hetero¬
zygous for the trait. (A. M. Winchester)

tion as the gene pool. We might con¬
sider that any individual is a random
sample of the genes present in his par¬
ticular population. How frequently
will a particular characteristic appear in
a population? This will depend on the
frequency in the population of the gene
associated with that characteristic.

To return to our survey of tasters
and nontasters of PTC, the fact that
both of these characteristics appear in
a population is evidence that both dom¬
inant genes (T) and recessive genes
(£) are present. Furthermore, if you
consider the national ratio of 70 percent
tasters to 30 percent nontasters, you will
see that this approximates a three
fourths to one fourth ratio character¬
istic of a monohybrid cross with domi¬
nance. We might assume, then, that
about one fourth of our population
would be homozygous tasters (TT),
one half heterozygous tasters {Tt), and
one fourth homozygous non tasters (tt).
This would indicate that the genes T
and t are present in about equal num¬
bers.

Might there be populations in

some parts of the world where only the
dominant gene is present in a popula¬
tion? All members of such a popula¬
tion would be tasters. Similarly, other
populations might be entirely recessive
for the trait.

As we examine various popula¬
tions, we can see evidence of a high fre¬
quency of certain genes. The Ameri¬
can Indian, for example, has character¬
istic dark, straight hair and facial fea¬
tures resembling the mongoloid peoples
of northeastern Asia to whom anthro¬
pologists believe they are related. Fol¬
lowing the migration of the Indians to
North America perhaps 15,000 to 20,-
000 years ago, various culture areas were
established from Alaska throughout
North America and into Central Amer¬
ica and South America. Certain gene
pools were established in these culture
areas and even in the tribes composing
each one. The Eastern Woodland In¬
dians had certain characteristics that
distinguished them from Plains Indians
or Southwest Farmers and Herders.
The Eskimos of the Far North had
characteristics that distinguished them

CHAPTER 11 GENES IN HUMAN POPULATIONS 159

11-3 Identical twins such as
these girls have the same
genetic makeup, while fra¬
ternal twins are usually no
more alike than ordinary
brothers and sisters. (Stick¬
ler from Monkmeyer)

from the Northwest Fishermen along
the Canadian Pacific Coastal region.
Similarly, the African Pygmy of the Bel¬
gian Congo area is quite a contrast to
the giant Watusi tribesman of the Lake
Victoria region.

Although certain genetic traits may
predominate in a population at a given
time, gene pools change as populations
shift. New gene combinations occur.
These may appear as variations in skin
color, hair color and texture, eye color,
height and body build, and blood type.
The more people that move from one
geographical area to another, the more
the gene pool changes.

Genetic studies of twins. Having con¬
sidered the frequency of genes in human
populations, let us now consider the ex¬
pression of genes in individual members
of a population. As we mentioned
earlier, it is difficult to distinguish be¬
tween genetic and environmental influ¬
ence on certain traits in humans. In an
effort to clarify the distinction, genet¬
icists have made studies of twins.
Twins are of two types. Fraternal
twins, the more common type, are two
entirely different individuals. Often
they are brother and sister. They de¬

velop from separate eggs which are fer¬
tilized by different sperm. They are no
more alike genetically and no more
closely related than other brothers or
sisters in a family. Most fraternal
twins live in similar environments.
Yet they may be totally different in
physical characteristics, personality,
emotional make-up, and mental ability.
These variations are valuable in help¬
ing to determine which characteristics
are hereditary and which are environ¬
mental.

Identical twins , on the other hand,
are nearly the same person in duplicate.
They started life as a single fertilized
egg. But after the first division, the
two cells separated and started growth
over again (Fig. 11-4). Sometimes this
happens several times, resulting in iden¬
tical triplets, quadruplets, or quintu¬
plets.

Having started life as the same
cell, identical twins have identical ge¬
netic make-up. Consequently, the sim¬
ilarities in identical twins indicate, for
the most part, characteristics controlled
by genes. Identical twins show a
marked likeness not only in appearance,
but in temperament, abilities, likes and

160 UNIT 2 THE CONTINUITY OF LIFE

One fertilized egg

Two cells separate

and then
7 each develops 7

Two girls
or

Two boys

IDENTICAL TWINS

Two fertilized eggs occur in the mother

at the same time

Each develops

separately
into an individual

any other combination

FRATERNAL TWINS

11-4 After having studied this diagram you will know that identical twins de¬
velop from a single egg, while fraternal twins develop from two fertilized eggs.

dislikes, and many other personality
traits.

Several studies have been made of
identical twins who were separated
earlv in life and reared in different en¬
vironments. In these instances, home
and family life, education, day-to-day
experiences, friendships, and other en¬
vironmental influences leave their mark
on the personality of each. But the two
persons remain amazingly alike in ap¬
pearance, basic personality traits, and
capacitv for learning. While they may
varv somewhat in intelligence as de¬
termined by scores on tests, these differ¬
ences indicate the influence of different
environments rather than changes in

basic capacity for learning. Unfortu¬
nately, the opportunities for studies of
identical twins are very limited.

Genes in human populations — inherit¬
ance of blood type. Having discussed
various methods used in the study of
human genetics, we are now ready to
consider a few hereditary traits in hu¬
mans and the mechanisms involved in
their inheritance. One such inherited
trait is your blood type.

In 1900, while working in a medical
laboratory of Vienna, Dr. Karl Land-
steiner made an important discovery in
a study of human blood. He found that
the mixing of blood from certain people
resulted in the clumping, or agglutina-

CHAPTER 11 GENES IN HUMAN POPULATIONS 161

tion, of red corpuscles. In further in¬
vestigations he found that the red cor¬
puscles of various people differed in
the presence of a protein substance on
the corpuscle surface. This substance,
which we know as an agglutinogen , is
one of two types. These were desig¬
nated as type A and type B. Some cor¬
puscles have one or the other, while
some have both and still others have
neither. Thus, we may designate all
human blood as type A, B, AB, or O.

In Chapter 42, when we study the
composition of blood, we shall deal with
the reaction that occurs when certain
different blood types are mixed. At
present, we are concerned with the in¬
heritance of blood type. Three genes
are involved in this inheritance. We
refer to them as multiple alleles, since

more than a single pair of genes is in¬
volved in determining the characteristic.
However, even though three genes are
associated with blood type, only two are
present in any single individual. A
gene we may designate as A produces
type-A corpuscles, gene B results in
type-B corpuscles, and gene O does not
produce either agglutinogen on the cor¬
puscles. Various combinations of pairs
of these three genes results in the four
human blood types as follows:

Genes AA or AO produce type-A cor¬
puscles

Genes BB or BO produce type-B cor¬
puscles

Genes OO produce type-O corpuscles
(no agglutinogen)

Genes AB produce type-AB corpuscles
(both agglutinogens)

11-5 Here a laboratory technician does A-B-0 blood grouping on samples of
blood collected in the Red Cross Blood Program. (American Red Cross)

162 UNIT 2 THE CONTINUITY OF LIFE

In order to identify the three genes
involved in corpuscle types as alleles,
geneticists write the dominant as I and
the recessive as i, and designate the ag¬
glutinogens they produce as IA, P, and
i. Using this system, then, type A
may be homozygous IAIA or heterozy¬
gous IAi. Type B may be homozygous
PP or heterozygous Pi. Type AB is
always heterozygous PP, while type O
is always homozygous ii. Determi¬
nation of results in crossing individuals
with different blood types is easily done
with a grid such as you used for a
monohybrid cross. A cross in which
one parent is heterozygous type A
(IAi) and the other is heterozygous
type B (Pi) is shown below.

RESULTS OF CROSSING
IAi AND IBi

Female

GENES

Male

IAIB

IAi

IBi

Notice that all four blood types
may result in a cross such as this. The
ratio of probabilities indicates that V\
of the offspring should be type A, 14-
type B, V4 type AB, and 14 type O.
This would be an unusual condition in
a family, but it could happen.

In recent years anthropologists
have conducted surveys of blood types
among populations of the entire world.
We have known the frequency with
which these blood types appear in our
own populations for many years. How¬
ever, it is only in recent years that we
have known how widely blood type fre¬
quencies differ in world populations.

Such variations occur both among ra¬
cial groups and in populations in differ¬
ent geographic regions. The following
table shows certain of these variations.
Inheritance of the Rh factor. Many
years after the discovery of the A-B-O
blood types, Landsteiner and Wiener,
an associate, discovered another protein
substance in the red corpuscles of a
Rhesus monkey. The protein was
named the Rh factor for the monkey.
Later it was found that 85 to 87 percent
of the human population in the city of
New York had corpuscles containing
the Rh factor. They are designated as
Rh positive. People who lack the fac¬
tor are Rh negative. Serious complica¬
tions result when Rh-positive blood is
mixed with Rh-negative blood that has
been sensitized against the factor, as
you will learn in Chapter 42. At pres¬
ent we are concerned only with the in¬
heritance of the factor.

At first it was thought that the Rh
factor was a simple trait, present in some
people and lacking in others. It was
also thought to be controlled by a sin¬
gle pair of alleles in which the gene for
the factor was dominant. However, we
now know that there are four or more
Rh factors believed to be determined
by as many as four pairs of alleles. For
this reason it is very difficult to diagram
the inheritance of various Rh factors.
You may be interested in knowing that,
of the several Rh factors that have been
discovered, only one, designated as the
Rhd factor, is of clinical importance.
Sex-linked genes in humans. Perhaps
the best known sex-linked character in
humans is red-green color blindness.
While it rarely occurs in females, it ap¬
pears in approximately 8 percent of the
male population. People with this con¬
dition cannot easily distinguish red and
green, both colors appearing as shades

CHAPTER 11 GENES IN HUMAN POPULATIONS 163

BLOOD TYPE PERCENTAGES IN VARIOUS REGIONS OF THE WORLD

Blood Type

A B

U.S.A.-white

41.0%

10.0%

4.0%

45.0%

U.S.A.-Negro

26.0%

21.0%

3-7%

49.3%

Swedish

46.7%

10.3%

5.1%

37.9%

Japanese

38.4%

21.8%

8.6%

31-2%

Hawaiian

60.8%

2.2%

0.5%

36.5%

Chinese

25.0%

35.0%

10.0%

30.0%

Australian aborigine

44.7%

2.1%

0.0%

53.1%

North American Indian

7.7%

L0%

0.0%

91.3%

of gray. The fact that it appears more
frequently in males than in females
would indicate that it is sex-linked.
The genes associated with red-green
color vision are located on the X chro¬
mosome. Furthermore, the gene for
normal color vision is dominant. Thus,
we may indicate various gene combina¬
tions as follows, using C for normal red-
green color vision and c for color-blind¬
ness:

XCXC, a normal female, homozygous for
color vision

XCXC, a carrier female, heterozygous
but with normal vision
XCXC, a color-blind female, homozygous
for color-blindness

XCY, a normal male with a single gene
for color vision

XCY, a color-blind male with a single
gene for color-blindness
By using a grid we can show how a
mother who is a carrier of color blind¬
ness but who has normal vision might
have a son who is color blind, even
though the father has normal vision.

The results of this cross indicate
that lA of the offspring would be nor¬
mal females, lA carrier females, lA nor¬

mal males, and lA color-blind males.
Perhaps you can determine a cross in
which a color-blind female might re¬
sult.

Hemophilia (hee-moh-/ee-lee-a),
or “bleeder’s disease,” is another sex-
linked character similar to color blind¬
ness in inheritance. Hemophilia is a
condition in which a blood substance
necessary for clotting is not produced
because of the lack of the gene neces¬
sary for its formation. Because of the
lack of this clotting substance, victims
of hemophilia bleed severely and may
even die from loss of blood as a result
of wounds which would be slight in a
normal person.

INHERITANCE OF COLOR
BLINDNESS

Female

SEX CHROMO

SOMES

Male

XCX°

XCXC

XCY

164 UNIT 2 THE CONTINUITY OF LIFE

11-6 The gene for color blindness is indicated in color, and the chromosome
that carries it is designated as X'. Notice that two such chromosomes are neces¬
sary for a color-blind female while only one is necessary in the case of a color¬
blind male.

Hemophilia tends to run in fam¬
ilies and to appear in males. It can ap¬
pear in a female only when the father
has hemophilia and the mother is a car¬
rier. Several genetic studies have been
made of families in which hemophilia
has occurred frequently. One of the
most famous of these is the study of the
family of Queen Victoria, shown in
Fig. 1 1—7. As you examine this family
history, notice how many males were
hemophiliacs. Then study the chart
and note how many females were car¬
riers.

Other characteristics associated with
sex. Baldness is an example of a human
sex-influenced trait. The gene for bald¬
ness is dominant in males but recessive
in females. Thus, a mother may trans¬
mit baldness to her son without show¬
ing it herself. If we represent the gene
for baldness as B and the one for nor¬
mal growth of hair as h, then a Bb male
would be bald, while a Bb female would
not be. However, BB would represent
a male or a female with baldness, while

bb would produce a normal male and
female.

Another type of inheritance, which
has been investigated in birds more
thoroughly than in humans, involves
sex-limited traits. In this type of in¬
heritance certain genes produce a char¬
acter in one sex or the other, but not in
both, even though they are carried in
both sexes. It appears that sex-limited
characters appear only in the presence
of sex hormones.

One example of a sex-limited char¬
acter is the bright plumage of certain
male birds, which does not appear in
females of the same species. The roost¬
ers of most breeds of chickens develop
a large comb and wattles and character¬
istic male plumage, while hens of the
same breed develop a different kind of
female plumage. Both result from the
action of genes in the presence of sex
hormones.

In humans similar sex-limited
genes mav determine such character¬
istics as the growth of a beard. A son

CHAPTER 11 GENES IN HUMAN POPULATIONS 165

Queen Victoria
1837-1901

O >

3 Q.

• •■fCinCrl# Crli ■

m 2, x

s <
*< —

” M -O Z3

a cu -T -i

° cr c *< A

— - CD on S

! Jr

CD C13 CO

-t D 7 » -

-- ^ Q. </> “

CD - r-*

■ 6666a •□a

M CD
a> cd
cr 3

NORMAL (FEMALE)
^ CARRIER (FEMALE)

■ NORMAL (MALE)

HEMOPHILIA (MALE)

— ■ CD

11-7 The English royal family of Windsor (the family name of Queen Elizabeth)
has an interesting hereditary trait — hemophilia.

might inherit a beard characteristic
from his mother, even though she did
not develop the characteristic because
of the lack of male hormones.
Inheritance of eye color and skin color.
The inheritance of eye color is governed
by multiple alleles. Blue eyes are
produced by a single pair of recessive

genes. The genes producing brown
pigment are dominant. The various
shades of iris pigmentation from hazel
to light brown or dark brown are ap¬
parently the result of the expression of
varying numbers of genes for brown pig¬
mentation. Other genes for pigmenta¬
tion produce shades of gray ranging to

166 UNIT 2 THE CONTINUITY OF LIFE

green. Loss of the eye pigment genes
resulting from mutations produces the
pink eye of the albino.

Skin color is thought to be deter¬
mined by several pairs of genes. Some
geneticists suggest that two pairs of al¬
leles are involved. Others believe that
four to perhaps as many as eight pairs
of genes may function. In any case,
the color of the skin from dark brown,
medium brown, light brown, and white
skin are the result of the expression of
varying numbers of genes for skin pig¬
mentation. Other pigment-producing
genes are responsible for the yellow skin
of oriental peoples and the reddish or
bronze skin of many Indians.

Are diseases inherited? The question of
whether diseases are inherited is of
great importance to the geneticist be¬
cause of its medical aspect. Many ge¬
neticists have explored the possibility
that certain diseases that appear to run
in families may be related to genes. In
infections such as tuberculosis, the dis¬
ease itself cannot be inherited. How¬
ever, evidence seems to indicate that
chemical conditions in the body tissues
important in resistance against infec¬
tions may be inherited. Lack of this re¬
sistance would increase the possibility
of contracting the disease sometime
during one’s life. However, proper pre¬
cautions can prevent tuberculosis.

On the other hand, diseases result¬
ing from abnormal structure or func¬
tion of body organs are more likely to
be hereditary. Such a disease is sugar
diabetes ( diabetes mellitus) in which
certain cells of the pancreas fail to se¬
crete sufficient insulin, a condition we
shall discuss more fully in Chapter 44.
Diabetes has been found to occur fre¬
quently in certain families. It is
thought to be due to a recessive gene.
However, the disease is not equally seri¬

ous in all people. Furthermore, regu¬
lation of the diet and body weight may
prevent or arrest the condition even
when the genes are present. There is
also evidence that multiple factors may
be involved in the inheritance of dia¬
betes. If this is the case, the serious¬
ness of the disease might be determined
by the number of genes present.

Case history studies of human fam¬
ilies indicate that many other human
diseases and disorders may be definitely
associated with genes. Among these
are respiratory allergies, asthma, bron¬
chitis, nearsightedness, farsightedness,
and night-blindness.

Is intelligence inherited? To what ex¬
tent is intelligence related to genes?
This is a very difficult question to an¬
swer since no one has yet given an ade¬
quate definition of intelligence. We
find many families in which parents
and children seem to be highly intelli¬
gent. Is this a genetic quality, or is it
the product of the home, the school,
and other environmental influences?

One of the standard methods of
measuring intelligence has been a test
that involves reasoning, memorizing,
calculating, visualizing, word recogni¬
tion, word usage, and other funda¬
mental thought processes. The score
on such an intelligence test is used to
determine the mental age. Intelligence
is then determined by dividing the
mental age by the chronological age, or
age in years. The result, multiplied by
100, is the intelligence quotient, or I.Q.
An average I.Q. is considered to be 100.

There is some evidence that intelli¬
gence is related to genes. Geneticists
believe that many alleles must be in¬
volved. Identical twins provide the
best evidence of such a genetic influ¬
ence. The similarity between their in¬
telligence test results is striking. There

CHAPTER 11 GENES IN HUMAN POPULATIONS 167

is much less similarity of results be¬
tween fraternal twins, and still less
among other brothers and sisters. The
decrease continues as relatives become
more distant. Thus, genes and gene
combinations, yet undiscovered, must
at least share with environmental influ¬
ences in determining intelligence.
Inheritance of mental disorders. Ex¬
tensive genetic studies have supplied
evidence that several kinds of mental
deficiency have a definite genetic basis.
Among these are forms of arrested men¬
tal development resulting in idiocy.
Such deficiencies may be the result of
complex chemical disturbances or the
lack of certain enzymes associated with
mental development. In some cases,
these disturbances are the result of mu¬
tations.

In recent years biologists have
found the genetic basis for Mongolian
idiocy. This unfortunate condition oc¬
curs in about one birth in one thou¬
sand in mothers under 35 years of age,
but increases to 20 to 30 per thousand
(2 to 3 percent) in mothers over 45
years of age.

The genetic basis for Mongolian
idiocy was discovered, probably by acci¬
dent, in studies of human chromo¬
somes. You will recall that the normal
chromosome number in humans is 46.
However, biologists found that the
body cells of some individuals con¬
tained 47 chromosomes, while others
had only 45. When the body cells of
Mongolian idiots were examined, there
were always 47 chromosomes present,
as shown in Fig. 11-8. Mongolian idi-

11-8 Nondisjunction in Mongolian idiocy. Compare this photograph with
Fig. 11-1 and determine the difference in chromosome content. (D. H. Carr and
M. L. Barr)

168 UNIT 2 THE CONTINUITY OF LIFE

ocy, then, seems to result from the pres¬
ence of an extra chromosome. This ex¬
tra chromosome is the result of nondis¬
junction during meiosis, a phenomenon
you studied earlier in a vermilion-eyed

female Drosophila.

Another type of arrested mental
development, known as phenylpyruvic
(fenl-py-roo-vik) idiocy , is due to the
lack of an enzyme. As you learned ear¬
lier, genes regulate the production of
enzymes. Enzymes in turn regulate
the production of amino acids. If a
gene or an enzyme in any chemical se¬
quence is lacking, the chain of reactions
cannot occur. Phenylpyruvic idiocy is
due to the inability of the individual to
convert the amino acid phenylalanine
to a similar amino acid, tyrosine. The
condition is inherited as a simple reces¬
sive trait. The enzyme necessary to
convert phenylalanine to tyrosine is
present in the liver of normal individ¬
uals. Those who lack the enzyme can¬
not oxidize phenylalanine during nor¬
mal metabolism. Thus, these individ¬

uals excrete the unoxidized amino acids
from the body in a condition called
phenylketonuria , or PKU. The inherit¬
ance of phenylpyruvic idiocy is further
evidence for the one gene-one enzyme
concept of gene action. It also illus¬
trates the vital role of enzymes in
growth and development, since only
one pair of genes and the lack of one
enzyme prevents the phenylpyruvic
idiot from developing as normal.

Geneticists have much to learn
about the relation of genes to mental
deficiencies. Is there some genetic lack
in the more than three million feeble¬
minded people in the United States to¬
day? Are certain mental illnesses re¬
lated to genes? Manic-depressive psy¬
chosis is thought to be hereditary, at
least to some degree. It seems to be re¬
lated to a dominant gene, although the
presence of the gene may merely pro¬
duce a tendency which can be avoided
by environmental influences. Simi¬
larly, a tendency to develop schizophre¬
nia may be associated with genes.

IN CONCLUSION

“Your genes are showing’’ might be an appropriate concluding statement to a
study of human genetics. They do show in many traits and features. 1 o-
gether with environmental influences, genes have been at work through the

years, determining what kind of a person you are.

Human genetics is of great importance in many aspects of human lite
True you cannot alter your genes, but you can make the most of your in eri
ance.’ Few people actually achieve this goal. From another standpoint, our
knowledge of human genetics has opened a new door for medical contribution
to humanity. “Living with genes” may mean compensating for some genetic
lack or problem. It might mean supplying a missing substance in the body
chemistry or preventing the development of an undesirable genetic trai or

problem, such as diabetes. . . , . .

We now shift our study to the application of the principles of genetics t

scientific plant and animal breeding. In this area of applied genetics we have
made astounding progress on our farms and ranches, our orchards, gardens, an

vineyards.

CHAPTER 11 GENES IN HUMAN POPULATIONS 169

BIOLOGICALLY SPEAKING

agglutinogen identical twins Rh factor

fraternal twins multiple alleles sex-influenced trait

gene frequency population genetics sex-limited trait

gene pool

QUESTIONS FOR REVIEW

1* List several of your physical characteristics you believe to be produced by
gene action.

2. Generally, what is population genetics?

3. How may PTC papers be used as a basis for sampling a population?

4. What is a gene pool?

5. When is a gene pool considered stable?

6. Distinguish between fraternal twins and identical twins.

7. Explain how A and B red corpuscle agglutinogens establish four basic hu¬
man blood groups.

8. What are multiple alleles?

9. In what way does the inheritance of blood type involve multiple alleles?

10. Generally, what are the Rh factors?

11. Why is color blindness more frequent in males than in females?

12. In what respect might we consider a gene for hemophilia a lethal gene?

13. Distinguish between sex-influenced and sex-limited traits, giving an exam¬
ple of each.

14. Explain how the inheritance of eye color involves multiple alleles.

15. Give several examples of a possible genetic relation to human diseases.

16. Explain how an intelligence quotient is determined.

17. What chromosome abnormality is believed to be the cause of Mongolian
idiocy?

18. What specific biochemical deficiency is believed to cause phenylpyruvic
idiocy?

APPLYING PRINCIPLES AND CONCEPTS

L Discuss environmental influences as complicating factors in the study of
human genetics.

2. Discuss the limitations of genetic studies in individual families.

3. How is random sampling applied to studies in population genetics?

4. Discuss various reasons for the shifting of genes in a gene pool.

5. How may identical twins supply valuable data in the study of human
genetics?

6. Discuss various problems in establishing proof of gene action in producing
intelligence.

7. How does phenylpyruvic idiocy bear out the one gene-one enzyme hy¬
pothesis?

CHAPTER 12

APPLIED

GENETICS

Plant and animal breeding — a time-
honored practice. Selective breeding of
plants and animals is an old practice.
For centuries man has made a constant
effort to improve the varieties of plants
and animals that supply his daily needs.
Wheat was grown as a cereal crop by
the early Egyptians. Flowers, fruit
trees, fowl, sheep, goats, and cattle have
been bred for domestication longer than
recorded history.

The science of genetics originated
largely to explain the results of plant
and animal breeding. Breeding was a
practice of chance selection rather than
scientific application of principles.
With the development of genetics as a
science, established laws have greatly
improved the efficiency of the process.
We must remember, however, that the
breeding of a plant or animal involves
the crossing of not one or two, but
hundreds of different characteristics.

While the laws of heredity apply, the
results are not always as predictable as
they were for Mendel’s seven traits in
peas.

Luther Burbank, the genius of Califor¬
nia. Plant breeding will always be as¬
sociated with the genius of Luther Bur¬
bank. He produced many new and dif¬
ferent plants on his farm in California.

Burbank’s brilliant work began in
the summer of 1871 in his native Massa¬
chusetts. While a young man he was
examining a crop of potatoes one day
and happened to notice a fruit matur¬
ing on one of the plants. This was an
unusual occurrence because the potato
plant flowers regularly but seldom bears
fruit. New plants are grown from cut¬
tings rather than from actual seeds.
Burbank saved that fruit. When the
seeds ripened he planted each one in
a separate hill. After the plants ma¬
tured, he dug up the potatoes and dis¬
covered that those from each plant were
different. Some were large, some were
small. Some hills had many tubers,
while others had only a few. One hill
had far better potatoes than any of the
others. These were large, smooth, and
numerous. Burbank sold them to a
gardener for $150 — his first profit from
plant breeding. They were named
Burbank potatoes in his honor, and
were the first of a strain that was des¬
tined to become popular all over the
country.

With the profit from his first
achievement, Burbank bought a ticket
to California, where he established the
farm that made him famous. From
his experimental gardens came such
varieties as the Shasta daisy and a new
strain of poppies. By combining vari¬
ous fruits, he produced the plumcot,
pitless plum, and the improved peach
plum. Another of his famous develop

170

CHAPTER 12 APPLIED GENETICS 171

12-1 Luther Burbank — the genius of plant
breeding. He is probably holding one of the
many root crops which his work did so much
to improve. (Bettmann Archive)

ments is the thin-shelled walnut. The
spineless cactus, used as fodder for cat¬
tle, is still another of his achievements.
Objectives in plant breeding. The
plant breeder has several purposes in
producing new strains or varieties of
plants. One of the chief objectives is
the production of more desirable vari¬
eties. Such characteristics as large fruit,
large and abundant seeds, vigorous
growth, early maturation of fruit, large
leaf area in leafy vegetables, and vigor¬
ous root growth in root crops are highly
profitable. Plant breeders work con¬
stantly to improve the quality and quan¬
tity of the yield of all crop plants. In
addition to the nature of the yield, re¬
sistance to disease is highly important.

Plant breeders also have been able to
produce many varieties of disease-
resistant crops.

A third objective is an extension of
crop areas through the production of
new varieties. Wheat is an example of
this extension through plant breeding.
Varieties of spring wheat grow well in
the northern sections of the nation,
while winter wheat favors the climate
of the central states. Wheat growing
has been extended even to the Great
Plains by the production of varieties of
hard wheat. In a similar way other
crops that were once limited to small
areas because of climatic requirements
or soil conditions have now been ex¬
tended to many other regions in the
form of new varieties.

Plant breeders have even been able
to develop entirely different kinds of
plants. The new plant may be the re¬
sult of crossing two strains or varieties
or two closely related species.

Obtaining desirable varieties by mass
selection. As the name implies, mass
selection consists of the careful selec¬
tion of parent plants from a great num¬
ber of individuals. Burbank practiced
mass selection when he discovered his
famous potato. He selected the most
ideal plant from all those he grew from
seed. The farmer who selects seed
from his own crop always picks the
most desirable plants for propagation.
Thus he takes advantage of any natural,
desirable variations that occur.

Mass selection is important, too,
in the production of disease-resistant
strains of plants. To show how mass
selection operates, let us assume that a
cabbage disease has swept into an area,
resulting in the destruction of almost
all the crop. As we examine the acres
of diseased plants, we find two or three
plants that, because of some unknown

172 UNIT 2 THE CONTINUITY OF LIFE

12-2 One way to control the deadly blue
mold of tobacco is by breeding disease-re¬
sistant strains. Here a technician is shown
putting pollen from a wild species, which is
immune to the disease, on the pistil of a
cultivated variety, which is susceptible.
(USDA)

sired results than selection of natural
variations.

In hybridization, characteristics of
two unlike but closely related parents
are combined in a new individual by
artificial cross-breeding. In getting a
new hybrid strain, we might choose one
parent because of vigorous growth. The
other one might be selected because of
the fine quality of its fruit or flower.
Often a hybrid possesses qualities not
shown in either parent, because of a new
combination of genes. This hybrid
vigor may include increased size, fruit¬
fulness, speed of development, and re¬
sistance to disease, insect pests, and
climatic extremes.

Line breeding — the opposite of hybrid¬
ization. After the desired characteris¬
tics have been obtained by mass selec¬
tion or hybridization, the next step is to

variation, have withstood the disease.
We carefully select the seeds from these
plants and sow them the following sea¬
son. Disease again strikes the crops,
but a few more plants remain than the
year before. These plants have in¬
herited the favorable genetic trait from
the parent. Again, we use these plants
for seed in the following season. Each
year more and more plants withstand
the disease. The genetic trait for dis¬
ease resistance, present in the original
plants, has become more common in
the offspring as they have been selected
and allowed to breed. Finally an en¬
tire strain is developed in which this
character appears.

What is outbreeding? We define out-
breeding as the crossing of two different
varieties to obtain a new one. This is
also called hybridization. It is like
mass selection in some ways because de¬
sirable traits are always sought. But it
is a more rapid way of getting the de-

12-3 Hybrid vigor is expressed in hybrid
corn. Note the large size of the plants and
the great yield. (USDA)

CHAPTER 12 APPLIED GENETICS 173

propagate these new and different
plants. This is a simple matter when
vegetative multiplication is involved,
because such asexual reproduction does
not involve the recombination of genes.

After Burbank had discovered his
potato, propagation was simple. He
used cuttings from the potato in order
to produce more plants exactly like the
parent. Had he been forced to grow
more potatoes from seed, the situation
would have been quite different because
chromosomes from two parents would
have been involved. In the same way,
the grower can propagate a new variety
of apple, peach, iris, or rose by grafting,
cutting, or budding without altering the
hereditary make-up.

In plants that are propagated by
seeds, like corn and wheat, the problem
is more difficult. Seed production in¬
volves a mixing of numerous characters.
Plants produced from seeds are not nec¬
essarily like the parent, especially when
they are crop plants that have been
crossed by man for centuries.

This difficulty can be overcome by
generations of line breeding , or inbreed¬
ing. This is the opposite of hybridiza¬
tion, or outbreeding. Self-pollination is
carried on to avoid introducing any new
characteristics from a new plant. Seeds
resulting from self-pollination are
planted and all individuals of the new
generation are carefully sorted. Only
those with the desired characteristics of
the parent are selected as seed plants for
the next generation. Again, self-pollina¬
tion is carried on, after which the re¬
sulting plants are sorted with the great¬
est possible care.

As you repeat this breeding method
generation after generation, more and
more plants bear the desired character¬
istics. Eventually a pure strain that will
be true to seed is established and is ready

for the market. Even then all plants
may not produce the pure-strain char¬
acteristics, but these individuals can be
readily sorted out.

The production of hybrid corn. Years
ago a farmer saved some of the best ears
from his corn crop as seed for the next
year. By the process of mass selection
he tried to produce more corn like his
best plants of the previous season. But
the plants that bore these ears were so
mixed in their heredity that only some
of the kernels bore the genes that had
made them productive. And with no
control over pollination, the farmer had
no idea about the quality of the other
parent. The seeds on a single ear might
produce many different varieties of corn,
some good and some poor. Some of the
kernels might have resulted from self-
pollination, while others were the result
of cross-pollination from fields some dis¬
tance away. It was not unusual to find
ears of corn with a mixture of yellow,
white, and red kernels. Often sweet
corn and popcorn were mixed with ker¬
nels of field corn. Under conditions
like these a yield of 20 to 40 bushels per
acre was all that could be expected.

Today hybrid varieties are produced
by scientifically controlled artificial pol¬
lination. When two varieties of corn
are cross-pollinated, the offspring are
usually more vigorous than either parent.
One of the reasons for this seems to be
that the desirable dominant characteris¬
tics from both parents mask many of the
undesirable recessives. The hybrids pro¬
duced today yield from 100 to 180 bush¬
els of corn per acre. These hybrids have

large root systems, sturdy stalks, broad
leaves, and large ears. Today the need
is for full, long ears.

Much of hybrid corn planted today
is the result of a double-cross in which
four pure-line parents are mixed in two

174 UNIT 2 THE CONTINUITY OF LIFE

crosses. Each pure-line parent has been
selected because of its vigor, resistance
to disease, or some other desirable trait.
Plants resulting from the double-cross,
however, are superior to any of the par¬
ent strains.

Figure 12-5 shows how hybrid corn
is produced by the double-cross method.
Four inbred plants, designated as A, B,
C, and D, serve as the foundation.
These varieties are the result of con¬
trolled self-pollination, or line breeding.
This is accomplished by covering the de-

12-4 Controlled pollination of corn. To pro¬
duce the necessary inbred lines required for
hybridizing, care must be taken to be sure no
natural pollination occurs. The photograph
at the top shows the plastic bag which is
used to cover the stigmas (“silk”) so they
will not be contaminated by other pollen.
At the bottom pollen previously collected
from another plant is carefully blown on the
stigmas and the bag is slipped back into
place. (Funk Bros. Seed Co.)

veloping ears with sacks until the silks
are ready to receive pollen. Then pollen
collected from the same plant is dusted
onto the silks. The plant breeder care¬
fully avoids any contamination of these
inbred varieties. During the first cross,
plant A is crossed with plant B, which
produces a single-cross hybrid, plant AB.
In making this cross, the tassel from
plant B is removed and the ear is covered
with a bag. When the silks are mature,
they are dusted with pollen from plant
A. A similar first cross is made between
plant C and plant D, resulting in a
single-cross hybrid, plant CD. The fol¬
lowing season plant AB is detasseled.
The developing ear is covered. This
plant is cross-pollinated with plant CD.
Kernels resulting from this double-cross
are designated as ABCD.

This seed is sold to the farmer for
planting. However, he cannot plant the
seed produced by this hybrid corn be¬
cause the genes will sort out in new
combinations in the next generation.
He might get plants that would have
different characteristics from those of
the hybrid parents.

Producing new plants by crossing dif¬
ferent species. One of the best known
of Burbank’s plant varieties is the Shasta
daisy. He produced this beautiful gar¬
den flower by crossing a native oxeye
daisy with a European variety. In a
cross between a plum and an apricot, he
produced the plumcot. Another hybrid

CHAPTER 12 APPLIED GENETICS 175

FIRST YEAR

Detasseled

SECOND YEAR

Detasseled

cxCL-1 -

(B x A) x (C x D)
Double-cross seed
for commercial planting

Single-cross plant (B x A)

Single-cross plant (C x D)

12-5 This diagram shows the method of crossing inbred corn plants and of
crossing the resulting single crosses to produce double-cross hybrid seed. The
four plants labeled A, B, C, and D represent the products of four different in-
bred lines. Strain A is crossed with strain B (A furnished the tassel and B was
detasseled). Likewise, strains C and D are crossed. The F1 generations of these
two lines are then crossed to produce the hybrid corn seed which is used by
farmers today. (U. S. Department of Agriculture)

176 UNIT 2 THE CONTINUITY OF LIFE

12-6 Left: an iris having the normal chromosome number. Right: a mutant
variety having the tetraploid number. (Walter Dawn)

plant was produced by crossing the
squash and the pumpkin. One of Bur¬
bank’s last experiments was an attempt
to cross a tomato and a potato to pro¬
duce a dual-purpose plant that would
bear fruit above ground all season and
form tubers that could be dug at the end
of the season. Unfortunately such a
cross was never perfected.

Plant varieties resulting from mutations.
While examining a bed of white tea
roses one day, a grower happened to
notice a branch that had produced a
pink flower. He carefully removed the
branch and set it in a cutting bed. The
plant that grew from the branch bore
all pink flowers. These were budded
onto understock and propagated as a
bud-mutanty or sport , of the white rose.
Such somatic gene mutations occur from
time to time in plants. In the tea rose,
a color gene had mutated in a cell of the
stem. All tissues of the branch that de¬
veloped possessed the mutant trait.

Other varieties that have resulted

from a bud-mutation include the Cali¬
fornia navel orange, the Delicious apple,
and the smooth-skinned peach, or nec¬
tarine.

Plant varieties with chromosomal aber¬
rations. Have you ever noticed blue¬
berries on a fruit counter that are twice
as large as native blueberries? These
olueberries have a double set of chromo¬
somes. This condition, in which plants
have some multiple of the diploid num¬
ber of chromosomes, is called polyploidy
(pahl- i-ploid-ee). Other fruits that have
been produced by plants with multiple
chromosome numbers include varieties
of plums, cherries, grapes, strawberries,
and cranberries. A similar increase in
the chromosome number occurred in the
McIntosh apple. Normal McIntosh
apple trees usually have 17 pairs; or 34
chromosomes. However, one variety
with a fruit more than twice as large as
the normal McIntosh has four sets- of
chromosomes (the tetraploid number
4n), or 68 in all.

CHAPTER 12 APPLIED GENETICS 177

A polyploid with an even number
of chromosome pairs (4 n, 6 n, 8 n, etc.) is
usually fertile. However, the formation
of gametes is interfered with in poly¬
ploids having an odd number of chromo¬
some pairs (3 n, 5 n, In, etc.). The lack
of fertility is not a handicap in obtain¬
ing new specimens with the same genet¬
ic makeup. The plant breeder can pro¬
duce these with cuttings and grafting.

It has been discovered recently that
polyploidy can be produced artificially
by the drug colchicine (kahl- chi-seen).
When shoots of plants are put in a weak
solution of colchicine, their chromo¬
some number is often doubled. The
drug is also available in salve form for
use on buds of plants. Blueberries, lilies,
and cabbage are a few plants that have
been improved by its application. Even

12-7 Selective breeding has produced the poultry shown here. The New Hamp¬
shire Red (top left) is suitable for both egg and meat production. The White
Cornish and the Light Brahma (top and bottom right) have both been bred for
meat. The Black Minorca (bottom left) is suitable for egg production. (Grant
Heilman)

178 UNIT 2 THE CONTINUITY OF LIFE

interspecies can be produced by using
colchicine. A cross between the ordi¬
nary cabbage, with 20 chromosomes, and
the Chinese cabbage, with 18 chromo¬
somes, yields a new type with only 19
chromosomes. This type is sterile.
After plant breeders treated it with col¬
chicine, it had 38 chromosomes, but it
continued to be sterile. But when plant
breeders crossed this plant with a ruta¬
baga having 38 chromosomes, a new
type of cabbage was produced. It also
had 38 chromosomes, but it was fertile
and bred true.

Many years ago almost all garden
iris plants were diploid, while most of
the modern ones are tetraploid. The
less desirable Einkorn wheat is diploid,
but the more desirable Emmer wheat is
tetraploid, while the vulgare wheat is 6n.
The large and beautiful varieties of roses
include almost every multiple of chro¬
mosomes up to 8n.

Animal breeding. The principles used
in plant breeding apply to animal breed¬
ing as well. Mass selection has long
been a method of producing highly de¬
sirable breeds of animals.

The results of years of selective
breeding are well illustrated in the mod¬
ern breeds of poultry. The Leghorn, for
example, has been bred for its ability to
lay large numbers of eggs. All its ener¬
gies are directed toward egg production
rather than the production of body flesh.
The Plymouth Rock has been developed
as a dual-purpose fowl and is ideal for
egg production and meat. Large breeds,
like the Brahma, Cochin, and Cornish,

12-8 Three breeds of cattle. Top: an impor¬
tant milk breed, the Holstein-Friesian cow;
middle: a dual-purpose breed for both milk
and meat, the Milking Shorthorn; bottom: a
prime beef breed, the Hereford steer. (Top:
USDA; middle: American Milking Shorthorn
Society; bottom: American Hereford Asso¬
ciation)

are famous for their delicious meat
rather than for egg production.

The modern turkev, with massive
body and broad breast covered with
thick layers of white meat, is quite a
contrast to the slender bird the Pilgrims
found in the New England forest. The
modern turkev has been bred for the

highest possible flesh production. It
spends its life, often on wire, eating a
scientifically prepared diet and building

CHAPTER 12 APPLIED GENETICS 179

up large, little-used muscles, better
suited to being eaten than to flying and
perching high in trees.

Improvement in livestock. Using simi¬
lar selective breeding methods, domestic
cattle have been developed along two
entirely different lines. Aberdeen-An-
gus, Hereford, and Shorthorn are breeds
of beef cattle. Their low, broad, stocky
bodies provide high-quality steaks and
roasts for the nation’s markets. Dairy
breeds, including the Jersey, Guernsey,
Ayrshire, Holstein-Friesian, and Brown
Swiss have been bred as milk producers.
A breed of Shorthorns known as Milk¬
ing Shorthorns, as well as Red Poll
cattle, are classified as dual-purpose
breeds because they were developed for
milk production as well as beef.

Swine raising is one of the most im¬
portant divisions of American agricul¬
ture, especially in the Corn Belt. Heavy,
or lard, type breeds include the Poland
China and Berkshire, Hampshire, and
Duroc-Jersey. The Yorkshire and Tam-
worth hogs have long slender bodies and
are classified as lean, or bacon, type hogs.

In livestock breeding, the records of

outstanding individuals used in breeding
are kept in pedigree and registration
papers. Purebred animals may be regis¬
tered at the headquarters of their respec¬
tive breeds. Papers must include the
names and registration numbers of both
sire (male) and dam (female) as well as
part of the ancestry. In this manner dif¬
ferent strains of the same breed may be
crossed without the danger of introduc¬
ing undesirable characteristics or losing
any good qualities.

The crossing of two different species.
The mule is an animal that has resulted
from the crossing of two entirely dif¬
ferent species. This hardy, useful ani¬
mal is produced by crossing a female
horse with a male donkey. The size is
inherited from the horse. From the
donkey the mule inherits long ears, sure¬
footedness, great endurance, and the
ability to live on rough food and to en¬
dure hardships. However, with all of its
hybrid vigor, the mule is usually sterile
— that is, unable to reproduce.

Several strains have resulted from
crosses between Brahman cattle from
India and domestic breeds. Tourists in

12-9 This hybrid Bran-
gus bull combines the
heat-resistant charac¬
teristics of the Brah¬
man with the excellent
beef qualities of the
Aberdeen-Angus. (USDA)

180 UNIT 2 THE CONTINUITY OF LIFE

the southern and southwestern states are
often surprised to see these large gray or
brownish animals with long, drooping
ears and shoulder humps. Brahman
cattle can endure the hot, humid climate
of the Gulf States as well as the dry
summer heat of the Southwest much
better than domestic breeds of beef

cattle. In addition they resist disease
and insect attacks.

A cross between Brahman and Aber-
deen-Angus cattle produced Brangus
cattle, one of the most popular of the
Brahman crosses. The Braford is
another crossbreed with Brahman and
Hereford cattle as parent stock.

IN CONCLUSION

Scientific breeding has brought rust-resistant asters to flower beds of your home
and city parks. Roses in clusters, long-stemmed tea roses, and climbing roses
in great variety leave the growers by tens of thousands each season to beautify
our gardens. Just name a size and color of tomato and the time in the summer
you want it to ripen and a grower will supply it to you. Beef cattle and dairy
cattle, fat hogs and lean hogs, horses for work and horses for pleasure — we
have them all.

While in this chapter we have discussed man’s artificial selection of plant
and animal traits, in the next we shall discuss a process of natural selection
that has been going on since life began.

BIOLOGICALLY SPEAKING

colchicine mass selection polyploidy

hybrid vigor outbreeding sport

inbreeding

QUESTIONS FOR REVIEW

1. What are four objectives of plant breeding?

2. Why is line breeding practiced in plant and animal breeding?

3. Make a comparison of hybridization and line breeding as to methods and
purposes.

4. How many pure-line parents are involved in the production of hybrid seed
corn by the double-cross method?

5. Name several hybrid plants produced by the genius of the late Luther
Burbank.

6. How is natural cross-pollination prevented during the growing of hybrid
com?

7. Name three general types of chickens and a breed representing each type.

8. Name a dual-purpose breed of cattle.

9. In what respect is the mule a true hybrid animal?

10. Why are Brahman cattle good parent stock for breeding purposes?

CHAPTER 12 APPLIED GENETICS 181

APPLYING PRINCIPLES AND CONCEPTS

1. If line breeding is practiced too long, offspring may become weak and in-
bred. How might this condition be remedied?

2. A farmer does not use seed from his hybrid corn for the next year’s crop.
Explain why.

3. Outline the method by which poultry breeders have been able to increase
egg production by developing 300-egg strains of chickens.

4. What is the importance of pedigrees and registration papers in breeding
livestock?

CHAPTER 13

ORGANIC

VARIATION

The study of evolution. Biologists have
long been aware that living things have
changed over the ages. You need only
to look at the reconstructed skeleton of
a dinosaur to realize that life on earth
todav is not the same as it was 100 mil-

lion years ago. Geologists, or earth sci¬
entists, estimate that the earth is approx¬
imately five billion years old, and that
life has been present for about two
billion of these. Fossil evidence indi¬
cates that during this time different
forms of life have been dominant, and
that the simpler forms evolved earlier
than the more complex forms of plants
and animals. From such evidence they
have constructed a geological timetable
such as that in Fig. 13-1. As you can
see, it is divided into long eras, which
are in turn subdivided into shorter pe¬
riods.

The study of the changing world
of life is called evolution. Whether ev¬

olution has occurred, how and why it
has occurred, and whether it is still going
on are questions that have perplexed
biologists for years. We shall first dis¬
cuss some evidences other than fossils
that indicate that evolution has occurred.
Evidences of common ancestry. In
both plants and animals we find parts
that are evidently of similar origin and
structure, although they may be adapted
for very different functions in different
species. These parts are called homolo¬
gous (hoh-mcz/iZ-uh-gus) structures. In
many plants leaves are found modified as
petals, tendrils, and thorns. Covering
tissue of animals may be modified as
hoofs, scales, nails, claws, feathers, and
hair. The bones of the bird’s wing, the
front leg of a horse, and the paddle of
the whale are so similar in structure
that, with slight exceptions, they are
given the same names.

In certain animals, structures exist
that are well developed and perform an
important function; while in other ani¬
mals a corresponding structure may be
present that is poorly developed or not
functioning. These remnants are called
vestigial organs. It is believed that such
structures are the remains of organs that
were well developed in common ances¬
tors and therefore offer further evidence
of the development of animal life and
its relationship. The appendix of some
mammals, for example, is a small struc¬
ture without any known function. In
others that eat a coarse diet with large
amounts of cellulose, this pouch serves
as an organ to store mixtures of food and
enzymes over a long period of time. In
rabbits, the appendix is the largest part
of the intestine.

Biologists often have compared the
developmental stages of present-day
mammalian embryos. The evidence
from embryology seems to indicate that

182

CHAPTER 13 ORGANIC VARIATION 183

Recent Period

PSYCHOZOIC ERA -about 25,000 years

Beginning of man’s dominance; domestication of
animals

Pleistocene Period

CENOZOIC ERA — about 60,000,000 years

Ice ages; extinction of mammoth and mastodon;
rise of modern horse; man uses fire and makes

Pliocene Period
Miocene Period
Oligocene Period

Eocene Period

implements

Rise of man; Pliohippus

Saber-toothed tiger; Protohippus; whale

Primitive anthropoids and Mesohippus

Primitive forms of modern mammals: sloths, arma¬
dillos, marsupials, Eohippus, rhinoceros

Cretaceous Period

MESOZOIC ERA -about 125,000,000 years

7 yrannosaurus and other dinosaurs became extinct ;
flowering plants; true trees; modern insects; true
birds

Jurassic Period

Giant dinosaurs dominant; birds with teeth; turtles
and flying reptiles; egg-laying mammals

Triassic Period

Rise of dinosaurs

Permian Period

PALEOZOIC ERA -about 350,000,000 years

Rise of insects, spiders, and primitive reptiles; extinc¬
tion of trilobites and other forms; glacial period

Pennsylvanian Period

Spore-bearing plants; sharks and large amphibians
(first land vertebrates); coal formed

Mississippian Period

Rise of crinoids, brachiopods, and sea urchins; dense
vegetation on land

Devonian Period

Tree ferns and other land plants; lung fish and
primitive amphibians; fish and invertebrates domi¬
nant

Silurian Period

First air-breathing animals; crinoids; primitive sharks;

Ordovician Period

scorpions; first land plants

Corals and clams; armored fish; starfish; first sea¬
weeds

Cambrian Period

Marine invertebrates: sponges, jellyfish, trilobites,
gastropods, brachiopods

Proterozoic Era and Archeozoic Era — about 1,450,000,000 years — Origin of life
in form of one-celled organisms; fossil evidence scanty
Total number of years since life began — about 2,000,000,000 years

13-1 The geological timetable. Geological eras are further subdivided into
periods. The Pennsylvanian and Mississippian periods are sometimes listed to¬
gether as the Carboniferous period.

184 UNIT 2 THE CONTINUITY OF LIFE

13-2 Nonfunctional vestigial organs are common among animals. These organs
provide one source of evidence that living things have changed through the

ages.

each animal in its individual develop¬
ment passes through stages that re¬
semble those of remote ancestors.

Not only do certain organisms re¬
semble one another in structure and de¬
velopment, there is also a marked sim¬
ilarity in function. For instance, all
organisms produce nucleic acids, espe¬
cially DNA, and all use ATP in energy
transfer.

Plant and animal breeding provides
other evidence that organisms may
change. Over 25 kinds of dogs have
been developed from wild wolflike an¬
cestors. A dozen kinds of chickens have
a common ancestor in the jungle fowl
of India. The story of plant and animal
breeding, while it does not prove that
similar changes have taken place natu¬
rally in past ages, does strongly point to
that possibility.

Lamarck’s theory of evolution. One of
the first and most interesting theories to
explain evolution was proposed by the
French biologist Jean Baptiste Lamarck.
His theory, presented in 1801, placed
great stress on the environment as the
mechanism responsible for change. Ac¬
tually, Lamarck’s idea of evolution in¬
volved three theories:

Theory of need — that the produc¬
tion of a new organ or part of a plant or
animal results from a need. For ex¬
ample, the early ancestors of the snake
had legs and short bodies. But with
the changes in land formations, it be¬
came necessarv for the snake to crawl

through narrow places. Then it began
to stretch its body and to crawl rather
than walk. This assumption formed the
basis which Lamarck used to formulate
his second theory.

CHAPTER 13 ORGANIC VARIATION 185

Theory of use and disuse — that or¬
gans remain active and strong as long as
they are used but disappear gradually
with disuse. Lamarck believed that the
snakes of each generation continued to
stretch and strengthen their bodies.
Furthermore, the legs were used less and
less because they interfered with crawl¬
ing, and they finally disappeared.

Theory of inheritance of acquired
characteristics — that all that has been
acquired or changed in the structure of
individuals during their life is trans¬
mitted by heredity to the next genera¬
tion. He believed that the modern
snakes evolved from the forms that had
lost their legs through disuse. Thus they
inherited the legless trait from their an¬
cestors.

Lamarck used other examples that
he could see in nature to explain his
theory of need. He believed that the
giraffe evolved from a short-legged, short¬
necked form. When competition for
low-growing grasses became too great,
the giraffe began to stretch its neck and
forelegs in order to reach the leaves of
trees. Each succeeding generation in¬
herited the longer neck resulting from
stretching until the trait was inherited
by our modern giraffe. Other beliefs
held by Lamarck included the idea that
frogs and ducks developed webbing in
the toes for swimming, and that the
heron and other wading birds developed
their long legs to keep their bodies out
of the water. He attempted to explain
the evolution of mammals into birds by
suggesting that the hair turned into
feathers as certain mammals attempted
to fly.

Many biologists have tested La¬
marck’s theory of inheritance, but each
experiment presents further evidence
that acquired traits are not inherited.
One experimenter tested the theory by

cutting off the tails of mice and then
mating them. The offspring of the tail¬
less mice had tails. He then cut off the
tails of the second generation and mated
them. He continued this experiment for
20 generations. The 21st generation still
had tails of the same length as the first.

The explanation of evolution given
by Lamarck may seem very reasonable.
Gradual modifications do appear in spe¬
cies from time to time. Parts of plants
and animals do change as a result of use
and disuse. For example, an athlete
develops strong muscles through use, but
muscles become weak from disuse when
contained within a cast. There is no
evidence, however, that these acquired
traits are handed on to offspring.
Darwin’s theory of natural selection. In
1859 Charles Darwin, an English scien¬
tist, published his Origin of Species by
Natural Selection. His theory of natural
selection, while confined to biology, has
also influenced other branches of sci¬
ence. The chief factors, according to
Darwin, that account for the develop¬
ment of new species from common an¬
cestry are listed on page 186.

13-3 According to Lamarck’s theory, mod¬
ern snakes might have developed from a
short-legged reptile like the common blue¬
tailed skink shown here attending her eggs.
(Dermid from National Audubon Society)

186 UNIT 2 THE CONTINUITY OF LIFE

1. That all organisms produce more off¬
spring than can actually survive.

2. That because of overproduction,
there is a constant struggle for exist¬
ence among individuals.

3. That the individuals of a given spe¬
cies vary.

4. That the fittest, or the best adapted,
individuals of a species survive to
transmit their traits to the next gen¬
eration.

Overproduction. A fern plant may pro¬
duce 50 million spores each year. If all
the spores resulting from this overpro¬
duction matured, they would nearly
cover North America the second year.
A mustard plant produces about 730,000
seeds annually, which, if they all took
root and matured, in two years would
occupv an area 2,000 times that of the
land surface of the earth. The dande¬
lion would do the same in 10 years.

At a single spawning an oyster may

shed 114,000,000 eggs. If all these eggs
survived, the ocean would be literally
filled with oysters. Within five genera¬
tions there would be more oysters than
the estimated number of electrons in the
visible universe! There is, however, no
such actual increase.

The elephant is considered to have
a slow rate of reproduction. An average
elephant lives to be 100 years old, breeds
from 30 to 90 years, and bears about
six young. Yet if all the young from
one pair of elephants survived, in 750
years the descendants would number
19,000,000.

The competition for life. We know
that in actuality the number of individ¬
uals of a species usually changes little in
its native environment. In other words,
regardless of the rate of reproduction,
only a small minority of the original
number of offspring reaches maturity.

Each organism seeks food, water.

13-4 Here you see three different stages in the development of five verte¬
brates. Note the similarities in the very early stages of all five. What differ¬
ences do you observe?

CHAPTER 13 ORGANIC VARIATION 187

air, warmth, and space, but only a few
can obtain these needs in struggling to
survive. This struggle for existence is
most intense between members of the
same species, as they compete for the
same necessities.

Variation among individuals. With the
exception of identical twins, every in¬
dividual varies in some respects from
other members of its species. Animal
breeders take advantage of this fact when
they choose the individuals with desir¬
able characteristics to breed. Nursery¬
men are able to produce disease-resistant
plants and control the size and color of
the bloom by careful study and cross¬
fertilization of particular individuals.
The variations within a species furnish
the material for nature to use in her
selection.

Survival of the fittest. If, among the
thousands of dandelion seeds produced,
for example, some have better dispersal
devices, these will be carried to a distant
place where they will be less crowded,
and so may survive. Those having poorer
adaptations will perish bv overcrowding.
In so severe a struggle where only a few
out of millions may hope to live, verv
slight variations in speed, or senses, or
protection may turn the scale in favor of
the better fitted individual. Those with
unfavorable variations sooner or later
will be wiped out.

In general, offspring resemble their
parents. If the parents reach maturitv
because of special fitness, those of their
descendants that inherit most closely
the favorable variation will in turn be
automatically selected by nature to con¬
tinue their species. Darwin called this
part of his theory the survival of the
fittest. In this way nature is selecting
the characteristics of a certain popula¬
tion by favoring even the slightest varia¬
tion.

Mutation theory of evolution. Muta¬
tion as a basis for evolution was proposed
by Hugo De Vries, a Dutch botanist,
when he presented his mutation theory
in 1901. De Vries had found two eve¬
ning primrose plants that were different
from their parent stock and that bred
true by producing these variations in off¬
spring. He experimented for many
years and found that, from 50,000 speci¬
mens of evening primrose, at least 800
plants showed striking noninherited vari¬
ations that were transmitted to offspring.
From his study of the evening primrose,
De Vries concluded that similar muta¬
tions occurred frequently in other or¬
ganisms and that this was the basis for
the evolution of life through the ages.
Mechanisms of evolution. As we re¬
view the work of Lamarck, Darwin, and
De Vries, we can see that each made a
significant contribution to modern con¬
cepts of evolution. Lamarck observed
a relation between the organism and its
environment. His observation was cor¬
rect. It was his conclusion that the en¬
vironment acted directly on the organ-

13-5 This little white fawn is an albino mu¬
tant. (Finke from National Audubon So¬
ciety)

188 UNIT 2 THE CONTINUITY OF LIFE

ism and produced hereditary changes in
relation to need that was in error.
Nevertheless, Lamarck did recognize
change in living things through the
ages, and thus provided evidence of evo¬
lution.

Darwin also recognized change, but
he reversed the theory Lamarck pro¬
posed. He established that the change
occurs in the organisms without respect
to need. He believed that the environ¬
ment determines whether a variation im¬
proves the chance for survival.

De Vries reinforced Darwin’s con¬
cepts of variation and survival of the fit¬
test and explained the mechanism of
change in terms of mutations and other
genetic alterations.

The modern biologist has been able
to go much further in exploring the
mechanisms of evolution. He has the
advantages of recent advances in genet¬
ics to account for variations, biochemis¬
try to explain gene action, anatomy and
physiology to establish relationships, and
ecology to give evidence of environmen¬
tal relationships, population movements,
and other factors operating on a large
scale. Generally, we may relate evolu¬
tion to the following mechanisms: gene
mutations, chromosomal mutations, and
recombination.

Gene mutations as a cause of change.
Mutations have been found in every
kind of plant and animal that has been
studied genetically — from columbines
to fruit flies and mice to men. In gene
mutations we find the basic source of
variations and the material for evolution.
In considering gene mutations as a mech¬
anism in evolution, we must examine
their nature and frequency more closely
to see how they relate to variations.

How frequently do gene mutations
occur? Genes tend to be stable. The
DNA molecules composing them tend

to replicate time after time without
chemical alteration. Still, gene muta¬
tions do occur, and at a rate that genet¬
icists have been able to predict. In
studies of mutation in corn, geneticists
have found that genes for seed color
mutate 492 times per 1,000,000 gametes.
This would average about one mutation
in 2,000 gametes. This may seem to be
a very slow rate of mutation. However,
when you consider that a single acre of
corn contains more than this number of
plants, you realize that the frequency of
mutations is greater than you might
think.

Many gene mutations are so slight
that there is no apparent difference in
the organism. The fruit fly is believed
to contain about 20,000 genes in its hap¬
loid chromosome number. Thus the
body cells contain as many as 40,000
genes. A mutation in a single gene
might not be noticeable. This may ac¬
count for the fact that change in a popu¬
lation may be detected only over a long
period of time, and not from one genera¬
tion to the next.

Mutant genes are nearly always re¬
cessive. For this reason a mutant char¬
acteristic does not appear in an offspring
in which the other allele is normal. For
example, in a pair of genes designated as
AA, let us assume that one gene mutates
to a. The mutant characteristic will not
appear in a heterozygous (A a) offspring.
However, if the frequency of the mutant
gene increases in the population, a ho¬
mozygous offspring with paired mutant
genes (aa) may appear and express the
new characteristic.

Genes often mutate, then mutate
back to the original expression.

Often gene mutations produce a
characteristic that cancels the survival
chance of an organism. If such a lethal
gene is recessive, many individuals in a

CHAPTER 13 ORGANIC VARIATION 189

population may possess the gene without
expressing the fatal characteristic. Since
homozygous individuals that express the
lethal trait die without reproducing,
there is a tendency for the frequency of
a lethal gene in a population to be re¬
duced. However, additional normal
genes may continue to mutate to the
lethal gene and perpetuate the problem.
Chromosomal changes. While chro¬
mosomal mutations occur less frequently
than gene mutations, their effects are
usually more noticeable since many
genes are involved. In your study of
genetics, you found various reasons for
changes in chromosomal structure. Gen¬
erally they include:

1. Polyploidy — gain in the number of
pairs of chromosomes resulting in
such abnormalities as 3 n (triploid
number) or 4n (tetraploid number).

2. Haploidy — loss of one entire set of
chromosomes.

3. Nondisjunction — gain or loss of part
of a set of chromosomes.

4. Translocation — exchange of parts or
segments of chromosomes between a
chromosome pair.

These and other changes in chro¬
mosome composition and number pro¬
duce new characteristics in offspring.
Thus chromosomal mutations are an im¬
portant mechanism in variation and re¬
sulting evolution of organisms.
Recombination as a source of variation.
You will recall, from your study of sexual
reproduction, that chromosome pairs
separate and reduce from the diploid
number (2 n) to the haploid number (n)
in the formation of gametes. During
fertilization new combinations of chro¬
mosomes are formed as the chromosome
number is restored to the diploid num¬
ber (2n). Recombination of chromo¬
somes and genes in sexual reproduction
is a mechanism second only to muta¬

tions in the production of variations.
Furthermore, it is by recombination that
mutant characteristics appear as varia¬
tions in populations.

To show how recombination greatly
hastens the appearance of a new charac¬
teristic, let us again represent a pair of
alleles as AA. One of these genes mu¬
tates. The gamete containing this mu¬
tant gene unites with a gamete contain¬
ing a normal allele, producing the gene
pair Aa. Since the mutant gene is reces¬
sive, the characteristic will not appear
in this individual. If this organism were
to reproduce asexually, all offspring
would be identical with the parent. The
only chance for the variation (a) to ap¬
pear would be for the second gene of the
pair (A) to mutate, thus producing the
alleles aa. The chance of this happen¬
ing in an organism has been calculated
to be about one in 10 billion!

Now consider what sexual repro¬
duction and recombination could ac¬
complish in this example. When the
pair of alleles A and a separate during
meiosis, half of the gametes produced
will receive A, while the other half will
receive the mutant gene a. The new
characteristic will not appear in the first
generation because the gene a must com¬
bine with a normal gene A. However,
the mutant gene a is increasing in fre¬
quency. It is now present in half the
offspring. As these heterozygous off¬
spring interbreed, they will produce one-
fourth AA, one-half A a, and one-fourth
aa, as you determined earlier in a mono¬
hybrid cross. Thus one fourth of the
offspring will express the mutation. If
this characteristic is favorable to survival
of these organisms, the mutant gene a
will increase in frequency in the popu¬
lation as a result of selection. In time
all members of the population may pos¬
sess the mutant characteristic.

190 UNIT 2 THE CONTINUITY OF LIFE

Variation, change, and survival. Muta¬
tions, chromosomal changes, and recom¬
bination provide the genetic basis for
variation in organisms. But it is the
environment that determines whether
variations are favorable to survival.
Thus, the environment determines the
direction as well as the rate of evolution.

Isn’t it logical to assume that a pop¬
ulation already established in a given en¬
vironment is well adjusted to the condi¬
tions of that environment? Such a
population is the result of natural selec¬
tion and survival of the fittest over a
long period of time during which un¬
desirable variants were eliminated. As
long as the environment remains un¬
changed, it is unlikely that further ge¬
netic variations will improve the already
adjusted population. But what if the
environment were altered? Then, ge¬
netic variations might improve the
chances of survival and lead to the es¬
tablishment of a new variety within a
species or even to a new species.
Changes in environmental conditions
may occur in two ways:

1. Migration of an organism to an¬
other locality.

2. Change in the environmental
conditions in a given locality.

Migration, variation, and the benefit of
change. When migration occurs so that
individuals of a population occupy new
areas and interbreed with other popula¬
tions of the same species, new gene com¬
binations are formed. Let us assume
that several members of an animal popu¬
lation migrate to a new area. They take
with them certain combinations of genes
characteristic of the population of which
they have been a part. Their arrival in
a new area introduces genetic character¬
istics that have been absent in this pop¬
ulation. By interbreeding, their off¬
spring are also receiving different genes

from the new population. The genetic
makeup of this entire population may
be altered by the migrating organisms.
Thus, one result of migration is the pro¬
duction of variations through new gene
combinations.

There is another important result.
Migration takes plants and animals into
new and different environments. This
introduces the benefit of change. New
characteristics that appear in offspring
through mutations and recombinations
of genes may result in adaptations that
are favorable to the new environment.
Migration and selection in animal popu¬
lations. The migration and redistribu¬
tion of camels in early times is a good ex¬
ample of the spreading of a population
and selection. At one time camels in
various forms were found throughout
Asia, Europe, North America, Central
America, and South America. Biologists
believe they had their origin in North
America and spread to Asia when the
two continents were connected by a land
bridge which at that time spread across
the Bering Strait region. Such a land
bridge that acts as a pathway for mi¬
grating animals is spoken of as a cor¬
ridor. The camel migrations extended
through North America and Central
America to South America. Then, with
the coming of the Ice Age, the camel
population was eliminated in most areas.
This resulted in widely separated camel
populations. Today, we have the Asian
camel with two humps and the African
(Arabian) camel with one hump. Both
are well adapted for life in the desert.
Relatives in far-off South America
evolved into quite a different animal,
lacking a hump but sure-footed and pro¬
tected with a dense coat of hair that
adapts it for life in the rocky, mountain¬
ous area — the South American llama.

The individual variations among

CHAPTER 13 ORGANIC VARIATION 191

the camel populations that reached the
Andes were probably not very great.
But the ones with the slightly heavier
coat had a better chance to survive cold
winters and to reproduce the following
year. Also, the animals with shorter
legs and sure-footedness had a better
chance to survive by running away from
enemies. The animals that did not pos¬
sess these favorable variations were
eventually wiped out.

The movement of organisms to
new environments sometimes involves
strange and interesting methods and de¬
vices. A mouse may float across a wide
body of water on a log and reach a new
environment. If other mice are present
in the new environment and are capable
of interbreeding with the new arrival,
new gene combinations may be formed
and mice may strike out in a new evolu¬
tionary direction. We refer to such dis-
tribution over strong barriers as sweep-
stakes dispersal.

The peppered moth and the impact of
environmental change. A moth popu¬
lation in England affords us one of the
best examples of the impact of a change
in the environment on a species. The
peppered moth ( Biston betularia) is na¬
tive to the region of Manchester, a Brit¬
ish industrial city. Two British scien¬
tists, R. A. Fisher and E. B. Ford,
conducted the original investigation of
a color change that occurred in this
moth within a period of just 50 years.
More recent studies were conducted by
Professor H. B. D. Kettlewell. The re¬
sult of these investigations has become
a classic example of evolution in action.

Prior to 1845 the peppered moth
was light colored with dark blotches and
spots. It could hardly be seen when
resting on the light gray bark of trees.
Then, in 1845, a black color variation of
the peppered moth was captured in

13-6 The African, or Arabian, camel has only
one hump, whereas the Asian camel has two.
(Top: Arabian American Oil Co.; bottom:
Ewing Galloway)

Manchester. A mutation had occurred
in a gene determining coloration. Was
this a favorable mutation? You would
expect a black moth resting on light
bark to be an easy prey for a bird. But
something else was happening in Man¬
chester. The city was rapidly becoming
an industrial center. Smoke poured
from factory chimneys, and soot turned
the light-colored bark of the trees nearly
black. The black moths could hardly
be seen on the dark, sooty bark. With
this environmental change the light
moths became easy prey for birds. In
the period from 1845 to 1895, the black
peppered moth population increased
from one known individual to 99 per¬
cent of the population!

This change in the peppered moth
population interested Professor Kettle-

192 UNIT 2 THE CONTINUITY OF LIFE

well, who continued the investigation.
He selected two entirely different areas
for study. One was in a bird reserve in
Birmingham, an industrial city similar
to Manchester. The other was a country¬
side near Dorset, an area where there was
no soot on the trees.

In the bird reserve in Birmingham,
Kettlewell released 477 black moths and
1 37 light moths. As he watched through
the day, birds fed on the moths as they
rested on tree trunks. That night he
recovered most of the remaining moths
by attracting them to a light. Altogether
he recaptured 40 percent of the black
moths but only 19 percent of the light
moths.

In the countryside near Dorset he
released 473 black moths and 496 light
moths. Again the birds destroyed large
numbers as they rested on trees of the
region. Only 6 percent of the black
moths were recaptured while 12.5 per¬
cent of the light moths survived.

Thus, we have an interesting dem¬
onstration of a principle biologists now
call industrial melanism ( meZ-a-niz-m ) .
A mutation, a resulting adaptation to a
changed environment, and a predator
combined to change an entire popula¬

tion of moths in 50 years. Industrial
melanism is a demonstration of the im¬
portance of natural selection in the proc¬
ess of evolution.

Isolation as a factor in evolution. Two
squirrel populations in the region of the
Grand Canyon of the Colorado River
illustrate the effect of isolation on the
evolution of different forms. On the
north rim of the Grand Canyon, we find
the Kaibab squirrel, with long ears,
white tail, and dark underparts. On the
south rim is a similar animal, the Abert
squirrel, with long ears but a gray tail
and light underparts. Biologists con¬
sider these squirrels to be different spe¬
cies. Why have they developed sepa¬
rate characteristics when the two
populations are so near? Between the
two rims of the canyon is a fast-flowing
river. In the depths of the canyon the
temperature may reach 120° F. These
factors have acted as barriers to the two
groups of squirrels, preventing them
from crossing the canyon and inter¬
breeding. Over a period of thousands of
years, the two populations have gradu¬
ally become different from each other,
although they probably started out as
the same species. In other words, the

13-7 Left: a black peppered moth and a light peppered moth against a light
surface. Right: the two phases against a dark surface. These illustrate the
principle of industrial melanism. (Michael Tweedie from Photo Researchers
Inc.)

CHAPTER 13 ORGANIC VARIATION 193

gene pools of the two groups were sepa¬
rated, so that as mutations occurred in
each group, the variations between the
two became more pronounced, until
they could be considered two species.

A mountain range, a dry plain, a
desert, or an ocean may act as a physical
barrier preventing plants or animals
from interbreeding. The populations
of islands give striking examples of isola¬
tion. With an ocean as a barrier there
is little or no opportunity for interbreed¬
ing with plants and animals on the
mainland. This accounts for the de¬
velopment of totally different popula¬
tions in the two areas.

Physical barriers are not the only
causes for isolation of a species popula¬
tion, however. Any factor that pre¬
vents interbreeding may cause isolation.
The sockeye salmon of the Fraser River
is a good example. This great Canadian
river has long been the ancestral spawn¬
ing area of this salmon. A sockeye
salmon hatches far up the river in shal¬
low, cold water. Gradually it works its
way downstream, finally arriving at the
mouth of the river, where it disappears
into the vast Pacific Ocean. Here it
lives three or four years until maturity
when a reproductive instinct urges it
back to the mouth of the Fraser. Fight¬
ing the currents and leaping waterfalls,
the salmon finally reaches its ancestral
home — perhaps the very pool in which
it hatched. Here it reproduces and dies.
In the Pacific Ocean the sockeye mingles
with other species of salmon. How¬
ever, it never interbreeds with other
species — only with other sockeyes and
always in the Fraser River. Thus the
sockeye salmon has remained an iso¬
lated breeding population. As might be
expected, this has resulted in differences
between the sockeye salmon and the
salmon that breed in other streams.

Other barriers to interbreeding in¬
clude such factors as variation in the
mating time of two populations, struc¬
tural differences that prevent mating,
and differences in mating habits. Some¬
times interbreeding between two popu¬
lations does occur but the resulting hy¬
brids are sterile.

The development of species. A species
is an individual kind of organism, dis¬
tinct from all other organisms and capa¬
ble of interbreeding with others of its

13-8 The Kaibab squirrel (top) occurs on
the north rim of the Grand Canyon while the
Abert squirrel exists on the south rim. The
evolution of these two species has been af¬
fected by the barrier of the canyon. (Top:
Union Pacific Railroad; bottom: Stophlet
from National Audubon Society)

194 UNIT 2 THE CONTINUITY OF LIFE

kind. All members of a species have
certain genetic likenesses. They have
the same number of chromosomes and
the same arrangement of genes on the
chromosomes. The development of a
species is a process called speciation
(spee-shee-dy-shun ) . All through the
ages species have been disappearing and
new species have been developing. New
species are being formed today, just as
they have in past ages.

The mechanisms of variation, mi¬
gration, or environmental change, selec¬
tion, and isolation that we have been
discussing result in speciation. To re¬
view how speciation may occur, let us
condense the changes of perhaps thou¬
sands of years into a few sentences. Or¬
ganism A is a member of a species popu¬
lation adapted to certain environmental
conditions. However, the environment
is such that migration can occur. Now
we will assume that a mutation occurs
in a member of the species popula¬
tion A. As the mutant gene increases in
frequency in the population, certain or¬
ganisms express the trait. We will
designate these variations as AB. As
both A and AB organisms migrate, they
occupy a new and different environ¬
ment. A is not entirely suited to the
new conditions and may perish. How¬
ever, the variation in AB is a favorable
adaptation and it survives. Additional
mutations occur. Those that are favor¬
able to the new environment are pre¬
served in offspring. Finally, a new spe¬
cies that we will designate as B is pro¬
duced. Variation and selection have
separated A and B to the extent that
they can no longer interbreed.

13-9 Acer saccharinum, the silver maple, is
shown here at the top, while Acer rubrum,
the red maple, is pictured at the bottom.
Account for the differences in speciation in
this genus. (U. S. Forest Service)

Speciation in maples. Maples are found
in many environments of North Ameri¬
ca. Several kinds have been introduced
from Europe and Asia and thrive in our
yards and parks and along city streets.
Altogether 60 or 70 maple species are
distributed through the Northern Hemi-

CHAPTER 13 ORGANIC VARIATION 195

sphere. Are all of these maples de¬
scendants of the same ancestral stock
that lived many ages ago? Similarities
in all of these maple species would in¬
dicate that they are. How, then, did
they become so different?

Suppose that some time in the past
a variation occurred in a maple that
adapted it for life in wetter surround¬
ings than maples had usually occupied.
This might have been a difference in
root structure. This new variety flour¬
ished in wet environments. Other varia¬
tions resulted in further adaptations to
wet surroundings. Finally a new maple
species evolved. We call it the silver
maple (Acer saccharinum) . Today the
silver maple is found throughout the
eastern part of the United States. It
towers to a height of 60 to 80 feet in
bottomlands, swamps, stream borders,
and river floodplains (lowlands that
flood seasonally). While it lives best
in lowlands, it may also tolerate much
drier situations and, for this reason, is
widely planted as a fast-growing shade
tree. Perhaps you know it as the soft
maple. It grows in shade in wet sur¬
roundings but requires more light in
drier situations. Foresters are familiar
with a variation of the silver maple that
has deeply cut leaves. The cut-leaf
maple grows more slowly and seldom
reaches the size of a silver maple. Can
you think of a reason why? Perhaps
the cut-leaf maple is a new species in
the making.

Maples in various forms occupy a
wide range of environments in North
America. The sugar maple and black
maple thrive in the forests of eastern
United States, while the moosewood
maple mingles with the pines and hem¬
lock of the northern forest. The striped
maple prefers the elevations of eastern
mountain ranges. The bigleaf maple is

limited to a narrow belt in the Pacific
coastal area from Alaska to California,
where it lives in moist situations in foot¬
hills and low mountains.

While all of the maple species have
distinguishing characteristics, they have
certain characteristics common to all
maples. All maples in the world pro¬
duce double-winged fruits known as
samaras. The arrangement of buds on
the stem is opposite in all maples. Their
leaves tend to have a distinctive vein
pattern. We refer to these similarities
as genus characteristics. They repre¬
sent a stable gene pool which has not
changed as maples have evolved during
speciation. Characteristics such as these,
in addition to indicating evolutionary
patterns, are important considerations in
classification, a topic we shall discuss in
Chapter 14.

Adaptive radiation. The evolutionary
pattern we have discussed in speciation
is called adaptive radiation. It consists
of a branching out of a population
through variation and adaptation to oc¬
cupy many environments. Speciation
is evolution on a small scale. Often
differences between species are slight.
However, at some time much greater
changes must have occurred and organ¬
isms must have branched out in a totally
different direction.

Early in the Cenozoic Era, approxi¬
mately 60 million years ago, primitive
mammals were replacing the reptiles of
earlier times. From these early mam¬
mals, three distinct lines arose by adap¬
tive radiation. These were the egg-
laying mammals, pouched mammals,
and placental mammals. At one time,
pouched mammals, or marsupials, were
prominent in the animal population of
the earth. In your study of mammals
in Chapter 38, you will discover why
the biologist considers marsupials more

196 UNIT 2 THE CONTINUITY OF LIFE

primitive and less favorably adapted to
a land environment than the more re¬
cent placental mammals. Most of the
mammals such as the dog, cat, horse
and others with which you are familiar
belong to this latter group.

For several million years placental
mammals have been replacing the mar¬
supials on most of the continents.
Marsupials could not maintain their
numbers against placental predatory
mammals, such as the fox, wolf, lion,
tiger, and leopard. But sometime many
million years ago, Australia became iso¬

lated as an island continent. Marsu¬
pials dominated the mammal popula¬
tion and placental mammals never
arrived to challenge them. As a result
marsupials in Australia underwent the
same adaptive radiation that was occur¬
ring among the placental mammals of
other continents. The result was wide
variation in form and in ecological re¬
lations.

When you think of Australia, per¬
haps you also think of a kangaroo.
What would you compare with a kan¬
garoo among North American mam¬
mals? If you look at the head of the
kangaroo and watch it graze, you would
probably say deer or antelope. But a
biologist would say the opossum, for
here is a true relationship, because the
opossum is a marsupial. A small bear¬
like marsupial lives in the eucalyptus
groves of Australia. We call it a Koala
bear. But is it a bear because it re¬
sembles one? The same adaptive radia¬
tion that produced the bear in other
parts of the world may have produced
the Koala in Australia. The same re¬
semblance of marsupials and placental
counterparts is to be found in the mar¬
supial moles and true moles, marsupial
Tasmanian wolf and wolves of other
continents, the wombat and the wood¬
chuck and the phalanger and the squir¬
rels. When you compare the kanga¬
roo, Koala bear, marsupial mole, Tas¬
manian wolf, wombat, and phalanger,
you see the great diversity among mar¬
supials resulting from adaptive radiation.

13-10 The wombat (top) is a beaver-like
burrowing marsupial whose geographical
habitat is the hilly country of eastern Aus¬
tralia. The groundhog (bottom) is a placen¬
tal rodent occurring in North America. Note
the similarity of form and function in spite
of differences in classification. (Top: Aus¬
tralian News and Information Bureau; bot¬
tom: Annan Photo Features)

CHAPTER 13 ORGANIC VARIATION 197

On the other hand, compare the deer,
bear, mole, wolf, woodchuck, and squir¬
rel, and you are impressed with the re¬
sults of adaptive radiation in placental
mammals.

Convergent evolution. Now let us con¬
sider a condition nearly the reverse of
adaptive radiation. In this case, the
environment is the same and organisms
of entirely different origin evolve in a

13-11 The sea lion above and the whale be¬
low show similar adaptations because both
inhabit the same general environment, even
though they belong to different orders of
mammals. (Top: Marineland of the Pacific;
bottom: N. Y. Zoological Society)

manner that results in certain similar¬
ities. We speak of this kind of change
as convergent evolution. Consider the
marsupials and placental mammals we
used as examples of adaptive radiation;
when we compare all members of either
group with each other, we can illustrate
convergent evolution by considering the
same organisms from a different point
of view. Consider the marsupial mole
and the true mole, a placental mammal.
Adaptation to similar surroundings and
mode of life has made these animals
similar, even though they have only
a distant biological relationship. The
whale and the seal are other good ex¬
amples of convergent evolution. Both
live in the open sea. They have flip¬
pers for locomotion. Both, while lung
breathers, can hold their breath and
submerge temporarily. A thick layer of
fat beneath the skin protects both ani¬
mals from loss of body heat in the cold
ocean water. The living world is full
of examples of convergent evolution in
which unrelated organisms develop
similar structures as adaptations to an
environment that they share.

IN CONCLUSION

For many millions of years organisms of the earth have been undergoing
change. With change may come improvement as the process of natural selec¬
tion weeds out the undesirables and poorly adjusted organisms. Thus life
forms evolve to occupy their respective places in the face of change.

Modern genetics has placed the mechanisms of evolution in a more logi¬
cal, reasonable, and comprehensible light. With gene and chromosomal mu¬
tations come variations — both good and bad. Natural selection operates to
sort the good from the bad. Change meets change. This is the story of
organic evolution.

198 UNIT 2 THE CONTINUITY OF LIFE

The evolutionary process has resulted in the great diversity of life on earth
today. In the next chapter we shall discuss the biologist’s methods of naming
and classifying these numerous and varied living things.

BIOLOGICALLY SPEAKING

adaptive radiation
barrier

convergent evolution
corridor
evolution
geological era

geological period
geologist

homologous structures
industrial melanism
isolation
migration

natural selection

speciation

species

survival of the fittest
sweepstakes dispersal
vestigial organ

QUESTIONS FOR REVIEW

1. List several biological evidences of change.

2. Describe the three theories in Lamarck’s explanation of evolution.

3. What concepts are included in Darwin’s theory of evolution?

4. What significant contribution did DeVries make to evolution?

5. Explain how gene mutations cause variations in organisms.

6. Explain why gene mutations seldom appear in the individual in which
they occur.

7. List several causes of chromosomal mutations.

8. Explain how recombination operates as a source of variation.

9. How are mutant traits expressed through recombination?

10. List two direct evolutionary results of migration.

1 1 . Why are corridors important in migration?

12. Explain how a black moth appeared in the peppered moth population in
Manchester, England, in 1845.

13. Describe several barriers that may isolate an environment.

14. How may isolation of a population lead to speciation?

15. Generally, what occurs in adaptive radiation?

16. Give an example of convergent evolution in organisms you have seen.

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the importance of structural, physiological, and biochemical sim¬
ilarities in determining paths of evolution.

2. Explain the development of the long neck of the giraffe according to
Lamarck, according to Darwin, and according to modern theory.

3. Discuss the genetic significance of the results of studies of the peppered
moth in England.

4. Discuss mutations, recombination, interbreeding, and selection as mech¬
anisms in evolution.

5. Compare adaptive radiation and convergent evolution as factors in evo¬
lution.

CHAPTER iU

THE DIVERSITY
OF LIFE

The science of classification. In our
study of evolution in the previous chap¬
ter, we discussed the various species of
maples and the way in which they have
probably evolved by adaptive radiation.
The science of genetics has given us a
foundation for understanding how such
evolution occurs. A study of evolution
has in turn enabled us to find natural re¬
lationships between living things. Early
biologists, however, attempted to group
and name living things without the
knowledge of genetics and evolution
that we have now. The classification of
organisms, a branch of biology known
today as taxonomy, has been of concern
to biologists since ancient times.

Aristotle, the Greek philosopher,
probably devised the first classification
system. He divided plants into three
groups: the herbs, with soft stems; the
shrubs, with several woody stems; and
the trees, with a single woody trunk.

He divided animals into three groups
on the basis of where they lived: the
water dwellers, the land dwellers, and
the air dwellers.

In the 18th century the great Swed¬
ish botanist Carolus Linnaeus (li -nee-
us) devised a classification system that
in many respects is still in use today, in
spite of the fact that he had no knowl¬
edge of genetics or evolution. Lin¬
naeus believed that the main aim of sci¬
ence was to find order in nature. He
recognized the species as the basic natu¬
ral grouping, and thought that species
were unchanging. He sought to group
all the known plants and animals into
a fixed number of species, according to
their structural similarities. He dis¬
regarded any organism that did not fit
into his species categories. Perhaps if
Linnaeus, like Darwin, had recognized
that these organisms that did not fit into
his categories were in the process of
change, the theory of evolution would
have been developed much earlier.

Linnaeus discarded the common
names of plants and gave each one a
scientific name made up of Latin words.
None of these names was taken from
his own language or from any other
modern language. There are several
reasons why Linnaeus chose Latin as
the language of classification. First,
since it was no longer in use, it was un¬
changing. Furthermore, many modern
languages contain words taken directly
from this ancient language. Latin was
understood by scientists of all countries.
Also, many descriptive Latin words are
ideally suited for identifying the char¬
acteristics of an organism.

Linnaeus published his list of plant
names in 1753 and his list of animal
names in 1758. Each scientific name
had at least two parts. Usually the
name referred either to some charac-

199

200 UNIT 2 THE CONTINUITY OF LIFE

14-1 The Latin name for the black bear (left) is Ursus americanus, while that
for the grizzly is Ursus horribilis. (Left: Annan Photo Features; right: Harrison
from Monkmeyer)

teristic of the organism or to the per¬
son who named it. Many of his names
are in use today and can be recognized
by the L. that appears at the end of
them.

How scientific names are written. Lin¬
naeus’ system of giving each organism
a scientific name of two or more parts
is called binomial nomenclature , or
“two-word naming.” The first name
refers to the genus and always begins
with a capital letter. The species name
follows and usually begins with a small
letter. The genus name is usually a
noun and the species name an adjective.
The placing of the noun before the
adjective is regular Latin order. We
use a similar system in official lists of
names where John Smith appears as
Smith, John. In addition, scientific
names are usually printed in italic type
or underlined.

The species is still the basic group
used in classifying organisms. The
members of a species are similar in
structural characteristics, and they can
mate and produce fertile offspring.

Thus, all domestic cats are of one spe¬
cies although they may differ in size,
color, and shape. All human beings
belong to the same species.

Closely related species are placed
together in the larger group called the
genus. For example, Pinus is the name
of the genus into which all pine trees
are grouped. There are many different
species of pine trees. Pinus resinosa,
for example, is the red pine, and Pinus
strobus is the white pine.

In devising his standardized nam¬
ing system, Linnaeus enabled biologists
to avoid the confusion and the mislead¬
ing nature of common names. The
mountain lion, for example, is also
known as the puma, or cougar. It is
called the panther, silver lion, Ameri¬
can lion, mountain demon, mountain
screamer, king cat, sneak cat, varmint,
brown tiger, red tiger, and deer killer.
Under the Linnaean system, however,
all scientists easily identifv this animal
as Felis concolor. The common house
cat is Felis domestica, as it belongs to
the same genus as the mountain lion

CHAPTER 14 THE DIVERSITY OF LIFE 201

but is of a different species. Felis onca
is the jaguar, while you will recognize
Felis leo as the real African lion.

Since organisms are classified ac¬
cording to structural similarity under
the Linnaean system, scientific names
are not as misleading as most common
names. For example, what is a fish? If
you think of a fish as an animal with
a backbone, scales, fins, and gills, you
are using the name correctly and scien¬
tifically. The perch, cod, halibut, bass,
and salmon are fishes. But what about
the silverfish? It is an insect. We call
clams and oysters shellfish. And we
call other animals in no way resembling
a true fish crayfish, jellyfish, and star¬
fish. We shall use these names, even
in the study of biology, because they are
familiar. But when you learn more
about these animals you will under¬
stand why the term “fish” is misleading
in referring to them.

The basis of scientific classification.
From the time of Linnaeus until re¬
cently, most scientific classification has
been based on structural similarity. For
example, the cow is structurally similar
to the bison and the deer. All are cud-
chewing mammals with large molar
teeth for grinding plant foods and two¬
toed hoofs adapted for weight bearing
and running over hard ground.

On this basis biologists have clas¬
sified organisms into groups, starting
with very large divisions and continuing
with smaller groups to a single species.
The smaller the classification group,
the more similar its members are.

14-2 Although the common names of these
animals are starfish, crayfish, and jellyfish,
not one of them is a true fish. They are not
even structurally like a fish, nor are they
closely related to one another. (Walter
Dawn)

202 UNIT 2 THE CONTINUITY OF LIFE

A MODERN CLASSIFICATION OF ORGANISMS

Kingdom — Protista

Organisms having a simple structure; many unicellular, others colonial or multicel¬
lular, but lacking in specialized tissue; both heterotrophic and autotrophic; neither
distinctly plant nor distinctly animal.

PHYLUM

ORGANISMS

Schizomycophyta

Bacteria

Cyanophyta

Blue-green algae

Chlorophyta

Green algae

Chrysophyta

Golden-brown algae, or diatoms

Pyrrophyta

Dinoflagellates and crvptomonads

Phaeophyta

Brown algae

Rhodophyta

Red algae

Mycophyta

Fungi

Myxomycophyta

Slime molds

Sarcodina

Amoeboid organisms

Mastigophora

Flagellates

Ciliophora

Ciliates

Sporozoa

Plasmodium

Kingdom — Plantae

Multicellular plants having tissues and organs; cell walls containing cellulose; chloro¬
phyll a and b present and localized in chloroplasts; food stored as starch; sex organs

multicellular; autotrophic.

PHYLUM

Bryophyta

SUBPHYLUM

CLASS

ORGANISMS
Liverworts, hornworts,
and mosses

Tracheophyta

Vascular plants

Psilopsida

Psilotum, Tmesipteris
(only living genera)

Lycopsida

Club mosses

Sphenopsida

Horsetails ( Equisetum ),
and calamites

Pteropsida

Filicineae

Ferns

Gymnospermae

Seed ferns, cvcads,
Ginkgo, and conifers

Angiospermae

Flowering plants

CHAPTER 14 THE DIVERSITY OF LIFE 203

Kingdom — Animalia

Multicellular animals having tissues and, in many, organs and organ systems; pass
through embryonic or larval stages in development; heterotrophic.

PHYLUM

SUBPHYLUM

ORGANISMS

Porifera

Sponges

Coelenterata

Coelenterates

Ctenophora

Comb jellies

Platyhelminthes

Flatworms

Nemertea

Ribbon worms

Nematoda

Roundworms

Nematomorpha

Horsehair worms

Acanthocephala

Spiny-headed worms

Trochelminthes

Rotifers

Bryozoa

Bryozoans, sea mosses

Brachiopoda

Brachiopods, or lampshells

Phoronidea

Phoronis

Chaetognatha

Arrow worms

Mollusca

Mollusks

Annelida

Annelids, segmented worms

Arthropoda

Arthropods

Echinodermata

Echinoderms

Chordata

Chordates

Hemichordata

Tongue worms, acorn worms

Tunicata

Tunicates

Cephalochordata

Lancelets

Vertebra ta

Vertebrates

THE CLASSIFICATION OF SIX DIFFERENT ORGANISMS

Man

Grasshopper

Dandelion

White pine

Ameba

Typhoid bacterium

Kingdom

Animalia

Plantae

Protista

Phylum

Chordata

Arthropoda

Tracheophyta

Sarcodina

Schizomycophyta

Class

Mammalia

Insecta

Angiospermae

Gymnospermae

Rhizopoda

Schizomycetes

Order

Primates

Orthoptera

Campanulales

Coniferales

Amoebida

Eubacteriales

Family

Hominidae

Acridiidae

Compositae

Pinaceae

Amoebidae

Bacteriaceae

Genus

Homo

Schistocerca

Taraxacum

Pinus

Amoeba

Eberthella

Species

\sapiens

americana

officinale

strobus

proteus

typhosa

204 UNIT 2 THE CONTINUITY OF LIFE

Recently, however, biologists have
considered other characteristics in clas¬
sifying organisms. One is cellular or¬
ganization, including nuclear structure,
plastids, and other cell organelles. An¬
other is biochemical similarity. For
example, cells of closely related organ¬
isms may synthesize the same organic
compounds. The most conclusive evi¬
dence of relationship between organ¬
isms, however, is in their genetic
makeup, since all the above character¬
istics are determined by genes. If two
animals have the same number of chro¬
mosomes, and the chromosomes of one
animal are identical or very similar in
structure to those of the other animal,
you can be sure that the two animals
are closely related.

Groupings in the Linnaean system. If

you had a specimen of each of the more
than one million kinds of plants and
animals known to exist and started
grouping them, where would you begin?
Wouldn’t it be best to divide them first
into plants or animals? This is where
the biologist begins his classification.
He separates living things first into very
large groups called kingdoms. Next he
divides each kingdom into smaller
groups known as phyla. Each phylum is
in turn divided into classes. A class con¬
tains many orders. A division of an
order is a family. A family contains re¬
lated genera, and a genus is composed of
more than one species. Sometimes in¬
dividuals of a single species vary slightly,
but not enough to be considered separate
species. We refer to them as varieties.

14-3 Similarity in structure and function
does not necessarily indicate close biologi¬
cal relationship. These are all flying ani¬
mals but the bat, the dragon fly, and the
bird are not at all closely related. (Top:
Davidson from National Audubon Society;
middle and bottom: Annan Photo Features)

CHAPTER 14 THE DIVERSITY OF LIFE 205

An organism considered a variety has a
third part to its scientific name.

As an example of how organisms
are classified into these groupings, let
us use man. The kingdom Animalia
includes all animals. The phylum
Chordata includes all animals having
backbones, with a few exceptions, which
we shall discuss later. The class Mam¬
malia includes all animals having mam¬
mary glands. The order Primates
includes only a certain group of mam¬
mals that stand nearly erect. Monkeys,
chimpanzees, and gorillas are primates.
The family Hominidae (hoh-mih-nih-
dee) separates early manlike forms from
the other primates. The genus Homo
includes all true men. The species
sapiens (which means “wise”) is the
only surviving species of man on earth.

Some biologists further divide man
into races, which are similar to varieties
in other organisms. The classification
of man and five other organisms may be
found in the table on page 203, bottom.
Problems in classification. As you ex¬
amine various biological references, you
may wonder at the wide variation in the
classification systems. How many king¬
doms do we recognize today? How are

the phyla of living things placed in
these kingdoms, and on what basis are
these distinctions made? As we attempt
to classify the diverse forms of life,
from the simplest bacteria and proto¬
zoans to seed plants and complex verte¬
brates, we must remember that any sys¬
tem of grouping is purely man-made.
We divide living things into classifica¬
tion groups for our own convenience.
In the evolution of life from simple to
more complex forms, nature did not
leave gaps for us to use in establishing
classification groups. The question
then is where do we draw the line in
forming these groups. What is a plant?
What is an animal? When is an or¬
ganism neither plant nor animal?

These questions did not bother
biologists much until recent years.
They recognized only two kingdoms —
plant and animal. The plant kingdom
contained four great phyla, while the
animal kingdom included 20 or more,
depending on the system used. This
traditional classification served very well
for many years. Since it is still the basis
for many of the books you will use as
references, we have included the two-
kingdom system in the Appendix.

14-4 Classify the dandelion and the grasshopper according to phylum. (Walter
Dawn)

206 UNIT 2 THE CONTINUITY OF LIFE

14-5 Here are four organisms: a date palm, a barberry sheep, a paramecium,
and a euglena. It is simple to classify the first two as either animal or plant,
but quite a different story to classify microscopic organisms into their respec¬
tive groupings. (Walter Dawn)

In the past few decades a great
amount of research has focused on sim¬
pler forms of life. Are these organisms
plants? Are they animals? Actually
they are neither. They are more closely
related to one another than to organ¬
isms we consider definitely plant or ani¬
mal. Traditional classification has pro¬
vided no real place for these “in-be¬
tween” organisms. Thus biologists have
recognized the need for classifying them

in a different way. Some systems place
the bacteria and the blue-green algae in
a kingdom called the Monera (moh-
ner- a) . These organisms have one strik¬
ing characteristic in common — they lack
an organized nucleus. Some biologists
even place the viruses in this kingdom,
while others present valid arguments
claiming that a virus isn’t even a living
organism.

A second kingdom, universally rec-

CHAPTER 14 THE DIVERSITY OF LIFE 207

ognized in more recent classifications, is
known as the kingdom Protista (proh-
tis- ta). There seems to be little doubt
that such a kingdom should be estab¬
lished, but there is disagreement as to
what phyla should be included. If the
kingdom Monera is not recognized,
then bacteria should certainly be placed
among the protists. Protozoans are
also placed in this kingdom. The real
problem arises in what to do with the
algae and fungi. The simpler members
of these groups seem logically to be pro¬
tists. But these same groups include
organisms such as the kelps and mush¬
rooms that are more like true plants.
On the basis of size and limited cell
specialization, however, it may be more
reasonable to place all the various algae
and fungi in the same kingdom. For
this reason, we shall use in this book
a three-kingdom classification system in
which the bacteria, fungi, algae, and
protozoans constitute the kingdom Pro¬
tista (see table on page 202). Remem¬
ber that this is not the only system, nor
is it by any means perfect. We cannot
say that one system is right and another
wrong. We are simply choosing one
system as a convenience in organizing
our study of the world of life.

The definition of a species. An even
greater problem in classification is an
exact definition of a species. We have
defined a species as an individual or dis¬
tinct kind of living thing. But this is
only a working definition because the
biologist cannot always be sure when to
classify two similar plants or animals as
separate species and when to classify
one as a variety of the other. Remem¬
ber, for example, our discussion of spe-
ciation in maples in Chapter 13. It is
difficult to determine exactly when to
name as two different species two types
of maples that are evolving by adaptive
radiation. Biologists usually agree, how¬
ever, on the following three general
characteristics of species:

1. A species is structurally different
from all other organisms except for
sexual differences (male and fe¬
male), and slight variations resulting
from environmental influences.

2. Members of a species can interbreed
and produce offspring that are capa¬
ble of further reproduction. In
other words, all members of a species
have the same genetic makeup.

3. Members of a species have come
from common ancestors and will
continue their species characteristics.

IN CONCLUSION

Although Carolus Linnaeus did not recognize the changing nature of species,
he may be considered one of the great biologists for his contribution in bring¬
ing order to the great diversity of life. Since the 18th century biologists
throughout the world have used his system of binomial nomenclature, and his

system of groupings from kingdom to species.

While classification is a convenience, it is important to remember that it
is of man’s devising, and that in the future we may learn new ways of grouping
organisms that will more accurately reflect their evolutionary relationships.

We are now ready to begin our study of groups of living things. We shall
begin with the viruses, which seem to be on the borderline between life and

nonlife.

208 UNIT 2 THE CONTINUITY OF LIFE

BIOLOGICALLY SPEAKING

binomial nomenclature

class

family

genus

kingdom

order

phylum

species

taxonomy

variety

QUESTIONS FOR REVIEW

1. What is the science of classification called?

2. On what bases did Aristotle attempt to classify plants and animals?

3. Why is Latin an ideal language for biological classification?

4. Explain the binomial system of naming organisms.

5. Discuss ways in which common names are both confusing and misleading.

6. On what characteristics of an organism is its classification based?

7. Name the classification groups from the largest to the smallest.

8. Discuss the problems biologists have encountered in trying to classify or¬
ganisms into kingdoms.

9. How do most biologists define a species?

APPLYING PRINCIPLES AND CONCEPTS

1. Give examples to show that size, habitat, and diet similarities show no true
animal relationships that could be considered in classification.

2. Make a list of plants and animals of your region that have more than one
common name.

3. The fox, wolf, and coyote are different species of the dog family. On the
other hand, the cocker spaniel, collie, and poodle are breeds of the domes¬
tic dog. Distinguish between a species and a breed, or variety.

4. How can the principles of scientific classification be made useful in areas
outside biological study?

RELATED READING

Books

Altenburg, Edgar. Genetics. Holt,
Rinehart and Winston, Inc., New
York. 1957.

Asimov, Isaac. The Genetic Code.

Orion Press, New York. 1962
Auerbach, Charlotte. The Science of
Genetics. Harper and Row, New
York. 1961

Bates, Marston and Humphrey, Philip

S., Editors. The Darwin Reader.
Charles Scribner’s Sons, New York.
1957

Beaty, John Y. Luther Burbank: Plant
Magician. Julian Messner, Inc.,
New York. 1943

Bonner, David. Heredity. Prentice-
Hall, Inc., Englewood Cliffs, N. J.
1961

Cain, A. J. Animal Species and Their

CHAPTER 14 THE DIVERSITY OF LIFE 209

Evolution. Harper and Row, New
York. 1960

Clare, June. The Stuff of Life. Prog¬
ress of Science Series, Roy Publish¬
ers, New York. 1964.

Darwin, Charles. Evolution and Nat¬
ural Selection. Beacon Publishers,
Boston. 1959

Darwin, Charles. Origin of the Species
by Means of Natural Selection.
Collier Books, New York. 1962
Darwin, Charles. Problem of World
Population. Cambridge University
Press, New York. 1958
Dickinson, Alice. Charles Darwin and
Natural Selection. Franklin Watts,
Inc., New York. 1964
Dobzhansky, Theodosius. Genetics and
the Origin of Species , 3rd Rev. Ed.
Columbia University Press, New
York. 1951

Dowdeswell, W. H. The Mechanism
of Evolution. Harper Torchbooks,
Harper and Row, New York. 1960
Fast, Julius. Blueprint for Life: The
Story of Modern Genetics. St.
Martin’s Press, Inc., New York.
1964

Gardner, Eldon. Principles of Genetics ,
2nd Rev. Ed. John Wiley and Sons,
Inc., New York. 1964
Goldschmidt, Richard B. Understand¬
ing Heredity. John Wiley and
Sons, Inc., New York. 1952
Goldstein, Philip. Genetics is Easy ,
2nd Ed. Viking Press, Inc., New
York. 1961

Huxley, Julian. Evolution in Action.
Mentor Books, New York. 1957
(Also Harper and Row, New York.
1953)

James, John. Create New Flowers and
Plants. Doubleday and Co., Gar¬
den Citv, N. Y. 1964
King, Robert C. Genetics. Oxford
University Press, New York. 1962

Lerner, Marguerite R. Who Do You
Think You Are? Prentice-Hall,
Inc., Englewood Cliffs, N. J. 1964

Ludovici, L. Links of Life: The Story
of Heredity. G. P. Putnam’s Sons,
New York. 1962

Montagu, Ashley. Human Heredity ,
2nd Rev. Ed. World Publishing
Co., Cleveland. 1964

Penrose, Lionel Sharpies. Outline of
Human Genetics , 2nd Ed. John
Wiley and Sons, Inc., New York.
1963

Penrose, Lionel Sharpies. Recent Ad¬
vances in Human Genetics. Little
Brown and Co., Boston. 1961

Randal, Judith. All about Heredity.
Random House, Inc., New York.
1963

Roberts, Elmer. Heredity: What and
How We Inherit. Bookmailer,
Inc., New York. 1959

Scheinfeld, Amram. The Human
Heredity Handbook. J. B. Lip-
pincott Co., Philadelphia. 1956

Scheinfeld, Amram. The New You and
Heredity. J. B. Lippincott Co.,
Philadelphia. 1951

Scheinfeld, Amram. Why You Are
You. Abelard-Schuman Ltd., New
York. 1958

Sheppard, P. M. Natural Selection and
Heredity. Harper and Row, Phila¬
delphia. 1960

Snyder, Laurence H. and David, Paul
R. The Principles of Heredity, 5th
Ed. D. C. Heath and Co., Boston.
1957

Sutton, Harry E. Genes , Enzymes and
Inherited Diseases. Holt, Rine¬
hart and Winston, Inc., New York.
1961

Webb, Robert. Gregor Mendel and
Heredity. Franklin Watts, Inc.,
New York. 1963

Winchester, Albert M. Genetics: A

210 UNIT 2 THE CONTINUITY OF LIFE

Survey of Principles of Heredity.
Houghton, Mifflin Co., Boston.
1958

Winchester, Albert M. Heredity and
Your Life: An Account by Every¬
day Human Inheritance. Vantage
Press, New York. 1956
Winters, Laurence M. Animal Breed¬
ing, 5th Ed. John Wiley and Sons,
Inc., New York. 1954

Articles

Beam, A. G. and German, J. L., III.
“Chromosomes and Disease,” Sci¬
entific American. November, 1961
Beer, Wolfgang and Ulrich Clever.
“Chromosome Puffs.” Scientific
American. April, 1964
Crick, F. H. C. “The Structure of the
Heredity Material.” Scientific
American. October, 1954
Crow, James F. “Ionizing Radiation

and Evolution.” Scientific Ameri¬
can. September, 1959

Deevey, Edward S. “The Human Popu¬
lation.” Scientific American. Sep¬
tember, 1960.

Dobzhansky, Theodosius. “The Ge¬
netic Basis of Evolution.” Scien¬
tific American. January, 1950

Hotchkiss, Rollin D. and Esther Weiss.
“Transformed Bacteria.” Scientific
American. November, 1956

Hurwitz, Jerard and J. J. Furth. “Mes¬
senger RNA.” Scientific Ameri¬
can. February, 1962.

Lack, David. “Darwin’s Finches.”
Scientific American. April,
1953

Mangelsdorf, Paul C. “The Mystery of
Corn.” Scientific American. July,
1950

Nirenberg, Marshall W. “The Genetic
Code.” Scientific American. March,
1963

UNIT THREE

MICROBIOLOGY

Your study of microbiology will take you to the fringe of life, where organic mole¬
cules first exhibit properties which we associate with the living condition. In the
composition of a virus, we find nuclear substances without a cellular organization.
Continuing from this primitive state, we find cells of increasing complexity which
constitute entire organisms. We cannot designate them as definitely plant or ani¬
mal and, therefore, we refer to them as protists. In this vast assemblage of lowly
organisms, we find cells which lead a solitary life, cells grouped in colonies, and
organisms which approach the multicellular organization of higher forms of life.

CHAPTER 15

THE VIRUSES

Viruses — living or nonliving. In be¬
ginning our study of microbiology with
the viruses, we are introducing one of
the most recent and exciting areas of
biology, the science of virology. We
are also presenting an entirely new con¬
cept of the living and the nonliving, for
a virus qualifies in both categories and
seems to shuttle back and forth be¬
tween the two.

Before biologists had explored vi¬
ruses and understood their structure

and activities, they had drawn a definite
line between nonliving substances and
living organisms. Then came our
knowledge of viruses to upset what we
had thought to be a clear-cut distinc¬
tion. In a virus we find a particle defi¬
nitely linked to biochemical processes
in its organization and unlike any non¬
living material, yet in itself not actually
living. But in the presence of a living
svstem within a cell, a virus seems to be
very much alive. Could it be that a
vims is alive at some times and not at

others? Who can say, when no one has
yet given an adequate definition of life?

Regardless of the classification of
viruses as living or nonliving, however,
it is appropriate to deal with them in a
unit on microbiology. If they are non¬
living, they are unique because their
influence on cell activities is different
from that of any other nonliving mate¬
rial. If they are living, they are cer¬
tainly the most basic organisms, repre¬
senting life at the molecular level.

What are viruses? At the mention of
the word virus, you probably think of
an agent of disease. In many respects
you would be right. You are familiar
with the polio virus and probably asso¬
ciate viruses with smallpox, chickenpox,
influenza, rabies, and the common cold.
These are but a few virus diseases.
More than 300 different viruses are
known to produce diseases in various
organisms. Scientists, too, thought that
all viruses were pathogenic agents until
recent years. Would it surprise you to
learn that many virus particles are ap¬
parently harmless and that few cells
escape some kind of virus invasion?

Virus particles are noncellular, or
perhaps we should say subcellular. That
is, they are below the level of cellular
organization. They have no nucleus, no
cytoplasm, and no surrounding mem¬
brane. They are larger than molecules
yet much smaller than the smallest cells.
The name filterable virus refers to the
fact that they pass through the extremely
small pores of unglazed porcelain filters
used in separating bacteria from fluids.

All but the largest virus particles
are invisible under even the highest
magnification of a light microscope.
For this reason little could be deter¬
mined about the structure of a virus
until the invention of the electron mi¬
croscope. With this instrument, even

212

CHAPTER 15 THE VIRUSES 213

the smallest viruses have been photo¬
graphed. We are now familiar with
the form of various viruses as well as
with the size of their particles.

Viruses tend to be one of four
shapes. Some are in the form of slen¬
der, needlelike rods, while others are
spherical or cubical or brick-shaped.
Still others have an oval or many-sided
head and a slender tail somewhat re¬
sembling a tadpole.

Viruses are measured in millimi¬
crons, for which we used the abbrevia¬
tion m/x- When you consider that one
millimicron is 0.001 micron and that one
micron is 0.001 millimeter, you can ap¬
preciate the extremely small size of virus
particles. Figure 15-1 shows the aver¬
age diameters of several viruses. The
smallest bacteria, which would appear
as tiny specks under the high power of
your microscope, range from 500 to 750
millimicrons.

The discovery of viruses. Scientists
worked with virus diseases long before
a virus was known to exist. Dr. Edward
Jenner performed the first vaccination
against smallpox in 1796 when he trans¬
ferred a virus-containing fluid from a
cowpox sore on the hand of a dairvmaid
to a scratch on the arm of an eight-year-
old boy. About a century later Louis
Pasteur discovered that the rabies in¬
fection centered in the brain and spinal
cord and successfully transmitted the
disease to a laboratory animal by inject¬
ing infected brain and spinal cord sub¬
stance. While both of these men made
significant medical contributions that
we shall discuss later, neither had any
concept of a virus as an agent of infec¬
tion. Other 19th and 20th century in¬
vestigators working with virus diseases
described virus-containing materials as
contagious fluids, destructive chemical
substances, and destructive enzymes.

In 1935 Dr. Wendell Stanley made
a significant discovery while working
with a disease of tobacco plants at the
Rockefeller Institute. The disease is a
virus infection known as tobacco leaf
mosaic, one of many similar plant in¬
fections. The name mosaic refers to a
curious pattern of light green and yel¬
low areas that appears in the leaves as
tissues are destroyed by virus attack.
As the disease progresses, the leaves be¬
come stunted and wrinkled. In the
study of this disease Dr. Stanley ex¬
tracted virus crystals from infected
leaves and found that they could be
stored in an apparently nonliving con¬
dition. When injected into a healthy

Diameter or
length X width
in millimicrons

750

Serratia marcescens
(a bacterium)

450

Psittacosis
(parrot fever virus)

270x230

Cowpox virus

115

Influenza virus

Equine

encephalomyelitis

virus

300x15

Tobacco mosaic

virus

Yellow fever virus

Polio virus

15-1 Comparative sizes of a bacterium and
selected viruses.

214 UNIT 3 MICROBIOLOGY

plant, the virus crystals produced the
leaf mosaic disease. For his outstand¬
ing contribution in isolating the first
virus, Dr. Stanley was awarded the
hiehlv coveted Nobel prize in chemistry
in 1946.

The composition of viruses. In recent
years scientists have determined the
chemical composition of virus particles.
A group of investigators at the Univer¬
sity of California analyzed the shell , or
outer covering, of the tobacco mosaic
virus and found it to be composed of
elongated protein fibers. Within this
protein shell is a core of nucleic acid.
In some viruses, including the tobacco
mosaic, the core is RNA. In others it
is DNA. It is interesting that the cells
of organisms contain both of these nu¬
cleic acids, while viruses contain one or
the other, but never both. The pres-

15-2 The rod-shaped particles of tobacco
mosaic virus, magnified 100,000 times. (C. E.
Hall)

ence of these genetic materials in vi¬
ruses tends to place them in the cate¬
gory of living organisms. The nucleic
acid structure of the influenza virus, for
example, gives it certain characteristics
that are duplicated in new influenza
virus particles as they are formed.
Properties of viruses. It is in the activity
of viruses that we find certain charac¬
teristics that make them different from
cells and of questionable status as liv¬
ing things. A virus particle may be ac¬
tive only in direct association with the
content of a living cell. Within the cell
a virus may alter the enzyme systems
and thus cause destruction of the cell.
Removed from a cell a virus ceases all
apparent activity but still retains its abil
ity to infect a cell.

A virus cannot reproduce actively.
That is, it cannot duplicate its own
structure in the manner in which cells
multiply by fission. A virus may repro¬
duce passively, however, by altering the
enzymes that control protein synthesis.
In this way the virus uses the machinery
of the cell to form virus particles rather
than cell proteins. Thus a virus has an
action similar to that of a gene. Could
it be then that a virus particle is a gene
without a “home” until it invades a
cell?

In the discussion of the living con¬
dition in Chapter 2, we referred to vari¬
ous processes we associate with life.
Cells carry on metabolic activities con¬
tinuously. They use organic molecules
for growth and oxidize fuel molecules
to supply the energy necessary to sup¬
port cellular activities. Are metabolic
activities essential to a virus? An iso¬
lated virus requires no metabolic activity.
Within a living cell it is capable of lim¬
ited metabolic activity made possible
by the machinery of the cell.

The properties of many viruses are

CHAPTER 15 THE VIRUSES 215

'' ' / "

Core of RNA
orDNA

15-3 This schematic drawing shows the
composition of a virus.

altered by the environment in which
they are grown. This applies to the
potency, or virulence (vz’r-yoo-lents), of
many disease-producing viruses. For ex¬
ample, when rabies virus is grown in
cells of the brain and spinal cord of
dogs, its virulence for man and dogs in¬
creases. But if the virus is grown in rab¬
bits it becomes less virulent for man
and dogs but increases in virulence for
rabbits. So, one might argue that the
structure of a virus may be altered by
the chemical nature and activity of the
cell that produces it.

Further variations in the structure
of viruses occur as mutations. More
than 50 mutant strains of the tobacco
mosaic have been discovered. These
mutant strains differ in virulence and
in the symptoms they produce in the
host plant.

You have heard of highly virulent
strains of influenza virus that strike
populations at various intervals and of
less virulent strains that cause much less
serious infections. Perhaps the virus
increases in virulence as it passes from
one person to another during an epi¬

Tail attached
to host

Shell of
protein

demic. Or we may be dealing with vari¬
ous mutant viruses that differ in viru¬
lence.

Classification of viruses. If a virus is
a product of the disorganized machinery
of a host cell, it is logical that a particu¬
lar kind of cell would be necessary to
produce a particular kind of virus. The
relation of viruses to host cells is in fact
highly specific. We classify viruses on
the basis of their hosts, as follows:

1. Bacterial viruses , which invade the
cells of bacteria.

2. Plant viruses , which live in the cells
of seed plants, especially flowering
plants.

3. Human and animal viruses , which
live in human and animal cells.

Actually the virus-host relationship
is even more specific than these large
groups would indicate. Bacterial vi¬
ruses invade only specific kinds of bac-

15-4 An electron micrograph of a virus
magnified 500,000 times. (T. F. Anderson)

216 UNIT 3 MICROBIOLOGY

15-5 An electronmicrograph of a strain of
bacteriophage attached to the cell of its
host, the colon bacillus, magnified 54,000
times. (T. F. Anderson)

teria. Similarly a plant virus may be
specific for the cells of flower petals,
leaf tissues, or stem tissues of a particu¬
lar kind of plant. A specific human or
animal virus may require the environ¬
ment of cells of the skin, the respiratory
organs, or the nervous system. It might
surprise you to know that some viruses
are even more specific than this. Polio
viruses attack only the cells of one kind
of nerve in the brain and spinal cord.
Similarly mumps is an infection of only
one pair of salivary glands, and never
invades the other pairs.

Bacteriophage viruses. Much of our
knowledge of viruses has come from
investigations of the bacterial viruses,
often referred to as phages (fayjs).
These are of the tadpole form, consist¬
ing of a round or many-sided head and
a slender tail. The shell of a phage
is composed of protein, while the core
is exclusively DNA. Recently, RNA
phages have been found.

The very first investigations of
phages were conducted by two scientists
working independently at about the
same time. F. W. Twort, working in
England in 1915, made an extensive
study of a peculiar phenomenon he ob¬
served in the type of spherical bacterium

called staphylococcus (staf-i-loh -kahk-
us). For some unexplained reason,
colonies of these bacteria growing in a
culture suddenly developed holes, or
plaques (plaks), as we call them today,
which spread until the entire colony was
destroyed. He found further that the
agent that caused destruction of the
bacteria could be transferred to other
colonies with an inoculating needle.
Two years later F. H. d’Herelle con¬
ducted similar studies of a mysterious
disease of the dysentery bacillus, a rod¬
shaped bacterium, in France. He de¬
tected the same plaques that Twort had
seen. d’Herelle first named this invisi¬
ble agent of bacterial destruction bac¬
teriophage ?, or “bacteria eater.”

The lytic cycle of a phage virus.
While early investigators were able to
demonstrate the action of a phage on
bacteria, it remained for the electron
microscope to reveal what actually hap¬
pens during the destructive attack.
Figure 15-5 is an example of an elec¬
tron micrograph showing a greatly en¬
larged bacterial cell with phage viruses
approaching it and others lodged, tail
down, on the cell wall. Biologists have
photographed all stages in the destruc¬
tion of a bacterial cell.

We refer to the disintegration of
a bacterium as a result of invasion by a
phage as lysis (ly- sis). A phage that
produces such a lytic cycle of destruc¬
tion is designated as a virulent phage.
As we describe the lytic cycle shown in
Fig. 15-6, check each numbered stage.

1. We begin with a normal, uninfected
bacterium.

2. A phage virus has attached, tail
down, to the cell wall of the bacte¬
rium by tiny hooks. An enzyme in
the tail of the phage is dissolving an
opening in the bacterial wall.

3. Having formed an opening in the

CHAPTER 15 THE VIRUSES 217

wall, the tail contracts and injects
the DNA of the phage core into the
bacterial cell. The empty protein
phage shell remains outside.

4. Within a few minutes the phage
DNA appears near the DNA of the
host cell, which normally controls
the formation of bacterial sub¬
stances. The phage DNA takes over
this control, and the machinery of
the bacterium is used to synthesize
phage DNA and protein molecules.
The bacterium has become a virus
factory.

5. Soon the bacterium contains 100 or
more phage particles.

6. The bacterial cell disintegrates, re¬
leasing its phage content to attack
100 or more other bacterial cells.

This entire cycle, from entry of

phage DNA to bursting of the bacterial
cell and release of phage particles, re¬
quires up to 45 minutes. If each phage
multiplies as much as 100 times in a
lytic cycle, you can see how an entire
colony of bacteria composed of many
millions of cells can be destroyed within
a few hours. The rate of bacterial de¬
struction by a phage can be demon¬
strated in a broth culture. When a
phage in the amount of one-billionth
the quantity of bacteria is added to
such a culture, all or nearly all of the
bacteria are destroyed within three to
fours hours.

Biologists have used the following
analogy to describe a lytic phage cycle
graphically. A tank moves up to the
wall of an automobile factory in which
an assembly line is in operation. The

$ ft

< _ /

2 ft ,

_ _ ^

s _ *

—

Bacterial cell

(^J Free phage

(^) Protein core
^ DNA core

15-6 The lytic cycle of destruction as produced by a virulent phage attacking
a bacterium. The steps are described on pages 216 and 217.

218 UNIT 3 MICROBIOLOGY

tank breaks through the wall and its
crew is discharged into the factory to
disrupt the assembly line. Machines
are reset and, using the same materials
that were to have been automobiles,
tanks begin to roll off the assembly line.
Soon 100 or more tanks are rumbling
through the plant. A wall is broken
through and the tanks move out to at¬
tack other automobile factories, disrupt
their assembly lines, and assemble more
tanks!

Biologically we can draw several
important conclusions from an under¬
standing of the lytic cycle of a phage.

1. The free phage cannot reproduce.
While it contains the DNA “blue¬
print” for synthesis of enzymes and
other proteins, it cannot organize
more of its own substance.

2. A phage particle is a nucleoprotein,
consisting of a protein shell surround¬
ing a DNA core. Both its composi¬
tion and its action are similar to
genes normally present in bacteria
and other cells.

3. Bacterial genes control the synthesis
of specific enzymes and other pro¬
teins characteristic of the particular
organism.

4. Phage DNA within a bacterial cell
functions as a gene and apparently
substitutes its control for the action
of bacterial genes.

5. The phage DNA alters the chemical
machinery of the bacterial cell, caus¬
ing it to synthesize phage particles.

Economic importance of bacterial vi¬
ruses. At one time biologists thought
virulent phages might offer a new and
powerful medical weapon in the treat¬
ment of infectious diseases. Destroy
disease-causing bacteria with a lytic
phage — fight disease with disease!
However logical and promising this pro¬
cedure might seem, there are many

problems and limitations. For one
thing it would be very difficult to intro¬
duce the phage at the site of an infec¬
tion, often deep in the body tissues. In
other diseases the infection is wide¬
spread and it would be difficult for a
phage to contact a sufficient number of
bacteria to be effective. Still another
problem is the fact that the human tis¬
sue environment may not be suitable
for phage action. The conditions in
the body are unlike those of a bacterial
culture where phage action may be effec¬
tive. For these reasons, and perhaps
others, the possible medical uses of
phages are not as promising as we once
thought.

Phages are more important, how¬
ever, in industries that require bacterial
action. In the cheese industry, for ex¬
ample, certain bacteria are involved in
the manufacturing process. Can you
imagine the problem that arises if a
phage is accidentally introduced and
the cultures of these necessary bacteria
are suddenly destroyed? This is a con¬
stant danger in industries that require
bacterial processes.

Lysogenic phages — seeds of destruc¬
tion. Certain phage viruses may invade
bacterial cells without causing imme¬
diate destruction. When this happens
the phage DNA discharged into the
host cell may attach to a bacterial chro¬
mosome and become a foreign gene.
When the bacterial genes replicate in
preparation for the splitting of chromo¬
somes during cell fission, the phage
DNA also replicates. Each resulting
daughter cell receives the phage DNA.
In this manner the phage DNA is multi¬
plied generation after generation as a
“stowaway” in bacterial cells, causing
no immediate damage but present as a
potential “seed of destruction.” At
some future time, if conditions within

CHAPTER 15 THE VIRUSES 219

the cell or in the cell environment are
altered, the phage virus may become ac¬
tive, alter the normal synthesis of the
cell, and produce a destructive lytic
cycle. We refer to the delayed action
of these phages as the lysogenic cycle.

Certain lysogenic phages seem to
produce two sets of bacterial descend-
ents. The DNA in these phages does
not replicate. When a division occurs
one cell receives the phage DNA at¬
tached to a chromosome, while the
other does not.

Plant viruses. Many viruses attack the
cells of plants, especially flowering
plants, causing serious damage and
often killing the plant. The core of
many of these viruses is RNA, rather
than DNA as in the phage viruses.
Plant viruses are often named for the
host plant, the specific tissues they at¬
tack, and the nature of the symptoms of
the infection.

We have already referred to the
tobacco leaf mosaic virus, the first dis¬
covered. Similar mosaic infections oc¬
cur in the tomato, potato, bean, and
cucumber. One of the most interesting
mosaic viruses causes an infection in the
cells of flower petals. This results in
light streaks or blotches that contrast
with the normal petal coloration. One
of the best examples of such a color
variation due to a mosaic virus is the
“broken” tulip. The unusual color pat¬
tern of these tulips has brought higher
prices among gardeners, who grew them
for many years without knowing that
they were the result of a virus infection.
Other plant virus diseases include the
potato leaf roll, curly top of beet, peach
rosette, aster yellows, and a serious dis¬
ease of elm trees known as phloem
necrosis.

Plant viruses may be spread in a
number of ways. Insects that suck

juices from leaves act often as agents
of infection. Among these insects are
plant lice, leaf hoppers, mealy bugs,
and thrips. Virus infections are some¬
times spread when gardeners handle dis¬
eased plants.

Human and animal viruses. Many
familiar diseases of man and animals
are caused by viruses. In most of them
the virus invades only specific primary
tissues in which the cell environment is
suitable for multiplication of the virus
particles. Symptoms of virus infections
are as different as the viruses that cause
them. Generally, however, a virus in¬
fection results in the disrupting of
metabolic processes of the cells involved
and damage or destruction of tissue.
A lasting immunity remains after re¬
covery from many virus infections, the
common cold and influenza being no¬
table exceptions. Biologists do not
know the basis for this immunity.

Following is a list of the better-
known virus diseases: smallpox, cow-
pox, chickenpox, shingles, cold sores
and fever blisters, warts, influenza,
measles, German measles (three-day
measles), virus pneumonia, common
cold, parrot fever, yellow fever, infec¬
tious hepatitis, infectious mononucle¬
osis (glandular fever), and mumps.

Are viruses associated with cancer?
One of the most encouraging develop¬
ments in cancer research in recent years
has been the discovery of a possible re¬
lationship between viruses and certain
forms of cancer. Such an association
might provide the basis for long-sought
preventive measures or even for cures
for certain forms of cancer. To date no
viruses have been definitely linked with
human cancers. But results of experi¬
ments with animals have been very en¬
couraging. Dr. Ludwik Gross of the
Veterans Administration Hospital in

220 UNIT 3 MICROBIOLOGY

New York City has found that a form
of leukemia can be produced in mice
by injecting an extract that contains a
virus. Other investigators have pro¬
duced more than 20 different kinds of
malignant tumors in mice, guinea pigs,
and hamsters with similar extracts.

When you think about what you

have learned in this chapter, you may
realize that it is reasonable to assume
that some cancers may be produced by-
viruses. Cancer is abnormal growth of
tissue, and growth is controlled by the
DNA molecule. Since viruses alter the
action of the DNA molecule, they may
very well produce cancers.

IN CONCLUSION

Which side would you take now in a discussion of the status of a virus? Is a
virus a living organism? Is it a by-product of life? Is it, perhaps, a cellular
material that should remain in a cell but escapes to invade other cells?

As we continue the study of microbiology, we take a long step from mole¬
cules to cells. This leads us to perhaps the most simple and primitive forms
of life, the bacteria. Simple as they are, however, we have no problem in con¬
sidering bacteria living organisms, for all have a cellular organization.

BIOLOGICALLY SPEAKING

bacteriophage lysogenic cycle

core lytic cycle

filterable virus plaque

QUESTIONS FOR REVIEW

1. Account for the name filterable virus.

2. Describe four shapes of virus particles.

3. From what source did Dr. Wendell Stanley isolate the first virus?

4. What organic substance forms the shell of a virus? the core?

5. What is meant by the virulence of a virus?

6. Classify viruses into three groups on the basis of the host organism.

7. What contributions in virology were made by F. W. Twort and F. H.
d’Herelle?

8. Describe the lytic cycle of a phage virus.

9. List several limitations in the medical use of a phage to treat infectious
diseases.

10. List several plant virus diseases.

11. Account for the unusual coloration of a “broken” tulip.

12. In what respect is a lysogenic phage a potential “seed of destruction ?

13. List several well-known human virus diseases.

shell

virulence
virulent phage

CHAPTER 15 THE VIRUSES 221

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss various factors that may alter the virulence of a virus.

2. Discuss several biological principles illustrated in the lytic cycle of a phage

virus.

3. Discuss passive reproduction of a virus as compared with active reproduc¬
tion of a cellular organism.

4. Account for the high degree of specificity of viruses.

5. Explain the delayed action aspect of a lysogenic phage.

6. Explain why it is biologically reasonable to assume that some forms of
cancer may be caused by viruses.

CHAPTER 16

BACTERIA
AND RELATED
ORGANISMS

Bacteria — primitive cellular organisms.

As we proceed from the study of viruses
to bacteria and their relatives, we shift
from molecular particles to primitive
cellular organisms. Many biologists be¬
lieve that bacteria were the first forms
of life on the earth, appearing late in
the Archeozoic era (see Fig. 13-1).
Long before there were green plants
capable of photosynthesis, certain primi¬
tive bacteria may have utilized energy
from iron, sulfur, and nitrogen com¬
pounds instead of the sun. Geologists
think that the extensive deposits of iron
ore we are using today are the result of
bacterial action during ancient geologic
times. Later, when green plants began
building up stores of organic com¬
pounds, other kinds of bacteria began
using them as a food supply. Still oth¬
er bacteria invaded the tissues of the

plants themselves as well as the bodies
of animals.

Bacteria have survived through the
ages and have increased their numbers
until today they are the most abundant
form of life. They live, invisibly, al¬
most everywhere. They thrive in the
air, in water, in food, in the soil, and in
the bodies of plants and animals. In
fact, any environment that can support
life in any form will have its population
of bacteria.

Louis Pasteur — the father of bacteri¬
ology. In the history of biology, there
have been a few truly great investigators
who have changed the direction of the
science and opened a whole new field
for experimentation. Louis Pasteur
was such a figure. This great scientist
of a century ago is no stranger to you.
Do you remember the goose-necked
flasks he devised to deal a crushing blow
to the belief in spontaneous generation?
This was only a small part of this great
man’s contribution to science. Let’s
take another look at the man who intro¬
duced microorganisms to the world, rev¬
olutionized the practice of medicine,
and provided much of the basis for the
modern science of bacteriology.

Pasteur was born in Dole, France,
in 1822. He graduated from college at
an early age with a brilliant record of
achievement in chemistry. In 1854,
while a young man of 32, he was ap¬
pointed professor of chemistry and dean
of the University of Lille (leel). It was
here that circumstances combined to
alter the course of biological science for
generations to come.

In the city of Lille, alcohol was
manufactured by fermenting sugar-beet
juice in large vats. Chemists of his day
thought that alcoholic fermentation was
a purely chemical process. They had
seen tiny yeast cells growing in ferment-

222

CHAPTER 16 BACTERIA AND RELATED ORGANISMS 223

16-1 The great French scientist Louis Pas¬
teur in his laboratory. (© 1962, Parke, Davis
& Co.)

ing beet and fruit juices but had always
considered them to be products of spon¬
taneous generation, in no way con¬
nected with the fermentation process.

Then, one day, a serious problem
arose at the fermentation plant. In
several of the vats, the juice was souring
rather than becoming alcoholic. What
had happened to change the chemical
process? Pasteur was called in to try to
find the answer.

In his laboratory, with his micro¬
scope, Pasteur examined the juice that
was fermenting as it should. Dispersed
through the liquid were numerous oval
yeast cells. Over a period of several
hours he watched the cells multiply by
forming chains of buds. As the cells
increased in number, the alcohol con¬
tent increased. Were the yeast cells
producing the alcohol? The answer to
this question might be found in the
sour juice. Accordingly, Pasteur ex¬
amined a drop of liquid from a vat of

sour juice. Here he found an entirely
different population of microorganisms.
Instead of yeast cells he found quivering
rods, smaller than yeasts but apparently
alive. He found that this liquid con¬
tained lactic acid rather than alcohol.
Furthermore, the lactic acid content in¬
creased with the germ population.

This discovery led Pasteur to more
extensive studies of fermentation.
Three years later he set up a small lab¬
oratory in Paris. Here he proved that
fermentations are associated with micro¬
organisms and that the products formed
depend on the organism involved.
Yeasts produce alcohol. Bacteria may
form lactic acid in beet juice and in
milk.

Fortunately Pasteur did not end
his explorations in microbiology with
fermentation studies. Far more impor¬
tant than fermentation was the presen¬
tation of his germ theory to the scien¬
tific world. If bacteria could ferment
beet juice, might other kinds cause dis¬
ease? These are accounts yet to be dis¬
cussed in the life of this remarkable
chemist who became one of the great¬
est biologists of all time.

What are bacteria? Since Pasteur ob¬
served bacteria, they have been consid¬
ered both animals and plants. Until
recently they were classified with the
fungi in the old phylum Thallophyta of
the plant kingdom. More recently bi¬
ologists have avoided calling them any¬
thing but organisms by placing them in
the kingdom Protista. Together with
close relatives they constitute the protist
phylum Schizomycophyta (skiz- o-my-
kahf- i-ta), a name meaning “fission
fungi.”

If you use a virus for comparison,
bacteria are large. You will recall that
a single infected bacterial cell may con¬
tain as many as 100 phage particles.

224 UNIT 3 MICROBIOLOGY

However, compared to the cells of most
other organisms, bacteria are extremely
small. We used millimicrons to meas¬
ure viruses. Bacteria are measured in
microns (one micron = 0.001 milli¬
meter). Spherical bacteria range in
size from about 0.5 to 1.5 /i in diameter,
while the rod-shaped forms average
from about 0.2 to 2 /jl in width and from
0.5 to as much as 10 /jl in length. The
average length of bacteria is approxi¬
mately 1.5 ju. Converting these figures
to inches, bacteria range from about
1/50,000 to 1/10,000 of an inch. If
these figures mean little to you, consider
that several thousand bacteria could be
placed on the period at the end of this
sentence or that a small drop of water
may contain as many as 50 million bac¬
teria.

You can see bacteria with the high-
power magnification of your microscope
(430X) and even determine their shape.
Additional magnification (1,000 to
1,5.00X) is necessary to see them clearly.
However, no light microscope can mag¬
nify sufficiently to reveal most of the
internal structures of bacterial cells.
For this we need the 50,000 to 100,000
enlargement of the electron micro¬
scope.

Forms of bacteria. While bacteria vary
greatly in size, their cells are of three
basic shapes. Some bacterial cells tend
to exist singly when grown in liquid
cultures or broths, while others often
remain attached after cell division and
form colonies of cells. We can classify
the basic cell shapes and groupings as
follows:

coccus (pi. cocci)— cells sphere¬
shaped or globular

diplococcus — cells often joined in
pairs or short filaments
streptococcus — filaments or strings
of cells

staphylococcus — clusters of cells
tetrad — groups of four cells
sarcina — cubes or packets of cells
bacillus (pi. bacilli) — cells elongated
or rod-shaped

streptobacillus — cells joined end to
end forming a filament or thread
spirillum (pi. spirilla) — cells in the
form of bent rods or corkscrews
Structure of a bacterial cell. Bac¬
terial cells are surrounded by a slime
layer of varying thickness. This gelat¬
inous coat may protect the cell from un¬
favorable environmental conditions and
enables it to stick to the surface of a
food supply or to a host cell or organism.
Some bacteria have a thick slime layer
referred to as a capsule. Perhaps you
remember from our discussion of trans¬
formation in Chapter 10 that the pneu¬
monia organism ( Diplococcus pneu¬
moniae) is an example of such an
encapsulated organism. It is interesting
that the presence of the capsule is an
indication of the virulence of the pneu¬
monia organism. Encapsulated forms
produce a serious infection. The cap¬
sule may protect these bacteria from the
normal body defenses against infectious
organisms. Strains of pneumonia or¬
ganisms lacking a capsule are low in
virulence or even noninfectious. Per¬
haps these organisms are easily destroyed
by the body defenses. Some bacterial
masses form gelatinous coatings. Did
you ever remove the gelatinous “mother
of vinegar” from a vinegar jar? This is
such a bacterial mass. In other species,
including iron bacteria, a sheath may
surround an entire filament of bacteria.
Iron compounds are often deposited in
the sheaths of these bacteria, forming
the reddish-brown, slimy masses often
seen in streams.

Beneath the slime layer or capsule is
an obvious cell wall. This gives the

CHAPTER 16 BACTERIA AND RELATED ORGANISMS 225

Diplococcus

(pneumonia)

Streptococcus
(scarlet fever)

Staphylococcus
(boils)

Anthrax with spores
Tetanus with spores

0*r \ \ C* Cr—

Spirillum

rubrum

'it- Yv^i

n ..... .......\ / /

. .. \ r

•• •• • L#'

SPIRILLUM

COCCUS

Streptobacillus

BACILLUS

16-2 This drawing illustrates the three great groups of bacteria with some ex¬
amples of each type.

cell its characteristic shape. A thin,
flexible plasma membrane lies just be¬
neath the cell wall and marks the outer
edge of the cytoplasm. Bacteria lack
a nuclear membrane. However, they
have been found to contain DNA in
genes that are concentrated in chro¬
matin bodies lying near the center of
the cell. Some biologists believe that
these bodies constitute a single bacterial
chromosome. Mitosis does not occur
in bacteria, although there is division
of the chromatin bodies each time a cell
divides. This results in equal distribu¬
tion of DNA in daughter cells.

Many bacteria contain granules of
stored food and other materials dis¬
persed through the cytoplasm. A few
also contain vacuoles of water and dis¬
solved materials. Mitochondria, the
vital centers of respiration in other cells,
are lacking in bacteria. Respiratory en¬
zymes, usually found in mitochondria,
seem to be concentrated on the cell
membrane of a bacterial cell.

Motility in bacteria. Various bacillus
and spirillum forms of bacteria are
equipped with threadlike whips, or fla¬
gella (fla-/eha), which propel the cell
through water and other fluids. Flagel¬

la may be found singly or in tufts at
either or both ends of a bacterial cell.
In some forms flagella are found all
around the cell. Flagella are visible only
when they are treated with special
stains. Under the highest magnifica-

16-3 An electronmicrograph of Diplococcus
pneumoniae. (Robert Austrian and Journal
of Experimental Medicine)

226 UNIT 3 MICROBIOLOGY

16-4 This electronmicrograph of Bacillis ce-
reus shows the cell wall and plasma mem¬
brane clearly. (C. F. Robinow)

tion of the light microscope, they ap¬
pear as minute threads. The electron
microscope shows them to be cytoplas¬
mic extensions that project through op¬
enings in the cell wall. Flagella are
strands of protein molecules resembling
the microscopic fibers composing mus¬
cle. Thus, in the beating flagella of
bacteria, we may have the basis for the
muscle contractions of animal organ¬
isms.

You may recognize motility in bac¬
teria by their random movement in a
microscopic field. All cells move inde¬
pendently in a quivering, twisting man¬
ner. This true movement by means of
flagella should not be confused with an
oscillating or bouncing motion, known
as Brownian movement. This motion
results from the jarring of very small
bacteria, especially coccus forms, by the

movement of molecules or other par¬
ticles in a fluid.

Conditions for growth of bacteria. In

discussing various environmental re¬
quirements for the growth of bacteria,
we need to make a distinction between
activity and survival. Bacteria have the
property of remaining inactive when
conditions for growth are lacking. For
example, they may float high in the air
or he on an object in an inactive condi¬
tion. Then, when environmental con¬
ditions are favorable, they may enter
a period of very rapid growth and re¬
production.

A suitable temperature is essential
for bacterial activity, but this tempera¬
ture varies greatly with different species.
The majority of bacteria thrive at mod¬
erate temperatures, ranging from 80 to
100° F. Those that cause human infec¬
tions grow best at 98.6° F (37° C), or
normal body temperature. But there
are many bacteria that grow best at
much lower temperatures, ranging from
32° F to an upper limit of 85° F.
These bacteria occur in ocean depths,
in the cold soils of the far north, and
high in the stratosphere. At the other
extreme are bacteria that thrive at tem¬
peratures ranging from 110° F to as
high as 185° F. These are found in hot
springs and in the hot environment of
decomposing sewage, silage, and other
plant materials.

Moisture is another growth re¬
quirement for bacteria. Bacterial cells
are 90 percent water. In dry surround¬
ings water loss makes the cells inactive
and prolonged dryness will kill them.

Darkness is also a condition for
best bacterial activity. Exposure to
sunlight may retard growth, and ultra¬
violet radiation actually kills cells. We
make use of radiation effects on bac¬
teria when we sterilize the air in hos-

CHAPTER 16 BACTERIA AND RELATED ORGANISMS 227

pital surgeries with lamps that emit ul¬
traviolet rays.

A suitable food source is still an¬
other requirement for the growth of
bacteria. In this respect bacteria vary
greatly. Some bacteria are highly spe¬
cific in their food needs. Most patho¬
genic bacteria require living tissues or
substances similar to them in chemical
composition. Many bacteria are more
tolerant and can live on a wide variety
of food materials.

Bacterial nutrition. The majority of
bacteria lack the property of synthesiz¬
ing the substances they require and
must therefore live in contact with pre¬
formed organic matter. In this respect
they are heterotrophic. The lack of
ability to carry on photosynthesis puts
them in direct competition with man
and other animals for food. Bacteria we
classify as saprophytes (sap-ro -fyts) uti¬
lize dead organic matter or nonliving
organic substances such as food products.

Parasites , on the other hand, invade the
bodies of plants and animals and take
their nourishment directly from living
tissue. We refer to the organism sup¬
porting the parasite as the host.

Bacteria secrete powerful enzymes.
These organic catalysts are essential in
causing chemical changes both inside
and outside the bacterial cell. Many of
these are digestive enzymes. An en¬
zyme acts only on one kind of food sub¬
stance. Thus, the food source or host
required by a specific kind of bacterium
is determined by the enzymes it pro¬
duces. Enzyme action simplifies com¬
plex organic molecules and converts
them to soluble substances that can be
absorbed through the cell wall and
membrane. Enzymes allow sapro¬
phytes to use a variety of organic mate¬
rials, many of which are useless to other
forms of life. Among these are wood
and other cellulose materials. Parasitic
bacteria lack many of the enzyme sys-

16-5 The flagella of Proteus vulgaris show plainly in this electronmicrograph.
(Houwink, Van Iterson and Biochimica et Biophysica Acta )

228 UNIT 3 MICROBIOLOGY

16-6 An interesting microorganism found in
a soil-sampling program is transferred from
the Petri dish on which it grew with other
types to a tube of nutrient medium of its
own. (Chas. Pfizer & Co.)

terns found in saprophytic organisms.
This explains why they must live in con¬
tact with living tissue and utilize en¬
zyme action of the host cells.

A relatively small number of bac¬
teria are autotrophic. These organisms
synthesize their own organic com¬
pounds. Certain of these organisms,
designated as chemotropic, or chemo-
synthetic, utilize inorganic compounds
as a source of energy. Some of these
oxidize iron compounds. Others use
sulfur compounds. Still others, includ¬
ing nitrite and nitrate bacteria, oxidize
nitrogen compounds. Perhaps the
most unusual bacteria are those capable
of photosynthesis. These photosyn¬
thetic bacteria contain bacteriochloro-
phvll, a pigment similar to the chloro¬
phyll of higher plants. The pigments

in these bacteria are dispersed on in¬
tracellular cell membrane systems rather
than localized in chloroplasts.

Bacterial respiration. Atmospheric oxy¬
gen is an important factor in the growth
of bacteria. Among the many kinds of
bacteria, we find both extremes in free
oxygen relations, some requiring its
presence and others requiring its ab¬
sence. Others are between these two ex¬
tremes.

Certain bacteria require atmospheric
oxygen for respiration in the same man¬
ner as most plants and animals. We
refer to these as obligate aerobes ( air -
ohbs ) , which include such organisms as
the diphtheria and tuberculosis bacilli
and cholera bacteria. These bacteria
split glucose molecules and form carbon
dioxide and water during respiration.

At the other extreme are organisms
classed as obligate anaerobes (an -air-
ohbs ) . These bacteria cannot grow in
the presence of atmospheric oxygen.
Among the best known obligate ana¬
erobes are the tetanus and botulism or¬
ganisms.

The greatest number of bacteria
are facultative anaerobes , which grow
best as aerobes but may grow, at least to
some extent, as anaerobes. Among these
organisms we find the common bacillus
of the human intestine (Escherichia
coli) [esh-er-ifc-ee-a koh- ly] and such
pathogens as the typhoid, diphtheria,
and scarlet fever bacteria. Much less
frequent are the facultative aerobesy
which are primarily anaerobes but are
able to maintain limited activity in the
presence of free oxygen.

We often refer to anaerobic res¬
piration as fermentation. The prod¬
ucts of fermentation vary with the or¬
ganisms involved. In the bacteria that
produce alcohol by fermentation, glu¬
cose molecules are split and a small

CHAPTER 16 BACTERIA AND RELATED ORGANISMS 229

amount of energy is released. Carbon
dioxide and pyruvic acid are formed.
Pyruvic acid is then reduced (hydrogen
is added) to form ethyl alcohol. We
can summarize this fermentation reac¬
tion in the following simplified equa¬
tion:

C6H1206 2C2H5OH + 2C02 + energy

glucose ethyl carbon
alcohol dioxide

Other bacteria produce lactic acid
by fermentation. In this reaction a glu¬
cose molecule is split with the forma¬
tion of pyruvic acid, which is then re¬
duced (hydrogen is added) to lactic
acid. A small amount of energy is re¬
leased with the splitting of the glucose
molecule. We may summarize this

process in the following simplified equa¬
tion:

C6H1206 -» 2C3H603 4- energy

glucose lactic
acid

A few bacteria carry on true anaer¬
obic respiration , which differs from fer¬
mentation. One of the more familiar
true anaerobic reactions occurs in the
formation of methane. In this reaction
hydrogen is combined with carbon di¬
oxide, forming methane and water and
releasing energy as follows:

4H2 + C02-^

hydrogen carbon
dioxide

CH4 4- 2H20 4- energy

methane water

Methane is called marsh gas be¬
cause it may be seen bubbling to the
surface from decomposing organic mat¬
ter accumulated in the bottom of bogs,

swamps, and ponds. It is also present
in natural gas.

Bacterial reproduction. Bacteria mul¬
tiply by dividing. They accomplish
this apparent mathematical impossibil¬
ity by fission, or cell division. They
multiply at an amazing rate when con¬
ditions for growth are ideal. Consider
that a cell may be formed by a division
of a mother cell, grow to maturity, and
start dividing again in a period of only
20 minutes! This may not seem alarm¬
ing until you begin to calculate the re¬
sults. Start with one cell that divides
and becomes two in 20 minutes. These
become four in 40 minutes and eight at
the end of an hour. How many would
be formed in 12 hours? in 24 hours? If
this rate of increase continued for 24
hours, a mass of bacteria weighing over
2,000 tons would be formed! If this
rate of increase continued for another
24-hour period, the mass would weigh
more than 20,000,000,000,000,000,000,-
000,000 tons! Why, then, don’t bac¬
teria cover the earth and crowd out all
other living things as you might believe
would happen?

Cell wall

16-7 Bacterial reproduction by fission. Note
that the chromosome divides first, followed
by a division of the cell proper.

230 UNIT 3 MICROBIOLOGY

Suppose we start with a single bac¬
terium in a Petn-dish culture contain¬
ing a culture medium such as nutrient
agar. Cultures such as this are com¬
monly used to grow bacteria in the lab¬
oratory. After a brief period of adjust¬
ment to the culture medium, the cell
divides. For a few hours growth and
cell divisions occur rapidly. If divisions
occur as rapidly as once in 20 minutes,
the original cell will produce 512 organ¬
isms at the end of three hours. As the
mass increases in size, the problem of
mechanical crowding also increases.
With this crowding comes competition
for food. There is also an accumula¬
tion of metabolic wastes, which must
be excreted through the cell membrane
and wall of each organism. Within a
short time these factors reduce the rate
of growth and reproduction. The mass
continues to increase in size, however,
and after 24 hours a visible colony com¬
posed of several billion cells is formed.
This colony will continue to increase
in size for perhaps another 24 hours,
but the rate of division during this pe¬
riod will be even more reduced. By the
end of 72 hours cells may be dying or
becoming inactive.

Spore formation. Many bacillus bac¬
teria form endospores when conditions
for growth and cell division become un¬

favorable. Endospores are formed sin¬
gly as the cell content, including the
genetic material, is drawn into a spheri¬
cal or oval mass which becomes encased
in a single or double protective mem¬
brane. Bacterial endospores can en¬
dure almost unbelievable conditions.
They may survive in extremely dry con¬
ditions and resist boiling water. Ex¬
treme cold does not destroy them. Sci¬
entists were surprised to find bacterial
spores in deep layers of ice in the Ant¬
arctic a few years ago. Apparently they
had been lying in the ice many thou¬
sands and perhaps millions of years.
Yet, when these spores were placed in
favorable growing conditions, they were
able to form active bacillus cells! This
is undoubtedly a record for existence of
living substances in a resting state.

Endospore formation is not a form
of multiplication. A cell forms a single
spore which may germinate after a pe¬
riod and become an active cell again.
But the number of bacteria is not in¬
creased by spore formation. Spores are
merely stages in which certain bacilli
resist unfavorable environmental condi¬
tions.

The resistance of spores creates
medical problems. Many procedures
used to destroy active bacterial cells do
not kill spores. This applies to boiling

16-8 A: Botulinus bacteria and spores. B: Tetanus spores. C and D: Spore
germination.

CHAPTER 16 BACTERIA AND RELATED ORGANISMS 231

and to the use of certain cleansing
agents and antiseptics. Tetanus spores
are a constant infection problem. Ac¬
tive tetanus bacteria are restricted to an¬
aerobic environments, which is why a
closed wound is an ideal site for tetanus
infection. Tetanus spores, however, are
abundant in the soil, on objects, and
even in the air.

Sexual reproduction in bacteria. Bac¬
terial cells can apparently reproduce by
fission indefinitely. For many years,
however, biologists have wondered if
sexual reproduction occurred in organ¬
isms as low in the scale of development
as bacteria. Recent research has estab¬
lished that sexual reproduction does oc¬
cur, at least in some species of bacteria,
and that it may occur in all bacteria.

Extensive studies of sexual repro¬
duction in bacteria have been con¬
ducted with E. coli, an organism often
referred to as the colon bacillus because
it normally lives in the human large in¬
testine. Two mating types of this or¬
ganism have been discovered. One is
designated as male, the other as female.
In this bacterium, maleness results from
the presence of genetic particles that
are not present on the chromosome.
Femaleness results from the lack of
these particles. When male and fe¬
male strains of E. coli are mixed in a
fluid suspension such as a broth culture,
the cells come together. A cytoplasmic
bridge joins the two cells. The male
cell injects its chromosome along with
its sex-determining particles into the fe¬
male cell. Soon after this process the
male cells, lacking genetic materials,
die. What do you suppose happens to
the female cells? Having received the
male genetic particles, they change to
male cells.

What is the significance of sexual
reproduction in bacteria? Remember

that when organisms reproduce sexu¬
ally, both parents contribute genetic
material. The resulting new genetic
combinations may produce offspring
that differ from either parent. These
offspring may be more favorably
adapted to their environment and have
better chances of survival. In the mat¬
ing process that occurs in E. coli, the
male cell that results contains genet’C
material from both the male and female
strains. The offspring arising from fis¬
sion of this cell will in turn contain
newly combined genetic material. It
may be that this sexual process has been
important in producing the many types
of bacteria existing today.

Beneficial activities of soil bacteria.
We have heard so much about the dis¬
ease-causing, or pathogenic (path-o-/en-
ik), bacteria that we easily get the im¬
pression that all bacteria are dangerous
to health. Quite to the contrary, the
majority of bacteria are entirely harm¬
less, having no direct relation to our
lives or to our economy. Many other
bacteria are beneficial to man. As a
matter of fact, our very lives depend on
the bacteria that live in the soil. To
understand the vital role these bacteria
play, we need to consider a much larger
segment of a biological society.

Green plants absorb minerals from
the soil and use them in synthesizing or¬
ganic compounds. Animals consume
plants and rearrange these compounds
to meet their specific needs. In this
manner many of the chemical require¬
ments for life come directly or indi¬
rectly from the soil. When a plant or
animal dies, the chemical substances
composing it are left in a complex form
— products of life, yet in no chemical
condition to supply the needs of an¬
other generation of plants and animals.
What if all of the organisms that have

232 UNIT 3 MICROBIOLOGY

populated the earth during the past few
thousands years were still lying about
in their original chemical condition?
Would there be any room on the
earth’s surface for new generations?
Would there still be sufficient chemical
supplies in the earth s crust to form
these new organisms?

You know, of course, that organ¬
isms decay after death. They are de¬
composed and their materials are re¬
turned to the earth and atmosphere.
Do you know, however, that this is
largely bacterial activity? During decay
and putrefaction, bacteria and other
soil organisms, including molds, break
down complex molecules in dead plant
and animal matter and form simpler
chemical compounds. In this way, the
matter that once composed a living or¬
ganism is broken down into substances
that can be absorbed by a plant root
and used in another series of building
activities in a new generation of organ¬
isms. Build up and break down — syn¬
thesize and decompose. Matter is com¬
ing and going constantly in chemical
cycles involving organisms. Bacteria
are vital in these chemical cycles. Since
chemical cycles involve many kinds of
organisms and the interrelation of their
lives, we have reserved a full discussion
of them for an ecological study in Unit
Eight.

Bacteria in industrial processes. Bac¬
teria plav an important part in the proc¬
essing of many foods and other prod¬
ucts of industry. We will discuss a few
of these processes briefly.

Vinegar making involves two kinds
of microorganisms: yeasts and bacteria.
In producing vinegar, yeasts ferment sug¬
ar in fruit juice and change it to alco¬
hol. Acetic acid bacteria (A cetobacter)
[as-ee-to-bdk-ter] then oxidize the alco¬
hol, changing it to acetic acid. Com¬

mercial vinegars contain about four per¬
cent acetic acid.

Sauerkraut making is another proc¬
ess involving bacteria. In preparing
sauerkraut, cabbage leaves are shredded
and put in an airtight jar. Anaerobic
bacteria ( Lactobacillus ) [Zczfe-toh-ba-siZ-
us] ferment the sugar in the cabbage
leaves and convert it to lactic acid. A
small amount of carbon dioxide and al¬
cohol are formed in the process.

Several kinds of bacteria are in¬
volved in the tanning of leather. After
a hide has been preserved by drying or
salting, it is soaked and scraped to re¬
move excess flesh. The hair is removed
either by bacterial action or chemical
treatment. The final tanning takes
place in large tanks or vats in which
bacteria and other organisms attack the
hides and make them pliable.

The retting of flax is a process
many centuries old. The slender stems
of the flax plant contain fibers that are
extracted and processed as linen.
When the stem is cut, the fibers are se¬
curely joined to other stem tissues. But
if the flax plants are tied in bundles and
submerged in tanks of warm water, sol¬
uble materials dissolve and form a cul¬
ture medium suitable for growing ana¬
erobic bacteria ( Clostridium ) [klah-
stnd-ee-um] . These organisms ferment
the pectin that holds the fibers in the
stem. After 10 to 14 days the fibers
may be extracted and processed as linen.

Silage (sy-lij) is a fodder for dairy
cattle. In making it, shredded corn
stalks or alfalfa, clover, and oat plants
are put in a silo. Bacteria (Lactobacil¬
lus) ferment the sugar in the plants and
form lactic acid. During the fermenta¬
tion, oxvgen is consumed and carbon
dioxide is produced, settling in the silo.
This anaerobic condition prevents de¬
cay of the plant materials. In the fer-

CHAPTER 16 BACTERIA AND RELATED ORGANISMS 233

16-9 These laboratory tech¬
nicians are examining colo¬
nies of bacteria that have
been cultured in Petri dishes
and test tubes which were
placed in an incubator. The
instrument pictured here is
used to count the number of
bacteria colonies per square
millimeter of culture. (Ewing
Galloway)

mentation of silage, which lasts three
weeks to a month, considerable heat is
produced. The lactic acid formed is of
great value in the milk production of
dairy cattle.

The tobacco industry uses bacterial
action in the curing process. Stalks and
leaves of the tobacco plant are har¬
vested and hung in special curing barns
where sweating occurs. Bacteria fer¬
ment carbohvdrates in the moist leaves,

producing special flavors.

Bacteria in the dairy industry. In
cheese making, bacteria are necessary in
the formation of lactic acid from milk
sugar, or lactose. Lactic acid causes
coagulation of the milk solids, includ¬
ing proteins, fats, and insoluble mineral
salts.

In making butter cream is sepa¬
rated from the other milk solids. After
separation the cream is pasteurized to
reduce the number of bacteria present.
The pasteurized cream is then inocu¬
lated with special bacteria that form
lactic acid and act on other ingredients

of the cream, giving desirable flavors.
After a period of bacterial activity the
cream is cooled and churned to butter.

Since milk, in addition to being a
nutritious food for human beings, is an
ideal culture medium for bacteria, great
care must be used in maintaining rigid
sanitary regulations throughout the dairy
industry. Without these regulations,
milk and ice cream would become a
source of epidemics. Pasteurization is a
method of partial destruction of bacteria
in milk as well as in fruit juices, wines,
and malt beverages. The principle of
pasteurization was discovered by Louis
Pasteur and was first used to prevent
souring in wines.

Milk is pasteurized by heating it to
a temperature of 140-150° F, holding it
at this temperature for 30 minutes, then
cooling it quickly. About 90 percent of
the bacteria present, including most of
the pathogens, are killed during the
process. Rapid cooling retards the
growth of surviving organisms. Thus
pasteurization not only reduces the dan-

234 UNIT 3 MICROBIOLOGY

ger of infection from contaminated
milk but also delays souring. However,
pasteurized milk must be kept refrig¬
erated to reduce bacterial growth and
should be covered to avoid introducing
additional bacteria from the air.

Food spoilage and preservation. Bac¬
teria are among our chief competitors
for food. We cannot even estimate the
amount of food lost each year because
of spoilage resulting from bacterial ac¬
tion. Carbohydrates ferment easily,
protein foods putrefy, and fats and oils
become rancid.

Most foods would remain edible
many months or even years if bacteria
were not present, or if they could not
multiply in them. Thus, there are two
general methods of preserving foods.
One is the destruction of all bacteria
present and the sealing of food in an ap¬
propriate container. The other in¬
volves environmental control. If foods
are held in conditions that will not al¬
low bacterial growth and activity, they
will not spoil even though bacteria are
present in an inactive or dormant con¬
dition.

Canning will preserve foods indefi¬
nitely if the canning is done properly.

Salt curing destroys bacteria by
plasmolyzing cells. This method has
been used for centuries in preserving
fish and pork and other meats.

Refrigeration, while it does not kill
bacteria, slows down the chemical ac¬
tivity of bacterial enzymes. This not
only retards growth and multiplication
of bacteria but also reduces their chem¬
ical activity, which causes food spoilage.

Quick freezing is a far more effi¬
cient method of preserving foods that
can be frozen solid. At temperatures
of 0° to minus 10° or minus 15° F, bac¬
teria cease all activity.

Dehydration involves the removal

of water from a food to the point that
bacteria cannot grow or secrete en¬
zymes. Active bacterial cells may be
killed by dehydration, although endo-
spores may survive. Dehydrated foods
are widely used today. They are easily
packaged and shipped and require no
refrigeration. They must be kept dry,
however, or spoilage will occur rapidly.
When ready for use they are rehvdrated
by adding water.

Chemical preservatives, once very
widely used, are largely discontinued or
forbidden by law today, although some
foods, including dried fruits, are still
preserved with harmless chemicals.

Radiation may be used much more
widely in years to come in the food in¬
dustries. If meats and other foods are
packaged and sealed, then irradiated to
kill all organisms present, they will keep
indefinitely without the necessity of dry¬
ing or refrigeration.

Relatives of bacteria — the rickettsiae.
The rickettsiae (rik-et-see-ay) are a
group of organisms that seem to be mid¬
way between the bacteria and the vi¬
ruses. If we consider viruses to be non¬
living or near-living, then the rickettsiae
are the smallest living organisms. Rick¬
ettsiae are tiny rod-shaped or spherical
organisms averaging 0.3 to 0.5 /x in size.
They are barely visible under the high¬
est magnifications of the light micro¬
scope.

Rickettsiae resemble bacteria in
that they are cellular and reproduce by
fission. They are similar to viruses in
that they can live only in direct contact
with a living cell. Thus they must be
cultivated in tissue cultures, chick em¬
bryos, or in the tissues of animals.

The rickettsiae were discovered in
1909 by Dr. Howard T. Ricketts, for
whom they were named. Dr. Ricketts
found the first rickettsial organism in

CHAPTER 16 BACTERIA AND RELATED ORGANISMS 235

the blood of victims of Rocky Moun¬
tain spotted fever. He also demon¬
strated that cattle ticks transmit this
rickettsial infection to human beings.
It is ironic that Dr. Ricketts died of
typhus fever in 1910, a victim of the
rickettsial organism he was investigating
at the time.

As far as we know, rickettsiae are
transmitted only by insects and their rel¬
atives, including the human body louse,
ticks, and mites. The organisms live in
cells of the carrier’s intestine but do not
cause disease. In the human body, how¬
ever, they invade cells and cause infec¬
tion. Those who recover from a rick¬
ettsial disease have an immunity to dam¬
aging effects of the organisms, although
they may remain in the tissues without
apparent damage.

In addition to Rocky Mountain
spotted fever and typhus fever, rickett¬
siae cause trench fever and the mysteri¬
ous Q fever. These infections are simi¬
lar in producing fever, skin rashes, and
dark blotches caused by hemorrhages be¬
neath the skin resulting from damage to
the cells lining small blood vessels.

The spirochetes. A group of microor¬
ganisms called spirochetes (spy- ro-keets)
seem to lie between the bacteria and the
more specialized one-celled protists we
call protozoans. Many of the spiro¬
chetes fall in the size range of bacteria,
with cells 3 to 15^ in length and 0.5 m in
thickness. However, certain spirochetes
reach a length of as much as 500 m-
The cells of spirochetes are long and
cylindrical. Some are in the form of
spirals while others are tight corkscrews.
So far as is known, no spirochetes have
an organized nucleus. Reproduction is
by transverse fission. Endospores are
not produced. The cell walls of spiro¬
chetes are flexible, permitting the organ¬
isms to move through fluids with a char¬

16-10 The tiny black bodies shown in this
photomicrograph are rickettsiae. They are
smaller than bacteria and larger than viruses.
(General Biological Supply House, Inc.)

acteristic quivering action. All spiro¬
chetes were thought to lack flagella until
recent studies of certain forms (Trepo¬
nema) [trep-o-nee-ma] with the electron
microscope revealed threadlike cytoplas¬
mic projections looking like flagella.

We are most familiar with the spiro¬
chetes that live in the human body and
cause disease. The best known patho¬
genic spirochete is the syphilis organism
(Treponema pallidum ), which lives in
the blood stream and may invade the
nervous system. Spirochetes are spread
in discharges from lesions or eruptions
in the skin and mucous membranes in
earlv stages of the disease.

236

UNIT 3 MICROBIOLOGY

IN CONCLUSION

If numbers of an organism are any indication of success, we must admit that
bacteria are the most successful. These lowly forms of life are not on y e
most abundant organisms, but they probably occur higher in the atmosphere,
deeper in the seas, and farther in the ground than any other group.

Their requirements vary widely. Some must live in contact with a living
host. Others can digest almost any form of organic matter. Still others utilize
inorganic compounds and could survive if there were no other forms of li e
on the earth. Some bacteria have still another great survival advantage,
conditions are not favorable for active life, they merely stop activity.

Having considered both beneficial and harmful activities of bacteria, we
now turn to the area of greatest concern about them — the production of in¬
fection in the human body. It is in this phase of bacteriology and medicine
that some of the greatest chapters in scientific progress have been written.

BIOLOGICALLY SPEAKING

bacillus

Brownian movement

capsule

coccus

endospore

facultative aerobes

facultative anaerobes

flagella

host

obligate aerobes
obligate anaerobes
parasite
pasteurization
pathogenic

rickettsiae

saprophyte

sheath

slime layer

spirillum

spirochetes

true movement

QUESTIONS FOR REVIEW

1. What difference did Louis Pasteur find in the population of microorgan¬
isms in the two samples of beet juice extract he examined?

2. Explain why bacteria are classified as protists rather than plants in the

recent classification of living things.

3. Expressed in microns, what is the average size of bacteria?

4. Classify bacteria into three groups, based on the shape of their cells.

5. Distinguish between a bacterial slime layer and a capsule.

6. What genetic material is present in the genes of bacteria?

7. Describe the structure and function of bacterial flagella.

8. Distinguish between true movement and Brownian movement of bacteria.

9. List four environmental requirements for the growth of bacteria.

10. Distinguish between heterotrophic and autotrophic bacteria.

11. Distinguish between aerobic and anaerobic bacteria.

12. Name several products of the fermentation of glucose by microorganisms.

13. Name two products of true anaerobic respiration.

14. By what method and how rapidly do bacteria multiply under ideal con¬
ditions?

CHAPTER 16 BACTERIA AND RELATED ORGANISMS 237

15. Why are certain bacillus bacteria more difficult to destroy than other forms
of bacteria?

16. List several industrial uses of bacteria.

17. Describe several uses of bacteria in the dairy industry.

18. Explain the slow method of milk pasteurization.

19. List several methods of preserving foods by killing bacterial cells; by con¬
trolling the environment and preventing growth and reproduction.

20. How are the rickettsiae transmitted to humans?

21. Name several infections caused by rickettsiae.

22. List several infections caused by spirochetes.

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the composition and organization of genetic materials in a bac¬
terial cell.

2. Discuss several factors that retard the rate of growth and multiplication
of bacteria in a culture.

3. Discuss endospore formation in bacilli, including the manner in which
spores are formed and the biological importance of the process.

4. Give evidence to support the recent claim that bacteria reproduce sexually.
What may be the significance of this type of reproduction in bacteria?

5. Discuss the position of the rickettsiae in relation to bacteria and viruses.

6. Compare bacteria and spirochetes on the basis of structure and activity.

CHAPTER 17

INFECTIOUS

DISEASE

Pathogenic organisms. Various kinds
of microorganisms are capable of pro¬
ducing infectious disease if they gain ac¬
cess to the body, multiply in the tissues,
and cause a host-parasite interaction.
We may include as infectious organisms:
viruses, rickettsiae, bacteria, spirochetes,
yeasts, molds and moldlike organisms,
protozoans (animal-like protist organ¬
isms), and parasitic worms. We shall
limit our discussion of infectious dis¬
eases in this chapter to those caused by
viruses, bacteria, rickettsiae, and spiro¬
chetes.

It is appropriate that we begin our
studv of infectious disease with the work
of an outstanding scientist who made an
investigation of an infectious disease
during the same period in the 19th cen¬
tury in which Louis Pasteur was con¬
ducting his studies in France. Prior to
this time the treatment of disease had
been based largely on superstition. Peo¬

ple had no basis for associating disease
with microorganisms. When Pasteur
proved that yeasts caused fermentation
of beet juice, he also suspected that the
rod-shaped organisms he found were
responsible for the abnormal souring of
the juice. He reasoned that if these
organisms could cause such an effect in
beet juice, perhaps they could also cause
disease in man. This is known as Pas¬
teur’s Germ Theory of Disease. Re¬
markable support for this theory came
from a German by the name of Robert
Koch.

Robert Koch, the father of bacteriolog¬
ical technique. Robert Koch (kok)
[1843-1910] was born in Germany, the
son of a poor miner and one of 1 3 chil¬
dren. His first outstanding contribution
to medicine was in the study of anthrax,
an epidemic disease of animals often
contracted by man. From early times
anthrax had spread through sheep, cat¬
tle, and other herds in epidemics. In
examining organs of animals that died
from anthrax, Koch found numerous
rod-shaped bacteria swarming in the
blood vessels. His next problem was to
find out if these organisms caused the
disease. He transferred some of the liv¬
ing bacteria into a cut made at the base
of the tail of a healthy mouse. The
mouse developed anthrax and died.
Koch found the same bacteria greatly in¬
creased in number in the blood stream
of the dead mouse.

Koch was not satisfied until he had
actually watched this multiplication.
Accordingly he obtained a drop of sterile
fluid from the eye of a freshly killed ox
and put into the drop a small portion of
the spleen of a mouse containing an¬
thrax germs. He patiently watched
through his microscope until the germs
spread entirely through the drop. He
transferred germs from one drop to an-

238

CHAPTER 17 INFECTIOUS DISEASE 239

17-1 The four types of bacteria shown here are all pathogenic. 1. Clostridium
tetani causes tetanus; note the drumstick appearance of the spore-containing
cells. 2. Diplococcus pneumoniae causes one form of pneumonia; note that
this type is found in pairs. 3. Streptococcus pyogenes is responsible for the
“strep throat” infection. 4. Mycobacterium leprae causes the disease known as
leprosy. (General Biological Supply House, Inc.)

other and succeeded in growing them in
the complete absence of any mouse
spleen or blood. His next step was to
try inoculating healthy mice with his
laboratory-grown organisms to see if
they would produce the disease. The
mice died soon after inoculation, and
microscopic examination of the blood
disclosed the same abundant rod-shaped
organisms. Koch thus concluded that
anthrax is indeed caused by the bacillus,
which provided significant proof of Pas¬
teur's Germ Theory.

Koch's procedure is summarized in
four steps called Koch’s postulates:

1. Isolate the organism probably causing

the disease. (Koch found anthrax or¬
ganisms in the blood stream of in¬
fected animals.)

2. Grow the organisms in laboratory cul¬
tures. (Koch used sterile fluid from
the eyes of oxen.)

3. Inoculate a healthy animal with the
cultured organisms. (Koch inocu¬
lated mice with the eye fluid contain¬
ing germs.)

4. Examine the diseased animal and re¬
cover the organisms that produced
the disease. (Koch found that the
organisms with which he had inocu¬
lated the mouse had multiplied enor¬
mously in the blood stream.)

240 UNIT 3 MICROBIOLOGY

The spread of infectious organisms —
food-borne infections. Bacteria and
other pathogenic organisms may be
spread in various ways. Contaminated
food is a common agent of infection.
Typhoid fever may be spread in food
through handling by infected persons.
Tuberculosis and various streptococcus
infections may travel in milk. Epi¬
demics of undulant fever have been
traced to raw milk from cows suffering
from a specific infection of cattle.

We must be careful to distinguish
between food infection and food poison¬
ing. In food infection food is merely
the agent in which pathogenic bacteria
enter the body. In food poisoning bac¬
teria grow in the food and produce poi¬
sonous products. The poisons are ab¬
sorbed into the blood stream from the
digestive organs. They act on the body
suddenly and within a period of a few
minutes to a few hours after eating.

Botulism ( bahch-u-\iz-m ) is the
most deadly of all food poisoning.
Most cases result from home-canned

foods, especially string beans, that are
eaten before thorough cooking. The
botulism organism, a close relative of
the deadly tetanus bacterium, thrives in
an airtight container. It gets into the
food as a spore before canning and mul¬
tiplies during the period of storage. Poi¬
sons are released from the bacteria into
the food. Symptoms of botulism usu¬
ally appear within 12 to 36 hours after
the food is eaten. They include double
vision, weakness, and paralysis that
creeps from the neck region to other
parts of the body. Death may result
from respiratory failure or heart failure.
Mortality occurs in about 65 percent of
the cases. Since the poison is destroyed
by heat, botulism can be avoided by
cooking home-canned vegetables before
eating them.

Other ways diseases are spread. Cer¬
tain bacteria remain alive for days or
even weeks in water. Most water-borne
infections are intestinal and are intro¬
duced into water through sewage con¬
tamination. Many lakes, streams, and

17-2 Left: Koch cultured this form of the bacterium that causes tuberculosis,
using agar with the addition of blood serum. Right: when he examined smears
of organs of animals that had died from anthrax, Koch saw a microscopic field
similar to this one containing Bacillus anthracis. (Left: National Tuberculosis
Association; right: General Biological Supply House, Inc.)

CHAPTER 17 INFECTIOUS DISEASE 241

shallow wells are dangerously contam¬
inated with the typhoid bacillus, the
protozoan that causes dysentery, and
other intestinal parasites.

Another very common means of
spreading disease is through the droplets
given off in sneezes and coughs, and in
turn inhaled by others. Diseases spread
through droplet infection include colds,
sinus infections, measles, scarlet fever,
and tuberculosis.

Certain diseases produce sores or
lesions on the skin. Direct contact with
material from these lesions or sores
spreads infection. Ringworm of the
scalp and impetigo may be spread by
such contact infection. Chickenpox vi¬
rus may be transmitted by direct contact
or through the air. Diseases spread by
direct contact also include syphilis and
gonorrhea.

The unbroken skin is an effective
barrier against the entrance of bacteria.
However, breaks in the skin may result
in wound infections unless they are
cleansed by bleeding and properly treat¬
ed with antiseptic. Puncture wounds
are especially dangerous because of the
possibility of tetanus infection. These
anaerobic bacteria enter the wound as
spores clinging to some object. When
the wound heals on the surface, it leaves
an airtight cavity ideally suited to the
tetanus organism. Various staphylo¬
cocci commonly infect wounds and pro¬
duce a characteristic yellow pus. Strep¬
tococcus wound infections are dangerous
and can lead to a fatal general infection.

Certain diseases are spread by hu¬
man carriers who are themselves im¬
mune but who harbor the organisms in
their bodies. In some diseases, patients
who recover carry the organisms in their
bodies for weeks or even months. The
carrier problem is especially great in ty¬
phoid fever.

Insects spread disease in two en¬
tirely different ways. Houseflies and
roaches carry germs on their feet and
body and thus bring them into our
homes. Other insects carry infectious
organisms internally and transmit them
in bites. In this way, typhus is associat¬
ed with the human body louse, bubonic
plague with the rat flea, and African
sleeping sickness with the tsetse fly. In
these cases the insect serves as a host
of the infectious organism.

How microorganisms cause disease.
Various microorganisms damage the
body in several ways. The damage may
be mechanical or chemical. Reaction
in the host may result in such abnormal
conditions as fever, pain, redness, and
swelling. It may occur at the place of
the infection or in other parts of the
body, depending on the kind of patho¬
gen present and the ability of the body
to control the infection.

Tissue destruction is a type of
damage illustrated by tuberculosis. The
organisms usually infect the lungs, al¬
though other organs may be affected.
As tuberculosis bacteria multiply in the
lung tissue, they destroy cells and pro¬
duce lesions. These allow blood to seep
from the capillaries into the air passages,
resulting in the hemorrhages character¬
istic of advanced tuberculosis. Many
organisms, including streptococci, de¬
stroy blood cells. During meningitis,
the membranes covering the brain or
spinal cord are attacked by bacteria.

Many bacteria produce chemical
substances that are absorbed by the sur¬
rounding tissue or are transported
through the blood stream with damag¬
ing effects. We refer to these poisons
as exotoxins. Such toxins may cause
serious damage far from the seat of the
infection. For example, tetanus or¬
ganisms living in a wound in the foot

242 UNIT 3 MICROBIOLOGY

produce toxins that cause paralysis in
the upper regions of the body. Other
diseases that involve exotoxins include
scarlet fever, diphtheria, streptococcus
infections, as well as botulism food
poisoning.

Endotoxins remain inside the bac¬
terial cells that form them. However,
these toxins are released with deadly ef¬
fect when the bacteria die and disinte¬
grate. Endotoxin diseases include ty¬
phoid fever, tuberculosis, cholera, bu¬
bonic plague, and dysentery.

Structural defenses against disease.
Even though the body is the normal en¬
vironment of an enormous number of
bacteria and other organisms that cause
no harm, there is a constant threat of
invasion of the tissues by organisms that
are capable of causing disease. To pre¬
vent such an invasion, the body is pro¬
vided with many defenses that function
at various stages of the attack. These
defenses are often compared to lines of
soldiers, each equipped with weapons.

The most effective way of avoiding
infectious disease is to prevent the or¬
ganisms from entering the body tissues.
This is the function of the structural de¬
fenses, composing the first line. Skin
covers all the external parts of the body
and, if unbroken, is bacteria-proof. In
addition to being a physical barrier, salts
and various fatty acids present in per¬
spiration are believed to destroy some
bacteria and make the skin an effective
defense against infection. However,
natural openings in the skin, such as
pores and hair follicles, may allow entry
of microorganisms.

The mouth, digestive tract, respira-
torv passages, and genital tract are lined
with mucous membranes. These mem¬
branes function much as skin does in
preventing microorganisms from invad¬
ing the body tissues. Mucus, secreted

Nucleus

Mucus

Cilia

Mucus

Epithelial cells

17-3 The mucous membrane is one of the
first-line defenses. Notice the cilia, which
aid in expelling foreign matter.

by cells of the mucous membranes, is a
slimy substance that traps bacteria on
the membrane surface. Cells forming
the lining of mucous membranes in the
respiratory tract are equipped with tiny
hairlike projections that sweep bacteria
and other foreign materials upward to¬
ward the throat. When such particles
irritate the mucous membranes of the
throat, a cough results and the particles
are blown out into the air. Irritation of
the membranes of the nasal passages re¬
sults in sneezing.

Tears flow over the eyes continually,
not only lubricating them but cleansing
them as well. Bacteria and other for¬
eign particles are washed into the tear
ducts, which empty into the nasal pas¬
sages. Recent studies have revealed
that both tears and mucus contain en¬
zymes known as lysozymes (ly- so-zvmz).
These enzymes seem to dissolve the cell
walls of many bacteria.

The acid secretion of the stomach
is another effective structural defense of
the body. Great numbers of bacteria
enter the stomach in the food we eat.
However, few can survive the high con¬
tent of hydrochloric acid secreted by
glands in the stomach wall.

CHAPTER 17 INFECTIOUS DISEASE 243

Little is known of the function of
other microorganisms as a first-line de¬
fense. The normal bacterial population
of the digestive system seems to inter¬
fere with the growth of invading organ¬
isms and thus protects the host. The
importance of these organisms is demon¬
strated when drugs destroy much of the
normal population of intestinal bacteria.
Before they can multiply and re-establish
their normal number, other bacteria re¬
produce in the intestine and cause dis¬
ease.

Cellular defenses. Once bacteria have
passed through the structural defenses
of the skin, mucous membranes, and
stomach, they are met by a second line,
which operates in the body tissues. The
principal defenders of this line are phag¬
ocytic (fag-o-sit-ik) cells which engulf
bacteria and digest them with enzymes,
including lysozymes. Among the phag¬
ocytic cells are certain of the white cor¬
puscles, or leucocytes. These cells pass
through the walls of capillaries and mi¬
grate through tissue fluids to the site of
an infection. Here they form a wall
around the invading organisms and be¬
gin to engulf them. Debris of the
battle, consisting of blood serum, di¬
gested bacteria, and degenerated leuco¬
cytes constitute pus.

During such an infection, which is
still local rather than general, the tissues
often swell and become inflamed. Red¬
ness results from increased flow of blood
to the region of the infection to promote
healing. Blood vessels enlarge and pour
lymph (a clear liquid present in blood)
into the tissue spaces. The lymph aids
the struggle by carrying bacteria and
leucocytes that have engulfed bacteria
to lymph nodes where they are filtered
out. All of these reactions are part of
the inflammation that occurs with body
defenses at the cellular level.

Further destruction of microorgan¬
isms occurs in the lining of small blood
vessels in the liver, spleen, lungs, and
bone marrow. Here, special cells com¬
posing the reticuloendothelial system ,
capture and engulf bacteria and leuco¬
cytes that contain bacteria.

During the struggle between invad¬
ing microorganisms and the cellular
body defenses, the body temperature
often rises. This temperature elevation,
or fever, is a host reaction that reduces
or inhibits the growth of many bacteria,
activates other body defenses, and in¬
creases the rate of body metabolism.
Thus fever is a beneficial reaction unless
it becomes too high or lasts for too long
a period. If this happens, it may dam¬
age or destroy cells of the host organism.

The influence of body temperature
on the growth of bacteria was demon¬
strated by Louis Pasteur many years ago.
Pasteur was attempting to infect ducks
with anthrax bacteria taken from dis¬
eased sheep. However, the bacteria
would not grow in the blood stream of a
duck, in which the body temperature is
about 3° C higher than that of a sheep.
In other words the normal body tem¬
perature of a duck is the equivalent of a
three-degree fever in a sheep. However,
when he lowered the body temperature
of a duck by immersing its feet in ice
water, the anthrax bacteria started grow¬
ing and produced the infection.
Chemical defenses. The body defenses
we have described so far have been non¬
specific in that they act on any invading
bacteria. We now reach the third line
of body defenses, which includes various
specific antibodies against disease or¬
ganisms and their products. Bacteria,
as well as their toxins, are protein ma¬
terials and as such are foreign to the
host organism. We refer to such pro¬
teins as antigens. They stimulate the

244 UNIT 3 MICROBIOLOGY

production of specific antibodies that
combine with the antigenic material in
the blood.

Investigations have shown that anti¬
bodies are protein molecules. They are
believed to come from the lymph nodes
and spleen, from which they enter the
blood and lymph. Antibodies against
disease are formed only in the presence
of specific bacteria or bacterial products
in the body. Their production begins
within a few hours after introduction of
antigens in the form of viruses or bac¬
teria or release of bacterial products.
Within a few days antibodies enter the
blood stream and continue to increase in
quantity for three or four weeks. This
period usually marks the highest level
of antibody production. Antibodies
may remain in the blood stream for
many weeks or even years. The level of
a specific antibody in the blood may de¬
cline slowly. A second exposure to the
antigen involved speeds up production
in the lymph nodes and spleen and
raises the antibody level rapidly.

Various antibodies act in interest¬
ing ways. Antitoxins combine with exo¬
toxins formed by organisms such as
diphtheria, scarlet fever, and tetanus.
These extremely poisonous bacterial se¬
cretions are neutralized when they are
joined to specific antitoxins. Aggluti¬
nins are antibodies which cause certain
bacteria to clump together, or aggluti¬
nate. In such masses they are more
easily destroyed by phagocytic cells.
Cytolysins, also referred to as bacteri-
olysins, are antibodies which cause cer¬
tain bacteria to dissolve. Precipitins
are little understood antibodies which
cause bacteria to settle out in the blood.
This action aids in filtering them out in
the lymph nodes, spleen, and other or¬
gans. Opsonins are believed to com¬
bine with certain substances in bacterial

cell walls and prepare them for inges¬
tion by phagocytic cells.

Immunity against disease. We refer to
resistance of the body against infections
as immunity. It may be present at
birth, which is the form called natural
immunity, or it may be acquired during
the lifetime of the individual.

For the most part, man has a natu¬
ral immunity to animal diseases because
conditions in the human body are not
suitable for the growth and activity of
these infectious organisms. This type
of immunity, found in all people, is
often called species immunity. How¬
ever, there are several notable exceptions
to species immunity. Tuberculosis and
undulant fever may be transmitted to
humans in milk from cattle. Anthrax
may be spread by contact with lesions
in the skin of infected sheep, cattle,
horses, and other animals. Tularemia
is spread by infected rabbits, while psit¬
tacosis may be contracted from infected
parrots and parakeets.

Acquired immunity may be active
or passive, depending on the way in
which it is established. Active immu¬
nity may be acquired naturally by re¬
covering from certain infectious diseases,
including diphtheria, scarlet fever, mea¬
sles, and mumps. During the infection
the body produces specific antibodies
against the pathogenic organisms or their
products. This antibody normally con¬
tinues after recovery, resulting in per¬
manent active immunity. Active im¬
munity may be acquired artificially bv
using biological preparations contain¬
ing dead or weakened pathogenic or¬
ganisms or their products. In this way,
the body is stimulated to form its own
antibodies without actually having the
disease.

Passive immunity is acquired arti¬
ficially by injecting antibodies which

CHAPTER 17 INFECTIOUS DISEASE 245

have been produced in other individuals
or in animals. This transfer of anti¬
bodies, while it provides immediate pro¬
tection against an infection, is only tem¬
porary, lasting from a few weeks to
several months in most cases.

Our knowledge of immunity and
the use of immune products in dealing
with infectious diseases is an important
part of medicine. This chapter of medi¬
cal achievement we refer to as immune
therapy represents the knowledge con¬
tributed by many great scientists in a
series of exciting discoveries of the past
two centuries. All of us have benefited
from the contributions they made.
Edward Jenner — country doctor. It
was during one of the most dreadful
smallpox epidemics in England that Ed¬
ward Jenner, a country doctor, made a
discovery that was to alter the course of
history. Epidemics took their greatest
toll in cities. Jenner noticed that the
disease seldom struck people who lived
in rural areas and worked around cattle.
Most farmers and dairy workers had con¬
tracted cowpox and had recovered with
nothing more serious than a pustule
which left a scar. Were they immune
to smallpox? If so, why not vaccinate
people with cowpox to protect them
from smallpox?

The first vaccination. On May 14,
1796, Dr. Jenner had a chance to test his
vaccination theory. His patient was
James Phipps, a healthy boy about eight
years old. James’s mother, with great
confidence in Dr. Jenner, allowed her
son to be used in the test with the hope
that he could be spared the danger of
smallpox. Dr. Jenner took his young
patient to a dairy maid, Sarah Nelmes,
who had a cowpox pustule on her hand
resulting from an infection from one of
her master’s cows. Dr. Jenner made two
shallow cuts about an inch long on

James Phipps’s arm and inoculated them
with matter taken from the cowpox sore.
A pustule developed on the boy’s arm,
formed a scab and healed, leaving only
a scar. Was James Phipps now immune
to smallpox? There was only one way
to find out. He must be inoculated
with smallpox.

In July of the same year Dr. Jenner
deliberately inoculated James with mat¬
ter from a smallpox pustule. During
the next two weeks the doctor watched
his patient anxiously for signs of small¬
pox. They did not appear. Several
months later he repeated the inocula¬
tion. Again, the disease did not devel¬
op. The vaccination was successful.
James Phipps was definitely immune to
smallpox!

Following this famous experiment
Dr. Jenner wrote a paper explaining his
method of vaccination. At first the doc¬
tors were hostile and would not listen to
such a ridiculous procedure. Many
townspeople even organized anti-vacci¬
nation campaigns. Gradually, however,
the doctors and their patients accepted
vaccination, and smallpox epidemics
were eliminated.

Pasteur’s famous immunization experi¬
ment. About 80 years after Dr. Jenner
vaccinated James Phipps, Louis Pasteur
conducted his famous immunization ex¬
periments. Previous to these experi¬
ments Pasteur had made a vaccine con¬
taining the weakened bacteria of chick¬
en cholera. He found that he could in¬
ject the vaccine into healthy chickens
and produce active immunity against
the disease. This led him to try a simi¬
lar procedure against anthrax at about
the same time Robert Koch was con¬
ducting his famous work on the disease.

Pasteur made an anthrax vaccine
from weakened bacteria taken from the
blood of infected animals. He claimed

246 UNIT 3 MICROBIOLOGY

17-4 James Phipps receives the first vaccination from Dr. Edward Jenner.
(© 1960, Parke, Davis & Co.)

this vaccine would immunize animals
against the disease. Scientists chal¬
lenged him to prove his theory. This
challenge was the opportunity he had
waited for. He selected 48 healthy ani¬
mals (mostly sheep) and divided them
into two groups — an experimental
group and a control group. He gave the
animals in one group injections contain¬
ing five drops of anthrax vaccine.
Twelve davs later he gave the same ani¬
mals a second injection of a vaccine.
Fourteen days later he gave all 48 ani¬
mals an injection of living anthrax bac¬
teria. Two days later the scientists met
at the pens to laugh at Pasteur. Imag¬
ine their amazement to find all Pasteur’s
immunized animals alive and healthy,
and all the untreated animals dead or

dying of anthrax. This famous experi¬
ment was an important milestone in the
conquest of disease.

Pasteur’s experimentation with rabies.
During the latter years of his life, Pas¬
teur turned his genius to experimenta¬
tion with one of the most dreadful of all
diseases, rabies, or hydrophobia. We
know today that rabies is caused by a
virus. Pasteur, of course, did not know
of the existence of viruses. But for the
sake of clarity we shall use the term in
explaining his work with the disease.
This disease was common among dogs,
wolves, and other animals during his
time. If the virus was transmitted to a
human being by a bite, the victim was
certain to suffer an agonizing death after
an incubation period of a few weeks to

CHAPTER 17 INFECTIOUS DISEASE 247

six months or more. During this time
the virus slowly destroyed brain and spi¬
nal cord tissue.

The restlessness, convulsions, great
thirst, and throat paralysis that climaxed
a rabies infection led Pasteur to believe
that the infection centered in the brain.
However, microscopic examination of
the brain tissue of an animal victim did
not reveal any microorganisms. We
can understand why today, for the rabies
virus is invisible under the ordinary
microscope.

Pasteur found that he could trans¬
mit rabies by injecting infected brain
tissue from a rabid dog to a healthy one.
He repeated the inoculations with rab¬
bits and discovered that the virus gained
strength as it was passed from one ani¬
mal to another. However, if spinal cord
tissue taken from a dog or rabbit was
dried for 14 days, the virus lost its
strength and could no longer produce
the infection. Virus 13 days old was
only slightly stronger. The discovery
that the virus weakened with drying and
aging led to experiments to find out if
rabies immunity could be produced.
Pasteur injected 14-day-old brain tissue
from a rabid animal into a healthy dog,
then followed this injection with 13-day-
old material.

The injections were continued day
after day until the dog was given an in¬
jection of full-strength virus in the 14th
injection. The animal suffered no ill
effects. The series of injections with
material of increasing strength had pro¬
duced immunity to rabies. The ques¬
tion now was whether he dare try the
series of injections in human victims.
A decision was forced on Pasteur a short
time later.

Pasteur’s treatment of rabies. On July
6, 1885, a frantic mother brought her
son to Pasteur’s laboratory, pleading

that he use any method to save her boy’s
life. The boy had been attacked by a
rabid dog two days before they reached
Pasteur’s laboratory. Pasteur had no
time to lose and no choice except to give
the boy the treatment that had worked
on dogs. The physicians and laboratory
assistants he consulted agreed with his
decision. On the evening of this impor¬
tant day, the boy was given an injection
of rabbit-grown virus that had aged 12
days. Injections were repeated each
day, using successively fresher virus. On
the 12th day he received full-strength
virus. After several weeks of observa¬
tion he was sent home, the first human
being immunized against rabies.

This series of injections, known as
the Pasteur treatment, is used today to
immunize victims of bites by rabid dogs.
The virus used in making the vaccine
was once grown in rabbits. However, a
recent method of growing the virus in
the embryos of developing duck eggs has
proved more satisfactory because there
is no danger of serious body reaction to
the substances present in nerve tissue of
the rabbit.

The conquest of diphtheria. From the
earliest times diphtheria was one of the
worst epidemic killers, especially among
children. The effects of diphtheria on
the body are twofold. The germs grow
in a thick, grayish-white membrane on
the back wall of the throat. As the
membrane spreads, it may block the
glottis opening and cause death by stran¬
gulation. In addition, toxins are given
off by the living bacteria and are ab¬
sorbed through the infected tissues into
the blood. They often cause severe
damage to the heart, nervous system,
and other organs.

The conquest of diphtheria some 50
years ago involved the work of several
scientists. One found the rod-shaped

248 UNIT 3 MICROBIOLOGY

17-5 Diphtheria antitoxin is prepared by first injecting quantities of diphtheria
toxin into the blood of a horse. After removal of blood from the external
jugular vein in the neck, it is processed to remove antitoxins from the rest of
the blood substances. (© 1957, Parke, Davis & Co.)

bacteria growing in the throats of pa¬
tients with diphtheria. Another worker
cultured the bacteria and developed a
stain used in microscopic study of the
organisms. However, much of the cred¬
it for the conquest of this disease be¬
longs to Emil von Behring (fahn foer¬
ing), a German bacteriologist.

Von Behring was puzzled bv the
fact that, even though diphtheria organ¬
isms remained in the throat, the effects
of the disease appeared in distant organs.
When bacteria were grown in culture
media, they produced a toxin which,
when injected into guinea pigs, produced
the symptoms of diphtheria even though
no germs were present.

While conducting such experiments
with diphtheria toxin, von Behring dis¬
covered that guinea pigs and rabbits
could be used only once. Thev devel-

* J

oped immunity to the disease. Could

this immunity be transferred to animals
that had never been given doses of toxin?
In an effort to answer this question, von
Behring took a fraction of the blood
from immune animals and injected it
into other animals. They too were made
immune to diphtheria toxin. Von Beh¬
ring named the substance that had pro¬
duced immunity antitoxin.

Sheep were used first in the produc¬
tion of diphtheria antitoxin. After ex¬
tensive testing of the antitoxin in guinea
pigs, it was first used with great success
in the Children’s Hospital in Berlin.
Von Behring found that immunity re¬
sulting from injections of sheep anti¬
toxin lasted only a few weeks. Appar¬
ently the antitoxin is destroyed slowly
in the human blood stream. If children
could be made to produce their own
antitoxin, immunity would be as lasting
as though they had recovered from diph-

CHAPTER 17 INFECTIOUS DISEASE 249

theria. To give diphtheria toxin would
be as dangerous as inoculating them
with the disease itself. Von Behring
reasoned that a mixture of toxin and an¬
titoxin might be safe to use. World
War I prevented von Behring from
finishing his work on such a toxin-anti -
toxin. However, it was completed in
the United States by Dr. William H.
Park and other workers. Toxin-anti¬
toxin was used in producing immunity
to diphtheria until it was discovered that
some people have a serious reaction to
it. This problem has been solved by
using a toxoid , which is a toxin that has
been weakened with heat and chemicals
to make it safe for injection in human
tissues.

Immune therapy summarized. The
work of Jenner, Pasteur, and von Beh¬
ring represents two different ways in
which artificial immunity may be pro¬
duced. Jenner was the first to use what
is now known generally as a vaccine ,
from the Latin word vacca , meaning
cow. Like Jenner’s vaccine, all vaccines
are preparations of the pathogenic or¬
ganisms, which stimulate the victim to
produce antibodies against the disease.
A vaccine may be made of weakened or
killed pathogens or their products. The
Salk vaccine, for example, is made from
polio virus weakened by treatment with
formaldehyde.

Von Behring’s work with diphthe¬
ria illustrates the second kind of im¬
mune therapy, which involves inject¬
ing the victim with antibodies specific
to the disease. These antibodies are
contained in serum , which is a blood
fraction. The gamma globulin that can
be given to produce temporary immu¬
nity to polio is administered in a se¬
rum. Serum usually comes from the
blood of an animal that has been inocu¬
lated with a disease and has produced

antibodies against it. The diphtheria
toxin-antitoxin is a preparation of horse
serum. A serum is used to reinforce the
body’s production of antibodies during
an infection. It is also used to give an
immediate immunity to someone who
has been exposed to a disease.
Chemotherapy. The conquest of dis¬
ease is not a war of biology alone, for
chemistry plays a very significant part.
Chemotherapy is a recent field in which
specific chemicals are used to destroy
germs. It is of great importance be¬
cause it assists the natural body de¬
fenses.

The development of chemotherapy
is associated with the work of a brilliant
German chemist, Paul Ehrlich (er-lik),
in connection with his long search for
a cure for syphilis. Ehrlich spent many
years attempting to discover a drug that
would kill the organisms in the blood
stream without damaging the blood or
other parts of the body. After 605 un¬
successful attempts he finally succeeded.
His 606th drug was an arsenic com¬
pound called salvarsan. It was used in
treating syphilis many years before peni¬
cillin was discovered.

Other scientists began experiment¬
ing with chemicals in the treatment of
disease. In 1932 Dr. Gerhard Domagk
(do/z-mag), another German scientist,
discovered that a red dye called prontosil
had remarkable germ-killing powers.
Soon after his discovery of prontosil, he
tried it on his own daughter, who was
dying of a streptococcic infection that
had progressed beyond medical control.
It proved to be effective in halting the
infection and saved the child’s life.

Further investigations on prontosil
proved that only a part of the drug had
germ-killing powers. This part was iso¬
lated and called sulfanilamide (sul-fah-
mZ-a-myd). It was the first of an im-

250 UNIT 3 MICROBIOLOGY

17-6 Each of these Petri
dishes contains the same
four strains of bacteria, ar¬
ranged in the same order.
An antibiotic has been
added to each culture, us¬
ing an increasingly potent
dose on the respective
dishes. The bacteria in the
upper right-hand corner of
each dish are least in¬
hibited by the antibiotic.
Which strain is most easily
inhibited? (Society of
American Bacteriologists)

portant familv known as the sulfa drugs.
There are many different ones now, and
they are used in the treatment of certain
infectious diseases. These drugs should
be taken only on the advice and recom¬
mendation of a physician. They are not
cure-alls and may be dangerous.
Antibiotic therapy. The chemical sub¬
stances called antibiotics are now com¬
monly used in the treatment of disease.
Antibiotics are products of living organ¬
isms. In this respect they are different
from the drugs used in chemotherapy.
We can sum up the use of antibiotics by
saving, “Bugs produce drugs that kill
bugs,” for our supply of these substances
comes from bacteria, molds, and mold-
like organisms.

The wonder drug of World War II,
penicillin, was the first of the antibiotics.
It was discovered accidentally by Sir
Alexander Fleming, a British bacteriol¬
ogist, in 1929. Fleming was working
with staphylococcus bacteria in a Lon¬
don hospital. While examining plate
cultures of staphylococci, he noticed
that several of them contained fluff}'

masses of mold, and that the mold was
stopping the growth of the bacteria.
Later the mold colonies turned dark
green and were identified as Penicillium
notatum, a relative of the mold found on
oranges, and its antibacterial secretion
was called penicillin. Since the mold
and the staphylococci were competing
for the same food supply, it seemed that
the secretion of the mold was an adapta¬
tion that destroyed the competition.

In the opening days of World War
II, Dr. Howard Florey and a group of
Oxford workers began a search for anti¬
bacterial substances that would be use¬
ful in combating wound infections.
Their attention turned to Fleming’s
work and, in cooperation with him, pen¬
icillin was developed and thoroughly
tested. The result of this work is his¬
tory.

Today a penicillin ten times as
powerful as Fleming’s is available in un¬
limited quantity and at low cost. Mu¬
tant strains of Penicillium notatum, pro¬
duced by exposure to X rays, yield far
more penicillin than earlier strains. Bio-

CHAPTER 17 INFECTIOUS DISEASE 251

logical companies have even produced
penicillin synthetically. It is given effec¬
tively in large doses by injection with a
slowly absorbed procaine salt. It can
be taken by mouth in tablet form and
inhaled into the nasal passages in pow¬
der form. Ointments are available for
use locally and in the eyes. However, it
should never be used in any form unless
recommended by a physician.
Streptomycin. Dr. Selman Waksman
became interested in the soil and its rela¬
tion to life when he was a boy in Eu¬
rope. Later he came to the United
States and enrolled in Rutgers Univer¬
sity. His interest in soil led him to the
New Jersey Agricultural Experiment Sta¬
tion at Rutgers. While still a student,
Waksman discovered a soil organism
which he named Streptomyces griseus.

After graduate study Waksman re¬
turned to Rutgers as a member of the
faculty. With the aid of students he
continued the investigation of soil or¬
ganisms. Together they studied the
problem of the disappearance of disease
organisms when the body of a diseased
animal is buried, and found that prod¬
ucts of soil organisms destroyed the
pathogens. After years of testing the ef¬
fect of soil organisms on various patho¬
gens, streptomycin, an antibiotic sub¬
stance produced by Streptomyces grise¬
us, was discovered.

Streptomycin proved to be an effec¬
tive drug against tuberculosis. In addi-

17-7 The top photograph shows a culture of
the mold Penicillium notatum growing on a
Petri dish culture. The antibiotic, penicillin,
is obtained from this mold as well as from
other species of the same genus. In the
bottom photograph, disks have been soaked
in four different strains of penicillin and
placed in a bacterial culture. Notice that
the strain at the lower left has no inhibiting
action on these particular bacteria. (Chas.
Pfizer and Co.)

tion, it is partially effective against
whooping cough, some forms of pneu¬
monia, dysentery, gonorrhea, and syphi¬
lis. The streptomycin industry grew
rapidly, and this valuable antibiotic took
its place with penicillin.

Other antibiotics. Today many new
antibiotics have been added. Among
these is terramycin (tair-uh-mys-in),
which promises to be of great value be¬
cause of its wide range of effectiveness
and low toxicity in the body. Erythro¬
mycin (eh-rith-roh-mys-in), one of the
more recent antibiotics, is similar in ac-

252 UNIT 3 MICROBIOLOGY

tion to penicillin. This substance is es¬
pecially valuable in dealing with organ¬
isms that have a resistance to penicillin.
Tetracycline ( te-tra-sy-kleen ) , another
recent one, is produced under a variety
of commercial names. The search for
new and better antibiotics continues.
An ideal antibiotic has a broad spec¬
trum, or range, of organisms against
which it is effective. It must also de¬
stroy pathogenic organisms without in¬
jury to the body tissues or disturbance
of bodv functions.

One reason for limiting the use of
antibiotics is because they sometimes
produce side effects. A severe allergic
reaction to penicillin is not uncommon.
Other antibiotics may create digestive
disturbances. This is often the result of
destruction of most of the organisms
that normally grow in the intestine

IN CONCLUSION

when the antibiotic is taken by mouth.

A second and perhaps most impor¬
tant reason for caution involves the proc¬
ess of adaptation and natural selection.
The bacteria causing a disease vary
somewhat in their characteristics, like
members of any species. When an anti¬
biotic is used, some of the population of
pathogenic bacteria may have adapta¬
tions that allow them to resist the ef¬
fects. Repeated use of the antibiotic
may kill off the other, more susceptible
bacteria while allowing the resistant
one to multiply and produce new
strains in a short time. This is becom¬
ing more and more of a medical prob¬
lem. For example, penicillin was very
effective against staphylococci for many
years. Then resistant strains became
more and more abundant until today
they are a great problem in hospitals.

You might think of your body as a fort, with defenses set up against invading
microorganisms. Microbe enemies are always with us. Normally, we hold
them in check. But when they overcome our defenses and multiply in our

tissues, an infectious disease may develop rapidly.

By understanding the nature of infectious disease — the host-parasite re¬
lationship and interaction — we may reinforce our natural body defenses. Bio¬
logical products are available to establish immunity before an infection develops
or to bolster the natural antibodies in dealing with infection. Chemotherapeu¬
tic and antibiotic drugs of many kinds are valuable weapons in the battle against
infection.

BIOLOGICALLY SPEAKING

antibiotic

immunity

reticulo-endothelial system

antibody

Koch’s postulates

serum

antigen

leucocyte

tissue destruction

antitoxin

lysozyme

toxin-antitoxin

botulism

mucus

toxoid

endotoxin

phagocytic cell

vaccine

exotoxin

pus

CHAPTER 17 INFECTIOUS DISEASE 253

QUESTIONS FOR REVIEW

1. List eight microorganisms that may produce an infection in the human
body.

2. List the Koch postulates and describe each as a step in the investigation
of an unknown disease.

3. Distinguish between food infection and food poisoning.

4. Name several diseases spread by droplet infection.

5. List several diseases spread by direct contact.

6. Name a disease associated with human carriers.

7. Distinguish between an exotoxin and an endotoxin.

8. List the principal structural defenses of the body.

9. In what way do lysozymes function in body defenses?

10. Give two examples of phagocytic cells that function at the level of cellular
defense.

11. In what way is fever an important body defense?

12. What is an antigen?

13. Distinguish between natural and artificial immunity.

14. Distinguish between a toxin, antitoxin, toxin-antitoxin, and toxoid from the
standpoint of origin and action.

15. What medical contributions were made by Ehrlich and Domagk?

16. How are antibiotics different from other drugs used in chemotherapy?

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the scientific contribution of Robert Koch in his investigation of
anthrax.

2. Discuss the specific and nonspecific body defenses.

3. Discuss the contribution of Edward Jenner to the rise of immune therapy.

4. What significant contribution did Emil von Behring make to our knowl¬
edge of immunity?

5. Discuss the principle involved in the Pasteur treatment for rabies.

6. Discuss the way in which natural selection produces bacteria resistant to
antibiotics.

CHAPTER 18

THE PROTOZOANS

The protozoans — four phyla of related
protists. Before biologists began to rec¬
ognize the need for a kingdom Protista,
they classified in a single phylum of the
animal kingdom a large number of re¬
lated one-celled organisms having spe¬
cialized cell organelles. Protozoa, the
name given to this phylum, means, liter¬
ally, “first animals/’ The protozoans
were in turn divided into four classes,
according to their means of locomotion.
Since many of these organisms are not
distinctly animal, today most biologists
classify them as protists and consider the
classes to be four separate phyla of that
kingdom. See the Appendix for the
names of these phyla and some exam¬
ples. Though only a few examples are
given in the Appendix, there are thou¬
sands of species of protozoans. While
some biologists estimate 15,000, others
believe that there are as many as 100,-
000 protozoan species.

The protozoans are fascinating to
study. Some have mouths and gullets,
while others merely flow around their

food. Some dart thither and yon by
means of tiny hairlike projections that
act like oars. Others wave flagella in
front of them and thus project them¬
selves forward and backward. Some
even sting their victims with poison
threads. The highly specialized organ¬
elles of the protozoans enable them to
live efficiently as single cells, carrying on
all the life processes in a microscopic
universe.

A mass of living jelly. The genus Ame-
ba includes several species of interesting
protozoans. An ameba might be de¬
scribed as “animated jelly.” On first
seeing it you might mistake it for a non¬
living particle. But this tiny blob of
grayish jelly moves of its own accord,
takes in food, and performs all the other
life processes.

These protozoans may be collected
by taking slime from the bottom of
streams and ponds and from the surface
of the leaves of aquatic plants. The
experience of collecting samples and
searching through the microscopic world
for an ameba is a rewarding one.

Under the microscope the ameba
appears as an irregular mass of jellylike
protoplasm surrounded by a thin mem¬
brane (Fig. 18—2). If you find an active
animal, you will notice that the cyto¬
plasm has a constant flowing motion.
This streaming cytoplasm presses against
the cell membrane and produces nu¬
merous projections called false feet, or
pseudopodia ( soo-doh-poh-dee-uh ) .
This type of locomotion is called ame¬
boid movement. It is responsible for
the classification of the ameba in the
protist phylum Sarcodina.

A closer look at the ameba will
show that the cytoplasm is of two dif¬
ferent types: a clear, watery ectoplasm is
found next to the cell membrane; the
endoplasm, found inside the ectoplasm,

254

CHAPTER 18 THE PROTOZOANS 255

Vorticella

Stentor

Stylonychia

18-1 Many of these interesting ciliated protozoans may perhaps be found in a
pond or stream near your home or school.

is denser and resembles gray jelly with
pepper sprinkled through it. The nu¬
cleus can be seen as a bronze-colored
mass that changes its position with the
flowing cytoplasm.

How the ameba gets food. When an
ameba comes in contact with a one-
celled green alga, a small diatom, or
another protozoan, it extends pseudopo¬
dia, which entirely surround the food.
Part of the membrane of the ameba now
becomes the lining of a food vacuole
inside the cytoplasm (Fig. 18-2). Di¬
gestion is accomplished by enzymes,
formed by the cytoplasm, which pass
into the vacuole and act on the food
substances. Digested food is absorbed
by the cytoplasm and may then be oxi¬
dized to release energy, or assimilated to

form additional protoplasm. Undi¬
gested particles remain in the vacuole
and pass out of the cell at any point in
the membrane.

The oxygen necessary to maintain
the life of these protists diffuses through
the cell membrane. Most of the carbon
dioxide and soluble wastes pass out
through the cell membrane. However,
with the intake of food, and by the
process of osmosis, much useless water
comes into the body of the ameba. If
the ameba did not have a method of
ridding itself of this water, it would
swell up like a balloon and burst. As
the excess water accumulates it forms a
contractile vacuole. When this vacuole
reaches maximum size, it discharges the
water through a temporary break in the

256 UNIT 3 MICROBIOLOGY

18-2 The structure of an ameba. As this
protist moves by its pseudopodia, it assumes
many different shapes.

cell membrane. In this way the or¬
ganism maintains a constant or nearly
constant internal water concentration.
Sensitivity in the ameba. The response
of the ameba to conditions around it is
a good example of the sensitivity of pro¬
toplasm. It has no eyes, yet it is sensi¬
tive to light and seeks areas of darkness
or dim light. It has no nerve endings
such as we associate with the sense of
touch, yet it reacts to jarring. It moves
away from the objects with which it
comes in contact in the water.

In order to see how the ameba re¬
sponds to food, put small amounts of it
in a culture. Watch how the ameba
cells flock to the food. The food, per¬

haps by means of chemical attraction,
acts as a stimulus to the cells. Unfavor¬
able conditions such as dryness or cold
cause some species of ameba to become
inactive and to withdraw into a rounded
mass. When favorable conditions re¬
turn, the organisms resume activity.
Reproduction in the ameba. In the
presence of abundant food and ideal
conditions for growth, the ameba rap¬
idly reaches maximum size. At this size
the membrane surface is not large
enough to supply the volume of cyto¬
plasm with adequate food and oxygen
or to remove waste. Reproduction now
occurs by the process of fission. The
nucleus divides by mitosis and its two
portions move to opposite ends of the
cell. The rest of the cytoplasm then
separates gradually, forming two distinct
masses. Each new mass has a nucleus
and is capable of independent life and
growth.

Biologists have found that, at a
temperature of 86° F, an ameba cell re¬
quires approximately 20 minutes in or¬
der to make a complete division.
When conditions for growth are ideal,
each cell matures and divides again after
about three days.

Paramecium, a complex protozoan.
Various species of the genus Para¬
mecium live in quiet or stagnant ponds
where scums form. In order to cul¬
ture them on the laboratory, submerged
pond weeds may be collected, put in
a jar with pond water, and set aside
in a warm place for a few days. As
the weeds decav, a scum forms on the
surface, and large numbers of paramecia
may be found.

When a drop of water containing
paramecia is placed on a slide under the
microscope, the most striking character¬
istic of this unicellular protist is its
movement. It appears to swim rapidly

CHAPTER 18 THE PROTOZOANS 257

through the thin film of water between
the slide and cover glass. Actually, its
rate of movement is quite slow — about
three inches per minute; but the micro¬
scope magnifies the speed to the same
extent as it does the object. A few
strands of cotton or filaments of algae
serve as effective barricades in preparing
a slide for examination of these moving
organisms. Methyl cellulose (which is
now used as a wallpaper paste) may also
be used to increase the viscosity of the
solution. This slows down the animals,
and allows an opportunity for the de¬
tailed study of structure.

The paramecium is shaped like a
slipper (Fig. 18-3). Although it does
not change its shape as an ameba does,
it is by no means rigid. It often bends
around an object it happens to meet
when swimming. The definite shape of
the cell is maintained by a thickened
cell membrane called a pellicle sur¬
rounding the cytoplasm.

Paramecia move by hairlike cyto¬
plasmic threads called cilia , which pro¬
ject through the cell membrane. These
cilia are arranged in rows and lash back
and forth like tiny oars. They cover
the entire cell, but are most easily seen
along the edges. Like flagella, cilia con¬
tain protein filaments, which biologists
believe contract alternately to produce
the beating. This form of movement
places paramecium in the protist phylum
Ciliophora.

Another striking feature of the para¬
mecium is a deep oral groove along one
side of the cell. This depressed area is
lined with long cilia that cause the ani¬
mal to rotate around its long axis as it
swims through the water. The para¬
mecium has a definite front end, or
anterior part, which is rounded, and a
more pointed rear end, or posterior part
— a perfect design in streamlining. The

oral groove runs from the anterior end
toward the posterior part. The action
of the cilia lining the oral groove and
the movement of the animal forward
force food particles into the mouth
cavity. The mouth cavity looks like
the opening of a funnel, and it leads to
a narrow tube, the gullet. Bacteria and
other food particles forced into the
gullet enter the cytoplasm within a
food vacuole. When the food vacuole
reaches a certain size, it breaks away
from the gullet, and a new one begins to
form.

ANTERIOR END

Trichocyst

Contractile
vacuole

Ectoplasm

Endoplasm

Food vacuole

Cilia

Oral groove

Micronucleus

Macronucleus

Mouth cavity
Gullet

Food vacuole
forming

POSTERIOR END

Anal pore

Canals of

contractile

vacuole

Pellicle

18-3 Compare this diagram of a paramecium
with that of an ameba in Fig. 18-2. How are
they alike and how do they differ?

258 UNIT 3 MICROBIOLOGY

The movement of the cytoplasm
carries the vacuole in a circular course
around the cell. During this circulation,
digestion and absorption occur as in the
ameba. Undigested food passes through
a special opening in the pellicle called
the anal pore. This tiny opening is
located near the posterior part of the
cell but is completely closed except
when in use and is quite difficult to see.

The two contractile vacuoles have
a definite location, one near either end
of the cell. Surrounding each vacuole
are numerous canals that radiate from
the central cavity into the cytoplasm.
The canals enlarge as they fill with wa¬
ter, after which their content is passed
to the central cavity and emptied out at
the surface through an opening.

As in the ameba the contractile
vacuoles of the paramecium serve pri¬
marily to remove excess water that has
entered with food and by osmosis.
Some soluble waste may be eliminated
by the contractile vacuoles, although
most of the waste products appear to
diffuse through the pellicle. Respira¬
tion in the paramecium is accomplished
by diffusion of oxygen and carbon di¬
oxide through the pellicle.

Sensitivity in the paramecium. The re¬
actions of paramecium cells to condi¬
tions around them are remarkable, con¬
sidering that this protist, like the ameba,
has no specialized sense organs. Except
when feeding, the cells swim constantly,
bumping into objects, reversing, and
moving around them in a trial-and-error
fashion. This response to the stimula¬
tion caused by bumping into objects is
called the avoiding reaction (Fig. 18-4) .
The reaction also occurs from such
stimuli as excessive heat and cold, chem¬
icals, and lack of oxygen. Paramecia
tend to move into regions of low acidity,
a response of value to the animal be-

18-4 This represents the avoiding reaction
of paramecium. Note how the protozoan
meets an obstacle, backs up, changes direc¬
tion, and tries again.

cause bacteria form an important source
of its food and bacteria accumulate on
decaying organic matter, causing the
water to be slightly acid.

The trichocystSy which normally ap¬
pear as minute lines just inside the pel¬
licle, are used as a means of defense.
When a larger protozoan approaches,
the trichocysts of the paramecium ex¬
plode special protoplasmic threads into
the water through tiny pores. These
threads are quite long and give the or¬
ganisms a bristly appearance. A bit of
acetic acid or iodine added to some wa¬
ter that contains paramecia will often
cause the trichocysts to discharge.
Reproduction in the paramecium. The
paramecium has two different kinds of
nuclei, both of which are located near
the center of the cell. A large nucleus,
or macronucleuSy regulates the normal

CHAPTER 18 THE PROTOZOANS 259

activity of the cell. Near the large nu¬
cleus is a small nucleus, or micronucleus ,
which functions during reproduction.
Some species of paramecia have more
than one micronucleus.

Reproduction may involve two dis¬
tinct processes: fission and conjugation.
Fission in paramecia involves the divi¬
sion of both the macronucleus and the
micronucleus, after which two daughter
cells form. Under ideal conditions fis¬
sion may occur twice a day. This
would produce over 700 generations per
year. If all the daughter cells were to
live and ideal conditions could be main¬
tained, the total mass of paramecia
would be many times greater than that
of the earth in five years!

After several months of cell divi¬
sions, especially in the same environ¬
ment, paramecia lose vitality and die un¬
less they undergo conjugation . This
process requires the mixing of two mat¬
ing types, or sexes, which may be desig¬
nated as + and — , or I and II. Ex¬
change of nuclear materials during con¬
jugation, resulting in revitalizing of the

18-5 A. Reproduction in the parameci
B. Conjugation of paramecia. Trace tf

cells, may be summarized in the follow¬
ing steps:

1. Two cells unite at the oral grooves.

2. The micronucleus in each cell di¬
vides. The macronucleus degener¬
ates.

3. The two micronuclei in each cell
divide, forming four microriuclei,
three of which degenerate.

4. The remaining micronucleus divides
unequally, forming a larger and
smaller micronucleus. The cells ex¬
change smaller micronuclei.

5. The larger (stationary) and smaller
(migrating) micronuclei fuse in each
cell.

6. The cells separate. The fused micro-
nucleus undergoes three consecutive
divisions, forming eight nuclei.

7. Of the eight' nuclei, four fuse and
form a macronucleus, three degen¬
erate, and one remains.

8. Two consecutive cell divisions occur,
producing four small paramecia.

A flagellated protozoan. Several species
of the genus Euglena live in fresh¬
water ponds and streams. Under the

5 6 7 8

B. Conjugation

i. Trace the steps involved in fission,
exchange of nuclear materials.

260 UNIT 3 MICROBIOLOGY

microscope a euglena appears as a pear-
shaped cell that swims about freely.
The anterior end is rounded, while the
posterior end is usually pointed (Fig.
18-6).

Since the euglenas possess some
characteristics of plants and some of ani¬
mals, they have been claimed by both
botanists and zoologists. Because of
one of their methods of locomotion, they
are often placed in the protist phylum
M astigophora ( mas-ti-gdhf-o-ra ) , al¬
though some biologists put them in a
phylum of their own. The organism
swims by means of a flagellum attached
to the anterior end and nearly as long
as the one-celled body. The flagellum is
held straight in front, and the tip is ro¬
tated, thus pulling the organism rapidly
through the water.

Unlike other members of the phy¬
lum Mastigophora, the euglena has a
second method of locomotion. This
type of movement is so characteristic of
the euglena that we call it euglenoid
(yu-gZee-noid) movement. It is accom¬
plished by a gradual change in the
shape of the entire cell. The posterior
portion of the body is drawn forward,
causing the cell to assume a rounded
form, after which the anterior portion
is extended, thus pushing the cell for¬
ward.

The internal features of the euglena
show an interesting combination of plant
and animal characteristics. The outer
covering is a thin, flexible membrane
like the membranes of typical animal
cells. At the anterior end of the cell is
a gullet opening which leads to an en¬
larged reservoir. Since the euglena has
never been seen to feed, the gullet prob¬
ably serves only as an attachment of the
flagellum. Near the gullet is a very
noticeable red eyespot. This tiny bit of
specialized protoplasm is especially sen¬

sitive to light and serves to direct the or¬
ganism to bright areas in its habitat.
Near the center of the mass of cytoplasm

is a large nucleus.

Perhaps the most striking charac¬
teristic of the euglena is the presence of
numerous oval chloroplasts scattered
through the cytoplasm of the cell.
Most species of euglena carry on photo¬
synthesis and thus live quite independ¬
ently of any outside source of food.
However, some species lose their chloro¬
phyll in periods of prolonged darkness
and begin to absorb dissolved organic
matter from the water in which they
live. This organic matter passes
through the cell membrane and into the
cell where it is digested. Thus these

CHAPTER 18 THE PROTOZOANS 261

organisms live an autotrophic life but
can revert to a heterotrophic type of
nutrition during unfavorable environ¬
mental conditions. Perhaps this is evi¬
dence that an organism like the euglena
was the ancestor of all present-day pro-
tists.

These organisms multiply rapidly
by regular cell division under ideal con¬
ditions. A mature organism splits
lengthwise, forming two new cells.
One euglena may give rise to teeming
millions within a few days if conditions
for growth are favorable. Euglenas are
often so numerous in ponds and streams
that the water is a brilliant green.

The table below, comparing the
ameba, euglena, and paramecium, may
help you to review the degree of spe¬
cialization of these three common pro¬
tozoans.

The spore-forming protozoans. The

spore-forming protozoans have no meth¬
od of locomotion. They are placed in
the protist phylum Sporozoa. In these
protozoans reproduction by spores is ac¬
complished in the following manner:
first the nucleus divides into many small
nuclei; then a small amount of cyto-

7 J

plasm surrounds each nucleus to form
a spore; finally the protozoan breaks
apart and releases these little spores.

COMPARISON OF THREE PROTOZOANS

Ameba

Paramecium

Euglena

Form

Variable

Slipper-shaped

Pear-shaped

Locomotion

Pseudopodia

(ameboid movement)

Cilia

Flagellum or

euglenoid movemen

Speed

Slow

Rapid

Rapid or slow

Food-getting

Pseudopodia

surrounding

food

Cilia in oral
groove

Photosynthesis
or absorption

Food taken in

Absorption

Through mouth
cavity and
gullet

Absorption

Digestion

In food vacuole

In cytoplasm

Respiration

Diffusion of

O., and C02
through membrane

Diffusion of

02 and C02
through membrane

Diffusion of

02 and C02
through membrane

Excretion

Through membrane

Through pellicle

Through membrane

Sensitivity

Responds to
heat, light,
contact

Responds to
heat, light,
contact,
chemicals

Eyespot sensitive to
light

Reproduction

Fission

Fission and

Fission

conjugation

262 UNIT 3 MICROBIOLOGY

IN LIVER CELLS

Malaria organism grows and develops
into infective stage in the mosquito.

Mosquito bites an infected human and
picks up the malaria organism. Later the
same mosquito bites a healthy person
and injects infective organisms.

Stomach

Piercing mouth parts

Anopheles sp.

_ / . .

- ’ - a

HUMAN SKIN

Blood vessels

yg®|»rprganisms which
gM^can infect mosquito

IN BLOOD

When the blood cells break open,
L the human patient has
faA chills and fever

Some organisms
° leave the liver
and develop
in red blood cells

18-7 The life cycle of Plasmodium is shown here. The Culex mosquito shown
at the upper right does not carry Plasmodium.

The spores may be enclosed in a re¬
sistant wall or they may be surrounded
only by a cell membrane.

Plasmodium ( plaz-mo/z-dee-um ) ,
the parasitic protist that causes malarial
fever in man and other warm-blooded
animals, is a good example of a spore¬
forming protozoan. When a female
Anopheles (a-n<j/r/-e-leez) mosquito
bites a person suffering from malaria,
some of these protozoans pass into the
insect’s stomach. They grow, repro¬
duce, and work their wav into the mos¬

quito’s blood stream where they travel
to the glands of the mouth. When this
infected mosquito bites a human being,
the Plasmodium is introduced into the
human blood stream. From the blood
stream it first invades the liver and then,
about two weeks later, a red blood cell
(Fig. 18-7). Here it forms so many
spores that the membrane of the red
blood cell breaks and liberates the young
spores, as well as waste products from
the parasite. Each spore finds a new
red blood cell and the cycle is repeated.

CHAPTER 18 THE PROTOZOANS 263

In about two weeks after the infectious
mosquito bite, there are about a billion
parasites in the body. The chills, high
fever, and sweating are caused by the
release of wastes into the blood stream
when the red blood cells burst.

There are three major species of
Plasmodium that cause malaria in hu¬
man beings. One species forms spores
and bursts the red blood cells every day,
so that the victim has chills and fever
every 24 hours. Another species forms
spores every other day, and a third spe¬
cies repeats its cycle every three days,
thus causing attacks of chills and fever
at intervals of 72 hours.

Other pathogenic protozoans. Many
people are surprised to find that the
great majority of human beings and
most animals are infected with some
type of protozoan. In man and most
animals, the favorite place for these in¬
fections is the intestine, where a flour¬
ishing collection of these animals may
be found. If you examine the intestinal
contents of a freshly-killed animal un¬
der the microscope, you will probably
see a great many protozoans. Some of
them are harmless and may even be
helpful, but many are pathogenic. The
latter live on material in the intestine,
robbing the host of its food. Some may
invade the intestinal wall, show up later
in the blood stream, and finally lodge
in some other part of the body.

Water or food can be the source of
infection by an ameba which causes
amebic dysentery , a form of diarrhea,

which results in poor nutrition and loss
of water from the tissues of its human
host. Although this disease more com¬
monly occurs in the tropics, it is fairly
common in the temperate zones. Afri¬
can sleeping sickness is caused by one of
the parasites that move by flagella, the
Trypanosoma (trip-cm-o-sohm-a) . It is
carried to a human host by the tsetse
fly. Many local and international
health organizations are waging a deter¬
mined battle to reduce the occurrence
of these two diseases in the world.
Economic importance of the proto¬
zoans. The protozoans that live in
fresh-water ponds and streams are of
great economic importance, for they are
the source of food for many small ani¬
mals. Some salt-water protozoans se¬
crete a hard wall made of calcium or sili¬
con. These substances may form elab¬
orately beautiful patterns. Members
of this group, called the foraminifers
and the radiolarians , are responsible for
the formation of many of the limestone
and chalk deposits throughout the world.
As they die their miniature skeletons
fall to the bottom of the sea, and there,
with billions of others, form a muddy
deposit. If, as the earth’s surface
changes, this deposit dries out, it be¬
comes hard.

Protozoans help to digest food in
the intestines of some animals. In cat¬
tle they play such a role; and in the in¬
testines of termites, protozoans are
chiefly responsible for digesting the
woody material the insects eat.

IN CONCLUSION

Protozoans are one-celled protists that function as complete organisms. The
ameba, a shapeless mass of living protoplasm, lacks the specialization of other
protozoan cells, yet, in its primitive way, performs all the processes of life.

Paramecia, on the other hand, illustrate a high degree of specialization.
The euglena may perhaps be a connecting link between the plant and animal
kingdoms, since it has certain characteristics of both groups.

264 UNIT 3 MICROBIOLOGY

Of great importance are the disease-producing forms of parasitic proto¬
zoans that live in the bodies of animals and man. Many of the protozoans
that cause disease, such as the malaria parasite, require two hosts in order to
complete the life cycle.

The next chapter will introduce us to some other very interesting protists
that, although they do not contain chlorophyll, are more plant-like than animal¬
like.

BIOLOGICALLY SPEAKING

ameboid movement
anal pore
anterior end
avoiding reaction
cilia

Ciliophora
conjugation
contractile vacuole
ectoplasm
endoplasm

QUESTIONS FOR REVIEW

1. What are the four protozoan phyla? What characteristic divides them
into phyla? Give an example of each.

2. Describe the wav the arrieba obtains its food.

3. Describe the way the paramecium obtains its food.

4. Explain how paramecium cells multiply. How are they rejuvenated?

5. In what ways does the paramecium show sensitivity?

6. List the plant-like and animal-like characteristics of the euglena.

7. Compare the locomotion of the ameba, the paramecium, and the euglena.

8. Why is the ameba considered simpler than the euglena and other proto¬
zoans?

9. Describe the life cvcle of Plasmodium.

10. Name some of the pathogenic protozoans and the diseases they cause.

11. Name ways in which protozoans are important to man.

APPLYING PRINCIPLES AND CONCEPTS

1. Biologists frequently say that understanding the life processes of a single
protozoan helps them to understand the life processes of complicated
organisms like man. Why is this probably true?

2. What reasons can you give for the fact that the paramecium has two con¬
tractile vacuoles rather than one, like the ameba?

3. What stage in development would you consider best for treatment, once
a person has become infected with the malarial parasite? Whv?

4. What useful functions do certain protozoans perform in the human in¬
testine?

5. In what way are protozoans important in a pond?

euglenoid movement

oral groove

eyespot

pellicle

food vacuole

posterior end

foraminifers

pseudopodia

gullet

radiolarians

macronucleus

reservoir

Mastigophora

Sarcodina

micronucleus

Sporozoa

mouth cavity

trichocysts

CHAPTER 19

THE FUNGI

What are fungi? When we speak of
fungi, we refer in a broad sense to two
phyla of protists that possess many
plant characteristics. Because the fungi
lack chlorophyll, they are either sapro¬
phytes or parasites and must live in
association with organic nutrients.
Fungi are common in all environments
where suitable organic compounds are
present. The great majority of them
are terrestrial, but there are many aquat¬
ic forms. All produce spores that are
carried either by air or by water.

In your study of the fungi, you will
find considerable variation in the struc¬
ture of the plant bodies as well as dif¬
ferences in reproductive structures. Bi¬
ologists recognize only two phyla of
fungi. The larger of the two, the My-
cophyta ( my-kahf-i-ta ) , consisting of the
true fungi, is divided into four classes
(see Appendix). The other phylum of
fungi, the Myxomycophyta ( mik-soh-
my-kahf-i-ta ) , is made up of curious or¬
ganisms called slime molds. At one
stage in their life, some of these con¬

sist of flagellated cells, but have feeding
stages in which large masses of proto¬
plasm with many nuclei are not divided
into separate cells. These molds are
placed with the fungi only because their
spores are borne in structures much
like those of the true fungi.
Characteristics of the true fungi. More
than 75,000 different species compose
the four classes of true fungi. They
vary in size from microscopic organisms
to structures as large as mushrooms and
puffballs.

Most of the true fungi have vegeta¬
tive bodies composed of whitish or gray¬
ish filaments, each of which is known as
a hypha. The hvphae vary in length
and contain many nuclei. Some hyphae
do not have their cells separated by cell
walls, while others do have such walls.
Therefore, you must think of a hypha as
a long, continuous structure composed
of many nuclei, but with or without cell
walls.

The total mass of hyphae is a myce¬
lium ( mv-see-lee-um ) . Although chloro¬
phyll is not present in the hvphae of a
mvcelium, certain fungi contain yellow,
orange, red, blue, or green pigments,
which give them a special color.

All true fungi reproduce asexually
by forming spores, which are carried to
new environments by wind or water or
contact with other agents. Most of
them reproduce sexually as well.

Because of the enormous number
of spores they produce, the fungi are
widespread. These spores float through
the air and lodge on objects. A suitable
food supply or host will, almost in¬
variably, be invaded by a fungus if con¬
ditions are suitable for growth. Fungi
have no light requirements and actually
thrive best in darkness. Moisture is,
however, a growth requirement, as are
warm temperatures, which usually favor

265

266 UNIT 3 MICROBIOLOGY

19-1 This photograph shows one of the slime
molds in its fruiting stage, with spores ready
to be released. (Walter Dawn)

the growth of fungi. However, most
fungi tolerate a wider temperature
variation than do the majority of bac¬
teria. Nearly all true fungi are aerobic,
although a few live as anaerobes.

Molds — some of the most familiar
fungi. We use the term mold to refer
to many kinds of fungi. Some of
these are Phycomycetes (fy-koh-my-
see- tees), and resembles algae in struc¬
ture; others are Ascomycetes (us-koh-my-

see-tees) or sac fungi, so named because
many of their group produce spores,
usually eight in number, in saclike struc¬
tures, each of which is called an asciis.

Molds thrive in dark, moist places.
While warmth stimulates the growth of
many of them, others grow well at tem¬
peratures near freezing, which makes
these organisms a serious problem in
cold storage plants and in home refrig¬
erators. Molds grow on nearly all foods
as well as on wood, paper, leather, and
many other organic substances. You
may even find mold on the shoes you
laid aside in your closet, if the summer
months were warm and humid.

It would be incorrect to brand all
molds as destructive. Certain of them
are used in ripening cheeses. As you
learned in Chapter 17, another mold,
Penicillium, is directly responsible for
the era of antibiotics.

Bread mold and its relatives. If you
moisten a piece of bread, expose it to
the air, and set it in a dark place for
several days or a week, bread mold is
almost certain to appear. Rhizopus ( ry -
zo-pus) is the genus name for this mem¬
ber of the Phycomycetes. The mold
starts as a microscopic spore that grows

Aspergillus Penicillium Saprolegnia Rhizopus

19-2 These are various types of molds, drawn as they appear under the micro¬
scope. What characteristics do all molds have in common?

4 Jyji

on the surface of the bread, forming a
network of silvery, tubular hyphae.
Within a few days, the mold covers the
surface of the bread, forming a cotton¬
like mass of hyphae, the mycelium. x

A portion of bread mold viewed
with a hand lens or under the low power
of the microscope reveals several distinct

' ^ y / / / / / /

kinds of hyphae composing the myceli¬
um. Those hyphae that spread over
the surface of the food supply are called
stolons (stoh- lonz). At intervals along
the stolons, clusters of tiny rootlike
hyphae, or rhizoids (ry- zoyds), pene¬
trate the food supply and absorb nourish¬
ment. Rhizoids secrete digestive en¬
zymes that act on the sugar, starch, and
anv other carbohvdrates in the bread.

J ' s / Jy ' / ^ y *

These digested foods are then absorbed
info the hyphae of tlie mold. The
flavor, odor, and color spots that mold
prodbces)on bread and other foods are
due to chemical changes resulting from
the action of these enzymes.

After a few days of growth on the
bread surface, black knobs appear among
the hyphae of bread mold. Each black
knob is a spore case, or sporangium
(spor-dn-jee-um), which is produced at
the tip of a special ascending hypha, or
sporangiophore. Each sporangium is a
thin-walled case containing thousands
of black spores. When the sporangium
matures and dries out, its wall splits and
releases the spores, which are blown
around by air currents. Each spore may
form a new hypha and, in a short time,
an entire mature mold.

Sexual reproduction also occurs in
bread mold by a form of conjugation.
Although the individual hyphae in
Rhizopus look alike, there are two physi¬
ologically different types, which may be
designated as + and — . When a + hy¬
pha touches a — hypha at its tip, conju¬
gation follows. Slightly back of the

CHAPTER 19 THE FUNGI 267

19-3 When you put a magnifying glass to a
piece of bread which is covered with bread
mold, you will see something like this. Note
the rootlike rhizoids extending into the
bread.

6 u

point of contact, walls are formed across
the tips of the two hypha. This area
at the tip, which has been cut off from
the rest of the hypha, becomes a gam¬
ete. Thus one gamete is + while the
other is — . Finally the contents of the
two gametes fuse as the two hypha tips
dissolve. The result of this fusion be¬
comes a thick-walled zygospore, which
rests until favorable conditions for ger¬
mination occur. It may be carried a
considerable distance by air currents or
wind before it lands on a suitable host.
When it does germinate, it produces a
small hypha at its tip, and from this a
sporangium develops. The sporangium

268 UNIT 3 MICROBIOLOGY

19-4 Sexual reproduction in bread mold,
m formation of the zygote. D shows the
nation of the mature zygospore.

A through C represent various stages
zygospore and E represents the germi-

wall eventually ruptures and several
spores are discharged, which then grow
into a new mold. Note that the new
spores are either + or - and that the
hyphae they produce are also + or

The water molds, belonging to the
genus Saprolegnia (sap-roh-Zeg-nee-uh),
are relatives of the bread mold and be¬
long also to the class Phycomycetes.

1 hey are similar to bread mold in hav¬
ing tubular hyphae. Certain of these
molds are saprophytes and live on the
bodies of dead insects and other animals
in the water. Other water molds are
highly destructive parasites. They in¬
vade the tissues of fish and other aquatic
animals where they form patches of
cottony tufts. These molds will, in
time, kill the host animal. They are a
constant problem to the aquarist and, in
addition, destroy large numbers of na-
ti\e fish. The spores of water molds
are equipped with two flagella which
propel them to new locations.

Blue and green molds. The blue and
green molds ( Penicillium and Asper¬

gillus) form the familiar powdery
growth on oranges, lemons, and other
citrus fruits. This powdery substance
consists of spores in tremendous num¬
bers, which form at the tips of hyphae.
The mycelia of these molds are deeply
embedded in the tissues of the food
source. These molds may live on meat
and other food products as well as on
citrus fruits.

In this group of molds we find sev¬
eral of the most valuable of all fungus
plants. Several species of Penicillium ,
one of the blue molds, are used in
the processing of fine-flavored cheeses.
Cheese manufacturers carefully grow
these molds and add them to the
cheeses at a certain point in processing.
During the aging period, the mycelium
of the mold grows through the cheese
and, by enzyme activity, forms sub¬
stances that add distinctive flavors.
Among the more popular mold cheeses
are Roquefort and Camembert. Both
arc cheeses in which Penicillium molds
are used.

CHAPTER 19 THE FUNGI 269

19-5 A water mold, Saprolegnia, produces
zoosporangia, one of which is shown here
releasing its zoospores into the water. (Wal¬
ter Dawn)

Today, however, Penicillium means
much more to us than an organism for
the flavoring of cheese. We associate
it with one of the most notable medical
advances of our time, the discovery of
the antibiotic penicillin , which we dis¬
cussed in Chapter 17.

Mildews — destructive plant parasites.
The downy mildews belong to the
Phycomycetes. Among the host plants
of these dangerous parasites are rad¬
ishes, mustards, white potatoes, sweet
potatoes, cereal grains, sugar cane, to¬
bacco, and lettuce. One of the downy
mildews caused a famine in Ireland in
1845-1846 when it destroyed nearly the
entire potato crop.

Many downy mildews form hyphae
which grow down among the cells of
the host plant. These hyphae absorb
nourishment directly from the tissues of

the host and thus are internal parasites.
Other species form downy patches of
surface hyphae, usually on leaves of the
host, and send short, branching hyphae
deep into the cells.

The powdery mildews belong to the
class A scomycetes and are not usually
as destructive as downy mildews, since
they are external parasites. They ap¬
pear as whitish or dark-colored patches,
usually on the leaves of the host plant.
Among the host plants of these mildews
are lilacs, grapes, roses, clover, apples,
gooseberries, and many other flowering
plants.

The familiar yeasts. The yeasts are
one-celled fungi belonging to the Asco-
mycetes. The cells are usually oval.
Each cell contains a nucleus and a vac¬
uole. Note that these cells are not hy¬
phae, since each yeast is a unicellular
plant. Yeast cells reproduce by bud¬
ding (see Chapter 8). When growth
conditions are favorable, a bud starts as
a small knob pushing out from the side
of a cell. This bud, or daughter cell,
grows rapidly and may produce another
bud while still attached to the mother

19-6 The powdery mildew has as its host
the common lilac bush. Why is it called an
external parasite?

270 UNIT 3 MICROBIOLOGY

19-7 Yeasts are extremely beneficial fungi
because of their ability to ferment sugars.
Note the buds which are here shown in vari¬
ous stages of growth.

cell. Thus a fragile chain of cells, like
a string of beads of varying sizes, results.
When growth conditions become un¬
favorable, budding does not occur. In¬
stead, a yeast cell may form spores,
usually four in number, and thus be¬
comes a spore-bearing sac, or ascus,
characteristic of the Ascomycetes.

In connection with our discussion
of Pasteur’s work with fermenting beet
juice, we mentioned that yeasts produce
alcohol bv fermentation. In this type
of anaerobic respiration, carbon dioxide
is given off as a fuel product. You can
observe this activity by adding commer¬
cial yeast to a 10 percent solution of
molasses (or some simple sugar) in
water. Within a few hours the yeast
cells multiply by budding in such num¬
bers that they give the solution a cloudy
appearance. Carbon dioxide rises
through the solution as tinv bubbles,
and you can smell the odor of the
yeast.

Both products of yeast fermenta¬
tion are valuable commercially. Yeast
in dough forms bubbles of carbon diox¬
ide which swell during baking and make
the loaf light. The alcohol is driven
off in the baking. Commercial alcohol
manufacturers use yeasts to ferment
various carbohvdrate mashes.

Yeasts are important, also, as pro¬
ducers of vitamin B2, or riboflavin (ry-
boh-flay-vin ) . This vitamin is essential
in normal growth and in the health of
the skin, mouth, and eyes. Riboflavin
remains in the yeast cells. To obtain it
we must either eat the yeast cells or use
a product made of ground yeast.

Wild yeasts are abundant in the air
and ferment sugars in natural fruit
juices. A few yeasts are pathogenic.
Other Ascomycetes. The cup fungus
and morel are relatives of the yeast.
These Ascomycetes are harmless sapro¬
phytes. Cup fungi grow on rotting
wood and leaves and on organic matter
in rich humus soils. Hyphae penetrate
the food supply and absorb nourish¬
ment from the food material. The
white, orange, or red cups are spore¬
bearing structures, composed of tightly
massed hyphae. Many of these hyphae
end in an elongated sac, or ascus, inside
of which are eight spores.

19-8 The morel is a highly prized edible
fungus. It is usually found in rich woods
or in various shady places. (Hugh Spencer)

CHAPTER 19 THE FUNGI 271

The morel, or sponge mushroom, is
highly prized for its delicious flavor.
This is one of the few mushrooms that
can be safely eaten. It is easily recog¬
nized and is not related to the true
mushrooms, many of which are in¬
edible or poisonous.

Some of our most serious plant dis¬
eases are caused by parasitic Ascomy-
cetes. Among these diseases are Dutch
elm disease, chestnut blight, apple scab,
peachleaf curl, and ergot disease of rye.
The basidiomycetes. Basidiomycetes
are often called club fungi. Both of
these names refer to a curious club-
shaped structure formed at the end of
certain hyphae. This basidium (ba -sid-
ee-um), as it is called, usually bears four
basidiospores externally. Four groups
of fungi compose the class Basidiomy¬
cetes. These include the rust fungi,
smut fungi, mushrooms and bracket
fungi, and puffballs.

Rust fungi cause many serious plant
diseases. There are more than 250
known forms of grain rust that attack
wheat, oats, barley, and other cereals.
They cause millions of dollars of damage
to these crops annually.

Wheat rust is one of the best known
of these plant parasites, and is a special
problem to farmers. It produces four
kinds of spores in a very complicated
life cycle that involves not just one but
two host plants.

The rust makes its appearance on
wheat during the late spring and early
summer months, while the wheat is
green and actively growing. The rust
forms a mycelium which grows among
the cells of the wheat stem and leaves.
Tiny blisters appear along the surface of
the stem and leaves where clumps of
hyphae grow to the surface and dis¬
charge their spores. These spores are
reddish-orange in color and are all one-

celled. Biologists refer to them as red
spores, or uredospores (yoo-reed-oh-
spohrz) . These uredospores can rein¬
fect wheat and spread the disease rapidly
by lodging on new plants. Later in the
summer, when the wheat is ripening and
the plants are turning yellow, a second
spore stage appears. The same hyphae
that produced red spores now produce
black spores, or teliospores (tee- lee-o-
spohrz). These are two-celled spores
with heavy, thick,, protective walls.

These black spores cannot reinfect
wheat. Instead, they remain dormant
through the winter on the wheat straw
or stubble, or on the ground. Early in
the spring, both cells of the teliospores
germinate, producing four-celled basidia.
One basidiospore forms on each of the
four cells of the basidium. These spores
are then carried away by the wind. If a
spore lodges on a leaf of the common
barberry (not the cultivated Japanese
barberry), the life cycle continues.
After complicated changes have occurred
in the tissues of the barberry leaf, tiny
cups appear on the leaf surfaces. These
contain rows of aeciospores (ee-see-oh-
spohrz), which drop from the cups and
are carried by the wind to young wheat
plants as much as 500 miles away. In¬
fection of the wheat and production of
red spores follows, thus completing the
life cycle.

Both the wheat and barberry are
necessary for completion of the cycle.
Thus destruction of the common bar¬
berry bush is essential in controlling this
disease, especially in the northern states
where winters are more severe. How¬
ever, in the southern states where there
are mild winters, red spores may not be
killed and can reinfect wheat directly.

Another rust, the cedar-apple rust ,
involves the red cedar tree and the ap¬
ple and its close relatives. The white-

272 UNIT 3 MICROBIOLOGY

Summer— WHEAT

Uredospores (will infect
other wheat plants)

BARBERRY

Fall — WHEAT
Teliospore (will lie
dormant till
spring)

Aeciospores (will
infect young wheat)

Winter-WHEAT STRAW

Basidiospores
l (will infect
barberry)

Teliospore

19-9 Left: the common barberry bush and the wheat plant are the two hosts
necessary for the completion of the life cycle of the wheat rust. Right: trace
the life cycle of the corn smut from the point at which masses of hyphae de¬
velop in the corn plant until the cycle is completed. How does this cycle com¬
pare with that of the wheat rust to the left?

CHAPTER 19 THE FUNGI 273

pine blister rust causes serious damage
to the white pine tree in one stage of its
cycle and lives on the wild currant or
gooseberry in another.

Smuts — parasites on cereal grains. The
smuts attack corn, oats, wheat, rye, bar¬
ley, and other cereal grasses, causing con¬
siderable damage.

Corn smut , one of the most famil¬
iar of this group, infects corn plants
when they are young. Some weeks lat¬
er, a grayish, slimy swelling appears on
the ear, tassel, stem, or leaf. These
swellings consist of hyphae. As the
corn matures the hyphae produce large
numbers of black, sooty spores, which
are carried by the wind. These spores
may infect nearby corn plants or he
dormant until spring. A germinating
spore produces a four-celled, or some¬

times three-celled, basidium. Each cell
of the basidium produces a single basid-
iospore, as in the rusts. These basidio-
spores are carried by the wind to corn
plants where they germinate and start
a new smut infection. Corn smut may
be controlled by burning infected
plants, plowing under the stubble after
the corn is cut, and destroying the un¬
used stalks and leaves after the corn is
picked.

Mushrooms — the best known of the
fungi. We find mushrooms in orchards,
fields, and woodlands, and popping up
suddenly after a warm spring or autumn
rain. We seldom see the vegetative
mushroom plant. It is a mycelium
composed of many silvery hyphae,
which thread their way through the soil
or the wood of a decaying log or stump.

19-10 This drawing illustrates the development of a mushroom as well as dif¬
ferent views of the mature structure. The inset is an enlargement of a gill
showing how the four basidiospores are attached to the basidium.

274 UNIT 3 MICROBIOLOGY

A mushroom mycelium may live many
years, gradually penetrating more and
more area of substrate. Digestive en¬
zymes secreted by the hyphae break
down organic substances in the host
and change them to forms that can be
absorbed and used as nourishment by
the mushroom.

At certain seasons, especially in the
spring and fall, small knobs develop on
the mycelium just below the ground.
These consist of masses of tightly-packed
hyphae. These buttons develop into
the familiar spore-bearing structure we
recognize as a mushroom. The mature
mushroom consists of a stalk, or stipe ,
which supports an umbrella-shaped cap.
While pushing up through the soil, the
cap is folded downward around the stipe.
After forcing its way through the soil,
the cap opens out, leaving a ring, or
annulus , around the stipe at the point
where the cap and stipe were joined.

Most mushrooms contain numerous
platelike gills on the under surface of
the cap, radiating out from the stipe
like the spokes of a wheel. On the
outside of each gill and extending all
around it are hundreds of small basidia,
each of which bears four basidiospores.
It is estimated that a single mushroom
may produce as many as 1,800,000,000
spores. These spores drop from the
basidia when mature and are carried by
the wind and air currents. Each spore
forms a new mushroom mycelium if it
happens to land in a new environment
favorable for growth. If it does not —
and not too many do — the spore dies.
Some basidiospores are black while oth¬
ers are brown, yellow, white, or pink.
If you turn the cap of a fresh ripe mush¬
room gills downward on a sheet of
brown paper, you will usually get a large
mass of spores and can identify their
color easily.

Growth characteristics of the fungi.

Many fungi grow in circles, which tells
us about their food relations. In the
fairy ring mushrooms, for example, the
mycelium of the original plant digests
the organic matter available at that spot.
As the mycelium expands into the un¬
used organic matter in the soil around
it, new mushrooms are produced at the
outer edge of this growing ring. You
may see the same principle in mold
growth in a culture dish. The newest
part of the mold is on the outside of the
circle, and the oldest portion is in the
center.

Poisonous and edible mushrooms. The

word “toadstool” is frequently used as
a popular term for poisonous mush¬
rooms. While many people claim to
have methods of distinguishing edible
from poisonous varieties, experts tell us
there is no certain rule or sign that can
be used to distinguish the two types of
mushrooms. Frequently some of the
most harmless-looking forms are poison¬
ous and produce severe or even fatal
effects if eaten. The only safe advice
that can be given is to leave them alone
unless you know exactly which forms
are edible and which are poisonous.
Bracket fungi. The bracket fungi are
the familiar shelflike growths seen on
the stumps or trunks of trees. They
may be either parasites on living trees,
or saprophytes, living on dead wood.
They are the most destructive of the
wood-rotting fungi. The mycelium of
the bracket fungus penetrates the woody
tissue of the host and causes it to dis¬
integrate internally. The shelflike re¬
productive body is telltale evidence of
the damage that is occurring within the
host. The bracket fungi are woody in
texture when old and remain attached
to the host year after year. New spore
producing hyphae form on the under-

CHAPTER 19 THE FUNGI 275

side of the shelf, forming layers or rings
of growth. Spores are discharged
through tiny pores located on the un¬
derside of the shelflike growth.
Puffballs. The puffballs resemble mush¬
rooms except that the reproductive
structure never opens to discharge the
spores. It merely dries and splits open,
thus releasing the spores into the air.
Puffballs are round or pear-shaped
growths, usually white in color. Nearly
all species are edible if collected when
young and before the spores mature.
No poisonous puffballs are known to
exist.

Imperfect fungi. The biologist places
all the fungi that do not belong in the
other classes in the class Deuteromyce-
tesy or imperfect fungi. They are con¬
sidered imperfect because they repro¬
duce only asexually. The majority of
imperfect fungi are most like the Asco-
mycetes.

. Many of the imperfect fungi are of
great importance as parasites in man,
animals, and plants. Among the plant

diseases associated with this group are
those of com, oats, wheat, citrus fruits,
tomatoes, cabbage, lettuce, beans, and
apples.

The ringworm fungi produce sev¬
eral skin infections in man, including
ringworm, barber’s itch (ringworm of
the scalp), and the all too familiar ath¬
lete’s foot. Thrush is a serious infection
of the mucous membranes of the mouth
and throat caused by an imperfect fun¬
gus resembling the yeasts.

Fungi and higher plants living together.
Biologists have been investigating an
interesting relationship between certain
fungi and the roots of higher plants.
Many Ascomycetes and Basidiomycetes
that live in the soil are parasites on the
roots of trees and shrubs. Some form
a mass of hyphae around a young root.
Others enter the root and live as in¬
ternal parasites. It is believed that the
fungi aid the root in absorbing minerals,
especially nitrogen compounds. The
fungi, of course, take nourishment from
the root tissues.

IN CONCLUSION

The fungi comprise two phyla whose members lack chlorophyll. Fungi must
therefore grow on organic materials because they cannot make their own food.
If the food source is living material, the fungus is a parasite. If the source is
nonliving, the fungus is a saprophyte.

Among the most important groups of fungi are mushrooms, bracket fungi,
molds, mildews, rusts, smuts, blights, and yeasts.

Fungi may be beneficial or harmful, depending on their source of food.
Among the valuable fungi are certain of the mushrooms, some of the molds,
and yeasts. Certain molds, mildews, rusts, and smuts are blight-causing organ¬
isms and among the most destructive in the entire plant kingdom.

In the next chapter we shall examine six phyla of aquatic or marine
protists (although a few are terrestrial) known as the algae. Here we find
seaweeds and kelps as well as many organisms that thrive in freshwater ponds,
lakes, and streams.

276 UNIT 3 MICROBIOLOGY

BIOLOGICALLY SPEAKING

aeciospore

annulus

Ascomycetes

ascus

Basidiomycetes

basidiospore

basidium

cap

Deuteromycetes

gill

hypha

mycelium

Mycophyta

Myxomycophyta

penicillin

Phycomycetes

rhizoid

sporangiophore

sporangium

stipe «

stolon

teliospore

uredospore

QUESTIONS FOR REVIEW

1. What one reproductive characteristic do all fungi have in common?

2. Name the classes of fungi. Which of these classes are included in the
true fungi?

3. Describe the mycelium of a true fungus.

4. What different kinds of hyphae make up a bread mold plant?

5. How can bread become inoculated with bread mold even though there
may be no molds close by?

6. Discuss several ways in which Penicillium molds are valuable.

7. Why are downy mildews more destructive to higher plants than the pow¬
dery mildews?

8. Discuss budding in yeast.

9. How are the products of yeast fermentation used commercially?

10. Describe the production of four kinds of spores by wheat rust.

11. What stage in the life cycle of corn smut relates it to the rusts?

12. Describe the structure of a mushroom reproductive body.

13. How are puffballs different structurally from mushrooms?

14. Name several human infections caused by imperfect fungi?

APPLYING PRINCIPLES AND CONCEPTS

1. Mildew diseases of higher plants are more prevalent some summers than
others. Account for this.

2. Sweet cider will ferment rapidly in a warm place even when it is in a
tightly closed container. Explain why.

3. Why are yeast preparations valuable in treating acne and other skin dis¬
orders?

4. Why is a severe epidemic of wheat rust likely to follow a mild winter?

5. Explain why certain kinds of mushrooms and molds often grow in a cir¬
cular ring.

CHAPTER 20

THE ALGAE

The occurrence and classification of
algae. Floating or submerged masses of
blue-green, green, or yellowish algae are
familiar to anyone who has walked
along the shore of a pond or stream or
rowed across a lake in spring, summer,
or fall. These are algae, which are the
most abundant form of vegetation in
aquatic environments. Some forms
cling to rocks along waterfalls or rapids;
others occur on seacoasts where they are
exposed during low tide. Still others
grow on the bark of trees, on fence
posts, or on wet stones.

Some algae are unicellular while
others are multicellular. Some one-
celled forms may be as small as 1/25,000
of an inch, while a giant kelp may be
over 100 feet long. Approximately
30,000 species of algae are known to the
biologist and more will probably be
found as new areas of the world are ex¬
plored for science. The classification of
the algae is difficult, and various systems
for grouping them have been proposed.
One commonly accepted today places

them into six phyla in the protist king¬
dom (see Appendix).

It is very likely that algae and fungi
evolved from a common ancestor. We
know that algae are ancient, for fossils
have been found in Archeozoic and
Proterozoic rock. They were dominant
during the Silurian period of the Paleo¬
zoic era.

The cells of all algae contain chlo¬
rophyll and carry on photosynthesis, just
as higher plants do. In fact, algae are
the principal food producers in water
environments. Furthermore, the oxy¬
gen released from algal cells during
photosynthesis dissolves in water and
becomes available for the respiration
needs of the many aquatic animals liv¬
ing among the algae. Thus this oxygen
is as important as the sugar, starch, oils,
and other organic products of the chem¬
ical activity of algal cells.

Structure and reproduction of algae.
Many algae are one-celled organisms.
Some float in the water or settle to the
bottom, while others swim about like
animals. Many algae form colonies
consisting of two or more individual
cells attached to each other. Even
though they are attached to one an¬
other, all the cells in a colony of algae
live independent lives. They do not
depend on one another as do the cells
of higher plants.

Many algae form threadlike colonies
in which the cells are attached end to
end. We refer to these linear groups of
cells as filaments. Other algae form
flat plates of cells or globular or spher¬
ical colonies. The cells of many species
secrete gelatinous cell coverings that
protect the cells from water loss and
unfavorable environmental conditions.
These gelatinous secretions make many
algae slimv in texture and difficult to
grasp in the water.

277

278 UNIT 3 MICROBIOLOGY

Many forms of reproduction are
carried on among the algae. All of
them carry on fission. When a one-
celled alga divides, two new organisms
result. Cell division in a colonial form
merely increases the size of the colony.

Colonies of algal cells are fre¬
quently broken apart by currents of
water, passing fish, or animals feeding
on them. We refer to this mechanical
separation of cells in a colony as frag¬
mentation. This breaking up of col¬
onies merely multiplies the number of
colonies, since the separated cells con¬
tinue normal growth and multiplica¬
tion.

Many algae reproduce asexually by
forming spores. These spores contain
a portion of the protoplasm of the
mother cell that formed them, and each
one may swim or float to a new location.
There the spore may grow immediately
into a new alga or may lie dormant for
weeks or months. When environ¬
mental conditions are suitable, it will

then form a new algal cell. By means
of spores, many algae spread over wide
areas in a single growing season.

Reproduction also occurs sexually
by both isogametes and heterogametes.
Often the zygote undergoes a resting
period after which it produces spores.
These spores then givo rise to a new
plant.

Because the cells of algae are so
small, and because the behavior of the
cells during reproduction is so specific,
a microscope is almost a necessity in
studying algae. If you plan to observe
living forms of algae, your microscope
will reveal fascinating activities in just
one drop of pond water.

Blue-green algae. The blue-green algae
are all one-celled protists which usually
form colonies. Some occur in filaments,
while others consist of slimy masses of
material in which the algal cells are
embedded. You can find various blue-
green algae in almost every roadside
ditch, in ponds, and in streams. To-

GLOEOCAPSA

20-1 This diagram shows the structure of four rather common blue-green algae.
What similarities and differences do you note?

CHAPTER 20 THE ALGAE 279

gether with the viruses and bacteria,
they are considered among the most
primitive of plants.

Blue-green algae thrive during the
hot summer months, and are a constant
problem in drinking water and swim¬
ming pools. They give water the odor
characteristic of stagnant pools and cer¬
tain streams. For this reason biologists
check the sources of public water sup¬
ply regularly for the amount of blue-
green algae it contains.

The phylum name Cyanophyta (sy-
a-mzhf-i-ta) refers to a blue pigment,
phycocyanin (fy-koh-sy-a-nin) . This,
together with chlorophyll, gives these
algae their characteristic blue-green
color. Colors of various species range
from bright blue-green to almost black.
However, a few species contain a red
pigment. One of these forms appears
periodically in the Red Sea, which gave
it its name in early times.

The cells of blue-green algae are
primitive and simple in structure. They
lack a nuclear membrane and the nu¬
clear material may appear to be scattered
throughout the protoplast. The chloro¬
phyll and other pigments are located on
membranes rather than in chloroplasts.
None of these blue-green algae repro¬
duce sexually as do the higher fungi.
Instead, their usual method of repro¬
duction is by simple fission.
Representative blue-green algae. We
have selected four common blue-green
algae as representatives of this important
group. One of the most curious is
Nostoc (nos- tok). You will find it in
mud and sand usually just at the point
where the ripples from a pond or lake
strike the shoreline. A Nostoc colony
looks like a small, gelatinous ball, rang¬
ing in size from a pinhead to a marble,
often likened to a peeled grape (Fig.
20-1 ) . The ball is composed of a gelat¬

inous matrix. Embedded in the matrix
are many curved and twisted filaments
made up of tiny spherical cells, each of
which resembles a pearl bead in a neck¬
lace. Distributed along the filaments
are curious empty cells with thick walls
and pores at either side where they join
other cells. These are heterocysts. Bi¬
ologists believe that they may be the re¬
mains of spores that have lost their con¬
tent and ceased to function. They en¬
able the filaments of Nostoc to break
into shorter pieces. Nostoc is, funda¬
mentally, a one-celled plant that forms
filamentous colonies surrounded by a
gelatinous matrix. Each time a cell
divides by fission, it forms two small
cells of equal size and thus increases the
length of the filament. This is the only
method of reproduction in this primi¬
tive alga.

A nabaena (an-a-foee-na) is a relative
of Nostoc, as seen by the similarity in
cells and filaments (Fig. 20-1). Fila¬
ments of A nabaena, however, are solitary
and do not occur together as the fila¬
ments of Nostoc do. Another difference
is the production of spores in Anabaena,
which can be recognized easily as en¬
larged, oval cells in the filaments. Each
spore is protected by a thickened wall
and contains an abundance of stored
food. Each spore eventually separates
from the parent filament and germi¬
nates in a new location.

Gloeocapsa ( glee-oh-kd/?-sa ) is one
of the most primitive blue-green algae
(Fig. 20-1). It can be found on wet
rocks and often grows on moist flower¬
pots in greenhouses, where it forms a
slimy, bluish-green mass. The individ¬
ual cells of this one-celled alga are
spherical or oval. The diffused blue
and green pigments lie in a zone near
the wall. Dark granules may be seen
deeper in the cell. Each cell of Gloe-

280 UNIT 3 MICROBIOLOGY

ocapsa secretes a slimy sheath. When
a cell divides, each new cell secretes its
own sheath within the old one, thus
forming characteristic layers of sheaths.
A colony of Gloeocapsa often contains
hundreds of cells, joined by their
sheaths in a gelatinous layer.

Oscillatoria ( os-il-a-tor-ee-a ) is a
filamentous blue-green alga composed of
many narrow, disk-shaped cells resem¬
bling a stack of coins. The tip, or apical
(up-ideal) celly is round on one side in
many species. Filaments of Oscillatoria
sway gently back and forth in the water,
a characteristic that gave the alga its
name. When one cell in the filament
dies, the ones on either side bulge into
its place and produce a curious concave
cell (Fig. 20-1). Concave cells are
weak places in the filament and cause
the plant to break into shorter pieces.
Oscillatoria becomes abundant in ponds
and streams during warm weather.
Green algae. The green algae belong
to the phylum Chlorophyta and vary
from one-celled forms to colonial forms
composed of a large number of cells.
They are for the most part fresh-water
algae, although certain members of the
group live in strange and interesting en¬
vironments. Many kinds live in the
ocean and a few live in bodies of water
even higher in salt concentration than
the ocean. Some, to the amazement of
scientists, thrive in hot springs. One
species grows on the hair of the South
American three-toed sloth and gives this
animal its peculiar green appearance.
Other strange environments include the
bodies of protozoans, sponges, and jelly¬
fish.

The cells of green algae have a defi¬
nitely organized nucleus. Carotinoid
pigments, including chlorophyll and
also the xanthophylls and carotenes,
are contained in the plastids. These

20-2 The round cells of the green alga Pro¬
tococcus may occur singly or in colonies of
two or more.

pigments combine to give the algae col¬
ors ranging from grass green to yellow¬
ish-green. Green algae form sugar dur¬
ing photosynthesis and convert it to
starch for storage in the cells. Repro¬
duction is both by fission and by asexual
spores, and some of the genera repro¬
duce by forming sexual gametes.
Protococcus, a common green alga.
Protococcus is one of the most common
of green algae, and is an exception
among algae in that it does not live in
water. Most of you may not have rec¬
ognized it as an alga because it grows
on the trunks of trees. You may also
find it on unpainted wooden buildings
and fences. During dry weather you
seldom see it, but in wet weather it is
very evident. It is most common on
the north side of tree trunks because
bark is more moist on the shaded side.

The cells of Protococcus are spheri¬
cal or somewhat oval (Fig. 20-2).
Each contains an organized nucleus
and a single large chloroplast. The
cells are so small that many thousands
cover only a few square inches of bark.
They may be carried from tree to tree
by birds and insects as well as by the

CHAPTER 20 THE ALGAE 281

wind during dry weather. Since Proto¬
coccus is a green protist, it requires no
nourishment from the tree on which it
grows.

Reproduction in Protococcus is by
fission only. Following divisions the
cells tend to cling together. This pro¬
duces the cell groups shown in Fig.
20-2.

Chlorella is a single-celled spheri¬
cal alga somewhat resembling Protococ¬
cus. It has a large, cup-shaped chloro-
plast. This alga is especially interesting
to the biologist because several of its
species live in the cells or tissues of
protozoans, sponges, and jellyfish. As
you learned in Chapter 6, it has also
been the subject for much research in
the study of photosynthesis as well as
in studying alga cultures as a possible
source of food and oxygen.

Spirogyra, a filamentous green alga.
Almost any pond or quiet pool will
have bright green masses of threadlike
Spirogyra (spy-roh-/y-ra) during the
spring and fall months. The un¬
branched filaments of this green alga
range from a few inches to a foot or
more in length. Under a microscope,
a thread of Spirogyra looks like a series
of transparent cells, arranged end to
end like tank cars in a train (Fig. 20-3).

Each cell has one or more spiral
chloroplasts which wind from one end
of the cell to the other. On the ribbon¬
like chloroplasts are small protein bodies
surrounded by a layer of starch. These
pyrenoids (py- re-noidz) are food reserves
of the cell. The nucleus is embedded
in cytoplasm and is suspended near the
center of the cell by radiating strands of
cytoplasm anchored to the pyrenoids.

20-3 Left: one cell of the green alga Spirogyra, showing the ribbonlike chloro-
plast. Right: conjugation in Spirogyra. Each step in the process is numbered
in sequence for your further study.

282 UNIT 3 MICROBIOLOGY

Most of the cytoplasm lies in a layer
close to the wall, leaving a large central
vacuole which fills with water and dis¬
solved substances. A thin, gelatinous
sheath surrounds each cell and gives the
filaments of Spirogyra a characteristic
slippery feeling.

On a bright day photosynthesis
takes place rapidly. Bubbles of oxygen
stream from the cells and collect among
the filaments, which cause a mass of
Spirogyra to float to the surface. Dur¬
ing the night the oxygen dissolves in
the water and allows the mass to sink.
You can see floating colonies of Spiro¬
gyra and other algae on the surface of
a pond, especially in the afternoon of a
bright day.

Spirogyra reproduces in two ways.
All the cells of a filament undergo fis¬
sion at certain time intervals. Since
the divisions are always crosswise, fis¬
sion adds to the length of the filaments.

The second method of reproduc¬
tion is by conjugation, which, as you
learned in Chapter 18, is sexual. It oc¬
curs when weather conditions are un¬
favorable for normal growth by fission.
You are likelv to see it in material col¬
lected early in the summer, at the close
of the spring growing season, and again
in the fall with the approach of cold
weather.

Conjugation involves two fila¬
ments of Spirogyra , which line up paral¬
lel to each other. A small knob grows
out from each cell on its inner side, as
shown in Fig. 20-3. Then each knob
grows until it touches the knob of the
cell across from it. Soon the tips of
the knobs dissolve and form a passage¬
way between the two cells, which re¬
sults in a ladderlike arrangement. The
content of one cell then flows through
the passageway and unites with the con¬
tent of the other cell. The fused con¬

tent of the two cells forms a spherical
or oval zygote, which is soon sur¬
rounded by a thick protective wall. It
is interesting to note that the content
of a cell in one filament moves across to
the cell of the other, thus producing
one filament of empty cells and another
containing rows of zygotes. This alga
reproduces much as does Rhizopus, the
bread mold, except that the filaments
are not definitely + or — . They do dif¬
fer physiologically, however.

Soon after conjugation is com¬
pleted, the zygotes fall from the cells
holding them and undergo a rest pe¬
riod. The thick wall protects the zy¬
gote from heat, cold, and dryness. As
a zygote, Spirogyra can survive a long,
cold winter, a summer drought, or may
even be transported from one pond to
another by a bird or other animal.
When conditions are favorable for
growth again, the content of a zygote
resumes activity. The nucleus divides
by meiosis, resulting in four nuclei,
each containing the haploid number of
chromosomes. Three of these nuclei
die. The remaining one becomes the
nucleus of the new Spirogyra cell that
grows from the zygote and establishes
a new filament by successive cell di¬
visions.

The life history of Ulothrix. Ulothrix
(yoo-loh-thriks) is interesting to the bi¬
ologist because it represents a group of
green algae with somewhat different re¬
productive processes. You can find this
alga attached to rocks in swift-flowing,
shallow water. The short filaments are
anchored by a special cell with finger¬
like projections, known as a holdfast.
The cells above the holdfast each have
a chloroplast shaped like an open ring
(Fig. 20-4).

Under ideal growth conditions, a
cell of Ulothrix undergoes a series of

CHAPTER 20 THE ALGAE 283

SEXUAL

REPRODUCTION

Gametes uniting
Zygote

20-4 The drawing on the left represents asexual reproduction in Ulothrix,
while that on the right illustrates sexual reproduction.

mitotic divisions followed by a splitting
up of the cell content into 2, 4, 8, 16, or
32 oval bodies. Each of these bodies is
a new cell because it contains a nucleus
and chloroplast. Each one also develops
four flagella. We refer to these tiny cells
as zoospores (zoh- uh-sporz) because
thev resemble one-celled animals. The
zoospores burst out of the encasing
mother cell and their flagella whip
them through the water. When a zoo¬
spore reaches a favorable place, it grows
into a holdfast cell. This holdfast cell
divides into two new cells of which the
upper one becomes an ordinary Ulo¬
thrix cell. This cell then forms a fila¬
ment bv further cell divisions.

At other times Ulothrix cells may
undergo similar nuclear divisions and
separation of the cell content into 8,
16, 32, or 64 bodies which are very simi¬
lar to zoospores except that they are
smaller and have only two flagella.
These bodies are isogametes, and have
definite sex even though they look alike.

They leave the parent cell and swim
away from the filament. If a gamete
meets another gamete of the opposite
sex, from another cell, they fuse and
produce a zygote. After a rest period
the zygote undergoes nuclear divisions
resulting in four nuclei, as in Spyrogyra.
In Ulothrix , however, all the nuclei live
and, with a portion of the zygote proto¬
plast, become zoospores. Each zoo¬
spore may produce a new holdfast cell
and thus give rise to a new filament.
Life history of Oedogonium. Oedogo-
nium ( ee-doh-goh-nee-um ) is a common
green alga found in quiet pools, where
it grows attached to rocks, sticks, and
other objects. Like Ulothrix , the plant
consists of an unbranched filament with
a basal holdfast cell. Each cell has a
single chloroplast composed of many
joined strands. Pyrenoids are numer¬
ous (Fig. 20-5).

Any cell above the holdfast may
convert its content into a large, single
zoospore. The zoospore is propelled

284 UNIT 3 MICROBIOLOGY

Sperm

Antheridium

Sperm entering pore

Egg

Oogonium

Vegetative cell

Pyrenoids
Nucleus
Chloroplast

20-5 Sexual reproduction in Oedogonium
involves heterogamy.

through the water by its ring of flagella.
When a zoospore reaches a suitable lo¬
cation, it settles down and modifies to
become a holdfast cell, thus establish¬
ing a new filament.

Sexual reproduction involves only
certain cells in a filament of Oedogo¬
nium. In this way it is different from
Spirogyra and Ulothrix. The proto¬
plasts of special cells distributed along
a filament each develops into a large,
single egg. Each of these special cells
•is called an oogonium (oh-uh-goh-nee-
um) and contains a tiny opening
through its wall. In most species of
Oedogonium two sperm develop in each
of several shortened cells that form in
groups in the filament. We refer to a
sperm-producing cell as an antheridium.

A sperm looks like a miniature zoo¬
spore. Eggs and sperm may develop in
the same or in different filaments, de¬
pending on the species of Oedogonium.
Within a short time (often only a few
minutes) after a sperm leaves an an¬
theridium, it swims to the pore of an
oogonium, enters, and fertilizes the egg.
This produces a zygote. When the
oogonium wall disintegrates some time
later, the zygote escapes and undergoes
a rest period. When conditions are
again favorable for growth, the zygote
forms four zoospores, which escape and
establish new filaments of Oedogoni¬
um. Oedogonium is considered one of
the more advanced algae because of
its specialized heterogametes and be¬
cause of the structural difference be¬
tween the female gamete, or egg, and
the male gamete, or sperm.

A closer look at the reproductive proc¬
ess in algae. In our study of certain
fungi and Spirogyra , we noted the be¬
ginnings of sexual reproduction in the
process of conjugation. In Ulothrix we
have seen a further development in sex¬
ual reproduction, as gametes are formed
that have definite sex although they
look alike. In Oedogonium, gametes
are formed that look distinctly different
and are designated as sperm and egg.
Remember that all these forms also un¬
dergo asexual reproduction by fission,
fragmentation, or spore formation.
Thus they have two wavs to reproduce.

Now let us examine the sexual
method more closely. In Ulothrix, for
example, the isogametes, like all gam¬
etes, contain the haploid number of
chromosomes. When two gametes fuse
in the process of fertilization, the diploid
number reappears in the zygote. This
In zygote is the stage, or generation, in
the life cycle referred to as the sporo-
phyte , because it produces spores. As

CHAPTER 20 THE ALGAE 285

Cladophora

Vaucheria

Chlamydomonas Meridion (diatom)

Eudorina

Chlorella

Zygnema

S> 9? ©

20-6 Algae, though among the simplest plants, are represented by a rich vari¬
ety of interesting forms.

286 UNIT 3 MICROBIOLOGY

we study the mosses, the ferns, and later
the seed plants, we will find that this
generation becomes increasingly promi¬
nent, until it is the only conspicuous
part of the plant. In JJlothrix, how¬
ever, it is present only in the form of
the zygote. Soon after fertilization, the
zygote undergoes a reduction division,
resulting in two nuclei having the n
number of chromosomes. This division
is followed by a mitotic division, re¬
sulting in four haploid nuclei which be¬
come four zoospores. These give rise
to new haploid JJlothrix filaments. Re¬
member that the filament, as well as re¬
producing asexually, may at times pro¬
duce gametes. For this reason it is
known as the gametophyte (ge-meet-uh-
fyt) generation and is the most con¬
spicuous stage in JJlothrix. The game¬
tophyte has the n number of chromo¬
somes, as do the gametes produced from
it by mitosis. The occurrence of two
distinct stages in the life cycle is known
as alternation of generations. You will
discover as you study the evolution of
plants that this alternation occurs
throughout the plant kingdom, even
though the relative importance of the
generations varies.

Since alternation of generations oc¬
curs so generally, we can probably as¬
sume that it is of some advantage to the
survival of the organism. Why might
this be true? Think back to the fact
that meiosis occurs in the zygote, or
sporophyte generation. Meiosis allows
for a reshuffling of the genes in chromo¬
somes, so that the resulting haploid
spores have various new combinations
of genes not present in the parent.
Some of these spores are probably bet¬
ter adapted than others to the environ¬
ment in which they develop. Thus
some of the spores have a survival ad¬
vantage and continue to produce the

species, while others die out. The hap¬
loid gametophyte (filament) that de¬
velops from the successful spores has
the same survival advantages. These
gene combinations are transmitted to
the gametes intact, as the gametes are
formed by mitosis.

At this point you might ask why
meiosis occurs at all. If a successful
combination of genes has been pro¬
duced, how does it happen that they are
again reshuffled? The answer probably
lies in the fact that environments are
constantly undergoing change of one
kind or another. As these changes take
place, various adaptations appear in the
populations. Some are successful and
some are not. Thus in alternation of
generations, the sporophyte produces a
variety of spores by meiosis. The ga¬
metophyte, on the other hand, transmits
intact the more successful combinations
of genes by mitosis. But the sporo¬
phyte stage still continues to supply
new combinations as environmental
conditions change.

Desmids. None of the green algae are
more beautiful and fascinating to study
than the group known as desmids.
These free-floating algae may be either
• solitary or colonial; some form filaments.
A desmid cell consists of two halves con¬
nected by a narrow isthmus in which
the nucleus is situated. This unique
structure makes them especially beauti¬
ful. You will find several desmids
among the algae shown in Fig. 20-6.
Diatoms — very common algae in both
fresh and salt water. The diatoms ( dy-
a-tahmz) belong to the phylum Chryso-
phyta ( kris-o-fy-ta ) and are one-celled,
free-floating algae varying from rectan¬
gular, round, triangular, or oval to spin¬
dle-shaped or boat-shaped forms. They
are sometimes green but more often
golden brown. They all contain chloro-

CHAPTER 20 THE ALGAE 287

phyll. The products of their photosyn¬
thesis are chiefly oils instead of carbo¬
hydrates. Diatoms are practically the
only plants that grow in open seas, and
next to the bacteria are the most nu¬
merous organisms in existence. Their
walls contain small amounts of silicon
dioxide and manganese. The wall is in
two sections or valves, one fitting over
the other like the top and bottom of a
pillbox. One might say that the dia¬
toms actually live in glass houses!

The shells of diatoms are pretty not
only because of their shapes, but also
because of the many fine lines that form
intricate and beautiful designs on their
walls. When they die, they fall to the
bottom of the pond, stream, or ocean
and may form deposits of diatomaceous
earth. In California and other parts of
the world, these deposits are thick.
They are mined and sold as ingredients
in various scouring powders, or used in
filters in the refining of gasoline.

The dinoflagellates and cryptomonads.
The phylum Pyrrophyta (pir-d/z/-i-ta) is
the smallest of the six algal phyla. The
members are nearly all unicellular with
two flagella, and live mostly in salt wa¬
ter, sometimes in fresh water. These
organisms are of great importance as a
source of food in the oceans. One of
the genera, Gymnodnium, during cer¬
tain times secretes a red pigment that
is extremely toxic to fish. When this
occurs fishermen speak of it as a “red
tide." Why it occurs is a question that
biologists have not yet been able to
answer.

The red and brown algae. The brown
algae belong to the phylum Phaeophyta
(fay-dh/-i-ta) and the red algae to the
phylum Rhodophyta (roh-dd/if-i-ta) .
Both are mostly salt-water forms, bet¬
ter known as seaweeds. Sizes vary from
some small threadlike red algae to the

giant brown species of the Pacific
Ocean, commonly called kelp. They
usually live in shallow water near the
shore, where light can reach them, but
some of the reds live in deeper water.
These red algae grow attached to solid
objects on the bottom or may grow in
mud or sand. Because they vary so
much in structure and reproduction and
are often difficult to obtain, we cannot
study any one type. A brown alga called
Fucus is a favorite for study because it
is so common along the seacoast (Fig.
20-7). You may find it as the packing
around oysters, lobsters, and other sea¬
foods that are shipped from the coast to
your town.

Economic importance of the algae.

Algae are the chief source of food and
energy for much of the animal life in
the environments in which they occur.
Although many small fish live entirely
on algae, one large group of mammals,
the whales, also exists primarily on them.
As we said at the beginning of this
chapter, algae are also a source of oxy¬
gen for aquatic animal life.

The use of marine algae as soil fer¬
tilizers has long been known. If sea¬
weeds are mixed with the soil, they not
only add organic matter to it but also
replenish the mineral salts that land¬
growing plants have removed. Algae
are particularly rich in iodine, a chem¬
ical element essential to plant and ani¬
mal life.

While algae are useful in certain
parts of the world in the preparation of
soups, gelatins, and other foods, we are
more likely to find them as a part of our
ice cream. A sodium compound ex¬
tracted from algae is added to keep the
ice cream smooth. This compound
may also be used as a stabilizer in choc¬
olate milk or as a thickener in salad
dressings. Agar-agar (cz/ig-ahr-cz/ig-ahr)

288 UNIT 3 MICROBIOLOGY

20-7 Fucus is a brown alga that inhabits rocks at the water line along the At¬
lantic Coast. At low tide, it covers the rocks and at high tide is under water.
(Walter Dawn)

is produced from red algae in the Indian
Ocean and is used in hospitals and lab¬
oratories as a base for culture medium
for bacteria. The list of other industrial
uses of algal preparations ranges from
cosmetics to leather-finishing.

An interesting new discovery is
that algae can be mass-cultured in plas¬
tic tubes or tanks. By supplying the
culture with all the best conditions for
photosynthesis, such as light, water, and
carbon dioxide, the algae multiply rap¬

idly. Periodically some algae are
strained out, dried, and then made
ready for use as flour in baking or as a
thickening in foods such as soup.

Some algae may become poisonous
when they die, and thus pollute the wa¬
ter. This not only makes the water un¬
fit for human beings, but also for fish
and other water life. Great care is
taken in fish hatcheries to prevent this.
A very weak solution of copper sulfate
put in the water will kill blue-green

CHAPTER 20 THE ALGAE 289

algae. This treatment, however, is not
recommended for home aquaria because
there is considerable danger of over¬
dosing, which would kill the fish.

Use of algae in space flights. One of the
problems that must be solved before
man can make long trips in space is
that of disposing of the waste product
of man’s respiration, carbon dioxide.
Recent research indicates that certain
forms of algae may be useful in accom¬
plishing this purification of the air in
a space ship. Algae, like all organisms
containing chlorophyll, use carbon di¬
oxide and give off oxygen as a waste
product in the process of photosynthesis.
There are certain hot-climate algae that
reproduce faster than cold-climate types.
They are, in fact, capable of multiply¬
ing a thousand-fold in 24 hours. These
types, of course, use up carbon dioxide
and produce oxygen faster than ordi¬
nary tvpes. It is therefore believed that
they might be effective both for purify¬
ing the air in a space ship and for sup¬
plying some of the oxygen needs.

Food is also a problem in space
travel. Chlorella is being considered as

20-8 Chondrus crispus, also called Irish
moss, is a marine red alga. It grows on
rocks along the cold North Atlantic Coast.
(Hugh Spencer)

20-9 These kelp are brown algae which live
on the bottom of the Pacific Ocean but in
sufficiently shallow water so that light can
reach them. (Cadbury from National Audu¬
bon Society)

a possible food source. Mice have been
fed these algae, and it is believed that
thev would be suitable for human use
in a space capsule. In one hour a
Chlorella cell can produce 18 grams
(drv weight) of food. Nitrogen is neces¬
sary for protein synthesis in algae as in
all living things. It has been found
that algae can utilize the nitrogen pres¬
ent in human urine to fulfill this need.
Lichens, curious plant relationships.
While lichens (ly- kenz) are often
grouped with the fungi, they can also
be placed with the algae, because an
alga and a fungus make up the plant
body of a lichen. While various lichens
differ in structure, the alga is usually
a green or blue-green form while the
fungus is one of the Ascomycetes.

The plant body of a lichen consists
of a mass of fungus hyphae among
which the algal cells are scattered.
Lichens are of three general types.
Some form a hard, granular crust ( crus -
tose lichens). Others resemble flat¬
tened, leathery leaves ( foliose lichens).

290 UNIT 3 MICROBIOLOGY

20-10 A lichen is actually two kinds of
plants living together by symbiosis, or mu¬
tual benefit. Here is*a green alga living
among fungal cells, the two combined
making up the lichen plant body.

Still others form a network of slender
branches ( fruticose lichens).

In a lichen both the alga and the
fungus benefit from the association. In
fact, neither could survive alone in the
environments where lichens live. The
fungus depends on the alga for food,

IN CONCLUSION

produced by photosynthesis. Although
the alga is in a sense a “slave”, it is pro¬
tected and kept moist by the fungus
hvphae. This relationship, in which two
organisms live together for the mutual
good of each, is symbiosis.

The biological teamwork illustrated
in a lichen allows it to live in places
where few other plants can survive.
Many lichens grow on tree trunks.
Others cling to rock surfaces far above
timber line in the alpine zone of high
mountains. These lichens are impor¬
tant pioneer plants. Gradually they
cause rock surfaces to crumble and, as
they add their own remains season after
season, produce organic matter that is
the basis for soil. This soil can then
support other plants.

Lichens are among the most abun¬
dant plants of the wind-swept areas of
the Far North. One of these lichens,
Cladonia, is so valuable as food for rein¬
deer that it is commonly called reindeer
moss.

Algae are among the most important protists. They are vital in water environ¬
ments as food for the animals there and as a source of oxygen.

Algae range from single-celled to many-celled forms and reproduce by
simple fission, thus forming new unicellular plants or adding to the size of
colonies of cells. Many algae also reproduce asexually by forming zoospores,
which swim about and form new plants directly when they germinate. Sexual
reproduction also occurs and involves isogametes or heterogametes.

BIOLOGICALLY SPEAKING

alternation of generations

antheridium

apical cell

Chlorophyta

Chrysophyta

concave cell

Cyanophvta

filament

fragmentation

gametophyte

heterocvst

holdfast

matrix

oogonium

Phaeophyta

phy cocyan in

pvrenoid

Pvrrophyta

Rhodophyta

sporophyte

zoospore

CHAPTER 20 THE ALGAE 291

QUESTIONS FOR REVIEW

1. How are the algae fundamentally different from the fungi?

2. In what respect are the algae simpler in structure than higher plants?

3. Explain how colonies of algae increase in size during the growing season.

4. Name six phyla of algae and give an example of each.

5. Why are blue-green algae an especially serious problem in a community

water supply?

6. Account for the color of blue-green algae.

7. Describe a heterocyst of Nostoc or A nabaena, and explain how it causes
filaments to break up.

8. Describe the composition of a ball of Nostoc.

9. Account for the slimy texture of Gloeocapsa.

10. What characteristic of Oscillatoria is referred to in its name?

11. What pigments other than chlorophyll are present in the cells of green

algae?

12. Describe the habitat of Protococcus.

13. Why is Chlorella of special interest to biologists?

14. How can you easily distinguish Spirogyra from other filamentous green
algae under the microscope?

15. Describe conjugation in Spirogyra.

16. How can you distinguish a Ulothrix spore from a gamete?

17. What is alternation of generations? Describe its occurrence in Ulothrix.

18. What characteristics distinguish diatoms from other algae?

19. What are some uses for diatomaceous earth?

20. Describe the typical habitat of red and brown algae.

21. List some of the reasons why algae are economically important.

22. How does a lichen show symbiosis?

APPLYING PRINCIPLES AND CONCEPTS

1. What evidence can you give that all algae contain chlorophyll even though
many are not green?

2. A colony of 50 algal cells is not a 50-celled plant. Explain why.

3. Both spores and gametes are reproductive cells. How are they different?

4. In what respect are the cells of blue-green algae more primitive than those

of green algae?

5. Why is conjugation in Spirogyra considered a primitive form of sexual
reproduction?

6. In what ways is sexual reproduction in Oedogonium more efficient and
more advanced than the sexual reproduction in Ulothrix?

7. Compare Spirogyra, Ulothrix, and Oedogonium in regard to specialization

of cells in a filament.

8. What is believed to be the significance of sexual reproduction in plant
evolution?

292 UNIT 3 MICROBIOLOGY

RELATED READING

Books

Alexopoulos, C. f. Introductory Mycol¬
ogy, 2nd Ed. John Wiley and
Sons, Inc., New York. 1962
Boettcher, Helmuth M. Wonder
Drugs: A History of Antibiotics.
J. B. Lippincott Co., Philadelphia.
1964

Clark, Paul F. Pioneer Microbiologists
of America. The University of
Wisconsin Press, Madison. 1961
Duddington, Charles L. Micro-organ¬
isms as Allies: The Industrial Use
of Fungi and Bacteria. The Mac¬
millan Co., Chicago. 1961
Fink, B. The Lichen Flora of the
United States. University of Mich-
igan Press, Ann Arbor. 1960
Fraenkel-Conrat, Heinze. Design and
Function at the Threshold of Life:
The Viruses. Academic Press, Inc.,
New York. 1962

Glasschiek, H. S., M.D. The March of
Medicine: The Emergence and Tri¬
umph of Modern Medicine. G. P.
Putnam’s Sons, New York. 1964
Hardy, Alister C. The Open Sea, Its
Natural History: The World of
Plankton. Houghton Mifflin, Bos¬
ton. 1956

Kalver, Lucy. The Wonders of Algae.

John Day Co., New York. 1961
Kalver, Lucy. The Wonders of Fungi.

John Day Co., New York. 1964
Kleijn, H. Mushrooms and Other
Fungi. Doubleday and Co., Gar¬
den City, N.Y. 1962
Landon, John F. and Sider, Helen T.
Communicable Diseases. F. A.
Davis Co., Philadelphia. 1964
Low, Robert Cranston. Atlas of Bac¬
teriology, 2nd Ed. Williams and
Wilkins Co., Baltimore. 1964
Prescott, Gerald W. Flow to Know the

Fresh-Water Algae. Wm. C. Brown
Co., Dubuque, Iowa. 1954

Prescott, Samuel Cate. Industrial Mi¬
crobiology, 3rd Ed. McGraw-Hill
Book Co., New York. 1959

Riedman, Sarah R. Portraits of Nobel
Laureates in Medicine and Physiol¬
ogy. Abelard- Schuman, Ltd., New
York. 1964

Riedman, Sarah R. Shots Without
Guns: The Story of Vaccination.
Rand McNally & Co., Chicago.
1960

Smith, Gilbert M. The Fresh-Water
Algae of the United States.
McGraw-Hill Book Co., New York.
1950

Stanier, Roger Y. The Microbial World.
Prentice-Hall, Inc., Englewood
Cliffs. N.J. 1963

Stanley, Wendell M. and Evans, G.
Valens. Viruses and the Nature of
Life. E. P. Dutton and Co., New
York. 1961

Starobinski, Jean. A History of Medi¬
cine. Hawthorn Books, Inc., New
York. 1964

Tiffany, Louis. Algae: The Grass of
Many Waters. Charles C. Thomas,
Springfield, Ill. 1958

Wyss, Orville and Williams, O. B. Ele¬
mentary Microbiology. John Wiley
and Sons, Inc., New York. 1963.

Articles

Burnet, Sir MacFarlane. “Viruses.”
Scientific American. May, 1951

Palmer, C. Mervin. Algae in Water
Supplies, an illustrated manual on
identification, Public Health Serv¬
ice Bulletin, Number 657. The
Supt. of Documents, Washington,
D.C. 1959

UNIT FOUR

MULTICELLULAR

PLANTS

Ages ago, modifications probably occurred in certain aquatic plants which allowed
them to invade a new environment — the land. Perhaps some ancestral alga grow¬
ing on moist soil developed an aerial shoot from a filamentous plant body and ex¬
tended root-like projections into the soil. The mosses are perhaps remnants of
these first land plants. However, they never became fully adapted to a terrestrial
environment. It remained for the seed plants, with vascular tissues for conduction
and support, and seeds to protect and nourish the embryo plant, to dominate the
land environments of the earth.

CHAPTER 21

MOSSES AND
FERNS

The great kingdom Plantae. The plant
kingdom includes the mosses and ferns
and their allies, and all the seed plants.
The many kinds of mosses belong to the
phylum Bryophyta (bry-uhf-i-ta) , but all
the other members of this kingdom are
placed in the phylum Tracheophyta
(trav-kee-d/rf-i-ta ) . This is because of the
fact that they have organized tissues for
conducting food and water through the
plant. Such tissue is called vascular
tissue. Although the mosses and ferns
are classified in two separate phyla, we
shall study them together because they
are both comparatively simple land
plants which differ greatly from the seed
plants. In addition, their reproductive
cycles are similar, and studied together
the mosses and ferns provide an interest¬
ing evolutionary picture of the transition
to land life.

The Bryophyta. The bryophvtes con¬
sist of two main groups of plants, the

mosses and the liverworts. They are
found in every region of the world from
the high mountains of the Antarctic
through the tropics of northern Green¬
land. They thrive equally well in the
coldest climates or the warmest. Al¬
though their preferred habitats are
chiefly terrestrial, they all require an
abundance of moisture; some even live
in hot springs. Early in the Paleozoic
era they were probably mostly aquatic,
but gradually they became adapted to
the land. Perhaps they were the first
plants to live exclusively on land.

No bryophyte plant body is very
large. Although they live on land, these
plants lack structures for efficient con¬
duction of water to the leaves. Nor do
they have true roots, so they cannot ab¬
sorb water except from the surface of the
soil. Thus, the bryophytes, while show¬
ing a more complicated reproductive
cycle than the algae, do not have cell
specialization for water movement on a
large scale. Such specialization is neces¬
sary for a large plant to live successfully
on land.

The mosses. You have seen mosses
growing in cracks in shaded sidewalks,
on moist ground under trees, or in
clumps in deep woods. What looks
like a tuft or carpet is actually a compact
clump of moss plants. Each plant has
its tiny “stem” with a cluster of “leaves”
encircling it. If you pull one of these
tiny plants from the soil, you will find
a group of hairlike rhizoids (ry-zoidz)
growing from the base of the stem.
Mosses do not have true roots contain¬
ing conducting tissues, as do higher
plants, but the rhizoids serve a purpose
by anchoring the plant and by absorb¬
ing some water and dissolved minerals
from the soil. Mosses do, however, have
structures that resemble the leaves and
stems found in higher plants.

294

CHAPTER 21 MOSSES AND FERNS 295

21-1 Mosses, shown here with two mush¬
rooms, are well adapted to moist woodland
environments. (Walter Dawn)

Life cycle of the moss. The moss plant
we commonly see is only one phase of
the life cycle. If you examine the dia¬
gram in Fig. 21-3, you will see that each
moss plant goes through a reproductive
cycle in which an asexual spore-produc¬
ing stage forms a sexual gamete-pro¬
ducing stage. This in turn forms the
spore stage again. You will recognize
this as the alternation of generations
we discussed in the last chapter.

Sexual reproductive organs develop
at the tips of the leafy stems of game-
tophyte moss plants, in clusters hidden
by leaves. Depending on the species of
moss, male and female organs may be
borne on the same plant, on different
branches of the same plant, or on sep¬
arate plants. Sperm are formed in club-
shaped, short-stalked antheridia. A cap
at the apex of the antheridium opens at
maturity, allowing the sperm to escape.
The female organs, or archegonia (ahr-
ki-go/i-nee-a ) , are borne on short stalks.
A swollen base contains a single egg.
Sperm swim to the archegonium after a
rain or even in a film of dew, but they

cannot move unless there is some water
present. A sperm passes through the
neck of the archegonium and fertilizes
the egg. This union results in the
zygote.

Fertilization starts the sporophyte,
or asexual phase, of the cycle. The
zygote always remains in the female or¬
gan. Soon the sporophyte begins to
grow and produces a slender stalk that
grows up and out of the leafy stem of
the plant. The top of the stalk swells
and becomes a large mass of tissue called
a capsule , which is covered with a thin
hood. Inside the capsule there are
formed many microscopic spores. These
are asexual. That is, they are neither
definitely male nor female. When the
spores are ripe, the hood falls off, the
capsule opens, and the spores escape.
They are carried off by the wind, and
when they fall on the ground they begin
to grow, provided environmental con¬
ditions are right. The sporophyte stage
has ended, and the gametophyte stage
begins. Notice that the spores are dis-

21-2 In this cluster of moss plants note the
capsules on their slender stalks. This is the
sporophyte stage. You can also see the
leafy gametophyte stage from which the
sporophyte grows. Inside the capsules are
the sporangia with their spores, ready to be
discharged when the capsule bursts open.
(Hugh Spencer)

296 UNIT 4 MULTICELLULAR PLANTS

21-3 Life cycle of the moss. Note in stages 1 and 2 the female gametophyte
and in 3 and 4 the male gametophyte plants. After fertilization, the sporophyte
generation (stages 5, 6, and 7) grows out of the top of the female plant. Spores
form in the sporangia inside the capsule at the top of the sporophyte (stage 8)
and when mature fall to the ground and give rise to a new generation of gameto¬
phyte plants (stages 9 and 10).

persed without the aid of water, while
the gametes still depend on water for
fertilization.

Recall that in the algae heterogamy
developed slowly but occurred in a wa¬
ter environment. As the mosses evolved
and became adapted to dry land, the
sporophyte generation was able to exist
in the new type of habitat. The game¬
tophyte generation, however, still re¬
quired water for the sperm to reach the
egg. We shall observe this evolutionary
process in the various groups of higher
plants.

Each spore produces a small thread¬
like structure called a protonema (proht-
o-nee-ma). All the cells that make up

the protonema have chlorophyll and can
make their own food. The resemblance
of the protonema to an alga is startling
and once caused many scientists to class
it as a close relative of the algae. Some
cells of the protonema produce short
buds that grow into a new moss plant.
Other threads enter the ground and be¬
come the rhizoids. Thus, a new moss
plant is formed and this gametophyte
will soon form sex cells.

In the moss life cycle, the sporo¬
phyte is diploid, having the In number
of chromosomes. Meiosis occurs in the
sporophyte, and haploid (n) spores are
produced. The haploid gametophyte
grows from the spore and produces hap-

CHAPTER 21 MOSSES AND FERNS 297

21-4 Sphagnum, or peat moss, is a valuable
mulch for gardens and lawns. It is also used
for packing delicate and fragile flowers for
shipment. (Hugh Spencer)

loid gametes. Fertilization restores the
diploid number of chromosomes to the
zygote.

Economic importance of the mosses.

The sphagnum , or peat-forming mosses,
are the most widely used. Sphagnum
grows in small lakes and bogs, where it
forms floating mats. These mats in¬
crease in size and thickness each year as
generation after generation occupies the
surface of the mat. Plants of previous
years decompose slowly, settle to the
bottom, and form peat. In time the
growth of sphagnum mats in a lake may
close the water entirely, causing the lake
to enter the bog stage. Eventually
what was once an open lake may become
a deep deposit of brownish-black peat.
Large sphagnum mats are often invaded
by larger plants — rushes, grasses, shrubs,
and even trees.

The absorbent quality of sphagnum
makes it valuable to the gardener as a
mulch. It can be worked into the soil
or placed on the surface to make the

soil loose and to hold water during the
usually drv summer months.

Other mosses are important to us
as pioneer plants in rocky areas. The
small amount of soil that collects in
cracks on the bare surfaces of cliffs and
ledges is sufficient to support moss
plants. The rhizoids break down rocks
gradually and thus form more soil. As
mosses die and decompose season after
season, they form enough soil to anchor
the roots of larger plants.

The liverworts. Much less familiar than
mosses are their relatives, the liverworts.
These curious small plants grow in wet
places, often along the banks of streams,
around the outlet of a spring, on rocky
ledges or in the water. The moist soil
and flowerpots in greenhouses are other
good places to find liverworts. They
look like thin, leathery leaves laid flat
against the ground. One of the com-

21-5 In this photograph of the liverwort
Marchantia, the antheridia are found em¬
bedded in the club-shaped disks. The arche-
gonia occur embedded in the star-shaped
structures. The “leaves” are merely lobed
disks which resemble a liver except that they
are green. (Carolina Biological Supply
House)

298 UNIT 4 MULTICELLULAR PLANTS

21-6 During the Pennsylvanian and Mississippian periods (about 300 million
years ago) a typical forest contained giant spore-bearing plants, some of which
have been redrawn in this picture from their fossil remains. (Yale Peabody
Museum)

mon liverworts resembles a thin tongue
with Y-shaped branches at the tip. Un¬
der ideal conditions a clump of liver¬
worts may cover a considerable area.
Figure 21-5 shows a common liverwort,
Marchantia ( mar-ferm-shee-a ) . The fe¬
male sex organs are formed in curious
structures like umbrellas, which rise
about an inch above the flat plant body.
Male sex organs form in disk-shaped
structures.

The Tracheophyta — vascular plants.
As you have already learned, all the
tracheophytes possess tissues for con¬
ducting food and water through the
plant. In this chapter we are concerned
only with one group of this large phy¬
lum — the ferns and their allies. To

appreciate this group fully, we should
have lived about 300 million years ago
during the Carboniferous period , which
includes the Pennsylvanian and Missis¬
sippian periods combined. Then they
were not limited to a few places in the
woods or swamps, some hillsides, and
flowerpots. They formed large forests
that covered the wet, marshy land com¬
mon at that time. Ferns much like
those of today flourished during this
past age, but tree ferns 30 to 40 feet high
were also abundant.

Although no man ever saw those
great forests of ferns, today we are reap¬
ing the benefits of their existence. Dur¬
ing this age, deep layers of plant re¬
mains accumulated in the swampy areas

CHAPTER 21 MOSSES AND FERNS 299

where they grew. Later the movements
of the earth compressed these layers
into layers of coal. It has been esti¬
mated that it took 300 feet of com¬
pressed vegetation to form 20 feet of
coal. When we consider what coal has
meant to industry and living, we might
almost conclude that the high civiliza¬
tion of modern America has sprung
from the vegetation of millions of years
ago.

Most of us are familiar with ferns
as clumps of plants with graceful, deeply-
cut leaves. In all but the few remaining
tree ferns of the tropics, the stems are
underground, creeping horizontally just
below the surface. These underground
stems, which are called rhizomes (ry-
zohmz), bear clusters of true roots that
spread through the ground, anchoring

the plant and absorbing water and dis¬
solved minerals.

Life cycle of the fern. The fern you
find in the shaded woods or growing as
a potted plant is the sporophyte stage of
the life cycle (Fig. 21-7). As in the
mosses it is diploid (2 n). When the
familiar fern leaves are mature, small
dots called sori appear on the lower side.
The sori differ in shape and location in
different kinds of ferns. Some people
become alarmed when sori appear and
remove them carefully, mistakenly think¬
ing the fern is diseased.

Each sorus consists of a cluster of
helmet-shaped sporangia , each of which
contains numerous spores. These are
the result of meiosis, as they are in the
mosses, and are haploid. The sporan¬
gium is attached to the sorus by a short

SPOROPHYTE

GAMETOPHYTE

.Young prothailus

Prothallus

Sporangium

Sorus

Archegonium

Antheridium

Prothallus

Young

sporophyte

Prothallus

21-7 The life cycle of the fern. Note the sori on the fern frond. These contain
the sporangia. The gametophyte generation develops on the ground and is so
small you can hardly see it.

300 UNIT 4 MULTICELLULAR PLANTS

stalk. Since the fern plant bears sporan¬
gia and spores, it corresponds to the
stalk and capsule of the moss.

When the spores are mature, the
sporangium bursts open and releases
them. The tiny spores are easily picked
up by the wind and carried to new lo¬
cations. If a spore falls in a moist place,
it germinates and grows into a short
filament of cells with rhizoids. The
gametophyte stage of the fern life cycle
has begun. This stage at first resembles
the protonema of a moss. However, un¬
like the moss, the filament broadens at
the tip and becomes a flat, heart-shaped
structure called a prothallium. When
fully grown, the prothallium may be a
quarter to a half inch in diameter. The
flat prothallium is green and clings close
to the ground, held by a cluster of
rhizoids that forms on the underside at
the rounded or lower end of the “heart.”
It would be an interesting project for
you to try to raise some fern prothallia.

Several antheridia containing sperm
develop among the rhizoids. A number

of archegonia form near the notch at
the upper end of the prothallium. Each
one contains an egg. In most ferns the
eggs and sperm mature at different
times. Thus, when sperm escape, they
swim in the film of dew to another
prothallium. Thus both mosses and
ferns, while largely land plants, still re¬
quire water for fertilization. When a
sperm fertilizes an egg, a diploid zygote
is produced. This marks the end of the
gametophyte stage and the beginning
of the sporophyte stage. It is interest¬
ing to note that this prothallium stage
of a fern corresponds to the leafy-shoot
stage of the moss.

Immediately after fertilization the
zygote grows into a young fern plant.
Soon the young fern becomes established
with a root, a leaf, or frond, and a stem.
The stem bears additional fronds and
the familiar fern clump results. Thus
the sporophyte is the most conspicuous
part of the life cycle of the fem.

In comparing the life cycle of the
fern with that of the moss, vou will see

7 J

21-8 The horsetail rush produces bushy, green vegetative shoots (left) and
brownish, cone-like structures which bear the reproductive organs. Note the
scale-like leaves which appear in whorls on the stem. (Left: Walter Dawn; right:
Hugh Spencer)

CHAPTER 21 MOSSES AND FERNS 301

21-9 Club mosses are closely related to the
ferns because their methods of reproduc¬
tion are similar. Because they are evergreen
and look like the leaves of fir and spruce
trees, they are picked for Christmas decora¬
tions and therefore are rapidly becoming ex¬
tinct. (Walter Dawn)

that the gametophyte generation is rela¬
tively insignificant. The sporophvte
generation has become the dominant
phase of the plant. This is also true in
the seed plants, which you will study
in the following chapters.

Relatives of the fern. Closely related
to the ferns are two groups of plants,
fairly common in some localities. The
equisetums (ek-we-see£-umz), or horse¬
tail rushes, are often seen in wet places
or around the margins of lakes. The
reproductive stems are slender, dark
green, and rodlike, and bear light-col¬
ored cones at their tips. There are also
green, bushy stems that give the plant
its name. The leaves are small scale¬
like structures occurring in circles
around the stem at regular intervals.

Club mosses resemble true mosses
only in general appearance and consti¬

tute another group of the fern relatives.
These plants bear cone-like reproductive
structures at the tops of certain of their
branches (Fig. 21-9). Various species
are found in rich, damp woods, or creep¬
ing along the rocky slopes of mountains.
Some species even occur in deserts.
Like the ferns, both horsetails and club
mosses are now merely remnants of
plant groups that were once more abun¬
dant. In prehistoric times they were
the size of present-day trees. You can
still see their remains in the form of leaf
or stem imprints in coal.

IN CONCLUSION

The two plant groups that are forerunners of the seed plants are the mosses
and the ferns. They both show a complicated life cycle in which one spore-
producing stage, the sporophyte, produces a gamete-producing stage, the game¬
tophyte, in an alternation of generations. These plants once covered the earth,
and millions of years ago with the remains of other plants, formed the coal we
use today.

Our survey of the plant kingdom thus far has shown us the simpler, flower¬
less, less-noticed forms of plant life. In the next chapter we will discuss the
great group of seed-bearing plants. They too are an important and interesting
collection of living organisms.

302 UNIT 4 MULTICELLULAR PLANTS

BIOLOGICALLY SPEAKING

antheridium

archegonium

Bryophyta

capsule

Carboniferous period

frond

prothallium

protonema

rhizoid

rhizome

sorus

sporangium

spore

Tracheophyta
vascular tissue

QUESTIONS FOR REVIEW

1. What prevents the bryophytes from growing into large plants?

2. Describe briefly the two phases in the life history of the moss.

3. What process separates the gametophyte from the sporophyte?

4. Describe briefly the formation of a peat bog.

5. In what ways are the mosses important to man?

6. Describe coal formation.

7. List the various structures in the order of their appearance in the life cycle
of the fern.

8. Describe the way in which the fern produces its spores.

9. In what ways are the gametophyte generation and the sporophyte genera¬
tion of the fern connected with each other?

APPLYING PRINCIPLES AND CONCEPTS

1. Why do you suppose that we seldom find more than one sporophyte grow¬
ing from a single leafy stalk of moss, although that stalk usually bears more
than one female sex organ?

2. What reasons can you give for the ferns dying out as the common plants
in the world?

3. Compare the sporophyte plant of a moss and a fern, and explain in what
ways the fern is more advanced.

4. List ways in which the fern is better suited to life on land than the moss.

CHAPTER 22

THE SEED
PLANTS

What is a seed plant? The phylum
Tracheophyta includes those plants hav¬
ing a vascular system with well-devel¬
oped cells for the conduction of food
and water. Included in this phylum
with the ferns and fern allies are the
great group of seed plants. Today these
plants are the dominant forms found on
land masses. Mosses and ferns, pre¬
dominant in the past, are restricted in
their environments to habitats where
water is available at some season of the
year, often in the shelter of large seed
plants. Their sperm still require water
to reach the eggs. Seed plants, on the
other hand, are free of this total de¬
pendence on water for the completion
of the reproductive process. Further¬
more, the development of the seed, with
its protection of the young sporophyte
generation, has been of vital importance
in the evolution of this great group of
plants.

The seed plants are divided into two

large classes: the Gymnospermae (plants
whose seeds are not enclosed in a fruit,
like the pine), and the A ngiospermae
(flowering plants like the apple) . Thus,
a pine tree and an apple tree are both
seed plants, but they differ chiefly be¬
cause the pine tree bears its seeds on the
scales of its cones, while the apple tree
bears its seeds enclosed in a fruit that
came from a flower.

A seed is an embryo plant covered
by one or more protective seed coats
(Fig. 22-2). Food stored in a seed
nourishes the young plant until it is
established in its new location. In a
sense, a seed is a packaged plant, ready
for delivery. A seed may travel through
the air, float in the water, be carried on
the fur of an animal, or it may he dor¬
mant for many months. When water
and temperature conditions are favor¬
able, the seed coats soften and the young
plant pushes out its root and its shoot.

22-1 Cycads resemble both tree ferns and
palm trees in appearance but their repro¬
ductive structures are like those of the gym-
nosperms. In early geological times they
were abundant but are now more or less re¬
stricted to regions of warm climates. (U.S.
Forest Service)

303

304 UNIT 4 MULTICELLULAR PLANTS

Bean pod is the fruit

Pine cone made up of spore leaves

Seeds carried
naked on
spore leaf

22-2 The main difference between the seed of an angioisperm like the bean and
that of a gymnosperm like the pine is that the bean seed is enclosed in a fruit,
while the pine seed is located on the scales of the pine cone and thus is not
enclosed in a fruit.

By means of seeds, plants spread to new
locations. Reproduction bv seeds is
highly efficient. It is one of the reasons
why the seed plants have gained domina¬
tion of the earth.

Characteristics of the gymnosperms.
This class is older and more primitive
than the angiosperms. The name
means ''naked seeds’’ and refers to the
fact that the seeds are not enclosed in
a fruit. Like the mosses and ferns,
many gymnosperms were in their prime
during the Carboniferous period. Most
of these ancient genera are now extinct,
so that we know about them only from
their fossil remains. However, many of
this class exist today and are familiar to
you as the group of conifers that in¬
cludes the pines, firs, cedars, spruces,
yews, and others.

Many years ago priests of China
found a curious tree in the forests of
the interior and planted it in the tem¬
ple gardens. Later it was introduced
into gardens in Japan. Little did these
ancient priests know that they had

found and preserved the last species of
an order of gymnosperms, the Gink-
goales (ging-koh-uy-leez) . You may
know the ginkgo tree ( Ginkgo biloba),
for it is now cultivated in the United
States and other countries.

The deciduous leaves of the ginkgo
are wedge-shaped and two-lobed, unlike
those of any other tree. Most of the
leaves grow in clusters at the tips of
curious spurs spaced along the branches.
Some of the branches lack these spurs.
The ginkgo is often called the maiden¬
hair tree because its leaves closely re¬
semble those of the maidenhair fern.
Its seed-bearing structures look like
fruits, but they are structurally more like
cones.

The ginkgo is a fine tree for plant¬
ing in the yard. Although slow grow¬
ing, it will eventually reach a height of
100 feet with a diameter of nearly four
feet. If you do plant a ginkgo, you will
have one of the rarest of all living plants
— a single species of a single genus —
the last survivor of an entire order.

CHAPTER 22 THE SEED PLANTS 305

Cone-bearing gymnosperms — the coni¬
fers. The name conifer refers to the
fact that trees of this class of gymno¬
sperms bear woody cones composed of
scales. These scales bear winged seeds
on their upper surface.

Conifer leaves are either in the
form of needles or scales. Most conif¬
erous trees keep some of their needles
all winter. The new ones that are
formed in the spring take the place of
the old ones that fall to the ground.
Exceptions to this are the bald cypress
and the larch. These two conifers lose
all their needles each autumn.

Of all the gymnosperms that ever
existed, the conifers are the best suited
to present world environments. Many
coniferous species have disappeared,
but others still flourish and are domi¬
nant in many parts of North America.
The table on page 306 lists the genera of

modern North American conifers. We
have included the scientific names of
these genera because you will see them
if you look at nursery catalogs. Small
conifers are widely used around houses.
Large conifers such as pines, spruces,
and hemlocks are widely planted as
windbreaks around farmhouses and as
trees in city yards. The Colorado blue
spruce is unsurpassed as a single tree
in a yard planting.

Conifers are most important as tim¬
ber trees. Many species thrive in sandy
and rocky soils, which are unsuitable for
the broad-leaved forest trees.

Before leaving the conifers, we
might mention that they hold the rec¬
ord for height, trunk diameter, and age
among trees. A giant redwood, in the
Calaveras Grove in California towers
300 feet above the ground. This tree
is over 30 feet in diameter and is esti-

22-3 The ginkgo tree was once found only in China and Japan, but is now a
familiar sight in the United States. The insert shows the seed-bearing struc¬
tures. (U.S. Forest Service)

306 UNIT 4 MULTICELLULAR PLANTS

CONIFERS TODAY

Pine ( Pinus )

Spruce ( Picea )

Fir (Abies)

Hemlock ( Tsuga )

Red cedar ( Juniperus )

White cedar, arborvitae (Thuja)
Bald cypress ( Taxodium )
Redwood (Sequoia)

Douglas fir (Pseudotsuga)

Yew (Taxus)

Larch, tamarack (Larix)

mated to be 4,000 years old. Even this
trunk diameter and age are surpassed
by a cypress tree, the Big Tree of Tule,
growing about 200 miles south of Mex¬
ico City. It is 50 feet in diameter and
is estimated to be over 5,000 years old.
Characteristics of the angiosperms.
Tire class Angiospermae includes the
great majority of green plants existing
today. The flowering plants probably
evolved from gymnosperm ancestors
during the Mesozoic era. Even some

22-4 Conifers have their leaves in the form
either of needles or small scales. They are
among our most valuable timber trees. Here
you see a stand of young loblolly pine trees.
(International Paper Company)

modern gymnosperms show characteris¬
tics transitional between the two.

All angiosperms have flowers of
some sort; all bear seeds enclosed in an
ovary wall; and the seeds contain an em¬
bryo that possesses either one or two
cotyledons. A cotyledon (kaht-il-ee-
don) is the first leaf of the embryo
plant; it may act as a food reservoir until
the new plant develops green tissue and
can synthesize its own food. On the
other hand, it may serve as the first
photosynthetic organ of the seedling.

The angiosperms are divided into
two subclasses, depending on the num¬
ber of cotyledons, as follows: the Mono-
cotyledonae ( mu/m-o-kaht-l-ee-duh-nee)
and the Dicotyledonae (dy- kaht-l-ee-
duh-nee) . As the names imply, a mono¬
cot embryo bears one while that of a
dicot has two cotyledons. As we study
the flowering plants, we shall point out
other differences between monocots and
dicots, such as in the arrangement of
root and stem tissues, the organization
of leaf veins, and the number of flower
parts. Some families of monocots and
dicots are listed on pages 308 and 309.
The plant body of a flowering plant.
Each organ of a flowering plant is highly
developed for performing certain activi¬
ties (Fig. 22-5). The root, stem, and
leaf are vegetative organs. They per¬
form all the processes necessary for life
except the formation of seeds. This
does not mean, however, that plants can-

CHAPTER 22 THE SEED PLANTS 307

not multiply directly from their roots,
stems, or leaves. If you have sprouted
roots on a pussy willow stem in a jar of
water, you know you can plant the stem
and grow a new plant. Root, stem, and
leaf cuttings, as well as budding and
grafting, are methods of vegetative re¬
production.

The root anchors the plant in the
ground. It spreads through the soil and
absorbs water and soil minerals. It con¬
ducts these to the stem for delivery to
the leaves. Manv roots store food and

return to the plant as needed.

The stem produces the leaves and
displays them to the light. It is a busy
thoroughfare, for it conducts water and
minerals upward and carries foods that
have been manufactured in the leaves
downward. Like the root the stem
often serves as a place of food storage.
In many plants green stems aid the
leaves in photosynthesis.

The leaf is the center of much of
the plant’s activity. It is the chief cen¬
ter of photosynthesis. It also exchanges
gases with the atmosphere in the proc¬
ess of respiration. Much of the water
absorbed by the root has a one-way trip
through the plant. It escapes from the
leaf as water vapor in the process called
transpiration about which you will learn
in Chapter 25.

After a period of growth a plant
usually starts reproduction. F lowers are
specialized organs for sexual reproduc¬
tion. They are followed by fruits with
their seeds. As the flower withers, the
fruit develops from certain of its parts.
Within the fruit are the seeds, each
containing an embryo plant, which are
ready to be carried to a new location.
There they sprout and establish a new
generation of the same kind of plant.
Tissues of flowering plants. The various
organs of higher plants perform their

22-5 This drawing of a bean plant has the
chief organs labeled. What is the function
of each?

activities with great effectiveness be¬
cause of the specialized tissues that
form them. We shall study these tis¬
sues more thoroughly when we discuss
each of the plant organs in greater de¬
tail in the chapters to follow. How¬
ever, before dealing with any individual
organ, you should be familiar with the
names of various tissues and the general
functions of each.

308 UNIT 4 MULTICELLULAR PLANTS

SOME FAMILIES OF MONOCOTS

Family

Familiar Members

Cattail

Common cattail

(Tvphaceae)

Grass

Cereal grains, bluegrass, sugar cane, bamboo, timothy

(Gramineae)

Sedge

Sedges

(Cyperaceae)

Arum

Indian turnip (Jack-in-th e-pulpit ) , skunk cabbage, calla lily

(Araceae)

Pineapple

Pineapple, Spanish moss

(Bromeliaceae)

Lily

Lily, onion, tulip, hyacinth

(Liliaceae)

Amaryllis

(Amaryllidaceae)

Iris

Flag, iris

(Iridaceae)

Orchis

Lady’s slipper, orchis, orchid

(Orchidaceae)

Palm

Coconut palm, date palm, palmetto

(Palmaceae)

The tissues of flowering plants are
summarized as follows:

Epidermis — an outer layer which
reduces loss of water, and protects
against injury and the entry of disease-
causing organisms.

Cork — a waterproof covering of
nonliving cells, especially in woody
plants. It serves the same general pur¬
pose as an epidermis but is even more
effective.

Parenchyma (pa-ren-kih-mah) — a
thin-walled, soft tissue of the type form¬
ing flower petals, leaf blades, and the
cortex and pith regions of stems and
roots. Food manufacture and storage
of food and water are functions of
parenchyma tissues.

Xylem (zy-lem) — a supporting and
conducting tissue. Large multicellular
structures called vessels and small cells
called tracheids are thick-walled con¬

ducting tubes. Xylem fibers give
strength, especially to stems and roots.

Phloem (floh-e m) — a tissue in¬
cluding long, multicellular structures
called sieve tubes , for conduction and
phloem fibers for support. Phloem
comprises what is called the inner bark
of trees.

Meristematic tissue — composed of
small, actively dividing cells. During
the growing season the cells in meri¬
stematic tissues divide often and give
rise to cells that mature into other
plant tissues. Meristematic tissue at the
tips of roots and in the buds of stems
forms the tissues that result in growth
in length. The cambium layer between
the bark and the wood causes growth in
diameter.

Herbaceous and woody plants. Any
plant with a stem that is not woody and
that dies to the ground at the end of the

CHAPTER 22 THE SEED PLANTS 309

SOME FAMILIES OF DICOTS

Family

Familiar Members

Willow

(Salicaceae)

Walnut

(Juglandaceae)

Birch

(Betulaceae)

Beech

(Fagaceae)

Pink

(Caryophyllaceae)

Water lily

(Nymphaeaceae)

Crowfoot

(Ranunculaceae)

Poppy

(Papaveraceae)

Mustard

(Cruciferae)

Rose

(Rosaceae)

Pulse (legume)
(Leguminosae)

Flax

(Linaceae)

Maple

(Aceraceae)

Mallow

(Malvaceae)

Parsley

(Umbelliferae)

Heath

(Ericaceae)

Mint

(Labiatae)

Nightshade

(Solanaceae)

Figwort

(Scrophulariaceae)

Composite

(Compositae)

Willow, poplar (cottonwood), aspen

Walnut, hickory

Birch, alder, hazel

Beech, chestnut, oak

Pink, carnation, chickweed

Water lily, pond lily

Buttercup, hepatica, columbine, delphinium, larkspur

Poppy, bloodroot

Mustard, radish, turnip, cress

Rose, apple, hawthorn, strawberry, pear, peach, plum, cherry

Bean, pea, clover, alfalfa, locust, redbud

Flax

Maple

Marshmallow, hollyhock, hibiscus

Parsley, parsnip, carrot, sweet cicely

Laurel, rhododendron, azalea, heather, blueberry, cranberry,
huckleberry

Catnip, spearmint, peppermint, sage

Tomato, potato, tobacco

Mullein, snapdragon, digitalis (foxglove)

Dandelion, daisy, sunflower, zinnia, aster, marigold, thistle,
dahlia

310 UNIT 4 MULTICELLULAR PLANTS

22-6 This is a highly magni¬
fied photograph of xylem tis¬
sue found in the ash tree.
The large openings are the
vessels while the smaller
ones are tracheids. (Interna¬
tional Paper Co.)

growing season is called herbaceous
(her-bczy-shus ) by the biologist. Garden
vegetables and flowers, cereal grains, and
many of our common weeds are her¬
baceous.

Woody plants include trees, shrubs,
and certain vines such as the wild grape,
Virginia creeper, and poison ivy. In a
woody stem growth in diameter is as¬

sured by the cambium tissue that pro¬
duces new cells year after year. Growth
in length of the stem continues each
year from meristematic tissues in vari¬
ous growing points.

Various life spans of the seed plants.

Plants that live for only one season are
called annuals. These plants, such as
the zinnia, marigold, bean, and pea.

22-7 The rosette of basal leaves on the left is the first year’s growth of foxglove,
which is a biennial. The second year’s growth of this plant produces an attrac¬
tive spike of flowers which mature into fruit containing the seeds. Then the
plant dies. (Left: Albert Towle; right: Walter Dawn)

CHAPTER 22 THE SEED PLANTS 311

grow from a seed, mature, reproduce,
and die in a single growing season.

Biennials live two seasons. The
first year they produce roots, stems, and
leaves only. The plant then grows dur¬
ing its second season and bears the
flowers; when seeds are produced, the
plant dies. Beets, carrots, and parsnips
are biennial vegetables. Among the
biennial garden flowers are the sweet
William, digitalis (foxglove), and Can¬
terbury bells.

Perennials live more than two sea¬
sons. Most perennials form roots,

stems, and leaves the first year, but do
not produce flowers until a season or
more later. Herbaceous perennials die
to the ground each year. The roots and
any underground stems remain alive
and give rise to new aerial stems
and leaves each season. Delphiniums,
lilies, columbines, and irises are herba¬
ceous perennials. Woody perennials in¬
clude the trees, shrubs, and many vines.
Once perennials have started to flower,
they usually continue season after sea¬
son, if environmental conditions remain
favorable.

IN CONCLUSION

The seed plants are the dominant form of plant life in the world today, having
replaced the mosses and ferns of an earlier era. The most familiar members
of the class Gymnospermae are the conifers. The class Angiospermae includes
all the true flowering plants. The vegetative organs of a flowering plant in¬
clude the root, the stem, and the leaf. The flower is the reproductive organ
from which fruits and seeds develop. In addition to these organs, flowering
plants have several highly specialized tissues and definite growing seasons.

What if the seed plants had not replaced most of the more primitive ferns
and mosses during the past ages? There would have been no cereal grains for
flour, no potatoes, tomatoes, lettuce, onions, radishes, and other vegetables in
our stores. Could our civilization ever have developed in such plant surround¬
ings? You might debate this point, but one thing is certain. Our plant and
animal industries, in fact, our very survival, is geared to the seed plants.

In the next chapter we shall start with the first root that grows from a
seed. In other chapters we shall study the vegetative organs and then the re¬
productive organs of flowering plants. We shall return to the seed again as
the climax of reproduction.

BIOLOGICALLY SPEAKING

Angiospermae

annual

biennial

cones

conifer

cork

cotyledon

Dicotyledonae

epidermis

flower

fruit

Gymnospermae

herbaceous

leaf

meristematic tissue

Monocotyledonae

parenchyma

perennial

phloem

root

seed

stem

vegetative organs

woody

xylem

312 UNIT 4 MULTICELLULAR PLANTS

QUESTIONS FOR REVIEW

1. On what basis are the seed plants divided into two great classes — gymno-
sperms and angiosperms?

2. Explain why we speak of the ginkgo tree as a “living fossil.”

3. Describe several uses made of conifers in yard plantings.

4. Name five well known families of monocots and five well known dicot fam¬
ilies.

5. Name the three vegetative organs of a seed plant, and describe briefly the
functions of each organ.

6. List five specialized tissues of a seed plant.

7. Distinguish between herbaceous and woody plants. Give several examples
of each type.

8. Discuss the life cycle of annual, biennial, and perennial seed plants.

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss possible reasons for the disappearance of most gymnosperms and
the rise of angiosperms through past ages.

2. Discuss various ways in which high development of the organs of angio-
sperm plants has given them an advantage over other kinds of plants in
the struggle for existence.

3. Many of our most beautiful garden flowers are annuals. Why are they
ideally suited to garden needs?

CHAPTER 23

ROOT STRUCTURE
AND FUNCTION

The origin of root systems. When a
seed first begins to grow, the primary
root pushes out through the seed coat,
lengthens rapidly, and quickly pushes its
way down into the soil. After a short
period of growth secondary roots begin
to appear. These tiny branches come
out first near the tip of the primary root
and farther down as the root grows.
From them arise still more secondary
roots, so that branching and rebranch¬
ing may develop a complicated root
system.

You have probably wondered how
a root system compares in size with the
stem and branches. There is no definite
rule, as the roots of different plants vary
widely, and the size of a root system
differs in various soil conditions. But
you can estimate that the average land
plant has about as much or more below
ground as above ground.

Types of root systems. If a primary
root continues to grow, it will always be

the largest member of the root system.
As such it is known as a taproot. Tap¬
roots may be round as in the beet, tur¬
nip, and radish, or long and slender as
in the carrot and parsnip. The taproot
of alfalfa grows to 15 feet or more.

Taproots are an ideal anchorage of
the plant. Furthermore, they can often
withstand prolonged drought because
they can get at the deep water supply.
This accounts for the fact that alfalfa
absorbs water after the grasses growing
with it may have turned yellow and
brown. It also explains why some
plants thrive on dry hillsides and why
others survive in arid regions. Tap¬
roots also serve as underground store¬
houses for the food supply of the plant.
For this reason many of these roots are
used as food by man.

In many plants the primary root
lives only a short time and is quickly re¬
placed by a whole mass of slender sec¬
ondary roots that branch and rebranch
in all directions. These are called
fibrous roots. Grasses, corn, wheat, and
many trees and shrubs have fibrous root
systems. Such systems are of great ad¬
vantage in absorbing water and minerals

Young shoot

Secondary root

— Root hairs

Primary root

23-1 Secondary roots branch out from the
primary root and grow until the complete
root system has formed.

313

314 UNIT 4 MULTICELLULAR PLANTS

23-2 Compare the fibrous root system of the
plant on the left with the taproot system of
the plant on the right. Which acts as the
better soil binder?

after a rain. During dry periods fibrous
roots cling to soil particles, preventing
the soil from being carried away by the
wind. For this reason, man uses plants
with fibrous roots to hold the soil in
sandy areas and on steep slopes.

The regions of a root tip. If you cut
off a young root half an inch or so back
of the tip, slice it thinly lengthwise, and
examine it with a microscope, you will
see the cells composing it and also
several important regions. The region
at the tip, the root cap , protects the
delicate end. As the cap is pushed
through the soil by the growing root
behind it, its outer surface is torn away.
The addition of new cells to the inner
surface, however, keeps it in constant
repair.

You mav wonder at the fact that

the delicate root tip can force its way
through soil without damage. The root

tip partly pushes its way through soil.
Root caps give off carbon dioxide, which
combines with soil water to form a weak
acid called carbonic acid. This car¬
bonic acid aids the progress of the young
root by dissolving certain minerals in
its path. When roots grow over lime¬
stone rocks, their pattern is often etched
into the rock surface by the carbonic
acid they form.

Immediately behind the cap, at the
tip of the root proper, is the meriste-
matic region , or growing point of the
root. Cells of this region are small and
are constantly dividing by mitosis, thus
giving rise to new root cells.

23-3 This drawing represents a root tip as
you see it under the microscope. Notice the
blunt, thimble-shaped root cap which pro¬
tects the delicate meristematic region from
injury by soil particles.

CHAPTER 23 ROOT STRUCTURE AND FUNCTION 315

23-4 This photograph of a radish seedling
gives you an idea of the number of root
hairs a very young root has. (Hugh Spencer)

Back of the meristematic region
cells gradually lengthen until they reach
full length a considerable distance from
the tip of the root. This lengthening
of cells marks the elongation region ,
which causes the forward movement in
the growth of the root tip.

After the cells have grown to full
length, they change further. Cells on
the surface give rise to tiny projections
called root hairs. Root hairs may be
seen as a white, fuzzy growth a short
distance back of the tip of a young root.
They should not be confused with sec¬
ondary roots, which, like primary roots,
are composed of many cells. Root hairs
are produced in a zone one to two
inches long. As the tip of the root
moves downward, new root hairs form
close to the tip and older ones wither
away. Thus they are constantly extend¬
ing into new areas of soil as the young
root pushes on. Root hairs are an ideal

adaptation for absorption. Because of
their small size, they come into intimate
contact with soil particles. Since they
are extensions of single cells and are
covered only with a plasma membrane
and cell wall, water passes into them
freely. Most importantly, they increase
the absorptive surface of the root.

While cells on the surface of the
young root give rise to root hairs, those
inside the root change somewhat as they
become special tissues of the mature
root. Modification of cells to form these
special tissues marks the maturation
region of the root.

Regions and tissues of a mature root.

Microscopic examination of a prepared
slide of a mature root shows its distinct
tissues. The epidermis appears as a
single layer of cells. Under the epider¬
mis are layers of rounded, loosely packed
parenchyma cells forming the cortex.
This is the principal food storage area
of the root, and is several cells thick.
The endodermis is a single layer of cells,
often with thick walls, located at the in¬
side edge of the cortex.

Vacuole

Nucleus

Nucleus
Cell wall
Nucleus
Vacuole

Cytoplasm

23-5 Note that each root hair is an exten¬
sion of one epidermal cell near the tip of a
root.

316 UNIT 4 MULTICELLULAR PLANTS

The central cylinder is the principal
conducting and strengthening region of
the root. It is composed of xylem and
phloem tissues, and may contain vas¬
cular cambium and pith tissues.

The pericycle is a layer of very thin-
walled cells lying at the outer edge of
the central cylinder and just inside the
endodermis.

The xylem is the water-conducting
tissue. Through the xylem water and
soil minerals travel upward to the stem
and leaves. The conducting portion of
the xylem is composed of various types
of large, thick-walled cells. They are
long, empty cells that resemble pipes.
These cells live only a short time but

continue to serve as channels of con¬
duction long after death. In addition,
the xylem contains numerous smaller
cells that give great strength to the
root. We commonly speak of the xylem
tissues of a mature plant as wood.

The phloem , or food-conducting
tissue, lies outside the xylem. If the
xylem is arranged in the form of a cross
as in some roots, like the buttercup
shown in Fig. 23-6, you can find the
phloem between the arms of the cross
in rounded groups. Foods, produced
in the stem and leaves, travel down¬
ward through the phloem cells.
Secondary thickening in the root. The
primary tissues we have described do not

Epidermis

Cortex

Intercellular space

Endodermis

Pericycle— i

Phloem

Vascular

cambium

Xylem

Central

cylinder

Secondary root

Secondary root cap

Cortex

23-6 This drawing of a young buttercup root shows the various tissues and also
the origin of a secondary root.

CHAPTER 23 ROOT STRUCTURE AND FUNCTION 317

Stem

Periderm

Vascular cambium

Secondary xylem

Secondary phloem

Secondary root

Periderm

Secondary phloem
Vascular cambium
Secondary xylem
Secondary root

23-7 As you study the tissues in the longi¬
tudinal section through the carrot root, com¬
pare the location of the same tissues in the
cross section.

increase as the root continues its growth
in diameter. However, the vascular
cambium adds secondary xylem on its
inner side and secondary phloem on its
outer side by continuous cell division
during the growing season. This pro¬
duces a root of which the carrot is an
example.

The outer edge is a tough layer, the
periderm, which develops from the peri-
cycle. Inside this layer is a thick one of
phloem, most of which has been formed
by the vascular cambium. The vascu¬
lar cambium appears as a layer of cells
at the inner edge of the phloem. The
center of the carrot is xylem, much of
which was formed by the vascular cam¬
bium during secondary thickening.
Notice that the secondary roots extend
from the xylem, thus joining the con¬
ducting vessels of the secondary roots
with those of the primary root. The
root tissues and their functions are sum¬
marized in the table on page 318.

An old root, such as the root of a
tree, contains many layers of xylem and
phloem, formed by the vascular cam¬
bium each season. The outside of the
root is covered with a thick layer of
bark. Such a root is an efficient organ
of conduction and anchorage, but the
thickened portion no longer absorbs
water.

Various root adaptations. Although all
roots have, in general, the tissues we
have described above, in different species
the root structure and behavior differs
in ways that adapt different species to
different habitats. Such plants as the
duckweed and the water hyacinth have
aquatic roots which usually lack root
hairs. They do not need extra surface
on the epidermis for absorption because
they live in a water environment.

The bald cypresses of the southern
United States grow in swamps and are

318 UNIT 4 MULTICELLULAR PLANTS

SUMMARY OF ROOT TISSUES AND THEIR SPECIAL FUNCTIONS

T issue

Function

Epidermis

Root hairs

Absorption and protection

Increase of absorption area

Cortex

Storage of food and water

Endodermis (boundary
layer)

Separation of cortex from central cylinder, and conduction

Central cylinder

Pericycle

Origin of secondary roots and formation of periderm

Xylem

Phloem

Vascular cambium

Conduction of water and dissolved minerals upward to the
stem and leaves

Conduction of manufactured food downward from the stem
and leaves

Production of secondary xylem and phloem

rooted in water-covered soil. To supply
the submerged roots with air, the roots
send structures called knees above the
water. Cypress knees contain air-con¬
ducting tissues that are connected with
the roots (Fig. 23-8).

Many plants in the tropics produce
aerial roots. Tropical orchids live on

23-8 What is the function of the bald cy¬
press knees shown? (Walter Dawn)

trees and absorb water from the humid
atmosphere. The various debris that
collects around the roots is enough to
supply the mineral needs. Aerial roots
have a thick, spongy cortex, making pos¬
sible rapid absorption of the rainwater
and dew that falls on them.

The roots we have discussed thus
far have developed from the primary
root or from one of its branches. How¬
ever, in some plants adventitious (ad-
ven-tish-us ) roots develop from the stem
or even from the leaves.

You have probably noticed the cir¬
cle of roots that grows from the joint of
the corn plant just above the ground
(Fig. 23-9). These are a kind of ad¬
ventitious root called prop roots , or
brace roots. They grow into the ground
and help the underground roots support
the stem. If soil is piled around the
stem, additional brace roots develop
from the next joint above the soil line.
In fact, all the roots of the corn plant
are really adventitious, since they come
from the stem rather than from the
short-lived primary root.

CHAPTER 23 ROOT STRUCTURE AND FUNCTION 319

23-9 The various types of roots shown here are adventitious; that is, they de¬
velop from stems.

Poison ivy, English ivy, and other
vines produce clusters of roots along
the stem. These roots cling to a wall
or to some other support and hold the
stem securely. Such plants also have
ordinary soil roots that absorb water
and dissolved minerals.

Vegetative reproduction by roots. We
do not ordinarily think of roots as be¬
ing organs of reproduction, but they
may act as such in certain plants. Some
perennial garden plants produce large
numbers of secondary roots each year.
When these are removed from the par¬

ent plant, they produce stems, leaves,
flowers, and fruit. Many garden peren¬
nials such as peonies, phlox, and shasta
daisies are produced commercially in
this way.

In certain plants, if the stem lies
in contact with the soil long enough,
roots will grow from the joints and pro¬
duce new plants. Climbing roses can
be started by burying a portion of a
stem until it has taken root. Raspberry
bushes root from the stem in a similar
way, except that the roots usually form
at the tip of the stem (Fig. 23-9).

320 UNIT 4 MULTICELLULAR PLANTS

Other examples of adventitious roots
for propagation include those that form
on pussy willow stems when cut and
put in a container of water, and the
roots that form on strawberry runners
as they grow into new areas.

The root as an organ of absorption. Al¬
though roots may anchor the plant and
even reproduce it, the principal function
of the root is absorption of water. To
understand how the root accomplishes
absorption, first review the osmosis ex¬
periment with the thistle tube that was
described in Chapter 5. Now substitute
a root hair for the thistle tube; the cell
content for the molasses; a living plasma
membrane for the membrane fastened
over the mouth of the thistle tube; and
the water in the soil for the jar of water
(Fig. 5-5). Inside the cell there are
solutions of various substances dis¬
solved in water. Cell vacuoles contain
solutions of minerals, food materials,

and other dissolved substances. The
soil water also contains dissolved min¬
erals, but normally not as much as the
cell content. In other words, the con¬
centration of water molecules outside
the cell is greater than inside. The
soil water is separated from the cell
content only by a thin, porous cell
wall and the differentially permeable
plasma membrane. Can you predict
what will happen? Water will move
from the soil into the root hair.
Successive osmosis. Now, think of the
whole root. The root hair is an out¬
growth of an epidermal cell. Inside are
many layers of cortex cells. When the
epidermal cell takes in water, the con¬
centration of water molecules becomes
greater than that of the cortex cell lying
against it, so that water passes from the
epidermal cell to the cortex cell. As
the water content in this cell increases,
the next cortex cell receives water from

Soil particle

Cortex cell

Epidermal

cell

Root hair

Film of
water

23-10 Water enters root hairs from the soil by osmosis. It moves through the
root by successive osmosis, as osmotic pressure is built up in each succeeding
cell.

CHAPTER 23 ROOT STRUCTURE AND FUNCTION 321

the first, the first one receives water
from the epidermal cell. The epidermal
cell in turn takes in more water from
the soil through its root hair.

This diffusion of water from cell to
cell continues to the xylem vessels in the
central cylinder of the root. Here water
and dissolved minerals move upward to
the stem. We may call this cell-to-cell
diffusion of water by the name of suc¬
cessive osmosis.

The above discussion is a very
much simplified one because the mech¬
anism of water transport is largely a
biophysical problem which becomes ex¬
tremely complicated. To understand
it adequately one needs a rather de¬
tailed knowledge of both chemistry and
physics.

Turgidity in plant cells. In Chapter 5
you learned that as water enters a cell by
osmosis, internal water pressure forces
the plasma membrane firmly against the
wall, causing stiffness known as turgor
(ter- ger). When turgor pressure, which
would tend to force water molecules out
of the cell, becomes as great as the
osmotic pressure causing them to move
into the cell, osmosis stops. Turgidity
(ter-/zd-i-tee) makes the cell firm, just
as you can make a plastic bag rigid by
filling it with water. Turgidity in the
cells in turn makes the whole plant stiff
and firm. It is very important in sup¬
porting tender plants whose stems are
not stiffened by woody fibers. When
turgidity is lost from lack of water, the
plant wilts.

When a plant is fully turgid, the
pressure in its cells may be as great as
60 to 150 pounds per square inch. Tur¬
gor permits the mushroom and the
delicate seedling to push through the
hard ground. Even concrete has been
known to crack from the push of a
growing plant, as shown in Fig. 23-11.

23-11 Turgor helps enable a plant to push
through surfaces as resistant as cement
sidewalks.

Absorption of minerals. Absorption by
the root concerns not only the intake
of water but the entry of dissolved min¬
eral substances as well. They pass
through the membranes of root hairs in
solution with water. Together with wa¬
ter they move through the cortex to the
inner tissues of the central cylinder
where they are conducted to other parts
of the plant.

The relation of dissolved minerals
to water in the soil solution is very
close. Biologists have found evidence
that mineral absorption may be inde¬
pendent of water intake. Remember
that the concentration of mineral ions
must be greater outside the cell than in¬
side if the ions are to enter by diffusion,
or passive transport. Root cells, how¬
ever, absorb mineral ions from soil solu¬
tions that contain fewer ions than the
cells already contain. Thus, active
transport must be operating. In other
words, the cells must be actively ex-

322 UNIT 4 MULTICELLULAR PLANTS

pending energy in absorbing minerals.

How rapidly do plants absorb soil
minerals? A very graphic answer was
given to this question by a representa¬
tive of the Oak Ridge Operations,
United States Atomic Energy Commis¬
sion. During an address on the use of
radioisotopes in biology, the speaker
produced two potted tomato plants.
One was watered with distilled water,
the other with water containing a radio¬
active phosphate (a soil mineral). In
less than 45 minutes, the water contain¬
ing the radioactive phosphate was ab¬
sorbed through the root hairs of the to¬
mato plant and moved up into the
leaves. This was proved by removing
a leaf and testing it for the presence of
radioactivity with a Geiger counter.
The plant had shown no radioactivity at
the beginning of the experiment. The
control plant showed no radioactivity
either before or after the experiment.
Responses of roots to their surround¬
ings. If you happen to plant a seed up¬
side down, will the roots grow out of
the soil? Is it mere chance that willow
or poplar roots enter cracks in a sewer
and clog it with a ball of roots? If a
root growing downward strikes a rock,
will the rock stop its progress in¬
definitely?

The various parts of a plant re¬
spond, rather rapidly, to certain stimuli
in their environment. These responses
are growth responses, referred to as
tropisms. External stimuli influence the
chemical activity of cells, especially in
the regions of roots and stems. As ex¬
ternal factors stimulate or reduce the
secretion of growth hormones, especially
auxins , the rate as well as the direction
of growth of the plant organ is in¬
fluenced.

If the growth response is toward
the stimulus, as when a root grows to-

23-12 Geotropism is shown in these draw¬
ings. A: the test tube is inverted at the be¬
ginning of the experiment. B: after several
days.

ward the earth, we call the response
positive. On the other hand, a stem
shows a negative response to gravity
when it grows away from the earth.

If a stem is lying horizontally,
auxins accumulate on the lower side
possibly due to the pull of gravity.
Growth stimulation on the lower side
bends the stem upward. On the other
hand, a root placed in the same posi¬
tion bends downward. It would seem,
then, that auxins accumulating on the
lower side inhibit the growth of root
cells. Thus different plant tissues vary
in growth response to auxins.

We shall discuss below responses
of the root which you may observe in
nature or demonstrate in the laboratory.

CHAPTER 23 ROOT STRUCTURE AND FUNCTION 323

In discussing tropisms, we must keep in
mind the fact that they are automatic
growth responses related to hormone
secretion. A root does not enter the
soil “in order to find water and min¬
erals” or “to get away from the light.”
Geotropism. The gravity of the earth
is a strong stimulus influencing root
growth. It is more than sheer weight
that causes the root to grow toward the
stimulus of gravity. The stem is
equally heavy, and yet it grows away
from the earth. Geotropism directs
the growth of the root toward the earth
and its supplies of soil water and dis¬
solved minerals. Without this impor¬
tant response, roots would grow in all
directions and only by chance reach
these necessary substances. Geotropism
is illustrated by the experiment shown
in Fig. 23-12.

Hydrotropism. Water is a very strong
stimulus and may cause a root to grow
toward it from a considerable distance.
This response is hydrotropism (Fig.

IN CONCLUSION

23-13 This experiment shows that water is
even a stronger stimulus to plants than
gravity.

23-13). It is shown in the vast number
of fine roots that force their way into
pipes or spread through the moist top¬
soil. They grow in these directions in
spite of the force of gravity, which at¬
tracts them downward. The response of
the roots of willows and elm trees makes
them undesirable for planting near
drains or sewers.

The entire contact of a plant with its soil environment depends on a healthy,
functioning root system. If this is upset drastically, the plant cannot survive,
for the root absorbs water and minerals, stores food, anchors the plant, and
sometimes acts as an organ of reproduction.

The root, important though it is, is only one of several different plant or¬
gans. True, the average seed plant could not exist without its root system.
But it would have just as much trouble living without its stem, as we shall see.

BIOLOGICALLY SPEAKING

adventitious root
aerial root
aquatic root
auxin

central cylinder
cortex

elongation region

endodermis

epidermis

fibrous root
geotropism
hydrotropism
maturation region
meristematic region
pericycle
phloem
plasmolysis
primary root

root cap

root hair

secondary root

successive osmosis

taproot

tropism

turgor

vascular cambium
xylem

324 UNIT 4 MULTICELLULAR PLANTS

QUESTIONS FOR REVIEW

1. From their origin, distinguish a primary root from a secondary root.

2. Describe the general form of taproot and fibrous root systems.

3. Why is carbonic acid important to the growth of a root?

4. Name four regions of the root tip and discuss the activity of each.

5. Root hairs are sometimes incorrectly called branch roots. Why is it in¬
correct to speak of a root hair as a branch or secondary root?

6. Name seven tissues of a mature root and state the function of each.

7. Distinguish between adventitious roots and normal roots.

8. Describe several special functions that are performed by adventitious roots.

9. Explain how successive osmosis takes place in the cells of a root.

10. What are some of the responses of roots to their surroundings?

APPLYING PRINCIPLES AND CONCEPTS

1. Hickory trees seem to thrive especially well on dry ridges and hillsides.
What kind of root system would you expect these trees to have?

2. A plant dug up with a large ball of dirt has a far better chance of living
than one taken out with the roots exposed, even though these roots are
not torn off. Why is this true?

3. Roots thicken by adding secondary tissues. How are these tissues formed?

4. Branch roots grow from the pericycle at the outer edge of the central
cylinder. Why is the attachment of the branch root to the central cylinder
extremely important?

5. Explain the importance of hydrotropism to the life of a plant.

6. Discuss the use of radioactive substances in mineral absorption by roots.

CHAPTER 24

STRUCTURE
AND FUNCTION

Herbaceous and woody stems. Those
stems that are usually soft and green are
classified as herbaceous stems. Some
examples of them are the stems of to¬
matoes, beans, peas, corn, grasses, and
lilies. Herbaceous stems lack most
woody tissues that give strength to trees
and shrubs. Most herbaceous stems
grow relatively little in diameter and
last only one season. In plants like the
hollyhock, columbine, delphinium, and
shasta daisy, the stem is herbaceous and
lives for only one season, while the root
is woody and perennial. Both the root
and the stem are annual in the mari¬
gold, zinnia, nasturtium, morning glory,
tomato, bean, and pea.

Woody stems , on the other hand,
are perennial. They grow in length, in¬
crease in diameter, and form new

branches season after season. The
woody tissues of these stems give them
great strength and allow them to reach
much greater size than a herbaceous
stem.

The external structure of a woody stem.
The twig of a tree is an ideal subject for
study of the external structure of a
woody stem (Fig. 24-1). In regions
where trees shed their leaves during
autumn, a dormant winter twig is es¬
pecially suitable.

Buds are perhaps the most notice¬
able structures on the dormant stem.
Each bud contains a growing point — a
place from which a new stem, leaves,
and flowers may develop. In cold cli¬
mates, winter buds are protected by
overlapping bud scales , which com¬
pletely enclose the tender growing point.
These bud scales serve to protect the
delicate tissues inside from drying out
and from mechanical damage.

We further classifv buds according
to their position on the twig. The ter¬
minal bud , not present on all twigs, is
located at the tip and contains the ter¬
minal growing point of the stem. Along
the sides are lateral buds , from which
branches may develop. They are usually
smaller than terminal buds and usually
different in shape.

At intervals along the twig, circular,
oval, or shield-shaped leaf scars mark
the point of attachment of leaf stalks
from previous seasons. On the leaf
scars minute dots called bundle scars
show the location of the conducting
bundles that carried water and dissolved
minerals into the leaf from the stem.
These bundle scars are of a definite num¬
ber and arrangement, depending on the
species.

You will note in examining the
lateral buds that they are usually just
above a leaf scar. When the leaf was

325

326 UNIT 4 MULTICELLULAR PLANTS

24-1 The external structure of a woody stem.

attached to the twig, these buds were in
an angle between the leaf stalk and the
twig. We call this angle the axil.
Lateral buds produced in the leaf axils
are given the special name of axillary
buds.

A node is the point at which leaves
or branches are produced from a stem.
The space between two nodes is called
an intemode. In examining several
twigs, note that a single leaf, two leaves,
or three or more leaves may develop
from a node. If a winter twig has one

leaf scar at each node, the leaves are
alternate. If there are two leaf scars,
the leaves are opposite. If three or more
leaf scars are present at each node, the
leaves are whorled.

Along the intemodes, especially on
young twigs, you can see tiny pores
called lenticels opening through the
bark. These let air enter and water
escape from the twig, especially while it
is young and active.

You can find other interesting struc¬
tures on certain twigs. When terminal
buds swell and drop their scales at the
beginning of the growing season, a series
of rings encircling the twig marks the
place where the bud scales were fastened.
These bud-scale scars , at intervals along
a twig, show the exact location of the
terminal bud during previous seasons.
Thus, by starting at the present terminal
bud and counting the sets of bud-scale
scars along the twig, you can find the
exact age of a twig.

Some twigs bear characteristic
thorns that make them very easy to
identify. These thorns may be either
short and broad, long and pointed, or
branching. In some species thorns are
outgrowths of the epidermis. They may
also be modified branches. The thorns
of hawthorn trees and the branching
thorns of the honey locust are examples
of stems modified into protective thorns.
The thorns of a rose, however, are out¬
growths of the epidermis.

How do stems grow? To show the way
in which stems grow, let us assume that
a tree is 30 feet high, one foot in diam¬
eter, and that the first branch is exactly
six feet above the ground. After ten
years we return to the tree and find
that the trunk is now 16 inches in diam¬
eter and 40 feet high. After this period
of growth, how far is the first limb from
the ground? The answer is still six feet.

CHAPTER 24 STEM STRUCTURE AND FUNCTION 327

24-2 The type of branching in a tree depends on the arrangement of the buds.
Left: opposite buds produce opposite branches. Right: alternate buds produce
alternate branches. (Left: Hugh Spencer; right: Albert Towle)

Growth in length has occurred at the
tips of all the branches. The stem re¬
gions below the tips have not length¬
ened at all.

In plants meristematic regions are
the places where growth begins. Only
in these special areas can new cells
form as a result of cell division. Fur¬
thermore, the growing regions of the
stem are of two distinct types: those
causing increase in length, and those
causing increase in diameter.

Stems grow in length by forming
new tissues at their growing points, lo¬
cated in the terminal bud. Branches
grow in length as meristematic cells di¬
vide and enlarge in the lateral buds.
You will remember that such growing
points are also found at the tips of
roots. The apical growing zone of the
stem is somewhat like that of the root,

except that it is longer and not pro¬
tected by a cap. The meristematic re¬
gion of a stem is microscopic but
adjacent to it is a region of cell enlarge¬
ment which is often several inches long.
The corresponding region of the root is
only a small fraction of an inch long.
As new tissues are produced at the stem
tip, they continue to grow until they
reach maximum size. Once they have
matured, they usually cannot lengthen
again. Growth in length is limited to
the actively-dividing cells of the grow¬
ing point.

Growth in diameter results from
the activity of the vascular cambium
located deep in the tissues of the stem.
We will study this important tissue
very soon.

Like growth responses in roots, ac¬
tivity of the growing points in stems is

328 UNIT 4 MULTICELLULAR PLANTS

controlled by auxins. Auxin production
by the meristematic cells of the terminal
bud inhibits the growth of lateral buds,
so that the plant does not become too
bushy. This effect of the terminal bud
on others is called apical dominance.
It means, literally, that the bud at the
apex (terminal end) dominates the
others. A gardener or florist takes ad¬
vantage of this when he snips off the
terminal buds of his plants. This allows
the lateral buds to develop, making the
plants bushy and producing more
flowers.

The growth response to light, called
phototropism , is another example of the
effect of auxins on stem growth. You
are familiar with the fact that the stems
of a plant kept on a windowsill will bend
toward the light. This bending is
caused by the fact that light reduces the
production of auxin on the bright side,

reducing the growth on this side of the
stem. The greater growth on the shaded
side causes stems to bend to the light.
Branching patterns. If a young tree has
a strong apical dominance and terminal
buds of other years escape injury, the
main stem will continue upward, form¬
ing a central shaft. Branches grow from
this as a result of the development of
lateral buds. We class such trees, char¬
acterized by branches which grow from
a central shaft, as excurrent (Fig. 24-4) .

Excurrent branching is shown in
the pine, fir, spruce, hemlock, redwood,
and cypress. These trees, unless in¬
jured or diseased, have a perfect cone-
shaped outline. A strong central shaft
rises to a point at the tip of the tree.
Branches grow horizontally at regular
intervals along the stem, decreasing in
length from bottom to top. This differ¬
ence in length is due to a difference in

24-3 Stems grow in
length by forming new
tissues at their growing
points, as this drawing
of the longitudinal sec¬
tion of a bud indicates.

CHAPTER 24 STEM STRUCTURE AND FUNCTION 329

24-4 Excurrent branching,
where lateral branches arise
from the main stem, or cen¬
tral shaft, is characteristic of
evergreens such as spruce,
fir, pine, and hemlock. They
can withstand large amounts
of snow without great injury.
(U.S. Forest Service)

age, the oldest branches being located
at the base, while the youngest are at
the top. Bv counting the number of
circles, or whorls, of branches along the
trunk, you can tell the age of the tree
with reasonable accuracy.

The willow, cottonwood, and elm
have quite a different form of branch¬
ing. These trees produce a single trunk,
which divides, usually rather low, to
form several large branches. The effect
is a spreading pattern of growth classed
as deliquescent (del-i-kwes-nt) [Fig.
24-5]. This growth results when twigs
lack strong apical dominance, so that
lateral buds form branches that equal
or exceed growth from the terminal bud.
In trees like the buckeve, horse chestnut,
and magnolia, terminal buds develop
flower clusters. Lateral buds form
branches, resulting in a spreading, deli¬
quescent pattern.

Branching form often determines
the timber value of trees. Quite obvi¬
ously, trees having excurrent branching
far exceed in timber value those having
deliquescent branching. The character¬
istic growth of pine makes it ideal for
lumber, while spruce and fir are suitable

for telephone poles, furniture, shingles,
and many other lumber products.

In the oak, walnut, hickory, maple,
and other forest trees, long trunks de¬
velop under forest conditions, although
the same trees branch more freely in
open places. Foresters have found that
the best timber trees are the types found
growing in dense stands.

The internal structure of a woody stem.
If you cut a young branch of a tree, you
can see three distinct regions, as shown
in Fig. 24-6. The outer region is the
bark, which is quite distinct from the
area of wood occupying the middle part
of the stem. The center of the stem is
called the pith, although it is hard to
find the pith of a large tree because it
is so very small in comparison with the
large amount of wood. A fourth re¬
gion, the vascular cambium, lies be¬
tween the bark and the wood and con¬
sists of a verv thin laver of delicate tis-

j J

sue. This is almost impossible to see
without a magnifying glass.

The wood in an old stem often ap¬
pears to be of two types. The outer
area is usually light in color and consists
of active, functioning wood, or sapwood.

330 UNIT 4 MULTICELLULAR PLANTS

24-5 An elm tree illustrates deliquescent branching in which the older branches
arise from the central shaft and many new branches arise from other branches.
(Saunders from Monkmeyer)

Sapwood is absolutely necessary for the
tree to live. Next to the sapwood is a
cylinder of darker wood called heart-
wood. This occupies the center of the
stem and surrounds the cavity where
the pith originally was. The tissues
composing the heartwood are dead and
often filled with gums, tannins, or resins
which give it a characteristic dark color.
Heartwood is not active in water trans¬
port but it does contribute to support for
a tree. It is the portion of the tree that
is used in making the finest quality of
furniture.

Annual rings form circles through
the wood, one outside the other, and
mark each season’s growth of wood.
You can determine the approximate age

of a stem by counting them. The
pith rays appear as lines radiating to the
outside of the wood like the spokes of a
wheel. Some types of wood, such as
oak, show especially prominent rays.
Bark — its structure and activity. The
term bark includes much more than just
the outer covering of a tree. It is a
region of the stem composed of several
kinds of tissues.

A young twig is covered for a time
by a thin epidermis that protects the
young stem from injury and disease.
Soon, however, the epidermis is replaced
by a tissue called cork , which forms the
outer covering we see on a branch or
tree trunk. Cork is composed of dead
cells arranged in layers. It protects the

CHAPTER 24 STEM STRUCTURE AND FUNCTION 331

stem from mechanical injury, from dis¬
ease, and from loss of water.

As stems grow in diameter, the
outer corky layer splits, so that it is con¬
stantly renewed. New cork is produced
by a special layer of cells called the cork
cambium. The cells that compose the
cork cambium divide frequently and add
new cork on the inside as it is destroyed
on the outside. The structure of the
cork cells and the continual splitting of
the cork layer due to the growth of the
stem result in the characteristic appear¬
ance of tree trunks — scaly, peeling,
grooved, or fissured. The cork tissue of
a tree is often called the outer bark.

Inside the cork and cork cambium
lie two other important bark tissues com¬
posing the inner bark. The outer one of
these, the cortex , is composed of large

thin-walled cells arranged like stones in
a wall. In young stems the cortex cells
contain chloroplasts and carry on photo¬
synthesis. As the stem matures and cork
begins to form, this function ceases, and
the cortex disappears.

Inside the cortex lies the innermost
layer of bark, the phloem. Tins im¬
portant tissue conducts food from the
leaves to the various parts of the plant.
Phloem consists of several different
kinds of cells. Sieve tubes are the most
prominent, appearing as rows of rather
large, elongate living cells that have
thin walls and protoplasm but no nu¬
cleus. Their end walls, and sometimes
their side walls, have small openings
through which sap, containing dissolved
food substances, flows easily. Next to
and bordering on sieve tubes are long,

Vascular cambium

-Xylem (wood)

Pith

Pith ray

Cork cambium |
- Cork j

Cortex

Phloem

— First annual ring
Second annual ring

- Third annual ring

Outer bark

I Inner bark

24-6 Diagram of the internal tissues of a three-year-old woody dicot.

332 UNIT 4 MULTICELLULAR PLANTS

24-7 A cross-section of a three-year-old
linden stem. (Walter Dawn)

narrow companion cells , which are also
living and which have nuclei. Just how
these companion cells aid in food con¬
duction is not clear, but they are always
present in phloem tissue. Additional
cells found in phloem are phloem fibers ,
which are slender, long cells with thick
walls, and phloem parenchyma cells, in
which food is temporarily stored. Linen
is made from the fibers of flax plants,
rope from those of Indian hemp.
Activities of the vascular cambium. A
ring of embryonic cells between the bark
and the wood, known as the vascular
cambium, is responsible for all increase
in diameter of the stem. During the
spring and summer this cambium is
active in producing new cells by division.
It forms new phloem tissues on its out¬
side surface and new wood tissues on its
inside surface. During one season of
cambium activity, many more wood cells
than phloem cells are formed. That is
why the wood area of a tree is always
much greater than its bark thickness.
The tissues composing wood. Wood, or
xylem tissue, serves two main functions:
conduction of water and dissolved min¬
erals upward, and support, and they have

special adaptations for these duties.
The largest xylem elements are the
vessels, which are long tubes made up of
nonliving cells joined end to end, with
large openings in their end walls. Thus
liquids can move easily from cell to cell
in a continuous stream. Next come the
tracheids (tray- kee-idz), which are long
and narrow and which die at maturitv.
They have rather heavy cell walls which
contain lignin and other substances
which give extra strength. In the walls
of tracheids are small unthickened areas
called pits. Because of the pits tracheids
are able to participate in the conduction
of water from cell to cell. With their
very thick walls xylem fibers contribute
to the strength of wood. Xylem paren¬
chyma cells serve as storage cells. Pith
rays are different from the other stem
tissues in that they contain protoplasm
and a nucleus. They conduct liquids
laterally rather than up or down.

As woody stems increase in thick¬
ness year after year, the wood formed by
the cambium is arranged in layers. Fre¬
quently, the cambium produces two
kinds of wood during the season: spring
wood, containing many large vessels
mingled with tracheids and fibers; and
summer wood, containing few vessels
and large numbers of fibers (Fig. 24-8).
This difference in texture between spring
and summer wood results in layers that
appear as the annual rings.

The pith region of the stem. The cen¬
tral core of pith is scarcely noticeable in
an old woody stem. In the young stem,
however, the pith occupies a proportion¬
ally large area and serves as an important
place of storage. Since pith is not pro¬
duced by the cambium, it never increases
in size. Regardless of the size to which
a tree may grow, its pith never increases
beyond the amount present during the
first year of growth.

CHAPTER 24 STEM STRUCTURE AND FUNCTION 333

The table below summarizes the
various tissues of the woody stem and
their functions.

The structure of a herbaceous stem.

Herbaceous stems differ from woody
stems chiefly in having much less xylem
and phloem tissue. These two occur in
most herbaceous stems in the form of
long strands called vascular bundles
which run lengthwise through the stem.

In our introduction to the seed
plants, we noted that the flowering
plants are divided into two large sub¬
classes — the Monocotyledonae and the
Dicotyledonae, according to the number
of seed leaves. Examples of dicots are

24-8 The cells at the left of this photomi¬
crograph were produced in the spring, while
those at the right were formed during the
summer months. The so-called line be¬
tween them is an annual ring. (International
Paper Company)

SUMMARY OF STRUCTURE AND ACTIVITIES OF A WOODY STEM

Region

T issue or

Cell Type

Activity

Bark

Epidermis (only
on voung stems)

Protection, reduction of water loss

Cork

Protection, prevention of water loss

Cork cambium

Production of cork

Cortex (only in
young stems)

Storage and food manufacture

Sieve tubes

Conduction of food, usually downward

Companion cells

Uncertain

Phloem fibers

Support

Phloem parenchyma

Storage

Vascular

cambium

Meristematic cells only

Formation of phloem, xylem (wood), and rays

Wood

Xvlem vessels

Conduction of water and minerals upward

Tracheids

Conduction and strengthening of wood

Xylem fibers

Support

Xylem parenchyma

Storage

Pith rays

Conduction laterally

Pith

Parenchyma

Storage

334 UNIT 4 MULTICELLULAR PLANTS

24-9 The approximate age of a tree may be
determined by counting the annual rings in
the trunk. (U.S. Forest Service)

the tomato, buttercup, and bean. The
monocots include the iris, orchids, lilies,
grasses, sedges, and corn. The her¬
baceous stems of these two groups show
definite differences in structure, espe¬
cially in the arrangement of their fibro-
yascular bundles.

The herbaceous dicot stem. If you sec¬
tion the stem of a herbaceous dicot
plant (your teacher will tell you a good
one to use in your community), you
will see several distinct regions. The
outer layer of the stem is a thin epider¬
mis, which serves for protection. Inside
the epidermis is a layer of cortex, com¬
posed of loosely-packed cells, often con¬
taining chlorophyll. These manufac¬
ture food as well as store it. Within
the cortex, the vascular bundles occupy
a ring-shaped zone. These bundles con¬
tain xylem and phloem tissues. In some
species the bundles are more or less
fused together into a continuous ring.
In others the bundles are separated
from one another bv broad pith rays.

Many herbaceous dicot stems de¬
velop a vascular cambium that consists

of a ring of cells extending through the
bundles. The vascular cambium sep¬
arates the phloem in the outer portion
of the bundle from the xylem tissues
within. The activity of this cambium
in producing new phloem and xylem
cells results in an increase in the diam¬
eter of the stem. But the herbaceous
dicot stem does not live through the
winter in cold climates, so the growth
in diameter of the stem is only for one
season.

The structure of a monocot stem. If

you cut a section across a corn stem,
you will find the tissues very differently
arranged from those in the dicot stem.
The outer covering is a tough epidermis
composed of thick-walled, hard cells.
Its functions are to support the plant
and to protect the other stem tissues.
The bulk of the stem consists of a pith
whose cells have thin walls. Through
the pith you will see numerous fibro-
vascular bundles scattered rather than
arranged in a ring. In other words,
monocot stems have scattered bundles
while dicots have their bundles in a
ring.

Monocots lack a cambium, so that
they usually grow in diameter only un¬
til their cells have reached a maximum
size. This is why they are generally long
and slender as in the iris, orchids, lilies, A
grasses, and sedges. Their leaves often
have sheathed bases that wrap around
the stems from one node to the next
lower one.

Some stem adaptations. Although we
think of a stem as growing entity above
the ground, not all of them do. Some
are entirely underground, and what we
see exposed to light are merelv clusters
of leaves. Various environments may
cause certain adaptations as in the iris
or lily of the valley, whose fleshy under¬
ground stems are called rhizomes. The

CHAPTER 24 STEM STRUCTURE AND FUNCTION 335

Pith cell

Cortex
Phloem fibers

Phloem

Xylem vessel

Vascularcambium
Mechanical tissue

Epidermis
Pith ray

Fibrovascular bundle

Cortex

Pith

Soft pith removed
leaving bundles

Phloem

Xylem

Vascular

cambium

Cortex

MONOCOT STEM AND BUNDLE

HERBACEOUS DICOT STEM AND BUNDLE

24-10 Compare these longitudinal and cross-sections of a monocot stem (corn)
and herbaceous dicot stem (bean). How do they differ?

potato is an enlarged rhizome called a
tuber , which is swollen with stored food.
The “eyes’' of a potato are buds. A
bulb ^is actually an underground bud,
with the stem of the plant reduced to
a small disk at the base of the bulb
surrounded by leaves. Daffodils and
tulips are examples of plants having
bulbs. The corm, found in the gladiolus
and crocus, is different from the bulb in

that most of its stem is covered with thin
scales.

Many stems, though they grow
above the ground, differ from ordinary
erect stems in their special adaptations.
The shortened stem of the carrot, for
example, is disk-shaped and grows just
above the root. Creeping stems, such
as occur in the strawberry, represent a
special adaptation for propagation.

336 UNIT 4 MULTICELLULAR PLANTS

Iris — Rhizome

24-11 Here are four types of underground stems. Why are they stems rather
than roots?

Climbing stems also lack woody tissue,
and may twine around an object for
support or produce tendrils that serve
as a means of grasping. The pole bean
and morning glory twine, while the pea
forms tendrils.

In short, stem adaptations make
possible the growth of plants in various
environments and enable them to com¬
pete favorably with other members of
the community. Figure 24-11 shows
some of these stem adaptations.

Plant propagation by means of stems.
While conduction, support, and storage
are the most outstanding functions of
a stem, reproduction of plants by stems
is another function. Such propagation
may occur either naturally or artificially.

J J J

Layering occurs naturally in many
plants when a stem comes in contact
with the ground. At the point of con¬
tact roots develop. If the stem is sev¬

ered, it becomes a fully independent
new plant. Horticulturists use layering
as a means of multiplying many plants.

As you are no doubt aware, the
stems of many herbaceous plants and
some woody plants can be rooted by
placing them in water or moist sand.
We call these rooted stems cuttings.
This is an important means of propaga¬
tion by stems.

Grafting consists of bringing into
close contact the vascular cambium of a
live, dormant twig and the vascular
cambium of the tree on which it is to
grow. This can be accomplished by
tapering the end of the twig, or scion
(sy- on), to be used and inserting it into
a slit prepared in the rooted branch, or
stock , which is to receive the graft. Such
a graft can be successful only if the
vascular cambiums of both scion and
stock are united with each other. Also,

CHAPTER 24 STEM STRUCTURE AND FUNCTION 337

grafting is successful only when stems
of the same species or closely related
species are united. We cannot graft an
apple twig to an oak tree, but we can
graft several varieties of apple trees onto
a single apple stock.

Budding is similar to grafting ex¬
cept that a bud rather than a twig is
united with the stock (Fig. 24-12). In
budding, a vigorous bud is selected and
removed with a piece of bark surround¬
ing it. The bud is united with the stock
by slipping the piece of bark under the
bark of the stock, which has been
loosened by a T-shaped cut. This
unites the vascular cambiums of scion
and stock. In both budding and graft¬
ing, the wound resulting from the oper¬
ation should be covered with wax to pre¬
vent the entrance of bacteria and fungi,
and to prevent local drying.

Pruning woody plants. When we
cut excess branches from trees and
shrubs the process is pruning. It is
done to change the shape of the plants,
to produce more and better quality of
fruit, to remove dead or diseased wood,
and to enable the plant to recover more
easily after being transplanted. It is
best to prune in winter when the sap is
not rising in the stem.

Movement of water in the stem. We
can understand how water and minerals
can rise from the roots to the leaves of
a small plant. But what about a col¬
umn of water rising 300 or more feet up
the trunk of a huge forest tree? The
answer to this question is not com¬
pletely known, but biologists agree that
at least four forces are involved.

You found in the study of water ab¬
sorption by the root that root cells nor¬
mally have a high osmotic pressure. As
water passes from the cells to the xylem
vessels extending through the root and
stem, it is forced upward with consider¬

able force. This force is great enough
to push water to the leaves of a small
plant. It can be observed in many
plants when the stem is cut off and
water bleeds from the severed xylem ves¬
sels of the stump. However, root pres¬
sure alone could force water but a short
distance through the stem of a tall plant.

In Fig. 24-13 we see a second force
known as capillarity. When the end of
a small tube is placed in a liquid, the
liquid rises in the tube to a level above
that at which it stands in the larger
container. The smaller the inside diam-

wood

Stock

Budding

Whip grafting

24-12 Budding and two types of grafting.

338 UNIT 4 MULTICELLULAR PLANTS

24-13 Each of these tubes has a bore of a
different size. In which one do you think
water will rise the highest?

eter of the tube, the higher the liquid
will rise. This rise is due to the attrac¬
tion of the liquid by the surface of the
tube. If you substitute the many tiny
xylem vessels in a stem for the capil¬
lary tubes, you can see why water rises
in a stem, even when it is cut off and
placed in a container of water, as in a
bouquet of flowers.

Biologists believe that the greatest
force involved in the rise of water
through a stem is a pull rather than a
push. This force is called the transpira¬
tion pull. During transpiration, leaves
lose water to the atmosphere by evapora¬
tion. As the cells closest to the atmos¬
phere lose water, they in turn take water
from the cells adjacent to them. Thus
a flow of water passes through the leaf
tissues to the atmosphere. As water is
lost from the leaves, a continuous col¬

umn, extending from the leaf through
the branches and trunk, is lifted upward.

The lifting of a column of water in¬
volves still another force, cohesion. You
are demonstrating cohesion when you
draw a liquid up through a soda straw.
As you remove liquid at the top of the
straw, you create a partial vacuum. To
fill this vacuum a column of liquid rises
through the straw. The particles of the
liquid cling together bv cohesion. Co¬
hesion results in the lifting of columns
of water and dissolved substances
through the xylem vessels of roots and
stems, as water is drawn upward by
transpiration pull.

The movement of foods in stems. The
movement of dissolved foods in plants,
chiefly through the phloem sieve tubes,
is referred to as translocation. The
forces involved in this movement are
not fully understood. The usual direc¬
tion of food translocation is from the
leaves downward through the phloem.
However, the movement may be up¬
ward from the leaves to flowers and de¬
veloping fruits situated above the leaves.
Furthermore, early in the spring, sap
containing dissolved foods moves up¬
ward from places of storage in the roots
and lower stem region to the branches
and developing buds of perennials.

The movement of foods is too rapid
to be explained by diffusion from one
phloem cell to another. Other forces
must be involved. Some biologists be¬
lieve that the pressure in the phloem
cells decreases from the leaves through
the stem, thus causing a flow toward
the cells of lower pressure.

IN CONCLUSION

The stem is a remarkable organ. One might say that it is a jack of all trades,
for it functions as an organ of conduction, support, storage, and even repro¬
duction.

CHAPTER 24 STEM STRUCTURE AND FUNCTION 339

Equally marvelous is the leaf, as you shall see in the next chapter. Tins
chemical factory lays the foundation for the entire living world by giving it the
basic food necessary for the maintenance of life.

BIOLOGICALLY SPEAKING

annual ring

dicot

rhizome

apical dominance

excurrent

root pressure

axil

fibrovascular bundle

sapwood

axillary bud

grafting

scion

bark

heartwood

sieve tube

bud

herbaceous stem

spring wood

bud scale

internode

stock

bud-scale scar

lateral bud

summer wood

budding

layering

terminal bud

bulb

leaf scar

tracheid

bundle scar

lenticel

translocation

capillarity

monocot

transpiration pull

cohesion

node

tuber

companion cell

phloem fibers

vascular cambium

cork

phloem parenchyma

vessel

cork cambium

phototropism

woody stem

corm

pith

xvlem fibers

cortex

pith rays

xylem parenchyma

deliquescent

pruning

QUESTIONS FOR REVIEW

1. Distinguish between herbaceous and woody stems, and give examples.

2. From what danger do bud scales protect the growing point of a twig dur¬
ing the winter season?

3. Explain how the branching pattern of a tree is determined by apical domi¬
nance.

4. Distinguish between sapwood and heartwood in regard to appearance and
use to the tree.

5. Name four tissues found in the bark region of a woody stem, and list the
functions of each tissue.

6. Explain how the cambium causes increase in the diameter of a woody stem.

7. In what direction are water, minerals, and dissolved foods moved through
the pith rays of a stem?

8. In many stems, spring wood is easily distinguished from summer wood.
Describe the difference in structure in these types of wood.

9. What three tissues form the fibrovascular bundles of herbaceous stems?

10. How can a herbaceous dicot stem be distinguished from a monocot stem

by the arrangement of its fibrovascular bundles?

340 UNIT 4 MULTICELLULAR PLANTS

11. Name three underground stems and give an example of a plant producing
each type.

12. Of what advantage to the plant is a twining stem?

13. What two tissues of a stem must be united if a graft is successful?

14. Name and define four forces operating in water movement through stems.

APPLYING PRINCIPLES AND CONCEPTS

1. Compare the way in which a tree grows with the growth of your body.

2. Why are forest-grown timber trees more valuable than the same species

grown in open places? v

3. Rabbits, beavers, horses, and deer often chew the bark of young trees. If
they destroy the bark all the way around on a portion of the trunk, the
tree usually dies within a few weeks or months. Explain.

4. Most plants with shortened stems grow in open fields or prairies. Why?

5. When the leaves of a tree drop off in the fall, much of the upward move¬
ment of water and soil minerals through the stem ceases. Explain why.

CHAPTER 25

LEAF STRUCTURE
AND FUNCTION

The leaf - a specialized organ for
photosynthesis. In Chapter 6 you
learned that the living world is powered
by sunshine. This energy from the sun
converts raw materials into food sub¬
stances in the green plant cell by the
process of photosynthesis. Although
some cells in the stem are green and
therefore carry on photosynthesis, the
leaf cells are the principal food makers
of the plant. The leaf is, in fact, a spe¬
cialized organ for photosynthesis. If
we examine a typical leaf for a moment
we will see what a perfect organ it is for
its function.

A leaf consists of a thin green por¬
tion called the blade. The blade may
vary in size and shape from species to
species, but it is always distinctive for
each species. The blade is strengthened
by a network of veins , which are really

vascular bundles entering from the stem,
much as blood vessels branch and re¬
branch in reaching all the tissues of our
bodies. In addition to strengthening
the blade, the veins carry water, dis¬
solved minerals, and food materials be¬
tween the leaf and the stem.

Usually the blade is attached to the
stem by a stalk, or petiole (pet-ee- ohl).
At its base the petiole joins the stem at
a node. Some leaves have no petiole.
Instead the blade is fastened directly to
the stem.

In most leaves, the principal veins
tend to be arranged in one of three
general patterns. The sycamore leaf
shows an arrangement in which several
large veins branch out from the tip of
the petiole, in much the way your fin¬
gers extend from your hand. This pat¬
tern is called palmate venation (Fig.

25-1).

Other dicotyledonous leaves, like
the willow and elm, have a single, large
vein called a midrib extending through
the center of the blade from the petiole
to the leaf tip. Smaller veins branch
from the midrib and run to the margins.
This second pattern of venation, resem¬
bling the structure of a feather, is called
pinnate venation.

Most monocotyledonous plants, like
the grasses, lilies, and iris, have several
large veins running parallel from the
base of the leaf to the tip. This third
pattern is called parallel venation.

The outline of a leaf depends some¬
what on the arrangement of its veins.
If the veins are parallel, the leaf is
usually long and slender. The forms of
the leaves are almost as varied as the
kinds of plants. Some have entire edges
(lily), others are toothed (elm), lobed
(maple), or finely divided (carrot).

When the blade of the leaf, even
though greatly indented, is in one piece,

341

342 UNIT 4 MULTICELLULAR PLANTS

25-1 Numerous veins conduct water to every part of a leaf. Identify the vari¬
ous types of venation and compounding shown in this drawing.

it is called a simple leaf. There are
many leaves, however, in which the
blade is divided into two or more parts.
Such a leaf is said to be compound, and
each separate part of the blade is called
a leaflet. When the leaflets radiate
from a common point, as in the clover

and horse chestnut, the leaf is palmately
compound. When the leaflets are ar¬
ranged opposite one another, as in the
black locust or hickory, or alternate on
the sides of a single midrib, the leaf is
pinnately compound. In certain leaves
it may not always be easy to tell whether

j J J

CHAPTER 25 LEAF STRUCTURE AND FUNCTION 343

Vein

Upper epidermis

( Xylem
l Phloem

Stomate

Cuticle

Palisade cell

Spongy layer
cells

Air space
Guard cell
Lower epidermis
Cuticle

25-2 This drawing represents the cross-section of a leaf with the cuticle and
epidermis folded back to show the various tissues.

the part is a leaflet of a compound leaf
or the blade of a simple leaf, except in
the fall when the whole leaf falls away
at the petiole.

Tissues of a leaf. If you cut across the
blade of a leaf and study it under a mi¬
croscope, you will see three distinct tis¬
sues. The first of these, the epidermis ,
consists of a single layer of cells, one
along the top of the blade and one
along the bottom. The cells of the
epidermis are covered by a layer of wax
called cuticle , which helps prevent loss
of water. The lower epidermis contains
numerous small pores called stomates ,
which regulate the passage of gases to
and from the inside of the leaf.

The mesophyll occupies the largest
area in the blade, and in its cells photo¬
synthesis occurs. The mesophyll con¬
tains two regions. The first of these is
made up of closely arranged, elongated
parenchyma cells with many chloro-
plasts, called palisade cells. Chloro-
plasts must be exposed to light, since
the chlorophyll they contain is essen¬
tial to food formation. But too much
light destroys chlorophyll. The shape

of the palisade cells permits the great¬
est light transmission throughout the
length of the cell. Below the palisade
layer is a layer of loosely packed paren¬
chyma cells called spongy cells with
large air spaces between them. Note
that these air spaces are connected to
the outside atmosphere by the stomates.
The veins, containing xylem and phloem
tissue, are scattered through the spongy
tissue. They carry water and food ma¬
terials and help to support the blade.
The stomates. The pores called sto¬
mates are only about one twentieth as
wide as this paper is thick. On each
side of the pore is a bean-shaped guard
cell containing chloroplasts. The guard
cells regulate the opening and closing
of the stomates. When the guard cells
are turgid, the stomate is open. As the
guard cells lose water their turgor de¬
creases and as a result the stomate
closes. The functions of the stomates
are threefold: 1. regulating the loss of
water vapor into the outside air; 2. ad¬
mitting carbon dioxide used in making
carbohydrates and releasing free oxy¬
gen, the by-product of food manufac-

344 UNIT 4 MULTICELLULAR PLANTS

25-3 This chloroplast of a corn cell is shown
magnified about 15,000 times by the electron
microscope. (A. E. Vatter)

ture; 3. admitting oxygen when needed
for respiration, and giving off of the car¬
bon dioxide formed by respiration. Sto-
mates would not be of much use if it
were not for the many air spaces into
which they open. By means of these
spaces, all parts of the leaf have access
to air for food-making, respiration, and
other processes.

The number of stomates varies
from 60,000 to 450,000 per square inch.
There are usually many more on the
lower surface than on the upper.
Leaves that float on water, however,
have all their stomates on the upper sur¬
face. In vertical leaves they are about
evenly distributed.

Leaf adaptations to light and water
conditions. No factor of the physical
environment has as great an influence
on the leaf as light. As a source of en¬
ergy necessary for food manufacture,
light has a direct bearing on the nutri¬
tion of the entire plant. The supply of
food depends on the extent to which a
plant displays its leaves to light.

Leaves are arranged on the stem in
a way that will expose each to the most
light. Each leaf is produced at a dif¬
ferent angle on the stem. For example,

two leaves arranged in a north-south di¬
rection will alternate with leaves ar¬
ranged in an east-west direction. Thus,
one leaf does not shade another grow¬
ing from the node under it.

The general arrangement of leaves
on the stem tends to put each in the
best position to get light. Any rigid
placing of leaves would not be very ef¬
fective among plants that must com¬
pete with one another for light. Indi¬
vidual leaves can adjust the position of
their blades by a bending of the petiole.
As you would expect, this bending is a
phototropic response like that of stem
growth toward light.

Light further influences leaf
growth in the make-up of the internal
tissues. Leaves exposed to bright light
usually develop one or more layers of
compact palisade cells on the upper
side. They also have many cells in the
spongy layer. Shaded leaves have fewer
layers of palisade cells, or none at all,
and they have fewer spongy cells than

Opening of stomate

25-4 Numerous stomates occur on the lower
epidermis of most leaves. What is the func¬
tion of these stomates?

CHAPTER 25 LEAF STRUCTURE AND FUNCTION 345

25-5 Compare these drawings of leaves taken from a plant growing in the shade
and a plant growing in direct sunlight.

leaves growing in bright light (Fig.

22-5) .

The critical relationship between a
leaf and light is shown in the influence
of light on leaf area. In places of re¬
duced light, as for example the inside or
lower branches of a tree, leaves tend to
be larger than those at the tips of
branches or at the top where abundant
light strikes them. Similarly, the leaves
of various species vary according to the
type of environment. The trees of the
dense tropical rain forests have leaves
with large blades for maximum ex¬
posure to light. The leaf blades in our
more open temperate forests, on the
other hand, are much smaller.

Like light, moisture affects the size
and growth of leaves. In regions of
heavy rainfall and moist atmospheric
conditions, leaves are often larger than
those of the same species in a drier cli¬
mate. As rainfall decreases and air be¬
comes drier, leaves tend to become
smaller. In extremely dry places plants
have hardly any leaves at all, as in the
cactus, in which leaves are reduced to
mere spines.

Other leaf modifications. Leaves are
frequently reduced to mere tendrils, or
they may develop as thorns. Some
plants, such as the sedums (seed- umz),
have leaves thickened with stored food
and water. Leaf cuttings of sedum or
begonia will reproduce the plant when
they are put in moist sand. Perhaps
the most curious adaptation of leaves is
found in the Venus’s-flvtrap, the sun¬
dew, and the pitcher plant, which have
leaves modified in various ways for cap¬
turing insects (Fig. 25-6). The insects
are then digested by special enzymes
secreted by the plant. Although the in¬
sectivorous plants are autotrophs and
can synthesize their own carbohydrates,
they usually live in areas scarce in nitro¬
gen. Thus, the insect diet supplies the
missing nitrogen.

Leaf coloration. During the late spring
and summer leaves are green because
chlorophyll is present in the chloro-
plasts. In addition to chlorophyll, the
chloroplasts also contain the yellow xan-
thophyll and orange carotene. Chlo¬
rophyll, however, masks these two other
pigments.

346 UNIT 4 MULTICELLULAR PLANTS

25-6 These photographs show the Venus’s-
flytrap leaf in the act of capturing an insect.
The bee in the top photograph is quite un¬
suspecting as he hopes for nectar in ap¬
proaching the trap. In the middle picture
he is trapped and the leaf is closing. At the
bottom he is completely engulfed and the
plant then will secrete special enzymes
which digest the insect. (United Press In¬
ternational)

With the coming of fall, the tem¬
perature is apt to drop below the point
necessary for chlorophyll formation.
Light destroys the remaining chloro¬
phyll, and the previously hidden yellow
and orange pigments become obviously
apparent.

Cool weather also promotes the for¬
mation of the red pigment anthocyanin
(ant-tho-sy-a-nin) in many leaves. This
red pigment does not form in the
chloroplasts but in the cell sap in vacu¬

oles of the leaf cells. It is formed from
food materials. This pigment accounts
for the red appearance of leaves of
many woody plants during the cool
spring and fall seasons.

Brown coloration results from the
death of leaf tissues and the production
of tannic acid inside the leaf.

The falling of leaves from their
branches. The natural fall of leaves is
caused by an abscission (ab-szz/r-uhn)
layer consisting of a layer of cells across
the base of the petiole (Fig. 25-7).
Soon after the layer forms, its walls sep¬
arate from the stem and leave the peti¬
ole attached only by its fibrovascular
bundles. In some trees the slightest jar¬
ring or gust of wind will cause the leaf
to drop. A thin layer of cork cells seals
the scar where the leaf was attached to
the stem. The leaves of some trees stay
on during the winter and do not drop
until the coming of spring.

While evergreen trees do not shed
all their leaves at any one time, new
leaves usually appear during the spring
and replace those of the previous sea¬
son.

The leaf and photosynthesis. Although
other plant cells contain chlorophyll,
the principal photosynthetic cells of the
green plant are the mesophyll cells.
These cells are truly little factories of
activity utilizing the water and dis¬
solved substances brought from the root
up the stem to the veins. The carbon
dioxide necessary for the process enters,
through the stomates, and excess oxy-
gen passes out through the same pores.
The resulting carbohydrates may be
stored in the palisade and spongy layers,
as well as in the guard cells, or they may
be used in respiration or in the synthe¬
sis of other carbohydrates, fats and
oils, or proteins in other parts of the
plant.

CHAPTER 25 LEAF STRUCTURE AND FUNCTION 347

25-7 During the usually cool weather of au¬
tumn, the cell walls of the abscission layer
across the base of the petiole separate from
the stem so that the slightest jarring will
cause the leaf to drop.

Since the stomates admit carbon
dioxide for photosynthesis, the regula-
tion of their opening and dosing is very
important to the plant! As we said ear¬
lier, the water content of the guard cells
determines whether the stomate is open
or closed. What in turn determines
the water content of the guard cells?
The answer lies in an interesting chem¬
ical regulation which allows for the
varying needs of the plant. An impor¬
tant factor in this chemical regulation
is the fact that carbpn_dioxide^combines
with water to form carbonic acid, and
that less starch is converted to sugar
in an acid environment. Now, think
back to the fact that stomates contain
chloroplasts, so that photosynthesis
takes place in them as it does in the
mesophyll cells. Carbon dioxide is, of
course, used in making sugar in the
process. As photosynthesis occurs in
the guard cells, then, carbon dioxide is
used up and the acid content of the

guard cells decreases. As a result of
this more starch is converted to sugar.
How does a higher sugar concentration
affect the water content of the cell?
Remember osmosis — if the concentra¬
tion of sugar molecules in the guard
cells increases, there must be a decrease
in the concentration of water molecules
relative to the surrounding cells. You
can predict what happens: water mole¬
cules will diffuse into the guard cells
from the cells around them. The re¬
sulting turgidity will in turn open the
stomates and more carbon dioxide
will be admitted for further photosyn¬
thesis. Thus carbon dioxide is acting
as a chemical regulator which controls
its own concentration. This control in
response to varying needs is another ex¬
ample of homeostatic regulation in liv¬
ing things: the organism maintains a
balanced, steady state in the face of
changing conditions.

If this series of events opens the
stomates in the morning, you would ex¬
pect that the reverse happens in the
evening, when the sun goes down.
This is exactly what happens. In re¬
duced light, photosynthesis stops and
the carbon dioxide content of the guard
cells increases. This in turn increases
the formation of carbonic acid, so that
less sugar is formed from starch. This,
in turn, reduces water absorption by
the guard cells, causing decrease in
turgor and change in the shape of the
guard cells, resulting in closing of the
stomates.

Respiration in green plants. Photo¬
synthesis occurs only during the sunny
hours, but respiration occurs night and
day. Respiration in the seed plant is
like that of other organisms. Glucose
is the principal fuel from which energy
is released, and glucose molecules are
oxidized by the removal of hydrogen, as

348 UNIT 4 MULTICELLULAR PLANTS

we explained in Chapter 7. Oxygen is
the ultimate hydrogen acceptor and
combines with it to form water, a prod¬
uct of respiration. Carbon dioxide is
released as the carbon skeleton of the
glucose molecule is broken down.

We may not be aware of plant res¬
piration because it does not involve
breathing. However, the process is just
as important to the plant as to the
animal, even though the rate is much
lower.

Plants require energv for all of their
internal processes. Some energy is used
in the streaming of cell protoplasm. In
addition, energy is required in the syn¬
thesis of carbohydrates other than glu¬
cose, fats, proteins, and nucleic acids.
Growth of the plant requires a constant
supply of energy.

Photosynthesis and respiration com¬
pared. The relationship between pho¬
tosynthesis and respiration is interest¬
ing. They would almost appear to be
opposite chemical reactions. However,
this is not the case. The enzymes and
chemical reactions involved in the two
processes are different. We can say,
though, that photosynthesis and respira¬
tion are complimentary processes. The
requirements of one process are prod¬
ucts of the other. This is an important
aspect of the biochemical balance main¬
tained in a biological society.

The complementary relationship
of photosynthesis and respiration be¬
comes more apparent when you com¬
pare the matter and energy changes in¬
volved in the two processes, as in the
table below.

As you compare the two processes,
notice that matter flows in a cycle.
That is, matter may be used over and
over. This cycle also involves an end¬
less change of inorganic molecules to
organic molecules, followed by a return
to the inorganic state. However, while
matter cycles, energy does not. Light
energy is used in photosynthesis, while
heat and useful energy are released in
respiration. Energy to support photo¬
synthesis must be received as light. For
this reason life will always be depend¬
ent on the sun for energy.

Perhaps you have wondered if pho¬
tosynthesis and respiration balance each
other in a plant. They may if the plant
is in very reduced light, which slows
down the rate of photosynthesis greatly.
However, in bright sunshine, photosyn¬
thesis occurs at a rate ten times that of
respiration, or more. At night, of course,
photosynthesis ceases while respiration
continues. However, the total photo¬
synthesis carried on by a plant far ex¬
ceeds the total respiration. This places
the green plants of the earth in the posi¬
tion of food and oxygen producers for

COMPARISON OF PHOTOSYNTHESIS AND RESPIRATION
Photosynthesis Respiration

Food accumulated

Energy from sun stored in glucose

Carbon dioxide taken in

Oxygen given off

Produces PGAL

Goes on only in light

Onlv in presence of chlorophyll

Food broken down (oxidized)

Energy of glucose released by oxidation

Carbon dioxide given off

Oxygen taken in

Produces C02 and H.,0

Goes on day and night

In all living cells

CHAPTER 25 LEAF STRUCTURE AND FUNCTION 349

25-8 A: water vapor from the plant is condensing on the sides of the bell jar.
B: the water vapor turns the indicator (cobalt paper) pink in 15 minutes. C: the
blue color of the cobalt paper in this control jar does not change.

the totally dependent heterotrophic
plants and animals.

Storage and translocation of foods.
During a bright warm day photosynthe¬
sis forms PGAL and glucose in leaf
cells much more rapidly than the plant
can remove it to other parts. As a re¬
sult, most leaves convert the sugar to
starch either immediately or soon after
it is formed. As the day’s food manu¬
facture progresses, starch grains become
more and more abundant. About the
middle of the afternoon, the starch con¬
tent reaches its peak.

In the evening light is reduced and
photosynthesis slows down. It stops
almost entirely at night but may con¬
tinue slightly on a clear, bright moon¬
lit night. Through the night the stored
starch in the cells of the leaf is con¬
verted to sugar, which dissolves in wa¬
ter. The translocation, or movement
of the sugar solution through the veins
into the stem, continues all night. At

daybreak, when photosynthesis begins
again, the food-making cells are cleared
of stored food and are ready for the
product of a new day’s activity.
Transpiration in plants. During the
growing season a plant conducts a con¬
tinuous stream of water up through its
roots and stem into the leaves. This
flow of water carries dissolved minerals
that are used in the manufacture of pro¬
teins, chlorophyll, and other products.
Some water is used for maintaining cell
turgor and some for photosynthesis.
With all of its uses, however, much
more water is absorbed than the plant
can use. The excess escapes through
the leaves (Fig. 25-8).

During the process known as trans¬
piration, water passes from the air
spaces in the spongy areas through the
stomates and into the air as a vapor.
While transpiration primarily involves
the leaves, other plant parts may be in¬
volved.

350 UNIT 4 MULTICELLULAR PLANTS

Transpiration is more than evap¬
oration. We see this in the different
rates at which it occurs under different
conditions. The rate of transpiration
is controlled to a major extent by the
opening of the stomates. Remember
that this opening is controlled by the
water content of the guard cells.
When the guard cells are full of water,
the stomate is open. When the guard
cells lose water, they change shape and
the stomate closes, thus reducing water
loss. Closed stomates will greatly slow

IN CONCLUSION

up transpiration but will not stop it.

That leaves cannot entirely stop
transpiration, even with their stomates
closed, is clearly shown in the wilting
that frequently occurs on hot days.
Such wilting ceases in the evening when
the atmosphere cools and absorption
makes up the water deficiency. Trans¬
piration is especially dangerous to
plants after transplanting. The remov¬
al of some of their leaves reduces the
evaporation of water vapor from the
leaves.

The leaf is the food factory for the plant, and indeed for most animal life.
Through leaves, plants respire and carry on photosynthesis, the latter produc¬
ing oxygen and carbohydrates.

At the end of a season of activity, many plants shed their leaves. New,
active leaves take up their work the following growing season. Even the ever¬
green plants lose their old leaves and grow new ones at regular intervals. Thus,
the food factories of seed plants remain young and active.

In the following chapter, we shall consider reproduction in flowering

plants.

BIOLOGICALLY SPEAKING

abscission layer
blade

compound leaf
cuticle
epidermis
guard cell
leaflet

mesophyll

midrib

palisade parenchyma
palmate venation
parallel venation
petiole

pinnate venation

simple leaf

spongy parenchyma

stomate

translocation

transpiration

vein

QUESTIONS FOR REVIEW

1. Why is it important that most leaf blades be thin and broad?

2. Name two important functions of leaf veins.

3. Distinguish between dicot and monocot leaves on the basis of their vein
structure.

4. Name the tissues of a leaf from top to bottom.

CHAPTER 25 LEAF STRUCTURE AND FUNCTION 351

5. Why are the numerous spaces in the spongy layer necessary for leaf ac¬
tivity?

6. Discuss the location, structure, and functions of the stomates of a leaf.

7. Discuss the relation of leaf size and numbers to atmospheric moisture.

8. Why do various pigments appear in leaves during the fall season?

9. How is sugar content related to the water content of guard cells? How is
water content related to the opening and closing of the stomates?

10. Compare photosynthesis and respiration with regard to substances neces¬
sary for the processes, waste products formed, and energy changes that oc¬
cur.

11. Explain how the rate of transpiration varies with the water content of a
plant and conditions of the atmosphere.

APPLYING PRINCIPLES AND CONCEPTS

1. Most of the cells of a leaf have thin walls. Why is this important to the
activities of a leaf?

2. In what way does carbon dioxide in the leaf act as a chemical regulator
that controls its own concentration?

3. If photosynthesis did not occur at a much greater rate than does respira¬
tion in green plants, there would be no animal life on earth. Explain why
this is true.

4. Do you agree or disagree with the belief that plants should be removed
from a sick room at night? Explain your opinion.

5. A nurseryman planted a tree in full leaf during the month of June. After
planting, he pruned back the branches and removed many of the leaves.
Why did this give the tree a far better chance to survive?

CHAPTER 26

REPRODUCTION
IN FLOWERING
PLANTS

What is a flower? If you asked this
question of a florist, you would get one
kind of an answer. A forester would
probably give you a different one, and
a biologist would surely offer still an¬
other. Using the biologist’s definition,
we may define a flower as a very spe¬
cialized structure of the angiosperms
which is adapted for reproduction of
the species. A flower is also a modified
branch in which the leaves are very
much altered to form the various parts.
The flower bud, for instance, contains
meristematic tissues as do the buds that
produce leaves and branches. The
flower is highly adapted for the type of
reproduction this group of plants em¬
ploys.

There is enormous variation among
flowers. The lily, rose, orchid and tulip
are handsome, colorful flowers. But

the flowers of the oak, elm, maple, and
birch are so inconspicuous as to be
hardly recognizable as flowers at all.
The flowers of grasses such as wheat,
oats, rye, and our common lawn grasses
are flowers in the botanical sense, but
you would scarcely think of them as
such.

Floral parts. A typical flower, such as
the geranium, apple blossom, snap¬
dragon, sweet pea, or petunia, has four
sets of parts (Fig. 26-1). These parts
grow from a special flower stalk, or pedi¬
cel (ped- i-sel), the end or tip of which
is the receptacle. The outer ring of
floral parts consists of several green leaf¬
like structures called sepals (seep- alz).
Together, the sepals form the calyx
(kay- liks) . The sepals cover and protect
the rest of the flower in the bud stage.
They also help to support the other
parts when the bud opens.

Inside the calyx is the corolla (ko-
rahl- a), which usually consists of one
or more rows of petals. These are often
but not always brightly colored. The
calyx and corolla frequently attract in¬
sects, as we shall see later. They may
also help to protect the inner parts. In
certain flowers, like the tulip, both the
calyx and corolla are the same color.
It is possible to miss the fact that both
parts are present.

Two kinds of essential parts are
concerned directly with reproduction :
the stamens and the pistil, located in
the center of the flower. Each stamen
(stay- men) consists of a slender stalk,
or filament , supporting a knoblike sac
called an anther. The anther produces
various colored grains called pollen ,
which are essential in reproduction.

The pistil consists of a sticky top,
called a stigma, a slender stalk, or styley
which supports the stigma, and a swol¬
len base, or ovary, which is joined to

352

CHAPTER 26 REPRODUCTION IN FLOWERING PLANTS 353

26-1 This is a complete flower, having all four parts.

the receptacle of the flower stalk. In¬
side the ovary are the ovules, which will
later become seeds. The ovules are at¬
tached to the ovary either at its base, or
along the side walls, or to a special

stalk running lengthwise from the base
of the ovary to the base of the style.
Ovules may number from one to sev¬
eral hundred, depending on the kind of
flower.

354 UNIT 4 MULTICELLULAR PLANTS

peach GERANIUM PEA

26-2 Various patterns of attachment of ovules to ovary. In the peach the one
ovule is attached to the base of the ovary. In the geranium several ovules are
attached to a special stalk in the center of the ovary. In the pea all ovules are
attached to the side walls of the ovary.

Variation in flower structure. Flowers
vary greatly not only in color, size, and
shape, but also in their reproductive
structures. Although many plants have
both stamens and pistil on the same
flower, some, such as the oaks, squash,
and corn, have these structures on sepa¬
rate flowers on the same plant. Other
species have flowers bearing pistils on
one plant and flowers bearing stamens
on another plant. Willows and cotton¬
woods are well-known and familiar ex¬
amples of this type.

In studying stems and leaves you
learned that dicots differ from mono¬
cots in the arrangement of their vascu¬
lar tissue. As you might expect, they
also differ in the structure of their flow¬
ers. Monocots, for example, usually
have flower petals and essential parts in
threes or multiples of threes, while di-'
cots usually have them in fours or fives
or their multiples.

Some flowers are not single flowers
in the biological sense but a whole clus¬
ter. The sunflower and daisy, for ex¬
ample, have dense clusters of reproduc¬
tive flowers in the center, surrounded
by so-called petals, which are really an¬
other type of flower and which usually
attract insects.

The anther and pollen formation. If

you examine a cross section of the de¬
veloping anther of a large stamen, such
as that of a lily or a tulip, you can see
four chambers, or pollen sacs clearly
(Fig. 26-3). During development of
the flower, each pollen sac is filled with
cells with large nuclei. These are
known as microspore mother cells.
Microspore mother cells contain pairs
of chromosomes, or the diploid number
(2n). As the anther grows, the nucleus
of each microspore mother cell divides
by meiosis forming first two daughter nu¬
clei which divide again in the second

CHAPTER 26 REPRODUCTION IN FLOWERING PLANTS 355

part of meiosis, resulting in four. Cell
walls form around each daughter cell,
resulting in a four-celled tetrad. The
cells are referred to as microspores.
Microspores are thus haploid, because
they always contain the n number of
chromosomes.

Each microspore develops into a
pollen grain. First the nucleus divides
by mitosis forming two. One is desig¬
nated as a tube nucleus and the other
as a generative nucleus. The wall of
the microspore thickens and becomes
the protective covering of the pollen
grain. At about this time the anther
ripens and the wall between two adja¬

cent pollen sacs disintegrates. The pol¬
len sacs burst open, and the pollen is
ready for distribution.

Development of the ovule. While pol¬
len grains are forming in the anthers,
changes are occurring in the ovary at
the base of the pistil. For the sake of
simplicity, we shall describe these
changes in an ovary containing a single
ovule, like the avocado pear. When
there are several or many ovules in
an ovary, the same general procedure
takes place.

An ovule appears first as a tiny knob
on the ovary wall. This swelling con¬
tains a single megaspore mother cell

Mega spore
mother cell (2n)

Megaspores (n)

( 3 disintegrate; 1 divides
to form daughter nuclei)

Micropyle

Daughter nuclei
in embryo sac

Synergid

Antipodals
Polar nuclei
Synergid
Egg

Pollen sac

Generative

nucleus

Tube

nucleus

Anther cross-section

26-3 In the drawings at the top, trace the steps in the formation of the ovule;
then do the same in the bottom drawing for the formation of pollen.

356 UNIT 4 MULTICELLULAR PLANTS

(Fig. 26-3). This cell contains the dip¬
loid (2 n) number of chromosomes. As
the ovule grows, it is raised from the
ovary wall by a short stalk through
which nourishment is received. One or
two protective layers form around the
ovule and enclose it completely except
for a tiny pore, or micropyle. The mi-
cropyle is usually on the lower side of
the ovule.

The megaspore mother cell divides,
followed by division of the daughter
cells, resulting in a row of four mega¬
spores. As in the formation of micro¬
spores, this double division is meiotic
and reduces the chromosome content
of the megaspores to the haploid num¬
ber (n). Of the four megaspores pro¬
duced in an ovule, one survives and the
other three disintegrate. Usually the
surviving megaspore is the one farthest
from the micropyle. This remaining
megaspore enlarges rapidly and forms an
oval embryo sac in which further de¬
velopment occurs in the following steps
(also see Fig. 26-3) :

1. The megaspore nucleus divides and
forms two daughter nuclei. Two ad¬
ditional mitotic divisions result in
eight nuclei.

2. Four nuclei migrate to each end of
the embryo sac.

3. One nucleus from each group of four,
designated as a polar nucleus , mi¬
grates to the center of the embryo
sac.

4. Each nucleus in the groups of three
at either end of the embryo sac is en¬
closed by a thin membrane.

5. One of the cells nearest the micro¬
pyle enlarges and becomes the egg.
The cells on either side of the egg
and partially surrounding it are
known as synergids (si-ner-jidz).

6. The three cells farthest from the mi¬
cropyle are known as antipodals.

The ovule is now ready for fertili¬
zation. However, only the egg and the
polar nuclei will be involved in this proc¬
ess. Both the synergids and the antip¬
odals are short-lived and have no ap¬
parent function. Before fertilization
can occur, a pollen grain must be trans¬
ferred to the stigma of the pistil by one
of the various agents in pollination.
Pollination. We can define pollination
as the transfer of pollen from the anther
to the stigma. In some plants pollen
is transferred from anther to stigma in
the same flower or to the stigma of an¬
other flower on the same plant. We
refer to this as self-pollination. If flow¬
ers on two separate plants are involved,
the process is called cross-pollination.
Cross-pollination requires an outside
agent. The chief ones are insects, wind,
and water. Curious adaptations of dif¬
ferent kinds of flowers are frequently
necessary to accomplish cross-pollina¬
tion.

Adaptations for pollination. Chief
among the insect pollinators are bees.
But moths, butterflies, and certain kinds
of flies visit flowers regularly and in so
doing carry on cross-pollination. Insects
come to the flower to obtain the sweet
nectar secreted deep in the flower by
special glands at the base of the petals.

Bees swallow nectar into a special
honey stomach where it is mixed with
saliva and converted into honey.
When they return to the hive, the bees
deposit the honey in six-sided cells of
the comb and later use it as food. The
plump hairy body of the bee makes it
an ideal pollinator (Fig. 26-4). To
reach the nectar glands at the base of
the flower, the bee must rub its hairy
body against the anthers. These are
usually located near the opening of the
flower. When the insect visits the next
flower, some of the pollen is sure to rub

CHAPTER 26 REPRODUCTION IN FLOWERING PLANTS 357

26-4 The bee is an
ideal pollinator be¬
cause of its plump
hairy body. (Eisen-
beiss from Photo Re¬
searchers)

against the sticky stigma of the pistil,
while a new supply is brushed off the
stamens.

Brightly-colored petals and sweet
odors aid insects in locating flowers.
Nectar guides in some flowers may be
brightly-colored stripes located on the
petals.

We must include at least one bird
in discussing agents of pollination.
Tiny hummingbirds feed on nectar of
certain flowers. Their long bills and
equally long tongues reach down to the
nectar glands while the bird hovers over
the flower.

The flowers of wind-pollinated
plants are much less striking than are
those pollinated by insects. They are
often borne in dense clusters near the
ends of branches. As a rule petals are
lacking and the flowers seldom have any
nectar. Frequently the stamens are
long and produce enormous quantities
of pollen. The pistils are also long, and
the stigmas are large and often sticky
to catch pollen grains that are blown

about by the wind. Cottonwood, wil¬
low, walnut, corn, oats, and other wind-
pollinated plants literally fill the air
with pollen when their stamens are ripe.
Growth of the pollen tube and fertili¬
zation. Once a pollen grain lodges on
the sticky surface of the stigma of the
pistil, it starts to form a pollen tubey
which penetrates the stigma surface.
As the tube lengthens, it grows through
the soft tissue of the style and reaches
the micropyle of the ovule. The tube
nucleus of the pollen grain often disin¬
tegrates early. As the generative nu¬
cleus moves into the tube, it divides
and forms two sperm nuclei , or male
gametes. After passing through the mi¬
cropyle, the pollen tube digests its way
through the thin wall of the embryo
sac. The tip of the tube ruptures and
the two sperm are discharged into the
embryo sac. Meanwhile the tube nu¬
cleus degenerates.

One sperm unites with the egg in
fertilization. This produces the ferti¬
lized egg, or zygote. Both the egg and

358 UNIT 4 MULTICELLULAR PLANTS

Generative

nucleus

Egg unites with one sperm
nucleus to form zygote

Pollen grain
Stigma

Ovule
Antipodals
Embryo sac
Integuments

26-5 The drawings on the left show how the pollen tube and sperm nuclei de-
velop from a pollen grain, while those on the right show in detail the ovule ot
a peach at the time of fertilization.

the sperm nuclei are haploid. Fertiliza¬
tion restores the diploid chromosome
number (2 n) in the zygote. Since the
zygote will form the embryo plant by
cell division, the cells of the plant will
possess this diploid number until flow¬
ers are produced and reduction division
occurs in the formation of microspores
and megaspores.

The second sperm unites with the
polar nuclei in the embryo sac and an
endosperm nucleus is produced. Since
both polar nuclei and the sperm con¬
tain the haploid chromosome number

(n), the endosperm nucleus is a triploid
(3n) structure. We refer to the union
of the three nuclei to form the endo¬
sperm nucleus as triple fusion. These
steps are summarized graphically in
Fig. 26-5.

Immediately after fertilization aux¬
ins that were produced in the pollen
grain and delivered to the ovule
through the pollen tube stimulate rapid
cell division and tissue growth within
the ovule. The zygote undergoes an
orderly development into the embryo
plant. Meanwhile the endosperm nu-

CHAPTER 26 REPRODUCTION IN FLOWERING PLANTS 359

cleus gives rise to a mass of tissue which
becomes the endosperm of the seed.
This functions as a storage area to nour¬
ish the developing embryo plant. In
some seeds the endosperm disappears
after a short time. In others it remains
as a part of the seed at maturity.

The gametophyte and the sporophyte
in flowering plants. In our discussion
of the algae, the fungi, and the ferns, we
noted that some of these organisms
have two distinct forms, the gameto¬
phyte and the sporophyte, in an alterna¬
tion of generations. In the algae and
the mosses, the gametophyte was the
most conspicuous form. In the fern,
the sporophyte has become the more
conspicuous of the two generations.
You may never have noticed the small,
inconspicuous heart-shaped gameto¬
phyte of the fern. In the seed plants
this trend has gone even further. The
sporophyte is the plant that we see.
The gametophyte has become com¬
pletely dependent on the sporophyte,
and is in fact microscopic. The male
gametophyte in seed plants consists
only of the pollen tube with its haploid
sperm and tube nuclei. The female
gametophyte consists of the embryo sac
with its haploid egg, polar nuclei, antip-
odals, and synergids, found in the ovule.
From this description of the gameto-
phytes in seed plants, you can see that
the vulnerable egg and sperm nuclei are
well protected from drying within the
tissues of the sporophyte. This evolu¬
tionary trend toward the more conspic¬
uous sporophyte seems to parallel the de¬
velopment of 1. vascular tissue; 2. roots;
3. epidermis; and 4. stomates, start¬
ing with the ferns and arriving at the
most efficient of the land plants, the
angiosperms.

From flower to fruit and seed. Fertili¬
zation brings a sudden end to the work

of the flower. As the sepals, petals, and
stamens wither, a group of special hor¬
mones force the plant to pour its full
energies into the development of the
ovary and the ovules inside. After a
few weeks the ovary and its contents
ripen. In many plants, other nearby
parts, such as the receptacle or the
calyx, enlarge and become part of the
fruit, so that we can define a fruit as a
ripened ovary , with or without associ¬
ated parts. A seed, on the other hand,
is a matured ovule that is enclosed in
the fruit.

Fruits, like the flowers from which
they develop, vary greatly in structure.
The way some of the common fruits are
classified according to structure is given
in the table on page 360. As you can
see, a fruit need not be fleshy, like an
apple, a peach, or an orange. A kernel
of corn, a hickory nut, a bean pod with
its beans, a sticky burr of burdock, and
a cucumber or pumpkin are just as
much fruits as the fleshy, juicy type.
Thus the biological meaning of the
word fruit is quite different from the
meaning used in a grocery store.

The relation of fruits and seeds. One
important fact about seeds: the new
plant grows from a seed, not from a
fruit. But the fruit is highly important
because it encloses the seed and pro¬
tects it from water loss, disease, insect
attack, and other dangers while it is de¬
veloping. Later the fruit serves as a de¬
vice for distributing the seeds.

When seeds mature they must be
carried from the parent plant by some
means or other. If they fall to the
ground close by, the parent plant will
be surrounded by struggling seedlings
and few of these will have much chance
of survival. Nature avoids this waste
by attempting to scatter seeds as far as
possible from the parent plant, a process

360 UNIT 4 MULTICELLULAR PLANTS

CLASSIFICATION OF FRUITS

Type

Structure

Examples

Pome

Fleshy Fruits

Outer fleshy layer developed from calyx and receptacle;

Apple, quince,

ovary forms a papery core containing seeds

pear

Drupe

Ripened ovary becomes two-layered — outer layer fleshy,

Plum, cherry,

inner layer hard, forming stone or pit, enclosing one

peach, olive

Berry

or more seeds

Entire ovary fleshy and often juicy; thin-skinned and

Tomato, grape,

containing numerous seeds

gooseberry

Modified

Like berry, but with tough covering

Orange, lemon,

berry

cucumber

Aggregate

Compound fruit composed of many tiny drupes clus-

Raspberry,

fruit

tered on single receptacle

blackberry

Accessory

Small and hard; scattered over surface of receptacle; edi-

Strawberry

fruit

ble portion formed from enlarged receptacle

Multiple

Compound fruit formed from several flowers in a cluster

Mulberry,

fruit

pineapple

Pod

Dry Fmits (dehiscent)

Ovary wall thin, fruit single-chambered, containing

Bean, pea,

many seeds; splits along one or two lines when ripe

milkweed

Capsule

Ovary containing several chambers and many seeds;

Poppy, iris,

splits open when mature

cotton, lily

7 J

Nut

Dry Fruits (indehiscent)

Hard ovary wall enclosing a single seed

Hickory nut,

Grain

Thin ovary wall fastened firmly to single seed

acorn, pecan
Corn, wheat,

Achene

Similar to grain, but with ovary wall separating from

oats

Sunflower,

seed

dandelion

Winged

Similar to achene but with prominent wing attached to

Maple, ash,

fruit or

ovary wall

elm

samara

we refer to as seed dispersal (Fig. 26-6) .

Sometimes only the seed is trans¬
ported, but often the entire fruit is car¬
ried to a new location. In some plants
seed dispersal is a mechanical process,
while in others an outside agent, such
as the wind, water, a bird, or some other
animal, is involved. We shall consider
some of the methods by which fruits

and seeds travel from the parent plant.

Pod fruits, like the bean and pea,
often twist as they ripen as a result of
changes in the amount of moisture in
the air. This causes a strain on the pod
so that it bursts open suddenly and
with enough force to throw the seeds
some distance from the parent plant.
Another interesting example of me-

CHAPTER 26 REPRODUCTION IN FLOWERING PLANTS 361

26-6 Describe the various ways in which these fruits and seeds are adapted for
dispersal.

chanical dispersal of seeds is the fruit of
the garden balsam, or touch-me-not.
When the fruits of this plant are ripe,
they open upon the slightest touch and
curl upward violently with the result
that the seeds may be thrown several
feet. Capsules, like the poppy, do not
split open along the sides, but holes
form around the top as the ovaries rip¬
en. They resemble salt shakers and as
the fruit sways back and forth on a long
and flexible stem in the breeze, seeds
sift out.

The delicious flesh of the apple,
grape, or cherry is a sort of biological
bribe. Birds and other animals feed on
the fruits and scatter the seeds. Often
the seeds pass through the digestive
tract of an animal unharmed because
their cellulose covers cannot be di¬

gested, and then they are deposited far
from the parent plant.

Animals aid in fruit and seed dis¬
persal in another way. Many plants
produce fruits with stickers or spines
that cling to the fur of animals. You
might have aided plants in seed dis¬
persal if you ever sat down after a hike
in the fall to pick off the beggar’s lice,
sticktights, and burdocks that were
stuck to your clothes.

Water is the agent of dispersal for
many seeds. The coconut palm, for in¬
stance, often lives close to the shore
and drops its fruit into the water. The
thick, stringy husk of the coconut is wa¬
terproof. When the seed germinates,
a sprout pushes through one of the
three “eves” on the end of the hard

covering. Grasslike plants, known as

362 UNIT 4 MULTICELLULAR PLANTS

sedges, are among some other plants
that may drop their fruits into the wa¬
ter. They, like the coconut, are gen¬
erally found along the shores of oceans
or the banks of rivers and streams where
their seeds may have found a foothold
on the land.

The wind is the agent of dispersal
for many fruits and seeds. When the
milkweed pod splits open, the wind
empties the pod of its seeds and each,
equipped with a miniature parachute, is
carried to a new location (Fig. 26-6).
You have probably blown the fluff off a
dandelion or thistle head. The fruits
of these plants often travel long dis¬
tances on their tiny tufts. In the spring
the cottonwood tree fills the air as
spring breezes empty its catkins of cot¬
tony tufted seeds. The winged seeds of
maple, ash, elm, and pine whirl in the
air like tiny propellers and are scattered
to a considerable distance from the
place where they develop.

What is a seed? We defined a seed as
a matured ovule and as the final prod¬
uct of plant reproduction. A seed con¬
tains a tiny living plant, the embryo ,
stored food, and the seed coats. The
stored food nourishes the young plant
from the time it starts to grow until it
can produce its own food by photosyn¬
thesis. The regions in which food is
stored may vary with different seeds. In
some seeds food is stored in thick “seed
leaves,” the cotyledons.

You may have seen thick cotyle¬
dons on the stems of such young plants
as the green bean or lima bean shortly
after they have pushed through the gar¬
den soil. They are located below the
foliage leaves and last for only a few
days before they wither and fall off.
The number of cotyledons in the seed
serves as the basis for the classification
of the angiosperms. Monocot plants

have only one cotyledon in their seeds,
while dicot plants have two.

Not all seeds have the same kind
of food stored in the cotyledons. A
grain of corn, for example, has its starch
and protein stored in the endosperm,
while the cotyledon contains oils and
proteins. The endosperm, filling much
of the corn seed, develops from the en¬
dosperm nucleus after it unites with one
of the two sperm during fertilization.
On the other hand the major part of
the bean seed is made up of the two
cotyledons, which store a large amount
of starch as well as proteins and oil.
Some seeds have a large endosperm,
while others have a very small endo¬
sperm or none at all. One of the latter
is the bean.

Seed coats cover the seed and pro¬
tect it from drying out and from other
dangers before it germinates. Usually
there are two seed coats, but some seeds
have only one. The outer coat is usu¬
ally tough and thick. The inner coat
is much thinner.

Structure of a bean. The bean seed is
usually kidney-shaped (Fig. 26-7). The
outer seed coat, the testa , is smooth
and may be white, brown, red, or other
colors, depending on the species. An
oval scar on the concave side, the hilum
(fiy-lum), marks the place where the
bean was attached to the wall of the
pod. Near one end of the point of at¬
tachment is the tiny pore, the micro-
pyle. The pollen tube grew through
this tiny opening in the wall of the
ovule just before fertilization. The in¬
ner seed coat of a bean is a thin, white
tissue which is difficult to separate from
the testa. Both of these coats have de¬
veloped from the wall of the ovule.

If you soak a dried bean and re¬
move the seed coats, the cotyledons will
separate easily. The cotyledons fill the

CHAPTER 26 REPRODUCTION IN FLOWERING PLANTS 363

Micropyle'
Hilum -

Exterior

Hypocotyl
Seed coat

Cotyledon •

With seed coat opened

,/ \

m i

Cotyledon scar

Epicotyl

Hypocotyl

Cotyledon

With cotyledons separated

26-7 This shows the external and internal
structures of the bean seed.

therefore it corresponds to the bean pod
and its contents rather than to the in¬
dividual bean seed. However, there is
only one seed in each grain and it com¬
pletely fills the fruit, the outer coat of
the kernel having been formed from the
flowers ovary wall. A very thin inner
seed coat is fastened tightly to the outer
one and is only one cell layer thick. It
developed from the wall of the ovule.

The micropyle is covered by the
fruit coat, but there is an obvious point
of attachment of the corn fruit to the
cob. This structure corresponds to the
stalk of the bean’s flower and is the
pathway through which the developing
fruit received its nourishment.

On one side of a grain of corn is
a light-colored, oval area that marks the
location of the embryo. This is plainly
visible through the fruit coat. Near
the top of the kernel, on the same side

space within the seed coats, and are
thick and fleshy and not at all leaflike.

Lying between the cotyledons are
the other parts of the embryo plant. A
fingerlike projection, the hypocotyl , fits
into a protective pocket of the seed
coats. It is that part of the embryo axis
between the cotyledons and the radicle
(embryonic root).

The epicotyl joins the upper end
of the hypocotyl. It consists of two
tiny leaves, folded over each other, and
between them lies the minute bud that
will later form the plant’s terminal bud
as the epicotyl develops into the shoot.
The location of the cotyledons is im¬
portant to the seedling. Both the hy¬
pocotyl and the epicotyl will grow rap¬
idly when the seed germinates and the
cotyledons supply nourishment to both.
Structure of the corn kernel. Each
corn grain is really a complete fruit, and

Silk scar

Endosperm

Cotyledon

Epicotyl

Hypocotyl
Root cap

Point of
attachment

26-8 The diagram shows the relation be¬
tween the surface features and internal
structure of the corn seed.

364 UNIT 4 MULTICELLULAR PLANTS

Ovary wall I S

and seedi4
coats

combined GRA|N_C0RN

SAMARA

MAPLE

I) POD—
y LIMA BEAN

•Style CA

ACHENE

SUNFLOWE

26-9 Various types of dry fruits.

as the embryo, is a tiny point, the silk
scar , where the style was attached.

If you cut a grain of corn length¬
wise through the region of the embryo,
you can see the internal parts clearly,
especially if you put a drop of iodine
solution on the cut surface. The endo¬
sperm fills much of the seed (Fig.
26-8). This part of the seed developed
from the endosperm nucleus after fertili¬
zation. The endosperm contains sugar
and starch, and will turn blue when

treated with iodine. This reaction in¬
dicates that there is a large quantity of
starch present. The embryo, however,
does not contain starch, but it does con¬
tain considerable protein. Sweet corn
stores sugar in the endosperm, but field
corn stores starch, which accounts for
the fact that we eat the garden variety
and not the other.

The embryo, consisting of a very
small hypocotyl, an epicotyl, a cotyle¬
don, and a radicle, lies on one side of

COMPARISON OF CERTAIN DICOTYLEDONOUS
AND MONOCOTYLEDONOUS SEEDS

Bean

Testa with hilum and micropyle plainly
visible

Two cotyledons
Large embrvo

No endosperm at time of dispersal

Epicotyl fairly large

Epicotyl leaves folded

The fruit is a pod, with several seeds

Corn

Hilum and micropyle covered by a three-
lavered fruit coat. The true seed coat
lies inside of it

One cotyledon

Small embryo

Large endosperm

Epicotyl rather small

Epicotyl leaves rolled

The fruit is a single grain, with one seed

CHAPTER 26 REPRODUCTION IN FLOWERING PLANTS 365

26-10 Various types of fleshy fruits.

366 UNIT 4 MULTICELLULAR PLANTS

the corn grain. The radicle points
downward, toward the point of attach¬
ment, and is surrounded by a protective
cap. The epicotyl is also protected by
a sheath or cap. The leaves of the epi¬
cotyl are rolled, not folded as they are
in the bean, into a compact spear.

The corn has only one cotyledon,
which is attached to one side of the very
short hypocotyl and the epicotyl and lies
against the endosperm. During germi¬
nation the cotyledon digests and ab¬
sorbs food from the endosperm, besides
furnishing some of its own, and supplies
it to the growing seedling. Notice that
in the corn grain, most of the energy-
producing food is stored outside the em¬
bryo rather than in the cotyledon, as we
found in the bean.

Dormancy in seeds. Many seeds go
through a rest period before they ger¬
minate. This rest period, or period of
dormancy , may be a few weeks, an en¬
tire season, or several years. Many
plants bear seeds in the fall and their
seeds are normally dormant throughout
the winter, but germinate during the
following spring or summer.

Drought, cold, and heat are all
enemies of the seedling, although it is
enclosed in protective seed and fruit
coats. When conditions are favorable
for growth of a particular seed, how¬
ever, the period of dormancy ends and
germination , or sprouting, begins.

While some seeds may lie dormant
for several years and still remain alive,
there is a limit to the length of this pe¬
riod. Some seeds may live for almost
100 years in a dormant state and then
germinate when conditions become sat¬
isfactory. On the other hand, some
seeds, like the maple, germinate almost
immediately after falling from the tree,
with the result that you frequently see
a large number of young maples start¬

ing to grow under the parent tree in the
late spring.

In annuals that grow in colder cli¬
mates, seeds are the only form in which
the plants can survive the winter
months. Their period of dormancy
normally extends from one growing sea¬
son to the next. The seeds of many
perennials likewise lie dormant through
the winter months and germinate the
following spring or summer.

The ability of seeds to germinate
is called viability. Seed viability de¬
pends on the conditions during dor¬
mancy and on the amount of food
stored in the cotyledons and endo¬
sperm. Cool, dry places are ideal for
storing seeds, while warmth and mois¬
ture lower viability. Commercial seed
growers run viability tests and mark the
results on various lots of seeds they sell.
If you check the reported viability test,
you can find out the percentage of ger¬
mination to expect. If a lot of seeds
has a viability of 92 percent, you can
expect 92 seedlings from each 100 seeds
you plant. Remember, however, that
viability may vary since only relatively
few representative samples are used in
each test.

Conditions for germination. For ger¬
mination, most seeds require at least
three conditions: moisture, the correct
temperature, and oxygen. The amount
of each of these required varies greatly.

Seeds of many water plants germi¬
nate under water where there is plenty
of moisture, a quite even temperature,
and oxygen dissolved in the water. The
seeds of most land plants cannot germi¬
nate under water.

Before a seed germinates it usually
absorbs considerable water, causing the
seed to swell and soften its seed coats.
But too much warm moisture during
the growing season encourages the

CHAPTER 26 REPRODUCTION IN FLOWERING PLANTS 367

growth of fungi, which may cause the
seeds to decay.

The temperature at which seeds
germinate best is also variable. A ma¬
ple seed can germinate on a cake of ice,
but growth will be slow and survival
very uncertain under these conditions.
Others, like com, require much higher
temperatures, with a range of between
60° and 80° F being the most suitable
for the majority of seeds.

During germination the cells of a
seedling are dividing very actively.
This increased activity requires a much
higher rate of respiration than that of
an older plant and you can see, there¬
fore, why the oxygen supply to a seed¬

ling is critical. That is the reason the
soil in a garden should be loose and the
seeds planted sufficiently near the sur¬
face to give them an ample supply of
oxygen.

Much of the food stored in the
cotyledons or endosperm of a seed is
starch. The plant changes this to sugar
by the action of an enzyme known as
amylase, and the cells of the embryo
absorb the sugar. This change accounts
for the sweetish flavor of sprouting seeds
and explains why sugar is extracted from
sprouting grain (malt) or why soybean
sprouts are sometimes used in cooking.
Growth of the seedling. The way in
which the seed germinates and the seed-

Cotyledons

Testa

Epicotyl

Cotyledon

Hypocotyl

Terminal bud

First true leaves

26-11 The various stages in germination of the bean seed. The cotyledons,
which are a source of food for the growing seedling, finally fall off when the
plant is able to produce its own food.

368 UNIT 4 MULTICELLULAR PLANTS

ling establishes itself varies in different
kinds of plants and in the location of
the seed during germination. If the
seed is lying on the surface, the root
must penetrate the soil from above, and
the epicotvl will grow freely upward.
If, on the other hand, the seed is com¬
pletely buried, the epicotvl must grow
through the soil and unfold its leaves
above the surface while the root grows
downward. We shall follow the stages
in the germination of a bean seed and
a grain of corn and see how this is ac¬
complished. Few of us realize what an
interesting process germination is be¬
cause we have seldom stopped to think
very much about it. But in a garden or
on a farm, it takes place almost without
our realizing it.

Figure 26-1 1 shows the stages in the
germination of a bean. After the bean
has absorbed water and softened its

seed coats, the hypocotyl grows out
through the seed coat. The root grows
downward and forms the primary root
of the seedling, while the hypocotyl is
growing upward and forming an arch
which pushes its way to the surface.
After the hypocotyl arch appears above
the ground, it straightens out and lifts
the cotvledons upward. The cotyle¬
dons turn outward and release the epi-
cotyl, which grows upward to form the
shoot. Then the minute leaves unfold,
forming the first foliage leaves of the
plant. These are true leaves and are
retained throughout the life of the plant
which, in this case, is an annual. The
plant dies after it has matured its
fruit.

The stem lengthens rapidly, devel¬
oping more leaves, and the small bud
that was between the epicotvl leaves of
the seed becomes the terminal bud of

26-12 In germination of the corn seed, neither the hypocotyl nor the cotyledon
grows above the ground.

CHAPTER 26 REPRODUCTION IN FLOWERING PLANTS 369

the plant. The cotyledons remain at¬
tached to the stem for a time, below the
true leaves. But as the plant becomes
better able to supply its own food by
photosynthesis, the cotyledons wither
and finally fall off.

The corn embryo also takes in wa¬
ter after it has been planted and its root
pushes through the softened fruit and
seed coats. This forms a temporary pri¬
mary root which is soon added to by
branch roots that develop from the pri¬
mary root and later from the bottom of
the stem. The leaves of the epicotyl,
which are tightly rolled and encased in

a sheath, penetrate the surface of the
soil. After reaching the surface, the
leaves unroll and the stem continues its
growth upward to form the cornstalk.
Neither the hypocotyl nor the cotyle¬
don of a corn grain grows above the sur¬
face of the soil, as was the case in the
germination of the bean (Fig. 26-12).

You need not worry about a root
growing upward and a shoot entering
the soil if you happen to plant a seed
upside down. The lower part of the
hypocotyl has a strong positive response
to gravity, and the epicotyl an equally
strong negative response.

IN CONCLUSION

The flower is an organ specially adapted for reproduction in angiosperms.
Pollen develops in the stamens, and cross-pollination may be brought about by
insects, wind, or water.

After fertilization the flower has served its function. The sepals and
petals fall off (though sometimes the sepals remain as part of the fruit). The
stamens wither, and all that is left is the pistil. Its ovary develops rapidly and
becomes the fruit. Inside the ovary most of the fertilized ovules mature and
become seeds. Each seed contains a tiny embryo and a substantial food
supply. Whether the embrvo plant will ever grow out of its covering depends
on the environmental conditions at its landing place.

BIOLOGICALLY SPEAKING

amylase

germination

pollen tube

anther

hilum

pollination

antipodals

hypocotyl

radicle

calyx

megaspore

receptacle

corolla

megaspore mother cell

seed dispersal

cotyledon

micropvle

self-pollination

cross-pollination

microspore

sepal

dormancy

microspore mother cell

silk scar

egg

ovary

sperm nuclei

embryo

ovule

stamen

embryo sac

pedicel

stigma

endosperm

petal

style

endosperm nucleus

pistil

svnergid

epicotyl

polar nucleus

testa

filament

pollen

tube nucleus

generative nucleus

pollen sac

viability

370 UNIT 4 MULTICELLULAR PLANTS

QUESTIONS FOR REVIEW

1. The sepals and petals of a flower are often spoken of as accessory parts.
What purpose do they serve?

2. How do microspore mother cells differ from microspores in chromosome
content?

3. Describe the formation of megaspores in an ovule.

4. Identify the eight nuclei present in the embryo sac at the time of fertiliza¬
tion.

5. Name three common agents of cross-pollination.

6. Describe the growth of the pollen tube after pollination.

7. Describe the fertilization of the egg and the triple fusion that produces
the endosperm nucleus.

8. What part of the seed is produced from the zygote? from the endosperm
nucleus?

9. Explain the relation between the flower and the fruit.

10. Name several ways in which the fruit serves the seeds that are enclosed
within it.

11. What method of seed dispersal is shown in pod fruits that twist and open
suddenly?

12. Describe some of the various modifications of fruits for dispersal by ani¬
mals.

13. Give several examples of fruits that are dispersed by the wind.

14. Name three conditions required for seed germination.

15. How does the young shoot of the corn plant force its way through the soil
in which it grows?

APPLYING PRINCIPLES AND CONCEPTS

1. Most seed plants produce large quantities of pollen grains. In proportion
to the amount of pollen produced, the number of ovules is very small.
Why?

2. Why do most wind-pollinated trees flower in the early spring?

3. Discuss various characteristics of insect-pollinated flowers that serve as
devices for attraction.

4. Discuss the importance of the pollen tube in reproduction of a flowering
plant.

5. Compare and contrast the gametophyte and sporophyte generations in the
mosses and in the angiosperms. Discuss the evolutionary significance of
the differences.

6. A seed will not germinate until it has enough water to soften the seed
coats. How is this an automatic safeguard against germination during un¬
favorable conditions?

CHAPTER 26 REPRODUCTION IN FLOWERING PLANTS 371

7. Explain why seeds planted in heavy clay soil or set too deep may germinate
very slowly, if at all.

8. Food is stored in many seeds as starch. During germination the starch is
changed to sugar, by the action of the enzyme, amylase. Why is this
change necessary?

RELATED READING

Books

Baldwin, Ernest. An Introduction to
Comparative Biochemistry. Cam¬
bridge University Press, New York.
1962

Bold, Harold Charles. Morphology of
Plants. Harper and Row, New
York. 1957

Bonner, James and Galston, Arthur W.
Principles of Plant Physiology.
W. H. Freeman & Co., San Fran¬
cisco, Calif. 1952

Brook, Alan. The Living Plant. Aldine
Publishing Company, Chicago.
1964

Coulter, Merle. The Story of the Plant
Kingdom. The University of Chi¬
cago Press, Chicago. 1964

Cronquest, Arthur. Introductory Plant
Science. Harper and Row, New
York. 1961

Fenton, Carroll Lane. Plants That
Feed Us. The John Day Co., Inc.,
New York. 1956

Galston, Arthur W. The Life of the
Green Plant. Prentice-Hall, Inc.,
Englewood Cliffs, N.J. 1961

Greulach, Victor and Adams, J. Edison.
Plants : An Introduction to Modern
Botany. John Wiley and Sons, Inc.,
New York. 1962

Hill, Albert F. Economic Botany: A
Textbook of Useful Plants and
Plant Products. McGraw-Hill Book
Co., Inc., New York. 1952

Hyde, Margaret O. Plants Today and

Tomorrow. Whittlesey House, Mc¬
Graw-Hill Book Co., Inc., New
York. 1961

Hylander, Clarence J. The World of
Plant Life, 2nd Ed. The Mac¬
millan Co., Chicago. 1956
Kreig, Margaret B. Green Medium.
Rand McNally & Co., Chicago.
1964

Lane, Ferdinand C. All About the
Flowering World. Random House,
Inc., New York. 1956
Meyer, Bernard S. and Anderson,
Donald B. Plant Physiology, 2nd
Ed. D. Van Nostrand Co., Inc.,
Princeton, N.J. 1952
Milne, Lorus J. and Milne, Margery.
Plant Life. Prentice-Hall, Inc., En¬
glewood Cliffs, N.J. 1959
Northern, Henry T. Introductory Plant
Science. The Ronald Press Co.,
New York. 1958

Overbeck, Johannes van and Wong,
Harry J. The Lore of Living Plants.
McGraw-Hill Book Co., Inc., New
York. 1964

Peattie, Donald Culross. A Natural
History of the Trees of Eastern and
Central North America. Houghton
Mifflin Co., Boston. 1950
Platt, Rutherford. 1001 Questions An¬
swered about Trees. Dodd, Mead
and Co., New York. 1959
Riedman, Sarah. World Provider : Phe
Story of Grass. Abelard-Schuman
Ltd., New York. 1962

372 UNIT 4 MULTICELLULAR PLANTS

Sterling, Dorothy. The Story of Mosses,
Ferns and Mushrooms. Doubleday
and Co., Garden City, N.Y. 1955
Stoutenburg, Adrian. Beloved Botanist.
Charles Scribner’s Sons, New York.
1961

Taylor, Norman. 1001 Questions An¬
swered about Flowers. Dodd, Mead
and Co., Inc., New York. 1964
Weisz, Paul. Science of Botany. Mc¬
Graw-Hill Book Co., Inc., New
York. 1962

Went, Frits W. and Editors of Life.
The Plants. Time, Inc., Books,
Chicago. 1964

Wilson, Carl and Loomis, Walter.
Botany, Rev. Ed. Holt, Rinehart
and Winston, Inc., New York. 1957

Articles

Grant, V. “The Fertilization of
Flowers/’ Scientific American. June,
1951

Roller, Dov. “Germination.” Scientific
American. October, 1950
National Geographic Society. The
World in Your Garden. The
Society, Washington, D.C. 1957
Salisbury, F. B. “Plant Growth Sub¬
stances.” Scientific American.
April, 1957

Schecken, V. “Plant Hormones.” Sci¬
entific American. May, 1949
Wald, George. “Life and Light.” Sci¬
entific American. October, 1959
Zahl, P. S. “The Evolution of Sex.”
Scientific American. April, 1949

UNIT FIVE

BIOLOGY OF THE
INVERTEBRATES

You are about to explore that great portion of the animal kingdom that lacks a
backbone. Among these many and diverse forms of life, known as the inverte¬
brates, you will find a variety of body structures and ways of life. The biologist
views invertebrates with interest because they provide living answers to many cjues
tions about animal life. Where did animal tissues originate? \\ hen did hearts
and circulatory systems, nervous systems, and digestive systems first appear? \\ hat
is the origin of kidneys? Somewhere among the invertebrates \ou may find these
and other answers to important questions about life.

CHAPTER 27

SPONGES AND
COELENTERATES

The advantage of association. Do cells
living in close association with one an¬
other have any advantage over those liv¬
ing independently? Perhaps we can an¬
swer this question by reviewing the
story of Robinson Crusoe. After Cru¬
soe’s ship was wrecked, he found him¬
self alone on an island. He had to
catch and prepare his food. He had to
make his clothes and shoes. He had to
build shelter and protect himself from
any enemies. Even though he learned
how to do all these things, he could not
devote enough time to any one job to
excel in it. He was like a protozoan, in
which one cell must perform all life ac¬
tivities without the aid of any other
cells. On the other hand, if there had
been ten men shipwrecked with Crusoe,
each could have specialized in a job.
One could have hunted food while an¬
other was building a house or making
clothes for the group. They could have

formed a small society having much
greater efficiency than that of Crusoe’s
simple solitary existence. The many-
celled animals that you will study in this
unit and the next have just such an ad¬
vantage over the protists — their cell
specialization permits greater efficiency
and in turn greater complexity.
Division of labor and interdependence.
As you learned in your study of the cell
in Unit One, increase in numbers allows
for division of labor , as it would have
on Robinson Crusoe’s island. In the
multicellular organisms, different cells
become specialists in performing certain
functions for the benefit of all the cells.
The modification of a cell to perform a
certain activity is specialization. As we
study animals, from the simplest to the
most complex, we shall find increasing
cell specialization. As a result of this
specialization, the animal is better able
to adjust to its environment.

Division of labor and cell specializa¬
tion result in the dependence of cells on
one another, or the situation called in¬
terdependence. The man who devotes
his life solely to one task is likely to lose
the ability to do other necessary things.
He depends on other specialists for help
in those phases of his life. When cells
are specialized for one activity, they be¬
come dependent on other cells for other
activities. The ameba can live inde¬
pendently in a pond. But a muscle,
nerve, or bone cell cannot live independ¬
ently when removed from your body.
Cell specialization is carried to the high¬
est degree in the vertebrates , the sub¬
phylum to which man belongs. The
vertebrates are animals with backbones;
the invertebrates , which you will study
in this unit, are animals without back¬
bones.

The animal phyla. In man’s attempt to
study and understand living things, he

374

CHAPTER 27 SPONGES AND COELENTERATES 375

searches for regularities. The regulari¬
ties of living things are their character¬
istics. For example, the backbone is a
characteristic conveniently used to sepa¬
rate the vertebrates from the inverte¬
brates. Since about 95 percent of the
animal kingdom is composed of inverte¬
brates, other characteristics must also be
used in classification. As you will learn,
these regularities are important to the
biologist in many ways. Similarities
may indicate evolutionary relationships,
but it should also be kept in mind that
all living things must satisfactorily per¬
form the life functions in order to sur¬
vive. Therefore similarities also indi¬
cate common solutions to the problems
of staying alive. As you read about the
various animals, ask yourself — What
important structural features place this
animal in its group? Where does it
live? How does it satisfy its organic
needs? How does the environment af¬
fect it? How does it affect other organ¬
isms in the environment?

We shall study ten large phyla (fy-
la) of animals. As you go along, you
may wish to use the Appendix — a Clas¬
sification of Organisms, found in the
back of the book, for reference and re¬
view of the characteristics of each group
and representative examples from them.
You will notice that there are many
more than ten animal phyla, and that
there are certain classes within the ten
phyla that we do not discuss. We shall
limit our study to the largest and most
important phyla, beginning our study
with the sponge phylum, the simplest
in the animal kingdom.

The sponges. Most sponges are marine,
although there are a few fresh-water spe¬
cies. Living sponges may vary in color
from white, gray, brown, red, orange,
and yellow to purple and black. They
may live singly or in colonies so massed

that they make an encrusting layer over
the surface of a rock. Sponges vary in
size from a fraction of an inch to two
yards in diameter.

When first looking at a living
sponge, you might conclude, as Aris¬
totle did, that it is an interesting plant.
You might change your mind if you put
a drop of India ink in the water near
the sponge. You would see the ink
particles pass into the body of the sponge
and then reappear as if being forcibly
expelled from the largest pore. The
sponge, then, is doing something to set
up currents. If you had time, a good
microscope, and several sponges, you
could determine that water passed into

27-1 Water is continually drawn into the
sponge by the flagella of the collar cells. It
passes through small pores into the spongo-
coel and out through the osculum.

376 UNIT 5 BIOLOGY OF THE INVERTEBRATES

27-2 The living brain cora I
and sea anemone shown
are invertebrates which
grow in the warm waters
around the Bahama Is¬
lands. (Walter Dawn)

the sponge's body through small incur¬
rent pores and out the larger excurrent
pore , or osculum (osk-kyoo-lum) . Fig.
27-1 shows the direction of water flow.
Because of their many pores, the
sponges are grouped into the phylum
named Poriferay which means “pore-
bearing."

Usually we think of an animal as
being visibly and actively engaged in
pursuing, catching, and eating its food.
The sponge, however, is sessile, which
means it is attached by the base, and
must attract its food. A single sponge
may circulate nearly a quart of water
through its body every hour. Diatoms,
small protozoans, bacteria, and other or¬
ganic particles are brought to the
sponge by the water. The sponge acts
as a living filter by removing food and
oxygen from the water. Carbon dioxide
and wastes leave the sponge in the wa¬
ter that passes out the excurrent pore.

Let us take a closer look at the
sponge to see how it is adapted for its
submerged, sedentary way of life. A
simple sponge consists of a hollow body,

whose wall contains many tubes. The
wall is formed by two layers of cells
separated by a jellylike substance, loose
cells, and spicules (spik- yools). The
spicules are noncellular structures that
form the skeleton and give support to
the body of the animal. The classifica¬
tion of the sponges is based on the com¬
position of these spicules. Some
sponges have spicules composed of sili¬
con, some of lime, and others of a tough
but flexible substance called spongin.

The outside layer or epidermis is
protective. Many cells of the inside
layer have curious cells with flagella
projecting through them. The flagella
of these collar cells set up the currents
that draw water into the sponge. As
food particles enter, they are caught by
the collars and are digested by enzymes
within the collar cells. From here di¬
gested food is absorbed by cells called
amebocytes, because they resemble ame-
bas. The amebocytes wander through¬
out the jellylike layer, transporting di¬
gested food and oxygen to the other
cells. These wandering cells also carry

CHAPTER 27 SPONGES AND COELENTERATES 377

wastes and carbon dioxide to the collar
cells for disposal. Although sponges are
often called loose aggregations of cells,
you can see that there is sufficient inter¬
dependence to classify them as real mul¬
ticellular animals.

Reproduction in the sponges. Sponges
reproduce both sexually and asexually,
in two ways. They may form buds ,
which are groups of cells that enlarge
and live attached to the parent for a
time, then break off and live independ¬
ently. Gemmules (jem- yoo-ls) are cell
masses surrounded by a heavy coat of
organic material. During periods of
freezing temperatures or drought, they
are formed from little groups of amebo-
cytes and a few spicules which break off
from the parent sponge. When favor¬
able conditions return, each gemmule
grows into another sponge. Reproduc¬
tion bv gemmules is usually character¬
istic of fresh-water species.

Sexual reproduction occurs when
eggs and sperm are produced. The
sperm are shed into the water and enter
another sponge through the incurrent
pores. They are taken into the cyto¬
plasm of the collar cells and then trans¬
ferred to the ova by the wandering ame-
bocvtes.

Sponges are able to regenerate
missing parts. For this reason sponge
growers are able to make sponges mul¬
tiply by cutting them in pieces, which
are then placed in special growing beds.
This is not a method of reproduction
for it is not normally used by the
sponge; this method is utilized by com¬
mercial sponge fishermen because it is
much faster than budding.

Fresh-water sponges are small and
of no commercial importance. Some
marine sponges with spongin skeletons,
especiallv those in warmer oceans, grow
to be verv large and are collected by

27-3 Simple sponges may be found growing
in colonies in warm waters. (Walter Dawn)

divers and by drag hooks. The sponges
are then piled on shore or hung on
the rigging of the boat until the flesh
has decayed. The remaining spongin
skeletons are washed, dried, sorted, and
sometimes bleached. They are then
ready for marketing. Famous sponge
fishing grounds include the Mediterra¬
nean and Red seas, the waters around
the West Indies, and Tarpon Springs,
Florida.

The coelenterates. Many a pleasant
swim in the ocean has turned into a
painful experience when the swimmer
was stung by a jellyfish. Strange indeed
are these pulsating creatures which bob
around in the ocean currents, dangling
long, stringy tentacles under a floating,
inflated sac. The phylum Coelenterata
(si-Zenf-e-ra-ta) also includes the hy-

378 UNIT 5 BIOLOGY OF THE INVERTEBRATES

27-4 Sea anemones live in tidepools on
rocky shores. (Ewing Galloway)

droids, corals, sea fans, sea anemones,
and Portuguese man-of-war. Members
of this phylum vary in size from micro¬
scopic forms to the largest jellyfish of
the North Atlantic, which may reach 12
feet in diameter. Coelenterates live ei¬
ther as individuals or united to form
colonies. All of these animals are
aquatic — a few live in fresh water but
most are marine.

The hydra. The characteristics of this

phylum can be seen in Hydra, a genus
of common fresh-water coelenterates.
There are white, green, and brown
species, which live in quiet ponds,
lakes, and streams. The body of the
hydra consists of two cell lavers separated
by a jellvlike material called mesoglea
(mez-uh-g/ee-a) . The outside layer is the

ectoderm , and the inner layer is the en-
doderm (Fig. 27-5). The baglike body
of the animal has a single opening that
is surrounded by tentacles bearing sting¬
ing cells, called nematocysts . Stinging
cells are found only in the coelenterates.
Hydras vary in size from less than a half
inch, including the tentacles, to about
an inch and a half. They are attached
to rocks or water plants by a sticky secre¬
tion of their basal disks. Because of
their small size, transparency, and habit
of contracting into little knobs when
disturbed, hydras are often overlooked.
Yet they are abundant and are the only
really successful organisms among the
few members of their phylum that have
invaded fresh water. A hydra mav

J J

leave one place of attachment and float
or move to another, or secrete a bubble

27-5 The external and internal structures of
the hydra. The animal has two layers of
cells with a jellylike material between them.
Two stinging cells are shown enlarged at
the upper right.

CHAPTER 27 SPONGES AND COELENTERATES 379

27-6 The hydra moves by a kind of somersaulting. It bends over, attaches its
tentacles to the bottom, loosens its basal disk, swings the disk over its mouth,
then attaches itself to the new bottom area. After loosening its tentacles
it repeats the process.

at the base and float to the surface up¬
side down. The hydra also moves in an

odd somersaulting fashion (Fig. 27-6).
How the hydra gets food. When a
small animal comes in contact with one
of the tentacles of a hydra, many nema-
tocysts explode and pierce the victim’s
bodv with tinv hollow barbs. Since

J J

each barb is attached to the tentacle by
a thin thread, the combined effect of
many threads prevents the escape of the

hapless victim. At the base of each
barb is a small poison sac which dis¬
charges its contents through the hollow
barb and into the prey, thus paralyz¬
ing it.

Once the prey has been stilled, the
tentacles bend inward and push it
through the circular mouth and into
the body cavity. Specialized endoder-
mal cells line this space and function in
digestion and absorption. For this rea-

27-7 The hydra uses its tentacles to capture and paralyze its prey. At the
right the victim is shown inside the body cavity. (Charles Walcott)

380 UNIT 5 BIOLOGY OF THE INVERTEBRATES

son the space is called a gastrov oscular
cavity. The digestive wastes are then
expelled through the mouth. The
mouth and digestive cavity of the coe-
lenterate are considered to be advances
over the feeding structures of the
sponge as they allow a greater range in
the types and sizes of food available to
the animal.

The behavior of the hydra. We have
now observed one reaction of a coelen-
terate which shows a definite advance
over the more primitive sponge. The
tentacles are coordinated in catching
and pushing food into the mouth. Also,
if you touch a tentacle of an extended
hydra with a needle, all the tentacles and
the body contract suddenly. The stimu¬
lus to one tentacle travels to cells of the
other tentacles and to the body through
a series of nerve cells, called the nerve
net , which lies in the mesoglea. The
contraction itself is accomplished by
slender fibers lying in the ectoderm, and
these can be compared to the muscle
cells of higher animals. The hydra has
no real nervous system such as that
found in higher animals, and it has no
brain.

Reproduction in the hydra. The hydra
accomplishes asexual reproduction by
forming buds. A bud appears first as a
knob growing out from the side of the
adult. Later this knob develops tenta¬
cles, and after a period of growth it
separates from the parent and lives in¬
dependently. In this method of repro¬
duction the bud is a small outgrowth of
endoderm and ectoderm and is capable
of growing into a new organism (Fig.
27-5 ) . In a plant, the bud is an unde¬
veloped shoot which will elongate the
stem or develop leaves or a flower; but it
will not become a whole new organism.

Also, like the sponge, the hydra has
remarkable powers of regeneration. If

a hvdra is cut into pieces, each piece
will regenerate the missing parts and be¬
come a whole animal.

Sexual reproduction usually occurs
in autumn. The eggs are produced
along the body wall in little swellings
called ovaries; the sperm cells are formed
in similar structures called testes.

The egg is fertilized in the ovary,
and the zygote grows into a spherical,
many-celled structure with a hard, pro¬
tective cover. In this stage it leaves the
parent and goes through a rest period
before forming a new hydra.

Two ways of life. The body form of a
coelenterate may be one of two types.
The polyp form is well illustrated by
the hydra, with its tubular body attached
to the bottom at one end with tentacles
at the other end. The bell-shaped, free-
swimming form found in the jellyfish is
called a medusa. A medusa swims by

27-8 The mouth of the jellyfish is in the
center of the "bell,” but is usually obscured
by the tentacles, which sting the victim to
be eaten. (Hermes from National Audubon
Society)

CHAPTER 27 SPONGES AND COELENTERATES 381

27-9 The life cycle of Aurelia is shown in this diagram.

taking water into the cavity of the bell
and then forcibly ejecting it. This jet
propulsion produces a jerky movement
through the water.

An interesting example of a coe-
lenterate that occurs in both the medu¬
sa and the polyp forms may be found in
Aurelia , a common jellyfish. The sex¬
ually-reproducing medusa has a scal¬
loped margin from which protective ten¬
tacles hang (Fig. 27-9). The male me¬
dusas shed sperm into the sea where the
sperm may enter the gastrovascular cav¬
ity of a female. Here they fertilize eggs
which have been released by the fe¬
male. The zygotes are protected for a
short time by folds of tissue surround¬
ing the mouth.

When the voung are released, they
are small, oval-shaped, ciliated larvae
called planulae (sing, planula). The
planula is the beginning of the asexual

phase in the life cycle of Aurelia. After
swimming about for a short period of
time, it attaches to a rock or seaweed,
develops tentacles, begins feeding, and
is then a polyp. The polyp grows and
forms more polyps by buds that form
at the base. In fall and winter, how¬
ever, the polyp elongates and forms
manv horizontal divisions until it re¬
sembles a pile of saucers. One by one
the uppermost “saucer” breaks loose and
swims away to develop into the adult
sexually-reproducing medusa. The Au-
relia is thus one coelenterate having two
different forms — the polyp and the me¬
dusa — both of which reproduce.

Other coelenterates. The coral is the
onlv coelenterate of economic impor¬
tance. Its body is a small, flowerlike
polvp only a fraction of an inch long.
Most coral polyps live in colonies and
build skeletons of lime which they ex-

382 UNIT 5 BIOLOGY OF THE INVERTEBRATES

27-10 The drawings show three different kinds of coral reefs. Left: the marginal
type grows around an island. Center: the barrier reef that is widely separated
from the land. Right: an atoll with an open lagoon.

tract from the sea. This lime skeleton
is firmly cemented to the skeleton of a
neighboring polyp. When one animal
dies, its skeleton remains for the attach¬
ment of another. Lime skeletons of
corals thereby increase in size until a
single mass may support many thou¬
sands of animals living on the surface
of the skeletons of their ancestors. In
some species, these masses are solid,
while other corals build delicate and
intricate fan-shaped structures.

Over a period of time, large coral
reefs are built up. These are most com¬
mon in the warm shallow oceans and
may be of three types: 1. the marginal
type, close to the beach; 2. the barrier
type, forming a ring around an island
with a wide stretch of water between
the beach and the reef; or 3. a ring with
an open lagoon in the center, called an
atoll (Fig. 27-10).

The Great Barrier Reef off the
northern coast of Australia extends
about 1,260 miles parallel to the coast.
It is about 50 miles wide. The extent
of coral formations became apparent
during World War II when coral was

found to be useful in the construction
of airstrips and roads. Coral jewelry
may be found in shops all over the
world. Some corals are also bleached
and dyed various colors to be used in
flower arrangements or for decorations
in homes.

After storms on the Pacific coast,
the beaches may be covered with bluish
membranelike animals measuring two
or three inches in length. They are
called purple sails, or Velella. Related
to the purple sail is the Physalia, or Por¬
tuguese man-of-war. These two organ¬
isms are actually colonies of coelenter-
ates hanging from a float which keeps
them near the surface and is moved
through the water by the wind. The in¬
dividual organisms are polyps, and each
has a special function to perform for the
colony. Some are feeding polyps and
digest food caught by the food-getting
polyp, while other polyps function in
gamete production. Although the Por¬
tuguese man-of-war is largely tropical or
semitropical, it is found in the Gulf
Stream and occasionally drifts to the
British coast.

CHAPTER 27 SPONGES AND COELENTERATES 383

IN CONCLUSION

Although the sponges are considered multicellular animals, they are not as
advanced as the coelenterates, in which you have observed cell specialization
allowing for coordinated action. With the development of tissues, the cells
become almost completely dependent upon the various activities and functions
of other cells.

In the next chapter we shall continue our consideration of the inverte¬
brate phyla with the three groups classified as worms. As you will see, an
elongated body is about the only characteristic the various worms have in
common.

BIOLOGICALLY SPEAKING

amebocyte

excurrent pore

nerve net

basal disk

gastrovascular cavity

ovaries

buds

gemmule

polyp

Coelenterata

incurrent pore

Porifera

collar cell

invertebrate

spicules

ectoderm

medusa

tentacles

endoderm

mesoglea

testes

epidermis

nematocyst

vertebrate

QUESTIONS FOR REVIEW

1. How does the multicellular condition permit such an efficient division of
labor?

2. What are the major structural characteristics which distinguish the sponges
from the coelenterates?

3. In what kinds of environments would you expect to find specimens of
sponges? Where would you expect to find specimens of hydra? Do you
think the size of the animal body has anything to do with the particular

habitats of these two groups of organisms?

4. Compare the feeding methods of a sponge and a hydra. How are they

alike and how do they differ?

5. Give examples of regeneration in sponges and coelenterates.

6. In what ways are the Aurelia and hydra similar? In what ways are they

different?

7. What is the function of the nematocysts?

8. Describe the formation of a coral reef. What are the three types of coral
reefs?

9. Why are the purple sails and the Portuguese man-of-war considered to be
so much more highly specialized than any of the other coelenterates you
have studied thus far?

384 UNIT 5 BIOLOGY OF THE INVERTEBRATES

APPLYING PRINCIPLES AND CONCEPTS

1. Compare the ways in which cell specialization is similar to division of labor
in modern civilization.

2. Why is it essential that a jellyfish be hollow?

x How is knowledge of the regeneration capabilities of sponges used com¬
mercially?

4. How do you account for the fact that some sponges die when exposed
briefly to air, even when promptly returned to water?

5. Would you consider a sponge to be a more primitive form of life than a
coelenterate? Give reasons for your answer.

6. How do the Porifera and Coelenterata spread to new habitats? What
conditions do you believe would be most favorable for their growth?

CHAPTER 28

THE WORMS

through the diameter of the disk and
through the central axis. Some simple
sponges, most coelenterates, and most
adult echinoderms (which we shall
study in Chapter 29) are radially sym¬
metrical. This type of symmetry allows
the stationary animal to encounter food
and enemies from all sides.

Bilateral symmetry is exhibited by
the higher animals. It means, literally,
“two-sided shape.” Only one plane can
separate animals with this kind of sym¬
metry into two similar parts. This plane
must pass through the longitudinal axis,
through the center of the back, and
through the center of the front. This
would not divide the animal into iden-

Body form in organisms. Let us begin
our study of the worms by reviewing the
general forms of organisms. Those that
have no definite shape, such as the
ameba and many sponges, are said to be
asymmetrical All other organisms have
a definite form and are symmetrical.
The radiolarians and Vo/vox are pro-
tists with spherical symmetry. They
may be divided into two equal parts by
any plane passing through the diameter
of the body. A baseball is a good ex¬
ample of a spherically symmetrical ob¬
ject. Organisms with spherical sym¬
metry often lack an efficient mode of
locomotion and usually float on or near
the surface of the water.

Radial symmetry is best demon¬
strated by the sea anemone, which is
pictured in Fig. 28-1. This coelenterate
has a central disk from which tentacles
radiate out like spokes of a wheel. The
central axis is formed by the mouth and
passes through the center of the body.
The animal may be divided into two
equal parts by any plane that passes

28-1 Three types of symmetry. Top: spheri¬
cal; middle: bilateral; bottom: radial.

385

386 UNIT 5 BIOLOGY OF THE INVERTEBRATES

tical parts, but the pieces would be
mirror images. Animals with bilateral
symmetry have a definite right and left
side; they have an upper, or dorsal , sur¬
face and a lower, or yentral , surface;
they have a definite anterior and pos¬
terior end. All the vertebrates and
many invertebrates have this type of
shape. The anterior end usually con¬
tains a concentration of nervous tissue
and the sense organs for testing the en¬
vironment.

Diversity among the worms. The
worms we shall study are divided into
three phyla (see Appendix). The least
complex of these is the Platyhelmin-
thes (plat-i-heZ-minth-ez), or flatworms.

These flat-bodied animals have two

•

layers of cells, the ectoderm and endo-
derm, like the coelenterates, but in ad¬
dition they have a middle layer called
the mesoderm. All the tissues and or¬
gans of the body develop from these
three layers of cells. This will also be
true in all the other animals we shall
study, including man. The flatworms
consist of three classes: the Turbellaria,
the Trematoda, and the Cestoda.

A free-living flatworm. The Turbellaria
are the free-living flatworms of which
Planaria is the most common example.
When biologists say that an organism is
free-living, they mean that it is not
parasitic. Planarians are aquatic and
are found under stones in fresh-water
ponds and streams. The next time you
are out collecting, tie a string to a piece
of liver and leave it on a rock for a few
hours. Bring the rock into class and put
it in an aquarium filled with pond water.
You will probably find many of these

28-2 The center drawing shows the diges¬
tive and nervous systems of the planarian
and that at the bottom a cross-sectional
view of the animal.

worms crawling on the glass of the
aquarium the following day.

Planarians range from a quarter to
a half inch long and vary in color from
black or brown to white. They are
bilaterally symmetrical, blunt at the an¬
terior end and pointed at the posterior.

Intestine

Pharynx-

Opening of
pharynx

Longitudinal
nerve

Transverse nerve

POSTERIOR

ANTERIOR
Eyes pot

Brain

CHAPTER 28 THE WORMS 387

The two eyespots on the anterior end
are responsible for its nickname, “the
cross-eyed worm.” These eyespots are
photosensitive — that is, light striking
them stimulates nerves in this area and
the animal can act accordingly. Pla-
narians usually avoid bright light, and
are therefore said to be negatively pho¬
totropic ( foht-uh-trohp-ik ) .

The pharynx (far- inks) is a tube
located on the ventral surface of the
animal (Fig. 28-2). When extended,
this tube sucks up microscopic particles
which may include tiny living organisms.
Since planarians clean up the water by
eating organic matter, they are consid¬
ered scavengers. When food is drawn
into the digestive cavity, it enters one of
the three main branches of the intestine
and then passes to one of the smaller
side branches. Cells lining the intestine
receive the food and digest it in vac¬
uoles. Digested food passes to all body
tissues by diffusion. Indigestible waste
materials are excreted through the phar¬
ynx and mouth opening. Cellular
wastes are collected by tubules that
branch throughout the animal and open
to the surface by several tiny pores.

Compared to the animals we have
studied so far, the nervous system is well
developed in planarians. A mass of
nervous tissue, the brain, lies just be¬
neath the eyespots. The concentration
of nervous tissue at the anterior end is
called cephalization (sef-a-\i-zay-shun) .
Many nerves from the anterior region
lead directly to the brain. In a bilater-
ally symmetrical organism this arrange¬
ment of nerves is very important. As
the worm moves, its head end is able
to receive stimuli of chemicals, water
currents, touch, light, and heat. The
planarian is thus able to test the en¬
vironment so that if unfavorable con¬
ditions exist, it can turn around. For

this reason the concentration of recep¬
tors (receivers of stimuli) and nerves in
the anterior end of a bilaterally sym¬
metrical organism is of survival value.

The longitudinal nerves run along
either side of the body near the ventral
surface. These nerve cords are con¬
nected by transverse nerves, giving the
nervous system a ladderlike appearance.
From the surface area many small nerves
go to the longitudinal nerves. This type
of nervous system allows the planarian
to have coordinated movement, as well
as to respond to stimuli on all parts of
the body.

When watching planaria in a glass
dish or an aquarium, you will see that
they have two distinct motions. One is
a muscular movement whereby the an¬
terior end of the body moves from side
to side. The other is a forward gliding
motion that is accomplished by almost
imperceptible muscular contractions
aided bv cilia on the ventral surface.

Reproduction in the planarian is ac¬
complished either asexually by fission
or sexuallv by gametes. Each animal is
hermaphroditic , which means that it
possesses both male and female repro¬
ductive organs. Cross-fertilization oc¬
curs, and the eggs are then shed in cap¬
sules. The capsules, usually containing
less than ten eggs, are often attached to
rocks or twigs in the water. In two or
three weeks the eggs hatch and minute
planarians emerge.

Like the sponges and many coelen-
terates, some planarians have remarkable
powers of regeneration. Some will re¬
generate complete worms from almost
any piece (Fig. 28-3). The parasitic
flatworms, however, have no ability to
replace lost parts. This is true of parasit¬
ic animals in general.

The parasitic way of life. Usually when
we speak of evolution we think of

388 UNIT 5 BIOLOGY OF THE INVERTEBRATES

28-3 Regeneration in the planarians.

forms increasing in complexity as they
adapt to environmental changes. But
a parasite living within the body of
another animal is faced with problems
not at all like those of its free-living rela¬
tives. The size of a parasite is limited
by the size of its host. If it were to
grow to a large size, it would kill its host
and find itself without a home. Intesti¬
nal parasites usually have hooks or suck¬
ers by which they can cling to the walls
of the intestine. This feature prevents
them from being swept away with the
movement of the intestinal contents.
The environment literally bathes the
parasite with already digested food from
which it can merely absorb its nutrients.
The parasite is protected against being
digested itself by a thick, resistant cu¬
ticle which its free-living relatives may
not have.

Since certain systems are reduced
or lost in the parasitic worms, we say
that the worms have degenerated. The
tapeworm, for example, has no digestive
system. This is actually a benefit to

the parasite, as it does not need a diges¬
tive system, and the resulting space may
be used for developing eggs.

The way in which the parasitic
worms develop is called a life history.
Dispersal is a problem to the internal
parasite, and larval forms may be free-
living or may involve a stage of life with¬
in another kind of organism. Let’s look
at the life histories of some common
parasitic worms.

The flukes. The class Trematoda in¬
cludes the flukes , which are parasites in
many animals, including man. Flukes
differ from planarians in that they have
no external cilia in the adult. Since cilia
are structures of locomotion, their lack
is in keeping with the parasitic way of
life. Flukes have a thick cuticle and
one or more suckers , which are used to
cling to the tissues of the host. The an¬
terior sucker surrounds the mouth, which
opens to a short pharynx. Although
the nervous system is similar to that of
the planarian, there are, as might be ex¬
pected, no special sense organs. Most
of the flukes have a highly developed
reproductive system and are hermaphro¬
ditic. Besides the fact that this sys¬
tem occupies a larger percentage of the
body, the fluke differs from the pla¬
narian in having a uterus. The uterus is
a long coiled tube which stores large
numbers of eggs until they are ready to
be discharged through the genital pore.

The flukes have complicated life
histories, usually involving a snail and
one or more other hosts. The sheep-
liver fluke, for instance, lives as an adult
in the liver and gallbladder of sheep.
Eggs pass from the gallbladder to the
intestine. If they fall into water after
they are eliminated by the sheep, they
hatch into young worms called larvae
(sing, larva), which enter the body of a
particular kind of snail. In the snail,

CHAPTER 28 THE WORMS 389

28-4 The life cycle of the sheep liver fluke.

the larvae pass through several stages
during which they increase in number
by asexual reproduction. They then
leave the snail, crawl on blades of grass
along the water, and form a cyst , or rest¬
ing stage. If a sheep eats this cyst, the
fluke enters the sheep’s liver and the
cycle starts again (Fig. 28-4).

Reactions of the host to the liver
fluke may cause inflammation, swelling,
general sickness, and irritability. In
cows, milk production is reduced. Nat¬
urally, fluke infestation renders the liver
unfit for human consumption. Other
types of flukes may live in the blood, in¬
testines, or lungs of animals. Many
flukes are found on the external gills of
fish and in cavities of other aquatic ver¬
tebrates, living on the epithelial tissue
and blood of these hosts.

Although human fluke infestations
are most common in the Orient, they
are found in other areas too. Cuba has
had several epidemics of human liver
fluke. In some areas of the Gulf Coast,
economic loss by liver-fluke infection of
cows, pigs, and sheep has been severe.
The best control of flukes is to elimi¬

nate one of the hosts in their life cycle.
Usually this is a snail of some particular
species.

The tapeworms. The tapeworms, the
best known of the parasitic flatworms,
are members of the Cestoda (ses-toh-
da), a third class of Platyhelminthes.
An adult tapeworm has a flat, ribbonlike
body and is grayish white in color (Fig.
28-5). The adult tapeworm has no
cilia. The knob-shaped head is equipped
with suckers and in certain species with
a ring of hooks. Below the slender neck
a number of nearly square sections ex¬
tend to a length of as much as 30 feet.
The worm grows by adding new sec¬
tions. Since new body sections, or pro-
glottids (proh-g/u/it-ids) , are formed at
the head end, the oldest sections are on
the posterior end. The proglottids are
essentially masses of reproductive organs.
Tapeworms are hermaphroditic, and
eggs formed in a proglottid are fertilized
there. When the eggs mature, the pro¬
glottids break off and are eliminated in
the feces [fee- seez] (solid wastes elimi¬
nated from the intestines). Proglottids
released in this way may be eaten by

390 UNIT 5 BIOLOGY OF THE INVERTEBRATES

28-5 The tapeworm can infect man only when he eats meat that is insufficiently
cooked. Rigid inspection of meat has reduced the number of victims of this
parasitic worm.

some animal such as a pig or cow. In
the body of the new host, the eggs hatch
into larvae, which burrow into the mus¬
cles and form cysts.

Tapeworms enter the human body
in the cyst stage when the improperly
cooked flesh of an infested animal is
eaten. Each cyst contains a fully devel¬
oped tapeworm head. In the human
intestine the head is released from the
cyst, attaches itself to the intestinal wall,
and begins to grow.

Since a tapeworm robs the host of
nourishment, the victim may lose
weight and vitality. In recent years hu¬
man tapeworm has been decreasing be¬
cause of improved detection and treat¬
ment and because meat is inspected for
tapeworm cysts.

The roundworms. The phylum Nem-
atoda (nem- a-toh-da) has only one
class, also called Nematoda. The
roundworms are long, slender, smooth
worms, tapered on both ends. They may
be as short as 1/125 inch or as long as
four feet. They occur in soil, fresh wa¬
ter, salt water, and as parasites in plants
and animals. The parasitic round-
worms include the hookworm, trichina
worm, pin worm, whipworm, A scaris
(tfs-ka-ris), and guinea worm. The nem¬
atodes are so abundant that —

If all the matter in the universe ex¬
cept the nematodes were swept away,
our world would still be dimly recog¬
nizable, and if, as disembodied spirits,
we could then investigate it, we should
find its mountains, hills, vales, rivers,
lakes, and oceans represented by a film
of nematodes. The location of towns
would be decipherable, since for every
massing of human beings there would
be a corresponding massing of certain
nematodes. Trees would still stand in
ghostly rows representing our streets
and highways. The location of the
various plants and animals would still
be decipherable, and, had we sufficient
knowledge, in many cases even their
species could be determined by an ex¬
amination of their erstwhile nematode
parasites. (Buchshaum, Ralph. Ani-
mals Without Backbones. The Uni¬
versity of Chicago Press, 1948.)

When you consider that over one
third of the human race, mostly in
warm regions of the world, is infested
with parasitic roundworms, their impor¬
tance can hardly be overestimated.
Harmless roundworms include the vine¬
gar eel and the numerous beneficial soil
nematodes.

Like the flatworms, the round-
worms are bilaterally symmetrical and
have three cell layers. They are, how¬
ever, more complex than the flatworms.
Their digestive system is a distinct tube

CHAPTER 28 THE WORMS 391

with an opening at each end, housed
in a long body which is also a tube (Fig.
28-6). This arrangement enables the
animal to take in food through an an¬
terior opening, the mouthy to digest it,
and to remove the usable parts as it
passes along the canal. Finally the un¬
digested material is eliminated through
a posterior opening, the anus. In the
hydra, one opening was used for both
the entrance of food and the elimina¬
tion of undigested substances. The ar¬
rangement of a tube within a tube, how¬
ever, makes possible the orderly pro¬
gression of material through the diges¬
tive tract and greater efficiency in han¬
dling it.

A scans is a large roundworm which
lives in the intestine of pigs, horses, and
sometimes man. The females are larg¬
er than the males, and may reach a
length of nearly 12 inches. A scans eggs
enter the human in contaminated food
or water. They do not hatch in the
stomach, but begin to hatch in a few

28-6 Side views of the male and female As-
caris are shown on the left. The top and
right drawings are diagrams of a dissected
female worm.

hours when inside the small intestine.
Once hatched, the larvae bore into the
intestinal wall to begin a ten-day jour¬
ney through the body. This journey
carries them into the blood stream and
to the lungs. When they reach the
lungs, these worms pass into the air
passages, up to the throat, and are swal¬
lowed, once again passing into the di¬
gestive tube. There they grow to ma¬
turity in about two and a half months.
After fertilization, the eggs are sur¬
rounded by a thick, rough shell and
passed out through the genital pore of
the female worm. A mature female lays
about 200,000 eggs each day. These
eggs pass out of the body of the host
with the feces, and the cycle continues.
A scans seems to be relatively harmless
in man, although occasionally a large
number of the adult worms twist to¬
gether and block the intestine and cause
death.

The hookworm of the southern
states and all semitropical and tropical
regions is a far more serious health men¬
ace. Larvae develop in the soil and en¬
ter the body by boring through the skin
of the feet. Then they enter the blood
vessels and travel through the heart to
the vessels of the lungs. In the lungs
they leave the blood vessels, enter the
air passages, and eventually reach the
windpipe and travel to the throat as
A scans larvae do. They are swallowed
and pass through the stomach to the in¬
testine, where they attach themselves
to the wall by means of jaws. In the
intestine the larvae grow to adult worms,
which suck blood from the vessels in
the intestine wall.

Loss of blood lowers the victim’s
vitality by producing anemia. A typi¬
cal hookworm victim may be quite shift¬
less and his growth may be retarded, al¬
though the latter is not always true. In

392 UNIT 5 BIOLOGY OF THE INVERTEBRATES

the intestine the worms reproduce, and
the fertilized eggs leave in the fecal
wastes. If these happen to lodge in
warm moist soil, thev develop into mi¬
nute larvae which can enter through
cracks in the feet and eventually return
to the intestine. Thus, three factors
are responsible for the spread of this
disease: 1. improper disposal of sewage;
2. warm soil; and 3. the practice of go¬
ing barefoot. Public health agencies
have done a remarkable job in reducing
the number of cases in the southern
part of the United States.

The trichina worm , or Trichinella,
is one of the most dangerous of the para¬
sitic roundworms. This roundworm
passes its first stage in the pig, dog, rat,
or cat as a cyst in the muscles. If raw
or uncooked scrap meat is fed to pigs, it
may contain some of these cysts. In
the intestine of a pig that eats infested
scrap meat, these larvae develop into
adult worms, mate, and produce micro¬
scopic larvae which pass into the blood
stream and into muscles where they

again form cysts. When a human eats
undercooked, infested pork, the same
thing occurs: the cysts are released and
the larvae mature in the intestine (Fig.
28-7). Each worm emits into the
blood stream about 1,500 young, which
eventually form cvsts in the muscles of
the human. This disease is known as
trichinosis. One method of preventing
it is to feed hogs only cooked scrap
meat. However, the best way to pre¬
vent this disease, as well as other para¬
sitic worm infections, is to cook all meat
as thoroughly as possible and thereby
avoid any risk.

By this time the importance and
widespread abundance of parasitic
worms should be obvious to you. Each
of these disease-producing worms is
spread and picked up as a result of poor
sanitary conditions. The eggs of the
worms can be killed by proper sewage
disposal. Proper inspection and thor¬
ough cooking of meat are measures that
have reduced the spread of these para¬
sites.

28-7 Diagram of the life cycle of the trichina worm.

CHAPTER 28 THE WORMS 393

28-8 The sandworm, Nereis, is one of the
primitive annelids. (Walter Dawn)

The common earthworm. The seg¬
mented worms are the most advanced
of the worms in body structure. They
belong to the phylum Annelida, which
is commonly divided into four classes
(see Appendix) . Most of the segmented
worms live in salt water; some live in
fresh water; and others, including the
common earthworm, live in the soil.
The annelids seem to be in a half-way
spot between the simple protists and
the highly complicated vertebrates. For
this reason, and the fact that they are
rather common, many biologists study
them closely. They are considered typ¬
ical invertebrates.

If you examine a common earth¬
worm, you will notice immediately that
its body consists of many segments. Y ou
will also see that the anterior end is more
pointed and darker than the posterior.
There is no separate head, nor are there
any visible sense organs. The mouth is
on the anterior end and is crescent-

28-9 In this diagram of the earthworm the
anterior portion is dissected to show the
well-developed nervous and circulatory sys¬
tems.

shaped, lying below a prostomium
(proh-sto/i-mee-um), which is a kind of
upper lip. The vertical slit at the pos¬
terior end, the anus , is the opening of
the intestine. The segments are often
numbered by biologists, starting with the
segment containing the mouth as num-

Seminal vesicles

Seminal

receptacles

Dorsal blood vessel

Crop

Nephridium

Gizzard

ntestine

Mouth

| Clitellum

Prostomium
Mouth cavity
c Brain

Pharynx
Ventral nerve cord

Ventral blood vessel
Esophagus

Aortic arches
(hearts)

394 UNIT 5 BIOLOGY OF THE INVERTEBRATES

ber one, in order to locate definitely any
special structure. On segments 32-37
there is a conspicuous swelling called
the clitellum (kli-te/-um), which is in¬
volved in the animal’s reproduction.

Four pairs of bristles, or setae (see-
tee), project from the under surface and
sides of each segment, except the first
and the last. The setae assist the earth¬
worm in movement and in clinging to
the walls of its burrow, as those who
hunt night crawlers can testify. The
earthworm moves by burying its anterior
setae into the soil, then shortening its
bodv bv a powerful series of longitudi¬
nal muscles which stretch from anterior
to posterior ends. The worm then sinks
its posterior setae into the soil, withdraws
its anterior setae, and pushes forward by
making itself longer. It does this by
constricting the circular muscles , which
are found around the body at each seg¬
ment, so that the worm becomes thin
and long.

As you study this animal you will
notice that it consists not only of many
cells, but of many kinds of specialized
cells. These specialized cells are
grouped together and each group per¬
forms the same function, so that they
make up a tissue. The tissues that are
grouped together to form larger struc¬
tures which perform a definite function
are organs. The earthworm is so ar¬
ranged that a whole series of organs
takes care of some fundamental body
process. These are systems. The earth¬
worm has well-developed digestive, cir¬
culatory, excretory, nervous, and repro¬
ductive svstems.

Digestive system of the earthworm. Be¬
low the prostomium is the mouth of the
earthworm. There are no jaws or teeth,
but the animal sucks in soil containing
food, by its muscular pharynx. The
food particles then pass through a long

Circular muscle

28-10 This is a cross-sectional drawing of
one of the abdominal segments of the earth¬
worm.

esophagus into a round organ called the
crop. This acts as a temporary storage
place for food. From the crop the food
is forced into a very muscular organ
called the gizzard. There, by rhythmi¬
cal contractions, the food is ground up
by grains of sand rubbing the food par¬
ticles. In the intestine, which stretches
from segment 19 to the end of the worm,
complete digestion takes place. En¬
zymes break down the food chemically,
and the blood circulating through the
intestine walls absorbs it.

The complex organs of the diges¬
tive system of the earthworm take up
most of the anterior region. The
earthworm consumes large quantities of
soil, which contains organic matter. The
useless inorganic matter passes through
the system largely unchanged and is de¬
posited on the surface of the ground
in the form of casts. This method of
feeding loosens the soil for air and wa-

CHAPTER 28 THE WORMS 395

ter penetration and is of great impor¬
tance to soil fertility.

Circulatory system of the earthworm.
As food is digested, the blood in the cir¬
culatory system picks it up for distribu¬
tion to all the cells of the body. In the
simpler animals we have studied so far,
the digested food had only to diffuse a
short distance to reach all the cells of
the body. But in higher forms the dis¬
tances are greater and more food ma¬
terial is needed by the many specialized
and very active cells of the body. In
these animals, therefore, we find a
special transportation or distribution
tissue — the circulating fluid called
blood.

The blood of the earthworm moves
through a series of closed tubes, or ves¬
sels. It flows forward to the anterior
end in a dorsal blood vessel and moves
to the posterior end in a ventral blood
vessel. Small tubes connect the dorsal
and ventral vessels throughout the ani¬
mal, except in segments 7-11. There the
five connecting tubes are large and mus¬
cular. Bv alternate contraction and re-

laxation, they keep the blood flowing.

They are not true hearts, but are called
aortic (ay-ort-ik) arches.

Respiration and excretion in the earth¬
worm. The earthworm absorbs oxygen
and gives off carbon dioxide through a
thin skin. This skin is protected by a
thin cuticle secreted by the epidermis
and kept moist by a slimy mucus also
produced by epidermal cells. A moist
surface is necessary for oxygen to be ab¬
sorbed and carbon dioxide to be given
off. If the worm is dried by the sun it
will die because the exchange of these
gases can no longer take place.

Nitrogen-containing waste materials
from cell activities are removed to the
outside of the body by little tubes.
There are two such structures, called
nephridia, in each segment except the
first three and the last. Each corre¬
sponds to a tiny kidney tubule in man.
Earthworm sensitivity. The nervous
system coordinates the movements of
the animal and sends impulses received
from sense organs to certain parts of the
body. There is a very small brain, or
nerve center, in segment 3. From it run
two nerves that form a connecting collar

28-11 Although the earthworm has the reproductive organs of both sexes, it ex¬
changes sperm with another animal as shown in this diagram. Sperm travel
from the seminal vesicles of one worm to the seminal receptacles of the other.

396 UNIT 5 BIOLOGY OF THE INVERTEBRATES

around the pharynx and join to become
a long ventral nerve cord. There are
enlargements called ganglia , or nerve
centers, in each segment. Three pairs
of nerves in turn branch from each gan¬
glion. The earthworm has no eyes or
ears, but is nevertheless sensitive to light
and sound. Certain cells in the skin are
sensitive to these stimuli, and the im¬
pulses are carried rapidly to the muscles
of the earthworm. Think how quickly
they can react to a flash of light at night
when you hunt them for tomorrow’s
fishing!

Reproduction in the earthworm is com¬
plex. Although earthworms are her¬
maphroditic, forming both eggs and
sperm, the eggs of one worm can be
fertilized only by sperm from another
worm. In Fig. 28-11 you will see sem¬
inal vesicleSy which extend from the
testis sacs and store sperm produced by
two pairs of testes within the sacs. The
seminal receptacles store sperm from
another worm.

Sperm from one worm travel from
the seminal vesicles, through openings
in segment 15, to the seminal recepta¬
cles of another worm, through openings
in segments 9 and 10. Here they are
stored until eggs are laid. When the
eggs mature later they pass from the
ovaries through openings in segment 14
and are deposited in a slime ring se¬
creted by the clitellum. As this ring
moves forward, sperm are released from
the seminal receptacles and fertilization
occurs. The slime ring slips from the

IN CONCLUSION

28-12 The leech is another example of a seg¬
mented worm. (Walter Dawn)

body and becomes the cocoon in which
the young worms develop.

The leech — another segmented worm.
The leech (Fig. 28-12), also called a
“bloodsucker,” is an annelid found in
streams and ponds. It is an external
parasite on fish and other aquatic ani¬
mals, but it may attach itself to your
skin while you are swimming.

In sucking blood a leech attaches
itself to some vertebrate by the posterior
sucker, applies the anterior sucker to the
skin, and makes a wound with the aid
of little jaws inside the mouth. The
salivary glands of the leech secrete a
substance that prevents the clotting of
blood while the worm is taking a meal.
Leeches were used frequently for me¬
dicinal purposes in the Middle Ages
and after, when it was thought benefi¬
cial to draw blood. Now the salivary
substance is extracted and used to slow
clotting after surgery.

This chapter has taken you through an interesting part of the animal kingdom.
You have seen animals whose bodies are not limited to the tissue level as are
the coelenterates, but are instead composed on the organ level of development.

CHAPTER 28 THE WORMS 397

The animals we have discussed in this chapter have three cell layers and bi¬
lateral symmetry. Some flatworms are free-living, while other degenerate forms
live as parasites which depend on the body activities of the host organism.
The roundworms are much more abundant than you had perhaps realized.
They too include harmless, free-living forms as well as the parasitic hookworm,
trichina worm, pinworm, whipworm, A scans, and guinea worm. The annelids,
organized at the system level of development, include the common earthworm
and the leech.

In the next chapter we shall study two groups of invertebrates that are
related to the worms and at the same time to higher forms — the soft-bodied
mollusks and the spiny-skinned echinoderms.

BIOLOGICALLY SPEAKING

Annelida

esophagus

posterior

eyespot

proglottids

anus

feces

prostomium

aortic arch

ganglia

radial symmetry

asymmetrical

gizzard

seminal receptacle

bilateral symmetry

hermaphroditic

seminal vesicle

Cestoda

intestine

setae

circular muscle

larva

spherical symmetry

clitellum

longitudinal muscle

transverse nerve

crop

longitudinal nerve

Trematoda

cuticle

mesoderm

Turbellaria

cyst

Nematoda

uterus

degeneration

nephridia

ventral

dorsal

pharynx

ventral blood vessel

dorsal blood vessel

Platyhelminthes

ventral nerve cord

QUESTIONS FOR REVIEW

1. Name and define the types of symmetry. Give an example of each.

2. Discuss regeneration in the planarians.

3. What is the significance of the three layers of cells found in flatworms?

4. How does the planaria test its environment?

5. In what respects are the flatworms more complex than the sponges and
coelenterates?

6. In what way does the tapeworm show degeneration?

7. Where are nematodes found?

8. How do the nematodes show an advance over the flatworms?

9. Describe the life cycle of A scans.

10. How does the trichina worm reach the human body?

398 UNIT 5 BIOLOGY OF THE INVERTEBRATES

11. How does the earthworm move?

12. Trace a particle of food through the digestive system of the earthworm,
naming the organs through which it passes.

APPLYING PRINCIPLES AND CONCEPTS

1. Why do we classify the various worms into three separate phyla?

2. What advantage does bilateral symmetry have over spherical symmetry?
What advantages does it have over radial symmetry?

3. Trace the path followed by Ascaris through the body by naming all the
structures through which it passes. At what stages during its development
are symptoms of disease most likely to be present?

4. Compare the advantages of the nervous system of the annelids over that of
the planarians.

5. What measures are important to take in an effort to control parasitic
worms?

6. Symptoms of tapeworm infestation usually include loss of weight and gen¬
eral tiredness. Account for these conditions.

7. Trichinosis can become a hopeless disease. Why is it almost impossible
to treat?

CHAPTER 29

MOLLUSKS AND
ECHINODERMS

Soft-bodied invertebrates. Although
some mollusks are often called shellfish,
they are really not fish at all. In fact
many do not even have shells. Mollusks
live in fresh water, marine, and terres¬
trial environments. Some are even
adapted for life buried in sand or mud
where the oxygen content may be too
low for a more active animal. In abun¬
dance of species, the mollusks are sur¬
passed only bv the phylum that includes
the insects. Although mollusks have
been used for food, money, eating uten¬
sils, jewelry, buttons, dyes, tools, and
weapons since earliest times, they have
been of value to man in still another
way. Since the shells of mollusks are
hard, they or their imprints may remain
in mud for thousands of years. These
fossils mav be used for comparison with
present mollusks, giving us an oppor-
tunitv to visualize the changes that have
occurred in this form of life. Where

layers of shells have accumulated in var¬
ious strata of the earth, the geologist has
been able to utilize them as a tool for
dating as well as to reconstruct changes
that have taken place in the earth’s sur¬
face. Aggregations of certain shells are
also used by engineers to determine the
possibility of finding oil in various re¬
gions. What conclusion might you be
able to draw if you found a large deposit
of mollusk shells while hiking in the
mountains?

The binding link of the mollusks. As

you have already seen in Chapter 14,
our system of classification is based on
morphology, the organisms with the
greatest resemblance being placed in
the same species. Perhaps you have
wondered why the clam, octopus, and
garden snail are placed in the same phy¬
lum when they are apparently entirely
different. In an attempt to place the
many diverse forms of life into groups,
we may theorize that organisms with

29-1 The trochophore larva is found in both
mollusks and annelids.

399

400 UNIT 5 BIOLOGY OF THE INVERTEBRATES

29-2 Although no living mollusk today looks like this drawing, the major char¬
acteristics of the phylum are shown. The arrows indicate direction of water flow
in the mantle cavity.

similar larval development are related.

A link that ties the mollusks to¬
gether as a group is the larval form called
the trochophore (trohk- o-for). The
trochophore has a tuft of elongated cilia
at one end and a ciliated band around
the equator (Fig. 29-1). The action of
these cilia propel the larva through the
water and bring food to the mouth.
The terrestrial mollusks and most ma¬
rine forms pass through the trochophore
stage while confined to the egg capsule.
The trochophore is also found in an¬
nelid development, and for this reason
biologists consider the segmented worms
and the mollusks to be related.

The general body plan. The body of an
adult mollusk consists of a head, foot ,
and visceral (vzs-e-ral) hump (Fig.
29-2 ) . The visceral hump contains the
digestive organs, excretory organs, and
the heart. It is covered by a mantle , or
thin membrane, which often secretes a
shell by taking lime from the water.
Where the mantle hangs down over the
sides and rear of the animal, the mantle
cavity is formed. The gills , which are
the respiratory organs, are in this cavity.
Undigested matter passes into the man¬
tle cavity through the anus before being
carried outside through the excurrent
siphon.

Since a current of water must pass
through the mantle cavity to bring in
fresh oxygen and food as well as to carry
away carbon dioxide and wastes, the
mollusks with shells have a sanitation
problem. The solution seems to be in
the growth of tubes, or siphons. The
incurrent siphon (or ventral siphon )
takes water into the mantle cavity while
the excurrent siphon (or dorsal siphon)
takes water out of the mantle cavity.

29-3 The shells of the bivalve mollusks are
composed of three distinct layers.

CHAPTER 29 MOLLUSKS AND ECHINODERMS 401

The major characteristic dividing
this phylum into classes is the type of
shell, if present. For the five classes into
which we divide the mollusk phylum,
see the Appendix.

Mollusks with two shells. The bivalves
include the clams, oysters, scallops, and
mussels. The shells, or valves , are con¬
nected by a hinge and are composed of
three distinct layers. The inside pearly
layer is smooth and glistening (Fig.
29-3). If a grain of sand or encysted
parasitic worm becomes lodged in the
mantle, this layer builds up around the
particle and forms a pearl. The middle
layer is composed of crystals of calcium
carbonate and is named the prismatic
layer. The outermost layer is very thin
and may be peeled off when the shell
dries. It resembles dried shellac or
varnish and is called the horny outer
layer. This substance forms the hinge
and also protects the middle layer from
being dissolved by small amounts of
acid that may be in the water.

The major muscles in the bivalve
are those holding the shells together.
If you have ever pried apart the valves
of an oyster or clam, you know their
strength. The muscle of the foot is re¬
sponsible for the scientific name of the
group, Pelecypoda, which means “hatch¬
et-footed” (Fig. 29-4). The foot ex¬
tends into the sand and spreads out to

form a hatchet-shaped anchor. Then,
when the muscle of the foot contracts,
the mollusk is pulled into the sand or
mud. The clam is a rapid digger as
some of you no doubt know!

As a clam is partially buried in the
sand or mud, the shells remain partly
open and the two siphons extend into
the water. Water is brought into the
mantle cavity by the incurrent siphon.
The water passes up through the gills
and then past the anus, where wastes
are excreted. The excurrent siphon then
carries the water out. As water passes
over the gills, two processes occur: 1. ox¬
ygen diffuses in and carbon dioxide dif¬
fuses out; and 2. small particles of or¬
ganic matter stick to a thin layer of mu¬
cus on the gills. Cilia on the surface of
the gills carry the mucus up to the dor¬
sal surface and then forward to the
mouth. Animals that feed in this man¬
ner are said to be mucus feeders. Many
marine worms also collect their food in
a mucus trap. Mucus feeders are scav¬
engers, taking advantage of dead and
decaying organic matter as well as the
numerous microscopic protists that set¬
tle to the bottom.

Bivalves have a well-developed nerv¬
ous system with several large ganglia.
The edges of the mantle contain sen¬
sory cells which are sensitive to contact
and light.

29-4 Note how its hatchet-shaped foot aids the clam in digging into the sand.

402 UNIT 5 BIOLOGY OF THE INVERTEBRATES

Dorsal siphon

Ventral siphon

Mantle cavity
Gills

Reproductive organs

adductor

muscle

Esophagus

Mouth

Mantle

Shell

Intestine

Foot

Shell
Mantle

Ventral siphon

Dorsal siphon

Posterior Heart

adductor
muscle

Anus

29-5 In this diagram of a dissected clam, the arrows indicate the direction of
water flow.

Clams, oysters, scallops, and mus¬
sels are edible bivalves. Since thev feed

on microscopic organisms, some of
which are poisonous to man, and since
they are often eaten raw, care must be
used in gathering them. Naturally mol-
lusks are not edible when their digestive
tracts contain poisonous protists.

In the Indo-Pacific region, an inter¬
esting relationship occurs between one-
celled algae and the giant clam, which

may be a yard and a half long. Im¬
mense numbers of these algae live in the
mantle of the clam, giving it a green
color. The algae manufacture starch
and oils and are often contained in
ameboid cells which carry them to the
digestive area of the clam.

Mollusks with one shell. Common uni¬
valves, or gastropods, include land and
water snails, conches, and abalones. If
a scientist were trying to reconstruct the

29-6 Compare the structure of the land snail shown here with the generalized
mollusk in Fig. 29-2, and with the clam in Fig. 29-5. In what ways does it differ
and in what ways is it similar?

CHAPTER 29 MOLLUSKS AND ECHINODERMS 403

way the snail developed from a hypo¬
thetical ancestor, he would compare the
two. Try to picture the following
changes in the generalized mollusk in
Fig. 29-2: rotate the visceral hump half¬
way around; remove the gills; put some
spirals in the shell — it would now re¬
semble a snail. The land snail is a good
mollusk to study for it is easily obtained
and moves around freely while being
observed. See if you can find the prom¬
inent structures on a land snail in Fig.
29-6.

The terrestrial snail moves at a
“snail’s pace” of about ten feet per hour.
The flat, muscular foot secretes slime
on which the snail travels in rhythmic
waves. The eyes of land snails are on
the tips of two tentacles. When
touched, the snail draws in these ten¬
tacles, somewhat as the toe of a stocking
vanishes when you turn it inside out.

The slug looks like a snail that has
lost its shell. It comes out at night,
leaving trails of slime wherever it goes.
Slugs eat plant leaves and cause con¬
siderable damage, as do snails.

The fresh-water snails in a school
aquarium are excellent for observing the
feeding mechanism of a gastropod. Here
you will see the mouth of the animal
open and a tonguelike structure scrape
the glass of the aquarium. This struc¬
ture is named the radula (md-uh-la),
which literally means “scraper.” The
radula is like a file, and actually files
algae from the glass sides of the aquari¬
um. Now you can understand how gar¬
den snails and slugs, also equipped with
radulas, can cause such damage to your
plants.

“Head-foot” mollusks. The cephalo -
pods (se/-a-luh-pahds) include the octo¬
pus, squid, cuttlefish, and chambered

29-7 The cuttlefish is one of the most highly developed of the invertebrates.
The shell of the animal is internal and consists of a calcified plate. (Annan

Photo Features)

404 UNIT 5 BIOLOGY OF THE INVERTEBRATES

29-8 The octopus is a cephalopod without a shell and with extremely well-
developed eyes. It moves by pulling itself over rocks with its tentacles. (Annan
Photo Features)

nautilus. The octopus has no shell; the
nautilus has an external shell; and the
squid and the cuttlefish have an internal
shell. The giant squid is probably the
largest of all invertebrates, sometimes
reaching a length of 55 feet and a weight
of two tons. The octopus has the worst
reputation, probably undeserved. As a
rule they are timid creatures. The larg¬
est species has a body not more than one
foot long, but its slender arms may
reach 16 feet in length. This species
lives along the Pacific Coast. There
are about 400 species of cephalopods to-
dav. When compared with the over
10,000 fossil forms, one might wonder if
they are to become extinct, as have
many other organisms.

The spiny-skinned invertebrates. The
starfish, brittle star, sea urchin, and sand
dollar are common members of the
phvlum Echiiiodermata (i-fey-nuh-derm-
a-ta). They have a hard, radially sym¬
metrical bodv covered with spines. The
spines may be long, as in the sea urchin,
or verv short, as in the sand dollar. All

the eehinoderms are restricted to the
marine environment. They have been
collected from the shallowest tide pools
to great ocean depths. The five classes
are listed in the Appendix.

Although the radial symmetry of
the eehinoderms is unlike that in any
other animal phylum, many biologists
would place them near the lower chor-
dates because of their larval develop¬
ment. The free-swimming dipleurula
larva is ciliated and contains a mouth,
a digestive tract, and an anus (Fig.
29-10). After swimming about for a
period of time, the larva settles on a
solid object and gradually changes
into the adult form.

The characteristics of the phylum
can be observed in one of its most famil¬
iar members — the starfish. In spite of
its name, the starfish is not a fish, but
possesses five (or rarely six) rays, which
radiate from a central disk. In a groove
on the lower side of each movable ray
are two rows of tube feet (Fig. 29-11).
These are part of a water-vascular sys-

CHAPTER 29 MOLLUSKS AND ECHINODERMS 405

29-9 The spiny echinoderm on the left is a sea urchin. The mouth of a sea
cucumber is shown on the right. (Left: Walter Dawn; right: Hermes from Na¬
tional Audubon Society)

tern. The tube feet are connected to
canals which lead through each ray to
a circular canal in the central disk. This
ring canal has an opening to the surface,
the sieve plate , on the dorsal side.
When the starfish presses its tube feet
against an object and forces water out of
the canals, the feet firmly grip by means
of suction. Return of water to the
canals releases the grip.

The starfish uses its water-vascular
system to open the shells of clams and
oysters, its principal food. The body
arches over the prey with the rays bent
downward. The shells of the clam or
oyster are gripped firmly by the tube
feet, and a steady pull is exerted at the
same time. The starfish secretes a sub¬
stance that paralyzes the clam or oyster.
After a while the shell muscles of the
prey tire, and the halves of the shell sep¬
arate. At this point the starfish pushes
its stomach out from a small opening in
the center of the lower side. The stom¬

ach, turned inside out, enters the shell
and digests the body of the clam or oy¬
ster, leaving only the shell. Although
starfish have extensive skeletons, they
are quite flexible and can bend very
easily around an oyster.

29-10 This dipleurula larva of an echino¬
derm is similar to that of some of the lower
chordates.

406 UNIT 5 BIOLOGY OF THE INVERTEBRATES

Sieve plate

Duct from reproductive organ
Reproductive gland

Tube feet

29-11 This is the dorsal view of a dissected starfish.

Food passes from the stomach of
the starfish into digestive glands in
which digestion is completed. Some
undigested matter passes out through
the mouth. Above the stomach is a
short intestine which ends in the anus
on the dorsal surface.

The starfish are either male or fe¬
male, and the reproductive organs lead
to the outside by small ducts. During
the reproductive season eggs and sperm
are shed to the outside, where fertiliza¬
tion takes place in the water. The fe¬
male starfish may produce as many as
200 million eggs in a season.

Oystermen are on constant lookout
for starfish in clam or oyster beds. An
active, adult starfish can destroy eight
to twelve oysters a day. People used to
tear starfish to pieces, thinking that they
were destroying the pests. Actually they
were multiplying their troubles, for a
starfish rav, with a portion of the central

disk, can regenerate and become an en¬
tire new starfish.

Although the echinoderms are not
an economically important food, some
are consumed. Sea urchins are some¬
times collected on beaches and their
eggs eaten. In the Orient, sea cucum¬
bers are collected, dried, and sold as
beche-de-mer or trepang (tri -pan) for
use in soup.

What is a phylogenetic tree? Before
proceeding with the largest invertebrate
phylum, the Arthropoda, let us look for
some regularities which may be useful
in understanding the course of animal
evolution. Figure 29-13 is a diagram¬
matic representation of how a taxono¬
mist might fit shreds of evidence to¬
gether to imagine how animal diversity
evolved. It is called a phylogenetic
tree. There are many such schemes,
and thev are used to point out relation¬
ships based on incomplete fossil records,

CHAPTER 29 MOLLUSKS AND ECHINODERMS 407

29-12 A starfish shown in the process of paralyzing and digesting a clam.
Note the extended stomach. (Charles Walcott)

morphological similarities of existing an¬
imals (both larval and adult), and im¬
agination. When we examine the larval
forms of the mollusks, for example, we
find that they are very similar. It can
be theorized, then, that they evolved
from a primitive ancestor which was
like the larval form. No one can say
for sure, nor can anyone disprove it.
Furthermore, if we find that annelids
also have a larval form very similar to
that of the mollusks, we can postulate
that these two apparently different phy¬
la developed from a trochophore-type
ancestor.

You will notice that in Fig. 29-13
there seems to be one main direction
until structures called protonephridia
(proh-toh-ne-fn’d-ee-a) develop. A pro¬
tonephron is a very primitive type of
kidney. According to the diagram,
then, all the organisms beyond this
point possess a protonephron. This is

true, but in the development of many
animals the protonephron undergoes
complicated changes before the adult
stage is reached.

One of the characteristics consid¬
ered in grouping animals is the type of
body cavity, or coelom (see-lom),
present in the adult. Among the bilat¬
erally symmetrical organisms, the flat-
worms do not possess a body cavity be¬
tween the internal organs and the body
wall (Fig. 28-2). As the body of an
animal increases in size, a solid struc¬
ture such as is found in the flatworms
would not be physiologically efficient.
A fluid-filled cavity allows for looped in¬
testines and aids in circulation of food
and oxygen, as well as waste removal.
For this reason, the majority of animals
have a coelom. The coelom develops
within the mesoderm of the embryo
and has a lining of specialized covering
cells. This lining surrounds the inter-

408 UNIT 5 BIOLOGY OF THE INVERTEBRATES

29-13 This phylogenetic tree shows a possible relationship of the various groups
of invertebrates, as well as their supposed relationship to the protists.

nal organs and covers the inner surface
of the bodv wall. It forms a membrane¬
like structure that suspends the intes¬
tine within the coelom.

Between the animals with a coelom
and those without, in our phylogenetic

tree, are the animals possessing a pseudo-
coel (sood- oh-seel), or false coelom.
This cavity is not lined with specialized
covering cells and therefore has no sus¬
pending membrane. The internal or¬
gans are free within the cavity.

CHAPTER 29 MOLLUSKS AND ECHINODERMS 409

IN CONCLUSION

In the study of the mollusks and echinoderms, you have investigated some of
the most prominent inhabitants of the seas and tide pools. Although the
mollusks live in marine, fresh-water, and terrestrial environments, the echino-
derms are the only major, exclusively marine phylum.

You have seen some of the ways in which data are gathered and used to
form hypotheses about the relationships and evolution of many diverse forms
of life. Observations and imagination are important biological tools.

In the chapters to follow we shall continue the study of invertebrates with
a phylum, which, if numbers indicate significance, is the most important of
invertebrate groups. Here are insects, spiders, lobsters, and many others.

BIOLOGICALLY SPEAKING

bivalves

cephalopod

coelom

Echinodermata
excurrent siphon
gastropod
gill

horny outer layer

incurrent siphon
mantle
mantle cavity
mollusks
pearly layer
phylogenetic tree
prismatic layer
radula

ring canal

sieve plate

tube foot

trochophore

visceral hump

water-vascular system

valves

QUESTIONS FOR REVIEW

1. What characteristics of the mollusks divide the phylum into classes?

2. What are the three characteristics all mollusks have in common?

3. Trace a particle of food from the outside of the clam to the digestive gland,
naming the organs through which it passes.

4. What layers are formed in the making of the shell?

5. How does the oyster form a pearl in its shell?

6. In what ways do the cephalopods differ from the gastropods?

7. What characteristics of the echinoderms distinguish them from other in¬
vertebrate animals?

8. Describe the movement of the starfish.

9. How does the water-vascular system assist the starfish in eating?

APPLYING PRINCIPLES AND CONCEPTS

1. What structural similarities relate the mollusks and the annelids?

2. What property' of mollusks makes them of help to the geologist?

3. In what ways have the mollusks been of value to man? In what ways have
they been a pest?

4. Shells of mollusks are frequently ground up and used as fertilizer. What
are some of the substances these shells add to the soil?

5. What data are used in making a phylogenetic tree?

CHAPTER 30

THE

ARTHROPODS

Arthropod characteristics. From the
standpoint of numbers the arthropods
are considered the most successful group
of animals. The phylum Arthropoda
includes the familiar insects, spiders,
centipedes, millipedes, crayfish, crabs,
and lobsters. Arthropods are found ev¬
erywhere. They serve as food for man
and compete with him for it. Many
live as parasites in or on other organ¬
isms, and some transmit disease.

To the casual observer the graceful
butterfly has little in common with the
crayfish lurking under a rock in a
stream. But careful study of these
quite different animals will show that
they have much in common. The char¬
acteristics that make the butterfly simi¬
lar in structure to the crayfish also re¬
late these creatures to spiders, scorpions,
and centipedes. All members of the
phylum Arthropoda are similar in hav¬
ing the following characteristics:

1. Jointed appendages , which include
legs and other body outgrowths.
This characteristic gives the phylum
its name, which means “jointed feet.”

2. A hard external skeleton, or exoskele¬
ton. , composed of a substance called
chitin (kyt- n), instead of the inter¬
nal support we have.

3. A segmented body, which refers to
the distinct divisions of the exoskele¬
ton.

4. A dorsal heart; that is, one that is lo¬
cated above the digestive system.

5. A ventral nervous system, with the
main nerves below the digestive sys¬
tem.

6. Muscles in groups.

7. Distinct jaws.

The development of the phylum. The
fact that the arthropod body is divided
into segments suggests a relationship to

COMPARISON OF ARTHROPODS AND ANNELIDS

Arthropod Characteristics

Legs divided into movable joints
Hard chitinous exoskeleton
Fewer segments, more highly specialized
Dorsal heart

Ventral nervous system with specialized
sensory receptors such as eyes and an¬
tennae

Muscles in groups

Annelid Characteristics

No legs

Flexible cuticle
Many segments alike
Dorsal heart

Ventral nervous system but sensory recep¬
tors simple

Muscles in sheets

410

CHAPTER 30 THE ARTHROPODS 411

the annelids. This relationship is often
expressed when biologists refer to the
“annelid-arthropod’' line of develop¬
ment (see the phylogenetic tree shown
on page 408) . When you compare char¬
acteristics of the arthropods and anne¬
lids, in the table on page 410, you may
understand why the arthropods were
able to become more successful than the
annelids. The increased complexity of
their sense organs allows for coordinated
responses to their environments. The
groups of muscles and jointed legs al¬
low for more coordinated movement in
the search for food and in escape from
enemies. As you see, the arthropod
characteristics permit greater efficiency
and adaptability to the environment.

The development of the hard exo¬

skeleton is a protective advantage over
the soft cuticle of the worm. It may
seem strange to find the skeleton on the
outside of the body of an animal. But
whether they are internal or external,
skeletons have the same function. They
give the body form, protect delicate in¬
ternal organs, and aid in motion by
serving as attachments for muscles.
Unlike an internal skeleton, however,
an exoskeleton is a size-limiting factor.
A large exoskeleton with the powerful
internal muscles required to move it
would crush the animal under its own
weight. Flying insects, for example,
could never attain the size of birds be¬
cause their exoskeletons would be too
heavy to be supported by wings.
Growth in the size of an organism with

CLASSES OF ARTHROPODS

Class

Body

Divisions

Appendages

Breathing

Examples

CRUSTACEA

2 — cephalo-
thorax,
abdomen

5 pairs or more

Gills

Lobster, crab,
water-flea,
sowbug, cray¬
fish

CHILOPODA

Head and

numerous
body seg¬
ments

1 pair on each
segment ex¬
cept first one
behind head
and last 2

Tracheae

Centipede

DIPLOPODA

Head and

numerous
body seg¬
ments

2 pairs of legs
on each body
segment

Tracheae

Millepede

ARACHNIDA

2 — cephalo-
thorax,
abdomen

4 pairs of legs

Tracheae
and book
lungs

Spider, mite,
tick, scorpion

INSECTA

3 — head,
thorax,
abdomen

3 pairs of legs;
usually 1 or 2

pairs of wings

Tracheae

Grasshopper,
butterfly, bee,
dragonfly,
moth, beetle

412 UNIT 5 BIOLOGY OF THE INVERTEBRATES

an exoskeleton can only take place by
molting, whereby the old skeleton is
shed and a new one is formed. This
process will be discussed in detail later
in this chapter.

Diversity among the arthropods. In
the classification of the arthropods, such
widely varied forms as butterflies and
crayfish, spiders and centipedes are seg¬
regated into classes. This large phylum
is commonly divided into five classes, as
shown in the table on page 411.

Each of these classes has all the
fundamental characteristics of arthro¬
pods, and, in addition, certain charac¬
teristics of the class to which it belongs.
For example, the Crustacea (kruh-stay-
shuh) have two pairs of antennae, or
“feelers,” on the front of the body, two
distinct body regions, five pairs of legs,
and a chitinous exoskeleton which con¬
tains lime, and often structures called
gills for respiration. The Insecta, on the
other hand, have one pair of antennae,
a body composed of three parts, three
pairs of legs, often two pairs of wings,
and an exoskeleton composed of chitin
lacking lime. They respire by means of
air tubes called tracheae (tray- kee-ee).

The arthropods show a great ad¬
vance over other animals already
studied. Worms, especially the earth¬
worm, show a high degree of specializa¬
tion of body parts and the presence of
specialized internal organs. In our study
of the Crustacea as typical arthropods,
we shall deal with animals such as the
crayfish, lobster, and crab, which are
adapted for aquatic life. Division of
labor among their various organs is car¬
ried to an even higher point than we
found in the earthworm. All of this
specialization has resulted in an efficient
animal, very well-adapted to our present
world. The segmented body is, of
course, common to both the arthropods

and the annelids. The ventral nervous
system first appeared in the worms.

The crayfish — a large fresh-water crus¬
tacean. Scientists often use the cray¬
fish to observe the features of the class
Crustacea because the crayfish is large
and easily obtained; it can be found in
nearly all rivers, lakes, and streams that
contain lime, which is used in harden¬
ing its exoskeleton.

The body is divided into two re¬
gions. The first of these, called the
cephalothorax (se/-al-oh-thor-aks), in¬
cludes the head and a second region,
the thorax. These are separate in many
arthropods. The abdomen, composed
of seven movable segments, is posterior
to the cephalothorax. Vital parts of
the cephalothorax are protected by a
shield called the carapace (kar- a-pays),
which extends forward as a beak, called
the rostrum. On either side of the ros¬
trum are the eyes. These are set on
short movable stalks and are composed
of numerous lenses. For this reason
they are called compound eyes.
Specialization in crayfish appendages.
The most anterior appendages of the
crayfish are the antennules, which con¬
tain the hearing and equilibrium ap¬
paratus. The large antennae attach
just posterior to the antennules. They
function as organs of touch, taste, and
smell. The next appendages are the
mandibles, or true jaws, which crush
and chew food, aided by two pairs of
maxillae ( mak-siZ-ee ) , or little jaws.
The jaws work from side to side and
not up and down — they are merely leg¬
like appendages adapted for chewing
and therefore continue to have a hori¬
zontal motion as do the legs.

The first appendages of the thorax
are three pairs of maxillipeds, or jaw
feet. They hold food during chewing.
The next and most obvious structures

CHAPTER 30 THE ARTHROPODS 413

Compound Antennule

Cheliped

Antenna

Uropod

Rostrum

Head

Thorax [ „

Base of antennule
Base of antenna
Compound eye
Mouth
Mandible
Maxilla
Maxilliped
Gills

VENTRAL VIEW
OF HEAD

DORSAL VIEW

1st Segment
2nd Segment
^ 3rd Segment
4th Segment
5th Segment
6th Segment
7th Segment (Telson)

Swimmerets

VENTRAL VIEW
OF TAIL

30-1 The crayfish (dorsal view on the left, ventral view on the right) is a com¬
mon crustacean.

are the chelipeds , or claw feet, which
function in food-getting and protection.
The next four pairs of legs are called
walking legs. Feathery gills , the respir¬
atory organs, extend under the cara¬
pace and are attached to all the thoracic
appendages.

The abdominal appendages of the
crayfish are called swimmerets. These
appendages are used in swimming and
in the female serve as a place of attach¬
ment for eggs. The sixth pair of append¬
ages is much larger than the first five
pairs. It is developed into a flipper, or
uropod (yur-o-pahd) . There is no ap¬
pendage on the seventh segment, which
is reduced to a flat, triangular structure
— the telson. Strong abdominal mus¬
cles can whip the sixth and seventh seg¬
ment forward, causing the animal to
shoot backward at a rapid pace.

Structures that are believed to have
developed from the same origin are
said to be homologous , regardless of
use. The appendages of the crayfish

are considered to be homologous organs.
Each segment except the telson bears a
pair of appendages. The antennae and
chelipeds of crayfish are homologous to
the swimmerets. Homology can also be
applied to different kinds of animals.
The foreleg of the horse and the arm of
the man, for example, are considered to
be homologous because they are devel¬
oped in a similar manner and from em-
bryological counterparts.

Analogous is another term fre¬
quently used in comparative anatomy
and indicates a similarity in function.
The wing of the bird and insect are
analogous because they both function
in flving. They are not homologous,
however, because each develops in a
different way and from different struc¬
tures. The gills of the crayfish and the
lungs of man are analogous because they
both perform the function of respira¬
tion. They are not homologous since
the gills are developed from the legs.
The lungs are outgrowths of the throat.

414 UNIT 5 BIOLOGY OF THE INVERTEBRATES

Digestion in the crayfish. After food
has been chewed by the mandibles and
maxillae, it passes through the mouth
and short esophagus to the stomach,
which is lined with hard, chitinous
teeth. These grind the food into small¬
er particles. When the particles of
food are finely ground, they pass through
folds of tissue, which act as a strainer,
into another portion of the stomach
where the ground food is mixed with
digestive juices. From here the digested
food passes into the digestive glands,
where absorption takes place. Undi¬
gested particles pass on through the in¬
testine, instead of entering the diges¬
tive glands, and are eliminated through
the anus (Fig. 30-2). There are also
excretory organs in the crayfish called
green glands, which lie anterior to the
stomach and open close to the base of
the antennae.

Circulatory and nervous systems. The
colorless blood of the crayfish is pumped
by the heart into seven large arteries
which pour the blood over the major
organs of the body. Blood is collected
in a number of spaces, or sinuses. One
large sinus surrounds the heart. From
here blood enters the heart through

three pairs of openings. This method of
circulation of blood is called an open
system .

The nervous system, though similar
to that in annelids, is more specialized.
Receptors sensitive to odors and flavors
are located in the antennae. The com¬
pound eyes each consist of numerous
lenses, but sight is probably not keen.
The hearing apparatus is located in a sac
in the basal segment of each antennule.
Hearing is poorly developed. Numerous
sensory bristles distributed all over the
body, especially on the surface of the an¬
tennae and other appendages, are sensi¬
tive to touch.

The sac at the base of each anten¬
nule is also concerned with balance.
The lining of this sac is called a stato-
cyst. At the time of molting the cray¬
fish normally places a grain of sand in
the statocyst, which is lined with
short hairs. Figure 30-2 shows the nerve
endings at these hairs. When the ani¬
mal is right side up, pressure on the
hairs by the grain of sand at the bottom
of the statocyst stimulates the nerves
and the crayfish senses its position in
the water. Similarly, if it is upside
down, the stimulation of the hairs in

“Ear”

sac

Green gland
Mouth
Mandible
Maxilliped

Esophagus
Ventral nerve cord
Ganglion of ventral nerve cord

Flexor muscle

30-2 In this longitudinal section of the crayfish, the term gonad refers to the
ovary in the female and the testis in the male.

CHAPTER 30 THE ARTHROPODS 415

the statocyst informs the animal to right
itself.

An interesting result occurs if a
crayfish molts in an aquarium in which
the sand has been replaced by iron fil¬
ings. After the shell hardens, the ani¬
mal is put in another aquarium. A
magnet will cause the iron filing to
stimulate the hairs in the top of the
statocyst. The crayfish will turn on its
back and stay upside down as long as
the magnet is present.

How the crayfish respires. Gas ex¬
change in protists is accomplished by
diffusion through the plasma membrane;
in worms, through the body wall. Cray¬
fish have gills, which are organs espe¬
cially adapted for the exchange of oxy¬
gen and carbon dioxide between the an¬
imal and its environment. These thin-
walled gills are richly supplied with
blood vessels to receive oxygen and car¬
ry it to all the cells, and also to liberate
carbon dioxide.

The gills are arranged to insure a
constant flow of fresh water over them.
They move in the water with every mo¬
tion of the legs or maxillipeds. The
gills are protected by the carapace,
which extends over them and forms a
chamber. This chamber can hold mois¬
ture for some time, thus keeping the an¬
imal alive when it is removed from the
water.

Reproduction and growth in the cray¬
fish. Crayfish usually mate in the fall,
at which time the sperm are stored in
small receptacles on the lower side of
the female’s body. As the eggs, which
number about one hundred, are laid in
the spring, they are fertilized by the
stored sperm and then attached to the
swimmerets. Here these berrylike struc-
tures are carried and protected for about
six to eight weeks until they hatch. As
in the development of many inverte-

30-3 The pattern of growth in the arthropods
is drawn in black; that of the vertebrates ap¬
pears in white. Why can we compare arthro¬
pod growth, or molting, to a flight of steps?

brates, the speed of development may
depend on external factors of the envi¬
ronment, such as temperature and water
conditions. The larvae are quite differ¬
ent from the parent, but during a series
of molts they reach the adult form.

From the time of hatching until
the adult size, molting occurs at in¬
creasingly long intervals. Most crayfish
molt seven times the first year and about
twice a year thereafter. The average life
span of the crayfish is three or four
years.

During the process of molting, the
cuticle secretes an enzyme which actu¬
ally digests the inside of the shell, thus
loosening it from the body. The cara¬
pace splits along the back, and water is
withdrawn from the tissues, causing
them to shrink. Next the animal liter¬
ally humps itself out of its former skele¬
ton. It also sheds the lining of its
stomach and its teeth. Immediately
following this process, water is absorbed
and the animal swells up. When the
lime is replaced, the exoskeleton hard-

416 UNIT 5 BIOLOGY OF THE INVERTEBRATES

ens and the animal has grown. Since
the crayfish is helpless at the time of
molting, the process usually takes place
quickly and in hidden locations.

During molting or in battle with
enemies, appendages are often lost or
injured. An injured limb is voluntarily
shed. A double membrane prevents
much loss of blood, and a whole new
appendage is gradually developed to re¬
place the injured member. This proc¬
ess is another example of regeneration
of lost parts.

Economic importance of various crus¬
taceans. Since the crayfish readily con¬
sumes dead organisms in any condition,
it is considered of benefit as a scavenger.
In certain parts of the country, espe¬
cially in the Mississippi River basin,
crayfish cause extensive damage by
making holes in earthen dams and levees
and by burrowing in fields, thus destroy¬
ing cotton and corn crops.

The spiny lobster is an edible crus¬
tacean which lives in warmer regions of
the oceans along the coasts of Florida,

California, and the West Indies. This
species lacks the large chelipeds pos¬
sessed by the cold-water, northern form.
The extremely long antennae and the
spines covering the anterior region of the
body are other characteristics of the
spiny lobster.

The blue crab, which inhabits shal¬
low, grassy ocean bays, is another highly
prized crustacean. If these are caught
immediately after molting, their cuticles
have not hardened into shells. In this
form they are called “soft-shelled crabs"
and considered a delicacy, as the animal
may be cleaned, fried, and eaten with¬
out being shelled. The body of the
crab is shorter and broader than that of
the lobster. The abdomen is reduced
in size and folded under the cephalo-
thorax. The cavity thus formed pro¬
tects the gills and also serves as a brood
pouch in which eggs are carried by the
female.

Another edible crustacean is the
shrimp. Shrimps have a highly muscu¬
lar abdomen and are stronger swimmers

30-4 A spiny lobster. (Kinne — Photo Researchers, Inc.)

CHAPTER 30 THE ARTHROPODS 417

30-5 These forms of small crustaceans are usually found in ponds, streams, or
moist places on land. What characteristics do they have in common?

than the bottom-dwelling crabs and lob¬
sters. They swim backward in true crus¬
tacean fashion. If alarmed, many
shrimps bury themselves in the sand
and thrust out their eyes and antennae
to maintain contact with their environ¬
ment. In the Gulf Coast states and in
California, the shrimp industry is very
important. Louisiana, Texas, and Cali¬
fornia supply much of the shrimp for
inland markets.

Crustacea are adapted for a variety of
environments. The edible marine
crustaceans are relatively insignificant in
terms of numbers of individuals. The
vast majority of crustaceans consist of
minute and even microscopic forms.
The major diet of the large whale is
tiny shrimps under an inch in length.
Small crustaceans may occur in such
tremendous numbers that the sonar ap¬

paratus of ships will give false readings
of the bottom depth.

A rtemia, the brine shrimp, inhabits
tide pools and may withstand a highly
concentrated salt-water environment.
Many tropical fish enthusiasts buy the
eggs and raise the shrimp for fish food.
Barnacles are sessile crustaceans which,
in larval form, settle down on a solid ob¬
ject, produce a shell, and use their feet
to kick food into their mouths. Bar¬
nacles may accumulate on the hulls of
ships in numbers large enough to reduce
the speed as much as 20 percent. They
also clog sea-water intake pipes and
grow on piers. Here the accumulation
of sand and decaying organic matter be¬
tween the barnacles provides a good
medium where bacteria live and grow.
In turn annelids and other smaller ani¬
mals feed on the bacteria. The bar-

418 UNIT 5 BIOLOGY OF THE INVERTEBRATES

nacles, usually associated with mussels,
actually change the environment of
wharf pilings by providing protection
against wave action, extreme tempera¬
ture changes, and drying at low tides.
These environmental changes, however,
do not make man happy, as they in¬
crease the speed of deterioration of the
wharves.

The crustaceans are by no means
limited to a marine environment. In
nearly every sample of fresh water,
manv tiny brownish specks will zig-zag
before your eyes. These organisms may
be Daphnia (water fleas) or ostracods.
The ostracods are tiny crustaceans
which, in addition to the chitinous exo¬
skeleton, secrete two shells resembling
those of clams. Other common minute
Crustacea are the copepods (koh- pe-
pahdz), which serve as an important
part of the diet of many fish.

Another interesting adaptation of
the crustaceans may be observed in the
terrestrial isopods. These animals are
the sowbugs and pillbugs. Since they
have seven pairs of identical legs, they

are called isopods, which means “same
feet.” If you do not live near a stream
where crayfish can be found or near the
ocean where other Crustacea can be
studied, these terrestrial crustaceans are
usually available. The gills of their
aquatic cousins are missing in these ter¬
restrial forms; but a series of plates
along the ventral surface of the abdo¬
men has tiny tubes in which air may
pass and through which respiration oc¬
curs. Since these plates must be kept
moist, sowbugs and pillbugs are usually
found under damp stones and logs.
Terrestrial isopods * may be kept for
weeks by keeping them in jars with po¬
tato or carrot slices.

“Hundred-leggers” and “thousand-leg-
gers.” The Chilopoda and the Diplop-
oda are often grouped together as one
class, the M yriapoda, which means
“many feet.” Perhaps you have won¬
dered how a centipede or millepede can
operate so many legs and not get them
tangled in one another. Their move¬
ment is certainly an excellent example
of coordination. These curious worm-

30-6 Centipede (left); millepede (right). How do they differ? (Walter Dawn)

CHAPTER 30 THE ARTHROPODS 419

like arthropods are often seen racing
away with a rippling motion when their
hiding place under a log, stone, or piece
of rubbish has suddenly been disturbed.

Centipedes belong to the class
Chilopoda and have bodies composed
of many segments. The head bears the
antennae and mouthparts; the first body
segment bears a pair of poison claws;
each succeeding segment, except the
last two, bears one pair of walking legs
(Fig. 30-6).

Millepedes, or “thousand-leggers,”
belong to the class Diplopoda and also
have bodies composed of many seg¬
ments. As in the centipede, the head
bears antennae and mouthparts; all the
body segments except the last two bear
two pairs of legs each. Hence the name
Diplopoda , which means “double feet.”
Millepedes are frequently slow moving
and are likely to roll into a spiral when
disturbed. Centipedes, on the other
hand, are fast moving and difficult to
capture. In tropical countries centi¬
pedes may measure 12 inches in length,
and their bites may be quite poisonous.
Some have as many as 173 pairs of legs,
but 35 is average.

Spiders — familiar arachnids. Unfortu¬
nately spiders are one of several groups
of valuable animals whose reputations
have been spoiled by a few undesirable
members. With a few exceptions spi¬
ders are extremely valuable because they
destroy harmful insects. They belong
to the class A rachnida (a-mk-nid-a) .
Some kinds of spiders, called orb weav¬
ers, spin elaborate webs of tiny silken
threads which are remarkable engineer¬
ing feats. The web serves as a trap for
capturing flying insects. When a vic¬
tim becomes entangled in the sticky
threads of the web, the spider races
from its hiding place along the margin.
Its bite poisons the prey. When the in¬
sect has become partially paralyzed, the
spider binds it securely in a case of
threads spun around the victim as the
spider turns it over and over. Other
spiders do not spin webs, but live as soli¬
tary individuals stalking their prey as
they roam about.

The spider resembles an insect but
differs in several respects (Fig. 30-7).
It has eight legs instead of six, and the
head and thorax are joined to form a
cephalothorax as in the Crustacea. The

Sucking

stomach

Heart

Abdomen

Digestive

gland

Spinnerets

Intestine

Cephalothorax

Simple

eyes

Pedipalp

'Ovarv / / I Branches of / \ Poison gland

n, L„ L / Lung book sucking / Chelicera
Oviduct j stomach Esophagus

Seminal receptacle

30-7 Longitudinal section of a female spider.

420 UNIT 5 BIOLOGY OF THE INVERTEBRATES

first pair of appendages, the chelicera,
serves as poison fangs, and for sucking
the juices from the victim’s body. The
second pair of head appendages in the
spider is called pedipalps. These ap¬
pendages are sensory and are used es¬
pecially in reproduction by the male
spider. On the tip of the abdomen of
many spiders are three pairs of spinner¬
ets. Each spinneret consists of hun¬
dreds of microscopic tubes through
which the fluid silk flows from the silk
glands. When the silk passes into the
air it hardens into a thread. This silk
is used in making the spider’s web, to
build cocoons for eggs, to build nests,
and as a guide so the spider can find its
way back home. Young spiders of
many species spin long silken threads
which catch in the wind and carry them
to distant places. This procedure is
known as ballooning.

30-8 The orange garden spider spins a beau¬
tiful orb web to trap insects. (Hugh Spen¬
cer)

30-9 The black widow spider is recognized
by its round, black abdomen and the vivid
red spot on its ventral surface. (Gerard from
Monkmeyer)

Most spiders have two types of re¬
spiratory organs. A pair of air-filled sacs
called lung books, situated on the lower
side of the abdomen, receive air through
slit-like openings. Numerous leaves, or
plates, of the lung books provide a large
surface for exposure to air. In addition
to lung books, many spiders have air
tubes resembling tracheae of insects
with openings situated on the abdomen.

The mature male spider can be dis¬
tinguished from the female usually by
size alone — the male is usuallv the

smaller. In species where there is no
size difference, the pedipalps of the
male are much larger near the tips. At
maturity the male transfers sperm to
special sacs at the tips of the pedipalps.
These sperm are then placed in the
seminal receptacle of the female, who
sometimes devours the unfortunate
smaller male afterward. At the time of
egg-laying, the eggs are fertilized as they
pass out the genital pore and into a nest
or cocoon the female has prepared.

Among the most famous spiders
are the tarantula ( ta-rcmc/i-uh-la ) , or

CHAPTER 30 THE ARTHROPODS 421

banana spider, the black widow, famous
for its very poisonous bite, and the trap¬
door spider of the western desert re¬
gions.

Other arachnids. Spiders are related to
many other forms of animal life. Scor¬
pions, found in southern and southwest¬
ern United States and in all tropical
countries, are provided with a long seg¬
mented abdomen terminating in a ven¬
omous stinger. The sting of a scorpion,
while painful, is seldom fatal to man.
Campers find scorpions annoying in
that they like to crawl in empty shoes
during the day to escape bright light.
Scorpions live solitary lives except when
mating, after which the female often
turns on her mate and devours him.
The young are brought forth alive and
spend the early part of their existence
riding on the mother’s back.

The harvestman, or daddy longlegs,
is one of the most useful of the arach¬
nids since it feeds almost entirely on

30-10 The sting of a scorpion, which is in¬
flicted from the end of the tail, is intended
for small prey but can be very painful to
man. (Albert Towle)

30-11 This photograph shows a chigger mag¬
nified over 50 times. The animal has eight
legs as do all arachnids and its bite causes
extreme discomfort in man. (American Mu¬
seum of Natural History)

plant lice. It leads a strictly solitary life,
traveling through the fields in search of
its prey.

Mites and ticks are among the
more notorious arachnids, causing con¬
siderable damage to man and other ani¬
mals. They live mostly as parasites on
the surface of the bodies of chickens,
dogs, cattle, man, and other animals.
Some forms, like the Rocky Mountain
tick, carry disease organisms.

Harvest mites, or chiggers, are im¬
mature stages of mites which attach
themselves to the surface of the skin
and insert beaks through which they
withdraw blood (Fig. 30-11). They are
almost microscopic in size and give no
warning of their presence until a swol¬
len area causes great itching and dis¬
comfort. After a few days the sore be¬
comes covered with a scab and dis¬
appears.

IN CONCLUSION

The arthropods are the largest single group of animals, with more different
forms than all other animal groups combined. They can swim, crawl, hop,

422 UNIT 5 BIOLOGY OF THE INVERTEBRATES

run, burrow, and fly. Their diversity has allowed them to utilize all environ¬
ments — fresh water, marine, and terrestrial — and to explore almost every
environment available to animals.

In the next chapters we shall consider representatives of the insects, the
most abundant class of arthropods.

BIOLOGICALLY SPEAKING

abdomen

Chilopoda

maxillae

analogous

chitin

maxillipeds

antennae

compound eyes

molting

antennules

Crustacea

open circulatory system

appendage

Diplopoda

rostrum

Arachnida

exoskeleton

statocyst

Arthropoda

gills

swimmerets

carapace

green glands

telson

cephalothorax

homologous

uropod

chelieera

Insecta

walking legs

chelipeds

mandibles

QUESTIONS FOR REVIEW

1. What are three external characteristics of an arthropod that distinguish
it from other animals?

2. Name the five principal classes of arthropods, and give examples of each.

3. In what ways are the arthropods similar to the earthworm?

4. What are some advantages and disadvantages of an exoskeleton?

5. How do the gills in the crayfish carry on the process of respiration?

6. To what stimuli is the crayfish sensitive, and what structures assist in the
sensitivity?

7. How does a centipede differ from a millepede?

8. Why are most spiders extremely beneficial animals? What reasons can
you give for the fact that many people are genuinely afraid of arachnids?

9. Of what value is ballooning to the spider?

10. What other animals besides the spiders are classified as arachnids?

APPLYING PRINCIPLES AND CONCEPTS

1. Why is it especially important for an armored animal like the crayfish to
have long antennae?

2. In which of the following localities do you think the crayfish would be
likely to produce weaker exoskeletons — in waters flowing through lime¬
stone rock or in waters flowing through granite? Explain.

3. Of what advantage to the young crayfish is its clinging to the adult’s swim-
merets until after the second molting?

4. How do barnacles do damage in other ways than reducing a ship’s speed?

CHAPTER 31

INSECTS-A

REPRESENTATIVE

STUDY

Vast numbers of species. The study of
insects is called entomologyy and the
scientists specializing in this field are
entomologists. Entomologists have al¬
ready recorded more than 675,000 spe¬
cies of insects, and they regard this as
not more than half of all insects in
existence! Three quarters of all classi¬
fied animals are arthropods, and of
these, the class Insecta is by far the
most numerous. Not only are there
many kinds of insects, but each kind
produces thousands of offspring. Con¬
sider, for example, the locusts and may¬
flies, whose swarms can darken the skies.
Thomas Huxley, the famous English
biologist, made the following calcula¬
tion:

I will assume that an Aphis weighs
one-thousandth of a grain, which is
certainly vastly under the mark. A
quintillion of Aphides will, on this es¬

timate, weigh a quatrillion of grains;
consequently, the tenth brood alone,
if all its members survive the perils to
which they are exposed, contains more
substance than 500,000,000 stout men
— to say the least, more than the
whole population of China!

So varied are the food requirements
of insects that they can maintain tre-
mendous numbers without interfering
with each other. There are leaf feed¬
ers, plant sap feeders, blood suckers,
flesh eaters, wood eaters, nectar and
pollen gatherers, and even cannibalistic
insects. Their small size, exoskeleton,
flight, and various colorings are but a
few of the protective mechanisms of
insects. Fortunately, as we shall see in
the final unit of this book, other factors
control this reproductive potential.

How to recognize an insect. Many
people speak of any small flying or
crawling animal as a “bug.” They are
wrong on two counts! First, a true bug
is a member of only one order of in¬
sects. Second, what some people call
a bug may be a spider or a centipede.
These, of course, are not insects at all.

Insects include that division of the
arthropods which has three separate
body regions: 1. head; 2. thorax; and
3. abdomen. The head bears one pair
of antennae and three pairs of mouth-
parts. The thorax of an insect bears
three pairs of legs and in many the
wings. The abdomen has as many as
11 segments and never bears legs. The
reproductive structures are usually found
on the eighth, ninth, and tenth seg¬
ments. Insects breathe by many
branched tubes called tracheae ( tray -
kee-ee ) .

A high degree of specialization. The

mouthparts of insects are adapted for
different kinds of food. Some insects
have chewing mouthparts with strong
jaws to grind up leaves. Some have

423

424 UNIT 5 BIOLOGY OF THE INVERTEBRATES

31-1 The life history of the grasshopper shows incomplete metamorphosis.

piercing and sucking mouthparts with
which they are able to suck plant juices
or blood from animals, as do the mos¬
quitoes. Some have a siphoning tube
for probing flowers to obtain the nectar.
You have all seen butterflies gathering
the nectar from flowers on warm days.

The wings and legs of many insects
are developed for swift locomotion.
Some insects are adapted for aquatic
life and some for burrowing in the
ground. Some insects live in colonies;
others fight their battles alone. Some,
such as the scale insects, have in the
process of evolution lost their legs en¬
tirely.

Insect metamorphosis. Most insects
undergo several distinct stages dur¬
ing development from egg to adult.
Such a series of stages in a life history
is called metamorphosis. Grasshoppers
and their relatives, true bugs, aphids,
termites, and many other insects have
incomplete metamorphosis. This is a
three-stage life history consisting of:
1. the egg; 2. the nymph ; and 3. the

adult. A nymph hatches from an egg
in a form resembling the adult except
for size, absence of wings, and lack of
development of the reproductive or¬
gans. In most species nymphs molt
five times, each time becoming more
like the adult.

Butterflies, moths, flies, and bee¬
tles are among the insects with com¬
plete metamorphosis. They pass
through four stages in their develop¬
ment: 1. egg; 2. larva; 3. pupa; and
4. adult. The larvae that hatch from
eggs are segmented and wormlike.
They are called by such names as cater¬
pillar, grub, or maggot, depending on
the kind of insect (see table, page 426).
After a period of feeding and rapid
growth, the larva enters the pupa stage.
This is often thought of as a resting
stage because changes occur within a
shell or case and cannot be seen ex¬
ternally. It is anything but a resting
period, however, because in this stage
all the tissues of the larva are trans¬
formed into those of the adult. The

CHAPTER 31 INSECTS — A REPRESENTATIVE STUDY 425

31-2 The Cecropia moth is a
familiar insect which shows
complete metamorphosis. (Top
left: Quilt; top right: Adams;
middle left: Chace; middle right:
Smith; bottom left: Smith; bot¬
tom right: Chace. All from Na¬
tional Audubon Society)

426 UNIT 5 BIOLOGY OF THE INVERTEBRATES

THE LARVA
OF THE

beetle

fly

mosquito

butterfly

moth

IS CALLED

grub

maggot

wiggler

caterpillar or “worm”
caterpillar or “worm”

change from caterpillar to butterfly or
grub to beetle is truly a marvelous event
in nature.

In recent years, biologists have been
investigating the role of hormones in in¬
sect metamorphosis. They have found
that two hormones, a juvenile hormone
and a growth hormone, are involved in
the transformation of a nymph of the
bug, Rhodnius, into an adult. These
hormones were found in a gland near
the brain and in a mass of tissue within
the brain.

In studies of the metamorphosis of
the Cecropia moth, it was found that a
juvenile hormone stimulates growth of
the larva but ceases to function when
the moth enters the pupa stage. Two
additional hormones, one secreted in the
brain and one in a gland of the thorax,
are necessary for development of the in¬
sect in the pupa stage.

A closer look at a representative insect.
By this time you are aware that a high
school course in biology cannot possibly
cover the entire subject. When a large
group of organisms is studied, we often
choose one that shows the character¬
istics of the group. We cannot say
that a grasshopper, for example, is a
typical insect because the diversity of
this class is unequaled in the animal
kingdom. The grasshopper, however,
is large enough to examine without high
magnification and is common enough to
be available in large numbers.

As a member of the order Orthop-
tera, which means “straight-winged,” the
grasshopper’s narrowly folded wings are

held straight along the body when not
actually used in flight. As in all arthro¬
pods the skeleton is external, but it dif¬
fers from the crayfish in that it contains
no lime. It consists entirely of the
light, tough substance called chitin.

Since the herbivorous grasshopper
feeds on blades of grass, let us examine
the mouthparts to see how they are
adapted for this food. The labrum is a
two-lobed upper lip used in keeping a
blade of grass at right angles to the
mandibles, which are toothed, horizon¬
tal jaws (Fig. 31-3). Lying posterior
to the mandibles are paired maxillae.
These are accessory jaws which aid in
holding and cutting food. The palpus

31-3 The mouthparts of the grasshopper
are especially adapted for chewing plant
materials. The mandibles are notched and
they move sideways.

CHAPTER 31 INSECTS — A REPRESENTATIVE STUDY 427

Compound eye

Tympanum

Spiracles

Coxa

Trochanter

Tarsus

Femur

Antennae
Simple eyes

Mouth parts
Foreleg

Jumping leg

Anus

Ovipositer

1 — Prothorax

2 — Mesothorax

3 — Metathorax

4— 13 — Segments of the abdomen

31-4 External structure of the grasshopper.

is an antennalike sense organ which is
part of each maxilla. Posterior to the
maxillae is the labium , or lower lip, also
provided with palpi. This organ func¬
tions in holding food between the jaws.
These mouthparts, like the legs of the
crayfish, are believed to be homologous,
as it is thought that each one developed
as an appendage of an independent em¬
bryonic segment.

Locomotion may be accomplished in
several ways. The grasshopper s loco-
motory appendages are located on the
thorax, which is divided into three seg¬
ments. The head and first pair of walk¬
ing legs are attached to the prothorax
(Fig. 31-4) . The first pair of wings and
the second pair of walking legs are at¬
tached to the mesothorax . The meta¬
thorax bears the second pair of wings
and the jumping legs.

The grasshopper’s wings may carry
it over dry fields for a short distance or
for many miles when they migrate in
swarms. The long, narrow, and stiff
anterior wings protect the delicate un¬
derwings when the grasshopper is at rest

or walking, but during flight or leaping
they act as planes. The posterior wings
are thin and membranous and supported
by many veins. These flying wings are
folded like a fan when not in use.

For shorter distances the large jump¬
ing leg is used to escape enemies, to
launch into flight, or to search for food.
The jumping legs are also used with the
walking legs to climb up plants in order
to feed on the tender leaves. When the
insect is jumping or walking, spines,
hooks, and pads of the foot, or tarsus ,
aid in gripping. The long joint next
to the tarsus is called the tibia. The
large muscles for jumping are contained
in the femur , the heaviest joint of the
leg. The trochanter (troh-kant-ex) joins
the coxa near the body and together
they act like a ball and socket joint in
providing freedom of motion.

A remarkable respiratory apparatus.
Each of the ten segments of the abdo¬
men consists of two curved plates. The
upper and lower plates are joined by
a tough but flexible membrane that al¬
lows the segment to expand and con-

428 UNIT 5 BIOLOGY OF THE INVERTEBRATES

Esophagus

Gizzard

Optic lobes
of brain

Antenna
Brain

Mouth

Labrum

Crop

Gastric

caecum

Rectum Anus

Heart

Intestine

First ganglion of
the nerve cord

Duct of
Labium salivary
gland

Stomach

Ventral Malpighian
Salivary nerve
gland cord

Oviduct

C°'on Genital

opening

tubules

31-5 Internal structure of the grasshopper.

tract in the process of respiration. The
flexible membrane also joins each seg¬
ment with the anterior and posterior
ones, allowing movement.

Eight of the abdominal segments
have pairs of tiny openings, the spiracles.
Spiracles are also found on the second
and third thoracic segments. The open¬
ings lead to the tracheae, or air tubes,
which form an amazingly complex net¬
work inside the animal. Air is pumped
in and out of the tracheae by action of
the wings and movement of the ab¬
domen. Very rapid diffusion of oxygen
into the tissues and carbon dioxide into
the tracheae accomplishes respiration.
Digestion, excretion, and circulation.
The horizontal action of the mandibles
pinches off bits of grass, which are then
sucked into the mouth. An esophagus
carries the food to a crop, where it may
be stored for a time. As in many ani¬
mals salivary glands secrete juices which
enter the mouth. These juices mois¬
ten the food to ease its passage to the
crop. You may have seen regurgitated
food from the crop when a grasshopper
is injured or disturbed, but normallv the
food passes on to the gizzard. Here
food is shredded by plates of chitin-bear-
ing teeth.

Partially digested food is screened
through thin plates and passes into the
large stomach. Figure 31-5 shows three
double pouches on the outside of the
stomach. These are gastric caeca (see-
ka), which produce and pour enzymes
into the stomach, where digestion is
completed. Digested food is absorbed
into the blood stream through the wall
of the stomach. The material remain¬
ing in the stomach passes into the in¬
testine, which is composed of the colon
and rectum and which terminates at the
anus.

Cellular wastes are picked up bv the
blood stream and collected by a series
of tubes called the Malpighian (mal-
pee- gee-an) tubules. The wastes are
then passed into the last part of the in¬
testine and out through the anus.

Spiracles

31-6 This diagram of the dissected abdo¬
men of the grasshopper shows the arrange¬
ment of the tracheal system.

CHAPTER 31 INSECTS — A REPRESENTATIVE STUDY 429

31-7 The upper photograph is of the simple
and compound eyes of the grasshopper. The
lower photo is a close-up of the compound
eye. (Hugh Spencer)

The circulatory system of the grass¬
hopper, like that of the crayfish, is an
open system. Blood forced out the
anterior end of the tubular, muscular
heart passes through the aorta and into
the body cavity near the head. As blood
flows toward the posterior region, it
bathes all the body organs and finally
returns to the heart for recirculation.
What are the grasshopper’s sensory
responses? Each side of the first ab¬

dominal segment bears a membrane-
covered cavity called the tympanum.
These sensory organs function in hear¬
ing. Touch and smell are perceived by
the many-jointed antennae. The grass¬
hopper has two kinds of eyes. Figure
31-7 shows the simple eyesy located
above the base of each antenna and in
the groove between them. The large
compound eyes project from a part of
the front and sides of the head and are
composed of hundreds of six-sided
lenses. The shape, location, and num¬
ber of lenses seem to adapt the insect
for sight in several directions at one
time, but the image formed is probably
not very sharp. Most insects are con¬
sidered to be nearsighted, yet they may
be able to distinguish colors. We know
that night-flying moths seek white flow¬
ers, while flies and some other insects
are attracted by red and blue.

The stimuli received through the
sense organs are then relayed by nerves
to certain parts of the body, such as a
muscle, which then contract. Nerve
centers called ganglia act as switches in
directing the message to the proper struc¬
tures for coordinated action. The brain
itself is an enlarged ganglion composed
largely of optic lobes. The sight of your
approaching hand is enough to cause the
grasshopper to start moving away, and
fast! This complicated activity is be¬
gun, controlled, and coordinated by the
nervous system.

Reproductive organs of grasshoppers.
In insects the sexes are separate. This
means that sperm cells are produced in
testes , found in the male. Egg cells are
produced in ovaries , found in the female.
The male deposits the sperm cells in a
special storage pouch, the seminal re¬
ceptacle , of the female. The sperm re¬
main there until the eggs are ready for
fertilization.

430 UNIT 5 BIOLOGY OF THE INVERTEBRATES

31-8 The cricket, the praying mantis, and the walking stick are relatives of the
grasshopper. Identify each of these in the photographs. (Top left and right:
American Museum of Natural History; bottom left: USDA)

The extreme posterior segments in
the female grasshopper bear two pairs
of hard, sharp-pointed organs called ovi¬
positors (o-vi-pahz-i-tors) . With these
she digs a hole in the ground and de¬
posits her fertilized eggs, which are pro¬
tected by a gummy substance. About
100 or more eggs are laid in the fall and
hatch in the next spring.

Relatives of the grasshopper. It is easy
to see that the meadow grasshopper and
the katydid are related. The wings of
the katydid look much like leaves, even
to the veining, so that these insects are
well protected from their enemies.
Other members of the Orthoptera are

the cricket, roach, walking stick, and
praying mantis. By destroying crops,
the grasshopper and its relatives are
harmful to man. Most of you have
read stories or seen movies in which
grasshoppers (“locusts”) have plagued
both the Old and New Worlds through¬
out history.

A useful relative of the grasshopper
is the praying mantis , which eats other
insects, many of which are harmful.
Mantis egg cases are sold at many seed
stores. When the eggs hatch in the
spring, nymphs emerge and remain in
the garden, where they mature and eat
other insects.

IN CONCLUSION

As a member of the largest class of arthropods, the grasshopper has been used
to illustrate the major characteristics of the insects. The grasshopper has the
high degree of specialization typical of most insects.

CHAPTER 31 INSECTS — A REPRESENTATIVE STUDY 431

The complex systems of the grasshopper allow it to respond to environ¬
mental stimuli in a coordinated manner. Its sense organs receive the stimuli,
and the information is relayed to an enlarged ganglion, the brain. Nerves from
the brain stimulate muscles to act. The action may be a slow and deliberate
motion toward food; it may be a quick jumping to avoid being caught; or it
may be a flight to carry it some distance.

To show what other variations this class of arthropods possesses, we shall
continue with other orders of insects in the next chapter.

BIOLOGICALLY SPEAKING

adult

labium

palpus

aorta

labrum

prothorax

colon

larva

pupa

complete metamor¬

Malpighian tubules

rectum

phosis

mandible

salivary glands

coxa

maxilla

simple eye

egg

mesothorax

spiracles

entomology

metathorax

tarsus

femur

nymph

tibia

gastric caeca

optic lobes

trachea

incomplete meta¬

Orthoptera

trochanter

morphosis

ovipositor

tympanum

QUESTIONS FOR REVIEW

1. What characteristics of the grasshopper make it similar to the crayfish?

2. List the characteristics of the grasshopper that relate it to the insects.

3. Most animals have sense organs located on the head. In what respect
is the grasshopper an exception to this rule?

4. How do the grasshopper’s legs illustrate adaptation?

5. What are the eight body systems of the grasshopper and their function?

6. What is the chief difference between the blood system of the grasshopper
and that of man?

7. What is the chief difference between the respiratory system of the grass¬
hopper and that of man?

8. What kind of metamorphosis does the grasshopper undergo? What hap¬
pens during this metamorphosis?

9. What protection is given the eggs of the grasshopper during the winter?

10. Which grasshopper relative is considered to be a useful animal? Why?

APPLYING PRINCIPLES AND CONCEPTS

1. What reasons can you give for the insects’ being able to withstand unusual
temperatures, pressures, and other environmental conditions?

2. Give reasons to support the statement, “Insects are the most successful

creatures in the world.”

3. What advantages are there to an insect in having complete metamorphosis?

CHAPTER 32

INSECT

DIVERSITY

A lesson in adaptation. Insects have
become adapted to a terrestrial life in
numerous ways. The success of the
Insecta can be attributed in part to a
behavior much more complicated than
the lower invertebrates that we dis¬
cussed in previous chapters. The com¬
plicated behavior has been possible be¬
cause of three major lines of develop¬
ment: 1. complex sense organs; 2. joint¬
ed appendages; and 3. a brain. The
sense organs receive many kinds of stim¬
uli from the immediate environment
as well as from some distance away.
These stimuli aid the animal in escap¬
ing enemies, finding food, finding a
mate, and locating a suitable place for
the development of eggs. The modi¬
fication of jointed appendages into com¬
plex mouthparts is another important
diversity of this class. Thus, insects
have been able to utilize many different
kinds of foods. The brain is complex
enough to allow the insect to integrate

impressions from the sense organs and
utilize it in coordinated muscular re¬
sponses.

With the development of these
systems the insects are able to perform
complicated, coordinated acts. They
are able to modify their environment
by building many kinds of homes and
nests. The paper wasp builds nests of
chewed woodpulp; the mud-dauber uses
mud; the tarantula hawk (a large wasp)
digs a burrow and stocks it with food
for the carnivorous larvae. Some ants
store grain in special passages made for
this purpose. The fungus ant has regu¬
lar underground gardens of a certain
species of fungus that are tended by
specialized workers. Before the ant
queen takes off on her nuptial flight,
she stores some of this fungus in a spe¬
cial pouch in her mouth so that she
can start the culture in a new home.

As you will see, many insects form
societies in which morphological diver¬
sity within the species allows for division
of labor. Some are adapted for gather¬
ing food, some for protection of the
home, some for tending the young, and
some for reproduction. Such insects,
which include the bees, ants, wasps, and
termites, are called social insects. The
activities of the social insects are not
planned and thought about by the indi¬
viduals performing the tasks. These
animals perform their life duties by in¬
stinct. That is, they have specialized
structures that are used in a behavior
pattern that is not taught to them. Ani¬
mal behavior studies form an interesting
field of current research.

Insect diversity. Since there are so
many insects, we will not be able to
study them all. We can, however, look
at the common orders and learn to rec¬
ognize the characteristics that are used
in their classification. These charac-

432

CHAPTER 32 INSECT DIVERSITY 433

teristics include types of mouthparts,
types of wings, and types of metamor¬
phosis. Insects are commonly divided
into a number of orders varying from
22 to 26. The difference in the number
of orders is that some entomologists
group two or more orders into one.
The table on page 444 lists ten of the
most common ones found in the tem¬
perate regions.

32-1 The egg of a lacewing, its larva being
preyed upon by another larva, and the adult
lacewing. This insect is carnivorous and
cannabalistic. (Walter Dawn)

You can easily find insects in your
home, in your school, outside in fields
and ponds or trees, under stones, and
hiding on plants with almost perfect
camouflage. A particularly valuable
project is to make your own insect col¬
lection. There is no better way to get
to know the insects firsthand.

The order Lepidoptera. Butterflies and
moths belong to the order Lepidoptera
(lep-i-dd/ip-ter-a ) , which means “scale¬
winged.” Their brilliant colors are due
to microscopic scales on their wings,
which make a mosaic pattern. If you
handle living moths and butterflies, be
sure to hold them by the thorax and
with only light pressure of the fingers.
Even a light touch on the wings will
remove some scales and injure the in¬
sect for flight.

Most people confuse butterflies
and moths. If you look at the table
on page 435, you will see that it is not
difficult to distinguish between them.
Head of a butterfly or moth. Unlike
the grasshopper, the head of a butterfly
or moth is hairy, often even shaggy, be¬
cause of the presence of scales. The
eyes are compound and large and
rounded, and the neck is flexible. The
mouthparts are different from those of
the Orthoptera because they are adapt¬
ed for sucking nectar from flowers. The
maxillae are enormously lengthened and
locked together to form the coiled pro¬
boscis. When extended it may equal
all the rest of the bodv in length, and
is thus able to reach the nectar glands
of the flowers these insects visit.

In most Lepidoptera the labium is
reduced in size to two feathery palpi.

434 UNIT 5 BIOLOGY OF THE INVERTEBRATES

32-2 Head of a butterfly. Notice the coiled
proboscis and the lance-shaped antenna.
(Annan Photo Features)

These mouthparts are homologous to
those of the grasshopper, but they are
adapted for different functions.

The thoracic and abdominal regions.
The legs of the Lepidoptera are small
and weak but have the same general
structure as all insects’ legs. Obviously
the butterfly spends much of its time
in the air and uses its legs only for
clinging to some surface. The wings
are large; the colored scales help the
few veins in giving strength to the
wing, and in some moths and butter¬
flies aid in protective coloration. The
butterfly, though easily supported bv its
large wingspread, is not a swift flier.

The abdomen resembles that of
the grasshopper but has fewer visible
segments, and, as in all insects, is the
least specialized body region.

Life history of butterflies and moths.
Most Lepidoptera deposit their eggs on
or near the material which is to be the
food of the young. Some eggs pass the
winter in this stage, but usually eggs are
deposited in the spring and develop into
caterpillars the following summer.

Since the Lepidoptera undergo com¬
plete metamorphosis, the egg does not

hatch into a form anything like the
adult. Instead it produces the larval
form called a caterpillar , with biting
mouthparts. The caterpillar has three
pairs of legs and some extra pairs of
fleshy legs at the end of the abdomen.
It eats ravenously, grows big and fat,
and molts several times. Because the
caterpillar needs tremendous amounts
of food to keep up this rapid growth, it
is during this stage that the insect does
extensive damage.

When full grown, the caterpillar
usually seeks a sheltered spot, hangs with
its head down, and becomes very quiet.
The body shortens and thickens; the exo¬
skeleton splits down the back and is
shed; and the animal now becomes a
pupa. The butterfly pupa rests in a
hardened case, often brown in color. It
is then called a chrysalis (kris- a-lis).
The moth larva usually spins a strong
case of silk, the cocoon.

The Lepidoptera usually spend the
winter in the pupal stage. In the spring
the insect emerges, totally changed, as
the adult butterflv or moth.

32-3 Head of a moth. Compare the antenna
of this insect with that of the butterfly.
(Charles Walcott)

CHAPTER 32 INSECT DIVERSITY 435

Notice that in a life history such

as that of the Lepidoptera, the larval
form has completely different mouth-
parts and therefore a completely differ¬
ent diet from that of the parents. The
butterfly larva feed on leaves, while the
adult sucks nectar from flowers. A life
history such as this allows an insect a
varied diet during its life and an abun¬
dant supply of usable food during its
period of most rapid growth. The adult
serves the species only in a reproductive
manner to continue and disperse the
species. There is no competition be¬
tween the young and adult for food.
The social insects. The order Hymenop-
tera includes the social insects such as
the ants, wasps, and bees, but it in¬
cludes numerous solitary forms as well.
Members of this order are characterized
by two pairs of membranous wings (if
wings are present). This characteristic
gives the order its name, which means
“membrane-winged.” The Hvmenop-
tera also have biting or sucking mouth-
parts, complete metamorphosis, and a
definite constriction between the thorax
and the abdomen.

The honeybee, a living powerhouse.

Many of you may water-ski. Have you
ever been towed by a boat with an un¬
derpowered engine? It will drag you
through the water, but even with the
planing surfaces of your skis at a sharp
angle, you cannot get up. If a jet air-

32-4 The termite belongs to the order Isop-
tera and although its damage to wooden
buildings is great, it is a social insect.
(Hugh Spencer)

plane, with its heavy body and short
wings, were underpowered it would not
be able to get up either. The power
and speed in both the skis and the jet
are able to create the lift. If you ex¬
amine a honeybee, you will see that it
has very small wings in comparison
with the size of the body. The anterior
wings are larger. Tiny hooks may at¬
tach the posterior wings to the anterior
ones. A first examination of the bee
might lead to the conclusion that this
insect could not possibly fly because its
wings are too small for its body. Might
you not conclude the same thing if you
saw a prepared water skier on the beach
or a jet airplane on the ground? The
bee makes up for its large body size
and short wings by having powerful
muscles in the large thorax. These mus-

COMPARISON OF BUTTERFLY AND MOTH

Butterfly

Moth

Flies during the day

Generally flies in the dark

Pupa in chrvsalis

Pupa in cocoon

Wings vertical when at rest

Wings held horizontally

Antennae knobbed

Antennae feathery

Abdomen slender

Abdomen stout

436 UNIT 5 BIOLOGY OF THE INVERTEBRATES

32-5 The large bee on the left, doing nothing, is a drone. The long, slender
bee with wings folded is the queen. The smallest bee, the worker, is dutifully
feeding the queen. (Annan Photo Features)

cles enable the wings to move at a high
speed, producing the familiar hum, and
making the bee a swift and enduring
flier.

The queen is the mother of the colony.

Perhaps nowhere else in the animal
kingdom is individual variation and di¬
vision of labor more noticeable than
in the Hymenoptera. Variation has
brought about three distinct forms: the
queen, the drone, and the worker (Fig.
32-5). The queen , which is nearly
twice as large as a worker, develops
from the special treatment of a fertile
egg. Workers enlarge a wax cell in
which the egg is to grow, and when the
grublike larva hatches, feed it with extra
portions of a high-protein food they

secrete called royal jelly. After five days
the larva spins a silken cocoon, changes
to a pupa, and is sealed in a large waxen
chamber by the workers. When the
mature queen emerges from her cell,
she seeks out other queen larvae in the
colony and kills them; or if she finds an¬
other adult queen, they fight until one
is killed. She never uses her sting ex¬
cept against another queen. If the
workers prevent the queen from destrov-
ing the other queens, she leaves the
hive, taking with her from 2,000 to
20,000 bees to seek a new home. By
this swarming new colonies are formed
and overcrowding is prevented.

After a few days the queen takes
a wedding flight up into the air where

CHAPTER 32 INSECT DIVERSITY 437

she mates with a drone (male bee), re¬
ceiving several million sperm cells.
Then she returns to the hive and begins
her lifework of laying eggs. This is no
small task as one queen may produce
as many as one million eggs per year
and often lives from five to ten years.
Although we call her a queen, she is in
no sense the ruler of the hive, but rather
its common mother.

The male bee. The drones , while larg¬
er than the workers, are smaller than the
queen and have a thick, broad body,
enormous eyes, and very powerful wings.
They develop from unfertilized eggs-
Their tongues are not long enough to
obtain nectar, so they have to be fed
by the workers. During the summer
a few hundred drones are tolerated in
the colony, because one of them must
function as a mate for the new queen.
The rest are of no use in the hive. This
easy life has its troubles, however.
With the coming of the autumn, when
honey runs low, the workers will no
longer support the drones, and starve
them or sting them to death. Their
bodies may often be found around the
hives in the early autumn.

The worker bees. The workers are by
far the most numerous inhabitants of
the hive. They are undeveloped fe¬
males, smaller than drones, and with
the ovipositor modified into a sting.
This is a complicated organ consisting
of two barbed darts operated by strong
muscles and enclosed in a sheath. The
darts are connected with a gland that
secretes the poison that makes a bee
sting painful. With the exception of
reproduction, all the varied industries
and products of the hive are the busi¬
ness of the worker. They attend and
feed the queen and drones. They act
as nurses to the hungry larvae, feeding
them with partly digested food from

their own stomachs. Workers clean
the hive of dead bees or foreign matter,
and they fan with their wings to venti¬
late the hive. All the time thousands
of workers are bringing in nectar and
pollen as needed for the use of the col¬
ony. In summer the workers literally
work themselves to death in three or
four weeks, but bees hatched in the fall
may live six months.

The worker has numerous special¬
ized structures that adapt her for her
tasks. The labium and maxilla together
form an efficient lapping tongue for
gathering nectar. On the four last
abdominal segments are glands that
secrete the wax used in comb-making.
The legs have various specialized struc¬
tures such as an antenna cleaner, pollen
packer, pollen basket, and pollen comb.
The structure and products of the hive.
The comb is a wonderful structure com¬
posed of six-sided cells in two layers.
It is so arranged as to leave no waste
space, and to afford the greatest storage
capacity using the least material.
Honey and beebread, a food substance
made by the worker from pollen and
saliva, is stored in the cells of the comb.
In a section of the hive called the
brood comb, the queen places one egg
in each cell.

Honey is made from the nectar of
flowers that is taken into the crop of
the bee. Here the sugars are changed
to a more easily digestible form which
is then emptied into the comb cells. In
these, the honey is left to thicken by
evaporation before the cell is sealed.
The removal of honey by man does not
harm the bees if enough is left for their
winter use. About 30 pounds are
enough to feed an average colony of
40,000 bees for an ordinary winter.

The language of the bees. One recent
discovery indicates that bees communi-

438 UNIT 5 BIOLOGY OF THE INVERTEBRATES

ABC

32-6 A. The dancing bee makes the diameter of the “round dance” upward on
the vertical surface of the comb. This indicates to the other bees that a source
of nectar is located in a direction toward the sun. B. This dance indicates
that the nectar is located in a direction away from the sun. C. Decipher this
message.

cate with one another bv a complicated
set of dances. When a worker returns
to the hive, she can inform the other
workers of the distance, direction, type,
and amount of nectar available. To do
this she dances on the vertical surface
of the comb. The dance consists of a
series of circles, with the bee making a
path through a diameter of the circle
each time she passes around one cir¬
cumference (Fig. 32-6). The direction
of the diameter indicates the direction
of the nectar source in relation to the
sun. If the food source is directlv to¬
ward or away from the sun, in relation
to the hive, the diameter of the circle is
in a line perpendicular to the horizon.
The dancing worker indicates a direc¬
tion toward the sun if she traces the
diameter while moving up, and thus
away from gravity. Forming the diam¬
eter while moving down indicates that
the food source is in a direction away

from the sun. If the food is 60 degrees
to the left of the sun’s direction, the di¬
ameter traced is 60 degrees to the left of
the vertical, and so on. Notice that
this is all the more remarkable because
the dancing bee is transposing the angle
between the direction of the sun and
that of the food source to a vertical
angle with respect to gravity.

As the other workers gather around
the dancing bee, they obtain the in¬
formation about the direction of a food
source. The type of food is determined
by smell. The distance of the food is
determined by the number of times the
bee waggles her body while tracing the
diameter of the circle. The greater the
amount of nectar available, the more
vigorous is her dance.

Other Hvmenoptera. Like the bees,
ants are social insects and the colonv
requires a queen. Unlike the bees,
most ants cannot sting, but they bite

CHAPTER 32 INSECT DIVERSITY 439

with jaws more powerful in proportion
to their size than those of any other in¬
sects. It is an interesting and generally
unknown fact that during the early
autumn, males and females will develop
wings and so are able to fly out to start
new colonies.

Wasps, both solitary and social,
and hornets, are interesting not only be¬
cause of the personal experiences we
may have had with their stings, but be¬
cause some of them are probably the
original papermakers of the world.
Their nests are made from a sort of
pulp obtained from strips of wood
chewed vigorously and mixed with se¬
cretions from the mouth.

Although the tiny ichneumon fly
usually goes unnoticed, it is very valu¬

able to us. These small insects lay their
eggs under the skin of living caterpillars.
When the larvae hatch they fed on the
tissues of the host, and bv the time they
pupate the caterpillar is dead.

The order Isoptera. Termites belong
to the order Isoptera , which means
“same-winged,” as they generally have
two pairs of similar wings. These social
insects are often thought of as harmful
because they destroy buildings, but as
you will see in Unit Eight, they also
perform an important function in the
forests by returning minerals to the soil.

Cellulose is a polysaccharide found
in the cell walls of plants, and is com¬
pletely indigestible by most animals.
In termites, however, an interesting as¬
sociation exists. The protozoan Tricho-

32-7 An old dried log has been opened to expose the home of carpenter ants.
Note the many runways and storehouses in which larvae and cocoons are safely
protected. (Chace from National Audubon Society)

440 UNIT 5 BIOLOGY OF THE INVERTEBRATES

nympha lives in their digestive tracts.
This protozoan produces an enzyme
that is capable of breaking down cellu¬
lose. The termite provides Trichonym-
pha with bits of wood and a place to
live. In turn the protist provides the
termite with digested cellulose, without
which the termite could not live.

The insect order Odonata. The name
of the order Odonata (o’hd- n-ah-ta)
comes from a Greek word meaning
“tooth.” It is probable that the larval
stage of these insects is responsible for
their name. The larva lives in streams
where it preys on the larvae of many
other insects. It catches them by rap¬
idly extending its elongated labium.
To some perhaps this lower lip resem¬
bled a big tooth that shoots out to catch
prey. The wings of the Odonata are
membranous and do not overlap. At
rest they are held at right angles to the
body. The abdomen is long, but there
is no stalk connecting it to the thorax.
These insects, exemplified by the drag¬
onfly and damsel fly, are very beneficial.
Not only are they predators when in the
larval stage, but even as adults they fly
over water, catching mosquitoes, gnats,
and other small insects.

The beetles. About 250,000 species of
beetles have been recorded, and most of
them can be easily recognized as beetles
because of their hard forewings which
fit closely over the body and resemble
a shell. They all have strong jaws and
undergo complete metamorphosis; they
belong to the order Coleoptera (koh-
lee-dhp-ter-a ) , which means “sheath¬
winged.”

Wood-boring beetles cause exten¬
sive losses; buffalo bugs are destructive
to carpets and furs; potato beetles rav¬
age gardens; weevils damage grain and
cotton. Texas alone has paid more
than $150,000,000 in attempting to con¬

trol the boll weevil. The Japanese
beetle, first discovered in this country
in New Jersey in 1916, has already be¬
come a great menace to fruit trees and
other crops. Carrion beetles, on the
other hand, are scavengers; ladvbugs eat
scale insects and thus aid the citrus fruit
industrv; and Calosoma beetles have
been introduced into New England and
elsewhere to help control the gypsy
moth.

The order Hemiptera includes many
pests. The “half-winged” insects have
sucking mouthparts and undergo in¬
complete metamorphosis. The edges of
their wings overlap, and only half of the
wing is thickened. One or two forms
are wingless. The insects belonging to
the order Hemiptera (hi-mzp-ter-a) are
the only insects constituting the true
bugs. Among them are many of our
worst pests, such as the chinch bug,
bedbug, squash bug, and stinkbug.
Others, less harmful to us, include
aquatic insects such as water striders,
backswimmers, water boatmen, and
water bugs.

The order Homoptera. Though some
of the members of the order Homoptera
(hoh-md/zp-ter-a ) are wingless, the name
means “similar wings.” When wings
are present they are held over the body
in an inverted V, like the roof of a
house. All have sucking mouthparts
and undergo incomplete metamor¬
phosis. Plant lice, scale insects, mealy
bugs, leaf hoppers, and others take a
huge toll of our wild and cultivated
plants. We are indebted, however, to
the lac insect, which alone of the
Homoptera is of economic benefit. We
get shellac from it, used throughout the
world as a base for lacquer and wood
finishes.

One species of cicada (si-kayd-a) , a
common representative of the Ilomop-

CHAPTER 32 INSECT DIVERSITY 441

32-8 The housefly carries disease-causing
organisms on the many hairs which cover its
body. (USDA)

tera, lives underground for two years;
another species, for 17 years. The ci¬
cadas then tunnel to the surface and
spend a week or two as adults. Their
high-pitched and strident notes, coming
from the treetops, are familiar on hot
summer evenings.

The Neuroptera. On warm summer
evenings in most parts of the United
States, lacewings can be seen fluttering
about. Their four delicate wings are
green and transparent. The veining of
the wings gives a network appearance so
that the name Neuroptera (n(y)u -rahp-
ter-a), which means “nerve-winged,”
seems appropriate. The lacewings have
long antennae and golden-colored eyes.
As larvae and adults they have chewing
mouthparts and an appetite for aphids,
which is why they are also called aphis
lions.

The two-winged insects. The insects
of the order Diptera have only two
wings. They have mouthparts suited
for piercing, rasping, and sucking.
Their metamorphosis is complete. This
order includes the mosquitoes, which
are known for their annoying habits as
well as their disease-carrying character¬
istics. The tsetse fly of Africa, responsi¬
ble for the transmission of the proto¬

zoan-caused sleeping sickness, and the
common housefly also belong to this
order.

One of our most notorious enemies
in the insect world is the housefly. Be¬
cause it breeds in filth, the housefly
often infects people with tvphoid, dys¬
entery, and other filth-borne diseases.
The housefly has large eyes, short, fleshy
antennae, and a club-shaped sucking
tube. It never bites, but the related
stable fly, or horsefly, bites cattle and
man. The wings of the housefly are
well developed and operate at high
speed because of the powerful muscles
of the thorax. The six legs are also well
developed, and the feet have claws and
sticky hairs that aid in clinging (Fig.
32-9). Unless these hair tips are free
from dust, they do not stick well and
the fly cannot walk easily on smooth sur¬
faces. You have probably noticed the
care with which a fly cleans its feet by
constantly rubbing them against each
other.

About 200 eggs are deposited in
horse manure or in similar matter by
the female. They hatch in one day into
the larval form called maggots , and in
this stage do some good as scavengers.
After eating and growing for five or six

Last

tarsal

segment

Hairs

Claws

Sticky

hairs

32-9 This enlarged drawing of the foot of
the housefly shows the projections and sticky
hairs which make it possible for the insect
to carry filth.

442 UNIT 5 BIOLOGY OF THE INVERTEBRATES

REPRODUCTION OF FLIES

1st generation

(eggs that hatch from

200 (100 females)

first female)

2nd

(100 females X

20,000 (10,000 females)

200 eggs each)

3rd

(10,000 x 200)

2,000,000

4th

200,000,000

5th

20,000,000,000

6th

2,000,000,000,000

2,020,202,020,200 in 12 weeks

days, the larvae pass into the pupal con¬
dition, inside the last larval skin, which
thus takes the place of a cocoon. From
this stage adults emerge in about a
week. The whole development from
egg to adult takes about two weeks.
Breeding begins early in spring and
continues until the cold weather starts.
Flies multiply at a tremendous rate. If
reproduction were unchecked and all
offspring survived (which fortunately is
not the case), one fly laying 200 eggs
would result in 2,020,202,020,200 flies in
12 weeks, as shown in the table above.
The mosquito. In the mosquito the
mouthparts — labrum, tongue, mandi¬
bles, and maxillae — are reduced to
sharp lancelike bristles. They are en¬
closed in the labium, which serves as a
sheath, and are ideally suited for pierc¬
ing and sucking. In order to dilute hu¬
man blood so that they can withdraw it
and prevent it from clotting in the pro¬
boscis, thev inject a little saliva. This
causes the irritation and swelling we call
a mosquito bite.

The female usually lays her eggs in
water. Ponds, rain barrels, and even
tin cans furnish ideal breeding places.
The eggs are deposited as single eggs or
in tiny rafts consisting of many eggs
covered with a waterproof coating. The
mosquito larva, called a wiggler , breathes

air, which it obtains through a tube pro¬
jecting from the posterior of its abdo¬
men. Often it can be seen with this
tube at the surface and the body hang¬
ing head downward in the water (Fig.
32-10).

The pupal stage is also passed in
the water; it differs from most insect
pupae in that it is active like the larva.
The adult emerges from the pupa,
whose shed skin acts as a raft. At this
critical time the mosquito must not fall
overboard or get its wings wet before
they expand, or it will die. There are
exceptions to this description, but it per¬
tains to most mosquitoes.

32-10 These larvae of the Culex mosquito
obtain oxygen by means of a breathing tube
which penetrates the surface of the water.
(Hugh Spencer)

CHAPTER 32 INSECT DIVERSITY 443

Our most common northern mos¬
quito, Culex , occasionally carries en¬
cephalitis, a form of sleeping sickness.
Anopheles (a-n<2/i/-e-leez) carries the
protozoan Plasmodium , which causes
malaria. Culex may be distinguished
from Anopheles by the fact that the
latter stands almost on its head when at
rest, while Culex holds its body more
nearly horizontal. Aedes ( ay-eed-eez ) ,
the mosquito that carries the virus of yel¬
low fever, is a tropical species and does
not usually invade our temperate regions.
The economic importance of insects.
Probably only a minority of insects are
distinctly detrimental to man. Yet
these obnoxious forms are so prominent
and well known that popular opinion is
apt to condemn all insects. Coping
with animals as numerous and active as
insects requires accurate knowledge of
their habits and life histories.

Their harmful activities are to:

1. Destroy grain, vegetables, and fruit
(numerous species).

2. Injure shade trees (tussock, gypsy,
and leopard moths).

3. Carry disease germs to animals and
man (fleas, lice, flies, mosquitoes).

4. Act as agents in the spread of plant
diseases (grasshoppers and aphids).

5. Destroy buildings and wood (beetles,
ants, termites).

6. Annoy and injure by bites and stings
(wasps, mosquitoes, gnats).

7. Affect food (beetles, cockroaches).

8. Destroy clothing and fabrics (clothes
moths, carpet beetles).

9. Act as parasites on domestic animals
and man (botflies, fleas, lice).

On the other hand, we owe to in¬
sects useful processes and products such
as the following:

1. Pollinating of flowers (bees, butter¬
flies, moths, certain types of flies).

2. Making silk (silk moth cocoon).

3. Furnishing of honey and wax (bees).

4. Furnishing of shellac (lac insect).

5. Supplying dye (cochineal insect).

6. Furnishing of material for ink (gall
insects) .

7. Acting as scavengers (maggots, bee¬
tles ) .

8. Killing of injurious insects (lady-
bugs, ichneumon flies).

The control of insect pests. While
most other forms of life have been un¬
able to maintain their numbers with the
spread of human civilization, insects
have continued to be a dominant form
of animal life. As you have learned in
your study of insects, their small size,
adaptability, and rapid rate of repro¬
duction have allowed them to become,
with man, the most biologically success¬
ful form of life. Since many insects
carry disease or destroy man’s crops and
stored foods, control of them is a con¬
stant concern. The war against insects
is fought with three principal weapons:
quarantine , chemical control, and bio¬
logical control.

The first weapon is administered
by the Bureau of Entomology and Plant
Quarantine operating under the U.S.
Department of Agriculture. Quaran¬
tine became necessary because at least
75 species of harmful insects have been
introduced into this country in the form
of eggs or larvae concealed on plants or
in fresh fruits. Today inspectors in ev¬
ery port of entry confiscate harmful in¬
sects or fungus pests. Quarantine has
also been used by many state govern¬
ments to help prevent widespread insect
destruction.

If quarantines had been in opera¬
tion earlier in our history, we might not
be struggling today with the European
corn borer, the Mexican bean beetle,
the Japanese beetle, the Oriental peach
moth, the gypsy moth, the cotton boll

444 UNIT 5 BIOLOGY OF THE INVERTEBRATES

SUMMARY OF CHARACTERISTICS OF INSECT

Order

Meta¬

mor¬

phosis

Mouthparts
in Adult

Economic
Importance
of Some

ORTHOPTERA

("straight-

winged”)

Incom¬

plete

Chewing

Damage crops,
act as pests

LEPIDOPTERA

("scale¬

winged”)

Com¬

plete

Siphoning

Pollinate flowers,
produce silk,
damage clothing
and crops

HYMENOPTERA

("membrane¬

winged”)

Com¬

plete

Biting or
sucking

Pollinate flowers,
act as pests,
parasitize other
pests,

make honey

ISOPTERA

("same¬

winged”)

Incom¬

plete

Chewing

Destroy buildings

ODONATA

(“toothed”)

Incom¬

plete

Biting

Destroy harmful
insects

COLEOPTERA

(“sheath¬

winged”)

Com¬

plete

Sucking or
chewing

Destroy crops,
act as pests,
prey on other
insects

HEMIPTERA

("half¬

winged”)

Incom¬

plete

Sucking

Damage plants,
act as pests,
carry disease

HOMOPTERA

(“like-

winged”)

Incom¬

plete

Sucking

Damage crops
and gardens

NEUROPTERA

/it

( nerve¬
winged”)

Com¬

plete

Chewing

Destroy harmful
insects

DIPTERA

(“two¬

winged”)

Com¬

plete

Sucking or
chewing

Carry disease,
act as pests

ORDERS

Examples

Grasshoppers,
crickets, katy¬
dids, locusts,
cockroaches

Butterflies,

moths

Bees, wasps, ants

Termites

Dragonflies,
damsel flies

Weevils, lady¬
birds, ground
beetles

Squash bugs, all
true bugs

Aphids, mealy
bugs, cicada,
scale insects

Dobson fly,
aphis lion

Flies,

mosquitoes

CHAPTER 32 INSECT DIVERSITY 445

weevil, and many other foreign insect
pests.

Chemical control. Chemical poisons
used to kill insects are called insecti¬
cides , which means, literally, “insect
killers.” Although expensive, chemical
control has been considered the most
effective method of reducing insect pop¬
ulations. Stomach poisons are used on
chewing insects such as beetles, grass¬
hoppers, and caterpillars. The poison
is consumed as the insect eats the leaves
of the plant. Some insects, such as true
bugs, aphids, lice, and scale insects, get
their food by sucking plant juices. A
thin coat of poison on the leaf does not
bother them as they push their beaks
through it. The insecticides used
against these pests are called contact
poisons. Many of these chemicals act
by clogging or poisoning the tracheae of
insects, causing them to suffocate.

DDT, a chemical insecticide. The field
of synthetic insecticides is best repre¬
sented by DDT (which is short for di-
chloro-diphenyl-trichloro-ethane) . DDT
is not a recent discovery, although we
have only used it extensively during the
past ten or twelve years. It was first
used effectively in World War II to
control body lice and the malarial mos¬
quito. Now we use it for fly and mos¬
quito control as well as for general gar¬
den and farm insect control.

DDT is a contact poison available
as powder and oil-solvent sprays. It is
extremely potent against many kinds of
insects, which it kills by its paralyzing
effect on the nervous system. The chi-
tin in the skeleton seems to attract and
hold the DDT. Roaches, mosquitoes,
and bedbugs succumb quickly m air
containing DDT mist. Important agri¬
cultural uses of DDT include spraying
of orchards and garden crops.

Both the powder and spray forms

of DDT can be absorbed through the
skin. Persons using it must take the
greatest care and must see that none
gets on any part of the body. Children
and pets should never be allowed where
it is being used.

Some disadvantages of chemical con¬
trol. Although the wide use of insecti¬
cides has been of considerable value in
controlling insect pests, many disadvan¬
tages are becoming more obvious.
Through natural selection many insect
strains have evolved that are resistant to
the effects of a particular chemical. For
example, successive generations of
houseflies produced from survivors of
DDT exposure have resulted in offspring
resistant to the poison.

Destruction of natural insect ene¬
mies of pest insects is one of the most
important and least recognized results
of large-scale application of insecticides.
Another is the accumulation of residues
of these poisons in mammals and birds
that feed on poisoned insects. This is
the chief way that so many animals are
affected — even those not sprayed or
those that are fairly invulnerable when
they are sprayed. But when they feed
on insects or other animals containing
DDT, the compound is stored up in the
body fat and hence may accumulate to
very high, and often lethal, levels.

In 1957 the larva of the gypsy
moth was spreading in the New Eng¬
land states. Since the larvae of this
moth eat leaves of oak and other hard¬
wood trees, its extermination was con¬
sidered desirable. DDT was sprayed
from airplanes over New York and Long
Island. Undoubtedly most of the gyp¬
sy moth larvae were killed, but so were
a multitude of plants, birds, crabs, fish,
beneficial insects, and mammals.
Truck gardeners lost many crops, and
even the milk from cows feeding on

446 UNIT 5 BIOLOGY OF THE INVERTEBRATES

contaminated plants contained DDT
and could not be sold.

Much of the damage from insecti¬
cides and many of the unexpected and
tragic results might have been avoided
had more attention been paid to the life
cycles and interactions among plants
and animals that you are now studying.
Biological control of insects. Since the
widespread use of insecticides has so
many disadvantages, entomologists are
constantly seeking other methods of in¬
sect control. We shall group these
methods under the general heading of
biological controls, since they utilize in¬
sect relationships with one another and
with their environment. Entomologists

have known for a long time, for exam¬
ple, that many male moths are attracted
to the female by a scent that is detect¬
able for great distances. The scent
from female gypsy moths has been used
to lure males into traps where they are
then killed by man. The lure scent has
now been synthesized from castor oil.
As little as 1/1000 gram proves to be
an effective and economic lure. The
search is continuing for similar attract-
ants in other harmful organisms.
Natural enemies as biological controls.
Perhaps you have wondered how, if Jap¬
anese beetles are so terribly destructive,
the Japanese are able to raise any plants
at all. The answer concerns natural en-

32-11 Eradication of such pests as the screwworm fly is possible only when
owners of infested animals report on the situation in their herds. Here, eggs
and larvae are being collected from an infested animal. (USDA)

CHAPTER 32 INSECT DIVERSITY 447

emies of insects. Insects were present
in America before our ancestors arrived
from other lands. Yet, even without
any control measures, these insects were
held in check by their natural enemies.
The fact that our worst pests are im¬
ported species is also explained in terms
of natural enemies. Introduced insects
are free from those animals that held
them in check in their native land.
Consequently, they have multiplied
here at a tremendous rate. Attempts
have been made to import the natural
enemies. Frequently, however, they
also become pests, thus creating a still
greater problem.

Birds are the most important natu¬
ral enemies of insects. Much of our
success in combating insects depends on
conservation of our bird life. We still
have not learned this lesson completely,
for we needlessly destroy necessary nest¬
ing places and feeding grounds for our
most important allies in the war on in¬
sects.

Snakes, frogs, spiders, and toads are
other valuable natural enemies of the
insect world. But our attitude toward
these creatures is far from enlightened.
How many of you have felt that you
rendered a valuable service by killing a
harmless garter snake, which lives largely
on small rodents and insects?

Insect control by environmental changes.
Man can control some insects by chang¬
ing their environment. For instance,
we can control certain insects by rotat¬
ing crops in one area. This method is
good for such localized pests as beetle
grubs and others, but it is ineffective
against those insects that migrate.

Another method is to drain those
places where certain pests breed. Ditch¬
es, ponds, and various shallow bodies of
water must be drained regularly to make
it impossible for these insects to com¬

plete their life cycle. This is especially
true of the mosquito. Drainage is a
costly process, however, and often re¬
sults in useless destruction of wildlife.
The water loss also hurts mankind far
more than the presence of mosquitoes,
which are more cheaply controlled by
other means.

Still another effective method of
environmental control is to correct
faulty methods of sewage disposal.
Flies often breed in sewage. If modern
disposal is practiced, the breeding places
are destroyed.

Equally important is proper gar¬
bage disposal. Again, some insects
breed in this type of filth; and the best
way to prevent this breeding is to burn
or treat garbage to be sure that none of
it is dumped where insects can get at it.
We should not forget man’s mechanical
methods of insect control, however.
The flyswatter, traps, flypaper, and elec¬
tric screens are still common ways of in¬
sect destruction.

Control by sterilization. Another suc¬
cessful method of insect control utilizes
radiation. About 25 years ago Dr. Ed¬
ward Knipling suggested sterilizing male
insects of a given harmful species. He
reasoned that, when released, the sterile
males would compete with normal
males. The females would then pro¬
duce infertile eggs after mating with the
sterile males. Since X rays have been
known to cause sterilization in insects,
Dr. Knipling proposed using this meth¬
od. He chose the screwworm fly for his
first experiment. The female fly of this
insect lays her eggs in any open wound
of a mammal. In the southern part of
the United States, Texas, Mexico, Cen¬
tral America, and South America, the
screwworm causes livestock losses esti¬
mated at manv millions of dollars.

Deer and other wildlife are also affected

448 UNIT 5 BIOLOGY OF THE INVERTEBRATES

by these insects, which are capable of
killing a full-grown steer in 10 days.

In 1954 Dr. Knipling began a test
of his theory on the isolated island of
Curasao in the Caribbean. Male screw-
worm flies were raised and treated with
X rays in Florida and then flown
and released from airplanes in Curasao.
The number and fertility of egg masses
on experimental goats decreased. Two
months after weekly distribution of irra-

IN CONCLUSION

diated flies, all the eggs were infertile.
The screwworm has been completely re¬
moved from Curasao. From 1957 to
1959 similar treatment has eliminated
the screwworm in Florida and parts of
Alabama and Georgia. Quarantine
methods are now used to prevent the
insect re-entering from infected areas of
the Southwest. Entomologists are now
determining whether similar steriliza¬
tion may be effective against other pests.

The great diversity in the structure of insects has allowed them to inhabit
almost every environment, utilize a variety of foods, and to compete with man
with considerable success. For these reasons insects are considered a biolog¬
ically successful group.

Because insects transmit disease, consume man’s crops and stored foods,
and damage gardens, several control measures have been employed. In order
to determine the most effective measures, however, the life history and habits
of each pest must be known. Often when using effective methods for control¬
ling one insect pest, we unwittingly destroy beneficial organisms or encourage
other harmful pests.

Our next unit will discuss the familiar animals with backbones, and per¬
haps some that are not quite so familiar. As you read about the various verte¬
brates, compare them with the invertebrates you have just studied.

BIOLOGICALLY SPEAKING

caterpillar drone Isoptera

chrysalis Hemiptera Lepidoptera

cocoon Homoptera maggot

Coleoptera Hymenoptera Neuroptera

Diptera insecticide Odonata

QUESTIONS FOR REVIEW

1. In what ways have insects adapted to terrestrial life?

2. Why are the mouthparts of the butterfly homologous to the mouthparts
of the grasshopper?

3. How are the Lepidoptera harmful in certain stages?

4. Many of the Hymenoptera may be considered well disciplined. Why is
this trait often confused with intelligence?

proboscis

queen

social insects

wiggler

worker

CHAPTER 32 INSECT DIVERSITY 449

5. Name the different types of bees and describe the functions of each in
the hive.

6. Do bees communicate with one another in the hive? Explain.

7. How are the Neuroptera helpful to man?

8. Give the life history of the housefly.

9. In trying to eliminate mosquitoes, why is prevention better than cure?

APPLYING PRINCIPLES AND CONCEPTS

1. In what ways can insects modify their environment?

2. Discuss further experiments that might be performed to understand com¬
munication of the bees.

3. Biologists state that the army ant of the tropics is entirely blind. Name
some other animals that lead successful lives in spite of the fact they are
missing one or more of the senses of higher animals.

4. Can a little fly grow to be a large fly? Explain.

5. What stage in the development of the housefly is easiest to control? Why?

RELATED READING

Books

Bandsma, A. T. and Brandt, R. T. The
Amazing World of Insects. The
Macmillan Co., Chicago. 1963
Berrill, N. J. The Living Tide. Dodd,
Mead and Co., New York. 1951
Borradaile, L. A. The Invertebrata
(3rd Ed.). Cambridge University
Press, New York. 1958
Buchsbaum, Ralph and Lorus J. Milne.
The Lower Animals: Living Inverte¬
brates of the World. Doubleday
and Co., Garden City. 1960
Carson, Rachel. The Edge of the Sea.
Houghton Mifflin Co., Boston.
1954

Chandler, Asa C. and Clark P. Reed.
Introduction to Parasitology (10th
Ed.). John Wiley and Sons, New
York. 1961

Darwin, Charles. Structure and Dis¬
tribution of Coral Reefs. Univer¬
sity of California Press, Berkeley.
1962

Edmondson, W. T. (ed) Ward and
Whipples Fresh-Water Biology.
John Wiley and Sons, New York.
1959

Emerton, James H. The Common Spi¬
ders of the United States. Dover,
New York.

Farb, Peter. The Insects. Time, Inc.,
New York. 1964

Green, James. A Biology of the Crus¬
tacea. Quadrangle Books of Chi¬
cago. 1961

Guberlet, Muriel. Explorers of the Sea :
Famous Oceanographers. Ronald
Press, Co., New York. 1964

Hegner, Robert. Parade of the Animal
Kingdom. The Macmillan Co.,
New York. 1935

Hylander, Clarence J. The Sea and
Shore. The Macmillan Co., Chi¬
cago. 1950

Jaques, Harry E. How to Know the In¬
sects ( 2nd rev.). Wm. C. Brown
Co., Dubuque, Iowa. 1947

450 UNIT 5 BIOLOGY OF THE INVERTEBRATES

Kamm, Josephine. Malaria Ross. Cri¬
terion Books, Inc., New York.
1964

Lane, Frank W. Kingdom of the Oc¬
topus: The Life of the Cephalop¬
oda. Sheridan House, Inc., New
York. 1960

McCarthy, Agnes. Creatures of the
Deep. Prentice-Hall, Inc. Engle¬
wood Cliffs, New Jersey. 1964
Metcalf, C. L. and W. P. Flint. De¬
structive and Useful Insects (4th
Ed. Rev.). McGraw-Hill Book
Co., New York. 1962
Miner, Roy Waldo. Field Book of Sea¬
shore Life. G. P. Putnam’s Sons,
New York. 1950

Morgan, Ann Haven. Field Book of
Ponds and Streams. G. P. Put¬
nam’s Sons, New York. 1930
Olsen, O. Wilford. Animal Parasites:
Their Biology and Life Cycles.
Burgess Publishing Co., Minneapo¬
lis, Minn. 1962

Swain, Ralph B. The Insect Guide.
Doubleday and Co., Garden City,
New York. 1948

Von Frisch, Karl. Bees , Their Vision,
Chemical Senses and Language.
Cornell University Press, Ithaca,
New York. 1950

Walford, Lionel A. Living Resources
of the Sea. The Ronald Press Co.,
New York. 1958

Articles

Singer, Marcus. “The Regeneration of
Body Parts.” Scientific American,
October, 1958

Waddington, C. H. “How Do Cells
Differentiate?” Scientific Ameri¬
can, September, 1953

Wenner, Adrian. “Sound Communica¬
tions in Honeybees.” Scientific
American, April, 1964

Williams, Carroll M. “The Metamor¬
phosis of Insects.” Scientific Amer¬
ican, April, 1950

UNIT SIX

BIOLOGY OF THE

VERTEBRATES

•»

This unit introduces the most advanced of all animals — a group which surpasses
others in structural organization and functional efficiency. The group is known
as the vertebrates because its members have spinal columns composed of bones
called vertebrae. The presence of a spinal column alone is not as important as the
nerve cord it encases and the highly developed brain this nerve cord joins. A most
efficient nervous system and the high-level responses and adjustments it permits are
the key to the biological supremacy of the vertebrates.

CHAPTER 33

INTRODUCTION
TO THE
VERTEBRATES

A quick review of invertebrate develop¬
ment. Nature seems to have tried sev¬
eral plans of body development in vari¬
ous invertebrate animals. In the proto¬
zoans the specialization of a single cell is
carried to the limit. Consider the para-
mecium with its cilia and trichocysts,
its gullet and contractile vacuoles. The
“slipper animalcule” is a truly marvel¬
ous cell, but it takes many cells to make
a complex animal.

You were introduced to a large
colony of cells in the sponges and coe-
lenterates. The beginning of tissues is
found in the ectoderm and endoderm
of these animals. In the flatworms and
roundworms there are organs that per¬
form such functions as digestion, repro¬
duction, excretion, and irritability with
much greater efficiency. The seg¬

mented worms reveal a more advanced
and efficient tubular digestive tract and
an elongated body divided into a series
of segments; a well developed circula¬
tory system transports blood through
closed vessels in this group of worms.

The clam and other mollusks have
the greatest possible protection for a
highly developed, soft-bodied animal.
Their shells are both a fort and a prison.
Mollusks, however, have not advanced
much in many millions of years.

The arthropods combine protec¬
tion with freedom of movement. But
an exoskeleton capable of supporting a
verv large arthropod would be too heavy
to move. Thus, arthropods have re¬
mained reasonably small. Numbers
alone maintain their place of importance
in the living world.

Why have the vertebrate animals be¬
come so important? The vertebrates
include the fishes, amphibians, reptiles,
birds, and mammals. They have
neither shell nor hard exoskeleton (with
the exception of certain reptiles and
mammals). A strong internal frame¬
work, or endoskeleton, supports a body
that is able to move freely and grace¬
fully. Although this prevents protec¬
tion of the soft body parts (except in
certain vertebrates such as the turtle
and armadillo, which have a hard outer
covering), the highly developed verte¬
brate brain and nervous system more
than make up for the lack of outer pro¬
tection. In escaping from enemies, the
vertebrate animals depend on their
keen sense organs and efficient move¬
ment as well as on their instinct and
intelligence.

Vertebrata — subphylum of the Chor¬
data. The Vertebrata (ver-te-bmy-tuh)
is one of four subphyla of the phylum
Chordata ( kor-cfoy-tuh ) . The other
three primitive subphvla give us some

452

CHAPTER 33 INTRODUCTION TO THE VERTEBRATES 453

Notochord

Dorsal nerve cord xGill slits

33-1 All chordates have a notochord in the early stages of their lives. Shown
above is Amphioxus, a primitive chordate that retains the notochord through¬
out its life.

idea of the animals from which verte¬
brates probably evolved. One sub¬
phylum includes two classes of worm-
like marine animals commonly called
acorn worms. Another is represented
by the strange tunicates, or sea squirts.
The fishlike Amphioxus (am-f ee-ahk-
sus), probably the best known of the
primitive chordates, represents the third
subphylum. Only because of the verte¬
brates, however, is the phylum Chordata
important today.

Early in life, all chordate animals
have a gristlelike rod running length¬
wise along the dorsal side of the body.
We call this rod a notochord , from
which the name of the phylum comes.
The more primitive chordates keep their
notochord throughout life. Some of
the lower vertebrates, such as the sea
lamprey, retain the notochord, but it
becomes surrounded by cartilage struc¬
tures of the spinal column. The noto¬
chord disappears early in the develop¬
ment of other vertebrates. It is re¬
placed by the vertebrae of the spine,
from which the name of the subphylum
comes. Thus we usually speak of verte¬
brates as animals that have backbones.

Other characteristics of chordates
include a nerve cord that runs down the
dorsal side of the body. In vertebrates
the bones of the spinal column enclose

a dorsal nerve cord, or spinal cord. In
invertebrates, such as the earthworm,
the main nerve trunk lies on the ventral
side of the bodv. The vertebrate nerv-
ous system is more complicated, larger,
and more specialized than that of any
other animal group.

All chordates have paired gill slits
that form openings in the throat. Like
the notochord, these disappear early in
the development of the land vertebrates
— the reptiles, birds, and mammals.
Another characteristic of vertebrates is
that they are able to produce antibodies
in their blood as protection against in¬
fectious organisms.

The rise of the vertebrates. Since
there is no direct fossil evidence of the
ancestors of the vertebrates, biologists
believe that they were soft-bodied lower
chordates whose remains would have
decomposed without leaving impressions
in rocks. Fossil records hint that some
lower chordates were passing through
evolutionary changes some half billion
years ago. Although biologists believe
that backboned animals lived before
the Ordovician period (Fig. 13-1, page
183), only a few fossils have been found
in rocks of this period. Vertebrate fos¬
sils become more numerous in the rocks
of the Silurian period. These aquatic
forms had unpaired fins and no jaws,

454 UNIT 6 BIOLOGY OF THE VERTEBRATES

like the modern lamprey and hagfish,
and a covering of tough, hard, bony
plates.

Sharks and other fishes with skele¬
tons of cartilage first appeared in the
Devonian period. These were followed
bv fishes with bony skeletons. Both
groups have survived to the present day.
The first land vertebrates, the amphib¬
ians, appeared at the end of the Devo¬
nian period. The first reptiles appeared
toward the close of the Carboniferous
period. Mammals first appeared about
the beginning of the Jurassic period,
and the oldest bird fossils are found in
rocks from the end of this time.

Fossil records show that many ver¬
tebrate forms appeared and disappeared.
It has been estimated that not more
than 1 percent of the amphibian, rep¬
tile, bird, and mammal species living in
the Jurassic period have living descend¬
ants today. Paleontologists believe that
there has been a tremendous turnover
in the kinds of animals that have in¬
habited the earth. Evolution of the
vertebrate animals will be discussed
more fully as we consider each class in
the following chapters. Living verte¬
brates are usually divided into seven
classes, as shown in the Appendix. The
classes are listed in order of their com¬
plexity.

Outstanding characteristics of verte¬
brates. Although fishes, frogs, reptiles,
birds, and mammals may seem very
different to you, they are actually sim¬
ilar in many ways. Their similarities
include the following:

1. A body with a head and trunk and,
in many, a neck and a tail.

2. Never more than two pairs of loco¬
motive appendages present. These
mav be fins, flippers, wings, arms, or
kgs.

3. Eyes, ears, and nostrils in the head.

4. Eyelids and separate teeth present
in most forms.

5. An internal skeleton ( endoskeleton )
of bone and/or cartilage (gristle).

6. A spinal column, or backbone, com¬
posed of vertebrae.

7. Two body cavities: a dorsal cavity
for the nervous system; and a larger
ventral cavity for the other internal
organs.

8. A heart on the ventral side of the
body; red blood corpuscles.

Specialized systems of the vertebrate

body. The vertebrate systems contain

many highly developed organs. These

systems include the following:

1. Integumentary system — the outer
body covering and special out¬
growths such as scales, feathers, or
hair for protection.

2. Muscular system — muscles attached
to bones for body movement; mus¬
cles that form the walls of the heart
and the digestive organs and blood
vessels.

3. Skeletal system — the bones and
cartilage that make up the body
framework.

4. Digestive system — the many spe¬
cialized organs concerned with the
preparation of food for use by the
body tissues.

5. Respiratory system — gills or lungs
and related structures used in the
exchange of gases between the or¬
ganism and its external environ¬
ment.

6. Circulatory system — the heart and
blood vessels, which function as the
transportation system of the body.

7. Excretory system — the organs that
remove wastes from the body.

8. Endocrine system — glands that pro¬
duce secretions necessary for the
normal functioning of the other
systems.

CHAPTER 33 INTRODUCTION TO THE VERTEBRATES 455

9. Nervous system — the brain, the
spinal cord, nerves, and special
sense organs — the most highly de¬
veloped system of a vertebrate.

10. Reproductive system — the male or
female organs of reproduction.
Lines of development in the verte¬
brates. The skeleton shows an interest¬
ing development in the vertebrate
classes. The lampreys, sharks, and rays
have a skeleton of cartilage throughout
life. Fishes, amphibians, reptiles, birds,
and mammals develop a bony skeleton.
These animals start life with a cartilag¬
inous framework. But early in life bone
cells replace most of the cartilage. Min¬
erals deposited in the bones make them
hard and strong.

The classes of vertebrates also show
an interesting change from water exist¬
ence to life on land. The lampreys,
sharks, rays, and bony fishes are adapted
only for life in water. Their limbs are
in the form of fins. Their gills absorb
dissolved oxygen from the water. Water
flows over the gills through gill slits in
the throat. After you have studied the
frog as a representative amphibian, you
will realize that it represents a transition
from water to land. During the tadpole
stage, a frog is a fishlike animal with
gills and a fin.

The vertebrate heart and brain show
great development over those of the
invertebrates. The fish heart has only
two chambers. One chamber receives
blood from the body, while the other
pumps blood to the gills. The frog has
a three-chambered heart and a more
complex circulatory system. Birds and
mammals have still more complex hearts
consisting of four chambers. One side
of the heart receives blood from the
body and pumps it to the lungs. The
other side receives blood from the lungs
and pumps it to the body. This heart

is really a double pump. Man’s heart
likewise consists of four chambers.

Similar advances can be seen in the
brain of the vertebrates. One brain re¬
gion, known as the cerebrum , is the cen¬
ter of instinct, emotion, memory, and
intelligence. This brain area increases
in relative size through the classes of
vertebrates. The brain of the mammal
has the largest cerebrum in relation to
the size of the body.

The highly developed behavior of ver¬
tebrates. Behavior is the way in which
an organism responds to stimuli. The
type of sense organs, nerve pathways,
and organs specialized for nervous con¬
trol determine the stimuli to which or¬
ganisms are sensitive and in many re¬
spects the responses they can make.
Protists and plants have no specialized
nerve tissue. Their responses are lim¬
ited to simple tropisms. The nerve net
of the hydra enables it to behave as
a unit. The sense organs, nerve cords,
and ganglia of the higher invertebrates
permit even more integrated behavior.
In the vertebrates, the highly developed
sense organs, brain, and nerves extend¬
ing to and from all parts of the body
provide the basis for complex behavior.

Much of the activity of a higher
animal is inborn, or innate behavior.
Since such behavior is inherited, it is
reasonable to assume that it is controlled
by genes. Reflexes are simple innate
responses. In a reflex, an animal auto¬
matically responds in a certain manner
to a given stimulus. For example, stim¬
ulation of the surface of the eve or eve-
lid will cause blinking. Reflexes are
involuntary; that is, the animal reacts
to the stimulus without any control on
its part. Generally reflexes protect the
organism from harm. Even man de¬
pends on reflex behavior for many of
his responses.

456 UNIT 6 BIOLOGY OF THE VERTEBRATES

Mammalia (leopard)

Amphibia (salamander)

Reptilia (turtle) #7

Reptilia (snake)

Osteichthyes (fish)

Chondrichthyes (shark)

Chondrichthyes (skate)

Cyclostomata (lamprey)

33-2 Here you see the representatives of the seven classes of vertebrates.
What characteristics do they all have in common?

CHAPTER 33 INTRODUCTION TO THE VERTEBRATES 457

33-3 Species preservation is one of the
strongest instincts in animals. These opos¬
sum babies ride safely on their mother’s
back until they are able to take care of them¬
selves. (Encyclopaedia Britannica Films)

The most interesting and the least
understood of the innate responses are
those that we call instincts. Instincts
are complex patterns of unlearned be¬
havior. They are involuntary, since the
animal performs them without a delib¬
erate decision.

Self-preservation is a basic instinct
in all vertebrates as well as in many
invertebrates. In times of danger an
animal will respond to the “flight or
fight” instinct of self-preservation. Have
you ever cornered an animal that would
normally flee? A seemingly harmless
animal like a squirrel will bite and claw
viciously if it cannot escape from an
enemy. The suckling instinct, another
example of self-preservation, directs the
nursing mammal early in life. The self-
preservation instinct causes the tiny bird
to pick its way through the shell at
the time of hatching.

A second instinct, that of species
preservation , directs animal reproduc¬
tion and care of the young. This is
the instinct that, for instance, drives
the Pacific salmon up the streams of the

Northwest to spawning beds. The adult
salmon lose their lives and a new gen¬
eration comes downstream to the ocean.
This instinct also causes the sunfish to
defend its nest against an intruder from
which it would normally flee.

Biologists have found that instinc¬
tive behavior is important to animals
with a short life span, such as the in¬
sects. Some insects have only a few
days or even a few hours in which to
reproduce their kind. If they had to
take the time to learn how to survive,
they would probably become extinct.

Because of their well developed
nervous systems, vertebrates are capable
of learned behavior as well as innate be¬
havior. A conditioned reaction is a
form of learned behavior common
among vertebrates. We say that a re¬
action becomes conditioned when a par¬
ticular behavior response continually
follows a specific stimulus, resulting in
habit formation. We develop this level
of behavior when we teach a dog to
heel at a command or shake hands at
a given signal. Even fishes in an aquar¬
ium can be trained to go to one special
corner of the tank when you approach,
if you always feed them in this particular
place.

Intelligent behavior is still more
complicated. An intelligent response
is a deliberate act that involves memory
of past experiences, association, and
judgment. Instinct can be observed in
all vertebrates. To a lesser degree most
of the vertebrates are capable of condi¬
tioned reactions. Birds and mammals
exhibit intelligent behavior to some
degree. Man, however, is supreme
among the vertebrates in development
of intelligence. He is also unique among
living things in his ability to learn to
communicate by symbols, both in speak¬
ing and in writing.

458 UNIT 6 BIOLOGY OF THE VERTEBRATES

IN CONCLUSION

As you study the vertebrates, you will see some characteristics developed to
the highest degree. The fish excels in swimming — its streamlined body cuts
through the water like a torpedo. The frog is at home both on land and in the
water. The bird has long been the model for life in the air.

The antelope and gazelle hold speed records among land animals. The
elephant is a symbol of strength. What about man? Man is no match for
other vertebrates in physical abilities, but his is the superior brain.

In the next chapter, we shall begin our study of vertebrates of the ocean
depths, lakes, rivers, and streams. Here the primitive fishlike vertebrates and
the many bony fishes, the most important of aquatic animals, are found.

BIOLOGICALLY SPEAKING

cartilage

cerebrum

Chordata

conditioned reaction
endoskeleton

innate behavior
instinct

intelligent behavior

notochord

reflexes

self-preservation
species preservation
vertebrae
Vertebrata

QUESTIONS FOR REVIEW

1. Distinguish the vertebrate skeleton from that of lower animals?

2. Name seven classes of vertebrates and give an example of each class.

3. Which is more efficient, the endoskeleton or the exoskeleton? Why?

4. Describe and locate the notochord.

5. How would you distinguish the vertebrates from the other chordates?

6. What are eight vertebrate characteristics?

7. Name ten vertebrate systems.

8. Which of the various brain regions is the center of instinct, emotions, and
intelligence?

9. What is the relation between a stimulus and a response?

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the development of vertebrates through the various classes, using
the skeleton, organs of respiration, heart, and brain as illustrations.

2. Self-preservation and species preservation are instincts. Which is stronger?
Give one or more illustrations to prove your answer.

3. How can you distinguish instinctive behavior from intelligent behavior in
observing the activity of various vertebrate animals?

4. Why are instinct and intelligence more vital to survival of a vertebrate
than to an invertebrate such as a clam, a starfish, an insect, or a cravfish?

CHAPTER HU

THE FISHES

Blood-sucking ‘‘vampires” of the Great
Lakes. About 40 years ago a deadly
vertebrate menace made its way from
the waters of Lake Ontario through the
Welland Canal at Niagara Falls and
into Lake Erie. The sea lampreys were
invading new waters. During a much
earlier period this species had become
established in Lake Ontario. Here their
movement was stopped by Niagara Falls.
But the Welland Canal, built to carry
shipping around the falls, gave them
passage into Lake Erie. Ten years later
the lamprey hordes had spread through
Lake Huron. They traveled through the
Straits of Mackinac into Lake Michigan
and through the locks at Sault Sainte
Marie into Lake Superior.

What sort of creature is this death¬
dealing sea lamprey? Biologists place
it in the class Cyclostomata (sy-klos-
toh-mczy-tuh ) , which means “round¬
mouthed.” This small class of primitive
vertebrates is also sometimes called
A gnatha ( ag-nuy-tha ) , which means
“jawless.” Ancestors of the cyclostomes
are believed to have been the first ver¬

tebrates, appearing in the Ordovician
period (Fig. 13-1, page 183).

The sea lamprey has a slender eel¬
like body. The mature lamprey reaches
a length of about two feet and a weight
of about one pound. Its skin is soft
and slimy, brownish-green, and blotched
or mottled. Paired fins are lacking in
the lamprey. Two single fins along the
back and a tail fin aid the lamprey in
swimming in its characteristic rippling
manner.

The head of a lamprey is curious
and quite different from that of a fish.
Instead of jaws the lamprey has a fun¬
nel-like mouth lined with sharp, horny
teeth (Fig. 34-1). A rasping tongue,
also bearing teeth, lies in the center
of the mouth. Small eves are situated
on either side of the head. Between
the eyes, on the top of the head, is a
nasal opening that leads to a sac con¬
taining nerve endings associated with
the sense of smell. Seven oval gill
slits, resembling portholes of a ship,
lie in a row on each side of the head
behind the eyes.

During its adult life the sea lam¬
prey is a very destructive predator. It
attaches its sucking mouth to the side
of a fish and gouges a hole through the
scales with its rasping teeth (Fig.
34-2). It feeds on the blood and body
fluids of its victim and may even suck
out internal organs. When it has killed
or weakened a host fish, the lamprey
moves on to another. The injury is
not always fatal, for many healthy fishes
with lamprey scars are found. Its fa¬
vorite host is the lake trout, one of the
most important commercial fishes of
the Great Lakes. When trout are not
available, the sea lamprey attacks white-
fish, pike, and other species.

The sea lamprey has exterminated
the lake trout in Lake Huron and Lake

459

460 UNIT 6 BIOLOGY OF THE VERTEBRATES

34-1 This close-up view of the sucking
mouth of a sea lamprey shows how it can
attach itself to the body of a fish and feed
on the blood and body fluids of its host.
(U.S. Fish and Wildlife Service)

Michigan and has seriously reduced the
population in Lake Superior.

Our hope of eliminating the lamprey
menace. The spawning habits of the

lamprey are helping us to destroy this
deadly menace. Sea lampreys reach
sexual maturity in the Great Lakes dur¬
ing the months of May or June. At
this time they enter fast-flowing streams
that feed the lakes. They lay their
eggs in circular depressions in the gravel
bottom of cold streams. An average
female lays from 50,000 to 100,000 eggs.
After about 20 days of development, the
eggs hatch into tiny, blind larvae. The
larvae leave the nest and float down¬
stream until they reach quiet water with
a mud bottom. Here they burrow into
the mud and start a period of inactive
life. During this period the larva lies
in a U-shaped burrow and feeds on
plant and animal matter drawn into
the mouth in a current produced by
moving cilia. After five or more years
in the stream, the larva changes to an
adult and starts its journey downstream
to the lake. It lives about one year as
an adult, feeding constantly on fish.

Two methods of lamprey control
have been used. One frequently used
is the lamprey trap, designed to cap¬
ture the adults as they migrate upstream
to spawn. Electrodes charged with elec¬
tricity are put in a row across the stream;
this charges the water and stops the
movement of all kinds of aquatic an¬
imals. The migrating lampreys and
fishes swim along the edge of the
charged area into traps. Here the lam-

34-2 This trout bears the scar left by a sea lamprey. (U.S. Fish and Wildlife
Service)

CHAPTER 34 THE FISHES 461

34-3 The shark has strong and sharp teeth which can inflict fatal wounds to
its prey. (Annan Photo Features)

preys are destroyed. The fishes are
caught and put back into the stream
above the traps.

A more recent method of lamprey
control that has replaced many electric
barriers makes use of a selective poison
which kills the larvae buried in the
streams. It has been used extensively
in Lake Superior, where it has reduced
the lamprey population about 80 per¬
cent, and is now being applied in Lake
Michigan.

Sharks and rays. To the class Chon -
drichthyes (kon-dn'k-thih-eez) , which
means “cartilage fishes,” belong the few
remaining fishes of those that controlled
the ancient seas. Sharks, rays, and
skates make up this class of fishes, which
is also called Elasmobranchii (i-laz-moh-
hrcmk-ee-ee ) , meaning “plated gills.”
The shark resembles the true fishes in
many wavs, but certain characteristics
put it in a separate class.

The body of a shark is torpedo¬
shaped. Its fins resemble those of true

fishes. The upper portion of the tail
fin is longer than the lower portion —
a characteristic of ancient fishes. The
shark’s mouth is a horizontal slitlike
opening on the ventral side of the head.
The jaws of most species are lined with
sharp razor-edged teeth. Water enters
the mouth, passes over the gills on each
side of the head, and is forced out
through pairs of gill slits. Gills, as you
probably know, are the special respira¬
tory organs of fishes and their relatives.
As the name of the class implies, the
skeleton of sharks and rays is composed
of cartilage rather than bone.

Sharks include the largest living
fishes. The whale shark, the giant of
sharks, reaches a length of 50 feet or
more and a weight of over 20 tons. The
great white shark, or man-eating shark,
may exceed 40 feet in length.

Reports of attacks by sharks have
increased as more people turn to the
water for recreation. Scientists are be¬
ginning to ask how the shark locates

462 UNIT 6 BIOLOGY OF THE VERTEBRATES

its prey. Does it depend on sight, smell,
or both? When opaque plastic eye
shields are put over a shark’s eyes, the
“blinded” shark takes longer to find
food. Thus sight must be important
in locating prey. It has been demon¬
strated that sharks can smell blood for
only a few hundred feet, but that they
can detect sound over some distance.
It may be that they are first attracted
to possible victims by the sound of
splashing water.

The rays and skates have broad,
flat bodies, with whiplike tails. Rays
swim gracefully through ocean waters,
moving their flat bodies like wings.
They often lie half buried in the sand
of the ocean bottom. The tail has a
sharp barbed spike near the tip that
causes a painful wound when driven
into a victim. Sting rays often come
close to shore. The torpedo ray has
an excellent means of defense. It is
equipped with electric organs so power¬
ful that they can knock a man off his
feet. This adaptation is probably used
for obtaining food.

The skate has a triangular-shaped
body with a long, thin tail. Locomo¬
tion is by the triangular pectoral fins,
which undulate from front to rear,
rather than flapping like wings. Two
fins attached to the rigid tail act as a
steering device.

Skates are well adapted for life on
the bottom of the ocean. Water is
taken in through two spiracles located
on top of the head, just back of the
eyes. Then the water passes out through
the gill slits underneath. Thus skates
avoid taking in debris when they
respire.

The true fishes. Biologists put all the
true fishes in the class Osteichthyes (os-
tee-zk-thih-eez), which means '‘bony
fishes.” Their bony skeleton distin¬
guishes them from the lampreys, sharks,
and rays of modern times. This class
is sometimes called Teleostomi (tel-ee-
os-toh-mee), which means “complete¬
mouthed,” to distinguish its members
from the cyclostomes. The bony fishes
first appeared in the Devonian period,
which is often called the age of fishes

34-4 The sting ray is found in warm waters and has a long whiplike tail pro¬
vided with a barbed stinger capable of inflicting a severe wound. (Annan Photo
Features)

CHAPTER 34 THE FISHES 463

because Devonian rocks contain the
greatest numbers and variety of fish fos¬
sils. This was a time of rapid evolution
for the fishes because their members
were so widely distributed then.

Bony fishes have gills as respiratory
organs. Limbs are in the form of fins.
Most fishes have an outer covering of
overlapping scales , or plates. Fishes are
ideally suited to aquatic life. In a wide
variety of forms they live in practically
every water environment of the earth.

The body of a fish is divided into
three regions: head, trunk, and tail.
In most species the body is perfectly
streamlined — tapered at both ends, or
spindle-shaped. The lack of a neck is
no disadvantage to a fish. It can turn
its body as easily in the water as most
other animals can move their heads.

Many people confuse the tail of a
fish with the tail fin. The tail is the
solid muscular region posterior to the
trunk. The tail fin is an outgrowth of
this region.

The body covering of fishes. Scales
grow from pockets in the skin and over¬
lap like shingles on a roof. Scales in¬
crease in size as a fish grows. In other
words a young fish has the same number
of scales it will have at maturity. As
scales grow, concentric rings are formed.
These rings are closer together in regions
of winter growth than in summer
growth, thus making it possible to dis¬
tinguish seasons of scale growth and de¬
termine the age of the fish.

A slimy secretion of the skin seeps
between the scales and forms a cover- v
ing that lubricates the body. This body
slime is important in locomotion and in
escape from enemies. It is important,
too, in protecting the fish from attack
by parasitic fungi, bacteria, protozoans,
and other organisms. If you handle a
fish with dry hands, you remove some

of the slime and expose the body to par¬
asites. You can avoid this by wetting
your hands before you pick up a fish.

Many fishes have bright colors,
often arranged in lines, bars, or spots.
Much of the coloration of fishes is due
to pigment granules present in special
cells of the skin known as chromato-
phores (kroh-mczt-uh-fohrz ) . Color may
also be caused by guanin (gwah- nin)
crystals , which are excretory products
found in the scales, skin, eye, and air
bladder. The chromatophores are sup¬
plied with nerves, but there is no nerve
supply to the guanin crystals.

Many fishes have a remarkable
ability to alter their colors in a short
time, as a means of camouflage. Light
stimuli received by the eye are trans¬
mitted to the brain. Then nerve im¬
pulses are sent to the chromatophores,
causing the pigment granules either to
spread out or clump together. Bright
or dark colors appear when the pig¬
ments spread out. The colors fade
when the pigments clump together.
Colors produced by granules in the
chromatophores show through the trans¬
parent scales. Other colors of fishes
are due to the reflection of light from
the scale surfaces and interference of
the guanin crystals.

Many fishes illustrate countershad -
ing, another means of camouflage.
Darker pigments on the dorsal side of
the body tone down the bright light
that strikes the fish from above. As
a result the upper side blends with the
lighter side, giving the body a uniform
appearance when viewed from the side.
The darker colors on the dorsal side
blend with the bottom or with deep
water when the fish is seen from above.
The light colors on the ventral side
blend with the bright light on the sur¬
face when the fish is viewed from below.

464 UNIT 6 BIOLOGY OF THE VERTEBRATES

34-5 This drawing shows the external structure of a bony fish, the yellow perch.

Head structures of the fish. Although
different species of fish vary greatly in
body form, many fishes are similar to
the yellow perch, shown in Fig. 34-5.
The head tapers toward the mouth, of¬
fering the least possible resistance as
the fish moves through the water. The
protective covering of the head is in the
form of plates instead of scales. The
mouth is large and is situated at the
extreme anterior end. Carnivorous fishes
such as the yellow perch have numer¬
ous small, sharp teeth extending from
the jawbones and from the roof of the
mouth. These teeth slant toward the
throat, making it easy for the yellow
perch to swallow a prey, but hard for
the prey to escape. The tongue is fas¬
tened to the floor of the mouth and is
not movable. It functions as an organ
of touch rather than one of taste.

Two nasal cavities lie on the top
of the head, anterior to the eves. Paired
nostrils lead to each nasal cavity. The
nostrils function in smell only. They
do not connect with the throat and are
not involved in respiration. The fish
has no external openings to the ears.

The ears are embedded in the bones of
the skull and probably function as bal¬
ance organs in addition to receiving vi¬
brations carried by the bones of the
skull.

The eyes of most fishes are large
and somewhat movable. Eyelids are
lacking. The pupils are large compared
with those of other vertebrates, and ad¬
mit the greatest possible amount of
light.

At each side of the head is a cres¬
cent-shaped slit that marks the posterior
edge of the gill cover, or operculum
(oh-per-kyoo-lum). This hard plate
serves as a protective cover for the gills
beneath it. By raising the unattached
rear edge of the operculum, you can see
the gills lying in a large gill chamber.
The edges of the opercula nearly meet
on the lower side of the fish, where the
head fastens to the trunk at a narrow
isthmus.

Structures of the trunk and tail. Vari¬
ous kinds of fins develop from the trunk
and tail. Each fin consists of a double
membrane supported by cartilaginous
or spiny rays. Fins serve a variety of

CHAPTER 34 THE FISHES 465

purposes in the fish and differ in form
in various species.

Two kinds of fins are paired. These
are considered homologous with the
limbs of other vertebrates. The pec¬
toral fins are nearest the head, and cor¬
respond to the front legs of other verte¬
brates. Posterior to these are the pel¬
vic fins , which correspond to hind legs.
The paired fins serve as oars when the
fish is swimming slowly. They also aid
in steering and in maintaining balance
when the fish is resting, and are used
in moving backward. The caudal fin
grows from the tail and aids in pro¬
pelling the fish.

Dorsal fins are situated along the
top middle line of the trunk. The an¬
terior, or spiny, dorsal fin of the perch
contains sharp projections that aid in
defense. The spines of this fin raise
upward toward the head, thus making
it difficult to swallow the perch tailfirst.
The posterior, or soft, dorsal fin lacks
these spines. Both dorsal fins serve as
a keel to keep the fish upright while

swimming. Another single fin, the anal
finy grows along the middle line on the
lower side. This fin, like the dorsal fin,
serves as a keel and helps to maintain
balance.

Powerful muscles, arranged in zig¬
zag plates, occupy the region of the
trunk above the spinal column. A
thinner muscle layer lies along the body
wall on the sides of the trunk. The
tail region is solid muscle with the
spine running through most of it.

If you examine the sides of a fish
closely, you will notice a row of pitted
scales extending from the head to the
tail fin. These make up the lateral line .
Nerve endings and a narrow tube he
under the scales. The line acts as a
sense organ as it is sensitive to low fre¬
quency underwater vibrations.

The digestive system of the fish. Many
fishes are vegetarians and feed on algae
and other water plants. Carnivorous
species eat other animals such as frogs,
other fish, and a wide variety of inverte¬
brates, including crayfish, worms, and

Cranial

cavity

Air bladder
Spinal cord
Ear

Intestine

Tongue
Pharynx
Gill filaments

Pericardial cavity

Kidney

Vertebra

Gall bladder

Pyloric caeca

Urinary opening
Opening from gonads
Anus

34-6 This is a lateral view of a dissected yellow perch.

466 UNIT 6 BIOLOGY OF THE VERTEBRATES

insects. Some fishes, like the bass and
pike, swallow fish almost as large as
themselves. Especially in carnivorous
fishes, the mouth is a large trap for cap¬
turing prey. The throat cavity, or phar¬
ynx, leads to the opening of the short
esophagus (Fig. 34-6). The esopha¬
gus in turn joins the upper end of the
stomach. The stomach is in line with
the esophagus, thus allowing a large
prev to extend from the stomach through
the mouth and even protrude for a
time as digestion occurs. A rather short
intestine leads from the lower end of
the stomach. Several short tubes called
the pyloric caeca (py-Zor-ik see- ka) ex¬
tend from the intestine and secrete di¬
gestive fluids. Digestion continues as
food moves through the short loops of
intestine. A well developed liver lies
close to the stomach. Digested food is
absorbed through the intestine wall. In¬
digestible matter leaves the intestine
through the anus on the lower side.
Circulatory system of a fish. The blood
of a fish is similar to that of other ver¬
tebrates. It contains both red and
white corpuscles. The heart (Fig.
34-7) pumps blood through a system
of vessels of three types. Arteries carry
blood from the heart to the gills, then
to all other regions of the body. The
arteries lead to thin-walled capillaries,
which penetrate all of the bodv tissues.
The capillaries come together to form
larger vessels called veins, which return
blood to the heart.

The heart lies in the pericardial
cavity on the lower side of the body just
behind the gills. A large vein, the car¬
dinal vein, receives blood from various
branches coming from the head, trunk
and tail, and the liver (Fig. 34-8). Just
above the heart the cardinal vein en¬
larges into a thin-walled sac, the sinus
venosus. This sac joins the first heart

34-7 The fish heart has only one atrium and
one ventricle.

chamber, or atrium, often referred to
as the auricle. From the atrium blood
passes into the ventricle, the thick-
walled, muscular pumping chamber of
the heart. Blood is pumped from the
ventricle with great force through the
ventral aorta, leading to the gills. This
arterv begins with a muscular bulblike
structure, the bulbus arteriosus, which
is attached to the ventricle. This struc¬
ture is very noticeable in the fish heart.
The ventral aorta branches to the two
sets of gills, then rebranches to form
arteries that lead to the four gills on
each side of the head. Another large
arterv, the dorsal aorta, receives blood
from the gills and, through its branches,
supplies the head, trunk, and tail. Blood
returns to the heart through the car¬
dinal veins, thus completing the circu¬
lation. Some of the blood returns
through veins from the digestive organs
and the liver. Various cell wastes are
removed as blood circulates through
the kidnevs.

The blood of a fish passes through
the heart once during a complete cir¬
culation. The heart receives deoxy-
genated blood from the bodv tissues
through the cardinal vein and pumps
it through the ventral aorta to the gills.
In circulating through the gills, the
blood discharges carbon dioxide and

CHAPTER 34 THE FISHES 467

Efferent Anterior Posterior

34-8 In this diagram, which shows circulation in the fish, note that the blood
flows in a single circuit: from the body to the heart, to the gills, and to the
body again.

receives oxygen. This blood, now oxy¬
genated, is received from the gills by
the dorsal aorta for circulation to the
body tissues.

The gills — organs of respiration. In a
bony fish such as the yellow perch, four
gills lie in a gill chamber on each side
of the head. A gill consists of a car¬
tilaginous arch to which is attached a
double row of thin-walled, threadlike
projections called gill filaments (Fig.
34—9 ) . These filaments are richly pro¬
vided with capillaries, so that the blood
is brought into close contact with the

water over a large surface. The gill
arches have hard, fingerlike projections
called gill rakers on the side toward the
throat. These prevent food and other
particles from reaching the filaments
and keep the arches apart to allow free
circulation of water.

Blood enters a gill at the base of
the arch through the afferent branchial
artery (af- fur-ent brcm-kee-al ar- tur-
rhee). Branches of this artery enter
each gill filament, where the blood en¬
ters a network of capillaries. Here car¬
bon dioxide is discharged from the

Efferent branchial artery

Anterior
gill arch

Afferent

branchial

artery

Gill raker

Efferent

branchial

artery

Anterior ^
gill arch

Afferent

branchial

artery

Capillaries

34-9 A sectioned view of a gill filament is shown at the top of the center draw¬
ing. The right-hand drawing shows a portion of a single filament, much
enlarged.

468 UNIT 6 BIOLOGY OF THE VERTEBRATES

blood, and oxygen is absorbed through
the thin walls of the capillaries and fila¬
ments. Oxygenated blood returns to
the gill arch and flows out the top of
the gill through the efferent branchial
artery to the dorsal aorta.

The fish requires a continuous flow
of water over its gills. Water is drawn
into the open mouth as the gill arches
expand and enlarge the cavity of the
pharynx. The edge of the operculum
is pressed against the body as water is
drawn in. The mouth is then closed,
the gill arches contract, and the rear
edge of the operculum is raised, thus
forcing the water over the gill filaments
and out of the gill chamber around the
raised edge of the operculum. The for¬
ward motion of the fish aids this process
when the fish is swimming.

The air bladder— pressure organ. A
thin-walled sac, the air bladder , lies in
the upper part of the body cavity of
a fish. In fishes that swallow air it con¬
nects with the pharynx by a tube. In
others it is inflated with gases (oxygen,
nitrogen, and carbon dioxide) which
pass into it from the blood. This sac
acts as a float and adjusts the weight of
the fish so that the weight of the animal
equals the weight of the water it dis¬
places. This equilibrium allows the fish
to remain at any desired depth in the
water with little effort.

Fishes live at various water levels
at different seasons of the year. The
air bladder adjusts to these variations by
losing air to the blood or receiving addi¬
tional air. When a fish is adjusted to
deep water and is caught and brought
to the surface suddenly, the air bladder
expands and may push the esophagus
into the mouth. One group of fishes
known as darters has no air bladder.
They sink to the bottom after each of
their jerky swimming motions.

Olfactory lobe
Cerebrum
Optic lobe

Cerebellum

Medulla
oblongata

Spinal cord

Olfactory nerve

Optic

nerve

Nerves
to gills

Nerve
to ear

Nerve
to heart
and
viscera

34-10 This is a dorsal view of the brain of
a fish. Compare its development with that
of some of the higher invertebrates.

The nervous system. The nervous sys¬
tem of the fish includes the brain, spinal
cord, and the many nerves that lead to
all parts of the body. The brain lies
in a small bony cavity, the cranial cav¬
ity. It consists of five distinct parts
(Fig. 34-10). At the anterior end are
the olfactory lobes , from which the
nerves, sensitive to odors, extend to the
nostrils. Behind these lobes are the
two lobes of the cerebrum , which con¬
trol the voluntary muscles. In these
lobes instincts are centered. Back of
the cerebrum are the optic lobes , the
largest of the fish’s brain. Optic nerves
lead from these lobes to the eyes. Be¬
hind them lies the cerebellum , which
coordinates muscular activity, and finally
the medulla oblongata, which controls
the activities of the internal organs.
The spinal cord passes down the back
from the medulla and is encased in the
vertebral column. Nerves connect the
spinal cord with all the various parts
of the body.

The fish’s brain is not highly devel¬
oped when compared to those of higher
vertebrates. It shows, however, a great

CHAPTER 34 THE FISHES 469

advance over the so-called brains of
invertebrates. As you study the brains
of other vertebrates, compare them with
the fish brain. The same regions are
present. There is, however, a gradual
increase in the size of the cerebrum in
proportion to the other brain regions
as vertebrates become more advanced.
As the cerebrum increases in size, there
is a corresponding increase in nervous
activity on higher levels, such as emo¬
tional responses, memory, and intel¬
ligence.

Sensations of the fish. The relatively
large optic lobes of the brain indicate
that fishes have a well-developed sense
of sight. However, vision at even mod¬
erate water depths is greatly reduced
because of insufficient light. Fishes are
known to be nearsighted and probably
do not see objects clearly at distances
greater than a few feet. The fish eye
focuses on objects by moving the nearly
spherical lens forward or backward
rather than changing the shape of the
lens, as in our eyes. Scientists are not
sure whether the fish sees colors or
lives in a world of black, white, and
shades of gray.

The internal ears are sensitive to
vibrations of a lower frequency than
those to which the human ear is sensi¬
tive. The bones of the skull, in which
the ears are embedded, function effi¬
ciently in transmitting vibrations from
the water to the sensitive ear structures.

Probably the fish’s most acute sense
is that of smell. Scientists have con¬
ducted extensive experiments to demon¬
strate the reaction of fishes to odors in
the water. It has been found that fishes
can distinguish the odors of many water
plants, even when these plants are
dipped for only a short time in pure
water. Similarly, they can detect the
odor of hands washed in a stream as

well as the odors of many animals, es¬
pecially mammals. Scientists now be¬
lieve that odors direct fishes to feeding
areas among water plants. It is possible,
too, that salmon find the mouths of
rivers and streams during the spawning
season by the odors of plants living in
these fresh-water bodies.

Reproduction in the fish. The repro¬
ductive organs, or gonadsy lie in the pos¬
terior region of the body cavity. The
opening from the gonads is just behind
the anal opening.

Eggs develop in the ovaries of the
female over a period of several months.
As the eggs enlarge the ovaries swell,
and may bulge the sides of the fish.
Sperm develop in the paired testes of
the male. Moments after the female
lays her eggs, or spawns , the male swims
over them and discharges a sperm-
containing fluid called milt . Sperm
swim to the eggs and fertilize them, and
the development of the embryos begins.
This development may require a few
days to many weeks, depending on the
species and the temperature of the wa¬
ter. The developing fish is nourished
by a large quantity of nonliving material,
the yolk, which is present in the egg.
A part of the yolk known as the yolk sac
remains attached to the young fish a
short while after it has hatched.

The sunfish, like many fishes, depos¬
its its eggs in a depression made in the
bottom of a pond. After spawning,
the male guards the nest, fighting off any
intruder. The male stickleback makes
a curious nest of bits of plants and rub¬
ble and drives the female into it for
spawning. Then he chases her away
and takes entire charge of the nest and
eggs. Channel catfish spawn in holes
in a bank or in a discarded can or other
receptacle they have found on the bot¬
tom.

470 UNIT 6 BIOLOGY OF THE VERTEBRATES

34-11 Some fish build nests on the shallow bottom. Compare the stickleback
nest (left) consisting of strands of algae with that of the sunfish (right).

Guppies, mollies, platys, and sword¬
tails are fresh-water tropical species com¬
monly reared in home aquariums.
These curious fish bear their young alive.
The female retains the eggs within her
body and receives sperm from the male
during mating. The young fish develop
internally and are brought forth alive.

Spawning is not a very efficient
process. Many eggs never receive sperm.
Large numbers are eaten by fishes and
other aquatic animals before they have

had a chance to hatch. After hatch¬
ing, the young fish are in constant dan¬
ger of being eaten by cannibalistic fishes
and other animals. Regardless of the
high mortality rate, the species survive
because of the tremendous numbers of
eggs laid. The number varies from
about 500 in the trout to six or seven
million in the codfish. Generally the
number of eggs is proportionate to the
amount of hazard encountered by the
embryos and young fish.

IN CONCLUSION

No vertebrate has challenged the fish for supremacy in the water. A stream¬
lined body with scales and fins, gills, and a two-chambered heart seem to be
perfectly adapted to an aquatic environment. Alligators and water-dwelling
snakes, swimming and diving birds, whales and seals share the water with
fishes, but none of these higher vertebrates compares with fishes in importance.

In your study of the next group of vertebrates, the amphibians, you will
find many fishlike animals. Others, such as the toad, are land dwellers. In
studying these animals, you will find the basic structures of fishes carried to
a higher degree of perfection in the development of land animals.

CHAPTER 34 THE FISHES 471

BIOLOGICALLY SPEAKING

air bladder

dorsal aorta

optic lobe

anal fin

dorsal fin

Osteichthyes

atrium

gill arch

pectoral fin

branchial artery

gill filament

pelvic fin

bulbus arteriosus

gill raker

pericardial cavity

cardinal vein

guanin crystals

pyloric caecum

caudal fin

isthmus

scales

cerebellum

lateral line

sinus venosus

Chondrichthyes

medulla oblongata

spawn

chromatophores

milt

spinal cord

countershading

olfactory lobe

ventral aorta

cranial cavity
Cyclostomata

operculum

ventricle

QUESTIONS FOR REVIEW

1. Describe the manner in which the sea lamprey attacks its prey.

2. What methods are being used to control sea lampreys?

3. What characteristics distinguish the shark from bony fishes?

4. How does the body slime protect a fish?

5. Describe countershading in the fish.

6. Name the fins of the yellow perch, and discuss the use of each.

7. Describe the organs of the alimentary canal of a fish in the order in which
food passes through them.

8. Describe the structure of the fish heart.

9. Trace a drop of blood from the ventral aorta through a gill to the dorsal
aorta, and describe changes in the blood during its circulation through a

gill-

10. Locate and describe the air bladder. What is its function?

11. Name the various regions of the fish brain and the kind of nervous activity
centered in each part.

12. Discuss the efficiency of the various sense organs of a fish.

APPLYING PRINCIPLES AND CONCEPTS

1. Explain how the body covering, limbs, and sense organs of a fish are ideally
suited to life in the water.

2. Why does a fish die in the air, even though the air contains more oxygen
than the water in which it lives.

3. Fishes lay enormous numbers of eggs, yet seldom overpopulate the waters
in which they live. Give several reasons to account for this.

CHAPTER 35

THE

AMPHIBIANS

The arrival of the amphibians. Biolo¬
gists believe that living things were con¬
fined to the water for millions of years.
Then, at the end of the Devonian period
(Fig. 13-1, page 183), changes must
have occurred that resulted in the evolu¬
tionary development of life on land.
The modern fishes we discussed in the
last chapter all live in the water, of
course, and breathe by gills. But re¬
member the air bladder they use as an
organ of balance? This organ is ac¬
tually similar to an interesting adapta¬
tion that occurred in some of the early
bony fishes. These fishes developed
lunglike structures that enabled them
to breathe air. There are even two
types of these primitive lungfishes sur¬
viving today. They are found in Aus¬
tralia, Africa, and South America —
all areas of seasonal drought. Func¬
tional lungs are absolutely necessary for
their survival. When the droughts
come, the lungfish digs a burrow in the
bottom of the pool, where it lives

curled up in a state of inactivity until
the rains come and the water returns.

Another unique characteristic of the
early lungfishes was their jointed, or
lobed, fins somewhat resembling legs.
Until about 25 years ago, it was thought
that lobed fins had disappeared by the
end of the Mesozoic Era. Fossils of
lobe-finned fishes have been found in
the rocks of the late Paleozoic and the
Mesozoic Eras, but there is no trace of
them in Cenozoic rocks. Biologists
believed that they had become extinct
some 70 million years ago. Then in
1939 a native South African commercial
fisherman caught a type of fish he had
never seen before. It was five feet long
and was covered with large bluish scales.
The fisherman gave the strange fish to
a local museum where it was mounted.
When it was shown to Dr. J. L. B. Smith
of Rhodes University, he recognized it
as a coelocanth, a lobe-finned fish that
was supposed to have been long extinct.
Since then many coelocanths have been
caught in the waters between Madagas¬
car and Mozambique. They retain
lungs as outgrowths of the throat, and
the fins resemble crude legs.

The development of lungs and leg¬
like fins in some of the early bony fishes
was probably an adaptation for survival.
There were great climatic changes in
the Devonian period. The ponds and
streams often dried up or became stag¬
nant, just as they do in the habitats of
the modern lungfishes. Typical fishes
could not have survived either condition.
A fish that possessed some type of lung,
however, could survive both drought and
stagnancy. Likewise, a fish that pos¬
sessed leglike fins could perhaps crawl
away from a dried-up pond to a pond
that still had water in it. This line of
reasoning has led biologists to believe
that the primitive lungfishes, with their

472

CHAPTER 35 THE AMPHIBIANS 473

35-1 Above left: an artist’s representation of how early lobe-finned fishes may
have looked when they first came out on land. Above right: early amphibians.
The drawings at the bottom show their skeletons, which have been recon¬
structed from fossil remains. Note the marked similarity between the two types
of animals. (American Museum of Natural History)

lobed fins, were transitional forms be¬
tween true fishes and amphibians.

The name Amphibia means, liter¬
ally, “having two lives.” It refers to
the fact that, although the amphibians
were able to develop some adaptations
for life on land, they have never become
completely free of water. They must
still return to the water for reproduction,
as their soft jellylike eggs would quickly
perish on dry land. Furthermore, the
young of all amphibians are completely
water-bound for a period of time, and
few adult amphibians can travel far
from shore because they must keep their
skins moist.

Amphibians were the dominant ver¬
tebrates into the Permian period. They
became reduced in number near the
end of the Paleozoic Era. The remain¬
ing large forms became extinct in the
Triassic period. The amphibians of
today, which include only three orders,
appeared after the Triassic. They are

not very significant vertebrates in our
time.

Characteristics of the amphibians.
The water-bound young of amphibians
are fishlike; but they change to land-
dwellers of quite different structure
when adult. This series of changes is
a metamorphosis, just as is the life
history of certain insects. In this tran¬
sition from water to land forms, many
strange combinations of gills and lungs,
fins and legs, occur. Gills are found
on animals with legs, and fins are some¬
times found on animals with lungs.

In general the Amphibia are dis¬
tinct from other vertebrate animals in
the following ways:

1. Body covered by a thin, flexible, and
usually moist skin, without scales,
fur, or feathers.

2. Feet, if present, often webbed.

3. Toes soft and lacking claws.

4. Immature or larval forms, vegetar¬
ian; adults, usually carnivorous.

474 UNIT 6 BIOLOGY OF THE VERTEBRATES

5. Heart, two-chambered in larvae;
three-chambered in adults; circula¬
tion well developed.

6. Eggs fertilized externally as soon as
laid.

7. Metamorphosis from young to adult
state.

Orders of Amphibia. The order A poda
(ap- oh-da) contains a few surviving leg¬
less amphibians of the tropics. These
strange, wormlike creatures are often
called caecilians (see-sz'Z-ee-anz) . A
second order, Caudata ( kaw-duy-tuh ) ,
includes amphibians which have tails
throughout life. Here we place the
familiar salamanders and newts. The
most familiar amphibians are the frogs
and toads, members of the order Sali-
entia ( sal-ee-en-tee-a ) . Frogs and toads
are different from other amphibians in
that they lack tails in the adult stage.
These animals, along with certain of
the salamanders, undergo an interesting
transition. They change from an aquat¬
ic life as a larva to a semiaquatic or ter¬
restrial life as an adult.

The salamanders. Modern salamanders
are very similar to their ancestors. The
only evidence of evolution has been
the replacement of cartilage with some
bone in the skeleton. You are prob¬
ably familiar with several of the sala¬
manders, although you may have called
them lizards. Many salamanders re¬
semble lizards in general form. Both
have elongated bodies, long tails, and
short legs; but a salamander has soft,
moist skin and lacks claws on its toes.
The lizard has a scale-covered body and
claws on its toes, characteristics of rep¬
tiles almost never found among the
amphibians.

Salamanders have very little protec¬
tion from their enemies. A few have
skin glands which secrete bad-tasting
substances. Others have color pigments

that change with their surroundings.
Salamanders cannot survive under dry
conditions. This is why they are found
under damp logs and stones or swim¬
ming about in water.

Salamanders range in size from a
few inches to species several feet in
length. The giant salamanders are rep¬
resented in the United States by the
American hellbender, which reaches a
length of two feet or more. This large
salamander, with loose grayish or red¬
dish-brown skin, lives in the streams of
the eastern United States. One of its
relatives, the giant salamander of Japan,
grows to five feet in length and is the
largest living amphibian.

Another large salamander of the
Middle West is the mud puppy or
water dog (Necturus) . Many an unsus¬
pecting fisherman has been startled
when he pulled one of these slimy sala¬
manders from a mud-bottom stream in
the late evening or night. The mud
puppy may reach a length of two feet.
It has a flattened, rectangular head,
small eyes, a flattened tail, and two pairs
of short legs. The most striking fea¬
ture of its body is the pair of dark red,
bushy gills attached at the base of the
head just above the front legs. The
presence of gills is a larval trait retained
throughout life.

The tiger salamander, shown in
Fig. 35-2, is found in most of the United
States. It is one of the larger salaman¬
ders, reaching a length of six to ten
inches. The bright yellow bars and
blotches on a background color of dark
brown give it its name. This sala¬
mander lives as an aquatic, gill-breath¬
ing larva for about three months, after
which it leaves the water and lives on
land. Both lungs and the thin, moist
skin function in respiration during the
land-dwelling stage.

CHAPTER 35 THE AMPHIBIANS 475

35-2 The tiger salamander is widely distributed in North America. (American
Museum of Natural History)

Certain tiger salamanders and
others of the same genus remain aquatic
throughout life and reproduce while
still in the larval stage. Larval sala¬
manders called axolotls (ak- sa-laht-ls)
have long been known in Mexico and
the southwestern part of the United
States. Their curious characteristic of
producing eggs or sperm in the larval
stage once led biologists to believe that
axolotls were separate species of sala¬
mander. But feeding experiments with
thyroid gland tissue or thyroid extracts
caused the animals to metamorphose
into adults. This revealed them to be
larval forms of the tiger salamander
and some of its near relatives. Research
revealed that the waters in which axo¬
lotls live are deficient in iodine. Iodine
is essential for the production of a thy¬
roid hormone needed for metamorpho¬
sis. Thus, axolotls are reduced to a
larval existence because of a thyroid de¬
ficiency.

The spotted salamander might
easily be confused with the tiger sala¬
mander, since the two species are sim¬
ilar in size and color. The spotted sala¬
mander, however, is shiny black with
yellow spots. Its tail is round, while
that of the tiger salamander is flattened
laterally.

We often speak of the land-dwell¬
ing stage of small salamanders as newts.
The crimson-spotted newt is especially
interesting because of its “triple life.”
This small salamander hatches, usually
in May, into a gill-breathing aquatic
larva. After about two months, it
changes to a land-dwelling stage with
lungs. The coral-red color of this stage
gives it the name red eft. One or two
years later, the skin color changes to
greenish-olive with crimson spots along
the sides. The newt returns to the
water and resumes aquatic life, breath¬
ing through its skin while under water
and using its lungs at the surface.

The salamanders we have discussed
are but a few of the many kinds you can
find under piles of wet leaves, under
rocks in stream beds, in abandoned
wells, and in other moist places. Aquatic
and land-dwelling stages of salamanders
make ideal specimens for aquariums and
moist terrariums. With a little coaxing
they will eat meal worms or small insects
from your hand.

Toads and frogs. The toads and frogs
that lived previous to the Jurassic period
all had elongated bodies and a long tail.
Biologists believe that the great changes
in body form were sudden. The most
conspicuous change was the disappear-

476 UNIT 6 BIOLOGY OF THE VERTEBRATES

ance of a tail in the adult. Other less
obvious changes made them better
suited to life on land. The hind legs
developed an extra joint and the ankle
bones became elongated. These adap¬
tations gave great power to the legs for
leaping. Although the front legs were
short, they were well suited for absorb¬
ing the shock of landing from a jump.
Modern frogs and toads have wide
mouths and front-hinged sticky tongues
that can catch insects with lightning
speed. These amphibians have lived
for more than 200 million vears and are

found over most of the earth in many
habitats. Biologists consider them
among the most highly successful ver¬
tebrates.

Although there is great structural
similarity, the frogs and toads differ in
some respects in their anatomy and be¬
havior. The toad is the most terrestrial
of all amphibians, and after leaving the
water early in life, never returns except
to lay eggs. The toad starts life as a
tinv black tadpole which soon grows
legs, absorbs its tail, and hops onto land
as a small, brown froglike creature with
the warty skin characteristic of its kind.
Adults of the common toad, Bufo
(by oo- foh), are usually reddish-brown
above and grayish-yellow beneath.

Toads sleep most of the day under
rocks or boards but are active at night,
snatching insects with their quick, sticky
tongue. When disturbed thev have no
choice but to he close to the ground.
The toad has lost the swimming ability
of other amphibians and on land moves
with clumsy motion. In its in-between
existence, this unfortunate creature lacks
efficient locomotion in anv environment
and is able to survive only because of
its protective coloration. Toads are
widespread, but there are none in Aus¬
tralia.

35-3 The drawing shows the front (A) and
the hind foot (B) of a frog. The fully webbed
hind foot enables the animal to be an excel¬
lent swimmer.

Interesting amphibians are the tree
frogs, of the genus Hyla. Most of them
have amazing protective coloration, and
several have the ability to change their
color. Members of the genus that live
in trees have a stickv disk on each toe
which enables them to cling to vertical
surfaces. One member of the genus
Hyla , the well known spring peeper,
lives in swamps and bogs rather than
in trees.

Peeper eggs are laid in early spring,
and the tiny tadpoles feed on algae and
protozoans. The adults eat mosquitoes
and gnats, which ought to give these
frogs a place in our affection. A curious
fact about their tadpole stage is that
they often leave the water before the
tail is entirely resorbed. Apparentlv
they are able to breathe air earlier in
their metamorphosis than the majority
of other frogs.

The most common frog in the
United States is the leopard frog, which
inhabits nearly everv pond, marsh, and
roadside ditch. It frequently travels
considerable distances from the water
and may be seen hopping through the
grass in meadows. Its name comes
from the large dark spots, surrounded
by yellow or white rings, that cover the
gravish-green background color of the
skin. The under surface of the leopard
frog is creamy white, so that it blends
with the light sky when viewed from

CHAPTER 35 THE AMPHIBIANS 477

below while it is resting on the surface
of a pond.

T he bullfrog , so named because its
sound resembles the distant bellowing
of a bull, is the most aquatic of all
frogs. It seldom leaves the water ex¬
cept to sit on the bank of a lake or
pond at night. The color of the bull¬
frog varies from green to nearly yellow,
although the majority of them are green¬
ish-brown. The under surface of the
body is grayish white mingled with nu¬
merous dark splotches.

The large fully webbed hind feet
of the bullfrog make it an excellent
swimmer. These legs are well devel¬
oped and ten inches long in large spec¬
imens. The bullfrog’s diet is quite
varied and includes insects, worms,
crayfish, small fishes, and even an oc¬
casional duckling.

The economic importance of frogs.

Much of the diet of frogs consists of in¬
sects. If frogs had no other value at all,
this service alone would justify their
protection. Many states have recog¬
nized their value and have passed laws
regulating the hunting of frogs and pro¬
hibiting their capture during the breed¬
ing season.

The large hind legs of the bullfrog
are a table delicacy. Frog farms, oc¬
cupying large marshy areas, supply much
of the demand for legs. The smaller
species of frogs are widely used by fisher¬
men for bait. As a biological specimen
for dissection in the laboratory, the
frog has long been a favorite. Since its
internal organs are arranged similarly
to those of the human body, dissecting
a frog is an excellent introduction to
human anatomy.

In recent years frogs have been used
for pregnancy tests. Hospitals and
clinics have become one of the best
customers of frog collectors. With all

of these uses, you can see that we must
guard our frog population and conserve
the lakes, marshes, and other watery
habitats in which they thrive.

Anatomy of the frog. Facing page 488
you will find a leopard frog as seen bv
the “Trans-Vision” process. The first
page (Plate I) shows the lower side of
the frog. The upper side is shown on
the last page (Plate VIII). As you
turn the pages between these, you will
see the internal organs at various depths
of the body. Pages on the right show
the ventral side of the organs. The
transparencies on the left show the dor¬
sal side of the organs.

As we discuss the structure of the
frog — its form and body covering, legs,
head structure, and internal organs —
find the various organs in the plates of
the “Trans-Vision.”

External structure of the frog. The

frog’s body is short, broad, and angular.
It lacks the perfect streamlined form we
find in the fishes. For this reason the
frog is not the graceful swimmer the
fish is, nor does its awkward hopping on
land compare with the graceful move¬
ment of most other land animals. This
is the price the frog must pay for liv¬
ing in two environments.

The skin is thin, moist, and loose.
It is richly supplied with blood vessels.
Glands in the skin secrete mucus,
which reaches the surface through tiny
tubes. This slimy substance makes the
frog difficult to hold. The skin lacks
any protective outgrowths such as the
scales and plates of fishes and reptiles.
Adaptations of the frog’s legs. The
front legs of the frog are short and weak.
Each has four inturned toes with soft
rounded tips, as shown in Fig. 35-3.
The front feet lack a web and are not
used for swimming. The inner toe of
a male frog is enlarged, especially during

478 UNIT 6 BIOLOGY OF THE VERTEBRATES

the breeding season. The front legs are
used to prop up the body on land and
to break the fall after a leap.

The hind legs are enormously de¬
veloped and adapted in several ways for
swimming and leaping. The thigh and
calf muscles are very powerful. The
ankle region and toes are greatly length¬
ened, forming a foot that is longer than
the lower leg. A broad flexible web
membrane lies between the five long
toes, making the foot an extremely effi¬
cient swimming organ. The hind legs
fold against the body when the frog is
resting on land. In this position the
animal is ready for a sudden leap.

The head and its structure. Probably
the most noticeable structures of the
head are the eyes. The eyes of frogs
and toads are among the most beautiful
of the animal kingdom. The colored
iris surrounds the elongated black pupil
opening. Muscles attached to the eye¬
ball rotate the eye in its socket. The
frog’s eyes bulge above the head, but
can be pulled into their sockets and
pressed against the roof of the mouth.
In this position they help to hold food
in the mouth.

When the eyes are pulled down,
the upper and lower eyelids fold over
them. The bulging eyes serve as peri¬
scopes when the frog is under water. It
can float just below the surface with its
eyes above water. A third evelid, the
nictitating (mk-tih-tay-ting) membrane ,
joins the lower lid. This thin covering
keeps the eyeball moist on land and
serves as a protective covering when the
frog is under water.

The nostrils are located far forward
on top of the head, allowing the frog to
breathe air with all but the top of the
head submerged.

The frog has no external ears. The
eardrum, or tympanic ( tym-pcm-ik)

35-4 The frog’s tongue is especially well
adapted for catching insects, because it is
both flexible and sticky. Note how it is at¬
tached at the front of the mouth.

membrane , lies on the surface of the
body just behind the eyes. The cavity
of the middle ear lies just below the
tympanic membrane. A canal, or Eu¬
stachian ( yoo-stay-kee-un ) tube , con¬
nects each middle ear with the mouth
cavitv. The inner ears are embedded
in the skull.

The frog’s mouth — an efficient insect
trap. The frog’s mouth extends literally
from ear to ear. If you watch a frog
catch a fly, you will discover why the
mouth must be so large — it serves as an
insect trap. The thick, sticky tongue is
attached at the front in the floor of the
mouth and has two projections on the
free end (Fig. 35-4) .

When a frog catches an insect, the
mouth opens wide and the tongue flips
over and out. The insect is caught on
the tongue surface and is thrown against
the roof of the mouth. The mouth
snaps shut and the insect is swallowed.
This happens so quickly you can hardly
see it. Two vomerine teeth , projecting
from bones of the roof of the mouth,
aid in holding the prey. The frog has no
teeth on the lower jaw. Small, conical
maxillary teeth, projecting from the
upper jawbone, also aid in holding prey.

Inside the frog’s mouth, as shown
in Fig. 35-5, you can see various open-

CHAPTER 35 THE AMPHIBIANS 479

ings. The internal nostril openings lie
in the roof near the front, on either side
of the vomerine teeth. Far back on the
sides of the roof of the mouth are the
openings of the Eustachian tubes. In a
corresponding position in the floor of
the mouth of a male frog are openings
to the vocal sacs. When a frog croaks,
air is forced through these openings into
bladderlike sacs which expand between
the ears and the shoulders. This action
adds resonance and volume to the sound.
When the frog croaks under water, air
is forced from the lungs, over the vocal
cords, into the mouth and back to the
lungs. The throat contains two single
openings — a large gullet opening leads
to the stomach; below the gullet open¬
ing is the slitlike glottis, the opening to
the lungs.

Digestive system of the frog. While
the diet of the adult leopard frog con¬
sists largely of insects and worms, it can
swallow even larger prey because of its
large, elastic gullet. The short gullet
leads to the stomachy an oval enlarge¬
ment of the food tube. The stomach is

Vomerine teeth

Internal nostril

Maxillary teeth

Eye socket

Eustachian tube
(to ear)

Gullet opening

Opening to vocal
sacs (male)

Glottis (opening
to lungs)

Tongue

Attachment of tongue

35-5 Here you see a diagram of the internal
structure of the frog’s mouth. Its rather
large size is an adaptation for the obtaining
of food.

large at the gullet and tapers at the
lower end. Here the stomach joins the
small intestine at a point referred to as
the pylorus. The stomach content
passes into the small intestine through
a muscular pyloric valve.

The small intestine lies in several
loops supported by a fanlike membrane,
the mesentery. The small intestine of
the frog is proportionally longer than
that of the fish. At its lower end, the
small intestine leads to a short, broad
colon, or large intestine. The lower end
of the large intestine, leading to the anal
opening, is termed the cloaca (kloh-uy-
ka). The walls of the cloaca contain
openings of the ureters from the kidneys,
the urinary bladder, and the oviducts of
the female frog.

The large three-lobed liver partially
covers the stomach. It is a storehouse
for digested food and also a digestive
gland which secretes bile. The bile col¬
lects in the gallbladder on the dorsal side
of the liver and passes into the upper
small intestine through the bile duct.
The pancreas , a second digestive gland,
lies inside of the curve of the stomach.
Pancreatic fluid passes into the small in¬
testine with bile through the bile duct.
Both of these fluids are necessary for
intestinal digestion. Mucous glands in
the walls of the stomach and intestine
secrete mucus, a lubricating fluid. Tiny
gastric glands in the walls of the stomach
secrete gastric fluid, another vital di¬
gestive fluid.

We find in the frog a digestive sys¬
tem like that of other vertebrates. A
long food tube, or alimentary canal, is
composed of specialized regions where
digestion and absorption of digested
food take place. The length of the
alimentary canal increases the general
efficiency of both these processes tre¬
mendously.

480 UNIT 6 BIOLOGY OF THE VERTEBRATES

The respiratory system of the frog.

Have you ever wondered how the frog,
an air breather, can stay under water
for long periods and lie buried in the
mud at the bottom of a pond through a
winter hibernation? The answer lies in
skin respiration. The skin of the frog
and other amphibians is thin and richly
supplied with blood vessels. While the
frog is in the water, dissolved oxygen
passes through the skin to the blood.
Carbon dioxide is given off. Respira¬
tion through the skin supplies the frog’s
needs as long as it is quiet. During
hibernation the body processes continue
at a very slow rate. The oxygen need
is very low. However, body activity such
as swimming greatly increases the need
for oxygen, and the skin cannot supply
enough. The frog then comes to the
surface and breathes air.

We inhale and exhale air bv in-

creasing and decreasing the size of our
chest cavities. This is accomplished by
movement of the ribs and diaphragm,
a muscular partition at the bottom of
the chest cavity. The frog has no dia¬
phragm and therefore has no chest cav¬
ity; nor does the frog have ribs. When
the frog lowers the floor of its mouth
with the mouth closed, air rushes in
through the open nostrils into the par¬
tial vacuum. When the floor of the
mouth springs up, air passes out through
the nostrils.

The lining of the mouth is well
adapted for respiration because it is
thin, moist, and richly supplied with
blood vessels. At this point we must
distinguish mouth breathing from lung
breathing. The frog may pump air in
and out of its mouth for some time with¬
out using its lungs at all. When the
lungs are used, the nostrils are closed bv
flaps of skin as the floor of the mouth
rises. The glottis opens and admits air

to the windpipe, or trachea , and lungs.
Then, with the nostrils still closed, the
mouth is thrust down and air passes out
of the lungs into the partial vacuum.
The upthrust of the mouth immediately
following this seems to be higher than
usual and forces air back into the lungs.
After exchanging air once or twice from
mouth to lungs and lungs to mouth, the
frog resumes mouth breathing through
the open nostrils.

Thus the frog depends on its lungs
only to supplement mouth breathing
of air. As you might expect, the lungs
are small when compared with higher
animals which depend entirely on lung
breathing. They are thin-walled sacs
that lack the spongy tissue ours have.
The circulatory system. The circulatory
system of the frog shows an advance over
that of the fish and a step toward the
complex system of the higher verte¬
brates. One of these advances is a
three-chambered heart, consisting of two
atria ( auricles ) and a muscular ventricle
(Fig. 35-6) . Deoxygenated blood enters
the right atrium from various parts of
the body. Blood from the lungs, which
is oxygenated when the lungs are in
use, enters the left atrium. The atria
contract simultaneously and fill the ven¬
tricle. Contraction of the ventricle
forces blood out a large vessel, the conus
arteriosus, which lies against the front
side of the heart. This large vessel di¬
vides at once into two branches like a
letter Y. Each of these branches divides
again into three arteries. The anterior
pair are the carotid arches, which trans¬
port blood to the head. The middle
pair, or aortic arches, bend to the right
and left around the heart and join just
below the liver to form the dorsal aorta.
This great artery supplies the muscles,
digestive organs, and other bodv tissues.
The posterior pulmo cutaneous arches

CHAPTER 35 THE AMPHIBIANS 481

Carotid arch

35-6 In the frog heart, three branches of the vena cava lead to the right atrium.
Pulmonary veins lead from the lungs to the left atrium. Blood from both these
chambers passes through the conus arteriosus. This large vessel divides above
the heart and gives rise to the right and left carotid, aortic, and pulmonary
arches. The right-hand drawing shows the heart viewed from the back side,
where the venae cavae enter the sinus venosus and the pulmonary veins enter
the left atrium.

form arterial branches that transport
blood to the lungs, skin, and mouth.

Blood returning from the body is
laden with carbon dioxide and other cell
wastes and has been relieved of much of
its oxygen. Three large veins, the venae
cavae , join a triangular, thin-walled sac,
the sinus venosusy on the back side of
the heart, which in turn empties into the
right atrium. Part of the blood return¬
ing to the heart from the lower parts of
the body flows through vessels of the di¬
gestive organs and absorbs digested food.
This blood flows through the hepatic
portal vein to the liver on its way to the
right atrium. During each complete
circulation of blood, some of the blood
passes through the kidneys where water
and nitrogen-containing wastes from
cell activity are removed.

The frog’s circulatory system shows
several advances over that of the fish.
Blood passes through the two-cham¬

bered heart of the fish only once in mak¬
ing a round trip through the body. The
three-chambered frog heart receives
blood from both the body and the
lungs, and pumps blood to the head and
body as well as to the various centers
where respiration takes place.

The excretory system of the frog. The
frog’s skin is a vital organ of excretion
since it is here rather than in the mouth
or lungs that most of the carbon dioxide
is discharged from the blood. The liver
removes certain wastes and eliminates
them with bile or converts them for re¬
moval by the kidneys. The large intes¬
tine eliminates undigested food and
other wastes. However, the kidneys are
the principal organs of excretion. They
receive wastes from the blood, which
flows into them through the renal arter¬
ies and out through the renal veins.
The kidneys are large, dark red organs
lying on either side of the spine against

482 UNIT 6 BIOLOGY OF THE VERTEBRATES

Fat body
i a cava
Testis
Kidney

Small intestine

Dorsal aorta

Urinary bladder

Large intestine

Ureters

Cloaca

Cloacal
opening

35-7 The urogenital organs of a male frog.

the back body wall. Urine collects in
the kidneys and flows to the cloaca
through tiny tubes, the ureters (yoo -ree-
terz), that you can see in Fig. 35-7.
The urine may be excreted immediately,
or it may be stored after being forced
into the urinary bladder through an
opening in the cloaca.

The frog’s nervous system. The frog’s
brain shows a considerable advance over
that of the fish. Olfactory lobes lie at
the anterior end of the brain (Fig. 35-
8). The elongated lobes of the cere¬
brum are proportionally larger than
those of the fish. Posterior to these are
the prominent optic lobes. The cerebel¬
lum is just behind the optic lobes. In the
frog it is a small band of tissue lying at
right angles to the long axis of the brain.
The medulla oblongata lies posterior to
the cerebellum and joins the short, thick
spinal cordy which extends down the
back. Pairs of spinal nerves branch from
the cord and pass to various parts of the
body through openings between the ver¬
tebrae. Extending from the brain are
ten pairs of cranial nerves.

The reproductive system. Since the re¬

productive organs of the frog are inter¬
nal, it is difficult to distinguish the sexes
except during the breeding season, when
the thumb of the male is enlarged. The
male reproductive organs are two oval,
creamy white or yellowish testes. They
lie in the back, one on each side of the
spine, above the anterior region of the
kidneys. Sperm develop in the testes
and pass through tubes, the vasa effe-
rentia (vah- sah ef-fur-ent-shah ) , into
the kidneys. When the sperm are dis¬
charged, they pass through the ureters
and on into the cloaca. Some species of
frogs have an enlargement, the seminal
vesicle, at the base of each ureter.

Eggs develop in a pair of large,
lobed ovaries in the female, which attach
along the back above the kidneys. Dur¬
ing the breeding season, the eggs en¬
large, burst the thin ovary walls and are
freed into the body cavity. Movement
of the abdominal muscles works the
eggs toward the anterior end of the body
cavity. Here are funnel-like openings of
the long coiled oviducts. The eggs are
fanned into the oviduct openings by

Olfactory nerve
- Olfactory lobe
Cerebrum

Nerve to eye
Optic lobe

Cerebellum
Cranial nerves

Medulla oblongata
- - Spinal cord

35-8 The brain of a frog. Compare this
with that of the fish in the previous chapter.

CHAPTER 35 THE AMPHIBIANS 483

cilia. Near their opening into the cloa¬
ca, the walls of the oviducts secrete a
gelatinous substance that surrounds each
egg. At the base of each oviduct is a
saclike uterus in which the eggs are
stored until they are laid through open¬
ings into the cloaca.

Fertilization and development of the
eggs. The female leopard frog usually
lays her eggs some time between the first
of April and the middle of May. The
male clasps the female at the time the
eggs are laid. The male may also press
down on the female, thus helping to
lay the eggs. As the eggs pass from the
cloaca of the female, the male spreads
sperm over them. As a result of this di¬
rect fertilization, most of the eggs re¬
ceive sperm.

The jellylike coat that surrounds
each egg swells in the water and joins
the eggs in a rounded, gelatinous mass.
In this clump the eggs look like small
beads, each surrounded by a transparent
covering (Fig. 35-9). Not only does
the jelly protect the eggs from injury,
but it makes them more difficult for a
hungry fish to eat. Also it serves as the
first food for the young tadpole.

The frog egg is partly black and
partly white. The white portion is the
yolk, or stored food material, which will
nourish the tadpole during develop¬
ment. The dark portion contains the
living protoplasm of the egg and a dark
pigment. The yolk is heavier than the
rest of the egg, causing the eggs to float
in the water dark side up. The black
pigment on the upper side absorbs heat
from the sun while the lighter lower half
blends in with the light from the sky
and makes the eggs hard to see from
below. The gelatinous covering holds
much of the heat in the mass. After
eight to twenty days, depending on the
weather conditions and water tempera-

35-9 This mass of frog’s eggs is lodged in
the leaves of a plant growing in a shallow
pond. (Hugh Spencer)

ture, the tadpole hatches and wiggles
away from the egg mass.

From tadpole to adult — the metamor¬
phosis of the frog. Just after hatching,
the tadpole is a tiny, short-bodied crea¬
ture with a disklike mouth. It clings to
the egg mass or to a plant as shown in
Fig. 35-10. Yolk stored in the body
nourishes the young tadpole until it
starts to feed. Soon after hatching, the
body lengthens and three pairs of ex¬
ternal gills appear at the sides of the
head. The tail lengthens and develops
a caudal fin. The mouth opens and the
tadpole begins scraping the leaves of
water plants with horny lips.

Soon after the tadpoles become free
swimmers, the horny lip disappears. A
long, coiled digestive tract develops, and
the tadpole starts living on vegetable
scums. Gradually a flap of skin grows

484 UNIT 6 BIOLOGY OF THE VERTEBRATES

Newly hatched tadpole

4-day-old tadpole

35-10 This diagram shows the life history of the frog. The length of time for
metamorphosis varies in different species of frogs.

over the gills (now only two pairs) and
leaves a small opening on the left side
through which water passes out of the
gill chambers. At this stage the tadpole
is a fishlike animal with a lateral line,
fin, two-chambered heart, and a one-
circuit circulation. The animal also has
a relatively long, spirally-coiled intes¬
tine.

The change to an adult frog is re¬
markable. The hind legs appear first.
The front legs begin to form at about

the same time but do not appear for
some time. They remain hidden un¬
der the operculum. Soon after the ap¬
pearance of the front legs, the tadpole
then starts resorbing (not shedding or
eating) its tail. Late in the metamor¬
phosis, the tadpole’s mouth broadens
and teeth develop. While these ex¬
ternal changes are taking place, equally
important internal changes occur. A
saclike chamber, resembling the swim
bladder of the fish, forms back of the

CHAPTER 35 THE AMPHIBIANS 485

throat. This divides into two sacs, which
become the lungs. The heart becomes
three-chambered, and the gill arteries
change to the carotids, aortic arches,
and the pulmocutaneous arteries. The
gills stop functioning and the tadpole
comes to the surface frequently to gulp
air. The thin skin and broad, flat tail
still play an important role in respira¬
tion during this extremely critical time
in its life.

Even before the tail is entirely re¬
sorbed, the tadpole leaves the water and
comes to land as a young frog. Devel¬
opment from this stage to the full-

grown adult frog usually requires about
a month. The metamorphosis of the
leopard frog varies from 60 to 90 days.
Full-grown adults usually appear about
the first of July. The bullfrog usually
spends two winters as a tadpole, and its
entire metamorphosis may last as long
as three years.

Regeneration in Amphibia. Many am¬
phibians, especially salamanders, have a
remarkable ability to regenerate lost or
injured body parts. A foot, a portion
of a limb, or part of the tail may be lost
in escaping from an enemy. Such am¬
putated organs may be regenerated rap¬
idly. The tadpole stages of frogs and
toads also have regenerative powers,

especially in the early phases. This
ability to regenerate disappears as the
tadpole matures and is lacking entirely
in the adult stage of all genera and
species of frog and toad.

Hibernation and estivation in the frog.
The frog, as well as the fish and reptile,
are “cold-blooded” vertebrates. This
does not mean that their blood is al¬
ways cold. It means that the bodv tem¬
perature of these animals varies with
the temperature of the surroundings.
Man maintains a constant average body
temperature of about 98.6° F bv regu¬

lating the rate of food oxidation and
resulting heat release in the tissues as
well as heat loss from the body surface.
The cold-blooded vertebrates carry on
much slower oxidation and do not main¬
tain a relatively constant body tem¬
perature.

With the coming of fall and the sea¬
sonal lowering of temperature, the body
temperature of the frog drops to the
point where it can no longer be very
active. It buries itself in the mud at
the bottom of a pond or finds shelter in
some other protected place in the water.
Heart action slows down to a point at
which blood hardly circulates in the ves¬
sels. The moist skin supplies the greatly
reduced oxygen necessary for keeping
alive. The tissues are kept alive by the
slow oxidation of food stored in the liver
and in the mass of yellow in the fat
bodies attached above the kidneys in
most frogs. Nervous activity almost
ceases, and the frog lies in a stupor.
This is the condition of the frog during
hibernation , or winter rest. With the
coming of spring, the warm days speed
up body activity and the frog gradually
resumes physiological and functional ac¬
tivities of normal life.

The hot summer months bring
other problems. Lacking a device for
cooling the body, the frog must escape
from the extreme heat. It may lie
quietly in deep cool water or bury itself
in the mud at the bottom of a pond, in
the condition of summer inactivity re¬
ferred to as estivation. Many smaller
ponds dry up during midsummer, and
the frogs and other cold-blooded animals
survive only by burying themselves in
the mud and estivating. With the com¬
ing of cooler weather and the return of
water to the pond, they come out of
estivation and continue normal activity
until hibernation.

486 UNIT 6 BIOLOGY OF THE VERTEBRATES

IN CONCLUSION

The frog has long been a favorite subject for biological study. Each in¬
dividual frog passes through various stages of development, from a fishlike
larva to an adult terrestrial amphibian. In the adult stage the organs and
systems of the frog are complex and efficient. Biologists still marvel at its
three-chambered heart with its complicated valves and arteries. Lungs, mouth,
and skin together compose an efficient respiratory team. Of course, if the skin
is to be used in respiration, it must be thin and moist. Thus the frog may not
leave the water or a moist environment entirely. Amphibians never did be¬
come successful on land, and today they form an insignificant part of the verte¬
brate population.

In the next class of vertebrates, you will find animals which, while similar
to the frog in many ways, are much better suited to life on land.

BIOLOGICALLY SPEAKING

alimentarv canal

gastric glands

Salientia

Amphibia

glottis

seminal vesicle

aortic arch

gullet

sinus venosus

Apoda

hepatic portal vein

spinal nerves

bile duct

hibernation

tadpole

carotid arch

maxillarv teeth

trachea

Caudata

mesentery

tympanic membrane

cloaca

mucous glands

ureters

colon

nictitating membrane

uterus

conus arteriosus

oviduct

vasa efferentia

cranial nerves

pancreas

vena cava

estivation

pulmocutaneous arch

vocal sacs

Eustachian tube

pyloric valve

vomerine teeth

fat body

pylorus

web membrane

gallbladder

renal vessels

QUESTIONS FOR REVIEW

1. Why do biologists believe that the early lobe-finned lungfishes were am¬
phibian ancestors?

2. What characteristics of amphibians distinguish them from other living
vertebrates?

3. How many orders of amphibians are represented by living members today?

4. In what ways do salamanders resemble lizards? Name several character¬
istics that make them different from lizards.

5. Explain why the axolotl does not undergo metamorphosis.

6. Describe the “triple life” of the crimson-spotted newt.

7. Describe the manner in which a frog catches a flying insect.

CHAPTER 35 THE AMPHIBIANS 487

8. How can a frog croak under water?

9. Name the organs forming the alimentary canal of a frog, in the order in
which they receive food.

10. Name the chambers of the frog’s heart.

11. What three arterial branches carry blood from the great artery leading from
the frog’s heart?

12. How is urine conducted from the frog’s kidneys to the cloaca and bladder?

13. Discuss in order of occurrence the changes during the development of a
tadpole.

APPLYING PRINCIPLES AND CONCEPTS

1. Explain why biologists believe that legs were developed to find water
rather than to leave it.

2. Although the amphibians became terrestrial, discuss why they were never
successful on land.

3. Discuss the problems in the life of a toad that result from its “in-between”
existence.

4. In what respect is the direct fertilization of the frog’s eggs more efficient
than spawning in fishes?

5. Explain how the frog shows a relationship to the fish in its early develop¬
ment.

6. In what ways are the heart and circulatory system more highly developed
in the frog than in fishes?

488 UNIT 6 BIOLOGY OF THE VERTEBRATES

Key to the Structures of the Frog

1. Transverse abdominal mus¬
cles

2. Vertical abdominal muscles

3. Muscles to floor of mouth

4. Sockets for attachment of arms

5. Shoulder muscles

6. Right atrium (auricle) of heart

7. Left atrium of heart

8. Ventricle of heart

9. Great veins to right atrium (au¬
ricle)

10. Great artery from heart (conus
arteriosus)

11. Liver

12. Stomach

13. Pancreas

14. Small intestine

15. Large intestine (colon)

16. Spleen

17. Mesentery

18. Abdominal vein

19. Leg muscles

20. Tongue

21. Glottis opening

22. Trachea

23. Lungs

24. Sinus venosus

25. Pulmonary veins

26. Gall bladder

27. Bile duct

28. Hepatic portal vein

29. Sockets for attachment of legs

30. Gullet

31. Vein from kidneys (posterior
vena cava)

32. Kidneys

33. Dorsal aorta

34. Fat bodies

35. Ovaries

36. Oviducts

37. Openings of oviducts

38. Egg sac (uterus)

39. Urinary bladder

40. Cloaca

41. Lining of mouth

42. Veins from legs to kidneys
(renal portal vein)

43. Ureters

44. Internal nostril openings

45. Vomerine teeth

46. Teeth of the upper jaw

47. Openings of Eustachian tubes

48. Eye sockets

49. Brain

50. Spinal cord

51. Spinal nerves

LAYER OF SKIN AND MUSCLES REMOVED FROM VENTRAL SIDE OF THE FROG.
Looking at this layer from the inside, you see the many blood vessels of the skin.
Notice the transverse abdominal muscles (1), and the vertical abdominal muscles (2).
The large muscles (3) which aid in mouth breathing have been cut. The ends which
attach to the floor of the mouth show in the next drawing. In the shoulder are the bones
which form the socket (4) for the attachment of the arms and the cut ends of some of
the shoulder muscles (5).

DMI .TMU1JI* (HM0lilV 2MA»!
MIlMO^ilW .33JUAWJIM

CUTAWAY VIEW SHOWING THE FROG LYING ON ITS BACK WITH FRONT BODY
WALL REMOVED. The heart is composed of a right auricle (6), a left auricle (7),
n rid a ventricle (8). Great veins (9) carry blood into the heart and a great artery (10)
carries blood away from the heart. The liver (ID covers most of the stomach (12) and
pancreas (13). The small intestine (14) leads from the lower end of the stomach to the
large intestine (15). The spleen (16) lies in the thin layers of mesentery (17) which
fasten the abdominal organs to the body wall. The large abdominal vein (18) carries
blood from the legs to the liver. Powerful leg muscles (19) enable the animal to swim
and jump.

CUTAWAY VIEW SHOWING THE FLOOR OF THE MOUTH AND ORGANS VIEWED
FROM THE BACK. The fl eshy tongue (20) nearly covers the floor of the mouth,
cords lie just inside of the slitlike glottis opening (21). A very short trachea (22)
leads to the lungs (23). On the back of the heart are the large veins which combine in a
thin-walled sac, the sinus venosus (24) before entering the right auricle. Above this, the
pulmonary veins (25) enter the left auricle. The gall bladder (26) receives bile from the
liver (ID and passes it to the small intestine (14) through the bile duct (27), The hepatic^
portal vein (28) carries blood from the small intestine to the liver. The stomach (12) and
large intestine (15) are cut open in this view. Sockets (29) which hold the leg bones

Vocal

are visible in the pelvic region.

□ CUTAWAY VIEW SHOWING THE ROOF OF THE MOUTH AND DEEPER ORGANS
AS SEEN FROM THE FRONT. The short, wide gullet (30) opens at the back of the

mouth and leads to the stomach (12). A large vein (31) collects blood from the kidneys

The dorsal aorta (33) carries blood to the internal organs and lower parts of the

body. Fat bodies (34) attach near the top of the ovaries (35). Eggs break out of the

ovaries and enter the oviducts (36) at openings (37) near the base of the lungs. Until

they are laid, eggs are stored in sacs (38) at the lower end of the oviducts. The urinary
bladder (39) attaches to the cloaca (40)

CUTAWAY VIEW SHOWING DEEPER ORGANS AS SEEN FROM THE BACK. The
lining of the mouth (41) shows its rich blood supply. You see the gullet (30)
and stomach (12) from the dorsal side. Veins (42) carry blood from the legs to the kid¬
neys (32). Near these are the ureters (43) which carry urine to the cloaca (40) which
is cut open in the drawing. Urine passes from the cloaca into the urinary bladder (39),
where it is stored.

VII

CUTAWAY VIEW SHOWING BACK BODY WALL AS SEEN FROM THE FRONT WITH

_ ORGANS AND SOME OF THE LARGE LEG MUSCLES REMOVED. Internal nostril

openings (44), vomerine teeth (45), teeth of the upper jaw (46), Eustachian tube open¬
ing (47) and eye sockets (48) can be seen in this view of the head. The cranium and
spine are shown as though they were transparent to show the brain (49) and spinal
cord (50). Spinal nerves (51) emerge from each side of the spinal cord. Dissection of
the lower leg muscles exposes bones and joints, blood vessels, and the dorsal wall of
the cloaca (40) with ureter and oviduct openings.

CHAPTER 36

THE REPTILES

The rise of the reptiles. Amphibians
were never successful land dwellers.
Most of the adults had to remain near
water. Both eggs and young required
either a water environment or at least
moist surroundings. Biologists believe
that millions of years must have elapsed
before a vertebrate evolved that could
live entirely on land. Two significant
advances probably made a completely
terrestrial life possible. The first and
perhaps most important was the develop¬
ment of an egg with a shell. The shelled
egg prevented drying, and freed animals
from the necessity of returning to the wa¬
ter for laying eggs. The eggs of modern
reptiles all have shells, and the develop¬
ing embryo within the egg is surrounded
by a thin membrane, the amnion. The
amnion is filled with a watery fluid, pro¬
viding the embryo with a liquid environ¬
ment even though the egg is laid on dry
land. A large yolk provides sufficient
nourishment for the reptile to develop
to a more advanced stage before it is
hatched. This eliminates the tadpole

stage and metamorphosis. The develop¬
ment of the amniote egg is one of the
most important adaptations of the class
Reptilia. Internal fertilization accom¬
panied the development of a shelled
egg. Otherwise a sperm would be
unable to penetrate the shell to fertilize
the egg nucleus. A second factor that
contributed to the success of animals on
land was the development of a more
suitable body covering. The skin of
reptiles resists loss of water by evapora¬
tion so effectively that they are even
found in deserts where the temperature
is high and the humidity is very low.

Unfortunately it is impossible for
paleontologists to determine exactly
when the transition from amphibian to
reptile took place. Fossil remains are
generally limited to hard parts such as
bones and teeth. But some of the im¬
portant differences between the two
groups are in the soft tissues, which do
not become fossilized. It is therefore
often impossible to be sure whether a
fossil is an amphibian or a reptile. Fos¬
sils of true reptiles have been found in
the late Carboniferous period (Fig. 13-1,
page 183). But there is some evidence
that there were animals with reptilian
characteristics before this period. Cer¬
tainly the first reptiles had little ad¬
vantage over their amphibian relatives.
They must have been nearly as restricted
to the water environment. There is lit¬
tle doubt that the transition to land
occurred slowly. Fossil evidence indi¬
cates that not until late in the Car¬
boniferous period did reptiles become
truly terrestrial.

The first reptiles were small and
were minor competitors of other animals
living in the Carboniferous swamps.
But during the Permian period, changes
in land formations created a variety of
environments. Animals equipped to

489

490 UNIT 6 BIOLOGY OF THE VERTEBRATES

live in these new habitats were assured
of life with little competition. Varia¬
tions in size, shape, and behavior seem
to have allowed the reptiles to live in the
different environments — a good exam¬
ple of adaptive radiation. From an un¬
important beginning in the Carbonifer¬
ous period, reptiles became the most im¬
portant animals on the land, in the air,
and perhaps even in the sea during the
Mesozoic Era — the age of reptiles.
The age of dinosaurs. Eggs and foot¬
prints preserved in rock and fossilized
bones are all that remain of the age
when dinosaurs roamed the earth. Yet,
with this evidence, gathered in many
parts of the world, and considerable
imagination, the paleontologist has been
able to piece together a vivid picture of
the earth during the time when dino¬
saurs dominated animal life.

Dinosaur is an appropriate name
for these ancient reptiles, for it means
“terrible lizards.” Many of the dino¬
saurs were no larger than our larger liz¬
ards today. But some of them were

giant beasts that would dwarf an ele¬
phant. These are the best known dino¬
saurs.

The largest of the dinosaurs was the
thunder lizard, or Brontosaurus. This
giant measured 75 feet in length, about
15 feet in height, and weighed 30 tons
or more. *It lived in shallow lakes and
marshes and fed on water plants. Its
enormously long neck was balanced by
an equally long and heavy tail. The
plated lizard, while smaller than the
thunder lizard, was one of the heaviest-
armored of the dinosaurs. Stegosaurus
(steg-oh-sor-us), as this 30-foot monster
has been named, had a double row of
plates projecting two feet from its back.
Pairs of spines near the end of the tail
served as deadly weapons with which
to lash out at an enemy. Stegosaurus
had a ridiculously small head with a
cranial cavity no larger than that of a
small dog. An auxiliary “brain” 20
times larger than the true brain con¬
sisted of a mass of nerve tissue formed
by the spinal cord in the hip region.

36-1 The herbivorous
dinosaur, Stegosaurus,
though it looked dan¬
gerous, had a small
brain in proportion to
the size of its body and
was unable to protect it¬
self against enemies.
Here it is being attacked
by a much smaller but
carnivorous dinosaur
with sharp teeth. (Ewing
Galloway)

CHAPTER 36 THE REPTILES 491

This second “brain” is thought to have
controlled the seven-foot hind legs and
ponderous tail of the animal.

The king of dinosaurs was the fe¬
rocious tyrant lizard, or Tyrannosaurus
(ty-ran-oh-sor-us), which is probably the
most terrible creature ever to roam the
earth. It walked erect on its powerful
hind legs and balanced its heavy body
with its long tail, much in the manner
of a kangaroo. Its front legs were short
but powerful, and its long claws could
tear most prey into shreds. This giant
flesh-eating reptile was nearly 50 feet in
length and towered 20 feet in height.
Its powerful jaws were rimmed with
double-edged teeth three to six inches
long that could rip the hide of even an
armored victim.

Classification and characteristics of liv¬
ing reptiles. Some 6,000 species of rep¬
tiles exist in the world today as rem¬
nants of the Age of Reptiles. They still
exceed the number of species of mam¬
mals, but are surpassed by the species of
birds or fishes. About 275 reptile spe¬
cies are found in the United States.
Some are much like their ancestors;
others have become greatly modified.
The geographic distribution of the rep¬
tiles indicates that the modern reptiles
originated in the tropics, but they have
migrated to some of the colder parts of
the earth. Only the icy regions, the
tops of mountains, and the ocean

depths are completely without reptiles.
Of the 16 orders of reptiles that once
existed, only the four orders that in¬
clude turtles, crocodiles, lizards, and
snakes are represented today (see table
below). One of these remaining orders
is so near extinction that it is repre¬
sented today by a single species.

Reptiles show the following charac¬
teristics:

1. Body usually covered with scales.

2. Skin dry, not moist and slimy.

3. Feet, if present, have claws on the
toes.

4. Eggs internally fertilized and, if laid,
have a protective shell. Certain spe¬
cies retain the eggs within the body
and bring forth the young alive.

5. No metamorphosis.

6. Gills never present, as both young
and adults breathe with lungs.

7. Body temperature changes with en¬
vironment ( cold-blooded ) .

Sphenodon, a relic of a bygone age.
One of the rarest animals on the earth
today is the sole surviving species of the
order Rhynchocephalia. This ancient
reptile, even older than the dinosaurs, is
the Sphenodon punctatus (sfee- noh-don
punk-fay-tus ) , or tuatara ( too-a-ta/zr-a ) .
Its relatives disappeared early in the
Mesozoic Era, probably because they
could not compete with more adaptable
lizards. This strange survivor of the
age of reptiles miraculously escaped ex-

ORDERS OF LIVING REPTILES

Name of Order

Representatives

Testudinata or Chelonia

Turtles and tortoises

Rhynchocephalia

Sphenodon (tuatara)

Squamata

Lizards and snakes

Crocodilia

Alligators, crocodiles, gavials, and caimans

492 UNIT 6 BIOLOGY OF THE VERTEBRATES

tinction through the ages in far off New
Zealand and neighboring islands. The
tuatara probably survived there because
of a total absence of mammals. Once
the English settlers had introduced rats,
wild pigs, cats, and weasels in New
Zealand, the tuatara became extinct on
the mainland. Today the last surviving
tuataras are found on a few small is¬
lands in the Bav of Plentv and in Cook

J J

Strait off the coast of New Zealand,
where they are protected by the govern¬
ment. It is a challenge to biologists to
preserve the remaining tuataras.

Because of a very limited evolution
in the Rhynchocephalia, today’s tuataras
retain many primitive features. Since
animals similar to the tuatara lived 170
million years ago, it is clear that the rate
of evolution in this order must have
been one of the slowest in the vertebrate
groups. The tuatara reaches a length of
about two feet and resembles a large
lizard (Fig. 36-2). Its skin is dark
olive, marked with numerous light
colored dots. Its eyes resemble those of
a cat. The most unusual characteristic
of the tuatara is a parietal eye (pa-ry-
et-1) in the top of its head. While not
a functioning sense organ, this strange
third eve has the remains of a retina

and other eye structures. The tuatara
hides in a burrow during the day, com¬
ing out at night to feed on insects,
worms, and other small animals. The
eggs are buried in a shallow depression
in the ground, where they remain al¬
most a year before hatching. The fe¬
male tuatara usually lays 12 to 14 eggs.
The fact that the tuatara lives well in
captivity may make it possible to pre¬
serve this rare species in the reptile col¬
lections of the world.

Snakes, the most widespread reptiles.
Snakes are relative newcomers. As with
the early ancestors of the reptiles, there

36-2 Sphenodon punctatus, the tuatara of
islands near New Zealand, is the only sur¬
viving species of a once flourishing order.
(American Museum of Natural History)

is no fossil record of snakes. But there
is little doubt that they evolved from
tvpical reptiles. In fact the boas and
pythons still have the remains of hind
limbs, indicating that they descended
from animals with legs. Snakes evolved
rapidly during the Tertiary period, at
the same time rodents and other small
mammals were developing. Snakes are
not only the most numerous reptiles to¬
day, they are also the most widely dis¬
tributed. Snakes are found in water, on
the high seas, among rocks, under
ground, and in trees. They are most
abundant in the tropical regions. Their
numbers reduce in cooler climates to
126 species in the United States and
only 22 species in Canada.

Of the more than 2,000 species of
snakes in the world, a relatively small
number are poisonous. The harm
caused by these dangerous snakes is far
outweighed by the valuable service all
snakes render in destroying large num¬
bers of insects and destructive rodents.
Body structure of a snake. If you ex¬
amine the elongated body of a snake
closely, you can distinguish the head,
the trunk, which contains the body cav-

CHAPTER 36 THE REPTILES 493

ity, and a tail, which extends beyond
the anal opening. As in all reptiles, the
snake’s body is covered with scales.
Those on the back and sides of the
body are small and oval, thus allowing
great flexibility. The heads of many
snakes, including our nonpoisonous spe¬
cies, are covered with plates. The
North American pit vipers, including
the rattlesnake, have scale-covered
heads. The scales on the lower side of
the body form broad plates, known as
scutes (skyoots).

Each season during the process of
molting, snakes shed the outer layer of
scales several times. As this thin layer
loosens all over the bodv, the snake
usually hooks a loose portion to a sharp
object such as a twig and works its way
out of it. After molting, the newly ex¬
posed scale surfaces are bright and shiny.
Structures of the head. The snake’s
mouth is large and is provided with a
double row of teeth on each side of the
upper jaws and a single row in the lower
jaw. The numerous conical teeth slant
backward toward the throat. None of
the teeth serve for chewing but are
necessary to hold the prey, which is
swallowed whole.

The sense of smell in snakes is very
acute. Olfactory nerve endings lie in
the nasal cavities, which open as paired
nostrils near the front of the head. The
sense of smell is made more acute with
the aid of the curious forked tongue,
which is thrust from a sheath in the floor
of the mouth close to the front through
a small opening that is left when the
jaws are closed. The tongue receives
dust and other odor-bearing particles
from the air and transfers them to tiny
pits close to the front of the roof of the
mouth. These Jacobsons organs , as
they are called, contain nerve endings
that are highly sensitive to odors.

The snake’s eves have no lids and

in this respect are different from those
of other reptiles. A transparent scale
covers the eye. This becomes cloudv

J J

just before molting and causes tempo¬
rary difficulty with vision. The eyeball
can be turned in its orbit. Movement
of the lens focuses the eye sharply on
objects, especially at close range. Many
snakes have round pupils. Biologists
have discovered that these snakes are
most active in daylight. Others have
elliptical pupils, similar to those of a
cat. These snakes are most active at
night.

The ears are embedded in the skull
and have no external openings. Thus,
the snake cannot hear vibrations trans¬
mitted by the air. Instead the skull
bones transmit vibrations resulting from
jarring to the highly sensitive ear mech¬
anisms.

Feeding habits of snakes. All snakes
feed on living animal prey. No vege¬
tarian snakes are known to exist. We
classify snakes into three groups, based
on feeding habits.

Many snakes, including most of our
nonpoisonous species, merely seize a
prey in the mouth and swallow it alive.
Most of these snakes feed on insects,
frogs, toads, lizards, fishes, and other
small animals.

The python, boa, king snake, bull
snake, and other large-bodied snakes
make use of a more specialized method
of food-getting. These snakes seize the
prey, usually by the head, wrap coils
around it with lightning speed, and kill
it quickly by constriction (Fig. 36-3).
The powerful coils squeeze the victim
with such force that its chest is com¬
pressed and breathing is stopped. In
addition the pressure cuts off the vic¬
tim’s circulation and stops the heart.
The shock kills the prey, often without

494 UNIT 6 BIOLOGY OF THE VERTEBRATES

breaking a bone. If, after the first con¬
striction, the snake feels a pulse in its
victim, it will squeeze again. Swallow¬
ing starts immediately after the prey is
killed. Biologists have found that
warm-blooded animals are killed much
more quickly by constriction than cold¬
blooded prey.

A relatively small number of snakes
poison the prey before swallowing it.
Poisonous snakes secrete venom in modi¬
fied salivary glands at the sides of the
head. When a poisonous snake strikes,
paired fangs are thrust into the victim.
Venom flows from the poison glands
through ducts that enter the fangs (Fig.
36-4). The hollow fangs, through
which venom flows into the victim, are
like hypodermic needles. The amount
of venom injected varies with the size
and species of snake. However, smaller
species, as well as the young of larger
species, have more concentrated venom.
Thus their bites may be just as danger¬
ous as those of larger poisonous snakes.
Adaptations for swallowing. Snakes are
infrequent feeders. They may go for

36-3 Pythons obtain their food by curling
their powerful bodies around their victim
and crushing it. (Lilo Hess)

36-4 The rattlesnake poisons its prey with
venom before swallowing it whole. The
head structures are modified for poisoning
and swallowing.

weeks or, under some conditions, as long
as a year without food. However, when
they do eat, it is amazing to watch a
snake swallow a prey four or five times
larger than its own body. This feat is
made possible by several modifications
of the jaws. The lower jaws are not
joined directly to the skull but are fas¬
tened to a separate quadrate bone , which
acts as a hinge and allows the jaw to
drop downward and forward (Fig. 36-5).
The two halves of the lower jaw are
fastened at the front by an elastic liga¬
ment , which permits each half to oper¬
ate independently of the other. The
numerous slanting teeth hold the prey
firmly during the swallowing process.

During swallowing, one side of the
lower jaw may pull the prey into the
mouth while the other side is thrust
forward for a new grip. By this see-saw
action, the prey is pulled down the
throat much as you might pull in a rope
hand over hand. The snake literally
crawls forward around its prey.

The process of swallowing takes so
long that special adaptations are neces¬
sary to permit the snake to breathe
while a large prey is in its mouth and
throat. The trachea extends along the

CHAPTER 36 THE REPTILES 495

36-5 What adaptations for swallowing large prey are shown in this photograph
of a snake’s skull? (American Museum of Natural History)

floor to a glottis opening near the front
rim of the lower jaw.

Locomotion in snakes. Several methods
of locomotion are found among snakes.
The most common method is lateral
undulatory movement. The body winds
from side to side, forming broad curves.
The snake pushes against irregularities,
which give it a grip in moving forward.
I he entire body follows along the same
track in this type of movement, used
when the snake is crawling rapidly.
T his type of movement is also used by
water snakes in swimming.

A snake may crawl slowly in a
straight line by caterpillar movement.
Scutes are pushed forward in several
sections of the body. The posterior
edge of each scute grips the ground while
the body is pulled forward by waves of
muscular contraction.

A third method of movement,
known as side winding , is used by snakes
of the sandy desert regions. Here, the

body is twisted into S-shaped loops and
is raised from the ground except for
two or three points of contact. The
sidewinder “walks” across the sand on
these loops.

You may be relieved to learn that
most snakes travel at a speed of less
than one mile per hour. The fastest
ones cannot exceed three miles per hour.
You can walk at this speed with ease,
and you can run a short distance at a
speed of from 10 to 20 miles per hour.
Thus you need have no fear of being
run down by a snake!

Internal organs of a snake. Over 300
pairs of ribs attach to the vertebrae of
the spine in a snake. These ribs are set
in muscle and are flexible, permitting
movement and allowing large prey to
pass through the body. The gullet and
stomach are highly elastic and the di¬
gestive secretions are powerful, in order
to accommodate whole prey which
snakes swallow for food.

496 UNIT 6 BIOLOGY OF THE VERTEBRATES

36-6 The garter snake is ovoviviparous, bringing forth its young alive. (Ameri¬
can Museum of Natural History)

The right lung is well developed.
The left lung is stunted or absent en¬
tirely. Flexible ribs permit expansion
of the body wall in breathing. The large
heart has a septum partially dividing the
ventricle into the two chambers.

Since the snake is cold-blooded, the
rate of oxidation is much lower than
that of a warm-blooded animal; and
much less heat is generated in their
bodies. Under resting conditions, a
snake is often slightly colder than its
surroundings. Also the heat they do
produce is quickly lost since they have
no coat of fur or feathers. The reptile
is at a disadvantage in being cold¬
blooded and living on land. In cold re¬
gions it cannot be active during the win¬
ter. During hot weather, it must seek
shelter through the day since it has no
method of cooling the body below the

level of whatever the outdoor tempera¬
ture happens to be.

Reproduction in snakes. The majority
of snakes lay eggs that resemble those of
the other reptiles. Each egg, enclosed
in a tough white shell, contains stored
food to nourish the young snake during
its development. The eggs receive no
care from the female after being laid
and no incubation except the warmth of
the sun. Egg-laying snakes are called
oviparous ( oh-vip-uh-rus ) ; they include
the black snake and blue racer.

A smaller group of snakes, includ¬
ing the garter snake and the copperhead,
bring forth their young alive, usually in
the late summer (Fig. 36-6). The eggs
are not laid but remain in the uterus
where they develop into young snakes.
During development, there is no nour¬
ishment provided from the mother’s

CHAPTER 36 THE REPTILES 497

36-7 Nonpoisonous snakes include the black snake and milk snake (top left
and right), and the boa constrictor and king snake (bottom left and right).
(American Museum of Natural History)

body as in the mammals. Snakes that
bring forth the young alive are classed
as ovoviviparous (oh-voh-vy-vi£-uh-rus),
as distinguished from the higher ani¬
mals, which are called viviparous (vy-
vzp-uh-rus) and which nourish their
young during development.
Nonpoisonous snakes. The snakes may
be divided into two distinct groups on
the basis of whether they produce toxins
or not. You are probably familiar with
the harmless garter snake, black snake,
and racers. We should be interested in
protecting these snakes as they destroy
insects, rats and other rodents. The
king snake is one of the most valuable
snakes, as it eats other snakes and ro¬
dents. The constrictors of South Amer¬
ica, Africa, and Asia, are also nonpoi¬
sonous snakes.

Poisonous snakes. The poisonous
snakes are grouped in four families, all
of which have developed specialized
teeth or fangs for the injection of
poison. The cobras are almost entirely
limited to the tropics of Africa and Asia.
Some of the members of the cobra fam¬
ily are deadly to man. The king cobra
of Siam is the largest of all poisonous
snakes. Some species of cobra spit
venom at their enemies. The fine spray
of poison can travel several feet with
surprising accuracy and can cause tem¬
porary or permanent blindness if it en¬
ters the eye. The coral snakes of Amer¬
ica are related to the cobras. They are
relatively small, with strikingly beauti¬
ful coloration. Their venom is very
potent, but they cause few fatalities be¬
cause they have very short fangs which

498 UNIT 6 BIOLOGY OF THE VERTEBRATES

36-8 The cobra is one of the most deadly poisonous of all snakes. (American
Museum of Natural History)

are unable to penetrate shoe leather or
heavy clothing.

The sea snakes are related to the
cobra but are usually placed in another
family. Although the sea snakes are
very poisonous, they seldom harm man
as they are strictly marine creatures.
Fishermen, however, fear these snakes,
for on rare occasions a specimen is
hauled aboard a fishing vessel. Sea
snakes inhabit the shallow waters of the
East Indies, and one species is fre¬
quently seen off the west coast of tropi¬
cal South America, Central America,
and Mexico.

The third group of venomous
snakes is the vipers. This family in¬
cludes all the poisonous snakes of Eu¬
rope, Africa, and Asia that do not
belong to the cobra family. The most
abundant viper is the common Euro¬
pean viper, which is known in England
as the adder. The largest number of
viper species occurs in Africa. Vipers
have large hollow fang teeth that are
held in an erect position when the

mouth is open but are folded flat when
the mouth is closed.

The fourth family, the pit vipers ,
is distinguished from the true vipers by
the presence of a highly specialized or¬
gan that is sensitive to temperature.
These pits are located on either side of
the head, in front of the eyes. Biolo¬
gists have determined that the pit senses
infrared rays. This enables the pit vi¬
pers to strike accurately at any prey
that produces heat. Since most of the
prey of the pit vipers are warm-blooded
animals, this adaptation gives them a
strong advantage in securing food. Al¬
though some pit vipers exist in southern
Asia, most species are found in North
and South America. The best known
are the copperhead, the cottonmouth
moccasin (both of North America),
the fer-de-lance and bushmaster of
South America, and the numerous rat¬
tlesnakes of both continents.

Rattlesnakes are the most widely
distributed poisonous snakes in the
world. Their range extends from north-

CHAPTER 36 THE REPTILES 499

era Argentina to southern Canada. Of
the 19 or more kinds of rattlesnakes
found in the United States, at least 12
species occur in the southwest. These
include the prairie rattlesnake, western
diamond rattlesnake, and horned rattle¬
snake, or sidewinder, of the desert re¬
gions. The range of the timber rattle¬
snake includes most of the eastern
United States. The largest of North
American rattlesnakes, the diamond-
back, lives in marshy areas of the South¬
east. Six- to eight-foot specimens have
been taken in southern swamps.

Rattlesnakes have a series of dry
segments, or rattles, on the end of the
tail. When the snake is disturbed, it
vibrates these rapidly, causing a whir¬
ring sound, which explains why few peo¬
ple are bitten by rattlesnakes even
though they are widely distributed.
Usually you can step away from danger
when you hear and recognize a rattle¬
snake’s warning.

The head of a rattlesnake is large
and triangular. The jaws are puffy, be¬
cause of the presence of poison glands
(Fig. 36-4). Near the front of the up¬
per jaw is a pair of large, hollow teeth,
or fangs. The fangs are fastened to a
bone that is hinged on the upper jaw
so that when the snake’s mouth is
closed, the fangs fold upward against
the roof of the mouth. They are pulled
down by muscles when the snake opens
its mouth to strike. The rattlesnake
can strike fiercely a distance of one
third the length of its body or more.
The fangs are driven deep into the flesh
of its victim, and poison flows from the
glands, through the fangs, and into the
wound. Both the length of the fangs
and the large amount of poison in¬
jected make the rattlesnake bite ex¬
tremely dangerous, especially when the
fangs happen to go into a vein.

Rattlesnakes, especially the dia-
mondback, have several economic uses.
The skin is used for purses, belts, and
other articles. In many regions the flesh
is eaten. The venom is taken from cap¬
tive specimens and is used for making
antivenin , a biological product used in
treating pit viper bites.

Two types of snake venom. The toxic
portion of snake venom is made up of
various complex protein substances.
One type of poison, called neurotoxin ,
affects the parts of the nervous system
that control breathing and heart action.
The second type, called hemotoxiny des¬
troys red blood cells and breaks down
the walls of small blood vessels. All
poisonous snakes contain both types, but
the proportion varies with the species.
The venom of cobras, coral snakes, and
sea snakes is usually mostly neurotoxic.
Vipers and pit vipers produce venom
that tends to be mostly hemotoxic.

The danger from snakebite varies
with the amount of venom injected;
the concentration of toxins in the
venom; the part of the body receiving
the bite; and whether the venom en¬
ters the main circulatory system rapidly
(through a blood vessel) or slowly
(through a muscle or fatty tissue). In
the tropics, snakebite is a serious prob¬
lem. As many as 40,000 people may die
annually from snakebite. Cobra bites
kill many people in India. In the tem¬
perate regions, few people die from
snakebite. Still, the bite of any poison¬
ous snake should be regarded with the
greatest possible seriousness and treated
at once.

Treatment of snakebites. We now
have efficient methods of treating the
bites of pit vipers. Treatment must be
started immediately since much of its
success depends on preventing the
spread of venom through the blood

500 UNIT 6 BIOLOGY OF THE VERTEBRATES

stream. The following steps should be
taken at once:

1. Keep the victim quiet and reassured,
to prevent speeding up of heart ac¬
tion.

2. Put a constricting band, made from
a piece of cloth, a cord, or a necktie,
between the wound and the heart.
Tighten it firmly, but not enough to
cut off the circulation completely.
The tourniquet should be loosened
by a doctor.

3. If ice is available, pack the area
around the wound in it. This meth¬
od of treatment is now regarded as
superior to binding with a tour¬
niquet as outlined in step 2.

The lizards. Of the more than 2,500
species of lizards, only a few are native
to the United States. Lizards are
chiefly tropical animals, although a few
species extend into the colder temper¬
ate regions. Many lizards are strange
and beautiful. Because of their resem¬
blance to dragons and dinosaurs, they
are feared by many people. But most
of them are shy, harmless creatures.
Of all the fierce-looking animals in¬
cluded in this suborder, onlv two are
poisonous. The lizards have evolved
from the Mesozoic Era to the present
time. Of all the modern reptiles, the
lizards have developed the greatest
number of adaptations for different en¬
vironments.

The iguanas (i-gwah- naz) are con¬
sidered primitive lizards. Thev tend to
possess the typical lizard appearance
and proportions; few have evolved anv
advanced characteristics. Included in
this family is the so-called horned toad,
which has a series of horny spines on
its head and back for protection. It
lives in the dry plains of the western
United States. The spinv skin, which
conserves water, is an excellent adapta¬

tion for an inhabitant of dry areas.
This small lizard survives because its
color blends with the sand and spiny
cacti of its environment.

While one tropical American
iguana grows to a length of seven feet,
the best known lizard of the Western
Hemisphere is probably the five-inch
anole. In parts of Florida, this graceful
lizard can be found in nearly every tree
and shrub. It is widely sold as a pet.
Because of its ability to change color,
the anole is usually called the chame¬
leon (ka-meeZ-yun), a name that may
properly be applied only to certain Old
World lizards. Under the influence of
light, temperature, or even its own emo¬
tions, the anole may be bright green,
brown, or gray. The beautiful collared
lizards, also iguanas, are found from the
southern United States, through Cen¬
tral America, into northern South
America.

The geckos (gek- ohz) are a group
of highly specialized lizards. The toes
of most geckos are expanded into cling¬
ing pads that enable them to run up
vertical surfaces and even walk on ceil¬
ings. Geckos are the only lizards that
can make loud noises. The skinks are
known for their shiny cvlindrical bodies
and generally weak legs. In the forests
of Africa and the East Indies, skinks are
the most abundant reptiles. In the
United States they are exceeded in num¬
bers onlv by the iguanas. Yet they are
not often seen because they are shy,
retiring animals.

The Gila (hee-\ a) monster, of
southwestern United States and Mexico,
and its close relative, the beaded lizard
of Mexico, are the only poisonous liz¬
ards. The name “monster” is mislead¬
ing, for a large Gila monster is less than
two feet long. Poison glands are sit¬
uated in the rear of the lower jaw.

CHAPTER 36 THE REPTILES 501

Grooved teeth are found in both jaws.
The venom does not flow through the
teeth (as in snakes), but the Gila mon¬
ster clings tenaciously and shakes its
head from side to side, thus allowing
the venom to enter the wound. The
toxins of the venom are neurotoxic, af¬
fecting the nerves that control the vic¬
tim’s breathing. Even though few
human beings die of Gila monster bites,
these lizards should be treated with
the same respect given to venomous
snakes.

The largest lizards in the world be¬
long to a family called the monitor
lizards. Monitors are closely related
to the ancient lizards that are thought
to have given rise to the snakes. Al¬
though they do not look like snakes,
monitors have several adaptations found
in snakes. Several of them are at home
in water. The largest monitor is the
Komodo dragon, which attains a length
of 10 feet and may weigh 300 pounds.
It inhabits Komodo Island and several
neighboring islets in the East Indies.
Except for an inability to spout fire,
monitor lizards have all of the classical
characteristics of the mythical dragons.
Two of the more interesting lizards are
depicted in Fig. 36-9.

The crocodilians. The crocodilians and
their extinct relatives reached their su¬
premacy in the late Mesozoic Era. They
were found in all types of environments,
but they thrived only in water. Croc¬
odilians like today’s alligators and
crocodiles appeared in the Cretaceous
period. The modern crocodilians have
survived from ancient times because
their ancestors developed a means of
breathing while in the water — even
with their mouths open. Raised nos¬
trils at the end of the snout are con¬
nected to the throat by an air passage in
the skull. At the back of the mouth

is a fleshy valve that prevents water
from entering the lungs when the mouth
is open. Thus, crocodilians can he in
the water with only the nostrils and
eyes above the surface. Unsuspecting
animals are often too close to escape by
the time they see the predator, or they
may be totally unaware of the crocodil¬
ian’s presence until they are grasped by
the animal’s powerful jaws.

Compared to the large number of
crocodilian species that inhabited the
seas in the days of the dinosaurs, only
a few survive. About twenty-five living
species of alligators , caimans (kay-
manz), crocodiles , and gavials make up
the order Crocodilia. All live in the

36-9 The collared lizard (top) and the
banded gecko (bottom) are shy, harmless
little reptiles. (American Musuem of Nat¬
ural History)

502 UNIT 6 BIOLOGY OF THE VERTEBRATES

36-10 The young Indian gavial feeding on a small fish and the sluggish-looking
caiman are representatives of the crocodilian order. (N.Y. Zoological Society)

tropical and subtropical zones. All are
very similar to one another, differing in
such small ways as length and width of
snout, the arrangement of scales, and
the arrangement of teeth. Crocodiles
are more aquatic than alligators. They
are distinguished from alligators by a
triangular head and a pointed snout,
and by a tooth on each side of the lower
jaw which fits into notches on the out¬
side of the upper jaw. Two members
of the order Crocodilia are shown in
Fig. 36-10.

Alligator hide is of great value for
making fine leather, and baby alligators
are in great demand as pets. Because
of the danger of exterminating the
American alligator, the United States
has outlawed the collecting of the ani¬
mals for hide or pets. Most “alligator”

hides and baby “alligators” sold in the
United States today are in reality the
skins and babies of the South American
caiman, which is disappearing from
large areas of South America. If kept
in an aquarium at room temperature
and fed regularly on insects, fish, or
pieces of raw meat, caimans will live
for a long time in eapitivity.

The turtles. Biologists speak of turtles
that live on land as tortoises. These
hard-shelled, slow-moving turtles have
strong feet and claws for walking on
land and digging. Most tortoises are
vegetarians, living on a variety of plant
foods. Several of the hard-shelled,
fresh-water, edible turtles are known as
terrapins. These turtles are found in
markets in many sections of the coun¬
try. The large ocean-dwelling forms

CHAPTER 36 THE REPTILES 503

with limbs in the form of flippers are
true turtles. However, for conveni¬
ence, we shall refer to all of them as
turtles.

If you want to know what the earli¬
est reptiles were like, examine a turtle.
It has changed but little in the past
200 million years. This ancient reptile
form outdates even the lizards and the
dinosaurs, first appearing in the Jurassic
period.

The turtle seems to have been na¬
ture’s experiment with armored verte¬
brates. The curious body structure was
highly successful in one sense, since the
animal has survived for such a long
time. However, of the more than 7,000
species of reptiles, only 300 are turtles.
It seems that the turtles have neither
increased nor decreased in number since
very ancient times.

It is possible that turtles actually
reversed the trend of vertebrates to move
from water to land. The ancestors of
turtles might have been land animals.
Although a few still live on land, most
are primarily water-dwellers. But since
they are reptiles, all turtles come to the
land to lay their eggs.

Several important adaptations have
made it possible for turtles to persist
for so many years. Most significant was
the development of a boxlike shell.
The upper and lower shells are con¬
nected to each other along the sides,
and many turtles can withdraw the
head and feet into the shell. Although
we often think of turtles as slow and
stupid, their adaptation for protection
at the expense of movement is the main
key to their success.

It is not certain just how much
shell the first turtles possessed, but some
modern turtles are almost entirely en¬
closed, while others have greatly re¬
duced shells. In the soft-shelled turtles,

the horny shell has been replaced by a
leathery skin, but there is a well-devel¬
oped bony shell beneath the skin. These
turtles have flattened bodies which en¬
able them to lie concealed at the bottom
of a lake or pond. A very long neck
enables these active predators to cap¬
ture unsuspecting prey. The upper shell
of the snapping turtle is an excellent
cover, but the lower shell is small. Still,
it has excellent protection with its large
head and powerful jaws. The typical
sea turtles have large upper shells, but
the lower shells are somewhat reduced.
They can escape from their enemies be¬
cause their flippers enable them to swim
efficiently. Only when they come ashore
to sun themselves or lay their eggs are
the sea turtles at the mercy of their
enemies. If they are turned on their
backs on land, they are totally helpless.

Turtles possess valuable adaptations
other than their shells. The horny
toothless beak is an efficient shearing
mechanism which permits the animal to
eat meat or plants. Their legs are
strong and heavy. Turtles can remain
completely submerged in water for long
periods of time. Some species have
developed a very efficient substitute for
gills. Sea turtles are able to absorb
oxygen from the water through mem¬
braneous areas in both the cloaca and
throat.

Although most turtles live in water,
the tortoise has become a land dweller,
with some living in dry desert condi¬
tions. It is a mvsterv how tortoises be-

J J

came established on islands far from
any mainland, but they are found on
islands in the Pacific and Indian oceans.
The giant tortoises of the Galapagos
Islands in the eastern Pacific were ex¬
tremely plentiful before the arrival of
civilized man and his domesticated ani¬
mals. A number of species of the

504 UNIT 6 BIOLOGY OF THE VERTEBRATES

36-11 The giant tortoise (left) of the Galapagos Islands off the South American
Coast and the gopher tortoise are representative turtles. (Left: American Mu¬
seum of Natural History; right: N.Y. Zoological Society)

Galapagos tortoises are found on the
islands. When Charles Darwin visited
these islands during his trip around the
world, he observed the different species.
These animals contributed to the devel¬
opment of Darwin’s theory of evolu¬
tion.

Structure of the turtle. The upper shell,
or carapace (kczr-a-paee) , of a turtle is
covered with epidermal plates, or
shields, arranged in a symmetrical pat¬
tern. While these shields vary in num¬
ber and arrangement in various kinds of
turtles, thev are the same in all members
of a species. The shields vary in color
and in markings. Beneath the shields
are bony plates which are fused together
to form a protective case. The shape
and arrangement of these bony plates
does not match the epidermal shields
above them. The lower shell, or plas¬
tron, has similar epidermal shields cover¬
ing bony plates. The carapace and plas¬
tron join on the sides in a bony bridge.

The head of a turtle is generally
pointed or triangular. The mouth lacks
teeth, but the margins of the jaws form
a sharp beak with which the turtle bites
off chunks of food. There is a pair of
nostrils at the tip of the head, making
it possible for a turtle to submerge in
the water, leaving only the tip of the
head above the surface for breathing air.
The eyes are well developed and are
protected by three eyelids. In addition
to fleshly upper and lower eyelids, the
turtle has a transparent nictitating mem¬
brane which closes over the eyeball from
the front corner of the eye. A smooth
tvmpanic membrane lies just behind
the angle of the upper and lower jaws.

The limbs of most turtles are short
and, in most species, have five toes pro¬
vided with claws. The feet vary in the
amount of webbing between the toes.
The skin covering the limbs is tough
and scale-covered. The tails of turtles
vary greatly in length.

IN CONCLUSION

Since the reptiles were able to occupy a wider range of environmental condi¬
tions than their amphibian ancestors, they have been more successful. While
they may equal the mammals in variety of adaptations, thev cannot compete
with either birds or mammals because they do not have the ability to regulate

CHAPTER 36 THE REPTILES 505

their internal temperature. They are further limited by their small brains.
Some early reptiles developed warm-bloodedness and larger brains, and are
thought to have given rise to the first mammals. Reptiles have been most suc¬
cessful in the tropics, but even in most tropical regions they are secondary to
the higher animals.

In the next chapter, we will study the birds, which, despite their covering
of feathers, can be thought of as reptiles with some significant improvements.

BIOLOGICALLY SPEAKING

amniote egg

lateral undulatory movement

Reptilia

antivenin

neurotoxin

scute

bridge

oviparous

septum

carapace

ovoviviparous

shield

caterpillar movement

parietal eye

side winding

elastic ligament

plastron

venom

hemotoxin

Jacobson’s organ

quadrate bone

viviparous

QUESTIONS FOR REVIEW

1. What factors could have enabled the reptiles to invade the land?

2. Why is it difficult to determine when the transition from amphibians to
reptiles occurred?

3. Name and describe a plant-eating and a flesh-eating dinosaur.

4. What four orders of reptiles are represented in the world today?

5. List seven characteristics that distinguish reptiles from other vertebrates.

6. In what respect is Sphenodon an unusual reptile?

7. What use does the snake make of its forked tongue?

8. Describe three methods used by various snakes in capturing prey.

9. How is the snake’s jaw structure adapted for swallowing large prey?

10. Describe three forms of movement in snakes.

11. Name the four families of poisonous snakes and differentiate them.

12. Why is the pit an important characteristic of the pit vipers?

13. Name the two types of toxin found in snake venom and describe their
effects on the victim.

14. List as many reasons as you can for the success of the crocodilians.

15. What adaptations have enabled the turtle to survive through the ages?

APPLYING PRINCIPLES AND CONCEPTS

1. In what ways are reptiles better suited to land life than are amphibians?

2. Of what significance is the shelled egg of reptiles when compared to the
eggs of fishes and frogs?

3. Make a list of possible reasons for the disappearance of the dinosaurs.

4. Account for the fact that many unusual and ancient animals are found
today only on islands.

CHAPTER 37

THE BIRDS

The origin of birds. The birds obvi¬
ously survived the forces that caused
their great reptile companions to dis¬
appear, but just how early could they
have evolved from the reptiles? In the
late 1800’s an explorer was digging for
fossils in 150-million-year-old Jurassic
stone in Bavaria, Germany. As he dug
through the layers of stone, he exposed
the most ancient bird fossil ever found.
Since this was a well-developed bird
type, scientists conclude that the first
birds appeared more than 200 million
years ago.

Fortunately, in addition to a print
of the skeleton, the Jurassic fossil in¬
cluded excellent impressions of the long
flight feathers on the wings and the
double row of tail feathers. Except
for these feather impressions, scientists
would doubtless have classified the fos¬
sil as just another reptile skeleton. Be¬
cause of its feather prints, the fossil of
Archaeopteryx (ahr-kee-u/zp-te-riks) is
one of the most important ever found.
It was a curious mixture of lizard and

bird characteristics. The front of its
skull and lower jaws were elongated and
narrowed into a beak armed with well-
developed reptilian teeth. The neck
was long and the body was supported on
a pair of strong hind limbs and a typical
reptilian tail. The modified forelimbs
were weak wings. Extending beyond the
feathers of each wing were three clawed
fingers, which were probably used in
climbing around in trees. The back of
Archaeopteryx was short and compact,
a characteristic that enables the modern
bird to fly. Scientists believe that Ar-
chaeopteryx was able to walk and run
in much the same manner as chickens.

During the same period that this
strange bird lived, other animals were
also taking to the air. These included
the Pterodactyls, or flying reptiles. Al¬
though they possessed the power of
flight, they cannot be considered birds.
Flight is not necessarily a bird charac¬
teristic. Many birds are flightless, and
even some mammals fly. The presence
of feathers distinguishes a bird from a
reptile. The Pterodactyls were covered
by reptilian skin. The flying reptiles
of the Jurassic period were probably
better fliers than the early birds. The
Pterodactyls and dinosaurs lived on the
earth for millions of years, but they
became extinct. Although all of the
birds of the first 150 million years died
out, their modified descendants are still
with us.

The birds of the Cretaceous period
had advanced far along the evolutionary
road that gave rise to modern birds.
Their fingers had grown together, mak¬
ing the wings stronger. They still re¬
tained teeth, but these gradually dis¬
appeared and their mouths developed
into horny beaks or bills.

Birds as we know them today had
evolved by the beginning of the Ceno-

506

CHAPTER 37 THE BIRDS 507

37-1 Left: fossil cast of Archeopteryx.
Right: an artist’s representation of how the
bird probably looked. (American Museum
of Natural History)

zoic Era. So far as biologists can tell,
there has been little or no structural
change in birds for more than 50 million
years. Most modern birds are very sim¬
ilar to one another. Their variations
are the result of adaptations to many
different types of life. Modern birds
have retained two very conspicuous char¬
acteristics from their reptile ancestors
— scales on their legs and feet and claws
on their toes. Although they never be¬
came a dominant form of animal life,
the birds must be considered highly
successful vertebrates.

Forms of life unchallenged in the air.
Birds have an advantage over all other
living things in that they are able to
change environments as conditions re¬
quire. They have been so successful
that they have spread their numbers
from the jungles of the tropics to the
wastelands of the polar regions, and
from mountaintop to valley.

Birds vary in size from the tiny
hummingbird to the ostrich. The food
of various birds includes everything
from flower nectar to mammals. The
variation in the form and use of the
beaks and feet, protective coloration,

nesting habits, care of the young, migra¬
tion, and many other phases of the lives
of birds are interesting studies in adap¬
tation.

Characteristics of birds. While birds
vary greatly in form, size, diet, and life
habits, they have certain characteristics
in common. The following distinguish
them easilv from the other vertebrates:

1. Body covering of feathers.

2. Bones light, porous, and air-filled.

3. Forelimbs (arms) developed as wings
for locomotion (in most birds), never
for grasping.

4. Body supported on two limbs.

5. Mouth provided with a horny, tooth¬
less beak.

6. Eggs covered by a protective shell;
in most birds, incubated in a nest.

7. Constant body temperature (warm¬
blooded).

8. Heart divided into four chambers.
Adaptations for flight. Did you ever
compare a bird to an airplane? We
probably got our first ideas for airplane
design from the birds. The body of
the flying bird is streamlined and cuts
through the air with a minimum of
resistance. The beak and head are

508 UNIT 6 BIOLOGY OF THE VERTEBRATES

pointed and also serve to reduce air
resistance. The body itself is made
smooth by feathers. The body tapers
at the tail, where large feathers act as
a steering device. The legs are attached
to the body above the center of gravity.
Fore-and-aft balance is provided by the
head and neck in front and by the legs
and tail at the posterior part of the
body.

The wings are rounded, thicker on
the front edge and tapered on the rear
edge. We have duplicated this general
shape in the airplane wing. The wings
of a bird can be tilted to give upsweep
or downsweep or act as a brake. The
wings of an airplane have ailerons (a
movable part that functions in lateral
control), and the airplane tail has a
rudder and elevators to accomplish the
same purpose. The porous bones of the
bird give maximum support with min¬
imum weight, while we have learned to
use aluminum and magnesium, both
strong, light metals, in airplane con¬
struction. Most birds pull their feet

against the body in flight. Did you
ever watch an airplane fold its landing
gear?

Structure and functions of feathers.
Strange as it may seem, feathers are
modified scales. Feathers develop from
pits in the skin. They grow in lines
that lie in only certain regions of skin.
But the feathers spread out to cover
featherless regions.

There are four kinds of feathers.
Soft down feathers form the plumage of
newly hatched birds. In older birds,
especially waterfowl, they form an insu¬
lation close to the skin. Down reduces
heat loss so efficiently that a bird can
fly through cold winter air and still main¬
tain a body temperature of over 100° F.
The slender hairlike feathers having
a tuft on the end are known as filo-
plumes.

Contour feathers cover the body
and round out the angles, giving the bird
a smooth outline. They also form an
effective shield against injury and pro¬
vide the coloration so important in the

CHAPTER 37 THE BIRDS 509

life of a bird. Often the female blends
more closely with the surroundings than
her brightly colored mate. Quill feath¬
ers grow in the wing and tail. These
large feathers provide the surface the
bird needs in flying and steering in
flight.

Figure 37-3 shows the structure of a
quill feather. A broad flat vane spreads
from a central axis, the rachis (ray- kis).
The rachis ends in a hollow quill. If
you magnify the vane, you can see the
many rays or barbs. Each barb is like
a tiny feather with many projections, the
barbules (little barbs). These are held
together with tiny interlocking hooks.
This complicated arrangement makes
the vane strong, light, and elastic. If
a vane is split, the bird shakes its feath¬
ers and locks the barbules together
again; or it may preen the feather by
drawing it through its beak, making
it whole again. The rachis is grooved

Opening of quill

Hollow part of quill

Vane

Rachis

Barb Barbule Hooks

37-3 This diagram shows the structure of
a quill feather. The enlargement on the
bottom illustrates a portion of the rachis and
the vane as seen under a microscope.

and the quill hollow, a condition that
gives a feather the greatest strength with
the least weight. At the base of the
quill is an opening through which nour¬
ishment is supplied while the feather
is growing.

The vane of the wing feather is
wider on one side of the rachis than
the other. When the wing strikes the
air in a power stroke, the vane turns up
and rests against its neighbor. On the
return stroke, it is free to turn back.
The air passes through the wing as each
feather turns slightly on its axis (feath¬
ering), and the wing meets less air
resistance.

You have probably noticed birds
oiling their feathers after a bath or a
swim. Many birds transfer oil from a
gland at the base of the tail and spread
it over the surface of the feathers, which
makes them waterproof. Oil on the
feathers is vital to swimming and div¬
ing birds such as ducks, geese, swans,
loons, and grebes. This oil not only
prevents water from penetrating the
feathers to the skin, it also makes the
birds buoyant and prevents chilling of
the body.

Molting in birds. The bird sheds its
feathers at least once a year. Feathers,
especially those of the wings and tail,
may be lost or broken, and since molting
usually occurs in the late summer, the
bird is provided with new quills before
the fall migrations. A second partial
molt often occurs in the spring before
the breeding season. This molt pro¬
vides the bright breeding plumage of
many birds. In some species, including
the ptarmigan, two complete seasonal
molts occur. The early summer molt
provides a plumage that blends with
rocks and soil. The fall molt arrays
the ptarmigan in a snow-white winter
plumage.

510 UNIT 6 BIOLOGY OF THE VERTEBRATES

Upper arm Wrist joint

Lower arm

Thumb bone

First finger bones

Second finger bone

Coracoid

Collar bone

Secondary feathers

Primary feathers

37-4 The wing of a bird shows a greatly lengthened “hand” region and a re¬
duced number of “fingers.”

The new feathers grow from the
same pits from which the old ones were
shed. In most species the wing feathers
are shed gradually and in pairs, thus al¬
lowing the bird to fly during the molt.
Many water birds, including ducks, lose
their flight feathers all at once. They
must hide from their enemies until new
feathers grow in.

A birds wing — a modified forelimb
adapted entirely for flight. You can see
the resemblance to your own arm if you
examine a chicken wing closely. The
upper arm is a large single bone which
is attached to the shoulder at a ball-
and-socket joint (Fig. 37-4). The part
corresponding to the lower arm has two
bones like your own. The end section
includes the wrist and the hand. This
is covered with skin and contains the
partial bone structure of three fingers.

The shoulders are braced by three
bones in a tripod arrangement: the
shoulder blades are embedded in the
muscles of the back above the ribs; the
collarbone (wishbone) extends from
each shoulder and fuses in front of the

breastbone, forming a V-shaped bone;
and the coracoid bones also brace the
shoulder against the breastbone. Thus
the wing is firmly braced to withstand
the tremendous force required in flying.

The muscles of the lower arm bend
the hand at the wrist. Those of the
upper arm move the lower arm. These
muscles are involved in folding the
wings. But the movement of the wings
in flight is largely a movement at the
shoulder. These muscles are enormous
and in many birds make up one third
or more of the whole body weight. The
breast muscles are attached to the
greatly enlarged breastbone. These
muscles of a chicken or turkey are ten¬
der and light in color because the birds
do not fly. Tendons from these muscles
pass over the shoulders like ropes over
pulleys, giving tremendous leverage.

The longest quill feathers, the pri¬
maries , grow from the end section of
the wing, or modified hand, where lever¬
age is the greatest. Secondaries grow
from the second section of the wing, or
modified forearm. The lower portion of

CHAPTER 37 THE BIRDS 511

both of these sets of quill feathers is
covered with smaller feathers known as
coverts. The secondaries in turn are
covered by other rows both above and
below. The outline of the wing as a
whole is concave on the lower side, thick
on the forward edge, and thin and flexi¬
ble on the rear edge and tip — a perfect
design for flight.

Motion of the wings in flight. We

might compare the motion of a bird’s
wings in flight to a horizontal figure
eight — down and back, up and for¬
ward. The down stroke is the power
stroke. The upward movement returns
the wing to position for another power
stroke. These two actions of the wing
require two sets of muscles, arranged
in layers on the breast. You may have
noticed that these layers separate on
the breast of a chicken. The tougher
muscles of the outer layer pull the wing
down in a power stroke. Those of the
more tender inner layer raise the wing
for the next stroke.

Flightless birds. In many parts of the
world, there are birds that have lost
the ability to fly. The best known are
the ostrich of Africa and the penguin of
the Southern Hemisphere.

Most flightless birds have succeeded
in life only because they live in areas
that are free of predators. Several, like
the giant elephant bird of Madagascar,
the dodo of the Mascarene Islands, and
the moas of New Zealand, became ex¬
tinct only after the arrival of man and
his pets. Others, like the ostrich, the
rheas, and the cassowaries survive among
predators only because they have excep¬
tionally strong legs and keen vision.

37-5 The cassowary and the emu of Austra¬
lia and the penguin are flightless birds.
(Top and middle: Walter Dawn; bottom:
Annan Photo Features)

512 UNIT 6 BIOLOGY OF THE VERTEBRATES

Penguins are interesting birds. Al¬
though they are flightless, their wings
are well developed. They use their
wings for swimming through the water.
Their webbed feet serve them well as
rudders. Penguins have the keeled
breast that most flightless birds lack.
This adaptation may be related to the
development of muscles used in swim¬
ming.

Adaptations for life — birds’ feet and
beaks. By examining the beak and feet
of a bird, it is possible to draw some
definite conclusions about the life and
diet of the bird. The feet of various
birds differ widely in structure, de¬
pending on the particular purpose for
which they are used. Birds may use
their feet for locomotion, to help ob¬
tain food, to aid in nest-building, for
offense and defense. Most birds walk
on their toes. What appears to be the
first leg bone is actually a foot bone.
The feet of ground birds are adapted
for scratching. Most swimming birds
have webbed feet, and the legs are set
far back on the bodv, which makes the
birds awkward on land. But in water,
these birds are very graceful. Birds
with very long legs are well adapted
for wading or walking in tall grasses,
and they have very long toes for bal¬
ance. Hawks and other flesh-eating
birds have strong feet with long sharp
claws or talons for capturing animals.
Some birds, such as the woodpecker,
have toes arranged two in front and two
in back for climbing.

In much the same way the beaks
of birds are adapted to perform different
functions. It is possible to learn much
about the diets of birds by studving the
shape of the beak. Seed-eating birds,
like finches, have strong thick beaks
for crushing the seeds. Thinner beaks
are found in insect-eating birds like

warblers. Probing birds such as the
woodcock have very long beaks. Birds
of prey have hooked beaks for tearing
flesh. Some ducks have beaks that are
adapted for straining food particles out
of mud. Hummingbirds and a few
others have long, slender beaks that can
sip nectar from the deep throats of
flowers. Birds belonging to entirely dif¬
ferent groups may have similar beaks
that perform similar functions. Unlike
beaks are sometimes found among mem¬
bers of the same group. Figure 37-7 il¬
lustrates the relationship between beaks
and feet in some birds.

In spite of all the differences in
feet and beaks, the birds have more
structural similarities than any other
class of vertebrates and this can be ex¬
plained. Isolation leads to more pro¬
nounced structural variation. Most ver¬
tebrates are isolated by oceans, moun¬
tains, and deserts. But birds, with their
ability to fly, are able to cross many of
the barriers that isolate other vertebrates.
Feeding activities. The bird is the
first warm-blooded animal we have stud¬
ied. A constant body temperature as
high as 112° F is maintained in some
species. The maintenance of this tem¬
perature, together with the tremendous
muscular exertion during flight, requires
a high rate of metabolism. This requires
a large amount of food — the fuel for
a metabolic furnace. Thus birds spend
much of their time eating. To say that
someone “eats like a bird” is actually
to say that he is a glutton.

Birds feed on a great variety of
animal and plant life. Some eat in¬
sects. Others eat seeds. Some destroy
rodents, fishes, and other birds. Some
eat carrion (decaying flesh), and others
sip the nectar of flowers or the honey
produced by bees. Among the birds
using animals for food are large birds

CHAPTER 37 THE BIRDS 513

37-6 The internal organs of the bird.

of prey, such as hawks and owls, which
help limit the population of rodents,
rabbits, and some smaller birds. The
loon, grebe, pelican, kingfisher, and sev¬
eral hawks feed largely on fish, which
they catch by diving. The best known
carrion eaters are the vultures. They
have keen vision, which enables them
to locate a dead animal as thev soar

through the air. Probably the largest
number of birds that enjoy an animal
diet five chiefly on insects, which they
catch in the air (swifts), in wood (wood¬
peckers), on the ground (robins), or
on trees (warblers).

Many birds live almost exclusivelv

j j

on seeds, doing much good by the de¬

struction of weed seeds, while others,
such as blackbirds and bobolinks, do
considerable damage by their preference
for grain, peas, and rice. Various kinds
of both wild and cultivated fruits, es¬
pecially berries, are preferred by certain
birds.

Sometimes birds eat the same seeds
or fruits that man raises, or they may
at times rob his vard of a stray chicken.
But careful study has proved that few
if any birds do more harm than good.
The rest repay manv times for eating
a small quantity of valuable food.

The digestive system of the bird. Food,
swallowed whole, passes down an esoph¬
agus into a crop, located just below

514 UNIT 6 BIOLOGY OF THE VERTEBRATES

HERON

QUAIL

HAWK

37-7 The feet and beaks of birds vary with their activity and their food. For ex
ample, a duck's feet are adapted for swimming while its beak is adapted for scoop

CHAPTER 37 THE BIRDS 515

ROBIN

WOODPECKER

PELICAN

DUCK

ing and straining. For what type of activity would you say each of the other feet
shown here is adapted? Also, what type of food would you say each bird ate?

516 UNIT 6 BIOLOGY OF THE VERTEBRATES

the base of the neck (Fig. 37-6). Here
the food is stored and moistened. From
the crop, food passes into the first divi¬
sion of the stomach where glands in the
thick walls add a digestive secretion.
The stomach content then passes into
the second stomach region, the gizzard.
The thick muscular walls of the gizzard,
aided by small stones in some species,
churn and grind the food. A U-shaped
loop of intestine joins a short rectum , or
large intestine, which leads to a cloaca
somewhat like that ot the frog.

The two-lobed liver is large and may
or may not have a gallbladder on the
lower side, depending on the species of
bird. Bile is poured into the small in¬
testine through two ducts. The pan¬
creas lies along the U-shaped portion of
the intestine and pours its secretion into
the intestine through three ducts.
Respiration, circulation, and excretion
in the bird. The lungs of a bird lie in
the back against the ribs, in the anterior
region of the body cavity. The capacity
of the lungs is greatly increased by a sys¬
tem of air sacs that extend from the
lungs into the chest area and the abdo¬
men and connect with cavities in the
larger bones (Fig. 37-8).

Air is drawn through the nostrils in
the beak and down the trachea and its
lower divisions, or bronchi , to the lungs
and air sacs by relaxation of the thoracic
and abdominal muscles. Contraction of
these muscles forces air out. Though
the lungs are small, a rapid rate of res¬
piration fills them often and supplies
the blood with the great amount of oxy¬
gen necessary to carry on the high rate
of oxidation in the body tissues.

The bird’s respiratory system is also
its principal temperature-regulating sys¬
tem. It has no sweat glands and cannot
eliminate heat through its skin. Most
excess heat is discharged from the body

37-8 Because birds are so active their re¬
spiratory systems are especially adapted to
accomplish rapid oxidation of food and re¬
lease of energy.

through the lungs. The air sacs are be¬
lieved to assist in heat elimination. You
may have noticed that birds often pant
with their mouths open on a hot day.
At times like these, the insulation pro¬
vided by feathers is more a liability than
an asset.

The lungs also supply air for sing¬
ing. The bird’s song is not produced in
the throat but at the base of the
trachea, where the bronchi begin. Here
the song box, a delicate and highly ad¬
justable structure, is located.

The kidneys are dark brown, three-
lobed organs lying along the back. They
excrete uric acid , a waste product of cell
activity. This is discharged with very

CHAPTER 37 THE BIRDS 517

little water through the ureters into the
cloaca, and is then eliminated along
with intestinal waste.

The heart of a bird is large and
powerful. It consists of two thin-walled
atria and two muscular ventricles. The
right side of this four-chambered heart
receives blood from the body and pumps
it to the lungs. Blood returns from the
lungs to the left side and is pumped to
the body. The heart of the bird beats
at an amazing rate. With the bird at
rest the beat is several hundred times per
minute. Under exertion the heart may
beat is many as a thousand times a
minute.

The nervous system of a bird. In birds
the brain is large and broad, completely
filling the cranial cavity. The olfactory
lobes are small, indicating a poorly de¬
veloped sense of smell (Fig. 37-9) . The
optic lobes are large, thus accounting for
the keen vision of the bird. The hem¬
ispheres of the cerebrum are the largest

37-9 The structure of the bird’s brain. Com¬
pare the relative size of the cerebrum to that
in the fish and frog brain. What do the optic
lobes indicate about the sense of sight?

of any animal we have discussed thus far.
The highly developed instincts of birds
center in this brain region. The large
cerebellum accounts for the excellent
muscular coordination of the bird, es¬
pecially in flight. The medulla joins the
spinal cord, which extends down the
back, encased in vertebrae.

Sense organs of the bird. The eyes are
large, and the sense of sight is very
keen. It is said that some birds, espe¬
cially hawks and owls, have vision eight
times as keen as that of man. Owls
and certain other birds have excellent
vision in reduced light. Birds have a
remarkable ability to judge distance,
both at close range and at great height.
They can drop out of the sky and light
on a rock in a stream, fly through a deep
woods, or swoop down onto a slender
perch. The eyes are protected by an
upper and a lower eyelid, as well as by
a thin transparent nictitating membrane.

Ear canals are covered by a tuft of
feathers. Eustachian tubes lead from
the ears to a single opening in the upper
wall of the throat. The sense of hear¬
ing is very keen, and the ears are espe¬
cially sensitive to high notes.

The senses of smell and taste are
very poor, a fact due in part to the
horny nature of the mouth. The tongue
of most birds is small and serves as an
organ of touch.

The reproductive system. The oval
testes of the male bird lie in the back in
about the same position we found them
in the male frog. Tiny tubes carry
sperm to openings in the cloaca. Dur¬
ing mating, sperm are deposited in the
cloaca of the female.

The female reproductive organs in¬
clude a single ovary in which eggs de¬
velop, and a long, coiled oviduct which
leads to the cloaca (Fig. 37-10). In
most birds the right ovary disappears

518 UNIT 6 BIOLOGY OF THE VERTEBRATES

early in life. The absence of one ovary
and oviduct is another of the interesting
modifications of birds for life in the air.

If you examine a hen you are pre¬
paring for dinner, you will find a mass of
orange spheres in the region of the back
— these are developing yolks. On the
surface of each yolk is a tiny egg cell
surrounded by protoplasm. When a
yolk has grown to full size, it is drawn
into the upper end of the oviduct by
lashing cilia. As the yolk travels down
the oviduct, it is surrounded with layers
of albumen, or egg white. Two enclos¬
ing membranes form around the albu¬
men. One of these is an amnion, such
as we found in the reptile egg. A hard
shell is secreted around the membranes
by lime-producing glands in the lower
part of the oviduct, before the egg is laid.

Thus, we must distinguish between
an egg and an egg cell. The tiny spot
surrounded by protoplasm on the side

Egg in ovary

Open end
of oviduct

Oviduct

Kidney

Reduced
right oviduct

Lime-

producing

gland

Opening of
ureter

Opening of oviduct

Ureter

Rectum

Cloaca

37-10 In this drawing of the reproductive
system of the hen, the ovary is shown con¬
taining immature eggs.

of the yolk will develop into a new or¬
ganism. It is the only living part of
an egg. The protein of the albumen
and oils of the yolk are stored nourish¬
ment for the developing embryo. The
shell prevents drying out, but must be
porous enough to admit air. The pores
are also large enough to admit bacteria,
which accounts for the spoilage of eggs,
especially in warm weather.

Incubation and development of a bird.
An embryo can develop in an egg only
if a sperm has fertilized the egg cell
before the shell is formed. Develop¬
ment begins as soon as the egg is in¬
cubated, or kept continuously warm.
The mother provides this warmth by
sitting on the egg. The incubation tem¬
perature of most birds is slightly above
100° F.

The time of incubation varies from
13 to 15 days in smaller birds to as
many as 40 to 50 days in a large bird.
The hen’s egg is incubated 21 days (Fig.
37-11). The egg of a duck requires 28
days. It is usually the female which
sits on the eggs. However, the male
bird takes his turn in some species, in¬
cluding the ostrich. Just before hatch¬
ing, the baby bird absorbs the remainder
of the yolk. The baby bird pushes the
shell halves apart and works itself out.
Some birds have hardly any body cov¬
ering when they are hatched. Others
are covered with a dense coat of down.
Egg number and parental care. Ducks
(except the wood duck), geese, quail,
grouse, turkeys, chickens, and other
fowl-like birds lay a large number of
eggs in a nest on or close to the ground.
The eggs are not incubated until the
last one is laid. As many as 12 to 15
birds may hatch at the same time. They
are fully covered with down and can
scratch for their food almost imme¬
diately. However, they must be warmed

CHAPTER 37 THE BIRDS 519

Yolk

Developing

embryo

Albumen

Shell membranes

Shell

1. FERTILE EGG — NEWLY LAID

Heart

Yolk

Embryo
Embryonic membranes

Embryo

k sac

Blood vessels
to yolk sac

3. SEVEN-DAY-OLD EMBRYO
(natural size)

2. THREE-DAY-OLD EMBRYO
(twelve times natural size)

Yolk sac

4. FOURTEEN-DAY-OLD EMBRYO
(natural size)

5. TWENTY-ONE-DAY-OLD EMBRYO
(natural size)

37-11 This drawing shows a sectioned view of a fertilized hen’s egg and also
illustrates some of the stages in the development of the chicken embryo.

and protected by their parents. Birds
like the robin, bluebird, sparrow, and
warbler usually lay fewer than six eggs.
Incubation starts when the last egg is
laid. Thus the baby birds hatch at
about the same time, but they are help¬
less and must be fed almost contin¬

uously by their parents. They remain
in the nest until they are able to fly.
Since these birds are protected by one
or both parents until maturity, the
chances of survival are much greater,
and there is need for fewer young to
insure continuation of the species.

520 UNIT 6 BIOLOGY OF THE VERTEBRATES

Hawks and owls usually lay only
one to four eggs and incubate them as
soon as each is laid. This results in a
stairstep family. The young of these
birds are also fed in the nest until they
are feathered and able to fly.

Most birds are very devoted to their
eggs and young. Some will protect the
eggs even when they may be stepped

on. Normally shy birds will swoop
down on an intruder and fiercelv attack

it, regardless of the size. The killdeer
will feign a broken wing and lure an
intruder awav from the nest. At the

other extreme is the cowbird, which lays
its eggs in other birds’ nests and de¬
pends on the other bird to incubate and
rear its young.

IN CONCLUSION

Birds show a distinct relationship to the reptiles, becoming a new class as a
result of adaptive changes believed to have occurred about 200 million years ago.
Birds exhibit the best example in all biology of adaptation for a special kind of
life. Their bodies are excellent examples of flight engineering. The special
structures they maintain for supremacy in the air has removed them from the
possibility of dominance on land.

Flightless birds survive only by possessing strong legs and keen vision, or
by living where there are no predators, and many flying birds survive in chang¬
ing environments only by periodically migrating to regions where there is suffi¬
cient food.

During the age of reptiles, while birds were developing their remarkable
adaptations for flight, the most efficiently organized animals ever known were
beginning a long evolutionary process that eventually gave rise to mammals.
In the next chapter, we will learn why the mammals have become the dom¬
inant form of animal life.

BIOLOGICALLY SPEAKING

air sac

albumen

barbs

barbules

bronchi

contour feathers
coverts

down feathers
egg cell
filoplume

lime-producing glands

primaries

quill

quill feathers

rachis

rectum

secondaries

song box

uric acid

vane

yolk

QUESTIONS FOR REVIEW

1. Explain why it was fortunate that feathers were fossilized with the bones
of Archaeopteryx.

2. What characteristics relate birds to reptiles?

CHAPTER 37 THE BIRDS 521

3. What are some characteristics of birds that make them ideally suited for
life in the air?

4. Give the functions of the four kinds of feathers.

5. Why is oil on the feathers of importance to water birds?

6. Where are the bird’s powerful flight muscles located?

7. Explain why the flightless birds are limited to certain parts of the world.

8. How can we learn about the diet and habits of birds by examining their
feet and beaks?

9. In what ways are air sacs important to the bird?

10. What advance in structure over the reptile is shown in the bird heart?

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss various reasons why birds have achieved success as a group.

2. Explain why the ostrich and other flightless birds are not considered primi¬
tive.

3. Compare the amount of food consumed and the rate of respiration in the
bird with other vertebrates you have studied.

4. Explain the meaning of the term warm-blooded as it applies to birds.

5. Discuss the relationship between parental care and the number of eggs laid.

CHAPTER 38

THE MAMMALS

The rise of mammals. Compared with
other animals mammals are a recent
form of life. Fossils indicate that the
earliest mammals appeared about 60
million years ago. At that time the
earth was undergoing geological changes
that marked the end of the Mesozoic
Era and the dawn of the Cenozoic Era
(Fig. 13-1, page 183). Prior to this time
a warm, humid climate had prevailed
over much of the earth. Shallow in¬
land seas and swamps were numerous
among the flat land masses. These con¬
ditions had favored the giant reptiles
that dominated animal life for ages.
Then, as though nature were closing
one chapter of life to make way for an¬
other, continents were uplifted and
mountain ranges and high plateaus
arose. These changes altered the cli¬
mate of the continents, resulting in sea¬
sonal temperature variations and dif¬
ferences in the precipitation in various
continental areas. Forest lands, prairies,
plains, arid lands, and deserts came into
being. Within a short time most of

the reptiles died out, yielding their dom¬
inance among land animals to the mam¬
mals.

The earliest mammals were small
and included forerunners of the squirrel,
rat, and mouse. Another of the earliest
mammals was a little ancient horse
known as Eohippus (ee-oh-hzp-us),
which was no larger than a small dog.
This curious ancestor of the modern
horse had five toes. Other early mam¬
mals resembled monkeys. One of the
few early mammals that has survived
to the present day is the opossum.

From these early beginnings mam¬
mals increased in size and became more
abundant. Hoofed mammals, includ¬
ing ancient species of the rhinoceros,
camel, wild pig, and a larger three-toed
horse roamed the forests and grasslands
in great herds. Still later, flesh-eating
mammals, including the great bear dogs
and terrible sabre-toothed cats, flour¬
ished, destroying large numbers of hoofed
mammals. Near the close of the Ceno¬
zoic Era, further cooling of the climate
resulted in the age of glaciers. Then
prehistoric elephants, called mastodons
and mammoths, migrated to Africa
through Europe, Asia, and the North
American continent.

Many species of ancient mammals
perished long ago. However, a large
number survived the changing condi¬
tions and served as the ancestral stock
of the mammals we know today.
Characteristics of mammals. Modern
mammals vary in size from the tiny
shrew, less than two inches long, to the
enormous blue whale, over 100 feet
long. In their widely varied forms,
mammals are found in all parts of the
world except a few Pacific islands.

1 hey are for the most part land animals,
although some, like the whale, porpoise,
and sea cow, are adapted to life in the

522

CHAPTER 38 THE MAMMALS 523

38-1 The mammoths are
prehistoric mammals that
became extinct, probably
because they could not
adapt to changes in cli¬
mate. (American Museum
of Natural History)

sea. The bats are the only mammals

capable of flight.

While mammals vary greatly in
body form, size, and mode of living, all
have certain characteristics in common:

1. Body mostly covered with hair.

2. Young nourished during develop¬
ment in the body of the mother and

born alive, in most mammals; hence,
viviparous.

3. Young nourished after birth by
milk secreted by the mammary
glands of the female, a characteristic
for which the class is named.

4. Cerebrum highly developed.

5. Diaphragm (breathing muscle) sep¬
arating the thoracic (chest) cavity
from the abdominal cavity.

6. Lungs used for breathing through¬
out life.

7. Four-chambered heart and high cir¬
culatory development with left aor¬
tic arch only.

8. Ability to maintain constant body
temperature (warm-blooded).

9. Seven cervical (neck) vertebrae, ex¬
cept in the sea cow (manatee) and
sloth.

10. Two pairs of limbs, except in the
whales, porpoises, and sea cow.

The egg-laying mammals. The most
primitive of mammals are the mono-
tremes (mahn- o-treems), represented by
the duckbilled platypus and the spiny
anteater. These animals are found in
Australia and New Guinea. Like their
reptilian ancestors, the monotremes lay
eggs. Their skeletons still have several
reptilian characteristics, and they lack
external ears. Biologists believe that
the monotremes descended from differ¬
ent reptiles than did other types of
mammals.

The duckbill is about 12 to 18
inches long. It has waterproof fur, a
horny ducklike bill, and feet modified
as paddles. Its home is a burrow, dug
several feet into a bank, which ends in
a grass-filled nest. The duckbill usually
lays two or three eggs, which resemble
those of reptiles. The eggs are retained
in the body of the female for some time
before thev are laid in a nest. Then

she clutches them to her body and rolls
into a ball to incubate them. After
hatching, the young are nourished on
milk from sweat glands of the mother
that are analogous to mammary glands.

The elongated toothless jaws of the
spiny anteater are well adapted for prob-

524 UNIT 6 BIOLOGY OF THE VERTEBRATES

38-2 The duckbilled platypus. (Australian
News and Information Bureau)

ing in ant hills. The animal is well pro¬
tected by a body covering of sharp
spines. It lays two eggs which are placed
in a brood pouch on the anteater’s lower
side. The eggs remain in the brood
pouch several weeks before hatching.
The pouched mammals. Perhaps the
most familiar examples of the marsupi¬
als, or pouched mammals, are the opos¬
sum and the kangaroo. The marsupial
egg does not contain enough yolk for
advanced growth of the embryo, and
there is no means of supplying nourish¬
ment from the mother to the develop¬
ing embryo. Hence the young, al¬
though they develop within the female’s
body, are bom prematurely. Marsupials
have a pouch with mammary glands,
into which the newborn young climb.

An opossum is born a tiny, hairless
creature about an inch long. From six
to fifteen young are produced in one
litter and a female usually bears two or
three litters a year. The young stay
in the pouch until they are large enough
to leave, which is about two months.
The opossum usually sleeps curled up in
a tree through the day. At night it
roams the countryside in search of small
birds and mammals, eggs, and insects.

The kangaroo is a helpless, naked
creature about an inch long at birth.

It spends four months in the mother’s
pouch before venturing out. Even then
it scampers back to the protection of
the pouch when frightened.

Marsupials seem to have occurred
over all the world during Cretaceous
times. They became greatly reduced
in the Cenozoic Era. Only in Australia
are many species of marsupials success¬
ful today. Perhaps you will remember
from your study of evolution in Chapter
13 that the reason for this is because
of Australia’s isolation as a continent.
The more successful placental mammals
never arrived to challenge the marsupi¬
als. As a result the marsupials evolved
into various forms specialized for dif¬
ferent ways of life.

At one time several marsupial forms
were abundant in South America. But
after South America became connected
to North America, more . suitably
adapted mammals preyed upon the mar¬
supials and successfully competed with
them. Today oppossums are the only
marsupials in the Western Hemisphere.
The placental mammals. The placen¬
tal mammals are so named because a
structure called the placenta (pla-sent-a)
joins the mother’s uterus to the em¬
bryo. The placenta permits oxygen
and nutrients to pass from the mother
to the developing embryo in exchange
for the embryo’s waste products. Thus
the young of the placentals are born in
a more advanced stage of development
than the marsupials. In addition, evo¬
lution of a larger brain case and the de¬
velopment of various specialized kinds
of teeth have contributed to the greater
success of the placentals. Of the 28
orders of placental mammals that
evolved during the Cenozoic Era, only
16 exist at the present time. We shall
discuss 13 of these orders, which are
listed on page 526.

CHAPTER 38 THE MAMMALS 525

38-3 The kangaroo is restricted in its native
habitat to Australia. (Leavens-Photo Re¬
searchers, Inc.)

The insect-eating mammals. The in¬
sectivores, represented by the shrews and
moles, were probably the first placental
mammals. All of today’s higher forms
of mammals probably evolved from this
order. The insectivores never became
very large. Their brains are small and
their teeth very primitive. The shrew
is the smallest of all mammals, and re¬
sembles both a mole and a mouse. It
is noted for its high metabolic rate,
which gives it a ravenous appetite for
insects, mice, and even other shrews.
Shrews are seldom seen because they
run along tunnels in the grass and hide
easily under leaves. Such secretive hab¬
its have probably been responsible for
the survival of shrews to the present.

The mole is well adapted for life
under the ground. Its powerful limbs
have long claws for digging, and the
greatly reduced eyes are covered with
skin. Its long nose is adapted for root¬
ing out grubs and worms in the soil.
While the mole is valuable in destroying
many harmful beetle grubs, it is a pest
in lawns and golf courses because it
digs up the turf.

The flying mammals. The bats are
classified in a different order from the
insectivores, even though most of them
eat insects. The chiropterans (ky -rahp-
ter-anz) include more than 600 species
of bats and vampires. These are the
only mammals that have developed
structures for flight. They have greatly
lengthened finger bones covered by
membranes. These structures probably
evolved during the Eocene age, and
there has been little change in the bats
to modern times. Bats fly mostly at
night when insects are abundant in the
air. They spend the days hanging in
a cave or hollow tree. They are nearly
helpless on the ground because their
hind limbs are so poorly developed and
their forelimbs are proportionally so
long. Bats are most numerous in the
tropics, but they occur in all temperate
zones as well. Vampires are members
of this order that pierce the skin of
cattle and other warm-blooded animals
including man, and draw blood flowing
from the wound.

The gnawing mammals. Rats and mice
are among the most common rodents.
Others include the squirrel, woodchuck ,
prairie dog, chipmunk, and gopher. The
beaver is the largest North American
rodent. The great value of beaver pelts
brought the early trappers to the North¬
west Territory. The muskrat is another
fur-bearing rodent.

Rodents are doubtless the most suc¬
cessful group of mammals. They out¬
number all other mammals combined,
and are found in nearly every area of
the world and in all climates. Most
rodents are terrestrial, tree-living, or
burrowing forms, but the beaver and
muskrat have developed a semiaquatic
existence, and the flying squirrel tends
toward flight as it glides from tree to
tree.

526 UNIT 6 BIOLOGY OF THE VERTEBRATES

ORDERS OF MAMMALS

Order

Representatives

Monotremata (egg-laying forms)

Duckbilled platypus
spiny anteater

Marsupialia (pouched forms)

Opossum, kangaroo

Insectivora (insect-eating forms)

Mole, shrew

Cheiroptera (flying forms)

Bat

Rodentia (gnawing forms)

Squirrel, rat, beaver

Lagomorpha (rodentlike forms)

Rabbit, hare, pika

Edentata (toothless forms)

Armadillo, sloth

Cetacea (marine forms)

Whale, porpoise

Sirenia (aquatic mammals)

Sea cow

Proboscidea (trunk-nosed forms)

Elephant

Carnivora (flesh-eating forms)

Dog, cat, bear, walrus

Ungulata (hoofed forms)

Horse, rhinoceros
goat, pig, deer

Primates (more or less erect forms)

Monkey, gorilla
chimpanzee, man

38-4 Beavers show remarkable ingenuity in
constructing dams and lodges. (McHugh
from National Audubon Society)

Why have rodents been successful?
Most of them are small, allowing them
to live in environments not suitable to
larger animals. They have a rapid rate
of reproduction, which enables them to
occupy new areas and adapt quickly to
changing conditions. There is very
little specialization in the body build.
All have strong chisel-shaped teeth.
These teeth have sharp edges that be¬
come even sharper with use because the
front edge is harder than the back edge,
causing the biting surface to wear at an
angle. The forelimbs of rodents are
adapted for running, climbing, and food-
getting.

The rodentlike mammals. For many
years biologists considered the rabbits,
hares, and pikas to be rodents because
they have enlarged incisor teeth for

CHAPTER 38 THE MAMMALS 527

LAGOMORPHA
Jack rabbit

RODENTIA

Squirrel

UNGULATA

Bison

CARNIVORA

EDENTATA

Armadillo

PRIMATES
Spider monkey

CHIROPTERA

Bat

INSECTIVORA

Shrew

MARSUPIALIA

Opossum

SIRENIA
Sea Cow

CETACEA
Blue whale

38-5 These are examples of the eleven orders of mammals in the Western
Hemisphere.

528 UNIT 6 BIOLOGY OF THE VERTEBRATES

gnawing. These mammals, however,
now called lagomorphs (Zczg-a-morfs),
have four incisor teeth in each jaw rather
than two as in rodents. They grind
plant foods with a characteristic side¬
ways motion of the lower jaw.

Pikas (pie- kaz) are small, with
short legs and short ears. The hares
and rabbits have long hind legs which
enable them to leap great distances.
The forelimbs are shorter and take up
the shock of landing. The long ears
give them an acute sense of hearing.

The cottontail rabbit is the most
widely hunted mammal of the United
States, and supplies more flesh than any
other wild mammal. But even though
preyed upon by man, predatory birds,
and other animals, these rabbits have
been able to hold their own in most
localities. The jack rabbit is common
in the broad expanses of the western
prairies and plains. It reaches a length
of nearly 30 inches and has character¬
istic long ears and large powerful hind
legs. Both sight and hearing are es¬
pecially keen, enabling it to escape
from its enemies.

The toothless mammals. Sloths, arma¬
dillos, and great anteaters belong to an
order of mammals that are toothless or
nearly so. These edentates ( ee-den -
tavts) have relatively small brains and
large claws, and some have nine neck
vertebrae instead of the seven usu¬
ally found in mammals. The arma¬
dillos, because of their protective armor,
have survived since the Tertiary7 period.
Their diet consists of insects, carrion,
bird eggs, grubs, worms, birds, and other
small animals. Various species of ar¬
madillo live in our southwestern states,
Mexico, and Central and South Amer¬
ica. Our North American species is
known as the nine-banded armadillo.
It hides in a burrow during the day and

spends its nights digging in the ground
for insects. The young of the armadillo
are identical quadruplets. They start
life as a single egg. Separation of cells
early in development results in four in¬
dividuals that ordinarily would have
been one.

The tree sloths are odd beaverlike
creatures of the jungles of Central and
South America. They spend most of
their time hanging upside down from
the branches of trees by their greatly
elongated limbs and hooked claws.
They feed on the leaves of the trees in
which they live. The hair of some
species is colored green by the algae
that live in it.

Anteaters probably evolved from
the same ancestors as the tree sloths.
The skull of the anteater is greatly
elongated, ending in a long snout. In¬
side is a sticky tongue that can be ex¬
tended to lap up the termites on which
the animal lives. The greatly enlarged
curved claws on their front feet are well
adapted for digging in a termite’s nest.
Locomotion, however, is difficult. The
animal walks and runs on the knuckles
of its front feet. Anteaters and tree
sloths are limited to tropical regions.
Armadillos have spread northward into
the temperate sections of North Amer¬
ica.

The aquatic mammals. The ceta¬
ceans (si-fuy-shunz) — the whales, dol¬
phins, and porpoises apparently evolved
from some land mammal during the
Tertiary period. They are in many
ways well adapted to life in the oceans.
The torpedo-shaped body and fishlike
tail and forelimbs provide for locomo¬
tion. These animals continue to use
lungs for breathing, but thev can take
in great quantities of air and hold their
breath for a long time. Thus they can

CHAPTER 38 THE MAMMALS 529

38-6 The cottontail rabbit is a very common
North American animal. (U.S. Fish and
Wildlife Service)

remain submerged for some time. The
young are born with the ability to swim,
and their mothers push them to the
surface immediately for their first
breath of air. Unlike man, they are
able to withstand sudden changes in
water pressure, and can dive to great
depths.

The blue whale is the largest living
animal and probably the largest that
ever lived. Specimens may reach a
length of 100 feet or more and weigh as
much as 150 tons. The head of the
sperm whale contains an enormous
reservoir of oil, which is used commer¬
cially as a lubricant and as a base for
cosmetic creams. The most valuable
product of this whale is ambergris, a
secretion of the intestine. For centuries
sailors have watched for ambergris float¬
ing on the ocean. Its principal use is in
the manufacture of perfumes.

Dolphins are smaller relatives of
the whale, usually under ten feet in
length. They travel in herds in the

ocean bays and mouths of rivers. Re¬
cent research in underwater sound has
shown that dolphins communicate with
one another. Porpoises also travel in
herds, often close to moving ships, and
delight the passengers with their grace¬
ful leaps. They feed on fish, which they
catch in their narrow toothlined jaws.

The whales, porpoises, and dolphins
are all strictly marine mammals; that
is, they live only in salt water. Another
order includes a mammal that lives in
fresh water: the sea cow, or manatee,
species of which inhabit the rivers of
South America and Africa. These sire-
nians (sy-ree-nee-anz) probably became
adapted for aquatic living during the
Eocene period. Sea cows reach a
weight of one ton. The large head re¬
sembles that of a walrus. The body is
streamlined, and the hind limbs are
lacking. The forelimbs of the sea cow
are modified to form flippers, and they
have a horizontal tail fin. Sea cows
have a tough skin with a sparse covering

38-7 Porpoises may be more intelligent
than we now realize. They seem to com¬
municate with each other under water.
(Annan Photo Features)

530 UNIT 6 BIOLOGY OF THE VERTEBRATES

38-8 The enlarged canines of this leopard
cat are typical of carnivores. (Van Nostrand
from National Audubon Society)

of hair. They spend most of their time
feeding on aquatic plants.

The trunk-nosed mammals. Only two
species of elephants remain as represent¬
atives of the order of trunk-nosed mam¬
mals, or proboscideans ( pro-bu hs-sih-
dee- anz). During glacial and preglacial
times, at least 30 species of elephant¬
like mammals lived in Asia, Europe,
Africa, and North America. The ex¬
tinct proboscideans are the mammoths
and mastodons. Biologists have not
been able to explain why they became
extinct.

Elephants are the largest of all liv¬
ing land dwellers. They may reach a
weight of seven tons or more. The
Asiatic elephant is the familiar perform¬
ing elephant of the circus. In many
parts of the world it is used as a beast
of burden. The African elephant is a
taller, more slender animal with a slop¬
ing forehead and enormous ears. Afri¬
can elephants travel in herds in the
deepest parts of Africa and are not as
easily domesticated as their Indian
cousins.

The flesh-eating mammals. The flesh-
eatmg mammals, or carnivores (kahr- ni-
vorz), evolved in Tertiary times, prob¬
ably from the insectivores. All of the
carnivores have strong jaws with en¬
larged canine teeth for piercing tough
skin and others for crushing bone.
They have a high level of intelligence,
which may enable them to outwit their
prey. They have a well-developed sense
of smell, and the sense of sight is usu¬
ally keen. Powerful bodies and limbs
and the presence of claws contribute to
their ability to overcome and devour
other animals. Variations in foot struc¬
ture are used in dividing the order into
families.

The dog and cat families are not as
well represented in America as in other
lands. The puma (pyoo- ma) [moun¬
tain lion or cougar] was found over
most of North America at one time,
but civilization has driven it to the re¬
mote regions of the southwest. Its
chief harm lies in its destruction of live¬
stock, especially young horses. Both
the bay lynx (links), or bobcat, and the
Canada lynx live in deep forests and are
seldom seen. The jaguars of South
America, the lions of Africa, and the
tigers of Asia are all well-known mem¬
bers of the cat family.

The gray icolf, or timber wolf, is
most frequently found in the northern
forests and may be dangerous during
the winter when it runs in packs. The
coyote (ky- oht), a prairie wolf, has
been more successful than its larger
cousin in surviving the effects of civi¬
lization. It is still abundant on the
western plains. However, in some re¬
gions too many have been destroved
and their natural prey, including ro¬
dents and jack rabbits, have become
pests. It has therefore been necessary
to import coyotes into these regions.

CHAPTER 38 THE MAMMALS 531

The red fox , together with the
other foxes that are color phases of the
red fox, ranges over much of the United
States. It is sly and a very fast runner
for short distances. The gray fox re¬
sembles the red fox but lives in the
warmer regions of the South.

The raccoon , with its black mask
and long-ringed tail, is a favorite of
many people because, if captured young,
it makes a nice pet. The raccoon pre¬
fers fish and clams as its food but will
eat other things as well if these are un¬
available. It has a habit of washing its
food before eating. This is probably to
moisten the food rather than to clean it.

The weasel family includes some
of the most blood-thirsty carnivores and
some of the most valuable fur-bearing
mammals. The mink especially is
prized for its fur. These long-bodied,
short-legged animals live along streams.
The ermine is an Arctic weasel which
grows a coat of white fur (except for
a black-tipped tail) in winter but is
brown in summer. The largest and
most destructive member of the family
is the wolverine of the northern forest.
Its body, including its bushy tail, is
about 36 inches long and weighs be¬
tween 30 and 33 pounds.

The bears were the last carnivores to
evolve, but they have changed little
since Pleistocene times. Bears have
teeth specialized for eating plant sub¬
stances as well as meat. The molar
teeth are elongated and the enamel of
the crown is wrinkled. Bears are widely
distributed, but are not found in Africa
or Australia.

Sea lions, walruses, and seals are
water-living carnivores. Their bodies
are streamlined for swimming, but they
have never developed the finlike ap¬
pendages of whales and dolphins. They
have webbed feet, the front ones serv¬

ing for balance and stability, and the
rear ones turned in such a way as to
provide an efficient means of propul¬
sion. Walruses probably evolved from
sea lion ancestors. Their canine teeth
have become long tusks, and they have
developed broad molars to facilitate
crushing and grinding the oysters and
other mollusks on which they feed.

The hoofed mammals. Man has lived
in close association with the hoofed
mammals, or ungulates (un- gva-layts),
since prehistoric times. The goat was
probably the first to be domesticated.
For ages man has depended on the
horse, camel, ox, llama, and other
hoofed mammals as beasts of burden.
The cow, pig, and sheep are our prin¬
cipal food animals. Deer, elk, caribou,
moose, and antelope are our most im¬
portant big game ungulates.

38-9 The giraffe, a cud-chewing ungulate,
is the tallest living four-footed animal.
Its elongated limbs are well adapted for
swift running. (Boker from National Audu¬
bon Society)

532 UNIT 6 BIOLOGY OF THE VERTEBRATES

The teeth of these animals have
become adapted to cropping and grind¬
ing grasses, leaves, and stems, as they
are all herbivores, or plant eaters. Some
of the hoofed mammals have elongated
limbs and feet that enable them to
cover hard ground rapidly. They fre-
quentlv walk on the tips of their toes,
with the wrist and ankle off the ground.
Hoofs, which are modified toenails, help
to absorb the shock of running.

About sixteen families of ungulates
evolved, probably from the insectivores,
in the Cenozoic Era. Biologists classify
the hoofed mammals as to whether they
have an even number of toes or an odd
number. Of the odd-toed ungulates,
only the horses continue to flourish, al¬
though the tapir and rhinoceros are also
odd-toed. The even-toed forms have be¬
come very numerous and are probably
at their peak today. These include the
pig and hippopotamus, and the cud-
chewers, or ruminants (roo-mi-nants) .
The cow, ox, bison, sheep, goat, ante¬
lope, camel, llama, giraffe, deer, elk, cari¬
bou, and moose are all ruminants.
These animals have four-chambered
stomachs. While grazing, they eat large
quantities of food, which passes into a
large paunch, or rumen, the first of the
stomach divisions. Here food is stored
for later chewing. Later the food is
forced back into the mouth for leisurely
chewing as a cud. After thorough chew¬
ing, the cud is swallowed into the second
stomach, where digestion begins. From
there it passes through the other stom¬
ach regions to the intestine.

The erect mammals. Superior brain de¬
velopment places the primates , or erect
mammals, at the top of all groups of
living organisms. Primates have well-
developed arms and hands. Their fin¬
gers are used for grasping, and one or
more fingers or toes are equipped with

nails. Most primates can walk erect if
necessary. Primates live in South
America, Africa, or the warm regions of
Asia. They tend to be tree-living, with
man the only one living exclusively on
the ground. The teeth of the primates
are less specialized than in any group
of mammals. They feed on plants and
flesh. Eyesight is well developed in the
primates, but the sense of smell is
poorly developed.

The apes are most like man in body
structure. They have no tails. The
arms are longer than the legs. When
walking, the feet tend to turn in. These
primates, especially the chimpanzee, re¬
spond to a very high degree of training.
They include the following:

1. The gorilla, the largest of the apes,
lives in Africa. It walks on two
feet and is one of the most powerful
animals.

2. The chimpanzee also lives in Africa.
It is smaller than the gorilla and
quite intelligent.

3. The orangutan lives in the East In¬
dies. It is a droll animal with red
hair.

4. The gibbon is a long-armed type
found in Asia.

5. The Old World monkeys have long
tails but do not use them in climb¬
ing. They sit erect. Food is stored
in cheek pouches. The baboon, an
Old World monkey, has a long, dog¬
like nose.

6. New World monkeys have flat, long
tails used for grasping. The septum
between the nostrils is wide, and
these monkeys lack cheek pouches.

7. Marmosets are small primates rang¬
ing from Central America to South
America. They resemble squirrels in
appearance and activity'.

8. Lemurs are primates found in Mada¬
gascar.

CHAPTER 38 THE MAMMALS 533

38-10 Primates are interesting mammals
because of their ability to grasp objects and
to walk on two feet if necessary. (New York
Zoological Society)

Diversity among the mammals. In your
survey of mammalian orders, you un¬
doubtedly noticed the diverse lines
along which mammals have developed.
Many adaptations have occurred in the
limbs, especially the forelimbs. We
found limbs for digging, hanging, fly¬
ing, running, defense, and capturing
prey. These limb modifications have
equipped mammals for life in nearly
all the environments of the earth. Their
numbers compete successfully with the
fish in aquatic environments, amphib¬
ians and reptiles on land, and with the
birds in the treetops.

Variations in tooth structure are
nearly as pronounced as those of limbs.
Well-developed incisors serve as chisels
in the mouths of rodents. Greatly en¬
larged canine teeth form the flesh-tear¬

ing fangs of cats and dogs and other
carnivores. Large molars provide the
grinding surfaces for grazing animals
like the deer, the cow, and the horse.
These modifications and others, includ¬
ing antlers and horns, and protective
coloration provided by pigmented hair
which blends with the environment, are
of great importance in the distribution
of mammals. However, to find the rea¬
son for the supremacy of mammals, we
need to take a closer look at the mam¬
mal’s internal development.

Regulations of the internal environ¬
ment. In the body of the mammal we
find tissue specialization and organ de¬
velopment at the most complex level
in all of life. With this specialization,
however, comes increasing cell depend¬
ence. Mammals could never have
reached their high level of development
without a controlled internal environ¬
ment. We refer to mammals as well
as birds as warm-blooded vertebrates.
Except during periods of hibernation
or inactivity the body temperature re¬
mains at a nearly constant level. The
maintenance of this uniform internal
temperature requires the combined ac¬
tivity of many highly developed organs
and organ systems. We might begin
with the release of heat during respira¬
tion in the tissues. Here we find the
rate of cell metabolism elevated far
above that of the cold-blooded verte¬
brates whose body temperature fluctu¬
ates with that of the environment. In¬
crease in the metabolic rate requires an
increase in the supply of nutrients and
oxygen to the cells. Highly efficient
digestive organs and well-developed
lungs are necessary to supply these
needs. The increased rate of metab¬
olism results in more metabolic wastes
which must be removed from the cells
constantly. Supply and waste removal

534 UNIT 6 BIOLOGY OF THE VERTEBRATES

Atrium

Incomplete septum

Ventricle

FROG

REPTILE

Ventricles

Ventricle

Septum

Atrium

Ventricle

Septum

38-11 Comparative heart structure of the vertebrate classes. Notice the de¬
velopment from two to four chambers.

for billions of cells composing the body
of a mammal require an extensive trans¬
port system and a powerful heart to
force blood through its many miles of
vessels. Kidneys which surpass those
of all other animals in functional effi¬
ciency remove the waste products of
cell metabolism in a complicated sys¬
tem of blood filters.

We find the basis for mammalian
structure and function in the lower
vertebrates you have studied. In fact,
we can trace the development of each
organ system through stages represented
in these lower forms.

Development of heart and lungs of a
mammal. In manv wavs the efficiencv

J J J

of the transport system is the key to the
development of warm-blooded animals

and regulation of the internal environ¬
ment. However, in considering the
heart and circulatory system of the
mammal we must also include the or¬
gans of respiration, for these two sys¬
tems are closely related. Let us think
back to the fish. Here a relatively small
heart composed of two chambers is suf¬
ficient to force blood through a circula¬
tory system consisting of a single circuit.
One ventricle provides sufficient pres¬
sure to force blood through the gills and
to the vessels of the body without re¬
turning to the heart between the two
circulations. Gills are relatively simple
organs of respiration which function
efficiently in a water environment.
Since the metabolic rate of a fish is
much below that of the higher verte-

CHAPTER 38 THE MAMMALS 535

brates and varies with the water tem¬
perature, a two-chambered heart is
adequate. However, neither gills nor
two-chambered hearts are suitable for
life on land.

In the amphibians, especially in the
adult stages of frogs, toads, and sala¬
manders, the heart has undergone a
structural change which permits life in
a land environment. Here we find a
separation of the upper heart chambers
into two atria. The presence of these
two chambers provides separation of
deoxygenated blood from the body from
oxygenated blood from the lungs.
However, mixing occurs in the single
ventricle which forces blood to the
body tissues as well as to the respira¬
tory organs. Such a heart could not
supply the requirements of a bird or
a mammal. For one thing, the pump¬
ing of mixed blood to the body tissue
could not supply the oxygen require¬
ment of the mammal. Furthermore,
the lungs of amphibians are not ade¬
quate to supply the needs of a bird or a
mammal. Remember that amphibians
still require skin respiration. This func¬
tion of the skin has been lost in the bird
and mammal with increase in thick¬
ness and development of the body cov¬
ering.

The reptile heart is still more effi¬
cient than that of the amphibian be¬
cause the ventricle is partially divided
by a partition, or septum. The mam¬
malian heart, like that of a bird, elimi¬
nates all mixing of blood because of a
complete septum forming two separate
ventricles. Thus it is really a double
pump. The right side receives the de¬
oxygenated blood from the body and
pumps it to the lungs, and the left side
receives oxvgenated blood from the
lungs and forces it throughout the en¬
tire body.

Development of the nervous system.
The high degree of development and
specialization of the nervous system is
the primary factor that has placed the
mammals above all other forms of life.
The brain of the mammal is larger, in
proportion to the body weight, than
that of any other animal. This high
development of the mammalian nervous
system could have occurred only in a
closely regulated internal environment.
Of all the body tissues, nerve tissue is
probably the most dependent and the
most rapidly affected by internal
changes.

We can learn a great deal about
the behavior and mental abilities of the
vertebrates by comparing their brain
structure. The brain of the most primi¬
tive vertebrates consists of three main
regions: a forebrain composed of the
olfactory lobes, or bulbs, and the two
hemispheres of the cerebrum; the mid¬
brain containing the optic lobes; and
the hindbrain composed of the cere¬
bellum and medulla. All vertebrates
have these brain regions, but their rela¬
tive size varies (Fig. 38-12). When we
compare the brain structure of the vari¬
ous vertebrates, we see an evolutionary
pattern in which the relative size of the
cerebrum increases and reaches its high¬
est development in the mammal. You
will recall from your studies of the brain
of the fish that the cerebral hemispheres
are relatively small. Large olfactory
lobes extend from the anterior region
of the cerebrum, indicating that the
sense of smell is well developed in the
fish. The largest brain regions in the
fish are the optic lobes. In the am¬
phibian the cerebral hemispheres are
proportionally larger. This increase in
the relative size of the cerebrum con¬
tinues in the reptiles and also in the
birds.

536 UNIT 6 BIOLOGY OF THE VERTEBRATES

FISH

Olfactory lobe
Cerebrum
Optic lobe
Cerebellum
Medulla
Spinal cord

FROG

Olfactory lobe
Cerebrum
Optic lobe
Cerebellum

Medulla
Spinal cord

REPTILE

BIRD

Olfactory lobe
Cerebrum -
Optic lobe
Cerebellum •

MAMMAL

Medulla

Spinal cord

38-12 Note the size of the cerebrum in the mammalian brain as compared with
that of the fish, frog, reptile, and bird.

It is interesting that various verte¬
brates differ also in the relative sizes of
the cerebellum. This brain area func¬
tions in muscle coordination. The cere¬
bellum is relatively large in most fish
but is greatlv reduced in amphibians.
In the birds and mammals it is large
and well developed.

In the mammalian brain the cere¬
bral hemisphere fills most of the cranial
cavity and is spread over the other brain
regions. The olfactory area consists of
two lobes extending from the anterior
regions on the lower side of the cere¬
brum. The optic region lies in the pos¬
terior region of the cerebral hemisphere.
The cerebellum lies below and posterior
to the cerebrum. The high develop¬

ment of this brain region accounts for
the excellent muscle coordination of
the mammal. The short medulla ob¬
longata, which controls vital body proc¬
esses, lies below the cerebrum and ex¬
tends to the spinal cord. Cranial nerves
extend from the brain to the sense or¬
gans and other head structures. The
spinal nerves extend from the spinal
cord to and from all regions of the
body.

Mammalian reproduction. Another line
of development that has occurred in the
vertebrates has been an increase in the
efficiency of reproduction and a corre¬
sponding decrease in the number of
young. Let us return to a consideration
of the fish, where species are preserved

CHAPTER 38 THE MAMMALS 537

only because of the enormous number
of eggs produced. Depending on the
species, a fish may lay from a few hun¬
dred to several million eggs. You will
recall that fertilization is external. The
male fish swims over the eggs and dis¬
charges milt containing sperm. Many
eggs are never fertilized. Many are
eaten before they hatch. In most fishes
the young are given little or no parental
care when they are hatched. As a re¬
sult, most of the young fall victim to
predators before reaching maturity.

Amphibians lay smaller numbers of
eggs which are fertilized directly as they
are laid. This insures the develop¬
ment of a much greater proportion of
the eggs laid. As in the fish, how¬
ever, amphibians must deposit eggs in
water, since they have no protection
against drying out

An abrupt change occurs in the
reptiles, the first vertebrates to lay eggs
on land. This advance requires internal
fertilization and the enclosure of the
egg with protective shells. Similar eggs
are produced by birds. With the great¬
er protection supplied by the eggs of
reptiles and birds, the number is greatly
reduced. Parental care, especially
among the birds, reduces the mortality
rate of offspring greatly.

In all mammals but the mono-
tremes, development is internal. As
you learned in your study of marsupials,

the young are born prematurely. How¬
ever, the pouch on the underside of the
female provides a protective “incubator"
where development may continue. The
mammary glands open into the brood
pouch and provide the young with
nourishment.

In the placental mammals, follow¬
ing internal fertilization, the develop¬
ing embryo attaches and embeds in the
wall of the uterus. Specialized tissues
composing the placenta exchange nu¬
trients, oxygen, and waste products be¬
tween the embryo and the mother.
Thus, the developing placental mammal
benefits from the functions of the or¬
gans of the female. As a result, the
mammal may develop longer and reach
a larger size before birth occurs.

The period between fertilization
and birth is called the gestation period .
The following table of gestation periods
shows the variation in intervals between
fertilization of the egg and birth among
various mammals.

Parental care. The fact that a young
mammal is nearly helpless and depend¬
ent on its mother’s milk for a time after
birth makes parental care necessary.
Male mammals are not usually involved
in the care of the young. Parental care
requires that the number of young be
smaller among mammals. This num¬
ber is usually proportionate to the
length of parental care. Mice, rabbits,

GESTATION PERIODS

Mammal

Period

Mammal

Period

Opossum

13 days

Human

40 weeks

Mouse

21 davs

Cow

4 1 weeks

Rabbit

30 davs

Horse

48 weeks

Cat

63 davs

Whale

20 months

Dog

63 days

Elephant

20 to 22 months

Pig

120 days

538 UNIT 6 BIOLOGY OF THE VERTEBRATES

and other small mammals bear several
litters, often numbering ten or more
each season. However, there are many
predators that feed on these mammals
and their chance of survival is not as
great as that of larger mammals. Larger
mammals produce smaller numbers of
young and provide parental care over a
longer period.

Mammals vary greatly in their de¬
gree of development at birth. Mice
and rats are born hairless and blind. A
baby bear weighs less than a pound when
it is born during winter hibernation.

IN CONCLUSION

On the other hand, a cow, horse, deer,
or bison is born in a much more ad¬
vanced condition and can stand and
walk with the mother a few hours after
birth. The young whale is born in a
well-developed condition after a long
gestation period. The newborn por¬
poise is half as long as its mother.

Of all the mammals, the human is
the most helpless at birth and requires
the greatest parental care for the longest
period of time. Much of this time is de¬
voted to training in which the child is
learning to walk, talk, think, and reason.

With the mammals, we have reached the peak of vertebrate development.
With their diverse adaptations, regulated internal environment, highly spe¬
cialized .internal organs, and high level behavior made possible by a superior
brain, mammals have evolved as the most successful land animals. Since life
on the earth began, organisms at various stations of development have had
their period of prominence. We speak of an age of invertebrates, an age of
fishes, an age of amphibians, and an age of reptiles. The age in which we live
might be called the age of mammals.

The next unit will present a more thorough study of mammalian anatomy,
physiology, and behavior. The subject will probably be the most interesting
of all to you — the human body.

BIOLOGICALLY SPEAKING

primate
proboscidean
rodent
sirenian
ungulate

QUESTIONS FOR REVIEW

carnivore

cetacean

chiropteran

edentate

gestation period

insectivore

lagomorph
mammary gland
marsupial
monotreme
placenta

placental mammal

1. List several early mammals that are now extinct.

2. According to biologists, which North American mammal is most ancient?

3. List ten characteristics of mammals that distinguish them from other
vertebrates.

4. In what wav are the duckbilled platypus and spiny anteater different from
all other mammals?

CHAPTER 38 THE MAMMALS 539

5. What characteristics of marsupials distinguish them from placental mam¬
mals?

6. Lagomorphs are similar to rodents. How may they be distinguished on
the basis of tooth structure?

7. In what respect is the armadillo an exception to the rule in the body cov¬
ering of mammals?

8. Describe adaptations of the cetaceans for life in the sea.

9. Name two elephants that have become extinct since glacial times.

10. List several North American carnivores.

11. Describe various adaptations of the hoofed mammals.

12. On what basis might we consider the primates the most highly developed
mammals?

13. What is the biological meaning of “warm-blooded?”

14. What other class of vertebrates is most like the mammals in heart struc¬
ture?

15. Which region is most highly developed in the mammalian brain?

16. Why must the young of marsupials spend the early part of their lives in
a brood pouch?

17. In what way is the number of young related to parental care?

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the relation of the regulated internal environment to the biological
development of the mammal.

2. Compare the structure and functional efficiency of the mammalian heart
with the hearts of other vertebrates.

3. Account for the high level of behavior of the mammal in terms of brain
structure and development.

4. What characteristics of mammalian reproduction give these animals an

advantage over other vertebrates?

RELATED READING

Books

Austin, Oliver. Birds of the World.

Golden Press, New York. 1961
Barker, Will. Familiar Animals of
America. Harper and Row, New
York. 1956

Bates, Marston. Animal Worlds. Ran¬
dom House, Inc., New York. 1964

Breland, Osmond P. Animal Life and
Lore. Harper and Row, New York.
1964

Carr, Archie. The Reptiles. Time,
Inc., New York. 1964
Carrington, Richard and Editors of Life.
The Mammals. Time, Inc., New
York. 1964

540 UNIT 6 BIOLOGY OF THE VERTEBRATES

Crompton, John. Snake Lore. Double¬
day and Co., Inc., Garden City,
N. Y. 1964

Detheir, V. and Stellar, E. Animal Be¬
havior. Prentice-Hall, Inc., Engle¬
wood Cliffs, N. J. 1961
Ditmars, Raymond L. The Reptiles of
North America. Doubleday and
Co., Garden City, N. Y. 1936
Earle, Olive L. Birds and Their Nests.
Wm. Morrow and Co., Inc., New
York. 1952

Eaton, Theodore. Comparative Anat¬
omy of the Vertebrates. Harper
and Row, New York. 1960
Guyer, Michael F. and Lane, Charles E.
Animal Biology. Harper and Row,
New York. 1964

Halstead, Bruce W. Dangerous Ma¬
rine Animals. Cornell Maritime
Press, Cambridge, Md. 1959
Hylander, Clarence J. Fishes and Their
Ways. The Macmillan Co., New
York. 1964

Kohn, Bernice. Marvellous Mammals:
Monotremes and Marsupials.
Prentice-Hall, Inc., Englewood
Cliffs, N. J. 1964

La Gorce, John Oliver, Ed. The Book
of Fishes, Rev. Ed. National Geo¬
graphic Society, Washington. 1952
Lanyon, Wesley E. Biology of Birds.
The Natural History Press, Double¬
day and Co., Garden City, N. Y.
1964

Life Nature Library. The Fishes.

Tinye, Inc., New York. 1963
Mayr, Ernst. Animals Species and
Evolution. Harvard University
Press, Cambridge, Mass. 1964
National Geographic Society. Wild
Animals of North America. The
Society, Washington, D. C. 1960
Norman, J. R. A History of Fishes,
5th Ed. Hill and Wang, New
York. 1960

Parker, H. W. Snakes. W. W. Nor¬
ton and Co., Inc., New York. 1964
Peterson, Roger Tory and Editors of
Life. The Birds. Time-Life
Books, Chicago. 1964
Pope, Clifford H. The Giant Snakes.
Alfred A. Knopf, Inc., New York.

1961

Raskin, Edith. Watchers, Pursuers and
Masqueraders: Animals and Their
Vision. McGraw-Hill Book Co.,
Inc., New York. 1964
Romer, Alfred S. The Vertebrate
Story, 4th Ed. University of Chi¬
cago Press, Chicago. 1959
Scheele, William E. Prehistoric Ani¬
mals. The World Publishing Co.,
Cleveland. 1954

Smyth, H. Amphibians and Their
Ways. The Macmillan Co., Chi¬
cago. 1962

Articles

Ayotte, A. E. “The Last Stand of the
Alligators.” Audubon Magazine.
July-August, 1964

Cone, Clarence, D., Jr. “The Soaring
Flight of Birds.” Scientific Ameri¬
can. April, 1962

Frieden, Earl. “The Chemistry of Am-
phibian Metamorphosis.” Scien¬
tific American. November, 1963.
Gilbert, Perry W. “The Behavior of
Sharks.” Scientific American.
July, 1962

Heald, Weldon F. “Snakes Are Inter¬
esting.” Audubon Magazine. July-
August, 1964

Kortlant, Adriaan. “Chimpanzees in
the Wild.” Scientific American.
May, 1962

Muntz, W. R. A. “Vision in Frogs.”

Scientific American. May, 1962
Shaw, Evelyn. “The Schooling of
Fishes.” Scientific American. June,

1962

UNIT SEVEN

THE BIOLOGY
OF MAN

Having followed the development of organs and organ systems through increasing
levels of advancement and efficiency in the vertebrate classes, it is fitting that oui
study of anatomy, physiology, and body chemistry concern the mammal. What
better example to use than the human body? Man, the most advanced living or¬
ganism, dominates the living world. Intelligence, ingenuity, and creative ability
have made him the master of every environment on the earth and in space beyond.

CHAPTER 39

THE

HISTORY
OF MAN

Man and mammals. The organs and
systems of the human body closely re¬
semble those of other mammals. The
structural similarity of man and the oth¬
er primates is especially striking. Al¬
most every anatomical detail is similar.
You can observe this similarity in the
skeleton and muscles, tooth structure,
position and structure of the eyes, form
of the hands and feet, and even in the
facial expressions. Similarities in the
internal structures of man and the other
primates are equally striking. They
may be seen in the structure of the heart
and blood vessels, lungs, digestive or¬
gans, excretory organs, glands, and
nearly all other internal organs.

Even the chemical secretions of
man and other mammals are similar,
and the digestive enzymes are the same.
The insulin used to save the lives of dia¬

betics is extracted from beef or hog
pancreas. The Rh factor, so important
in matching human blood types, was
discovered originally in the blood of the
Rhesus monkey.

Although man is structurally and
biochemically similar to the other mam¬
mals, he has significant differences that
have allowed him to become biologically
superior. He is the only mammal with
truly upright posture. This is possible
because he has a shortened and flat¬
tened pelvis which holds the abdominal
organs. The upright posture freed
man’s hands so that they could be used
entirely for manipulation. At the same
time the action of man’s thumb op¬
poses that of the fingers, so that his
hand is ideally suited for grasping. Be¬
cause of their many sensory receptors,
the hands are highly sensitive to touch
and are grooved so that they can grip
smooth objects.

Both of man’s eyes see the same vi¬
sion from slightly different angles, re¬
sulting in an improved sense of depth.
The combination of intelligence, acute
vision, and efficient hands is ideal for
the use of tools, an ability peculiar to
man. The characteristic that has prob¬
ably made the most difference in man’s
development, however, is his capacity to
use symbols in the form of the written
and spoken word. This ability to com¬
municate with others of the same spe¬
cies by symbols and to pass learning on
from generation to generation has al¬
lowed man to undergo a rapid cultural
evolution not possible in other species.
Theories about man’s development. In
1871 the English biologist Charles Dar¬
win published his famous book entitled
The Descent of Man. Darwin pro¬
posed that the same forces operating to
bring about changes in plants and ani¬
mals could also affect man and his de-

542

CHAPTER 39 THE HISTORY OF MAN 543

velopment. Biologists classify man as a
primate because of his many structural
similarities to the monkey, gorilla, chim¬
panzee, orangutan, and gibbon. In his
book, Darwin pointed out these simi¬
larities, and some people interpreted his
comparisons as suggesting that man
evolved from monkeys. Actually this
unfortunate misinterpretation was far
from Darwin’s intent. As vou know, it
is the less specialized primitive forms
that are most able to move into new en¬
vironments and evolve into new species.
The orangutans and chimpanzees of to¬
day are highly specialized forms that
have probably evolved from more prim¬
itive ancestors. Likewise, modern man
has probably evolved from primitive,
more generalized ancestors. If we were
able to trace the history of the primates
back perhaps ten million years, we
might find a generalized primate that
was common ancestor to both modern
man and the modern primates. It is
most likely, however, that the two lines
of descent separated at a very early date
and gradually evolved into the forms of
today.

At the time Darwin lived, no re¬
mains of early man had been found.
Since then hundreds of bones have been
unearthed. They have been dated and
studied by anthropologists , who special¬
ize in the history of man. This evi¬
dence still does not provide us with a
complete picture of man’s development,
but the fossils serve as clues from which
hypotheses can be formed. A fossil skull
indicates the size of the brain, shape of
the head, and age of the man at the
time of death. The jaw structure indi¬
cates whether its owner had the capacity
for speech. It would not mean, how¬
ever, that a language was used.

A study of the environment of
primitive man also helps to piece to¬

gether man’s development. What did
early men eat? Wherever the anthro¬
pologist finds evidences of primitive
man, he searches for fossil remains of
animals that might have been used for
food. Charred bones indicate that he
had learned to use fire to cook meat.
The type of food also indicates the cli¬
mate and nature of the surroundings.
The anthropologist is also interested in
evidence of tools, since their use is
unique to man. If similar tools are
found over a wide area, some type of
communication must have occurred.
The material used and the complexity
of the tools are indications of how
highly developed the men were.

How fossils are dated. In Chapter 13
we discussed the dating of fossils by the
determination of the relative ages of the
layers of the earth. The radiocarbon
method is also used to date fossils. This
method was made possible by the dis¬
covery in 1930 of a radioactive isotope of
carbon with an atomic weight of 14 in¬
stead of 12. In living organisms there
is about one carbon-14 atom to one tril¬
lion carbon-12 atoms.

Radioactive isotopes spontaneously
give off radiations at a constant rate,
changing eventually to the stable form
of the element. The time required for
half of a sample of a radioactive isotope
to decay to the stable form is called the
half-life. The half-life of carbon-14 is
5,568 years.

As long as an organism is living it
incorporates carbon compounds into its
body, but at death this process ceases.
Thus, if a sample of bone is examined
and found to contain half the amount
of carbon-14 that occurs in organisms
today, its age would be estimated at
5,568 years. The radiocarbon method
is considered to be accurate to about
50,000 years.

544 UNIT 7 THE BIOLOGY OF MAN

Recently another method of dating
has been developed. Similar to the car¬
bon-14 technique, the potassium-argon
method depends on the fact that potas¬
sium-40 breaks down into calcium-40
and argon-40 at a slow but constant
rate. This method has its shortcom¬
ings because it involves the presence of
argon-40 in a rock sample. In dating a
fossil by this method, a rock that was
formed at the same time the organism
was alive has to be found.

Putting together the pieces. In 1924 a
perceptive worker in a South African
quarry noticed a small fossilized skull
that had been dislodged by blasting.
When the fossil was given to a South
African medical school, Professor Ray¬
mond Dart, an anatomist, recognized its
resemblance to human skulls. Dr. Dart
named the primate Australopithecus
(au-stmy-lo-pith-i-kus) africanus , which
means “southern ape from Africa.” Since
then other bones of this primate have
been found.

Dr. and Mrs. L. S. B. Leakey have

39-1 Dr. L. S. B. Leakey, the eminent British
anthropologist who unearthed the remains of
the earth’s earliest known manlike creature
in East Africa, examines the skull of his
1,750,000-year-old discovery. (© National Geo¬
graphic Society)

spent nearly 35 years excavating and
studying fossils. Much of their effort
has been near the Olduvai Gorge in Tan¬
ganyika, now Tanzania. They first found
many pebble tools thought to have been
used for cutting instruments. In 1959
the Leakeys unearthed some teeth, which
led to the excavation of an area believed
to be a campsite of ancient man. Since
then, skull fragments have been pieced
together. Bones from a foot, fingers,
and a lower jaw with well preserved teeth
have provided us with clues about the
structure of the primate that was named
Zinjanthropus (zin-/cm-thro-pus) . By
using the potassium-argon method, sci¬
entists at the University of California
have placed its age at 1,750,000 years.

The bones of Zinjanthropus and
Australopithecus are similar and look
like those of modern apes. The point¬
ed fanglike teeth of modern apes are
not found in these forms, however.
Also, the shape of the pelvis and open¬
ing for the spinal cord in the skull indi¬
cate an upright posture. Anthropolo¬
gists are not agreed as to whether these
forms should be placed in the ape fam¬
ily or in the family that includes man.

In mid-1964 Dr. Leakey published
an account of the discovery of the re¬
mains of still another primitive manlike
form in the Olduvai Gorge. He has
named this form Homo habilis , and
dating evidence indicates that it lived at
the same time as Zinjanthropus. Homo
habilis , however, is more manlike. He
seems to have walked and run erect, to
have had a well opposed thumb, and to
have eaten both meat and plant foods,
like modern man. Dr. Leakey now be¬
lieves that Homo habilis was the ances¬
tor of modern man, while Zinjanthro¬
pus and Australophithecus were evolu¬
tionary dead ends. Dr. Leakey’s the¬
ory indicates that the genus Homo was

CHAPTER 39 THE HISTORY OF MAN 545

present on earth about 1,200,000 years
earlier than anthropologists had previ¬
ously thought.

Early forms of man. In 1891 a part of a
skull, a piece of a jaw, and an upper leg
bone were discovered in an excavation
on the island of Java. These remains
lay in a deposit of sand and gravel trans¬
ported by an early glacier, so that the
period could be roughly dated. Similar
remains were found in the same general
region in 1937. Java man, as he was first
called, is believed to have walked erect,
so that he is now called Pithecanthropus
erectus. He is thought to have lived
500,000 years ago. He had a slanting
forehead and heavy brow ridges. The
skull of Java man indicates that his
brain, while only about half the size of
modern man’s, was more than one third
larger than that of the present-day goril¬
la. Anthropologists believe that Java
man learned to make use of crude stone
weapons and of fire. Pithecanthropus
remains have also been discovered in
excavations of ancient caves near Pe¬
king. Many of the skulls of Peking man
that were found are broken near the
base, suggesting he was a head hunter.
Neanderthal man. More is known
about Neanderthal (nee-dn-der-thol)
man , who lived in Europe, Asia Minor,
Siberia, and Northern Africa. Scien¬
tists believe that Neanderthal man dis¬
appeared about 25,000 years ago, near
the close of the glacial age. Informa¬
tion about him has been acquired from
careful study of almost 100 skeletons,
many of them nearly complete. He was
about five feet tall and walked in a
stooped position. His bone structure
indicates that he was powerfully built.
His facial features were coarse. Like
Pithecanthropus, his forehead sloped
backward from heavy brow ridges. His
mouth was large, and he had little chin.

39-2 Above.- a drawing showing how Zinjan-
thropus may have looked. Below: the loca¬
tion in Africa where Zinjanthropus was
found.

Neanderthal man lived in caves
from which he journeyed on hunting
expeditions in search of the hairy mam¬
moth, saber-toothed tiger, and woolly
rhinoceros. His brain was as large or
larger than that of modern man. Nean¬
derthal man used stone tools and weap¬
ons, made use of fire, buried his dead,
and lived in a family group.

546 UNIT 7 THE BIOLOGY OF MAN

Cro-Magnon man. Anthropologists
place Cro-Magnon man in the same spe¬
cies as modern man, Homo sapiens ( hoh -
moh sayp-ee-e nz). He lived in Eu¬
rope, especially in France and Spain,
about 50,000 years ago. He had a high
forehead and a well-developed chin, and
lacked the heavy brow ridges of most of
the more primitive men. Several caves
along the southern coast of France have
yielded Cro-Magnon weapons of stone
and bone as well as skeletons. The
walls of these caves bear beautiful draw¬
ings of animals of the region. These
drawings have been valuable in dat¬
ing the period. Anthropologists believe
that Cro-Magnon man, living at the
same time as Neanderthal man and be¬

ing superior to him in intelligence, may
have exterminated him. There is also
evidence that Neanderthal man mixed
with Cro-Magnon man and in time lost
his identity.

Modern man. All people living today
belong to the species Homo sapiens.
Anthropologists sometimes divide this
single species into three or four racial
groups, according to features that are
common within each group. They rec¬
ognize, however, that there is great vari¬
ation within a racial group, and that all
men are more alike than they are dif¬
ferent.

The Mongoloid type is represented
by people from most of Asia, the East
Indies, and the Philippines. This type

39-3 This chart represents Dr. Leakey’s current theory of man’s ancestry.

CHAPTER 39 THE HISTORY OF MAN 547

39-4 At the upper left is a gorilla skull and
at the right is the skull of Zinjanthropus.
Below them is the skull of modern man.
How do these three differ? What does the
structure of the teeth tell you about the pos¬
sible method of defense of Zinjanthropus as
compared to the gorilla? (© National Geo¬
graphic Society, left; American Museum of
Natural History, right and bottom)

is well represented in the Western Hem¬
isphere bv the Eskimos, and by the In¬
dians of North America as well as Cen¬
tral and South America. Anthropolo¬
gists believe that the Mongoloid people
came to North America from Asia in
very early migrations through Alaska.

Natives of North Africa, South
Africa, East Africa, and the Congo pig¬
mies comprise the Negroid type. Ne¬
groid stock is also found in New Guinea,
the Philippines, and islands in the region
of Australia.

People of the Caucasoid type are
largely from Europe, Southwestern Asia,
and North Africa. Caucasoid people
vary greatly. They include the Teuton¬
ic types of Northern Europe and Ice¬
land as well as the southern Europeans,
Slavs, Hindus, Gypsies, Arabs, Jews,
Egyptians, and Ethiopians.

A fourth type of modern man is rep¬
resented by the Australoid type. These
people do not fit any of the three major
types. This small group includes the
aborigines (original people) of Austral-

548 UNIT 7 THE BIOLOGY OF MAN

39-5 Various types of prehistoric man as compared to modern man. Top left:

Australopithecus; top right: Pithecanthropus; bottom left: Homo neander-
thalensis; bottom right: Homo sapiens.

V V,
4N

CHAPTER 39 THE HISTORY OF MAN 549

ia and Ceylon. These primitive peo¬
ple still maintain a life typical of the
early Stone Age. The sloping forehead
and prominent brow ridge characteristic
of the Australoids suggest a relationship

to ancient man. However, the hand
structure and body form are definitely
like those of modern man. Anthropol¬
ogists still have much to learn about
these interesting people.

IN CONCLUSION

Intelligence combined with highly efficient hands and the ability to commu¬
nicate separate man from the other mammals. It is believed by many an¬
thropologists that, although man evolved along separate lines from the primates,
the two forms may have had a common, generalized ancestor in the remote
past.

An anthropologist is truly a detective. Although fossils are few and scat¬
tered, they provide valuable information regarding life in the past. By dating
the fossils and comparing the structure of bones, the anthropologist is able to
form hypotheses regarding primitive man, the food he ate, the tools he used,
and the environment in which he lived. The comparison of manlike types does
not mean, however, that one developed from another. There is no conclusive
evidence to link the forms that have been found, dated, described, and com¬
pared. Modern man may have developed from forms that have not yet been
located.

The next chapter will describe the structure of man as he exists today. It
will be of vital concern because it is about you.

BIOLOGICALLY SPEAKING

anthropologist
Australoid
Australopithecus
africanus
Caucasoid
Cro-Magnon man

half-life
Homo habilis
Homo sapiens
Mongoloid
Neanderthal man
Negroid

Pithecanthropus

erectus

potassium-argon

method

radiocarbon method
Zinjanthropus

QUESTIONS FOR REVIEW

1. What characteristics separate man from other animals?

2. What did Charles Darwin believe about man’s development, and what was
the misunderstanding about his theory?

3. Why is the finding of an old skull important?

4. What can be learned from a study of the lower jaw and teeth?

5. How can we determine what type of environment occurred in a given area
thousands of years ago?

550 UNIT 7 THE BIOLOGY OF MAN

6. How can we determine the diet of primitive man?

7. Describe two methods of dating fossils.

8. Describe Australopithecus.

9. Describe Pithecanthropus erectus.

10. Name the racial groups found in the world today, and give their place of
origin.

APPLYING PRINCIPLES AND CONCEPTS

1. Do you think that tropical forests would provide good fossils? Explain
your answer.

2. In what ways can the intelligence of primitive man be judged from fossil
evidence?

3. Explain the theory that reduction of the canine teeth is related to tool
making.

4. Compare the skull of modern man with those of Java, Neanderthal, and
Cro-Magnon men.

5. Did Cro-Magnon man have any methods of communication? Explain.

6. How could agriculture have developed from a settled life?

CHAPTER UO

THE BODY
FRAMEWORK

Tissues of the human body. No topic
is more important to you than your own
body. Like any other organism the hu¬
man body is made up of cells and their
products. All the cells of the body can
be placed into four groups: 1. connec¬
tive tissue, 2. muscle tissue, 3. nerve tis¬
sue, and 4. epithelial tissue (see table
on page 553). You will study muscle
tissue in this chapter and nerve tissue in
a later chapter.

Connective tissue lies between
groups of nerve and muscle cells. It
fills up spaces in the body that are not
occupied by specialized cells, and it
forms protective layers (Fig. 40-1).
Connective tissue also binds together
many softer tissues and gives them
strength and firmness. Fibrous tissues
in the walls of organs, the tendons of
muscles and ligaments binding bones, as
well as the bones themselves are all
types of connective tissue. Blood and
lymph are also connective tissue.

Epithelial (ep-i-t/iee-lee-al) tissue is
the type that covers the body surfaces,
both inside and outside. For example,
certain flat epithelial cells cover the
blood vessels and heart. Another type
of epithelium lines the stomach. Some
cells of this lining are modified to se¬
crete mucus and other stomach secre¬
tions. Still another epithelial tissue
forms the skin, and another the ciliated
lining of the trachea.

Tissues are organized into organs and
systems. Familiar examples of organs
in the human body include the arms,
legs, ears, eyes, heart, liver, and lungs.
Each of these organs is specialized to
perform a definite function or a group
of related functions involving several
different tissues. The arms, for exam¬
ple, are composed of epithelial tissue,
bone, blood, lymph, cartilage, muscle,
and nerve tissues. All of these function
together to perform such acts as grasp¬
ing, writing, and sewing.

Organs are grouped together into
ten systems, as follows:

1. Skeletal (bones and cartilage)

2. Muscular (muscles)

3. Digestive (teeth, mouth, esophagus,
stomach, intestines, liver, pancreas)

4. Respiratory (lungs, trachea, nose,
pharynx)

5. Circulatory (heart, arteries, veins,
capillaries)

6. Endocrine (ductless glands)

7. Excretory (kidneys and bladder)

8. Integumentary (skin and hair)

9. Nervous (brain, spinal cord, nerves,
eyes, ears)

10. Reproductive (testes, ovaries, uter¬
us, oviducts)

The body regions in man. The general
form of the human body is similar to
that of the other vertebrate animals. It
includes the limbs (in the form of arms
and legs), the head, neck, and trunk.

551

552 UNIT 7 THE BIOLOGY OF MAN

Connective
tissue cell

Connective
tissue fiber

Drop of fat
in fat cell

Nucleus
of fat cell

Elastic

fibers

40-1 When viewed under the microscope,
connective tissue appears like this.

40-2 This photomicrograph shows simple
columnar epithelial cells in the lining of the
small intestine. (Walter Dawn)

The head includes the cranial cavity ,
which is formed bv the bones of the
skull and safely encloses the brain. The
head also contains the sense organs,
which are located close to the brain, to
which they transmit impulses.

The thoracic cavity is formed by
the ribs, breastbone, and spine. It en¬
closes the lungs, the trachea, the heart,
and the esophagus. A dome-shaped par¬
tition, the diaphragm, separates the

thoracic cavity from the abdominal cav-

ity, which is included in the lower part
of the trunk. Inside the abdominal
cavity are the stomach, liver, pancreas,
intestines, spleen, kidneys, and in the
female, the ovaries. While the abdomi¬
nal organs lack the bony protection of
the cranial and thoracic cavities, they
are protected by the vertebral column
along the back and by layers of skin and
muscle on the front.

The body framework. In building a
model airplane the framework of the
body is usually built first. Then comes
the covering and painting, and finally the
motor, wheels, and accessories. The
strength of the entire structure depends
on the framework to which all the other
parts are fastened. Man and the other
vertebrates, like the model airplane, have
a very efficient system of support in the
form of an internal skeleton, or endo-
skeleton. You will recall that the ar¬
thropods have an exoskeleton. Man’s
bony framework gives him the greatest
support with the least amount of weight.
It also permits more efficient movement
than any other type of framework. The
animal with an internal skeleton is,
however, at one great disadvantage. It
does not have much of the protection
against injury from the outside that is
given by an external skeleton. Many
soft parts of the body are exposed. Con¬
sequently, the organism must rely on its
nervous system and sense organs to make
up for the protection the skeleton does
not provide.

The functions of the skeleton. The
functions of the bones of the bodv are
classified as follows: 1. support and form
for the body; 2. place for the attach¬
ment of muscles; and 3. protection for
delicate organs. Many of the 206 bones
composing the human skeleton have
more than one function. For example,

CHAPTER 40 THE BODY FRAMEWORK 553

TISSUES IN THE HUMAN BODY

T issue

Occurrence

Function

I. CONNECTIVE TISSUE

A. Bone

B. Cartilage

C. Dense fibrous connec¬
tive tissue

1. Regularly arranged

2. Irregularly arranged

D. Loose fibrous connec¬
tive tissue

1. Fibroelastic
(elastic — strong,
closely woven)

2. Fibroareolar (areo¬
lar — loosely
woven )

3. Reticular

4. Adipose

E. Liquid tissue

1. Blood

2 . Lymph

II. MUSCLE TISSUE

A. Smooth

B. Skeletal

C. Cardiac

III. NERVE TISSUE

IV. EPITHELIAL TISSUE

Skeleton

Outer ears, ends of long bones,
larynx, tip of nose, between
vertebrae, juncture of ribs
and breastbone, trachea

Tendons, ligaments

Membrane around bone (per¬
iosteum), one of the mem¬
branes around spinal cord
and brain (dura mater), in¬
ner layer of skin

Capsules of organs
Facial area beneath skin

Surrounding individual cells
and muscle fibers
Around organs, beneath skin

In heart and vessels (arteries
and veins)

Fluid in tissue spaces between
cells, cerebrospinal fluid

In internal organs
Attached to bones, tendons,
and other muscles
In heart

Brain, spinal cord, nerves

Composes framework and
allows for movement

Acts as cushion, lends rigid-
idv to structures that lack
bones, provides slippery sur¬
face to some joints

Joins muscles to bones or bone
to bone to aid in move¬
ment

Provides protection and carries
blood supply

Holds organ together
Acts as filler tissue

Acts as filler tissue

Cushions and insulates, stores
fat

Has essential part in: respira¬
tion, nutrition, excretion,
regulation of body tempera¬
ture, protection from dis¬
ease

Bathes the cells, has part in
nutrition and protection
from disease

J Produces either voluntary or

j involuntary movement

Carries impulses that cause
muscles to contract, carries
messages to brain to inform
individual about the en¬
vironment

1. Covering surface of body Provides protection, produces
(skin), lining nose, throat, secretions

and windpipe, lining all of
digestive tract

2. Many glands

Cranium

Clavicle
Sternum
Rib bone

Rib cartilage

Humerus
Xiphoid process

Elbow joint

Radius

Ulna

Hip joint

Wrist joint
Carpals

Femur

Knee joint

Patella

Fibula

Tibia

Ankle joint
Tarsals
Metatarsals
Phalanges

Vertebra

Lumbo-sacral

joint

Sacrum

Pelvis

Metacarpals

Phalanges

Pubic arch

40-3 Ligaments are in place on
the figure’s right side, showing the
way in which they support and
bind the bones together at the
joints. Ligaments are removed on
the left side to show the bony
structure of the joints. The
shaded contour of the body shows
the way in which the skeleton
supports the body and gives it
form, and also indicates the rela¬
tive amount of soft tissues which
overlie the bones.

Shoulder joint

Maxilla

Mandible

554

Occipital ridge

Mastoid process

First rib

Clavicle

Suture lines

Cervical vertebrae (7)

Scapula

Humerus

Ulna

Radius

Femur

40-4 In this view, the ligaments
are in place on the left side of the
figure while those on the right
side have been removed. This
view of the skeleton reveals the
sutures of the cranial bones, the
scapulae with their broad surfaces
for muscle attachment, and the
sacro-iliac joint with the liga¬
ments which bind the sacrum and
pelvic bones together. The ridges
which are visible on many of the
bone surfaces are for the attach¬
ment of muscles.

Tibia

Fibula

Thoracic
vertebrae (12)

Lumbar
vertebrae (5)

Sacro-iliac joint

Sacrum

Coccyx

555

556 UNIT 7 THE BIOLOGY OF MAN

40-5 This is an X-ray photograph of a com¬
plete fracture and separation of the lower
end of the thighbone (femur) just above the
knee joint. (Indiana University Medical
Center)

the vertebral column, the shoulder gir¬
dle, the hip girdle, the bones of the legs,
and those of the arms both support the
body and give it definite form. Some of
these bones also have muscles attached
to them, permitting the many types
of movement (Figs. 40-3 and 40-4,
pages 554 and 555). Certain delicate
organs lie under special protective bones.
Examples are the brain, which is en¬
cased by the cranial bones; the heart,
which lies under the sternum; and the
lungs, which are protected by the ribs.
The development of bone tissue. We
use the expression “drv as a bone/’ and
assume that living bone is like a dried-
out bone. Actually, living bone is far
from dry. It is moist and active and re¬

quires nourishment as does any living
organ. True, part of what we call bone
is nonliving, for bone tissue is a peculiar
combination of living cells and their
products and mineral deposits.

Among some of the lower verte¬
brates the skeleton is composed entirely
of cartilage, which lasts throughout their
lives and results in a tough, flexible
skeleton. In the early stages of the de¬
velopment of the human embryo, the
skeleton is also composed almost en¬
tirely of cartilage, with a few membranes
taking the place of bone in some regions.
After about the second month of de¬
velopment, however, certain of the car¬
tilage cells disappear and are replaced
by bone cells. These cells deposit min¬
erals in the form of calcium phosphate
and calcium carbonate in the spaces be¬
tween them. This process is called
ossification (ahs-i-fi-fozy-shun) and oc¬
curs throughout childhood. Even in
the adult, however, some cartilage is not
replaced by bone. Such permanent car¬
tilage is found in the end of the nose,
the external ear, and the walls of the
voice box and trachea.

Since ossification involves the de¬
posit of calcium compounds between
the bone cells, it results in an increase
in the strength of the bone. Naturally,
this deposition cannot occur unless the
proper minerals are present. Calcium
compounds enter the body with food
and are carried to the bone tissues by
the blood. The diet, especially in child¬
hood, is therefore an important factor
in governing mineral deposition in bone.
Milk, the natural food of all young
mammals, is the ideal source of calcium
compounds. Developing bone tissue
must assimilate the minerals after they
have been supplied by a proper diet.
Certain vitamins, especially vitamin D,
are necessary for the normal growth of

CHAPTER 40 THE BODY FRAMEWORK 557

bone. We shall study these in Chapter
41 under vitamins.

Bones grow along lines of stress.
This means that they become heaviest
and strongest where the strain is great¬
est. This fact is important in dealing
with bone fractures. If a broken bone
is protected by a cast and is unused dur¬
ing the period of repair, the fact that it
is under no stress delays healing. For
example, if a leg bone is broken the pa¬
tient is provided with a walking cast,
which puts a broken bone under limited
stress during the healing period and
speeds up the repair process. If, on the
other hand, a limb is paralyzed or made
useless, the minerals are reabsorbed by
the blood and deposited elsewhere.

The structure of a bone. If a long
bone, such as a bone from the leg or
thigh is cut lengthwise, several distinct
regions can be seen (Fig. 40-6). The
outer covering is a tough membrane

called the periosteum. This membrane
aids in nourishing the bone (because of
its rich blood supply) and in repairing
injuries; it also provides a surface to
which muscles are attached. Beneath
the periosteum is a bony layer contain¬
ing the deposits of mineral matter. This
layer varies in hardness from an ex¬
tremely hard material in the middle re¬
gion to a porous and spongy material at
the ends. The bony layer is penetrated
by numerous channels, the Haversian
canals , which form a network extending
throughout the region. These canals
carry nourishment to the living cells of
the bony layer, by means of blood ves¬
sels that connect with those of the outer
membrane.

Many bones have hollow interiors
and contain a soft tissue called marrow.
The marrow is richly supplied with
nerves and blood vessels. There are
two distinct types of marrow. The red

Ball for ball-and
- socket joint

Haversian canal

Periosteum

Haversian canal

Spongy bone

Yellow marrow

[Sjp ' Layers of
calcium
compounds
making up the
hard part
of the bone

Haversian canal

Periosteum

Marrow cavity

40-6 A cutaway view of the human femur appears to the left. The center draw¬
ing shows an enlarged section of that bone. On the right we see the Haversian
canal system as well as an enlarged view of a single bone cell.

extends and raises leg

Sartorius

flexes lower leg

Abductors

move legs apart

40-7 The muscles are intact on the fig¬
ure’s right side. On the left side, some
of the overlying muscles have been re¬
moved to show deeper muscles and
parts of the skeleton to which they
attach. The tendons attaching muscles
to bones are visible in many areas. The
sheath of the rectus muscle of the ab¬
domen has been removed on the left
side to show the division of this muscle
into segments connected by areas of
tendon. The slender cable-like tendons
in the region of the wrist contrast with
the broad, flat tendons in the region of
the knee which are designed for more
powerful muscles. Some of the larger
and more familiar muscles are labeled.
In labeling, only the main action is de¬
scribed. Many have other functions and
also work differently in groups than they
work singly.

Anterior tibial

flexes ankle and raises foot

Minor pectoral

assists major pectoral

Serratus

moves scapula forward

Intercosta Is

breathing

Abdominal rectus

flexes trunk,
raises pelvis,
compresses abdomen

Muscles Of
abdominal wall

3 layers

Circular muscle of eye

closes eyelids

Muscles of facial expression

Biceps

flexes arm

Biceps tendon

Flexors of hand

Adductors

move legs together

Sterno-mastoid

turns head sideways

Trapezius

raises shoulder

Deltoid

raises upper arm

Major pectoral

moves arm across chest

Trapezius

raises head and shoulders

Deltoid
Muscles of scapula
Latissimus dorsi

moves arms backward

Triceps

extends arm

40-8 The muscles are intact on
the figure’s left side. Some of the
overlying muscles have been re¬
moved on the right side. The
attachments of muscles to the
scapula are shown on the right
side as are the deeper muscle at¬
tachments to the pelvic bones and
hip joint. The tendon of Achilles,
extending upward from the heel is
shown in both lower legs. A di¬
vided muscle, joining the tendon
has been removed to expose the
full length of the Achilles tendon
and the deeper muscle it also
joins.

Extensors of

Flexors of hand

Lesser gluteals

rotate thigh

Greater gluteal

extends thigh and raises trunk
from stooping position

Hamstrings

flex lower leg

Gastrocnemius

extends or depresses foot in
walking or standing on tiptoe

Soleus

acts with gastrocnemius

559

560 UNIT 7 THE BIOLOGY OF MAN

marrow is found in flat bones such as
the ribs and sternum, as well as in the
ends of long bones and vertebrae. It is
active in forming the red corpuscles and
most of the white corpuscles. The yel¬
low marrow fills the central cavity of
long bones and extends into the Haver¬
sian canals of the bony layer. It is nor¬
mally inactive and primarily composed
of fat cells, but may produce corpuscles
in time of great blood loss and in cer¬
tain blood diseases.

The smaller bones are solid rather
than hollow and vary considerably in the
amount of spongy bone tissue present.
Although they are solid, they are com¬
pletely penetrated with blood vessels.

elbow ribs and vertebra

CRANIUM VERTEBRAE

40-9 The five types of joints.

The joints of the body. The point at
which two separate bones meet is called
a joint. The various bones of the hu¬
man body are connected by several dif¬
ferent kinds of joints (Fig. 40-9). The
elbow is an example of a hinge joint.
Such a joint moves as a hinge in one
plane only, but may give great power
because there is little danger of twisting.
When the biceps muscle of the upper
arm contracts, the lower arm is pulled
upward only. The knee is another ex¬
ample of a hinge joint. The hip and
shoulder joints are examples of ball-and-
socket joints. Here the bone of the
upper arm ends in a ball which fits into
a socket of the shoulder girdle. Such a
joint has the advantage of movement in
any direction within the limits imposed
by the muscles. The hip joint is similar
to that of the shoulder, with a ball on
the end of the femur , or thighbone, fit¬
ting into a socket of the hipbone, the
pelvis. Ball-and-socket and hinge joints
are held in place by tough strands of
connective tissue called ligaments. Lig¬
aments may be stretched with exercise,
thus loosening joints and permitting
freer movement.

The ribs are attached to the verte¬
brae by joints that are only partially
movable. Long strands of cartilage at¬
tach the ribs to the breastbone in front
to allow for chest expansion during
breathing. The junction between the
spine and pelvis, the sacroiliac joint, is
a well-known example of partial mov-
abilitv. It is frequently injured in sud¬
den falls. Some joints, such as those in
adult skull bones, are immovable. Oth¬
er joints include the angular joints of
the wrists and ankles, the gliding joints
of the vertebrae, and the pivot joint of
the head on the spine.

The inner surfaces of the joints are
covered with layers of permanent carti-

CHAPTER 40 THE BODY FRAMEWORK 561

lTTTl

TTT

rrrr

■n

■

[HI

:nrr

mi:

Muscle fiber
at rest

Muscle fiber
contracted

40-10 This diagram illustrates the current
theory of muscle contraction. It is believed
that the tiny myofibrils slide together in
the manner shown, resulting in a shortening
of the muscle.

lage. A secretion called synovial (se-
no/z-vee-al) fluid serves to lubricate the
joints. In some joints, such as the knee
or shoulder, a sac called the bursa serves
as a cushion between the bones.

How muscles produce movement.
Bones, even in a living body, have no
power to move by themselves. Muscle
cells, however, are specialists in motion
because of their ability to contract.
Grouped in bundles, these cells accom¬
plish such mechanical activities as walk¬
ing, grasping, breathing, heartbeat, and
movement of the digestive organs.
There are about 400 different muscles
in the human body, making up about
one half of the body weight (Figs. 40-7
and 40-8, pages 558 and 559).

Muscle tissues of man and other
higher animals are composed of bundles
of long slender cells often called muscle
fibers. Each of these consists of numer¬
ous fine threads called myofibrils , which
lie parallel and run lengthwise in the fi¬
ber. The myofibrils in turn are bundles
of two distinct kinds of still smaller pro¬
tein filaments — thick and thin. These

are arranged in the myofibril in a defi¬
nite pattern (Fig. 40-10).

When muscle cells, or fibers, are
supplied with energy from ATP and ac¬
tivated by a nerve impulse, they con¬
tract. The actual mechanism by which
muscle cells contract is not known, but
a widely accepted current theory states
that the thick and the thin protein fila¬
ments slide past each other, in a man¬
ner that would shorten the myofibrils.

Each nerve cell that carries im¬
pulses to a muscle branches to supplv a
small number of muscle fibers. The
nerve cell and the individual fibers it
stimulates comprise what is called a mo¬
tor unit. Each fiber, when stimulated
to contract, will do so to its fullest. The
factor that determines whether a move¬
ment will be a very delicate, precise one
or a verv forceful one is therefore the
number of motor units that are called
into action. Muscle fibers may also be
stimulated to contract by heat, light,
chemicals, pressure, and electricity.
Types of muscle cells. The three types
of vertebrate muscle cells are shown in
Fig. 40-11. Smooth muscle forms the
walls of many interna] organs. Each
cell in smooth muscle is an elongated
spindle containing one nucleus usually
situated near the center of the cell.
The stomach and intestinal walls con¬
tain layers of smooth muscle cells which
contract in waves to churn food or pass
it along through the digestive tract.
Artery walls also contain layers of
smooth muscle. Impulses from the
nervous system cause the artery walls to
constrict and raise the blood pressure
during danger or emotional upset. All
action of smooth muscle is controlled
by parts of the nervous system over
which we have no conscious control, so
that smooth muscle is a type of involun
tary muscle.

562 UNIT 7 THE BIOLOGY OF MAN

The skeletal muscles may be con-

trolled at will and are therefore volun¬
tary muscles. Each fiber is a long cyl¬
inder with tapering ends and contains
many nuclei situated near the periphery
of the cell throughout its length. Skele¬
tal muscle fibers do not run the entire
length of most muscles but are bound
together in small bundles by connective
tissue sheaths. These small bundles are
then held together by a heavier sheath
which encloses the entire muscle. This
structure gives most voluntary muscles
a spindle shape.

Some skeletal muscles attach di¬
rectly to bones, some attach to other
muscles, and some attach to bones by
inelastic tendons extending from the
tapered end. These tendons are dense
bands of fibrous connective tissue.
Since muscles are organs of movement,
they must attach at two points. The
origin is stationary while the insertion
is the attachment of the muscle on the
movable part. Be sure to note the origin
and the insertion of the biceps in Fig.
40-12.

The skeletal muscles that move
joints of the trunk and limbs are always
arranged in pairs. Muscles that bend
joints are called flexors, while those that
straighten them are called extensors.
For instance, when you bend your el-
bow joint, the tendon of the contracted
biceps muscle raises the radius bone of
the forearm. The other end of this
muscle is securely anchored at the shoul¬
der. During this contraction you can
feel the biceps muscle swell on the front
side of your upper arm. The extensor
muscle involved in this movement is

40-11 Top: photomicrograph of stained
skeletal muscle; middle: cardiac muscle;
bottom: smooth muscle. (Walter Dawn)

CHAPTER 40 THE BODY FRAMEWORK 563

called the triceps. It is on the back side
of the upper arm. When you lower the
arm, the triceps contracts and the bi¬
ceps relaxes. If you straighten your arm
completely, you can feel this muscle
contract.

Even when a joint is not being
moved, flexor and extensor muscles op¬
pose each other in a state of slight con¬
traction called tone. Increased use of
muscles results in enlargement and in¬
creased tone. When totally unused,
muscles become weak and flabby, de¬
crease in size, and lose tone.

Cardiac muscle , the involuntary
muscle found in the heart, is made up
of the third kind of contractile cell in
the body. Structurally cardiac muscle
resembles skeletal muscle except that
the fibers branch and join others to
form an interlacing meshwork. Thus,
when cardiac muscle fibers contract, the
cavities of the heart are squeezed and
blood is forced out through the vessels.

The action of heart muscle is un¬
like that of any other muscle. It has an
automatic beat which is conducted from
cell to cell throughout the muscle. The
beat originates in a small mass of tissue
in the wall of the right atrium of the
heart, called the sinoatrial node. From
this point the beat is carried through

Tendons of
origin

Bellies

Tendons of
insertion

Biceps muscle

Triceps

muscle

40-12 In the upper drawing the arm is
straightened by the contraction of the triceps
muscle, while the biceps is relaxed. In the
lower drawing the biceps contracts to flex
the arm while the triceps relaxes.

the muscle of the upper chambers to
another node, the atrioventricular
node , where it is relayed through the
muscles of the lower chambers. Con¬
duction of the beat through the cells of
heart muscles results in the characteristic
rhythmic wave of contraction.

IN CONCLUSION

The human body is organized into tissues, organs, and systems. The body
framework consists of an internal skeleton composed of bones that give the
body form, act as levers for muscles, and protect the more delicate organs.

Muscles produce movement. Those that are attached to bones by means
of tendons are skeletal muscles. Smooth muscles form layers in the walls
of such internal organs as the stomach, intestines, and the arteries. Their
control is involuntary.

Our attention will next be focused on the nature of foods and nutrition in
a living organism. If we know what foods are and what they do, we can then
study the digestive system with better understanding.

564 UNIT 7 THE BIOLOGY OF MAN

BIOLOGICALLY SPEAKING

abdominal cavity

atrioventricular node
bony layer
bursa

cardiac muscle
connective tissue
cranial cavity
diaphragm
endoskeleton
epithelial tissue

QUESTIONS FOR REVIEW

1. How do the tissues, organs, and systems of the human body illustrate di¬
vision of labor?

2. What are the three body cavities? By what structures are they enclosed?
Name some of the organs found in each.

3. What are the principal functions of bones? Give an example of a bone
serving each purpose.

4. What are some important functions of the Haversian canals?

5. Describe some of the tissues surrounding a joint.

6. Describe four kinds of joints found in the body.

7. How do we classify muscle cells as to appearance, control, and location?

8. Why are muscles often found in opposing pairs in the body?

9. Describe the contraction of a striated muscle.

APPLYING PRINCIPLES AND CONCEPTS

1. Explain why an organism with an internal skeleton must have a highly
developed nervous system.

2. Proper diet alone does not insure good teeth and healthy bones. What
other factors are involved?

3. How does a walking cast speed up the repair of a bone fracture?

4. Why can the heart of some lower animals be removed and kept beating in
a nutrient solution?

extensor

flexor

Haversian canal

insertion

joint

ligament

origin

ossification

periosteum

red marrow
sinoatrial node
skeletal muscle
smooth muscle
synovial fluid
tendon

thoracic cavity
tone

yellow marrow

CHAPTER Ui

What is food? Some time after you eat
a sandwich and drink a glass of milk,
these substances are still in your tissues.
But in what form — as bread, beef, milk,
and butter? No. These foods have
been changed to glucose, amino acids,
and fatty acids. How did this happen?
As the foods passed through the ali¬
mentary canal, a 30-foot tube, several
enzymes broke them down chemically in
a series of changes that make up the
process of digestion. Your digestive sys¬
tem is like an assembly line in reverse.
It starts with the many complex foods
you eat and simplifies them to a few
nutrients. Food is any substance which,
when absorbed into the body tissues,
yields materials for the production of
energy, the growth and repair of tis¬
sue, and the regulation of life proc¬
esses, without harming the organism.
This is another way of saying that you
eat to be active and to grow and to
maintain your body. Six classes of sub¬
stances meet the requirements of this
definition: 1. water; 2. minerals; 3. car¬

bohydrates, 4. fats; 5. proteins; and 6. vi¬
tamins. The table on page 567 lists the
food substances, their functions, and
their sources.

Perhaps this would be a good time
for you to review the importance to the
living organism of the first four of these
substances and their chemical structures,
as outlined in Chapter 3.

Water has many uses. Water is in¬
organic, and does not yield energy to the
tissues. However, it is so vital in the
maintenance of life that a person de¬
prived of it dies sooner than he would if
deprived of other types of food.

If you weigh 100 pounds, your body
contains between 60 and 70 pounds of
water. Much of this water is organized
into your body protoplasm and into the
spaces between the cells. As water is
lost, first from the intercellular spaces
and then from the cells themselves, the
protoplasm becomes more and more
solid and finally dies. This water loss is
part of the process called dehydration.

The fluid part of blood, called plas¬
ma , is 91 to 92 percent water. Water
is essential in the plasma as a solvent
for the food and waste products that are
transported to and from the body tissues.
Water serves further as a solvent in the
movement of dissolved foods from the
digestive tract to the blood and in the re¬
moval of tissue wastes from the skin and
kidneys. The kidneys alone pass two to
five pints of excess water daily; this wa¬
ter contains many cellular wastes which
the body must eliminate.

The flow of sweat is essential in the
regulation of heat loss from the body.
Evaporation or the change of a sub¬
stance from a fluid to a gas, requires
heat. When perspiration evaporates
from the body surface, heat resulting
from internal oxidation is lost.

Water requirements of the body are

565

566 UNIT 7 THE BIOLOGY OF MAN

met in three ways: 1. some water is
present in the food you eat; 2. some is
a by-product of oxidation in cells; and
3. some is consumed as drinking water.
The amount required varies with the
temperature and humidity of the air and
the amount of body activity.

The importance of mineral salts in the
body. Table salt, or sodium chloride ,
is consumed directly and in considera¬
ble quantities in the diet. Other salts
(chemical compounds composed of a
mineral and one or more other elements)
are also present in food. Since salts are
lost in perspiration, persons exposed to
excessive heat over long intervals must
either increase the salt in the diet or
supplement the normal diet with salt
tablets.

Animals require calcium and phos¬
phorus in greater abundance than other
mineral elements. Calcium is necessary
for proper functioning of plasma mem¬
branes, while phosphorus is a compo¬
nent of ATP, DNA, and RNA. Cal¬
cium phosphate is important in the for¬
mation of bones and teeth. These two
elements form about 5 percent of ani¬
mal tissue when combined with others
in the form of proteins. Milk is an
ideal source of these two elements; oth¬
er sources include whole-grain cereals,
meat, and fish. Calcium is also neces¬
sary to insure the proper clotting of
blood and, together with magnesium , to
nerve and muscle action. Potassium
compounds are essential to growth.

Iron compounds are essential for
the formation of red blood corpuscles.
Meats, green vegetables, and certain
fruits such as plums or prunes and rai¬
sins are important sources of iron in the
diet. Iodine salts are essential in the
formation of the thyroid gland’s secre¬
tion. Iodine may be obtained from
drinking water or by eating sea foods.

Minerals are vital to the body in
many ways. Each of them, however,
must be in a compound form before it
can be used by the body. Eating chem¬
ically pure elements such as sodium or
chlorine would be fatal. When these
elements are in a compound form, such
as sodium chloride, however, they are
harmless and in fact essential to the
body.

What are organic nutrients? We call
carbohydrates, fats, and proteins or¬
ganic nutrients because they are origi¬
nally formed by living cells and contain
the element carbon. Carbohvdrates

and fats supply energy. The tissue¬
building value of foods cannot be meas¬
ured except by observing growth in
animals when they are fed. But the
energy value can be measured in heat
units, called Calories. A Calorie (large
Calorie) is the amount of heat required
to raise the temperature of 1,000 cubic
centimeters (about one quart) of water
one degree centigrade. This is 1,000
times as great as the small calorie used
in physical measurements of heat. No¬
tice that the two are distinguished by
the large Calorie capitalizing.

The number of Calories required in
an average day’s activity varies with the
kind of activity and with the age and
body build of the person concerned. A
daily requirement of 2,500 to 3,500 Cal¬
ories is probably above average.

More than half of your total diet is
carbohydrate food. Regardless of this
high percentage, the accumulated carbo-
hvdrate reserve in vour bodv is less than

J J J

one percent of your total weight. This
is evidence that carbohydrates are pri¬
marily fuel foods, and that they are oxi¬
dized rapidly to supply the energy re¬
quired for body activity.

The significance of carbohydrates in
the diet. Many different kinds and forms

CHAPTER 41 NUTRITION 567

FOOD SUBSTANCES

Substance

Kind of
Substance

Essential For

Source

Water

Inorganic com¬
pound

Composition of pro¬
toplasm and blood
Dissolving sub¬
stances

All foods (released
during oxidation)

Sodium compounds

Mineral salts

Blood and other
body tissues

Table salt, vegetables

Calcium compounds

Mineral salts

Deposition in bones
and teeth

Heart and nerve
action

Clotting of blood

Milk, whole-grain
cereals,
vegetables,
meats

Phosphorus com¬
pounds

Mineral salts

Deposition in bones
and teeth

Formation of ATP,
nucleic acids

Milk, whole-grain
cereals,
vegetables,
meats

Magnesium

Mineral salts

Muscle and nerve
action

Vegetables

Potassium com¬
pounds

Mineral salts

Blood and cell
activities

Growth

Vegetables

Iron compounds

Mineral salts

Formation of red
blood corpuscles

Leafy vegetables,
liver, meats,
raisins, prunes

Iodine

Mineral salts

Secretion by thyroid
gland

Sea foods, water,
iodized salt

Carbohydrates

Organic nutrients

Energy (stored
as fat or glycogen)
Bulk in diet

Cereals, bread,
pastries, tapioca,
fruits, vegetables

Fats

Organic nutrients

Energy (stored
as fat or glycogen)

Butter, cream,
cheese, oleomar¬
garine, lard, oils,
nuts, meats

Proteins

Organic nutrients

Growth,

maintenance, and
repair of
protoplasm

Lean meats, eggs,
milk, wheat,
beans, peas,
cheese

Vitamins

Complex

organic

substances

Regulation of body
processes

Prevention of
deficiency diseases

Various foods,
especially milk,
butter, lean meats,
fruits, leafy vege¬
tables; also
made synthetically

568 UNIT 7 THE BIOLOGY OF MAN

of carbohydrate foods are included in
the average diet. Some are easily di¬
gested and are transported to the tissues
with little chemical change. Others re¬
quire more chemical simplification for
tissue use. One group, while indigest¬
ible, provides necessary bulk, or rough-
age, in the diet. All carbohydrates
which are digestible reach the body tis¬
sues as glucose , or dextrose.

Many simple sugars are present in
the foods you eat. These include glu¬
cose, fructose, and galactose. As you
learned in Chapter 3, these sugars are
classed as monosaccharides because they
consist of single hexose molecules with
the chemical formula C6H1206. These
sugars are quick-energy sources because
they require little or no chemical change
before they are absorbed by the blood
from the digestive organs.

Sucrose (cane sugar), lactose (milk
sugar) and maltose (malt sugar) are di¬
saccharides, composed of two hexose
units. These double sugars require di¬
gestive action which reduces them to
simple sugar molecules for absorption.

Starches , or polysaccharides, com¬
pose a large part of the carbohydrate
portion of the diet. They are abundant
in cereal grains such as wheat, com, rye,
barley, oats, and rice, in addition to po¬
tatoes and tapioca.

As you learned in Chapter 3,
starches are composed of large chains
of glucose units, each represented as
C6H10O5. Starch digestion involves the
addition of a water molecule to a glu¬
cose unit. Maltose is formed during
starch digestion. Later this double sugar
is reduced to glucose and absorbed by
the blood.

Much of the glucose received by
the blood and transported to the liver is
converted tempo rarilv to animal starch,
or glycogen. As glucose is oxidized in

the body tissues, glycogen is changed
back to glucose (dextrose) and released
into the blood stream. In this manner,
the level of blood sugar is maintained.
Were it not for this action of the liver,
we would have to eat a small quantity of
carbohydrate foods constantly.

Celluloses are complex carbohy¬
drates present in the cell walls of vege¬
table foods. These materials cannot be
digested in the human system. How¬
ever, as roughage in the digestive system
they expand the intestines and stimulate
muscle contractions of the walls result¬
ing in movement of the food content.
This muscular activity is necessary for
normal digestion.

Fats are highly concentrated energy
foods. Fats and oils yield more than
twice as much energy as carbohydrates.
Common sources of these foods in the
diet include butter, cream, cheese, oleo¬
margarine, lard and other shortenings,
vegetable oils, nuts, and meats.

Fats undergo slow digestion by en¬
zyme influence during digestion. The
complex fat molecules are split into
molecules of glycerin (glycerol) and
fatty acids. Water molecules combine
with fat molecules in the process.

Body fats are formed by the con¬
version of excess carbohydrates. The
chief storehouses of body fat are the
tissue spaces beneath the skin, the re¬
gion of the kidneys, and the liver. Ex¬
cess body fat is detrimental to health.
For this reason both the carbohydrate
and fat content of the diet should be
carefullv regulated.

Proteins and their uses. You learned
in Chapter 3 that proteins are com¬
plex organic molecules of almost un¬
limited chemical structure. They are
composed of large numbers of units
known as amino acids. The proteins
you consume as food are foreign to your

CHAPTER 41 NUTRITION 569

body and cannot be used in your tissues.
However, reduced to amino acids during
digestion, they supply the units required
by your cells in synthesizing your own
specific protein molecules. Both growth
and repair of body substances depend on
protein intake in your diet.

Certain of the amino acid mole¬
cules absorbed by the blood are not used
in cell protein synthesis. These are
broken down by chemical activity of the
liver, called deamination , into two parts.
One part contains carbon and is sent to
the tissues as glucose. The nitrogen-
containing part is synthesized as urea, a
waste product received by the blood and
transported to the kidneys for excretion
in the urine. This urea is in addition to
that formed during the breakdown of
tissue protein.

The most valuable protein sources
in the diet include lean meat, eggs (al¬
bumen), milk (casein), cheese, whole
wheat (gluten), beans, and corn. These
are body-building foods, essential in
growth during childhood and young
adult life and in the maintenance and
repair of protoplasm in the mature years.
Vitamins — organic compounds essen¬
tial to proper body functioning. In 1911
Dr. Casimir Funk found that certain
substances, apart from ordinary nutri¬
ents, are present in very small amounts
in foods. They seemed to be necessary
for normal growth and body activity and
in the prevention of certain diseases
called deficiency diseases. He called
these substances vitamins. Vitamins
were first designated by letters — A, B,
C, etc. Later it was discovered that cer¬
tain vitamins thought to be simple were
made up of many different components
such as, for example, the vitamin-B com¬
plex. Then such names as Bx, B2, and
so forth were adopted. Today most of
the vitamins have names that indicate

their chemical composition, although
letters are still used as a means of easy
and simple reference. Vitamins are or¬
ganic compounds that act as catalysts.
In this respect they are similar to the
digestive enzymes. The best sources,
functions, and deficiency symptoms of
•the better-known vitamins are summa¬
rized in the table on page 570.

Some vitamins may be stored in the
body, while others must be supplied con¬
stantly because the excess in the diet is
excreted in the urine. Vitamin D can
be produced in the skin. Other vita¬
mins or their precursors must be sup¬
plied by the diet or taken in the form
of extracts, if the normal diet lacks
them. But the best source of vitamins
is a balanced diet.

Synthetic vitamins. Most of the vita¬
mins listed in the table on page 570 may
be purchased in highly concentrated
synthetic form. These preparations are
important in supplementing the natural
vitamins of the diet when deficiency oc¬
curs. However, even with all the pub¬
licity given commercial vitamin prepa¬
rations, remember that a normal, bal¬
anced diet is much more desirable for
good health than supplementary doses.
Your doctor can diagnose vitamin defi¬
ciency and prescribe concentrated vita¬
mins if he thinks you need them. If a
proper diet is followed, additional vita¬
mins are probably a waste of money
and are unnecessary for the average per¬
son.

The phases of digestion. There are two
reasons why tissues cannot use most
foods in the forms in which you eat
them. First, many substances are in¬
soluble in water and could not enter the
plasma membranes of the cells even if
they reached them. Second, these
foods are too chemically complex for
tissues to use, either in oxidation or for

570 UNIT 7 THE BIOLOGY OF MAN

FUNCTIONS AND IMPORTANT SOURCES OF VITAMINS

Vitamins

Best Sources

Essential For

Deficiency

Symptoms

Vitamin A

Fish liver oils

Growth

Retarded growth

(oil soluble)

Liver and kidney
Green and yellow
vegetables

Yellow fruit
Tomatoes

Butter

Egg yolk

Health of the eyes
Structure and func¬
tions of the cells of
the skin and mu¬
cous membranes

Night blindness

Susceptibility to infec¬
tions

Changes in skin and
membranes

Defective tooth forma¬
tion

Thiamin (Bi)

Sea food

Growth

Retarded growth

(water soluble)

Meat

Soybeans

Milk

Whole grain

Green vegetables
Fowl

Carbohydrate metab¬
olism

Functioning of the
heart, nerves, and
muscles

Loss of appetite and
weight

Nerve disorders

Less resistance to fa¬
tigue

Faulty digestion
(beriberi)

Riboflavin (B2)

Meat

Growth

Retarded growth

(water soluble)

Soybeans

Milk

Green vegetables

Eggs

Fowl

Yeast

Health of the skin and
mouth

Carbohydrate metab¬
olism

Functioning of the

eyes

Dimness of vision
Inflammation of the
tongue

Premature aging
Intolerance to light

Niacin

Meat

Growth

Smoothness of the

(water soluble)

Fowl

Fish

Peanut butter
Potatoes

Whole grain
Tomatoes

Leafy vegetables

Carbohydrate metab¬
olism

Functioning of the
stomach and intes¬
tines

Functioning of the
nervous system

tongue

Skin eruptions

Digestive disturbances
Mental disorders
(pellagra)

Vitamin B12

Green vegetables

Preventing pernicious

A reduction in number

(water soluble)

Liver

anemia

of red blood cells

Ascorbic acid (C)

Citrus fruit

Growth

Sore gums

(water soluble)

Other fruit
Tomatoes

Leafy vegetables

Maintaining strength
of the blood vessels
Teeth development

Gum health

Hemorrhages around
the bones

Tendency to bruise
easily (scurvy)

Vitamin D

Fish liver oil

Growth

Soft bones

(oil soluble)

Liver

Fortified milk

Eggs

Irradiated foods

Regulating calcium
and phosphorus
metabolism

Building and main¬
taining bones, teeth

Poor teeth development
Dental decay
( rickets )

Tocopherol (E)

(oil soluble)

Wheat-germ oil
Leafy vegetables

Milk

Butter

Normal reproduction

Undetermined

Vitamin K
(oil soluble)

Green vegetables
Soybean oil
Tomatoes

Normal clotting of
the blood

Normal liver func¬
tions

Hemorrhages

CHAPTER 41 NUTRITION 571

growth and repair by protein synthesis.
Digestion brings about changes in both
of these conditions, with the result that
cells can absorb and use the products.
Thus in digestion complex foods are
broken down into smaller molecules of
water-soluble substances that may be
used by the body cells.

The first part of the change that
occurs during digestion is mechanical.
This phase involves the chewing of food
in the mouth, and the constant churn¬
ing and mixing action brought about by
the muscular movement of the walls of
the digestive organs. The breakdown
of food into small particles and the thor¬
ough mixing with various juices aid the
second phase of digestion, which is
chemical. This phase is accomplished
by digestive enzymes that are present in
various secretions produced by the di¬
gestive glands. In studying the entire

process of digestion, you will find it
helpful to refer to the “Trans-Vision"
of the human torso between pages 584
and 585.

The digestive system includes the
organs that form the alimentary canal ,
or food tube (Fig. 41-1). It also in¬
cludes those organs that do not actually
receive undigested food, but that act on
foods in the alimentary canal by means
of secretions delivered to it by various
ducts. Ducts are tubes extending from
certain glands into the digestive organs.
The mouth — the first digestive struc¬
ture. The mouth is an organ of sensa¬
tion and of speech, but its chief func¬
tion is to prepare food for digestion.
The hard palate forms the roof of the
mouth in the chewing area. It consists
of a bony structure covered with several
membranes. The soft palate lies just
back of the hard palate. It is formed

Liver

Colon

(traverse)

Colon

Duodenum

(ascending)
Caecum

Small intestine

Colon
(descending)

Appendix

Rectum

Colon (sigmoid)

41-1 The organs of digestion in the human body.

Salivary glands

Gall bladder

Stomach

Common bile duct

Pancreas

Mouth
Tongue

Esophagus

572 UNIT 7 THE BIOLOGY OF MAN

bv folded membranes that extend from

the rear portion of the hard palate and
fasten along the sides of the tongue.
You can see a knoblike extension of the
soft palate called the uvula (yu-vyu-la)
when the mouth is opened wide (Fig.
41-2).

The back of the mouth opens into
a muscular cavity called the pharynx
(fair-inks) . This cavity extends up¬
ward, above the soft palate, to the nasal
cavity. The soft palate partly separates
the nasal cavity from the mouth cavity
and extends into the pharynx, some¬
what like a curtain, as you can see in
Fig. 41-2. The inside of the cheeks
forms the side walls of the mouth cav¬
ity. The cheek linings are mucous
membranes, containing numerous mu¬
cous glands. Mucus , a lubricating se¬
cretion, mixes with food in the mouth
and aids in chewing and swallowing.
The lining of the mouth turns outward
to form the lips.

The salivary glands. The parotid glands
are the largest of the salivary glands .

One lies on each side of the face below
and in front of the ears. Ducts from
these glands empty saliva into the
mouth, opposite the second uppei
molars. An infection of the parotid
glands, causing swelling and irritation,
is the disease called mumps. The sub¬
maxillary glands lie within the angles of
the lower jaws. The sublingual glands
are embedded in the mucous membranes
in the floor of the mouth, under the
tongue (Fig. 41-3). Ducts from both
of these glands open into the floor of
the mouth under the tongue. The
smell of food, the sight of it, the pres¬
ence of it in the mouth, and the taste
of it stimulate the secretion of saliva.
In other words your mouth “waters.”
The tongue and its functions. The
tongue lies in the floor of the mouth
and extends into the throat. This mus¬
cular organ performs several different
functions, as follows:

1. It acts as an organ of taste. Scattered
over the surface of the tongue are nu¬
merous tiny projections. These pro-

Incisors

Canine

Premolars

Hard palate
Soft palate
Uvula

Tonsil
Pharynx
Tongue

Papilla on tongue

Submaxillary gland

Duct of the
submaxillary gland

Duct of the
sublingual gland

Sublingual gland

Parotid gland

Duct of the
parotid gland
(to cheek)

41-2 Digestion begins in the mouth, the 41-3 Three pairs of salivary glands secrete

most anterior organ of the alimentary canal. saliva into the mouth.

CHAPTER 41 NUTRITION 573

Nasal cavity
Hard palate

Nostril
Mouth cavity

Teeth

Lower lip

Lower jaw bone

Soft palate

Adenoid

Opening to the
Eustachian tube

Uvula

Pharynx

Tonsil

Epiglottis

Neck vertebrae

Esophagus

Tongue

Vocal cord

Trachea

41-4 This cutaway diagram shows the internal structure of the mouth and throat.

jections contain taste buds , which
have nerve endings at their base.
When food is mixed with mucus and
saliva, it makes contact with the taste
buds, thus stimulating the nerve end-
ings.

2. The tongue aids in chewing by keep¬
ing the food between the teeth.

3. During swallowing, food is worked to
the back of the tongue. When the
tongue is jerked downward, food
lodges in the pharynx and passes into
the esophagus opening. The open¬
ing of the trachea is closed by the
pressure of the tongue and breathing
ceases for a moment during the proc¬
ess of swallowing.

4. The tongue keeps the inner surface

of the teeth clean, because you roll
it around in your mouth.

5. The tongue is essential in speech.
In forming certain word sounds, the
tongue acts together with the lips,
teeth, and hard palate. Without
such interaction these sounds could
not be formed into words.

The types of teeth. If you start be¬
tween the two front teeth and count
back, your permanent teeth are arranged
in the following order: the first two are
the flat incisors, with sharp edges for
cutting food (Fig. 41-2). Next, near
the corner of your lips, is a large conical
canine, or cuspid, tooth. These teeth
are often called eye teeth, although they
have no connection with the eyes. Be-

574 UNIT 7 THE BIOLOGY OF MAN

hind the canine teeth are two premo¬
lars, or bicuspids. Next come three mo¬
lars (two, if you have not cut your wis¬
dom teeth ) . The premolars and molars
have flat surfaces and are adapted for
grinding and crushing. Manv jaws are
too small to provide space for the third
molars, or wisdom teeth. In such jaws
they may grow in crooked, lodge against
the second molars, or remain impacted.
The structure of the teeth. A tooth is
composed of three general regions. The
exposed portion above the gum line is
called the crown. A narrow portion at
the gum line is called the neck, while
the root is encased in a socket in the
jawbone and holds the tooth securelv in
place. Roots vary in form in the differ¬
ent kinds of teeth. They may be long
and single, or they may consist of two,
three, or four projections. The crown
is covered with a hard white substance,

41-5 This vertical section through a canine
tooth shows the various parts.

the enamel. The covering of the root
is called cementum, which holds the
tooth firmly together. The root is an¬
chored firmly in the jaw socket by the
fibrous periodontal membrane.

If you cut a tooth lengthwise, you
can see the dentine beneath the protec¬
tive layers of enamel and cement (Fig.
41-5). Dentine is a softer substance
than enamel and forms the bulk of the
tooth. The pulp cavity lies inside the
dentine area.

The structure of the esophagus and
stomach. After a food mass is ground
between the teeth, rolled on the tongue,
and mixed with saliva, it passes through
the pharvnx to the esophagus. This is
a tube about a foot long that connects
the mouth to the stomach. Food trav¬
els to the stomach with the aid of lavers
of smooth muscle in the wall of the
esophagus. One layer is circular and
squeezes inward. The other laver is
longitudinal and contracts in a wave
that travels downward, pushing the
food ahead of it.

The stomach lies in the upper left
region of the abdominal cavity just be¬
low the diaphragm. The stomach walls
contain three lavers of smooth muscle,
each arranged differently. One layer is
longitudinal, one is circular, and one is
angled, or oblique. Contraction of the
smooth muscle fibers of the various lav-

ers in different directions causes a twist¬
ing, squeezing, and churning movement
of the stomach.

The lining of the stomach is a thick,
wrinkled membrane in which numerous
gastric glands are embedded. Each
of these glands is a tiny tube with an
opening that leads into the stomach.
The walls of each gland are lined with
secretory cells. There are three kinds of
glands. One kind secretes an enzyme;
a second secretes hydrochloric acid; and

CHAPTER 41 NUTRITION 575

a third mucus. Together these secre¬
tions form gastric fluid , which passes
directly into the stomach.

Food usually remains in the stomach
two to three hours. During this period
rhythmic contractions of the stomach
muscles churn the food back and forth
in a circular path. This action sepa¬
rates the food particles and mixes them
thoroughly with the stomach secretions.
At the completion of stomach digestion,
the valve at the intestinal end, the py¬
loric valve , opens and closes several
times. With each opening of the valve,
food moves into the small intestine.
Finally the stomach is relieved of its
contents and begins a period of rest.
After several hours without food, con¬
tractions start again and cause the sensa¬
tion of hunger.

The small intestine. When food leaves
the stomach through the pyloric valve,
it enters the small intestine , a tube about
one inch in diameter and 23 feet long.
This part of the alimentary canal is the
most vital of all digestive organs. The
upper ten inches of the small intestine
are referred to as the duodenum (doo-
oh-dcc-num ) . The duodenum curves
upward, then backward and to the right,
beneath the liver. Beyond the duode¬
num is a second and much longer re¬
gion, the jejunum. This portion, about
seven feet long, is less coiled than the
other regions. The lower portion of the
small intestine is referred to as the ile¬
um, which is about 15 feet long and
coils through the abdominal cavity be¬
fore joining the large intestine.

Embedded in the mucous lining of
the small intestine are many tiny intesti¬
nal glands. These glands secrete in¬
testinal fluid , containing four enzymes,
which passes into the small intestine.
The liver — the largest gland in the
body. The liver weighs about three and

41-6 This X-ray photograph is of a normal
stomach that has been filled with barium
to make the various parts visible on film.
Note contractions (1); pyloric valve (2); be¬
ginning of small intestine (3); and region of
villi in the small intestine (4). R indicates
right side; L indicates left side.

a half pounds and is a dark chocolate
color. It lies in the upper right region
of the abdominal cavity and secretes
bile. Bile is a brownish-green fluid
which passes from the liver through a
series of bile ducts forming a Y. As bile
is secreted in the liver, it passes down
one branch of the Y, then travels up the
other branch to the gallbladder. Here
the bile is stored and concentrated as
part of the water is removed. The base
of the Y is the common bile duct, which
carries bile from the gallbladder to the
duodenum. If the common bile duct
becomes clogged by a gallstone, or a
plug of mucus, bile enters the blood
stream and causes a yellowing of the
eyes and skin, known as jaundice.

The pancreas and pancreatic fluid. The
pancreas is a many-lobed, long, whitish
gland, quite similar in general appear¬
ance to a salivary gland. It lies behind

576 UNIT 7 THE BIOLOGY OF MAN

the stomach and the upper end of the
small intestine, against the back wall of
the abdominal cavity, and it performs
two entirely different functions. The
production of insulin by the pancreas
will be discussed in Chapter 46. Pan¬
creatic fluid , a digestive secretion, passes
into the small intestine through the
pancreatic duct , which leads to a com¬
mon opening with the bile duct in the
wall of the duodenum.

The large intestine, or colon. The
small intestine ends at a junction with
the large intestine, or colon , in the lower
right region of the abdominal cavity.
Below the point of junction is a blind
end of the large intestine called the
caecum (see-kum). The vermiform ap¬
pendix is a fingerlike outgrowth of the
caecum. Appendicitis is an inflamed
condition of the appendix resulting from
infection.

The colon is usually five to six feet
long and about three inches in diameter.
It forms an inverted U in the abdomi¬
nal cavity. The ascending colon runs
upward along the right side, where it
curves abruptly to the left to form the
transverse colon. This portion extends
across the upper region of the abdomi-

41-7 This diagram shows the stomach, pan¬
creas, gall bladder, and duodenum.

nal cavity. Another curve leads to the
descending colon on the left side. At
its lower end the descending colon
forms an S, called the sigmoid colon.
The rectum is a muscular cavity at the
end of the large intestine. The lower
end of the rectum forms the anal open¬
ing. A valvelike muscle in the lower
end of the rectum controls the elimina¬
tion of intestinal waste.

The chemical phases of digestion. As
foods move through the organs of the
alimentary canal, a series of chemical
changes occurs in the step-by-step proc¬
ess of simplification. Each of these
changes requires a specific enzyme in a
digestive secretion. The chemical
changes generally involve hydrolysis,
since water molecules interact with mol¬
ecules of the various food materials.
Digestive enzymes are therefore hydro¬
lytic enzymes, each associated with the
splitting of specific molecules. Diges¬
tion in the alimentary canal is neces¬
sarily extracellular. That is, enzymatic
action takes place outside the cells rath¬
er than inside, as in some other forms.
Digestion in the mouth. The chemical
action on food begins in the mouth
where an enzyme in saliva begins the
hydrolysis of starch. Saliva is a thin,
alkaline secretion of the salivary glands.
It is more than 95 percent water and
contains mineral salts, lubricating mu¬
cus, and the enzyme ptyalin (fy-a-lin),
sometimes called salivary amylase (am-
i-layze). This enzyme converts cooked
starch to maltose, a disaccharide. It is
necessarv to cook starchv foods such as

J J

potatoes to burst the cellulose cell walls.
This allows the ptyalin to contact the
starch grains. Because of the short time
food is in the mouth, starch digestion is
seldom completed when food is swal¬
lowed. However, ptyalin continues its
action in the stomach.

CHAPTER 41 NUTRITION 577

The action of gastric fluid. The prin¬
cipal enzyme in gastric fluid is pepsiny
sometimes referred to as gastric protease.
This enzyme acts on protein, splitting
the complex molecules into simpler
groups of amino acids known as pep¬
tones and proteoses. This is the first in
a series of chemical changes involved in
protein digestion.

Hydrochloric acid , in addition to
providing the proper medium for the ac¬
tion of pepsin, dissolves insoluble min¬
erals and kills manv bacteria that enter
the stomach with food. It also regu¬
lates the action of the pyloric valve,
which opens at the completion of stom¬
ach digestion and allows food to pass to
the small intestine.

The food passing from the stomach
to the small intestine contains the fol¬
lowing: 1. fats, unchanged; 2. sugars,
unchanged; 3. the starches that were not
acted on by ptyalin; 4. maltose formed
by the action of ptyalin; 5. coagulated
milk casein; 6. those proteins that were
unchanged by the pepsin of the gastric
fluid; and 7. peptones and proteoses
formed from pepsin acting on protein.
Functions of the liver and bile. The
liver performs several vital functions.
In receiving glucose from the blood and
changing it to glycogen, it serves as a
chemical factory. It serves also as a
storehouse in holding reserve carbohy¬
drates as glycogen. In acting on amino
acids and forming urea, it is an organ of
excretion.

As a digestive gland, the liver se¬
cretes bile, which acts on food in the
small intestine. In the formation of
bile, the liver plays a part in using what
might otherwise be discarded as waste.
Part of the bile is formed from worn-out
hemoglobin that the blood system can
no longer use. Bile has the following
important functions:

1. It is partially a waste substance con¬
taining material from dead red blood
corpuscles filtered from the blood
stream by the liver.

2. It increases the digestive action of li¬
pase, an enzyme produced in the
pancreas, by breaking globules of fat
into small droplets.

Actually, bile is not a digestive se¬
cretion. In splitting large fat particles
into smaller ones, a milky colloid called
an emulsion is produced. In this form
pancreatic fluid can act on fats more
readily.

The role of the pancreas in digestion.
Pancreatic fluid acts on all three classes
of organic nutrients. Pancreatic flu¬
id contains the following three enzymes:

1. trypsin; 2. amylase; and 3. lipase (lyp-
ays). Trypsin continues the breakdown
of proteins that began in the stomach
by changing peptones and proteoses to
still simpler amino acid groups called
peptides. In addition, trypsin may act
on proteins that were not simplified dur¬
ing digestion in the stomach. Peptides
are not the final product of protein di¬
gestion, because one additional step is
necessary to form the amino acids used
in protein synthesis by the cells. Amyl¬
ase duplicates the action of the ptvalin
in saliva by changing starch into malt¬
ose. This is how the potatoes you did
not chew enough are changed to sugar.
Lipase splits fat into fatty acids and
glycerin , both of which can be absorbed
bv the body cells.

Digestion in the small intestine. The
intestinal fluid secreted by the intesti¬
nal glands is highly alkaline and con¬
tains four principal enzvmes: 1. erepsin;

2. maltase; 3. lactase; and 4. sucrase.
Erepsin completes protein digestion by
changing peptides, formed by the pan¬
creatic fluid, to amino acids. Maltase
splits the disaccharide maltose into the

578 UNIT 7 THE BIOLOGY OF MAN

SUMMARY OF DIGESTION

Place of
Digestion

Glands

Secretion

Enzymes

Digestive Activity

Mouth

Salivary

Mucous

Saliva

Mucus

Ptyalin

Changes starch to maltose,
lubricates

Lubricates

Esophagus

Mucous

Mucus

Lubricates

Stomach

Gastric

Gastric fluid

Pepsin

Changes proteins to pep¬
tones and proteoses

Hydrochloric

acid

Activates pepsin

Dissolves minerals

Kills bacteria

Mucous

Mucus

Lubricates

Small intestine

Liver

Pancreas

Bile

Pancreatic

fluid

Trypsin

Amvlase

Lipase

Emulsifies fats

Activates lipase

Changes proteins, peptones,
and proteoses to peptids
Changes starch to maltose
Changes fats to fatty acids
and glycerin

Intestinal

glands

Intestinal

fluid

Erepsin

Maltase

Lactase

Sucrase

Changes peptids to amino
acids

Changes maltose to glucose

Changes lactose to glucose
and galactose

Changes sucrose to glucose
and fructose

Mucous

Mucus

Lubricates

Large intestine
(colon)

Mucous

Mucus

Lubricates

monosaccharide glucose, the final prod¬
uct of carbohydrate digestion. Lactase
has a similar action on lactose, or milk
sugar, in changing it to glucose and ga¬
lactose. Sucrase acts on sucrose and
changes it to the simple sugars glucose
and fructose.

Thus, with the combined action of
bile, pancreatic fluid, and intestinal
fluid in the small intestine, all three

classes of foods are completely digested.
As soluble substances in the form of
simple sugars, fatty acids and glycerin,
and amino acids, they leave the diges¬
tive system and enter the blood and
lymph.

Absorption in the small intestine. A
magnified portion of the small intestine
shows that its irregular lining gives rise
to great numbers of fingerlike projections

CHAPTER 41 NUTRITION 579

41-8 The villi greatly increase the absorp¬
tion surface in the small intestine.

called villi. These projections are so
numerous that they give a velvety ap¬
pearance to the intestinal lining. With¬
in the villi are branching lymph vessels
called lacteals and blood vessels (Fig.
41-8). The villi bring blood and
lymph close to the digested food and
increase the absorption surface of the
intestine enormously. Absorption is in¬
creased further by a constant swaying
motion of the villi through the intesti¬
nal content.

Glycerin and fatty acids enter the
villi and are carried away by the lymph.
They eventually reach the general circu¬
lation and travel to the tissues. Glu¬
cose and amino acids, however, enter
the blood vessels of the villi. From the
villi they are carried directly to the liver
through the portal vein.

Water absorption in the large intestine.
The large intestine receives a watery
mass of undigestible food bulk from the
small intestine. As this mass progresses
through the colon, much of the water is

absorbed and taken into the tissues.
The remaining intestinal content, or
feces (fee- ses), becomes more solid as
the water is absorbed. The feces pass
into the rectum, from which they are
eventually eliminated through the anal
opening.

The chart at the left summarizes
the digestive processes occurring in the
various organs.

IN CONCLUSION

Food is any substance which, when absorbed into the body tissues, yields ma¬
terials for the production of energy, the growth and repair of tissue, and the
regulation of life processes, without harming the organism. Six classes of sub¬
stances meet the requirements of this definition: water, minerals, carbohydrates,
fats, proteins, and vitamins.

The digestive system is a tube divided into various regions. Each par¬
ticular region of the tube is a specialized organ adapted for carrying on certain
phases of the digestive process. Many glands pour their enzymatic secretions
into the digestive tract. These enzymes bring about the chemical changes in
foods, while muscular contractions bring about the mechanical changes.

In the next chapter we shall investigate the way digested food is trans¬
ported in the bodv. We shall also study the way in which waste materials are
excreted by means of the kidneys and skin.

580 UNIT 7 THE BIOLOGY OF MAN

BIOLOGICALLY SPEAKING

alimentary canal
amylase
anal opening
bile

caecum

Calorie

colon

deficiency diseases

dehydration

digestion

duodenum

esophagus

evaporation

feces

gallbladder
gastric fluid
gastric gland
hard palate
intestinal fluid
intestinal gland
liver
mucus

organic nutrients
pancreas

pancreatic fluid
pharynx
pyloric valve
rectum
saliva

salivary glands
soft palate
taste buds
uvula

vermiform appendix
villi

vitamins

QUESTIONS FOR REVIEW

1. List six classes of foods and the general use of each in the body.

2. In what two general ways must foods be changed during digestion?

3. List, in order, the divisions of the alimentary canal.

4. Discuss five or more ways in which the tongue is used.

5. Name and locate the various salivary glands.

6. Name the regions that can be distinguished in a tooth cut lengthwise.

7. Why is it especially important that you chew bread and potatoes thor¬
oughly?

8. Suppose that a person had a glass of milk and a sandwich consisting of
bread, butter, and ham. Tell what would happen to each of these foods
as they go through the process of digestion.

9. List four functions of bile.

10. Name two important functions of the large intestine.

APPLYING PRINCIPLES AND CONCEPTS

1. Explain how a vitamin deficiency is possible even if an adequate amount
of all the vitamins is taken daily.

2. Why is food acid in the stomach and alkaline in the small intestine?

3. Explain how interference with the rhythmic waves of the walls of the large
intestine may cause either constipation or diarrhea.

4. Why is it easier to digest sour milk than fresh milk?

CHAPTER 42

TRANSPORT
AND EXCRETION

The circulatory system. The flow of
nutritive fluids, waste materials, and
water in living organisms is called circu¬
lation. The sponges accomplish cir¬
culation by literally pumping the ocean
into their bodies! The sea water sup¬
plies each cell with its individual oxygen
needs and washes its wastes away. Ac¬
tually the cells in man’s body are bathed
in a fluid with a salt content very much
like sea water. We call this solution
tissue fluid. However, man’s circulatory
system is far more complex than that of
the invertebrates. Man produces his
own “sea water” and adds other vital
substances to it. Then it is piped
through his body and circulated with a
pump — the heart. If the pump stops
working, man’s cells are in the same
predicament that a sponge would be if
thrown up on the beach.

Blood — a fluid tissue. Blood is a pe¬
culiar type of connective tissue in that
the cells are scattered among the nonliv¬

ing substances composing the fluid por¬
tion. The average person has about 12
pints of blood, which composes about
9 percent of the body weight. The
fluid portion of the blood is the plasma ,
and the blood cells are called the solid
components.

If you remove the cells from whole
blood, the straw-colored, sticky plasma
remains. Nine-tenths of plasma is wa¬
ter. The proteins in plasma give it the
sticky quality. One of them, fibrinogen
(fy-brin-o-jen), is essential in the clot¬
ting of blood. When fibrinogen is re¬
moved from plasma, two other groups
of proteins remain. One is serum al¬
bumin , which is necessary to normal
blood and tissue relationships during ab¬
sorption. The other is serum globulin ,
which gives rise to antibodies that pro¬
vide immunity to various diseases. Pro¬
thrombin (proh-f/ird/zm-bin), an en¬
zyme found in plasma, is produced in
the liver when vitamin K is present in
the body. It is inactive normally, but
active during clotting.

The following additional materials
are also present in plasma:

Inorganic minerals , dissolved in
water, give plasma a salt content of ap¬
proximately 1 percent, while that of sea
water is approximately 3 percent. These
compounds include carbonates, chlo¬
rides, and phosphates of the elements
calcium, sodium, magnesium, and po¬
tassium. They are absolutely essential
to the blood and to the normal func¬
tioning of body tissues. Without cal¬
cium compounds, blood will not clot in
a wound.

Digested foods are present in
plasma in the form of glucose, fatty
acids and glycerin, and amino acids.
These are transported to the liver and
other places of storage and to the body
tissues.

581

582 UNIT 7 THE BIOLOGY OF MAN

42-1 The solid components of the blood
include red corpuscles, white corpuscles,
and platelets.

Nitrogenous ( ny-trahj-i-mis ) wastes ,
resulting from protein metabolism in
tissues, and urea , produced largely in
the liver during the breakdown of amino
acids, travel in the plasma to the organs
of excretion.

The solid components of blood. The
red blood corpuscles (red blood cells,
or erythrocytes), the white corpuscles
(white blood cells, or leucocytes), and
the platelets (thrombocytes) are the
three solid components of blood (Fig.
42-1). The red corpuscles are shaped
like disks with both sides concave.
Sometimes they travel in the blood in
rows that resemble stacks of coins, al¬
though they may separate and float in¬
dividually. The red cells are so small
that ten million of them can be spread
in one square inch. They are so nu¬
merous that, placed side by side, they
would cover an area of 3,500 square
yards. It is estimated that the blood of
a normal person contains 25 trillion red

blood cells, or enough to go around the
earth four times at the equator, if they
were laid side by side. The pigment in
the red blood cell is hemoglobin. This
protein substance gives blood its red
color and is essential to life.

The red blood cells are produced in
the red marrow of bones. During their
development they have nuclei, as do all
other cells. Normally, by the time they
are ready to be released into the blood
stream, they have lost these nuclei.
The average life span of a red corpuscle
is 20 to 120 days. Worn-out red cells
are filtered out of the blood in the
spleen and liver. At the same time cer¬
tain valuable compounds are released
into the blood stream and used in the
manufacture of new red blood cells.
What do the red blood cells do? The
hemoglobin within the cell membrane
of the red blood cell is a complex pro¬
tein containing iron. This element
gives hemoglobin the ability to carry
oxygen. Perhaps you have seen an iron
nail turn red with rust. It has oxidized,
which means that it has combined with
the oxygen in the air. The iron of he¬
moglobin combines with oxygen in the
lungs. But there is an important dif¬
ference. The iron of the rusty nail does
not easily give up its oxygen. The iron
in hemoglobin, however, gives up its
oxygen at the proper time and place in
the body. In this form the hemoglobin
is a bright red color and is called oxy¬
hemoglobin. Oxygen is carried from
the lungs to the tissues in this form. In
the tissues the pigment gives up its oxy¬
gen. The carbon dioxide formed in the
tissues now combines with the hemo¬
globin, and the product is called carb-
hemoglobin. In this form much of the
carbon dioxide is carried to the lungs
where it is released, and the cycle is re¬
peated.

CHAPTER 42 TRANSPORT AND EXCRETION 583

The white blood cells. White corpus¬
cles are on the average larger than red
blood cells and differ from them in three
ways: 1. White corpuscles have nuclei.
2. White corpuscles do not contain
hemoglobin and are therefore nearly
colorless. 3. Some white corpuscles can
move much like the ameba. The white
blood cells are less numerous than the
red cells, the ratio being about one
white cell to every 600 red cells. White
corpuscles are formed in the red bone
marrow and in the lymph glands. Nor¬
mally there are about 8,000 white cor¬
puscles in one cubic millimeter of blood
as against four and a half to five million
red cells.

The white blood cells that can
move about are able to ooze through
the capillary walls into the tissue spaces.
Here they engulf solid materials, in¬
cluding bacteria, and thus are an im¬
portant defense of the body against in¬
fection. Whenever an infection devel¬
ops in the tissues, the white-cell count
may go from 8,000 to more than 25,000
per cubic millimeter. White corpuscles
collect in the area of an infection and
destroy bacteria. The remains of dead
bacteria, white corpuscles, and tissue
fluid is pus.

SUMMARY OF COMPOSITION
OF BLOOD

Plasma

Solid Components

Water

Red corpuscles

Proteins

White corpuscles

Fibrinogen

Serum albumin,

Platelets

globulin

Digested foods
Mineral salts

Organic nutrients
Cell wastes

Platelets are irregularly shaped,
colorless bodies, much smaller than the
red corpuscles. They are probably
formed in the red bone marrow. Plate¬
lets are not capable of moving on their
own but float along in the blood stream.
They have an important function in the
formation of a blood clot.

Blood is the transporting medium
for all body substances. Its functions
are shown in the table on page 586.
How blood clots. When you cut small
blood vessels in a minor wound, blood
oozes out. Such an injury is not alarm¬
ing because a clot will soon form and
the blood flow will stop. You probably
take this for granted, without consider¬
ing what would happen if the flow did
not stop.

Clotting results from chemical and
physical changes in the blood. When
blood leaves a vessel, the platelets dis¬
integrate and release thromboplastin.
This substance reacts with prothrombin
and with calcium to form thrombin.

42-2 This diagram shows the microscopic
changes which occur during the clotting of
blood. A: before clotting begins; B: forma¬
tion of threads of fibrin; C: shortening of the
fibrin threads and trapping of the blood cells.

584 UNIT 7 THE BIOLOGY OF MAN

Description of the Anatomical Transparencies

Plate 1 This view shows the skin and muscle walls of the front of the body removed
from the figure on the opposite page. You are looking at the inner side of the front
half of the chest and abdominal cavities. The muscles in the neck, including the
sterno-mastoid (1), which turns the head, and the pectoral muscles (2) of the chest,
have been sectioned with the ribs (3) and clavicles (4). You are seeing the inside of the
sternum (5) to which the rib cartilages (6) attach. The pleural membrane (7) lining the
chest cavity is intact on the left side of the figure and has been removed on the right
side to show the intercostal muscles (8) between the ribs. The diaphragm (9), a dome¬
shaped muscle, divides the chest and abdominal cavities. The abdominal cavity is
lined with a membrane, the peritoneum (10), which is shown on the left side. It has
been removed on the right side to expose the transverse abdominal muscle (11) and the
vertical rectus muscle (12), which is partially enclosed in tendonous sheaths (13) pierced

by the umbilicus (14).

Plate 2. In this view, most of the skin has been removed from the head to show the
temporal (15) and masseter (16) chewing muscles attached to the skull and the large
parotid salivary gland (17) with its duct. Around the mouth are the muscles of facial
expression. The neck muscles have been removed to expose the thyroid cartilage (18)
surrounding the larynx, the thyroid gland (19), the carotid artery, and the internal
jugular vein. The numerous veins of the arms and legs lying close to the skin are shown
in this view. With the front portion of the pleural membrane (7) removed, the lobes
of the lungs (20) and the pericardium (21) covering the heart are visible. Below the
diaphragm (9), in the abdominal cavity, are the organs of digestion. On the right
side are the liver (22) and gall bladder (23). The stomach (24) is on the left side. The
ascending, transverse, and descending regions of the colon (25) surround loops of the
small intestine (26).

Plate 3. In this view, the head is sectioned along the midline and you are looking
toward the inside of the right half of the cranial cavity (29) and the spinal canal (30)
with its dural lining intact. On the side wall of the nasal cavity (31) are the three
nasal turbinates. Two sectioned nasal sinuses are above the nasal cavity. The sectioned
tongue (32) reveals its muscular structure. The pharynx (33) leads to the esophagus
and the larynx at the upper end of the trachea. At the top of the larynx (34) are the
vocal folds and epiglottis. The lungs (20) are sectioned. The pericardium (21) has been
lifted off the heart and you are seeing its inner surface. The superior vena cava (35)
has been cut above the heart. The back view of the liver (22) shows the cut ends of the
hepatic veins above and the end of the portal vein entering the liver from below. The
gall bladder (23) and bile ducts are clearly shown. The stomach (24), a portion of
the transverse colon (25), and the small intestine (26) with its blood supply and anchor¬
ing root of the mesentery (36) are viewed from the back. The muscles shown are portions
of the deltoid (27) of the shoulder and the sartorius (28) and a side muscle of the thigh.
These are removed to expose deeper views on Plate 6.

Plate 4. This view shows the brain. You are seeing the outer surface of the right
hemisphere of the cerebrum (37) and the cerebellum (38). The pons, medulla, spinal

( Continued on page following Anatomical Transparencies )

THE HUMAN BODY

IN ANATOMICAL TRANSPARENCIES

By GLADYS McHUGH, Medical Artist

Associated with the University of Chicago Clinics

The “Trans-Vision” process presents the human body in a unique manner
in which you can perform a “dissection” and proceed through the
depths of its structures by turning transparent pages. You can see organs
overlying other organs in a three-dimensional effect. As you turn the
pages, a layer of anatomy is removed and a deeper layer comes into
full view. The right pages give you a front view of the structures. To see
the same structures from the back side, you turn the page. Thus, you can
see the relation of organs to the body as a whole and to each other.
You can single out an individual part for more detailed observation.

The pages preceding and following the anatomical transparencies
give you a description of each view— how it was made and what it
shows. The numbers you find on many of the structures refer to an
identification key on the back of Plate 6. This will identify any structure
you wish quickly and easily. Numbers have been omitted where they
would detract from structural detail. Many such structures are referred
to in the description of the various plates.

The structures shown are detailed and accurate. This presentation of
the human body will serve as an adequate basis for anatomical study in
any degree of thoroughness and complexity you may desire.

Key identifying numbered structures on back of Plate 6.

Key to the Structures of the Human Body

1. Sterno-mastoid muscle, used in
turning the head

2. Pectoral muscle, used in moving
the arm across the chest

3. Rib

4. Clavicle

5. Sternum

6. Rib cartilage

7. Pleural membrane

8. Intercostal muscles, used in
breathing

9. Diaphragm

10. Peritoneum

11. Transverse abdominal muscle,
supporting the abdominal wall

12. Rectus abdominal muscle, used in
flexing the trunk

13. Rectus sheath

14. Umbilicus

15. Temporal muscle, used in chewing

16. Masseter muscle, used in chewing

17. Parotid salivary gland

18. Thyroid cartilage

19. Thyroid gland

20. Lung

21. Pericardium

22. Liver

23. Gall bladder

24. Stomach

25. Colon

26. Small intestine

27. Deltoid muscle, used in raising
the arm

28. Sartorius muscle, used in crossing
the leg

29. Cranial cavity

30. Spinal canal

31. Nasal cavity

32. Tongue

33. Pharynx

34. Larynx

35. Superior vena cava

36. Root of the mesentery

37. Cerebrum

38. Cerebellum

39. Nasal septum

40. Trachea

41. Heart

42. Aorta

43. Pulmonary artery
44.. Inferior vena cava

45. Esophagus

46. Duodenum

47. Spleen

48. Pancreas

49. Urinary bladder

50. Cerebral septum

51. Biceps muscle, used in flexing
the arm

52. Flexor muscles of front of thigh

53. Brachial plexus

54. Kidney

55. Ureter

56. Renal artery

57. Renal vein

58. Adrenal gland

59. Iliac artery

60. Inguinal ligament

61. Femoral artery

62. Crest of hip bones

63. Lumbo-sacral joint

64. Pubis

65. Rectum

66. Femur

67. Humerus

68. Brachial muscle, which works with
biceps as a flexor of the arm

69. Extensor muscles of hand

70. Flexor muscles of hand

71. Adductor muscles which bring legs
together

CHAPTER 42 TRANSPORT AND EXCRETION 585

cord, and spinal nerves are visible. The nasal septum (39) is the central partition of the
nasal cavity. The trachea (40) is an airway to the lungs, where it divides into the
bronchi. The sectioned lungs reveal the numerous branches of the bronchial tree
(white), the pulmonary arteries (blue), and the pulmonary veins (red). You are looking
at the heart (41) and its great vessels: the aorta (42), pulmonary artery (43), superior
vena cava (35), and inferior vena cava (44). The cut ends of the hepatic veins are seen
entering the inferior vena cava just below the diaphragm. In removing the stomach,
the lower end of the esophagus (45) and the beginning of the duodenum (46) were cut.
The spleen (47) and pancreas (48), along the back wall of the abdominal cavity, are
visible. After removal of the transverse colon and the small intestine, the peritoneum
(10), lining the back of the abdominal cavity and carrying blood vessels to the colon,
can be seen. The lower end of the small intestine (26) is sectioned close to its junction
with the colon. Just below this junction is the appendix, which extends from the caecum,
or beginning portion of the colon (25). The lower end or sigmoid portion passes behind
the urinary bladder (49).

Plate 5. In this view, the hemispheres of the cerebrum are divided along the midline.
You are seeing the cerebral septum (50), which divides the hemispheres. Below the
cerebrum you see the sectioned interbrain, pons, medulla, cerebellum (38), and spinal
cord. You are viewing the lungs (20) from the back with sections removed to show the
bronchial divisions of the trachea (40). With the posterior peritoneum removed, the
spleen (47), pancreas (48), and duodenum (46) are visible from the back. The portal
vein, which receives blood from these organs and the intestinal tract, is seen entering
the liver close to the bile duct. A portion of the pancreas has been dissected to show

the pancreatic duct, which enters the duodenum beside the common bile duct. The

biceps muscle (51) and three thigh muscles (52) are removed and seen from the back.

Plate 6. In this view, the cerebral septum is removed and you are seeing the inner
side of the left hemisphere of the cerebrum (37) with its surface blood supply. Notice
the spinal nerves emerging from between the vertebrae of the neck and forming the
brachial plexus (53), the nerve supply to the shoulders and arms . The arteries supplying
these regions follow the same course as the nerves. With the lungs and trachea re¬
moved, you can see the full length of the esophagus (45) and its relation to the
aorta (42). The inferior vena cava (44) is cut at the entrance to the heart, just above
the junction with the hepatic veins. The kidneys (54), ureters (55), and bladder (49) form

the urinary tract. A renal artery (56) enters each kidney from the aorta (42). The

renal veins (57) drain into the inferior vena cava (44). On top of the kidneys are the
adrenal glands (58). In the lower abdominal region, the aorta divides into the iliac
arteries (59), which divide further into internal and external branches. The internal
iliac arteries supply the pelvic organs. The external iliacs encircle the pelvic region,
pass under the inguinal ligaments (60) in company with the femoral veins and nerves,
and emerge as the femoral arteries (61). Portions of the bone structure of the pelvis
are visible as the crests of the hip bones (62), the joint between the sacrum and spine
(63), and the pubis (64). The upper end of the rectum (65) can be seen behind the
bladder. The shoulder and hip joints are partially exposed on the left side, showing
bone and muscle structures not visible on the right side. Additional muscles and bones
of the arms and legs are identified in the Key to the Structures of the Human Body.

586 UNIT 7 THE BIOLOGY OF MAN

BLOOD AS A TRANSPORTING MEDIUM

Transporta¬
tion of

From

For the Purpose of

Digested

food

Digestive
organs and
liver

Tissues

Growth and repair of cells, supplying
energy, and regulating life processes

Cell

wastes

Active

tissues

Lungs, kid¬
neys, and
skin

Excretion

Water

Digestive

organs

Kidneys,
skin, and
lungs

Excretion and equalization of body
fluids

Oxygen

Lungs

Tissues

Oxidation

Heat

Tissues

Skin

Equalization of the body temperature

Secretions

Ductless

glands

Various

organs,

glands

Regulation of body activities

The thrombin changes fibrinogen, a
blood protein, to fibrin. The fibrin is a
network of tiny threads that trap blood
cells, thus forming a clot, and prevent
further escape of blood (Fig. 42-2).
These trapped corpuscles dry out and
form a scab. Healing takes place as the
edges of the wound grow toward the
center. If any of the substances men¬
tioned above is not present, clotting will
not occur. Clotting is summarized in
the table below.

If blood vessels are broken under
the skin in a bruise, a black-and-blue
spot may appear, because clotting oc¬
curs under the skin. Gradually the
clotted blood is absorbed and the color
of the bruise changes and finally dis¬
appears.

THE CLOTTING OF BLOOD

Thromboplastin -f prothrombin

-f calcium = thrombin
Thrombin + fibrinogen = fibrin

Blood transfusions have saved many
lives. Conditions like hemorrhage,
wound shock, severe burns, and various
other illnesses may require blood trans¬
fusions. If whole blood is used, the
patient receives both the necessary plas¬
ma and blood cells. However, the blood
of the donor must be typed and matched
with that of the patient. The matching
is done by adding a drop of a test
serum to a drop of the donor’s blood. If
the red cells agglutinate , or clump to¬
gether, the bloods are incompatible (Fig.
42-3). If, however, the red cells remain
in suspension, the samples are compat¬
ible, and it is possible to perform the
transfusion. Blood types are designated
A, B, AB, and O. You can see why
using the wrong type might result in
serious blood reactions, clotting, and
death of the patient.

Often the patient needs an imme¬
diate increase in the volume of liquid
in the blood stream and does not re¬
quire additional blood cells. This con-

CHAPTER 42 TRANSPORT AND EXCRETION 587

42-3 This diagram of a blood smear shows
the agglutination reaction that occurs when
incompatible types of blood are mixed. When
two compatible types are mixed there is no
such reaction.

dition is called shock. The red cor¬
puscles form rapidly if the blood volume
is maintained. At such times plasma
may be transferred in preference to
whole blood. Typing is not necessary
when plasma is used for transfusion be¬
cause of the absence of the cells.

The Rh factor in blood. A factor in
blood independent of the A, B, AB, and
O blood groups is called the Rh factor ,
after the Rhesus (ree- sus) monkey in
which it was discovered. About 85 per¬
cent of the people in the United States
have this factor in their blood and are
designated as Rh positive. The other
15 percent are Rh negative. The Rh
factor, like the other blood types, is in¬
herited, and is actually any one of six
protein substances called antigens.

If a patient is Rh negative and re¬
ceives Rh-positive blood in a transfu¬
sion, he produces an antibody against
the factor. This particular antibody
causes the corpuscles of the Rh-positive
blood to agglutinate and to dissolve.

There is little danger during the first
transfusion because the antibody is not
present when the Rh-positive blood is
added. But the patient now builds
antibodies against this Rh-positive
blood, so that a second transfusion may
result in serious or even fatal complica¬
tions.

How the Rh factor may affect child¬
birth. Complications from the Rh fac¬
tor occur with childbearing in about
one in three or four hundred mothers.
When the mother is Rh negative and
the father is Rh positive, the child may
inherit the Rh-positive factor from the
father. During development, blood
from the child, containing the factor,
may seep into the mother’s circulation
through tiny ruptures in the membranes
that normally separate the two circula¬
tions. Blood from the mother seeps into
the child through the same channels.

Since such seepage is uncommon,
many Rh-negative mothers bear normal
Rh-positive children. However, if seep¬
age occurs again with a second Rh-posi¬
tive child, the antibody in the mother’s
blood, produced in the first pregnancy,
enters the child’s circulation and causes
serious damage. Occasionally the child
dies before birth. But if the damage to
the child is not too extensive, an imme¬
diate transfusion after birth may save
its life. Sometimes the child’s blood is
almost entirely removed and replaced
by transfused blood. Blood used in
such a complete transfusion is Rh nega¬
tive but does not contain the antibodv.

In other words the donor has never re¬
ceived positive blood and his blood is
not sensitized against the factor.

The structure of the heart. The heart
is a cone-shaped muscular organ situated
under the breastbone and between the
lungs (Fig. 42-4). It is enclosed in a
sac called the pericardium (paii-i-kahrd-

588 UNIT 7 THE BIOLOGY OF MAN

ee-um) . It usually lies a little to the left
of the midline of the chest cavity, with
its point extending downward and to
the left between the fifth and sixth ribs.
Since the beat is strongest near the tip,
many people have the mistaken idea
that the entire heart is on the left side.

The heart is composed of two sides,
right and left. The two halves are en¬
tirely separated by a wall called the
septum. Each half is composed of two
chambers, a relatively thin-walled
atrium , and a thick, muscular ventricle.
The two atria act as reservoirs for the
blood entering the heart. Both contract
at the same time, filling the two ven¬

tricles rapidly. Next the thick muscular
walls of the ventricles contract, forcing
the blood out through the great arteries.

Flow of blood from the ventricles
under pressure and maintenance of pres¬
sure in the arteries between beats require
two sets of one-way heart valves. The
valves between the atria and ventricles,
called the atrioventricular valves, or a-v
valves , are flaplike structures which are
anchored to the floor of the ventricles bv

tendonlike strands (Fig. 42-5). Blood
passes through these valves freely into
the ventricles. The valves cannot be
opened from the lower side, however,
because of the tendons anchoring them.

42-4 This X-ray photograph shows a normal heart and lungs as seen from the
front. Notice that the heart points toward the patient’s left side. The curved
area below the heart is the diaphragm. (Indiana University Medical Center)

CHAPTER 42 TRANSPORT AND EXCRETION 589

Opening to left coronary artery

Semilunar valves of the aorta
Superior vena cava

Right pulmonary
arteries

Right pulmonary
veins

Right atrium

Opening from
coronary vein

Right A-V valve

Inferior
vena cava

Opening from pulmonary veins

Aorta

ulmonary
arteries

Left
monary
veins

Left atrium
Left A-V valve

Opening
into aorta

Right ventricle-
Semilunar valves of the pulmonary artery

Left ventricle

42-5 Notice the location of the valves in relation to the heart chambers and
blood vessels. The arrows indicate the direction of blood flow.

Thus blood is unable to flow backward
into the atria during contraction of the
ventricles. Other valves, called the semi¬
lunar valves , or s-l valves , are located at
the openings of the arteries. These cup¬
like valves are opened by the force of
blood passing from the ventricles into
the arteries, and they prevent blood from
returning to the ventricles.

Circulation of blood through the heart.
Blood first enters the right atrium of the
heart by way of the superior vena cava
(vee-na kahv- a), and the inferior vena
cava. The superior vena cava carries
blood from the head and upper parts of
the body. The inferior vena cava

returns blood from the lower body re¬
gions. From the right atrium blood
then passes through the right a-v valve
into the right ventricle. When the
right ventricle contracts, blood is forced
through a set of s-l valves into the pul¬
monary artery , which carries the blood
to the lungs. After the blood has passed
through the lungs, it is returned to the
heart through the right and left pul¬
monary veins. These vessels open into
the left atrium, from which the blood
passes through the left a-v valve into the
left ventricle. From here blood passes
out the aorta (ay-ort-a), and is distrib¬
uted to all parts of the body.

590 UNIT 7 THE BIOLOGY OF MAN

Although the heart is an organ
filled with blood, its muscle layers are
too thick to be nourished by this blood.
The cells receive special nourishment
through arteries called coronary arteries.
There is an enlargement of the aorta at
the point where it leaves the heart. This
is called the aortic sinus. From here,
the right and left coronary arteries
branch off. These arteries curve down¬
ward around each side of the heart,
sending off smaller vessels that penetrate
the heart muscle.

The heart, a highly efficient pump. A

complete cycle of heart activity, or beat,
consists of two phases. During one part
of the cvcle, called systole , the ventricles
contract and force blood into the arter¬
ies. During the other part, called dias¬
tole. , the ventricles relax and receive
blood from the atria.

The sounds vou hear in a stetho-

scope when you listen to a normal heart

sound like the syllables “lub” and “dup”
repeated over and over in perfect
rhythm. The “lub” is the sound of the
contraction of the muscles of the ven¬
tricles and the closing of the a-v valves
during systole. The “dup” is the
closing of the semilunar valves at the
base of the arteries during diastole.

At rest the heart of an average
man beats about 70 times per minute.
During strenuous work or exercise the
heart rate may be as high as 180 beats
per minute.

With the body at rest the heart
pumps about 10 Vi pints of blood per
minute. Your body contains 12 pints of
blood, so that all the blood makes a
complete circulation through the body
in slightly over a minute. However,
mild exercise such as walking speeds
the heart output to about 20 pints per
minute, and strenuous exercise mav in-

7 J

crease it to as much as 42 pints per

Serous membrane
Smooth muscle tissue,
Connective tissue,

•Whitens

corpuscle

Serous membrane cell

Red corpuscles

Serous membrane

Smooth muscle

ARTERY

Connective tissue

CAPILLARY

42-6 Notice the difference in structure in these three types of blood vessels.
In the drawing of the capillary, the white corpuscle is squeezing through the
wall between cells and will then pass into the tissue spaces.

CHAPTER 42 TRANSPORT AND EXCRETION 591

minute. If you were to use a hand
pump to move blood, you could not
possibly keep up with a heart under
exertion. This will give you some idea
of the efficiency of this organ which
weighs only a quarter of a pound and
works every minute of every day and
night of your life.

The blood vessels. Blood moves in a
system of tubes of varying sizes (Fig.
42-6 ) , which have been classified as
follows: arteries and arteriolesy vessels
carrying blood away from the heart;
capillaries , very small, thin-walled ves¬
sels; and reins and venules , vessels car¬
rying blood back toward the heart.

The aorta branches into several
large arteries. These arteries branch
and become arterioles. As the arterioles
branch, they soon become capillaries,
which are the smallest vessels in the
body. After passing through a tissue or
organ, the capillaries come together to
form venules. As the venules join, they
become veins, which take the blood to¬
ward the inferior or superior vena cava
and into the right atrium of the heart.

Arteries have elastic, muscular walls
and smooth linings. Because of their
elasticity, arteries can expand and absorb
part of the great pressure resulting from
contraction of the ventricles at systole.
The pressure in the aorta leading from
the left ventricle is greater than that in
the pulmonary artery pumped by the
smaller right ventricle. If the aorta
were cut, blood would spurt out in a
stream of six feet or more. When the
ventricles contract, arterial pressure is
greatest and is called systolic pressure.
The elasticity of the artery walls main¬
tains part of this pressure while the ven¬
tricles are at rest. This is the time of
lowest pressure in the arteries, or dias¬
tolic pressure. The bulge in an artery
wall caused by systolic pressure can be

felt in the wrist or any part of the body
where an artery is near the surface. This
pulse has the same rhythm as do the
heart beats.

What are capillaries? As arterioles
penetrate the tissues, they branch into
capillaries (Fig. 42-6). Capillaries dif¬
fer from arterioles in that their walls are
only one cell layer thick. Capillaries
are only slightly greater in diameter
than the red blood cells. Red corpus¬
cles must pass through them in single
file and may even be pressed out of
shape by the capillary walls.

Dissolved foods, waste products,
and gases pass freely through the thin
walls of capillaries and in and out of
the tissue spaces. Tiny openings in the
walls are penetrated by white corpus¬
cles as they leave the blood stream and
enter the tissue spaces. Also in the
capillaries, part of the plasma diffuses
from the blood and becomes tissue fluid.
Thus, all the vital relationships between
the blood and the tissues occur in the
capillaries and not in arteries and veins.
Vein structure and function. On leav¬
ing an organ, capillaries unite to form
veins. Veins carry dark-red blood; that
is, blood containing less oxygen. In
the skin the veins have a bluish color
because the skin contains a yellow pig¬
ment that changes the appearance of
the dark-red blood. The walls of veins
are thinner and less muscular than those
of arteries, and their internal diameter
is proportionally larger. Many of the
larger ones are provided with cuplike
valves that prevent the backward flow
of blood.

Veins have no pulse wave, and the
blood pressure within them is much
lower than that of arteries. Blood pres¬
sure resulting from heart action is al¬
most completely lost as blood passes
through the capillaries. Blood from

592 UNIT 7 THE BIOLOGY OF MAN

Capillaries of
head and arms

Superior vena cava

Capillaries of
right lung

Right

pulmonary vein
Inferior vena cava

Liver

Hepatic
artery & vein

Portal vein

Right kidney
Renal artery and vein

Capillaries of legs and feet

Pulmonary artery

Capillaries of
left lung

Left

pulmonary vein

Abdominal aorta
Stomach

Gastric artery
Left kidney
Renal artery and vein

42-7 This drawing is a diagrammatic representation of the various circulations
in the human body. Identify pulmonary, systemic, coronary, renal, and portal
circulations.

the head may return to the heart with
the aid of gravity, but in the body re¬
gions below the level of the heart other
factors are required. Venous flow from
these regions is aided by the working
muscles and by the vacuum created in
the chest during inspiration.

Circulations in the body. A four-cham¬
bered heart, such as that in the human
body, is really a double pump in which
the two sides work in unison. Each side
pumps blood through a major division
of the circulatory system. The right
side of the heart receives dark, deoxy-

CHAPTER 42 TRANSPORT AND EXCRETION 593

genated blood from the body and pumps
it through the arteries of the pulmonary
circulation. The great pulmonary ar¬
tery, extending from the right ventricle,
sends a branch to each lung (see the
“Trans-Vision” between pages 584 and
585). These arteries in turn branch
within the lungs, forming a vast num¬
ber of arterioles. Here the blood dis¬
charges carbon dioxide and water and
receives oxygen. Oxygenated blood,
now bright scarlet in color, leaves the
lungs and returns to the left atrium of
the heart through the pulmonary veins.

Oxygenated blood passes through
the left chambers of the heart and out
the aorta under great pressure. The
blood is now in the systemic circulation ,
which supplies the body tissues. This
extensive circulation includes all of the
arteries that branch from the aorta, the
capillaries that penetrate the body tis¬
sues, and the vast number of veins that
lead to the venae cavae. The systemic
circulation also includes several shorter
circulations that supply or drain special
organs of the body.

The coronary circulation , referred
to in the discussion of the heart muscle,
supplies the heart itself. This short but
vital circulation begins at the aorta and
ends where the coronary veins empty
into the right atrium. Every beat of
the heart depends on the free flow of
blood through the coronary vessels.

The renal circulation starts where a
renal artery branches to each kidney
from the aorta. It includes the capil¬
laries that penetrate the kidney tissue
and the renal veins , which return blood
from the kidneys to the inferior vena
cava. Blood on this route nourishes the
kidneys and discharges water, salts, and
nitrogenous cell wastes. Thus, even
though it is low in oxygen content,
blood in the renal veins is the purest.

The portal circulation includes an
extensive system of veins that lead from
the spleen, stomach, pancreas, small in¬
testine, and colon. The large veins of
the portal circulation unite to form the
portal vein, which enters the liver.
Blood flowing from the digestive organs
transports digested food and water.
Blood laden with food for the body tis¬
sues flows from the liver in the hepatic
veins, which in turn empty into the in¬
ferior vena cava, thus ending this vital
branch of the systemic circulation.
Return of tissue fluid to the circulation.
The tissue fluid that bathes the cells is
collected in tubes and is then called
lymph (limf ) . These tiny lymph vessels
join one another and become larger
lymph vessels in the same way that
capillaries join to form venules. Lymph
nodes, which are enlargements in the
lymph vessels, are located along the ves¬
sels much like beads on a string. In
these lymph nodes, the lymph tubes
break up into many fine vessels once
again. Here certain white corpuscles
collect and destroy bacteria that may be
in the lymph. The lymph glands, then,
act to strain, or to purify, the lymph be¬
fore returning it to the blood. The
greatest concentrations of these lymph
nodes are in the neck, the armpit, the
bend of the arm, and the groin. Often
when there is an infection in the hand
or arm, the lymph nodes of the armpit
swell and become painful. Both the
tonsils and adenoids in the throat are
merely masses of lymphatic tissue that
often become inflamed during child¬
hood and have to be surgically removed.

The lymph of the right side of the
head, neck, and right arm enters into a
larger vessel named the right lymphatic
duct. This vessel returns the lymph to
the blood by opening into the right sub¬
clavian vein. The lymphatics from the

594 UNIT 7 THE BIOLOGY OF MAN

42-8 The lymphatics course throughout the
entire body. At what point does the lym¬
phatic system join the bloodstream?

rest of the body drain into the thoracic
duct , which in turn empties into the
left subclavian vein (Fig. 42-8).

In the walls of the larger lymph ves¬
sels are valves to control the flow of
lymph. These valves are similar in
structure to those found in the veins.
In inactive tissues lymph flows very
slowly or is completely stagnant. When
activity increases, the fluid flows faster.
The return of lymph to the blood stream
is aided by the contracting movements
of many of our body muscles.

Removal of wastes from the circulating
fluids. The oxidation of foods involved
in metabolism produces waste products
that must be given off by the body in the
process called excretion. In protein
metabolism, waste products result from
the separation of the carbon and nitro¬
gen parts of amino acids before oxida¬

tion of the carbon part. Other waste
products result from the synthesis of
proteins from amino acids during growth
processes. These nonprotein nitroge¬
nous wastes include urea and uric acid.

Any great accumulation of wastes
in the tissues, especially nonprotein ni¬
trogens, causes rapid tissue poisoning,
starvation, and eventually suffocation.
Tissues filled with waste products can¬
not absorb either food or oxygen.
Fever, convulsions, coma, and death are
inevitable if nonprotein wastes do not
leave the tissues.

Further complications arise if min¬
eral acids and salts accumulate in the
body because of excretory failure. This
accumulation disturbs certain delicate
acid-base balances in the body and also
upsets the osmotic relationships be¬
tween blood and lymph and the tissues.
When excess salts are held in the tis¬
sues, water accumulates and causes
swelling.

One-celled organisms and animals
like the sponge and jellyfish discharge
their cell wastes directly into a water
environment. However, when many
millions of cells form an organism, as
in higher animals, the removal of cell
waste products becomes a complicated
process involving many organs. Each
cell discharges its waste materials into
the tissue fluid, which in turn reaches
the blood stream. The blood trans¬
ports the cell wastes to excretory organs
such as the kidneys, lungs, and skin for
elimination.

The kidneys — the principal excretory
organs. The kidneys are bean-shaped
organs, about the size of your clenched
fist. They lie on either side of the spine,
in the “small” of the back. Deep layers
of fat around them form a protective
covering (see the “Trans-Vision” be¬
tween pages 584 and 585). If you cut a

CHAPTER 42 TRANSPORT AND EXCRETION 595

kidney lengthwise (Fig. 42-9), you can
see several different regions. The firm,
outer region which composes about one
third of the kidney tissue is called the
cortex. The inner two thirds, or me¬
dulla, contains conical projections called
pyramids. The points of the pyramids
extend into a saclike cavity, the pelvis
of the kidney. The pelvis in turn leads
into a long, narrow tube (one for each
kidney) called the ureter (yoo-reet-er) .
The two ureters empty into the urinary
bladder.

Each kidney contains about 1,250,-
000 tiny filters called nephrons. The
function of these nephrons is to control
the chemical composition of blood.
Each nephron consists of a small, cup¬
shaped structure called a Bowman’s cap-

42-10 This diagram shows the close rela¬
tionship of nephron and the blood vessels by
which materials are reabsorbed into the
blood.

42-9 In the drawing at the upper left the
kidney has been cut away to show its inter¬
nal structures. The cortex contains millions
of glomeruli.

sule (Fig. 42-10). A tiny, winding
tubule comes from each capsule. This
tubule becomes very narrow as it
straightens out and goes toward the
renal pelvis. This tubule widens out
again into a loop called Henle’s loop
and goes back into the cortex. Once
this tubule has passed back into the
cortex, it becomes very crooked again
and then enters a larger straight tube
called the collecting tubule. The col¬
lecting tubule is a straight tube which
receives the tubules of many nephrons.
It carries fluid to the renal pelvis. If
all these tubules were straightened out
and put end to end, they would extend
over 200 miles.

How does the nephron function?

Blood enters each kidney through a
large renal artery , which branches di¬
rectly from the aorta. It is the largest
artery in the body in proportion to the
size of the organ it supplies. In the
kidney the renal artery branches and
rebranches to form a maze of tiny arte-

596 UNIT 7 THE BIOLOGY OF MAN

rioles, which penetrate all areas of the
cortex.

Each arteriole ends in a coiled,
knoblike mass of capillaries, the glomer¬
ulus (glah-mer-[y]e-lus) . Each glomer¬
ulus fills the cuplike depression of the
Bowman’s capsule. In the first stage
of removal of waste from the blood, far
too much of the blood content leaves
the blood stream and enters the Bow¬
man’s capsule. However, this is soon
corrected in a second stage, in which
valuable substances return to the blood.
The first stage takes place in the coiled
capillaries of the glomeruli. Here water,
nitrogenous wastes, glucose, and mineral
salts pass through the walls of the capil¬
laries and into the surrounding capsule.
This solution resembles blood plasma
without blood proteins. Complete loss
of this much water, glucose, and min¬
erals would be fatal. However, after this
fluid leaves the capsule through the
tubules, it passes a network of capillar¬
ies. Here, many of the substances are
reabsorbed into the blood. Only nitrog¬
enous wastes, excess water, and excess
mineral salts pass through the tubules to
the pelvis of the kidney as urine.

Some recent studies of kidney func¬
tion indicate that, for every 100 cc of
fluid that pass from the blood in the
glomeruli into the capsules, 99 cc are
reabsorbed. The urine passes from the
pelvis of each kidney through the ureters
to the urinary bladder. Blood leaves
the kidneys through the renal veins and
returns to the general circulation by way
of the inferior vena cava. As was said
above, the blood in these veins, while it
is deoxygenated, is the purest blood in
the body.

The formation of urine by the kid¬
ney is a constant filtration process. After
urine leaves the kidneys through the
ureters, it collects in the muscular uri¬

nary bladder. Contraction of the uri¬
nary bladder at intervals expels the urine
through the urethra (yoo-ree-thra) . The
two kidneys have tremendous reserve
power. When one is removed, its mate
enlarges to twice its normal size and
assumes the normal function of two
kidneys.

The skin — a supplementary excretory
organ. The skin helps the kidneys in
the excretion of water, salts, and some
urea, in the form of perspiration. This
fluid is, however, much more important
in regulating body temperature than it
is as an excretory substance.

Skin consists of an outer portion,
or epidermis , composed of many layers
of epithelial cells (Fig. 42-11). The
outer cells, or horny layer , are flattened,
dead, and scalelike. The inner ones, or
germinative layery are more active and
larger. The epidermis serves largely for
protection of the active tissues beneath
it. It is rubbed off constantly, but ac¬
tive cells in the lower layers replace cells
as fast as they are lost. Friction and
pressure on the epidermis stimulate cell
division and may produce a callus more
than a hundred cells thick. Hair and
nails are special outgrowths of the epi¬
dermis. The dermis lies under the epi¬
dermis. It is a thick, active layer, com¬
posed of tough, fibrous connective tis¬
sue, richly supplied with blood and
lymph vessels, nerves, sweat glands, and
oil glands.

The functions of the skin. The varied
functions of the skin include the fol¬
lowing:

1. Protection of the body from mechan¬
ical injury and bacterial invasion.

2. Protection of the inner tissues against
drying out. The skin, aided by oil
glands, is nearly waterproof. Little
water passes through it, except out
through the pores.

CHAPTER 42 TRANSPORT AND EXCRETION 597

Hair shaft

Nerve ending (pain receptor)

Epidermis

Perspiration pore

Dermis

Capillaries
Muscle

Oil gland

Nerve ending
(pressure
receptor)

Sweat gland

Fatty
layer
(subcutaneous)

Connective tissue

Fat cells

Blood vessels

42-11 A thin section of skin shown highly magnified.

3. Location of the nerve endings that
respond to touch, pressure, pain, and
temperature changes.

4. Excretion of wastes present in sweat.

5. Control of the loss of body heat
through the evaporation of sweat.

This last statement needs further
explanation. In an earlier discussion
about water and its uses, we mentioned
that heat is used during the change of
liquid water to water vapor. Thus as
sweat evaporates from the body surface,
heat is withdrawn from the outer tis¬
sues. The skin is literally an automatic
radiator. It is richly supplied with
blood containing body heat withdrawn
from the tissues. As the body tempera¬
ture rises, the skin becomes more flushed
with blood, and heat is conducted to the

surface. At the same time, secretion
of sweat increases and bathes the skin.
This increases the rate of evaporation
and the amount of heat loss.

Other organs of excretion. During ex¬
piration the lungs excrete carbon diox¬
ide and considerable water vapor. The
excretory function of the liver in form¬
ing urea has been discussed earlier. The
bile stored in the gallbladder is also
a waste-containing substance.

The large intestine removes undi¬
gested food. This, however, is not cell
excretion in the strict sense, since the
food refuse collected there has never
actually been absorbed into the tissues.
As you will recall, excretion was defined
as the giving off of waste products re¬
sulting from metabolism.

598 UNIT 7 THE BIOLOGY OF MAN

IN CONCLUSION

The circulator}' system is the transportation system of the body and its vehicle
is blood. Blood is a fluid tissue, composed of plasma and solid components.
Plasma contains water, blood proteins, prothrombin, inorganic substances,
digested foods, and cell wastes. The solid parts of the blood are of three types:
red corpuscles, white corpuscles, and platelets. Red corpuscles are essential in
carrying oxygen to the body cells and carrying carbon dioxide away as a waste
product. White corpuscles aid in fighting disease bacteria. Platelets are an
essential factor in the process of blood clotting.

The heart is a pump that forces blood through the arteries to all parts of
the body. It consists of two atria, which receive blood from the veins, and two
ventricles, which force blood through the arteries by contractions. Arteries
carry blood from the heart to the tissues, and veins return it. The arterial and
venous systems are connected by countless microscopic networks of capillaries.

Once part of the plasma has seeped into the tissue spaces, it is collected as
lymph and filtered by the lymph nodes. Then it is returned to the blood.

Various wastes that result from metabolism are removed from the body
through actions of the kidneys, skin, lungs, and liver. The kidneys, the most
vital organs of excretion, serve as blood filters. They are responsible for the
removal of practically all the nitrogenous wastes resulting from protein me¬
tabolism, excess water, and mineral acids and salts. Skin also has a role in
excretion and in eliminating heat during evaporation of sweat.

In the next chapter you will learn about respiration, a process that releases
energy for the body’s use.

BIOLOGICALLY SPEAKING

a-v valves

agglutination

aorta

aortic sinus

arteriole

artery

atrium

blood

Bowman’s capsule

capillary

circulation

coronary circulation

cortex

dermis

diastole

epidermis

excretion

germinative layer

glomerulus

hemoglobin

horny layer

inferior vena cava'

lymph

medulla

nephron

nitrogenous wastes
pelvis

pericardium

plasma

platelets

portal circulation

pulmonary circulation

pulse

pyramids

red corpuscle

renal circulation

Rh factor
s-1 valves
septum
shock

solid components

superior vena cava

systemic circulation

systole

tissue fluid

tubule

ureter

urethra

urinary bladder

urine

vein

ventricle

venule

white corpuscles

CHAPTER 42 TRANSPORT AND EXCRETION 599

QUESTIONS FOR REVIEW

1. What materials are found in blood plasma?

2. What is the origin of the various blood cells?

3. What condition in the body does a high white-blood count usually indicate?

4. What are the various steps in the clotting of blood?

5. Why is plasma more quickly and easily used in a transfusion than whole
blood? Which method do you think is better and why?

6. Trace the path of a drop of blood from the right atrium to the aorta.

7. Name the major circulations which begin at the heart ventricles.

8. Why can you feel the pulse in an artery and not a vein?

9. What is tissue fluid? How does it get back to the blood stream?

10. How does lymph differ from blood?

11. In what way do the kidneys regulate blood content?

12. What are the differences in composition between the glomerular fluid and
the urine that finally leaves the kidneys?

APPLYING PRINCIPLES AND CONCEPTS

1. What is the basis for the saying: “A man is as young as his arteries”?

2. Alcohol dilates the arteries in the skin. What would be its effect, then, on
the temperature control of the body?

3. Why might a second transfusion with Rh-positive blood be fatal in an
Rh-negative patient, while the first transfusion with Rh-positive blood
caused no complications?

4. How does the manufacture of red blood cells demonstrate conservation of
resources by the body?

5. Why is increased salt intake recommended in hot weather?

6. What happens to water in the blood if it does not enter the glomerulus?
if it enters the glomerulus but is reabsorbed? if it enters the glomerulus but
is not reabsorbed?

CHAPTER 4 3

RESPIRA TION
AND ENERGY
EXCHA NGE

Respiration — a life process common to
all living things. Each living cell takes
in oxygen, uses it in the oxidation of
foods, and gives off carbon dioxide and
water. This vital process supplies the
cell with energy to carry on its life proc¬
esses. We may define respiration as
the intake of oxygen and elimination
of carbon dioxide associated with en¬
ergy release in living cells. In some of
the simple organisms such as the pro-
tists, sponges, and jellyfish, the cells are
in direct contact with the environment,
and an exchange of gases between the
cells and their surroundings occurs di¬
rectly. Plant cells also respire in this
way. In an insect air is delivered di¬
rectly to the tissues through the
tracheae. However, as animals become
more complicated in their structure, the
cells are deprived of direct contact with

the external environment. Some means
of receiving oxygen at one place and
carrying it to the body tissues becomes
necessary.

Two phases of respiration in man. Ex-
ternal respiration concerns the exchange
of gases between the atmosphere and
the blood. This phase involves the
lungs. Internal respiration concerns
the exchange between the blood and the
body tissues. It occurs in every living
cell. Breathing is merely a mechanical
process involved in getting air that con¬
tains oxygen into the body and air that
contains waste gases from respiration out
of the body.

We can divide the organs con¬
cerned with breathing and external res¬
piration into two groups. The first
group includes the passages through
which air travels in reaching the blood
stream: the nostrils, nasal passages,
pharynx, trachea, bronchi, bronchial
tubes, and lungs (Fig. 43-1). The sec¬
ond group is concerned with the me¬
chanics of breathing by changing the
size of the chest cavity. This group in¬
cludes the ribs and rib muscles, the
diaphragm, and the abdominal muscles.
The nose and nasal passages. Air enters
the nose through two streams, because
the nostrils are divided by the septum.
From the nostrils air enters the nasal
passages, which lie above the mouth
cavity. The nostrils contain hairs which
aid in filtering dirt out of the air. Other
foreign particles may lodge on the moist
mucous membranes in the nasal pas¬
sages. The length of the nasal passages
warms the air and adds moisture to it
before it enters the trachea. All these
advantages of nasal breathing are lost
in mouth breathing.

The trachea. From the nasal cavitv the
air passes through the pharynx (far- inks)
and enters the windpipe, or trachea.

600

CHAPTER 43 RESPIRATION AND ENERGY EXCHANGE 601

Epiglottis Larynx

Pharynx

Trachea

Esophagus

Pleural membrane

Right lung
(middle lobe)

Esophagus
Diaphragm
Abdominal cavity

Septum of nasal cavity
Mouth cavity
Tongue
Epiglottis
Larynx
Esophagus
Trachea

Pleural membrane
Left lung
Left bronchus
Bronchial tube

Bronchiole

Air sacs

43-1 The air passages increase in number and decrease in size as they enter
the lungs. They end in tiny membranous air sacs. The two enlargements show
details of the areas indicated.

The upper end of the trachea is pro¬
tected by a cartilaginous flap, the epi¬
glottis. During swallowing the end of
the trachea is closed by the epiglottis.
At other times the trachea remains open
to permit breathing. The larynx , or
“Adam’s apple,” is the enlarged upper
end of the trachea. Inside it are the
vocal cords. The walls of the trachea
are supported by horseshoe-shaped
rings of cartilage which hold it open for
the free passage of air. The trachea and
its branches are lined with cilia. These
are in constant motion, and carry dust
or dirt taken in with air upward toward
the mouth. This dust, mixed with mu¬
cus, is removed when you cough, sneeze,
or clear your throat.

The bronchi and air sacs. At its lower
end the trachea divides into two

branches called bronchi. One extends
to each lung and subdivides into count¬
less small bronchial tubes. These in
turn divide into many small tubes called
bronchioles , which end in air sacs.
These air sacs are composed of protru¬
sions called alveoli and compose most
of the lung tissue. The walls of the air
sacs are very thin and elastic. Through
their thin walls gases are exchanged be¬
tween the capillaries and the air sacs
(Fig. 43-2). Thus the lungs provide
enough surface to supply air by way of
the blood for the needs of millions of
body cells having no direct access to air.
The total area of the air sacs in the
lungs is about 2,000 square feet, or more
than 100 times the body’s surface area.

The lungs fill the body cavity from
the shoulders to the diaphragm, except

602 UNIT 7 THE BIOLOGY OF MAN

43-2 This diagram represents external res¬
piration and shows an enlargement of a
single alveolus. The solid oval structures
in the capillaries represent red blood cor¬
puscles.

for the space occupied by the heart,
trachea, esophagus, and blood vessels.
They are spongy and consist mainly of
the bronchioles and air sacs and an ex¬
tensive network of blood vessels and
capillaries, held together by connective
tissue. The lungs are covered by a
double pleural membrane. One part
adheres tightly to the lungs, and the
other covers the inside of the thoracic
cavity. These membranes secrete lubri¬
cating mucus so that there is free motion
of the lungs in the chest for breathing.
Gas exchange in the lungs. The pul¬
monary artery brings dark (deoxvgen-
ated) blood to the lungs. There it di¬
vides into an extensive network of capil¬
laries, completely surrounding each air
sac (Fig. 43-3). The thin moist walls
of both air sac and capillary aid the gas¬
eous exchange of oxygen from air to
blood and of carbon dioxide and water
from blood to air. The pulmonary
veins return to the heart the bright (ox¬
ygenated) blood for the tissues.

The concentration of oxygen in the
air sacs of the lungs is higher than that
in the lung capillaries. Hence oxygen
is absorbed by the lung capillaries where
it combines with the hemoglobin of
the red blood corpuscles. In the tissues,
where the concentration of oxygen is
low, hemoglobin releases its oxygen.
The affinity of hemoglobin for oxygen
decreases with increasing acidity. This
is an important characteristic, because
during violent exercise lactic acid is pro¬
duced by the active muscle cells. This
increases the acidity of the blood and
causes the hemoglobin to release more
of its oxygen than it would normally.

The movement of carbon dioxide
and water in the lungs is the opposite
of that of oxygen. Carbon dioxide and
water in the lung capillaries have a
higher concentration than in the con¬
necting air sacs. Hence they diffuse
outward into the area of lower concen¬
tration in the air sacs, and are exhaled.

Capillary net

43-3 Three clusters of alveoli. Refer to Fig.
43-2 for an enlargement of one alveolus.

CHAPTER 43 RESPIRATION AND ENERGY EXCHANGE 603

Trachea

Left lung

Left pulmonary veins

Right pulmonary veins

Left

pulmonary

artery

Left
bronchus

Left
atrium

Right lung

Right

pulmonary

artery

Right
bronchus

Right

atrium

Coronary arteries & veins

Inferior vena cava

43-4 In this back view of the lungs and heart, you can see the branches of the
pulmonary artery and the pulmonary veins. Which vessels are carrying oxygen¬
ated blood?

The mechanics of breathing. Many
people think that the lungs draw in air,
expand, and bulge the chest. Actually
this is the opposite of what happens.
The lungs contain no muscle, and can¬
not expand or contract of their own ac¬
cord. They are spongy, air-filled sacs,
anchored in the chest cavity. Breath¬
ing is accomplished by changes in size
and air pressure of the chest cavity.
This fact can be shown by substituting
apparatus for the body parts (Fig.
43-5).

A Y-tube (trachea and bronchi) is
inserted in a stopper and set in a bell
jar (chest). Balloons (lungs) are fas¬
tened to the Y-tube. A piece of rubber

sheet (diaphragm) is fastened securely
to the open base of the bell jar. When
you pull the rubber sheet downward,
you increase the volume of the bell jar
and decrease the pressure within it. Air
moves through the Y-tube and inflates
the balloons. When you release the
rubber sheet, the volume of the bell jar
is decreased and the pressure within it is
increased. Again air moves to equalize
the pressure. But this time it leaves
the balloons and passes through the
Y-tube to the atmosphere.

Although the apparatus described
above is a good model, notice that only
one thing is being changed — the rub¬
ber sheet representing the diaphragm.

604 UNIT 7 THE BIOLOGY OF MAN

43-5 Identify the parts of the respiratory sys¬
tem that are represented in this apparatus.
What breathing structures are not properly
represented?

As you will see, the ribs also play an im¬
portant part in changing the size and
pressure of the chest cavity.

Breathing movements. Inspiration , or
intake of air, occurs when the chest
cavity is increased in size and therefore
decreased in pressure. Enlargement of
the chest cavity involves the following
movements:

1. The rib muscles contract and pull the
ribs upward and outward. If you in¬
hale with force, you carry this action
even further with the aid of the
shoulder muscles.

2. The muscles of the resting, dome¬
shaped diaphragm contract. This
action straightens and lowers the
diaphragm and increases the size of
the chest cavity from below.

3. The abdominal muscles relax and
allow compression of the abdominal
organs by the diaphragm.

The enlargement of the chest cav¬
ity results in decrease of the air pressure
within. In an equalizing of outer and
inner pressure, air passes down the
trachea and inflates the lungs.

Expiration , or the expelling of air
from the lungs, results when the chest
cavity is reduced in size. The action
involves the following four movements:

1. The rib muscles relax and allow the
ribs to spring back.

2. The diaphragm relaxes and rises to
assume its dome-shaped position.

3. The compressed abdominal organs
push up against the diaphragm.
This action is increased during forced
exhalation by contraction of the ab¬
dominal muscles.

4. The elastic lung tissues, stretched
while the lungs are full, shrink and
force air out.

The control of breathing. The factors
that control breathing and breathing
rate are both nervous and chemical.
There are nerves leading from the lungs,
diaphragm, and rib muscles to a respira¬
tory control center at the base of the
brain. When the lungs expand in in¬
spiration, impulses pass from nerve end¬
ings in their tissues along the nerve
leading to the control center. The con¬
trol center in turn sends impulses to the
rib muscles and diaphragm, causing
them to return to their resting position
in the act of expiration. When expira¬
tion is complete, the process is reversed
and the control center sends impulses
to the rib muscles and diaphragm that
cause them to contract, so that the lungs
expand, and the cycle starts again. In¬
spiration and expiration occur from 16
to 24 times per minute, depending on
the body activity, position, and age.
The air capacity of the lungs. When
the lungs are completely filled they hold
about 300 cubic inches of air. But only
about 30 cubic inches are involved each
time we inhale and exhale. The air
involved in normal, relaxed breathing
is called tidal air. Forced breathing in¬
creases the amount of air movement.

CHAPTER 43 RESPIRATION AND ENERGY EXCHANGE 605

43-6 These two X-ray photographs show the chest during exaggerated breath¬
ing. The left-hand one represents inhalation; the right one represents exhala¬
tion.

To illustrate the effects of forced
breathing, inhale normally without any
forcing. Your lungs now contain about
200 cubic inches of air. Now exhale
normally. You have moved about 30
cubic inches of tidal air from the lungs.
Now, without inhaling again, force out
all the air you can. You have now ex-
haled an additional 100 cubic inches of
supplemental air. The lungs now con¬
tain about 70 cubic inches of residual
air, which you cannot force out.

When you inhale normally again,
you replace the supplemental and the
tidal air, or about 130 cubic inches. If
you inhale with force you can add 100
cubic inches of complemental air , rais¬
ing the total capacity of the lungs to
about 300 cubic inches. We can say the
total capacity of the lungs consists of:

Tidal air 30 cu. in.

Supplemental air 100 cu. in.

Complemental air 100 cu. in.

Residual air 70 cu. in.

300 cu. in.

Artificial respiration. Any stopping of
the breathing motions can be serious
because the blood will then lack oxygen
and the cells will suffer. Artificial respi¬
ration is simply a method of artificially
forcing the lungs to inspire and expire
air rhythmically. It can be described
more accurately as artificial breathing.
The back pressure-arm lift method is
one approved by the American Red
Cross. A more recent method now
strongly recommended by the Red Cross
is the mouth-to-mouth method, shown
in Fig. 43-7. This method gets more
air into the victim’s lungs than any oth¬
er method of artificial respiration. The
operator places his mouth over the
mouth or nose of the victim and
breathes into the victim until he sees
the chest rising. The rate at which this
is repeated depends on whether the vic¬
tim is an adult or a child.

Paying off a debt. In the tissues, oxygen
is used by each cell in the oxidation of
food to release energy. This process is
especially rapid in active muscle tissue.

Total capacity

606 UNIT 7 THE BIOLOGY OF MAN

43-7 Mouth-to-mouth artificial respira¬
tion. Before starting the mouth-to-
mouth method of artificial respiration,
look to see if there is any foreign mat¬
ter in the victim’s mouth. If so, wipe
it out with your fingers, then proceed ac¬
cording to the following steps:

First, tilt the head back so the chin is
pointing upward (7). Pull or push the
jaw into a jutting-out position (2 and 3).
These maneuvers should relieve ob¬
struction of the airway by moving the
base of the tongue away from the back
of the throat.

Second, open your mouth wide and
place it tightly over the victim’s mouth.
At the same time pinch the victim’s nos¬
trils shut ( 4 ) or close the nostrils with
your cheek (5). Or close the victim’s
mouth and place your mouth over his
nose. Blow into the victim’s mouth or
nose.

Third, remove your mouth, turn your
head to the side, and listen for the re¬
turn rush of air that indicates air ex¬
change. Repeat the blowing effort. For
an adult, blow vigorously at the rate of
12 breaths per minute. For a child, take
relatively shallow breaths appropriate
for the child’s size at the rate of about
20 per minute.

Fourth, if you are not getting air ex¬
change, recheck the head and jaw posi¬
tion. If you still do not get air ex¬
change, quickly turn the victim on his
side and administer several sharp blows
between the shoulder blades to dislodge
any foreign matter in the airway. (Re
drawn from American Red Cross)

CHAPTER 43 RESPIRATION AND ENERGY EXCHANGE 607

When sufficient oxygen is supplied to
muscle tissues, it combines with hydro¬
gen released during fuel breakdown and
forms water. This water, together with
carbon dioxide released as a fuel by¬
product, is discharged as a waste prod¬
uct of aerobic respiration.

During times of muscular exertion,
the need for oxygen in the tissues is
greater than the body can possibly sup¬
ply. The lungs cannot take in oxygen
nor can the blood deliver it rapidly
enough. All of us at some time or an¬
other have had the experience of run¬
ning or swimming in a race, playing
tennis, climbing a mountain, or running
to board a train or a bus. Perhaps you
can remember feeling, for a second, that
you just couldn’t finish that race or
catch that bus. But with a final surge
of reserve energy, you succeeded. You
felt limp, lightheaded, and completely
exhausted. You may recall how your
heart pounded and your breathing was
deep and rapid. After 20 or 30 minutes
you were probably ready for another
race, another tennis match, or the long
hike back.

What happens in one’s body to
cause these changes, and how is this
rapid recovery brought about? During
mild exercise the supply of oxygen meets
the demands of the cells. During exer¬
tion, however, there is not enough oxy¬
gen. Then, respiration becomes an¬
aerobic and pyruvic acid becomes the
hydrogen acceptor. This produces lac¬
tic acid , which accumulates in the tis¬
sues and causes fatigue. Like carbon
dioxide, lactic acid signals the respira¬
tory center in the brain. Breathing be¬
comes rapid and the heart speeds up in
order to supply the tissues with enough
oxygen. As you learned above, hemo¬
globin releases more oxygen because of
the acidity of the blood. But even with

43-8 During excessive muscular exercise
such as a track event, a considerable oxygen
debt is built up. How and when is this debt
paid off? (Shelton from Monkmeyer Press
Photo Service)

these efforts, the lactic acid accumu¬
lates in the body, and the body is in a
state of oxygen debt. During a half-
hour rest, some of the accumulated lac¬
tic acid is oxidized, and some is con¬
verted to glycogen. Carbon dioxide and
excess water are excreted, and the debt
is paid. The body is then ready for
more exercise.

Body metabolism and its measurement.
In Unit One you learned that the sum
of all the processes occurring in a cell or
an organism is called metabolism. The
constructive phase of metabolism in¬
cludes carbohydrate and protein syn¬
thesis, while the destructive phase in¬
cludes oxidation and energy release.
The rate of metabolism increases in pro¬
portion to the activity of the body. This
activity may be muscular, as in walking,
running, or some other form of exertion,
or it may be mental. Other factors gov-

608 UNIT 7 THE BIOLOGY OF MAN

erning the metabolic rate include ex¬
posure to cold and activity of the diges¬
tive organs during digestion of food.
One way to measure the metabolic rate
of the body is to measure the rate of
oxidation by determining the amount
of heat given off from the body surface.
This may be measured by a device
called a calorimeter.

The person to be tested enters a
closed compartment that is equipped to
accurately measure all the heat given
off by his body. He may lie quietly in
bed during the process, or he may sit
in a chair, or exercise vigorously, de¬
pending on the nature of the activity to
be tested. The amount of heat energy
given off during each type of activity is
a direct indication of the rate of oxida¬
tion in the body tissues. Calorimeter
tests are important in determining the
energy needs of various individuals in
order to adjust a diet to their specific
requirements.

Even when the body seems com¬
pletely inactive, as it does in sleep, res¬
piration, oxidation, and energy release
are continuing. With the cessation of
muscular and, to a great extent, nervous
activity, the rate of oxidation is greatly
reduced. The activities required to
maintain the body and to supply energy
necessary to support the basic life proc¬
esses are included in the term basal
metabolism. The rate at which these
activities occur is termed the basal met¬
abolic rate, or BMR.

BMR may be determined by means
of the calorimeter test. Another meth¬
od, widely used in hospitals, measures
the amount of oxygen consumed in a
definite period (Fig. 43-9). The pa¬
tient rests for at least an hour before the
test. The test is usuallv run in the

morning, and the patient is instructed
to eat no food until after the test is

completed. After a rest period, during
which the body is completely relaxed,
the nose is plugged to avoid breathing
from the atmosphere. A mouthpiece
connected to a tank of oxygen is fitted
into the mouth. Thus all oxygen in¬
haled during the test period is from the
measured tank. The amount of oxygen
used from the tank is recorded on a
graph. From these data the rate of oxi¬
dation is determined. The rate of basal
metabolism is calculated from the rate
of oxidation in the tissues during com¬
plete rest.

External influences on breathing and
respiration. External factors such as
temperature, moisture in the air, and
oxygen and carbon dioxide content of
air, are very important influences on
the rate of breathing and respiration.
Certain of these factors are involved in
ventilation. Stuffiness in a room is due
mainly to increase in the temperature
and moisture content of the air, rather
than to accumulation of carbon dioxide

43-9 A basal metabolism test determines
the rate at which oxidation occurs in the
body. (Sanborn Co.)

CHAPTER 43 RESPIRATION AND ENERGY EXCHANGE 609

and decrease of the oxygen content.
Movement of air in a ventilating system
increases its flow over the body surfaces
and speeds up the evaporation of perspi¬
ration. Modern air-conditioning sys¬
tems not only circulate air but remove
moisture and heat as well.

The air in most homes, especially
those equipped with a central heating
system, becomes too dry during the cold
months. This condition dries out mu¬
cous membranes and lowers their re¬
sistance to infection. For this reason
the moisture content of the air should
be kept as high as possible by means of
humidifiers or other devices.

Many people carry the ventilation
of bedrooms at night to extremes. Your
body requires less oxygen while you are
asleep than at any other time. If the
windows are open too much during cold
weather, your body may chill during
the night. There is little logic in piling
covers on a bed to keep part of the body
warm while at the same time chilling
the exposed parts with cold air from an
open window.

Carbon-monoxide poisoning. Far too
often we read of people who have died
in a closed garage where an automobile
engine was running or in a house filled
with gas from an open stove burner or
a defective furnace. The cause of death
is given as carbon-monoxide poisoning.
Actually the death is not due to poison¬
ing but to tissue suffocation. Carbon
monoxide will not support life. Yet it
combines with the hemoglobin of the
blood 250 times more readily than oxy¬
gen. As a result the blood becomes
loaded with carbon monoxide and de¬
creases its oxygen-combining power. As
tissues suffer from oxygen starvation,
the victim becomes lightheaded and
ceases to care about his condition. Soon
paralysis sets in, and he cannot move

43-10 This spaceman’s suit has been de¬
signed to provide a comfortable level of
pressure and oxygen for high-altitude flying.
(National Aeronautics and Space Adminis¬
tration)

even if he wants to. Death follows
from tissue suffocation.

Respiration problems at high altitudes.
At sea level the air pressure remains at
approximately 14.7 pounds per square
inch. In a sense we are living at the
bottom of a large sea of air. With in¬
creased altitude the amount of air pres¬
sure is reduced. The pressure of the
air is an important factor in breathing
and in how oxygen combines with the
hemoglobin of the blood. For this rea¬
son mountain climbers and airplane
pilots experience increasing difficulty in
breathing and progressive weakness as
they increase their altitude. At eleva¬
tions near 12,000 feet many people fa¬
tigue easily.

A pilot can fly much higher than a
man can climb because the plane motor
is doing the work. But when a pilot
nears 20,000 feet, the pressure becomes

610 UNIT 7 THE BIOLOGY OF MAN

so reduced that he experiences diffi¬
culty in seeing and hearing. This con¬
dition, called anoxia , is the result of
oxygen starvation in the tissues. It will
cause death if not corrected within a
short time. Anoxia may be avoided by
equipping the pilot with an oxygen tank
and a mask.

Passengers in modern airliners can
fly at high altitudes in the safety and
comfort of pressurized cabins. These
cabins maintain an internal pressure
and oxygen content equivalent to an
altitude of approximately 5,000 feet.
Respiration — a vital problem in space
travel. The astronaut encounters the
problems of people at high altitudes to
a much greater degree. At 16 miles
above the earth, the density of the air
is only about 4 percent of its density
at sea level; beyond an altitude of 70
miles there is practically no atmosphere
and therefore practically no oxygen. It
is now known, however, that after train¬
ing, man can exist in an atmosphere of
about half the normal oxygen content
on earth without discomfort, and still
be reasonably efficient. Investigators
learned this by studying a tribe of In¬
dians who live at high altitudes in the
mountains of Peru. These Indians are
able to carry on normal physical activ¬

ities that would quickly exhaust a
healthy person accustomed to the at¬
mosphere at sea level. On further in¬
vestigation the scientists learned that
these tribesmen have greater lung capac¬
ity and a higher red blood cell count.
They are able to take in more air with
each breath, and the extra red cells dis¬
tribute oxygen more efficiently. Space
scientists have found that similar
changes occur in men trained in an
atmosphere of reduced oxygen. But
even for trained astronauts, the entire
cabin of a spaceship must be supplied
with oxygen or the astronauts must wear
masks connected to oxygen tanks and
pressure suits.

On earth the carbon dioxide in the
atmosphere is used by plants in the
process of photosynthesis. In a sealed
spaceship, however, it is necessary to
dispose of the carbon dioxide exhaled
by the travelers. Algae are now being
considered for use in prolonged space
travel. The green alga Chlorella shows
promise because of its high photosyn¬
thetic rate and rapid reproduction.
Chlorella may also serve as an excellent
source of oxygen to replenish the supply
on the spaceship. There is also the
possibility that Chlorella can serve as a
source of food for the passengers.

IN CONCLUSION

Respiration involves the exchange of gases between living matter and its sur¬
roundings and the chemical process of oxidation, in which energy is released
during the breakdown of foods. In lower forms of life individual cells are in
direct contact with their surroundings. In higher animals blood is the conduct¬
ing medium between the body tissues and respiratory organs in contact with
the outer environment. The movement of air in and out of lungs is accom¬
plished by the mechanical process known as breathing, which consists of in¬
spiration and expiration. External respiration and internal respiration are

CHAPTER 43 RESPIRATION AND ENERGY EXCHANGE 611

concerned respectively with the actual exchange of gases between the lungs and
blood and between the blood and body tissues.

Metabolism includes respiration, oxidation, and the growth processes. The
rate at which these processes occur during rest is expressed as the basal meta¬
bolic rate.

In the next chapter we shall examine other processes that are controlled
by the nervous system, and we shall see how various stimuli are received.

BIOLOGICALLY SPEAKING

air sacs

alveoli

BMR

breathing

bronchi

bronchial tubes
bronchioles

epiglottis
expiration
external respiration
inspiration
internal respiration
larynx

pharynx

pleural membrane

respiration

septum

trachea

vocal cords

QUESTIONS FOR REVIEW

1. What are the differences between respiration and breathing?

2. List the major organs of the respiratory system.

3. Describe gas exchange in the lungs, naming the structures involved and
explaining why the exchange occurs.

4. How do pressure changes within the chest cavity cause inspiration and
expiration?

5. What factors influence the rate of breathing?

6. What is the purpose of artificial respiration?

7. How do you build up an oxygen debt? How is it repaid?

8. Define BMR and give two ways in which it may be measured.

9. Explain the physiology of carbon-monoxide poisoning.

10. Compare respiration problems encountered on a high mountain to those in
space travel.

APPLYING PRINCIPLES AND CONCEPTS

1. If plants produce oxygen in photosynthesis, how do you explain the fact
that they also respire?

2. What changes would you expect to occur in the blood if you were to hold
your breath for a period of time? if you were to breathe rapidly and deeply
for a period of time?

3. People who live in dry climates, such as the southwestern parts of our
country, report that high temperatures there are easier to take than the
same temperatures in the more humid parts of the United States. Why?

CHAPTER 44

BODY

CONTROLS

The nervous system. Your nervous sys¬
tem receives impressions from your sur¬
roundings, stores them in the brain,
originates activity, and carries impulses
to all parts of the body. It coordinates
the activity of several million cells into
a single functioning unit — the control
center for body activities. The nervous
system’s activities involve impulses car¬
ried along nerves in a two-way commu¬
nication system. Impulses are sent
from the body tissues and organs to
nerve centers, and from these centers to
the tissues and organs.

The brain and spinal cord comprise
the central nervous system. They com¬
municate with all parts of the body by
means of the nerves of the peripheral
system. Another division is the auto¬
nomic system , which regulates certain
vital functions of the body almost in¬
dependently of the central nervous sys¬
tem.

The nerve cell. Nerve cells are called
neurons. Each has a rounded, star¬
shaped or irregular cell body, containing
a nucleus and cytoplasm, from which
threadlike processes, often called nerve
fibers, extend (Fig. 44-1, page 613).
The branching treelike fibers that carry
impulses to the cell body are called
dendrites. The number of dendrites
entering a cell body may range from one
to 200. Impulses travel from the cell
bodv along a single fiber. These out¬
going processes are called axons. Den¬
drites and axons branch at their tips in
tiny brushlike structures. The fibers of
many brain cells are short. Those lead¬
ing from the spinal cord to muscles and
glands, however, may be half as long as
your body.

Impulses travel along nerve fibers
in one direction only. The fibers of
sensory neurons always carry impulses
to the brain or spinal cord. Those of
motor neurons always carry impulses
from the brain or spinal cord. The end¬
ings of sensory and motor neurons in
the spinal cord and brain mingle with
many central neurons that have short
processes.

The processes of one neuron never
touch those of another neuron. The
space between endings of neuron proc¬
esses is called a synapse (sin- aps). Im¬
pulses must pass over these synapses as
they travel from one neuron to another.
Furthermore, an impulse never travels
from one motor neuron to another. It
is received by the dendrite of a sensory
or a central neuron. Nor do impulses
travel from one sensory neuron to
another.

The threadlike nerves that branch
through your body are bundles of fibers
arranged like wires in a cable. In the
peripheral nervous system, any nerve
composed only of the fibers of motor

612

CHAPTER 44 BODY CONTROLS 613

neurons is a motor nerve. Any nerve
composed only of the fibers of sensory
neurons is a sensory nerve. Some
nerves contain both motor and sensory
fibers, and are called mixed nerves. If
all the body tissues other than nervous
tissue were dissolved, the outline of the
body would be preserved by the nerves
that would remain.

Nerve impulses. A nerve impulse is
known to be an electrochemical im¬
pulse, which brings about a change in
the nerve fiber. It is not a flow of elec¬
tricity, for nerve impulses are much
slower. A nerve impulse travels at a
rate of about 300 feet per second, while
electricity travels at a rate of 186,000
miles per second. Also, when a nerve
impulse passes along a nerve, carbon di¬
oxide is liberated, which indicates that
a chemical reaction is involved.

For a long time scientists were in
doubt as to how a nerve impulse causes
a muscle to contract. Now we know
that the stimulation is indirect. An im¬
pulse traveling along the axon of a mo¬
tor neuron ends at the motor end plate
at the tips of the brushlike structures.

Here the impulse causes the release of
a minute amount of a chemical called
acetylcholine (a-seet-i\-koh-\een) . This
substance transmits the impulse to mus¬
cle fibers, which begin the process of
contraction we discussed in Chapter 40.
Following a brief period of contraction,
the nerve releases another substance,
cholinesterase (koh-li-nes-te-rays), which
neutralizes acetylcholine and causes the
muscle fibers to relax, all in 0.1 second
or less.

The brain and its membranes. The
brain (Fig. 44-2) is probably the most
highly specialized organ of the human
body. It weighs about three pounds and
fills the cranial cavity. It is composed
of soft nervous tissue covered by three
membranes, together known as the me¬
ninges (meh-nm-jeez) . The inner mem¬
brane, or pia mater (pee- a mah- ter), is
richly supplied with blood vessels which
carry food and oxygen to the brain cells.
It is a delicate membrane which closely
adheres to the surface of the brain and
into all the grooves. The middle mem¬
brane, or arachnoid mater , consists of
fibrous and elastic tissue. This mem-

614 UNIT 7 THE BIOLOGY OF MAN

44-2 Longitudinal section of the brain
showing the regions and the meninges.

brane does not dip down into the
grooves of the brain but bridges them.
The space between these two mem¬
branes is filled with a clear liquid, the
cerebrospinal fluid , which, as the name
implies, is also found around the spi¬
nal cord. The outermost layer of pro¬
tective membranes is a thick, strong, fi¬
brous lining, the dura mater. This
layer serves as a lining for the inside of
the cranium as well as a membrane of
the brain. The meninges protect the
brain from jarring by acting as a cush¬
ion. A concussion is a brain bruise re¬
sulting from a violent jar that causes
damage in spite of the protective me¬
ninges. These three membranes ex¬
tend down the spinal column to cover
and protect the spinal cord.

Cavities of the brain. There are four
spaces called ventricles within the
brain. Two lateral ventricles open into
the third ventricle, which leads to the
fourth ventricle. From the fourth ven¬

tricle the cavity is continuous with the
subarachnoid space and the central ca¬
nal of the spinal cord. The cavities of
the brain and the central canal are lined
with ciliated epithelium, which keeps
the cerebrospinal fluid, with which the
cavities are filled, in motion.

The cerebrum — the largest of the brain
regions. The region of the brain called
the cerebrum is proportionally larger in
man than in any animal. It consists of
two halves, or hemispheres, securely
joined by tough fibers and nerve tracts.
The outer surface, or cortex , is deeply
folded in irregular wrinkles and furrows,
the convolutions, which greatly increase
the surface area of the cerebrum.
Deeper grooves divide the cerebral cor¬
tex into lobes (Fig. 44-3).

The cerebral cortex is composed of
countless numbers of neurons. We fre¬
quently refer to this area as gray matter
because of the color of these cells. The
cerebrum below the cortex is composed
of white matter, formed by masses of
fibers covered by sheaths and extending
from the neurons of the cortex to other
parts of the body.

The functions of the cerebrum. Differ¬
ent activities are controlled by specific
regions of the cerebrum. Some of the
areas of the cerebral cortex are motor
areas, which means that they are cen¬
ters that control voluntary movement.
The motor area of the cerebrum con¬
trols the muscles of the legs, trunk,
arms, shoulders, neck, face, and tongue
in this order, from the top of the lobes
downward. Some of the areas of the
cerebral cortex are sensory areas, which
means that the various senses, such as
seeing, hearing, touching, tasting, and
smelling, are interpreted here. For ex¬
ample, we interpret what our eyes see
in the vision center of the occipital
(ahk-sip-it-1) lobes. If these lobes were

CHAPTER 44 BODY CONTROLS 615

destroyed, we would not be able to see
anything, although our eyes might be
perfect. We know also that the frontal
lobes are centers of emotion, judgment,
will power, and self-control. These
functions, however, are shared by other
areas of the cerebral cortex.

The things we see and hear and
feel are registered as impressions in the
sensory areas of the cerebral cortex.
The things we do are controlled by the
motor areas. These areas are in turn
connected by a vast number of associa¬
tion areas. Thoughts are the result of
associations of impressions. Your intel¬
lectual capacity is determined by the
ability of your cerebral cortex to register
impressions, the activity of your associa¬
tion areas, and the sum of your past ex¬
periences.

The functions of the cerebellum. The
cerebellum is a structure lying below the
back of the cerebrum. Like the cere¬
brum it is composed of hemispheres,
but its convolutions are shallower and
more regular than those of the cere¬
brum. The surface of the cerebellum
is composed of gray matter. Its inner
structure is largely white matter, al¬
though it contains some areas of gray
matter. Bundles of nerve fibers con¬
nect the cerebellum with the rest of the
nervous system.

In a sense the cerebellum acts as
an assistant to the cerebrum in control¬
ling muscular activity. Nervous im¬
pulses do not originate in it nor can one
control its activities. The chief func¬
tion of the cerebellum is to coordinate
the muscular activities of the body.
Thus, without the help of the cerebel¬
lum, the impulses from the cerebrum
would produce uncoordinated motions.

The cerebellum functions further in
strengthening impulses to the muscles.
This action is a little like picking up a

LOBES

D Neck D'

E Face E'

F Tongue F'

G Motor speech

44-3 These are the control areas of the cere¬
brum. Notice that some are for the origin of

action and some for the termination, or in¬
terpretation, of senses.

weak radio or television signal and am¬
plifying it before broadcasting it.

Another function of the cerebel¬
lum is maintenance of tone in muscles.
The cerebellum cannot originate a mus¬
cular contraction, but it can cause the
muscles to remain in a state of partial
contraction. You are not aware of this
because the cerebellum operates below
the level of consciousness.

The cerebellum functions also in
maintaining balance. In this activity
it is assisted by impulses from the eyes
and from the organs of equilibrium of
the inner ears. Impulses from both of
these organs inform the cerebellum of
your position in relation to your sur¬
roundings. The cerebellum in turn
maintains contractions of the muscles
necessary to balance your body.

616 UNIT 7 THE BIOLOGY OF MAN

44-4 In this "air” X-ray
photograph of the brain, the
cerebrospinal fluid has been
drained through the spine
and replaced by air. Since
the patient was under an an¬
esthetic, the white “airway”
was placed in the mouth to
avoid having the tongue fall
back and cut off the air sup¬
ply. (Indiana University Med¬
ical Center)

The brain stem. Nerve fibers from the
cerebrum and cerebellum enter the brain
stem, an enlargement at the base of the
brain. The lowest portion of the stem,
the medulla oblongata , is located at the
base of the skull and protrudes from the
skull slightly where it joins the spinal
cord. Another part of the brain stem is
the pons (ponz), which receives stimuli
from the facial area.

There are twelve pairs of cranial
nerves connected to the brain. These
are part of the peripheral nervous sys¬
tem and act as direct connections with
certain important organs of the body.
One pair, for example, connects the
eves with the brain. Another cranial
nerve connects the brain with the lungs,
heart, and abdominal organs.

The medulla oblongata controls
the activitv of the internal organs. The
respiratorv control center we discussed
in the last chapter is located here.
Heart action, muscular action of the
walls of the digestive organs, secretion
in the glands, and other automatic ac¬
tivities are also controlled by the me¬
dulla oblongata.

The spinal cord and spinal nerves. The
spinal cord extends from the medulla
oblongata through the protective bony
arch of each vertebra, almost the length
of the spine. Its outer region is white
matter, made up of great numbers of
nerve fibers covered by sheaths. Neu¬
rons, composing the gray matter, lie in¬
side the white matter in a form similar
to the shape of a butterfly with its wings
spread (Fig. 44-5). The pointed tips
of the wings of gray matter are called
horns. The posterior pair points to¬
ward the back of the cord while the
anterior pair points toward the front of
the cord.

Thirty-one pairs of spinal nerves
branch off the cord between the bones
of the spine. Along with the cranial
nerves, these nerves and their branches
form the peripheral nervous system.
One of a pair goes to the right side of
the body. Its mate goes to the left side.
Spinal nerve branches begin in the neck
and continue the full length of the
cord. These large cables are mixed
nerves. Some of their many fibers are
sensory and carry impulses into the

CHAPTER 44 BODY CONTROLS 617

spinal cord, while others are motor and
lead impulses away from it. Each spi¬
nal nerve divides just outside the cord.
The sensory fibers that carry impulses
from the body into the spinal cord
branch to the posterior horns of the
gray matter. This branch of each spi¬
nal nerve has a ganglion (near its point
of entry to the cord), in which the sen¬
sory cell bodies are found. The other
branch at the junction leads from the
anterior horns of the spinal cord, in
which the motor cell bodies are located.
The motor fibers of this branch carry
impulses from the spinal cord to the
body.

If the spinal cord were cut, all parts
of the body controlled by nerves below
the point of severance would be totally
paralyzed. Such an injury might be
compared to cutting the main cable to
a telephone exchange.

Nervous reactions. Nervous reactions
vary greatly in form and complexity.
The simplest of these is the reflex c iction
(Fig. 44-6). It is an automatic reac¬
tion involving the spinal cord or the
brain. The knee jerk is an excellent ex¬
ample of a simple reflex action. If you
allow your leg to swing freely and strike

44-5 This drawing shows in cross-section
the structures of the spinal cord. Notice
that, unlike in the brain, the outer region is
the white matter. What do you think the
small arrows indicate?

the area just below the kneecap with a
narrow object, the foot jerks upward.
This reaction is entirely automatic.
Striking the knee stimulates a sensory
neuron in the lower leg. An impulse
travels along the dendrite to the spinal
cord. Here the impulse travels to a
central neuron. This neuron in turn
stimulates a motor neuron extending
to the leg muscles, causing a jerk. The
entire reflex takes only a split second.

Axon \
Cell body
Dendrite

Sensory neuron

nerve

Ganglion

Axon /
Dendrite

Motor neuron

Receptor (as in skin)

Central neuron

Cell body

Motor end plate
in muscle^

44-6 Trace the path of this impulse from the stimulus to the response. Why
are reflex actions said to be automatic?

618 UNIT 7 THE BIOLOGY OF MAN

When you touch a hot object, you
experience a similar reflex. Your hand
jerks away almost instantly. After the
reflex is completed, the impulse reaches
the brain and registers pain. However,
if the muscle response had been delayed
until the pain impulse had reached the
brain and a motor impulse traveled
down the spinal cord from the cerebral
motor area, the burn injury would have
been much greater. Other reflex ac¬
tions include sneezing, coughing, blink¬
ing the eyes when the cornea is touched,

laughing when tickled, and jumping
when frightened.

The autonomic nervous system. The
autonomic nervous system is entirely in¬
voluntary and automatic. It is com¬
posed of two parts, one of which is
called the sympathetic system. This
system includes two rows of nerve tis¬
sue, or cords, which he on either side of
the spinal column. Each cord has gan¬
glia, which contain the bodies of neu¬
rons. The largest of the sympathetic
ganglia is the solar plexus , located just

Eye

Nose /.
Salivary glands

Bronchi
and lungs

Esophagus
and stomach

Liver

Small intestine

Large intestine

Kidney
and bladder

Uterus

Cerebrum

Cerebellum

Medulla

Vagus nerve

Sympathetic cord
(that from one side
only is shown)

Ganglia of the
sympathetic cord

Solar plexus

Sympathetic system

Parasympathetic system

Pancreas

44-7 The autonomic nervous system regulates the internal organs of the body.
What are the functions of its two divisions?

CHAPTER 44 BODY CONTROLS 619

44-8 The five types of recep¬
tors found in the skin. The
nerve fibers are drawn in
solid black; the accompany¬
ing structures in gray.

Touch

Pressure

Heat

below the diaphragm. Another is near
the heart; a third is in the lower part of
the abdomen; and a fourth is in the
neck. Fibers from the sympathetic
nerve cords enter the spinal cord and
connect with it and with the brain, as
well as with one another. The sym¬
pathetic nervous system helps to regu¬
late heart action, the secretion of duct¬
less glands, blood supply in the arteries,
the action of smooth muscles of the
stomach and intestine, and the activity
of other internal organs.

The parasympathetic system op¬
poses the sympathetic system and thus
maintains a system of check and bal¬
ance. The principal nerve of the para¬
sympathetic system is the vagus nerve, a
cranial nerve that extends from the me¬
dulla oblongata, through the neck, to
the chest and abdomen. The check-
and-balance system is illustrated by the
fact that the sympathetic system speeds
up heart action, while the vagus nerve
slows it down.

The sensations of the skin. The skin
has five different types of nerve endings,
each associated with a different sensa¬
tion. The nerve endings in the skin are
terminal branches of dendrites of sen¬
sory neurons and are called receptors.
Some receptors are many-celled; some
consist of only one specialized cell; and
some are the bare nerve endings them¬
selves. Each receptor is suited to re¬
ceive only one type of stimulus and to
start impulses to the central nervous sys¬
tem. The skin has five different types
of receptors. Certain of these respond
to touch, while others receive stimuli of
pressure, pain, heat, or cold (Fig.
44-8).

Normally no one receptor reacts to
more than one stimulus, and thus the
five sensations of the skin are distinct
and different. The pain receptor, for
example, is a bare dendrite. If the
stimulus is strong enough, a pain recep¬
tor will react to mechanical, thermal,
electrical, or chemical stimuli. The

620 UNIT 7 THE BIOLOGY OF MAN

sensation of pain is a protective device
used to signal a threat of injury to the
body. Pain receptors are distributed
through the skin.

The sensory nerves of the skin are
distributed unevenly over the skin area
in spots and lie at different depths in
the skin. For instance, if you move the
point of your pencil over your skin very
lightly, you stimulate only the nerves of
touch. These nerve endings are close
to the surface of the skin in the region
of the hair sockets. The fingertips, the
forehead, and the tip of the tongue con¬
tain abundant nerve endings that re¬
spond to touch.

Nerve endings that respond to
pressure lie deeper in the skin. If you
press the pencil point against the skin,
you feel pressure in addition to touch.
Since the nerves are deeper, a pressure
stimulus must be stronger than a touch
stimulus. You may think that there is
no difference between touch and pres¬
sure. But the fact that you can distin¬
guish the mere touching of an object
from a firm grip on it indicates that
separate nerves are involved.

Heat and cold stimulate separate
nerve endings, which is an interesting
protective device of the body. Actually

cold is not an active condition. Cold
results from a reduction in heat energy.
If temperature stimulated a single
nerve, impulses would be strong in the
presence of great heat and would be¬
come weaker as heat decreased. There
would be no impulses in greatly re¬
duced heat (intense cold). However,
since some nerves are stimulated by
heat and others by the absence of it, we
are constantly aware of both conditions.
The sense of taste. Taste results from
the chemical stimulation of certain nerve
endings. Since nearly all animals prefer
some food substances to others, we must
assume that they can distinguish differ¬
ent chemical substances. The sense of
taste in man is centered in the taste buds
of the tongue. These flask-shaped
structures, containing groups of nerve
endings, lie in the front area of the
tongue, along its sides, and near the
back. Foods, mixed with saliva and
mucus, enter the pores of the taste buds
and stimulate the hairlike nerve end¬
ings (Fig. 44-9) .

Our sense of taste is poorly devel¬
oped. We recognize only four com¬
mon flavors: sour, sweet, salty, and bit¬
ter. Taste buds are distributed un¬
evenly over the surface of the tongue.

44-9 Diagram A shows how a
section of the membrane of the
nose would appear under a micro¬
scope. Diagram B shows a sec¬
tion of the tongue through a taste
bud.

Taste receptors

Membrane cells

Nerve fibers Supporting cells

Cerebrum

44-10 This diagram shows the cen¬
tral nervous system and the ma¬
jor nerve trunks of the peripheral
nervous system. For clarity, only
a few of the branches from the
major nerve trunks and nerves are
shown. If all the nerve branches
were shown, they would solidly
fill this outline of the body, so
that if a pin were put anywhere
on this outline it would strike a
nerve. The central nervous sys¬
tem and posterior peripheral
nerves are drawn in gray; the an¬
terior ones are in brown.

Lumbar plexus

Sacral plexus

Sciatic nerve

Cerebellum
Spinal cord
Cervical plexus

Brachial plexus

Sympathetic
ganglionic chain

Intercostal

nerves

622 UNIT 7 THE BIOLOGY OF MAN

Those sensitive to sweet flavors are at
the tip of the tongue. Doesn’t candy
taste sweeter when you lick it than
when you chew it far back in the
mouth? The tip of the tongue is also
sensitive to salty flavors. You taste sour
substances along the sides of the
tongue. Bitter flavors are detected on
the back of the tongue, which explains
why a bitter substance does not taste
bitter at first. If a substance is both
bitter and sweet, you sense the sweet¬
ness first, then the bitterness. Sub¬
stances such as pepper and some other
spices have no distinct flavor, but irri¬
tate the entire tongue and produce a
burning sensation.

Much of the sensation we call taste
is really smell. When you chew foods,
vapors enter the inner openings of the
nose and reach nerve endings of smell.
If the external nasal openings are
plugged up, many foods lack the flavor
we associate with them. Under such
conditions onions and apples have an
almost identical sweet flavor. You
probably have noticed the loss of what
you thought was taste sensation when
you had a head cold and temporarily
lost vour sense of smell.

Mouth cavity

Branches of olfactory nerve

Upper
turbinate

_ Middle

^ ...turbinate

I Lower
turbinate

Opening of Eustachian tube

44-11 Here you see the surface of the inner
wall of the nose. What function does the
Eustachian tube perform?

The sense of smell. Like taste, smell
results from the chemical stimulation of
nerves, except that odors are in the form
of gases. The nasal passages are ar¬
ranged in three tiers, or layers, of cavi¬
ties, separated by bony layers called
turbinates. The upper turbinate con¬
tains branched endings of the olfactory
nerve , which is a cranial nerve (Fig.
44-1 1 ) . Stimulation of these endings by
odors results in the sensation of smell.
Receptors that are exposed to a particu¬
lar odor over a long period of time be¬
come deadened to it, although they are
receptive to other odors. Nurses are
not usually aware of the odor of iodo¬
form in a hospital, but a visitor is.

The structure of the human ear. Our
ears and those of other mammals are
wonderfully complex organs. The ex¬
ternal ear opens into an auditory canal
embedded in the skull (Fig. 44-12).
The canal is closed at its inner end by
the eardrum , or tympanic membrane,
which separates it from the middle ear.
The middle ear connects with the throat
through the Eustachian ( yoo-stay-shun )
tube. This connection equalizes the
pressure in the middle ear with that of
the atmosphere.

When the connection between the
middle ear and throat is blocked by a
cold that involves the Eustachian tube,
the pressures outside and inside do not
equalize. For this reason divers and
fliers do not work when they have a
cold. The outside pressure increases
during a dive. But with a blocked Eu¬
stachian tube, the middle-ear pressure
would not be equalized and the differ¬
ence might burst the eardrum. The
flier’s situation would, of course, be the
reverse, in that the pressure would
be less outside than in the middle ear.

Three tiny bones, the hammer ,
anvil , and stirrup, form a chain across

CHAPTER 44 BODY CONTROLS 623

External ear

Middle ear

Auditory nerve

Sound

waves

To brain

Tympanic
membrane
(eardrum) Eustachian tube

Cochlea
To throat

44-12 The structure of the human ear.

the middle ear. They extend from the
inner face of the eardrum to a similar
membrane that covers the oval window,
which is the opening to the inner ear.

The inner ear is composed of two
general parts. The cochlea ( kahk-\ee-a )
is a spiral passage resembling a snail
shell. It is filled with a liquid and is
lined with nerve endings that receive
the sound impressions. The auditory
nerve leads from the cochlea to the
brain. The semicircular canals consist
of three loop-shaped tubes, each at
right angles to the other.

How we hear. An object vibrating in
air produces regions in which the air
molecules are squeezed together (com¬
pressions) and regions in which they
are farther apart (rarefactions). This
regular pattern produced by any vibrat¬
ing object in air or other matter is
called a sound wave. When sound
waves reach the ear, they pass through
the external ear into the auditory canal
and to the eardrum. Here they cause

the eardrum to vibrate in time with
the compressions and rarefactions of the
sound wave. The vibration of the ear¬
drum in turn causes the hammer, anvil,
and stirrup bones of the middle ear to
vibrate. The vibration is transmitted to
the membrane of the inner ear and sets
the fluid in the cochlea in motion. Vi¬
bration of the fluid in turn stimulates
the nerve endings in the cochlea. Im¬
pulses travel through the auditory nerve
to the cerebrum, where the sensation of
sound is perceived. If the auditory re¬
gion of the cerebrum ceases to function,
a person cannot hear, even though his
ear mechanisms receive vibrations nor¬
mally.

The sense of balance. Our sense of
equilibrium, or balance, is centered in
the semicircular canals (Fig. 44-13) of
the inner ears. These canals lie at
right angles to one another in three dif¬
ferent planes. Their position has been
compared to the parts of a chair. One
canal lies in the plane of the seat, an-

624 UNIT 7 THE BIOLOGY OF MAN

44-13 The semicircular canals in the inner
ear function in maintaining balance. Notice
that each canal is at right angles to the
other two.

other in the plane of the back, and a
third in the plane of the arms.

The semicircular canals contain a
great number of receptors and a fluid
similar to that of the cochlea. When
the head changes position the fluid
rocks in the canals and stimulates these
receptors. Impulses travel from them
through a branch of the cerebellum.
Thus the brain is made aware of the
position of the head. Since the canals
he in three planes, any change in posi¬
tion of the head moves the fluid in one
or more of them. If you spin around
rapidly, the fluid is forced to one end
and impulses travel to the brain.
When you stop spinning, the fluid
rushes back the other way, giving you
the sensation of twirling in the opposite
direction, so that you feel dizzy. Regu¬
lar rhvthmic motions produce unpleas¬
ant sensations that involve the whole
body. These sensations are what we
call motion sickness. Disease of the
semicircular canals results in temporary

or permanent dizziness and loss of equi¬
librium.

The structure of the human eye. The
normal eye is spherical and slightly flat¬
tened from front to back (Fig. 44-14).
The wall of the eyeball is composed of
three distinct layers. The outer, sclerot¬
ic layer (skle-ru/zt-ik) is tough and
white. This layer shows as the white of
the eye. It bulges and becomes trans¬
parent in front, and is called the cornea.

The middle, choroid layer is richly
supplied with blood vessels. It com¬
pletely encloses the eye except in front
where there is a hole. This small open¬
ing, lying behind the center of the
cornea, is the pupil. Around the pupil
the choroid contains pigmented cells.
This area is the iris and may be colored
blue, brown, hazel, or green. Change
in size of the circular pupil is accom¬
plished by muscles in the iris. This ad¬
justment in size of the pupil opening to
the intensity of light is an automatic
reflex. When the light is reduced, the
pupil becomes large, or dilates. In
bright light it constricts, or becomes
small. The eye doctor uses drops to
block the automatic iris reflex so that
he can use a bright pinpoint of light to
see inside the eye. This is the only
place in the body where the blood ves¬
sels may actually be seen. The black
pigment of the choroid layer may also
be seen inside the eye. This black pig¬
mentation prevents reflection of light
rays within the eye.

A convex, crystalline lens lies be¬
hind the pupil opening of the iris. The
lens is supported by the ciliary muscles
fastened to the choroid layer. Contrac¬
tion of these muscles changes the shape
of the lens. In this way varying light
rays are focused on the surface of the
retina in the eye.

The space between the lens and the

CHAPTER 44 BODY CONTROLS 625

cornea is filled with a thin, watery sub¬
stance, the aqueous humor. A thicker,
jellylike transparent substance, the vit¬
reous humor , fills the interior of the
eyeball. This fluid aids in keeping the
eyeball firm and preventing its collapse.

The inner layer, or retina , is the
most complicated and delicate of the
eye layers. The terminals of the optic
nerve , a cranial nerve, are found in the
retina. This large nerve extends from
the back of each eyeball to the vision
center in the occipital lobe of the cere¬
brum. Some of the fibers cross as they
lead to the cerebrum. This means that
some of the impulses from your right
eye, for example, go to the left occipital
lobe and some go to the right occipital
lobe. Thus, what you see with each eye
is interpreted in both lobes.

Eye movement and protection. The
eye rests in its socket against layers of
fat which serve as cushions. Move¬
ments of the eyeball are accomplished
by pairs of muscles which attach to its
sides and extend back into the eye

socket (Fig. 44-14, left). The sclerotic
layer is supplied with nerve endings
that register pain when a foreign object
touches it. T he eye is further protected
by its location deep in the recesses of
the eye socket, by bony ridges, bv the
eyelids, and by the tear glands that keep
its surface moist. Tears wash over the
eye and drain into the tear ducts in the
lower corner of the eye socket, which
leads to the nasal cavity. Because tears
contain an antibacterial enzyme, they
are mildly antiseptic.

The structure of the retina. The retina
is less than 1 /80th of an inch thick. Yet
it is composed of seven layers of cells,
receptors, ganglia, and nerve fibers. The
function of all the structures of the eye
is to focus light on the retina. The
specialized receptors that are stimulated
by light are called photoreceptors and
are of two types, cones and rods (Fig.
44-15). Cones are sensitive to bright
light and are responsible for color vision.
The rods act in reduced light but do not
respond to color. Perhaps the phrase

Optic

nerve

Fat layers

Pupil

Sclerotic
layer

Choroid layer

Retina

Muscles

Cornea

Ciliary muscles

Crystalline lens

44-14 Left: a view of the human eye showing the muscles and socket. Right: a
cutaway diagram showing the various internal structures.

626 UNIT 7 THE BIOLOGY OF MAN

44-15 This drawing shows the structures at the back of the eye. The enlarge¬
ment on the right illustrates the arrangements of the rods and cones of the
retina.

“cones color, rods reduced” will help
you to keep the two straight. They lie
deep in the retina, pointing toward the
back surface of the eyeball. Impulses
from the cones and rods travel through
a series of short nerves with brushlike
endings to ganglia near the front part
of the retina. More than half a million
nerve fibers lead from the ganglia over
the surface of the retina to the optic
nerve. There are no rods or cones at
the point where the end of the optic
nerve joins the retina. Since there is
no vision at this point, it is called the
blind spot.

How we see. Cones occur throughout
the retina, but are especially abundant
in a small, sensitive spot called the
fovea (foh-vee-a) . When we see in day¬
light, light rays pass through the cornea,
aqueous humor, pupil, lens, and the

vitreous humor to the cones of the ret¬
ina. The lens focuses rays on the fovea,
the point at which we see an object
clearly. As light rays pass through the
lens, thev cross and strike the retina in
an inverted position (Fig. 44-16).
Cones outside the fovea register vision
only indistinctly. Thus, if you focus

Retina

44-16 How we see. Light rays enter the eye,
cross in the lens, and focus on the retina.
Why is the image inverted on the retina?

CHAPTER 44 BODY CONTROLS 627

44-17 Study these two drawings, which represent parts of the human eye and
those of a camera. Then, indicate exactly how the parts of the camera are simi¬
lar to those of the eye and how these parts differ.

your eyes on an object, you see it
clearly. In addition, you see indis¬
tinctly objects contained in a hemi¬
sphere of vision, or “out of the corner
of your eye.”

During the late evening or at night,
the light is too reduced to stimulate the
cones. This quality of light stimulates
the rods. Rods produce a substance we
call visual purple , which is necessary for
their proper functioning. Bright light
fades visual purple, which causes the
rods to be insensitive to further stimu¬
lation by light. This explains why,
when you leave a bright room at night,

you are temporarily night blind. As
visual purple is restored, the rods begin
to function, and you can see objects in
dim light.

The human eye contains fewer
rods than many animal eyes, so that our
night vision is relatively poor. The cat,
deer, and owl see well at night because
they have many rods. The owl, how¬
ever, lacks cones and is day blind.

The fovea of your retina contains
many cones but no rods. This explains
why you can see an object “out of the
corner of your eye” at night, but when
you focus on it, it disappears.

IN CONCLUSION

The brain and spinal cord compose the central nervous system. They com¬
municate with all parts of the body by nerves. The cerebrum controls con¬
scious activities. It is the center of intelligence and contains both sensory and
motor areas. Impulses from the cerebral motor area pass through the cerebel¬
lum, where coordination of impulses takes place. The medulla oblongata
controls the activity of internal organs and is the control center of respiration.

Sensory nerves carry impulses from their receptors in sense organs to the
central nervous system. The numerous minute sense organs of the skin re¬
spond to touch, pressure, pain, heat, and cold. The endings involved in the
sense of smell contact odors that have reached the upper turbinate region of
the nasal passages. The ears are highly developed sense organs which receive
air-borne vibrations and carry them to the receptors in the cochlea in the inner

628 UNIT 7 THE BIOLOGY OF MAN

ear. They also contain the semicircular canals, which control our sense of
equilibrium.

The eye is the most highly specialized of the sense organs. It receives
light rays through the pupil and directs them to the retina by means of the
lens. In the retina, the photoreceptors send impulses through the optic nerve
to the visual center in the occipital lobes of the brain.

In the next chapter we shall see how the nervous system and sense organs
are affected by alcohol, narcotics, and tobacco.

BIOLOGICALLY SPEAKING

anvil bone

dendrite

pons

aqueous humor

eardrum

pupil

auditory canal

Eustachian tube

receptor

auditory nerve

fovea

reflex action

autonomic nervous

hammer bone

retina

system

iris

rod

axon

medulla oblongata

sclerotic layer

central nervous

meninges

semicircular canals

system

motor neuron

sensory neuron

central neurons

nerve

spinal cord

cerebellum

nerve impulse

spinal nerve

cerebrospinal fluid

neuron

stirrup bone

cerebrum

olfactory nerve

sympathetic nervous

choroid layer

optic nerve

system

ciliary muscles

oval window

synapse

cochlea

parasympathetic

taste buds

cone

nervous system

turbinate

cornea

peripheral nervous

visual purple

cranial nerves
crystalline lens

system

vitreous humor

QUESTIONS FOR REVIEW

1. Name the three main divisions of the nervous system and state the func¬
tions of each.

2. Why are peripheral nerves containing only axons considered to be motor
nerves?

3. What occurs at the endings of a motor neuron that causes a muscle fiber
to contract? What causes it to relax?

4. Name the parts of the brain and state the functions of each.

5. In what way is the autonomic nervous system really two systems?

6. Name the five sensations of the skin. In what ways are the receptors dif¬
ferent?

CHAPTER 44 BODY CONTROLS 629

7. Account for the fact that we think we distinguish more than the five tastes
the tongue can perceive.

8. Describe how a sound wave in the air is able to stimulate the receptors in
the cochlea.

9. How can an infection in the middle ear produce temporary deafness?

10. Describe the movements of the head that would be necessary to stimulate
each semicircular canal separately.

11. Why is our vision at night relatively poor when compared to the eyes of
an owl?

12. Why is it that you can see an object out of the corner of your eye at night,
but when you focus on it, it disappears?

APPLYING PRINCIPLES AND CONCEPTS

1. What is intelligence?

2. Explain the activity that occurs after a chicken has had its head cut off.

3. Explain the fact that the sympathetic nervous system is sometimes called
“the system for fight or flight.”

4. How would you go about designing an experiment to prove whether or not
the eye really receives an image upside down, while the brain interprets it
oppositely?

CHAPTER 4 5

ALCOHOL,

NARCOTICS,

AND

TOR A CCO

Three social and health problems.

Problems resulting from alcohol and
narcotics are as old as civilization. But
they are more acute today because auto¬
mobiles, firearms, and other mechanical
devices become implements of destruc¬
tion in the hands of an intoxicated or
drug-addicted person.

We shall consider alcohol, tobacco,
and narcotics together, because they are
all harmful substances when used habit¬
ually. Young people who see a large
proportion of adult society smoking
without any apparent effect may arrive
at the wrong conclusions, because the
long-term effects of tobacco are not al¬
ways visible. While there is no real
justification for the use of tobacco, its
influence on the human mechanism is

not to be compared with the social,
emotional, and mental damage result¬
ing from the habitual and excessive use
of alcohol or narcotics. This chapter
will show you some of the effects of
these substances on the body. It will
be up to each of you, as an individual,
to form your own opinion and your
own personal habits.

Alcohol in the body. Alcohol does not
have to be changed in form or composi¬
tion in the stomach before absorption
occurs. It starts to enter the blood
within two minutes and is rapidly ab¬
sorbed and delivered to the tissues.
This absorption is even more rapid
when the stomach is empty than when
it contains food.

Oxidation in the tissues begins im¬
mediately, and large amounts of heat
are released. The body tissues oxidize
alcohol at the rate of approximately one
ounce in three hours. Because this rate
cannot be changed according to the
energy needs of the body, alcohol has
no value as a food. The excess heat is
picked up by the blood and delivered to
the skin, where it causes the character¬
istic alcoholic flush. Since the recep¬
tors of heat are in the skin, the rush of
blood to the skin gives a false impres¬
sion of warmth. Actually the internal
organs are being deprived of adequate
blood supply and become chilled.

Some effects of alcohol on the body
organs. Alcohol is absorbed by all the
body organs, so that all of them are af¬
fected by its presence. But some or¬
gans seem to be affected more than oth¬
ers. The oxidation of alcohol produces
water, which is excreted in large quan¬
tities by the skin during heat elimina¬
tion. The tissues become dehydrated,
and this loss of water concentrates ni¬
trogenous wastes in the kidneys, inter¬
fering with normal elimination.

630

CHAPTER 45 ALCOHOL, NARCOTICS, AND TOBACCO 631

Vitamin-deficiency diseases are com¬
mon among alcoholics, as they often
starve themselves during long periods
of excessive drinking. In addition, dur¬
ing these fasts the liver is deprived of
its stored food and swells as the carbo¬
hydrates are replaced by fats. This con¬
dition, known as fatty liver , is found in
75 percent of alcoholics. A more serious
degeneration of the liver, called cirrho¬
sis (si-roh-sis), occurs eight times more
frequently in alcoholics than in other
people.

Another organ frequently affected
by excessive alcohol is the stomach. Al¬
cohol causes increase in stomach secre¬
tions, which often leads to a painful in¬
flammation of the stomach lining called
gastritis.

Effects of alcohol on the nervous svs-

tem. Alcohol is a depressant , as it has
an anesthetic effect on the nervous svs-

tem. You may have heard it referred
to as a stimulant because its anesthetic
effect releases inhibitions. It is, how¬
ever, exactly the opposite of a stimu¬
lant.

The first effects of alcohol occur in
the cortex of the brain. Loss of judg¬
ment, will power, and self-control oc¬
cur. Cares seem to vanish, and the per¬
son becomes gay and lightheaded. In¬
fluence on the frontal lobe alters emo¬
tional control and may lead to a feeling
of great joy, shown by foolish laughter,
or to sadness and weeping. As the ef¬
fects of alcohol progress through the
brain tissue, the vision and speech areas
of the cerebrum become involved.
There is blurred or double vision, lack
of ability to judge distance, and slurred
speech.

As the cerebellum becomes in¬
volved, coordination of the muscles is
affected. The victim becomes dizzv
when standing, and if he is able to walk

45-1 Drunkometer tests are given to motor¬
ists in an effort to reduce highway injuries
and deaths. (Wide World Photos)

at all, he does so with a clumsy, stagger¬
ing gait.

In the final stages of drunkenness
a person becomes completely helpless.
The brain cortex ceases activity, result¬
ing in complete unconsciousness. The
skin becomes pale, cold, and clammy.
Heart action, digestive action, and res¬
piration slow up and the victim lies
near death.

Alcoholism is a disease. Six to seven
percent of the adult users of alcohol de¬
velop an abnormal, chronic dependence
on it. This disease, called alcoholism ,
may begin with occasional social drink¬
ing. As distressing situations and prob¬
lems arise and life seems temporarily
unpleasant, the individual uses alcohol
as an escape from realitv. The prob¬
lems remain unsolved, and alcohol is
used for a definite purpose — to try to
escape from the problems. With loss
of judgment and will power, the
chances of solving these problems are

632 UNIT 7 THE BIOLOGY OF MAN

further reduced. The alcoholic then
resorts to solitary drinking for the pure
effects of alcohol. The destruction of
his personality has begun.

If alcoholism is allowed to progress
to an acute condition, serious deteriora¬
tion of brain tissue may result. This
may cause terrifying hallucinations
known as delirium tremensf or “D.T.’s.”
The victim has visions of snakes, rats,
and other vermin crawling over his body,
and becomes violent with fear. By this
time his alcoholism has reached the pro¬
portions of alcoholic insanity.

Alcohol is the tool and not the un¬
derlying cause of alcoholism. Thus the
alcoholic seeking a cure must first find
the reason for his problem drinking.

Then he must attempt to solve the
problem, not resort to alcohol as an es¬
cape from it. Sympathetic understand¬
ing and the cooperation of his family
and friends help greatly in overcoming
the problem.

Thirty-four states and the District
of Columbia have clinics and hospitals
to assist alcoholics in curing their con¬
dition. They supply both medical
treatment and counseling necessary to
deal with the problem. Alcoholics
Anonymous is a voluntary organization
that has resulted in the recovery of
350,000 alcoholics.

Alcohol and the length of life. Life
insurance companies ask applicants
about their use of alcohol and drugs,

HOW ALCOHOL AFFECTS A DRIVER

Less dependable responses

Over-confidence and recklessness

CHART BY GRAPHICS INSTITUTE, N.Y.C.

45-2 Alcohol lowers every aspect of driving efficiency.

CHAPTER 45 ALCOHOL, NARCOTICS, AND TOBACCO 633

which is evidence that heavy drinkers
are poorer risks than those who abstain.
It is difficult to say that limited or mod¬
erate use of alcohol shortens life. But
no one can deny that even moderate
drinking of alcoholic beverages in¬
creases the possibility of accidental
death. It also lowers body resistance
and increases the possibility of death
from infectious disease, especially tuber¬
culosis. There is no question that
heavy drinking shortens life consider¬
ably.

Alcohol and society. The effects of
alcohol are much more far-reaching
than damage to the habitual drinker
himself. His family and all society pay
a price for his shortsightedness. Often
an alcoholic will neglect his family to
satisfy the desire for alcohol. Child
neglect, loss of job, divorce, and other
acute domestic problems frequently re¬
sult. Anxiety, frustration, and insecu¬
rity in children are a terrible price to pay
for alcoholism. Alcohol must also an¬
swer for much of the crime committed
in America. In a recent study of the
records of 13,402 convicts in 12 states,
alcohol was found to be a contributing
factor in 50 percent of the crimes com¬
mitted, and a direct cause of 16.8 per¬
cent of the crimes.

Alcohol and driving. Important experi¬
ments have recently been carried on in
Pennsylvania to test thoroughly, under
actual road conditions, the relation of
drinking to driving a car. Motorists
who have been given measured
amounts of alcohol but who were not
drunk (all but one passed the standard
police sobriety tests) were found to
make all sorts of accidental errors. Not
only did most of these drivers have a
slower braking reaction time, but they
were also inaccurate in performance.
Yet every one of these drivers thought

he was doing well. The fundamental
trouble, graphically proved by psycho¬
logical tests, was found to be the im¬
pairment of judgment after only one or
two drinks.

The death toll from automobiles is
over 100 per day with over 3,000 more
people injured per day. Safety officials
directly attribute from 7 to 10 percent
of fatal highway accidents to the use of
alcohol, while competent traffic officials
state that a third of these accidents is
indirectly caused by the driver’s alco¬
holic indulgence.

The effects of alcohol on a driver
of a car are as follows:

1. Less attention to signals and driving
hazards.

2. Slower responses of eyes, hands, and
feet, due to increased reaction time.

3. Increased self-assurance, which causes
a driver to take chances and be less
considerate of other drivers.

Narcotic drugs. By discussing problems
relating to alcohol and narcotic drugs in
the same chapter, we do not mean to
imply that the problems are similar.
Many people consume alcoholic bever¬
ages without becoming habitual drink¬
ers or alcoholics. Continued use of nar¬
cotic drugs, however, results in both
mental and physical addiction. That is,
the victim becomes dependent on the
mental and emotional effects of the drug
and his body develops a need for it.
When not under the influence of a nar¬
cotic, the addict develops violent with -
drawal symptoms. These include sleep¬
lessness, difficulty in breathing, irregular
heart action, and acute suffering. Men¬
tal symptoms include severe depression
and derangement. Withdrawal sickness
may be severe enough to cause death.
The longer an addict uses a narcotic,
the greater amount he must take to ward
off the withdrawal sickness.

634 UNIT 7 THE BIOLOGY OF MAN

45-3 “If you drive, don’t drink; if you drink, don’t drive.” Traffic officials esti¬
mate that one third of all automobile accidents are either direct or indirect
results of drinking. (Wide World Photos)

A narcotic is a drug designated as
such by the federal government be¬
cause it results in addiction if used with¬
out medical direction. The sale of nar¬
cotics, except on a doctor’s prescription,
is illegal.

Opium is the source of a family of
narcotic drugs. It is extracted from the
juice of the white poppy. Morphine
and codeine are derived from opium.
Heroin is a synthetic compound pre¬
pared from morphine. Morphine is
used to reduce pain. Codeine has simi¬
lar uses and is an ingredient in special
kinds of medicines, including some
cough syrups. Heroin is so dangerous
and addicting that its possession or use
in the United States, even for medical
purposes, is illegal.

Cocaine is a narcotic drug ex¬
tracted from the leaves of the South
American coca plant (not connected
with the beverage cocoa). Cocaine
deadens skin and mucous membranes.
A doctor may use it to deaden the area
around a wound before he cleanses it
or takes stitches. When taken inter¬
nally, cocaine causes a temporary stimu¬
lation of the nervous system and a feel¬
ing of pleasure. Later, however, the
victim is seized bv a feeling of great
fear and may even become violent.

A weed called marijuana (mar-i-
wha- null) often receives considerable at¬
tention in the newspapers as a narcotic.
It is usually mixed with tobacco and
smoked. The marijuana user develops
an emotional addiction to the effects of

CHAPTER 45 ALCOHOL, NARCOTICS, AND TOBACCO 635

the drug but does not experience physi¬
cal addiction. However, marijuana ad¬
dicts usually continue down the nar¬
cotic road to ruin, as they eventually
turn to opiates.

The narcotic menace. People may be¬
come narcotic addicts in several ways.
Some people become addicted after an
illness in which a narcotic drug was
used medically to relieve pain. Highly
nervous or distressed people may use
certain narcotics as depressants and con¬
tinue purchase of the drugs illegally.
Still others who are emotionally dis¬
turbed or maladjusted deal with dope
peddlers who are associated with the
organized and unlawful sale of nar¬
cotics.

Once a person starts to use an ad¬
dicting drug such as heroin, he starts
down the shortest and surest road to
ruin. The process takes only a few
weeks. The narcotic addict becomes
absolutely useless to himself or anyone
else. He cannot hold a job. He with¬
draws from his family and friends to
live alone with his drug and needle.

There are approximately 60,000 ad¬
dicts in the United States today. For
most of these people crime is a neces¬
sary way of life. They are forced to
steal to pay the fantastic prices exacted
in the illegal narcotics traffic. Of the
addicts who are treated in the two fed¬
eral narcotics hospitals, less than one
fourth stay off drugs after their release.
Many of these are teen-agers, some as
young as 13 or 14.

Several things can happen to an
addict. He can ask for treatment,
which is often unsuccessful. He can
be arrested for illegal possession of nar¬
cotics. He can die from an overdose,
an infected needle, or suicide. The
only way to avoid these tragedies is
never to start taking a narcotic.

The barbiturate problem. The barbi¬
turates are synthetic drugs that act as
sedatives. They are commonly called
sleeping pills. Although it is not gen¬
erally known, hundreds of thousands of
people in this country use barbiturates
habitually. Barbiturates are not addict¬
ing in the sense that narcotic drugs are,
but victims develop a psychological de¬
pendence on them and may even have
withdrawal symptoms when they are
taken away. Aside from the degrading
dependence, there are two severe haz¬
ards of barbiturates: they are fatal in
combination with excess alcohol, and
they are frequently used for suicide.
Tobacco — the nation’s leading habit.
More than 70 million people in the
United States use tobacco in some form.
The great majority of these are ciga¬
rette smokers. The smoker actually
becomes a slave to two habits: the smok-

45-4 Painful, grotesque withdrawal symp¬
toms occur when a narcotic addict is com¬
pletely taken away from his drug. (Wide
World Photos)

636 UNIT 7 THE BIOLOGY OF MAN

ing habit and the tobacco habit. The
first involves reaching for a cigarette at
regular intervals, lighting it, and going
through the various movements asso¬
ciated with smoking. Heavy smokers
often light a second cigarette even be¬
fore finishing the first one. They also
acquire a physiological craving for the
nicotine in tobacco — the tobacco habit.

Most young people who start smok¬
ing feel that it makes them seem more
mature. Yet, if they asked the advice

of an older person who has smoked for
several years, his advice would undoubt¬
edly be not to start. Certainly several
things should be considered before de¬
liberately starting a practice as habit¬
forming and as dangerous to health as
smoking.

The effects of smoking. In 1964 the
Public Health Service published a re¬
port on the effects of smoking based on
experiments with animals, clinical and
autopsy studies in man, and studies of

INHALATION

CD ° 3

2 E of
<s) <d

1.00

NUMBER DAILY

-O —

S> o 3
^ w «

1.00

-4— *

_ro

(?)

1.86

1.98

45-5 These graphs show ratios of death rates in the age group 40-69. The
death rate for nonsmokers is given as 1.00; the other numbers are ratios com¬
pared to this base number. (Adapted from a report by E. C. Hammond)

CHAPTER 45 ALCOHOL, NARCOTICS, AND TOBACCO 637

the occurrence of disease in the popula¬
tion. Included in the report were the
following findings:

Tissue damage: Sections of lung tis¬
sue from thousands of smokers have
been examined after death. Even in
individuals who did not die of cancer,
abnormal cells were found in the lungs.
Enlarged and ruptured alveoli and thick¬
ened arterioles were observed. In the
trachea and bronchi, cilia and the pro¬
tective cells of the mucosa were de¬
stroyed. Remember that these struc¬
tures normally cleanse and lubricate the
respiratory tract and help to prevent in¬
fection.

Increased death rate: For the pur¬
poses of this study the number of deaths
among a large sample of nonsmokers
was compared to the number of deaths
among a similar sample of smokers.
There were 70 percent more deaths from
all causes among smokers than among
nonsmokers. The greater number of
deaths from certain diseases among
smokers was particularly marked. There
were 1000 percent more deaths from
lung cancer and 500 percent more from
chronic bronchitis and the degenera¬
tive lung disease called emphysema.
The death rate was also considerably
higher for cancer of the tongue, larynx,
and esophagus, for peptic ulcer, and for
circulatory diseases.

Increase of death rate with the
amount smoked: In general the greater

the number of cigarettes smoked, the
higher the death rate. For men who
smoke less than ten cigarettes a day, the
death rate is about 40 percent higher
than for nonsmokers. For those who
smoke 40 or more it is 120 percent
higher. The same kind of relationship
exists between the number of years of
smoking and the death rate.

It should be obvious from these
findings that smoking is a health haz¬
ard. To put the figures on lung cancer
in another way, 95 percent of the vic¬
tims of lung cancer are heavy smokers.
One half of one percent are nonsmok¬
ers. Lip cancer has definitely been
traced to irritation from a pipe or cigar.
Lung and lip cancer, however, do not
account for a large percentage of deaths
in this country. Heart and circulatory
diseases are the number one cause of
death in the United States. The death
rate for these diseases is 200 percent
higher among smokers.

Aside from the long-term effects,
smoking has many short-term disad¬
vantages. It is an expensive habit, and
gets more so every year. It is messy
and often annoying to other people.
Smoking stains the teeth and fingers
and makes the breath unpleasant. Cer¬
tainly no one's appearance is improved
by a cigar or cigarette hanging from his
mouth. Smoking causes constant throat
irritation, stomach discomfort, nervous¬
ness, and sometimes headaches.

IN CONCLUSION

Alcohol is a depressant that has no relation to the energy needs of the body.
Its habitual use may lead to alcoholism, various organic diseases, and even

death. . .

Narcotic addiction is one of the worst medical and social problems in the

country today. The narcotic road is one-way. It leads all too suddenly to

complete physical, moral, and mental ruin.

638 UNIT 7 THE BIOLOGY OF MAN

While tobacco is not addicting in the sense that narcotic drugs are, the
various ways in which cigarette smoking is dangerous to health are becoming
more obvious with each year of further research.

BIOLOGICALLY SPEAKING

addiction

alcoholism

barbiturate

cirrhosis

cocaine

codeine

delirium tremens
depressant
fatty liver
gastritis
heroin

marijuana

morphine

narcotic

opium

withdrawal symptoms

QUESTIONS FOR REVIEW

1. Why is alcohol not considered a food?

2. What happens when alcohol enters the body?

3. What organs of the body are especially affected by alcohol?

4. Why is alcohol not considered a stimulant?

5. Where in the nervous system does alcohol first have an effect? What are
the results of this effect?

6. Why is it dangerous for a person who has had an alcoholic drink to drive
an automobile?

7. Name six definite narcotic drugs as defined by the federal government.

8. In what ways is it possible to become a narcotic addict?

9. What are some of the findings concerning the death rate among smokers?

10. What are some of the short-term disadvantages of smoking?

APPLYING PRINCIPLES AND CONCEPTS

1. Why is drinking alcohol on an empty stomach more injurious than drink¬
ing with meals or after eating?

2. Why does the presence of alcohol in the body give a person a feeling of
warmth?

3. Why do life insurance companies always ask the applicant if he drinks, or
smokes, and to what extent?

4. Explain the possible relationship between drug addiction and juvenile
delinquency. How do drug addicts and alcoholics show character weak¬
ness?

5. Why is inhaling smoke more injurious than not inhaling?

CHAPTER 46

BODY

REGULA TORS

What are the ductless glands? You are
already familiar with some glands, such
as the salivary glands of the mouth and
the gastric glands of the stomach. These
pour secretions into the digestive tract
through ducts. The ductless glands,
which we shall study in this chapter, are
entirely different from the digestive
glands. The name ductless indicates
that they have no ducts leading from
them; their secretions enter the blood
stream directly. With blood as a trans¬
porting medium, these secretions reach
every part of the body and influence all
the organs. Ductless glands are also
called endocrine (en- do-krin) glands.

We call the secretions of ductless
glands hormones. These chemicals are
formed from substances taken from the
blood. They regulate the activity of all
the body processes. Thus the circula¬
tory system is vital to the endocrine sys¬
tem, both in supplying the raw mate¬
rials and in delivering the finished prod¬

uct. For the most part the endocrine
glands are small. Their size is entirely
out of proportion to the vital influence
they exert on the body.

As you study the various glands,
you may want to refer to Fig. 46-1 and
to the summary table on page 645.
The thyroid gland and its hormone.
You are probably more familiar with the
thyroid than with any of the other en¬
docrine glands. The gland is relatively
large and lies close to the body surface,
in the neck near the junction of the
lower part of the larynx and the trachea.
The thyroid consists of two lobes con¬
nected by an isthmus (Fig. 46-2).
The lobes lie on either side of the tra¬
chea and extend upward along the sides
of the larynx. The isthmus extends
across the front surface of the trachea.
As Fig. 46-2 shows, the complete thy¬
roid gland somewhat resembles a butter¬
fly with its wings spread.

The thyroid hormone contains a
substance called thyroxine, which has
the highest relative concentration of
iodine of any substance in the body.
From this you can understand why io¬
dine is essential for normal function¬
ing of the body. Commercially the thy¬
roid hormone is prepared by extraction
from the thyroid glands of sheep. After
being purified it is called thyroid extract
and is used in treating thyroid disor¬
ders. Thyroid extract is the least expen¬
sive of any commercial endocrine prep¬
aration.

The thyroid and metabolism. The thv-
roid hormone regulates the rate of me¬
tabolism. In this way it influences
growth and oxidation. Overactivity of
the thyroid gland, or hyperthyroidism ,
increases the rate of oxidation and raises
the bodv temperature. Heart action in¬
creases and blood pressure rises. Sweat¬
ing, when the body should be cool, is a

639

640 UNIT 7 THE BIOLOGY OF MAN

Pineal

body

Pituitary

gland

Adrenal

glands

Pancreas

Thyroid
gland -

Parathyroid

glands

Thymus
(in babies
and

children)

46-1 Diagram showing the location of the
endocrine glands in the female. Why are
they called ductless glands?

common symptom of this condition.
The individual also becomes extremely
nervous and irritable. Some victims de¬
velop characteristic bulging eyes and a
staring expression.

Hyperthyroidism used to be treated
solely by surgery, but recently treatment
with a drug called thiouracil (thy-oh-

yur-a-sil) has been found to be effective.
Another treatment for hyperthyroidism
consists of dosage with radioactive io¬
dine. This is picked up by the gland as
ordinary iodine would be. It then bom¬
bards the gland with radioactivity and
destroys some of the gland tissue, as
surgery formerly did.

Underactivity of the thyroid gland,
called hypothyroidism , produces the op¬
posite symptoms. The rate of oxida¬
tion is decreased and activity of the
nervous system is reduced. This pro¬
duces characteristic physical and mental
retardation. Heart action decreases and
in many cases the heart enlarges. Both
overactivity and underactivity of the
thyroid gland may be determined by
measuring the rate of basal metabolism.
Hypothyroidism may be treated with
thyroid extract.

If the thyroid is defective during
infancy, cretinism results. This condi¬
tion is characterized by stunted physi¬
cal and mental development. The face
usually becomes bloated, the lips greatly
enlarged, and the tongue thick and pro¬
truding from the mouth. If the cretin
passes from infancy to childhood with¬
out treatment with thyroid extract, the
dwarfism and mental deficiency can
never be corrected.

If the thyroid stops working during
adult life, myxedema (mik-se-dee-ma)
results. This causes coarsening of the
features and swollen eyelids. Often
mental ability suffers. Like cretinism,
myxedema can be corrected with thyroid
extract if treatment is started early.

Iodine deficiency is the major cause
of enlargement of the thyroid gland,
known as simple goiter. This condition
is rare along the seacoast where people
eat an abundance of seafoods contain¬
ing iodine. It is more common in
mountainous regions and in the Great

CHAPTER 46 BODY REGULATORS 641

Lakes basin where the iodine content
of the soil is low. The addition of io¬
dine compounds to table salt and to the
water supply in certain regions is an ade¬
quate preventive measure.

The parathyroid glands. The parathy¬
roids are four small glands embedded in
the back of the thyroid, two in each
lobe (Fig. 46-2). Their secretion, para-
thormone , controls the use of calcium
in the body. Bone growth, muscle tone,
and normal nervous activity are abso¬
lutely dependent on a constant, stable
calcium balance.

The pituitary gland. The small pitui¬
tary , a gland about the size of an acorn,
lies at the base of the brain. It was
once called the “master gland,” since its
secretions influence the activity of all
other glands. It is now known that
other glands, especially the thyroid and
adrenals, in turn influence the pituitary.

The pituitary gland consists of two
lobes: anterior and posterior. The an¬
terior lobe secretes several different hor¬
mones. One of these, the somatotropic
hormone , or growth hormone , regulates
the growth of the skeleton. Other secre¬
tions of the anterior lobe of the pituitary
gland, the gonadotropic hormones , in¬
fluence the development of the repro¬
ductive organs. It also influences the
hormone secretion of the ovaries and
testes. The gonadotropic hormone, to¬
gether with the sex hormones, cause the
sweeping changes that occur during ado¬
lescence, when the child becomes an
adult.

Other secretions of the anterior
lobe of the pituitary gland include hor¬
mones that stimulate the secretion of
milk in the mammarv glands (lacto¬
genic hormone), and the activity of the
thyroid gland ( thyrotropic hormone).

Thyroid cartilage

Right lobe of
thyroid gland

Left lobe of
thyroid gland

Parathyroid glands
(on dorsal side)

Isthmus of the
thyroid

Trachea

Vein from
thyroid gland

46-2 Note the position of the thyroid gland
in relation to the trachea. Note also the
four parathyroid glands embedded in the
back of the thyroid.

46-3 A section of human thyroid tissue.
The large areas are follicles into which the
hormone is secreted from surrounding cells.
(Walter Dawn)

642 UNIT 7 THE BIOLOGY OF MAN

46-4 If the anterior lobe of the pituitary
gland produces too little of the somatropic
hormone, a midget may result. What hap¬
pens when there is considerable oversecre¬
tion of this growth hormone? (United Press
International)

ACTH ( adrenocorticotropic hor¬
mone) is another secretion of the an¬
terior lobe of the pituitary gland; it stim¬
ulates the outer part, or cortex, of the
adrenal glands. ACTH has been used
in the treatment of leukemia and more
successfully in the treatment of arthri¬
tis. Good results in the treatment of
asthma and other allergies with ACTH
have also been reported. Even though
ACTH may not permanently cure these
diseases, its use may lead the way to the
discovery of their actual causes.

The posterior lobe of the pituitary
gland produces two hormones: oxytocin ,
which helps regulate the blood pressure
and stimulates smooth muscle; and vaso¬
pressin, which controls water resorption
in the kidneys. Oxytocin is adminis¬
tered during childbirth to cause con¬
traction of the uterus.

Vasopressin deficiency causes a con¬
dition called diabetes insipidus (in -sip-
id-us), in which the craving for water is
constant. This disease should not be
confused with true diabetes, which we
shall discuss in connection with the
pancreas.

Disorders of the pituitary gland. The
most frequent disorder of the pituitary
involves the somatotropic hormone. If
an oversecretion of this hormone occurs
during the growing years, a giant may re¬
sult. There are circus giants over 8 feet
tall who weigh over 300 pounds and
wear size 30 shoes. If the oversecretion
occurs during adult life, the bones of
the face and hands thicken, since they
cannot grow in length. The organs and
the soft tissues enlarge tremendously.
7 his condition is known as acromegaly
(ak-roh-meg-a-lee) . Victims of this dis¬
order have greatly enlarged jawbones,
noses, and hands and fingers.

Somatotropic hormone deficiency
results in a pituitary dwarf, or midget.
These individuals are perfectly propor¬
tioned men in miniature. They are
quite different from the thyroid dwarf
in that they have normal intelligence.
The glands of emergency. The adrenal
glands, also called suprarenals, are lo¬
cated on top of each kidney (Fig. 46-5).
They -are composed of an outer region,
the cortex, and an inner part, the me¬
dulla. Unlike the adrenal medulla the
adrenal cortex is absolutelv essential for

life. It secretes a hormone complex
called cortin. These hormones are re-

CHAPTER 46 BODY REGULATORS 643

sponsible for the control of certain
phases of carbohydrate, fat, and protein
metabolism as well as of the salt and
water balance in the body. The adrenal
cortex also yields hormones that control

the production of some types of white
corpuscles and the structure of connec¬
tive tissue.

Addison's disease results from
damage or destruction of the adrenal
cortex, which often occurs as a result
of tuberculosis. Symptoms include fa¬
tigue, nausea, loss of weight, general
circulation failure, and changed skin
color. Treatment with the compound
in cortin called cortisone is effective.

The medulla secretes a hormone
called epinephrine ( ep-i-ne/-reen ) , or
adrenaline. The adrenal glands have
been called the glands of emergency be¬
cause of the action of this hormone.
Many people have performed super¬
human feats of strength during periods
of anger or fright, with the help of
epinephrine. This strength of despera-

46-5 Notice the position of the adrenal
glands in relation to the kidneys. What two
important hormones do these glands secrete?

tion results from a series of rapid
changes in body activity.

1. The person becomes pale, because of
constriction of the blood vessels in
the skin. The rapid movement of
blood from the bodv surfaces reduces
loss of blood if there is a surface
wound. It also increases the blood
supply to the muscles, brain, heart,
and other vital organs.

2. The blood pressure rises, because of
constriction of surface blood vessels.

3. The heart action and output are in¬
creased.

4. The liver releases some of its stored
sugar and provides material for in¬
creased body activity and oxidation.

The pancreas. The production of pan¬
creatic fluid in connection with digestion
is only part of the function of the pan¬
creas. Special groups of cells, called
islets of Langerhansy secrete the hor¬
mone insulin. This hormone enables
the liver to store sugar as glycogen and
regulates the oxidation of sugar.

A person who lacks insulin cannot
store or oxidize sugar efficiently. Thus
the tissues are deprived of food, and
sugar collects in the blood. As the
blood sugar rises, some of it is excreted
in the urine. Doctors call this condition
diabetes mellitus (me-Zy-tus). Diabetes
mellitus, however, is probably not only
simple failure of the islet cells of the
pancreas to produce insulin. The pitu¬
itary, thyroid, and adrenal glands, as
well as the liver, are known to play an
important part in the disease. Body
weight also influences the appearance
of this condition. Diabetes mellitus is
definitely hereditary. If you have it in
your family, regular periodic checkups
for sugar in the urine should be made
by your family doctor. There is no
cause for alarm if the disease appears,
for once discovered, it can usually be

644 UNIT 7 THE BIOLOGY OF MAN

46-6 A diabetic learns to inject himself with
insulin and also to control his diet so he may
lead a normal life. (New York Diabetes As¬
sociation, Inc.)

controlled successfully. If treatment is
begun in the early stages, the patient
can lead a perfectly normal life.

Production of excess insulin results
in a condition called hypoglycemia ,
which means, literally, ‘‘low blood
sugar.” Excess insulin in the blood
causes sugar that should be delivered to
the cells to be stored in the liver. The
fatigue caused by hypoglycemia is
treated by a diet controlled in its car-
bohvdrate content.

The ovaries and testes. The ovaries of
the female and the testes of the male
have dual functions. We think of them
primarily as organs for the production of
eggs and sperm. However, certain cells
of the ovaries and the testes serve as
ductless glands. These ovary cells
secrete the female hormones estrogen
(es-troh-jen) and progesterone (proh-
jes- ter-ohn). Special cells of the testes
produce the male hormone testosterone

(tes-tos-ter-ohn ) . This hormone can
now be produced artificially, and is used
in treating sex hormone disturbances in
both males and females. Furthermore,
the production of this hormone is not
limited to the testes. It is secreted by
the cortex of the adrenal glands in both
males and females. In the female the
estrogen secreted in the ovaries nor¬
mally neutralizes the effects of the
testosterone from the adrenal glands.
However, if the estrogen secretion in the
ovaries is reduced, the female may be¬
come mannish. Similarly, reduced pro¬
duction of testosterone in the testes of
the male can result in feminine tend¬
encies. Thus different individuals may
represent various degrees of maleness
and femaleness.

Sex hormones control the develop¬
ment of the secondary sex characteristics
that appear in the change from child¬
hood to adulthood. These changes first
appear with the maturation of the
ovaries and testes during the period
called puberty. In the animal world
these characteristics may appear as the
large comb of the rooster, the bright
plumage of most male birds, and the
antlers of the deer. Many second¬
ary characteristics are appearing and
have appeared in your own body. As a
boy approaches pubertv, his voice
cracks and then deepens. His beard ap¬
pears along with a general increase in
body hair. The chest broadens and
deepens. Rapid growth of the long
bones adds to his height. As a girl ma¬
tures, her breasts develop and her hips
broaden, because of the formation of
fat deposits under the skin, and men¬
struation begins. These physical
changes in both boys and girls are ac¬
companied by sweeping mental and
emotional changes. Compare your pres¬
ent personality with that of a child ten

CHAPTER 46 BODY REGULATORS 645

DUCTLESS GLANDS AND THEIR SECRETIONS

Gland

Location

Hormone

Function of Hormone

Thyroid

Neck, below

Thyroid

Accelerates the rate of metabo-

larynx

hormone

lism

Parathyroids

Pituitary

Back surface of
thyroid lobes

Base of brain

Parathormone

Controls the use of calcium in
the tissues

Anterior lobe

Somatotropic

hormone

Gonadotropic

hormone

ACTH

Lactogenic

hormone

Thyrotropic

hormone

Regulates growth of the skele¬
ton

Influences development of sex
organs and hormone secre¬
tion of the ovaries and testes
Stimulates secretion of hor¬
mones by the cortex of the
adrenals

Stimulates secretion of milk by
mammary glands

Stimulates activity of the thy¬
roid

Posterior lobe

Oxytocin

Vasopressin

Regulates blood pressure and
stimulates smooth muscles
Controls water resorption in

Adrenal

Above kidneys

the kidneys

Cortex

Cortin (a hor¬
mone com¬
plex)

Regulates metabolism, salt, and
water balance

Controls production of certain
white corpuscles and struc¬
ture of connective tissue

Medulla

Pancreas

Below and be¬
hind stomach

Epinephrine

Causes constriction of blood
vessels, increase in heart ac¬
tion and output, stimulates
liver and nervous system

Islets of
Langerhans

Ovaries

Pelvis

Insulin

Enables liver to store sugar and
regulates sugar oxidation in
tissues

Follicular

cells

Testes

Below pelvis

Estrogen

Progesterone

Produces female secondary sex
characteristics; influences

adult female body functions

Maintains growth of the mu¬
cous lining of the uterus

Interstitial

cells

Testosterone

Produces male secondary sex
characteristics

646 UNIT 7 THE BIOLOGY OF MAN

to twelve years old and you will see how
these glands have influenced you. These
glands will be discussed further in the
next chapter.

The pineal body and thymus. The
pineal body is a mass of tissue about the
size of a pea, located at the base of the
brain. It lies directly behind the junc¬
tion of the spinal cord and brain tissue.
This body may or may not be a duct¬
less gland. No hormone secretion from
the pineal body has been discovered,
and we do not know what its func¬
tion is.

The thymus , too, may be a ductless
gland, but no endocrine secretion has
been identified with it. It lies just
above the heart, under the breastbone.
At birth the thymus weighs less than
half an ounce. It increases in size dur¬
ing childhood and reaches its maximum
size between the ages of 12 and 14.
During the time when it is maximum
size, the thymus gland usually weighs
about one ounce, or twice its weight
at birth. During adulthood it gradu¬
ally grows smaller and finally shrinks
to the size it was at birth. The thymus
is a center of the production of cells
called lymphocytes. Recent research
indicates that these cells may be the
parent cells of those that produce anti¬
bodies in the lymph nodes and spleen.
Thus the thymus may be important in
the body’s defenses against disease.
Dynamic balance in the endocrine
glands. We have seen that too much or
too little of a hormone can upset the
balance that the endocrine glands nor¬

mally maintain in the body. We have
also seen that one gland can influence
the activity of others. Besides the in¬
fluence of glands on one another, there
are two other factors operating to pro¬
duce the delicate check-and-balance sys¬
tem in body chemistry.

In the first of these factors, called
feedbacky the accumulation of a sub¬
stance in the blood automatically cuts
down its production by the endocrine
gland. For example, remember that
parathormone regulates the level of cal¬
cium in the body. The concentration
of calcium in turn regulates the produc¬
tion of parathormone. When the cal¬
cium level in the body drops, the secre¬
tion of parathormone increases to re¬
store the calcium level. When the
proper level is reached, the calcium in¬
fluences the parathyroids to decrease
their secretion. This same kind of feed¬
back occurs in the other glands and be¬
tween various glands. In this way a
balanced state is automatically main¬
tained in the normally functioning
body.

The endocrine glands are also af¬
fected by the activity of the nervous
system, which acts as a monitor of both
internal and external conditions. The
adrenal medulla, for example, may be
stimulated to produce epinephrine as
the need is signaled by the sympathetic
nervous system. Nervous control and
feedback are further examples of home¬
ostatic mechanisms operating in the
body to maintain a steady state in the
face of constantly changing conditions.

IN CONCLUSION

Ductless glands secrete hormones directly into the blood stream. These
hormones influence body metabolism, growth, mental capacity, chemical bal¬
ance in the body fluids, and many other functions. Glands are controlled
by feedback and by nervous control, as well as by one another.

CHAPTER 46 BODY REGULATORS 647

In this chapter you have studied some of the endocrine functions of the
ovaries and testes. In the chapter to follow you will study the primary func¬
tion of these organs, that of producing the sex cells for reproduction.

BIOLOGICALLY SPEAKING

acromegaly

giant

parathyroid glands

Addison’s disease

gonadotropic hormones

pineal body

adrenal cortex

hormone

pituitary gland

adrenal glands

hypoglycemia

progesterone

adrenal medulla

hyperthyroidism

puberty

ACTH

hypothyroidism

simple goiter

cortin

insulin

somatotropic hormone

cretinism

islets of Langerhans

testes

diabetes insipidus

midget

testosterone

diabetes mellitus

myxedema

thymus

endocrine gland

ovaries

thyroid gland

epinephrine

oxytocin

thyroxine

estrogen

feedback

pancreas

parathormone

vasopressin

QUESTIONS FOR REVIEW

1. What role does the blood play in the function of endocrine glands?

2. How does the thyroid gland regulate the rate of metabolism?

3. Explain how hyperthyroidism and hypothyroidism can affect personality.

4. In what ways do the pituitary and thyroid glands influence growth?

5. How does the pituitary gland affect the sex glands?

6. Compare the body characteristics of a thyroid dwarf and a pituitary dwarf.
How do they differ and in what ways are they similar?

7. What is ACTH? What is its function?

8. In what ways are puberty and adolescence a result of glandular activity?

9. Why does sugar appear in the urine of a diabetic?

APPLYING PRINCIPLES AND CONCEPTS

1. What gland has a hormone that may influence intelligence?

2. How do you account for the fact that the heartbeat of a basketball player
increases a great deal before the game as well as during it?

3. Why is a study of the endocrine glands often included with a study of the

nervous system?

4. What hormone injected into the blood stream of a male rat will often
result in a mothering instinct? Why?

5. Discuss dynamic balance in the endocrine system resulting from feedback.

CHAPTER U7

REPRODUCTION

AND

DEVELOPMENT

The significance of sexual reproduction.
At this time it might be a good idea to
review the significance of the process
of sexual reproduction, which you first
studied in Unit I. The union of two
gametes to form a zygote that is capable
of growing into an organism resembling
the parents constitutes sexual reproduc¬
tion. Gamete production occurs in the
gonads. During the meiotic division
of gamete formation, the chromosome
pairs separate, resulting in haploid cells.
The mechanism of the separation of
chromosome pairs assures variation in
the genetic composition of the gametes.
Figure 8-7, page 111, shows how meio-
sis produces the haploid number of
chromosomes in gamete production.
When fertilization forms the zygote,
the diploid number of chromosomes is
restored, with each parent contributing

one chromosome to each pair. The
genes then begin their influence on the
development of a new organism. All
the zygotes of a species contain genes
that control the method of development
to create an individual resembling the
species. At the same time the combina¬
tion of genes for unlike characteristics
produces offspring that vary from each
parent.

Organisms that reproduce asex-
ually, in a one-parent system, do not
have this possibility of variation that
results from the combination of unlike
genes from two parents. Remember
that variations in offspring sometimes
result in favorable adaptations that im¬
prove the species. Although favorable
mutations may occur occasionally in
asexual organisms, their ability to adapt
is greatly lessened by the one-parent sys¬
tem.

The male reproduction system. The

male gonads are the testes and are
located outside the body in a pouch of
skin called the scrotum (Fig. 47-1).
Although these paired organs produce a
male hormone concerned with the de¬
velopment of secondary sex characteris¬
tics (see Chapter 46), the testes have
the other important function of produc¬
ing sperm. The highly coiled tubes
within the testes are the seminiferous
tubules (sem-i-mf-e-rus too-byoolz).
The haploid sperm, formed by meiotic
divisions of the cells in the testes, are
carried to the epididymis (ep-i-did-i-mis)
for storage, by the action of ciliated cells
of the seminiferous tubules. The vas
deferens (vas def-e- renz) is a duct that
carries the sperm from the epididvmis
past the seminal vesicle. A short tube
connects the seminal vesicle to the
urethra by passing through the prostate
gland and another small organ called
Cowper’s gland. These three structures

648

CHAPTER 47 REPRODUCTION AND DEVELOPMENT 649

Urethra

Seminal vesicle

Seminiferous
tubules

Epididymis

Prostate gland

Cowper’s gland

Scrotum

Ureters

Testes

Bladder

Vas deferens

47-1 Diagram of the male reproductive system.

add their activating secretions to the
sperm as they pass by. The fertilizing
fluid consisting of sperm and fluids from
the seminal vesicle, prostate gland, and
Cowper’s gland is called semen. In the
male the urethra serves as a duct for the
passage of semen and carries urine from
the bladder for excretion.

Figure 47-2 contrasts sizes of the
human ovum and sperm. The male
gametes are very small and are motile.
Each sperm is able to move by the beat¬
ing of its flagellum. It has been esti¬
mated that 130 million motile sperm
are necessary to insure fertilization of
one egg. The human sperm consists of
a head, neck, connecting piece, and tail.
The head is a flattened oval-shaped part
that is propelled by the lashing motion
of the tail. It contains the haploid
number of chromosomes. When the

47-2 This unusual photograph shows a hu¬
man ovum at the time of fertilization. Al¬
though the ovum and sperm vary greatly in
size and shape, they carry the same number
of chromosomes. (L. B. Shettles, Ovum Hu-
manum, Hafner Publishing Co., New York)

650 UNIT 7 THE BIOLOGY OF MAN

Fallopian tube

Cervix -

Vagina

Urethra

Ureter

Uterus

Bladder

47-3 Diagram of the female reproductive system.

sperm penetrates an ovum at the time
of fertilization, the tail separates. The
head and connecting piece enter the
ovum and the zygote is formed.

The female reproductive system. The
paired ovaries of the female are located
in the abdominal cavitv on either side
of the midline. They are about 1V4
inches long and Vs of an inch wide.
The ovaries are not connected directly
to the oviducts, or Fallopian tubes.
When an ovum is released, the motion
of ciliated cells lining the Fallopian tube
causes it to be drawn into the tube.
Then the ovum passes into the uterus ,
or womb.

The uterus is a hollow, thick-walled
muscular organ. The mucous mem¬
brane lining the uterus contains small
glands and many capillaries. If the
ovum is not fertilized, it passes through

the narrow neck of the uterus, called
the cervix, and into the vagina, from
which it is discharged.

The ovarian and uterine cycle. The de¬
velopment of the ovum and of the
uterus are coordinated bv hormones.

The human ovaries usually produce only
one egg during each 28-dav cycle of
activity. The mass of ovarian cells pro¬
ducing an ovum forms a follicle (Fig.
47-4). The cycle is controlled by a
hormone called the follicle-stimulating
hormone, or FSH, which is produced in
the anterior lobe of the pituitary gland.
As the egg reaches maturity, the follicle
becomes filled with a fluid containing
the hormone estrogen. Estrogen, as dis¬
cussed in Chapter 46, brings about sec¬
ondary sex characteristics in the female.

After the ovum has been dis¬
charged, the follicle becomes yellowish

CHAPTER 47 REPRODUCTION AND DEVELOPMENT 651

. . . . *

47-4 Diagram of a section through the human ovary. Stages 1-5 show maturing
of the ovum. Ovulation is taking place in stage 6, while in stages 7, 8, and 9 the
corpus luteum is forming.

in color; it is now called the corpus
luteum. The development of the cor¬
pus luteum is controlled by another hor¬
mone of the pituitary gland — the lu¬
teinizing hormone, or LH. The corpus
luteum in turn produces another hor¬
mone, progesterone. The function of
progesterone is to maintain the growth
of the mucous lining of the uterus. If
the ovum is not fertilized, however, the
corpus luteum degenerates, progesterone
is not produced, and the inside mem¬
brane of the uterus sloughs off. The
breakdown and discharge of the soft
uterine tissues and the unfertilized egg
is called menstruation. Figure 47-5
shows the events of ovum production
correlated with changes in the mucous
membranes of the uterus.

The uterine cycle consists of four
recognizable stages: 1. menstruation,

averaging about 5 days; 2. the follicle
stage, occurring from the end of men¬
struation to the release of the ovum —
about 10 to 14 days; 3. ovulation, the
release of a mature ovum from the
ovary; and 4. the corpus luteum stage,
lasting from ovulation to menstruation
— about 10 to 14 days.

Fertilization of the ovum. Immedi¬
ately upon entrance of a sperm into the
ovum, a membrane forms around the
zygote (zy-goht). This is called the
fertilization membrane , and its forma¬
tion prevents other sperm from entering
the ovum. Fertilization usually occurs
in one of the Fallopian tubes, and it
brings about several important changes.
The corpus luteum of the ovary con¬
tinues to develop and produce proges¬
terone, which influences the uterus.
The membrane of the uterus continues

652 UNIT 7 THE BIOLOGY OF MAN

to thicken, and many small glands and
capillaries form throughout the tissue
in preparation for the zygote.

The zygote reaches the uterus in
three to five days, and during this time
it continues its growth. Following the
re-establishment of the diploid number
of chromosomes, the zygote begins a
series of mitotic divisions, and in a few
days it consists of a mass of cells.
Development of the zygote. The term
embryo refers to a developing organism

when its body form is not recognizable
as a member of a certain species of ani¬
mal. In the human being this stage lasts
from six to eight weeks after fertiliza¬
tion. At the end of six weeks the em¬
bryo is about two-thirds of an inch
long, but its growth rate increases and
human characteristics become obvious.
From this time until birth it is called a
fetus.

Repeated division of the fertilized
ovum results in a hollow sphere of cells

Follicle-stimulating
hormone — FSH
Which acts on

Inhibits FSl^ production

T/Qrion f /-'J 1 1 ^ I ^

Prod

Ovarian follicle
causing its
enlargement

Luteinizing hormone — LH
which acts on ovarian follicle
causing its { aversion to

o O O

Estrogen

Rupture of
follicle and
release of egg

corpus luteum

Procfcces

\ ♦

Progesterone

Causing increased

vascularization Which #ts on

and glandularization

1 4

Menstruation

28 1 4

Menstruation

47-5 This diagram shows the relationship of the pituitary gland to the uterine
cycle.

CHAPTER 47 REPRODUCTION AND DEVELOPMENT 653

Fertilized egg

Many cells

Two cells

Blastula

Ectoderm

Blastocoel

Endoderm

Gastrocoel

Blastula changes to gastrula

Four cells

Eight cells

Ectoderm
Mesoderm
Endoderm

Three layers are formed

Endoderm

Ectoderm

Mesoderm

Coelom

47-6 Immediately following fertilization, cells divide repeatedly and finally form
the blastula, which resembles a hollow ball. This continues its division until
the gastrula is formed. In this stage the three germ layers that will give rise
to tissues and organs differentiate. The beginnings of the body cavity, or
coelum, may be seen in the final drawing.

known as a blastula (Fig. 47-6). One
side of the blastula folds inward to form
a shape called a gastrula. The edges of
the gastrula then become fused together,
and the embryo resembles a tube within
a tube. In this stage the outer layer of
cells is the ectoderm and the inner layer
the endoderm. The space between the
ectoderm and endoderm is the blasto¬
coel (bias- to-seel), and the central cavity
is the gastrocoel ( gus-troh-seel ) . The
gastrocoel develops into the digestive
tract of the fetus.

As the embryo develops, cells of the
endoderm begin to grow into the blasto¬
coel and fill it. These cells form the

mesoderm. Tire mesoderm then forms
two layers of cells. The innermost layer
adheres to the endoderm to form the
wall of the digestive system, and the
outermost layer forms part of the body
wall. The space between the mesoder¬
mal layers forms the body cavity, or
coelom (see-lom). The formation of
a coelom permits the digestive tract to
move freely.

The ectoderm, endoderm, and mes¬
oderm are called the primary germ lay¬
ers , as they form the various tissues and
organs of the body. Structures of the
body formed from the specific germ lay¬
ers are summarized on page 655.

654 UNIT 7 THE BIOLOGY OF MAN

47-7 Growth of the embryo. A: 3 days old; B: 6 days old; C: 35 days old; D: 42
days old. (L. B. Shettles)

Attachment of the embryo. At the
same time the embryo is passing down
the Fallopian tube toward the uterus, a
membrane forms around the mass of di¬
viding cells. This is the first of several
membranes that will form, and since it
will not become part of the embryo it¬
self, it is called an extraembryonic mem¬
brane. The first membrane, the chorion
(kor-ee- on), forms manv small fingerlike
projections, the chorionic villi. En¬
zymes produced bv the chorionic villi
enable them to sink into the uterine
membrane to make close contact with
the capillaries and provide nourishment
for the embrvo.

Soon another extraembryonic mem¬
brane, the amnion, develops. The fluid

in the cavity formed by the amnion
protects the developing embryo from
mechanical injurv and keeps it moist.
The yolk sac is the third extraembryonic
membrane. In animals that hatch from
eggs, this provides the food for the em¬
bryo. The human yolk sac is small and
not significant. The allantois ( a-lant -
was) is the fourth extraembryonic mem¬
brane, and it is present in man for only
a short time. In birds and reptiles this
membrane serves as an embryonic lung.

Once the chorionic villi become
embedded in the uterine wall, capillar¬
ies break down and form blood sinuses
around the villi. There is no direct con¬
nection between the blood of the
mother and the embryo, but food and

CHAPTER 47 REPRODUCTION AND DEVELOPMENT 655

STRUCTURES FORMED FROM
SPECIFIC PRIMITIVE GERM
LAYERS

ECTODERM

Skin and skin glands
Hair

Most cartilage
Nervous system
Pituitary gland

Lining of mouth to the pharynx
Part of lining of rectum
Adrenal medulla

MESODERM
Connective tissue
Bone

Most muscles
Kidneys and ducts
Gonads and ducts

Blood, blood vessels, heart, and lym¬
phatics

ENDODERM

Lining of alimentary canal from pharynx
to rectum

Thyroid and parathyroids
Trachea and lungs
Bladder

waste exchange occurs by diffusion
through the thin membranes. In the
uterus the area of the chorionic villi
and maternal blood supply forms the
large thin membrane called the pla¬
centa.

With growth the area that attached
the embryo to the yolk sac and allantois
lengthens to form the umbilical cord.
Two umbilical arteries, one umbilical
vein, and the allantoic duct connect the
developing embryo to the placenta.
Birth of the child. The period of fetal
development ends with the birth of the
child approximately forty weeks after
fertilization of the ovum. The smooth
muscles of the uterus begin to contract,
and when the membrane of the amnion
breaks, the fluid is discharged through
the vagina. Muscles of the cervix of the
uterus and the vagina relax to increase
the size of the opening. Further uterine
contraction forces the child from the
uterus. At this time the umbilical cord
still attaches the baby to the placenta.
To prevent loss of blood from the child
through the umbilical vessels, the cord
is tied and cut on the side of the pla-

47-8 Birth of the child. A: head is deep in the birth canal; B: head is begin¬
ning to emerge. Muscular contraction of the uterus expels the fetus during
labor.

656 UNIT 7 THE BIOLOGY OF MAN

centa. The navel is a scar on the abdo¬
men that marks the location where the
umbilical cord attached to the fetus.
Shortly after the child is born, the
placenta and remains of the amnion,
now called the afterbirth, are expelled.

Until the time of birth, the baby
receives its nourishment and oxygen
through the placenta. During develop¬
ment, movement of the thoracic mus¬
cles of the fetus draws fluid into the
lungs, which aids in their expansion.
The first cries of the infant remove the
fluid and fill the lungs with air.

Another highly important change
occurs at the time of birth. During
fetal life, the blood does not circulate
through the lungs. Instead, as blood
leaves the right ventricle, it goes through

a vessel called the ductus arteriosus ,
which takes the blood to the aorta. In
this way the lungs are short-circuited dur¬
ing the life of the fetus. At birth, how¬
ever, the ductus arteriosus closes off and
the blood flows through the pulmonary
arteries to the lungs. This aids the
lungs in their expansion as the baby be¬
gins to breathe for itself.

Occasionally the ductus arteriosus
fails to close off completely. When this
happens, all the blood does not get to
the lungs. Thus the carbon dioxide
content of the blood is higher than nor¬
mal. This condition is one of the
causes of blue babies. Often the vessel
will close off naturally, but sometimes
surgery has to be performed and the
vessel is tied.

IN CONCLUSION

In placental animals, such as the human being, internal fertilization occurs.
A balance of hormones controlling the development of the uterine wall pre¬
pares it to receive the embryo. By the time the zygote reaches the uterus it
consists of many cells and an extraembryonic membrane that aids in attach¬
ment to the uterus. The total dependence of the fetus on the mother termi¬
nates when the offspring is born.

BIOLOGICALLY SPEAKING

blastocoel

blastula

cervix

coelom

corpus luteum
Cowper’s gland
ductus arteriosus
embryo
epididymis

extraembryonic membrane
Fallopian tube

fertilization membrane

fetus

follicle

FSH

gastrula

gastrocoel

menstruation

navel

placenta

primary germ layers

progesterone
prostate gland
scrotum
semen

seminal vesicle

seminiferous tubules

umbilical cord

uterus

vagina

vas deferens

CHAPTER 47 REPRODUCTION AND DEVELOPMENT 657

QUESTIONS FOR REVIEW

1. Of what advantage is sexual reproduction to a species?

2. Describe the passage of sperm and the production of semen in the male
reproductive system.

3. How does the quantity of sperm in the human being compare to the quan¬
tity of ova? Explain the significance of this variation.

4. Describe the passage of an ovum in the female reproductive system.

5. How do hormones influence the reproductive cycle in the female?

6. Name and define the four stages in the uterine cycle.

7. Describe the changes occurring in the formation of the embryo from the
zygote.

8. Name and tell the function of the extraembryonic membranes.

9. Name several structures that are produced from the primary germ layers.

10. Name the components of the umbilical cord.

11. What important change occurs in the circulatory system after birth?

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the importance of size and other structural differences in a human
ovum and sperm.

2. Why must fertilization occur in a Fallopian tube prior to movement of
the ovum into the uterus?

3. Explain why hormone imbalance that alters the normal function of< the
pituitary gland might result in interruption of ovarian function.

4. If an egg is fertilized, menstruation does not occur and the lining of the
uterus is prepared for implanting of the embryo. Account for this in
terms of hormone secretion.

5. Discuss the importance of the ductus arteriosis as a bv-pass from the
pulmonary circulation to the systemic circulation in a fetal heart.

RELATED READING

Books

Anthony, Catherine P. Structure and
Function of the Body , 2nd Rev. Ed.
C. V. Mosby Co., St. Louis. 1964
Asimov, Isaac. The Human Body: Its
Structure and Operation. Hough¬
ton Mifflin Co., Boston. 1962
Cain, Arthur H. The Cured Alcoholic :
New Concepts in Alcoholism. John
C. Day, New York. 1964

Calahane, Dorothy and Payne, A. The
Great Nutrition Puzzle. Charles
Scribner’s Sons, New York. 1956
Carlson, A. and Johnson, V. The Ma¬
chinery of the Body , 5th Ed. Uni¬
versity of Chicago Press, Chicago.
1961

Carrington, Richard. A Million Years
of Man. World Publishing Co.,
Cleveland. 1963

658 UNIT 7 THE BIOLOGY OF MAN

Chandler, Asa C. Introduction to Par¬
asitology, with Special Reference to
the Parasites of Man. John Wiley
and Sons, New York. 1949
Clark, W. E. LeGros. History of the
Primates : An Introduction to the
Study of Fossil Man. University of
Chicago Press, Chicago. 1959
Clarke, Robin. The Diversity of Man.

Roy Publishers, New York. 1964
Clymer, El. The Case of the Missing
Link. Basic Books, Inc., New York.
1962

Dobzhansky, Theodosius. Evolution,
Genetics and Man. John Wiley
and Sons, Inc., New York. 1955
Faber, Doris. The Miracle of Vitamins.
G. P. Putnam’s Sons, New York.
1964

Haller, Albert von. The Vitamin Hunt¬
ers. Chilton Book Div., New York.
1962

Hammond, Winifred. Plants, Food and
People. The John Day Co., New
York. 1964

Hyde, Margaret. Your Brain — Master
Computer. McGraw-Hill Book
Co., New York. 1964
Kimber, Diana C., et al. Anatomy and
Physiology, 14th Ed. The Macmil¬
lan Co., New York. 1961
Lasker, Gabriel. Human Evolution:
Physical Anthropology and Origin
of Man. Holt, Rinehart and Wins¬
ton, Inc., New York. 1963
Lehrman, Robert L. The Long Road to
Alan. Basic Books, Inc., New York.
1961

Life Nature Library. Evolution. Time,
Inc., New York. 1964
McBain, W. N. and Johnson, R. C. The
Science of Ourselves. Harper and
Row, Publishers, New York. 1964
Osborn, Robert and Benton, Fred W.
Dying to Smoke. Houghton, Miff¬
lin Co., Boston. 1964.

Poole, Lynn. Carbon 14. McGraw-Hill
Book Co. (Whittlesey House),
New York. 1961

Riedman, Sarah R. Our Hormones and
How They Work. Abelard-Schu-
man, Ltd., New York. 1956
Scheele, William E. Prehistoric Man
and the Primates. The World Pub¬
lishing Co., Cleveland. 1957
Schenk, Gustav. The History of Man.
Chilton Company, Philadelphia.
1961

Schneider, Leo. You and Your Cells.
Harcourt, Brace and World, Inc.,
New York. 1964

von Koenigswald, G. A. R. The Evolu¬
tion of Man. University of Michi¬
gan Press, Ann Arbor. 1962
Wilson, Mitchell. The Human Body :
What It Is and How It Works.
Golden Press, New York. 1959

Articles

Deevey, Edward S. “The Human Popu¬
lation.” Scientific American, Sep¬
tember, 1960

Slaughter, F. G. “Heart Surgery.” Sci¬
entific American, January, 1950
Surgenor, D. M. “Blood.” Scientific
American , February, 1954

UNIT EIGHT

ECOLOGICAL

RELATIONSHIPS

Thus far we have analyzed a great variety of organisms as units of life. We have
studied the substances composing them, the cells that make them up, their organs
and their systems. Now we are ready to assemble these many organisms into living
societies determined by the conditions they require for life. Our study now broad¬
ens to include the whole living world — its forests, grasslands, deserts, lakes, and
oceans. In all of these communities, organisms with like requirements interact with
conditions of the environment and with each other in a dynamic system. The ex¬
ploration of these communities is one of the most fascinating areas of biology.

CHAPTER 4 8

INTRODUCTION
TO ECOLOGY

The science of the environment. No
plant or animal is independent of its en¬
vironment. Organisms are products of
their surroundings. These surroundings
must provide conditions suitable for
maintaining life and carrying on all the
life activities. The study of the relation¬
ships of living things to their environ¬
ment and to each other is the branch of
biology known as ecology. The word
means, literally, “the study of houses.”
The field or pond or forest in which a
plant or animal lives out its days is in a
very real sense its “house.”

Ecologists work in everv region of
the earth — from equatorial forest to
polar outpost and from ocean depths to
mountaintop — studying the relation¬
ship of plants and animals to their en¬
vironment. Any forest, field, pond,
lake, or ocean is an outdoor laboratory
to the ecologist.

The biosphere — the layer of life. The
area in which life on our planet is possi¬

ble is called the biosphere. Life exists
only a few feet below the surface of the
earth, on the surface, in the oceans and
other .bodies of water, and in the lower
atmosphere. This comparatively thin
layer of life is affected by many factors.
The most important single factor affect¬
ing the biosphere is radiations from the
sun. These enable green plants to man¬
ufacture food. Heat from the sun is
vital to the existence of many organ¬
isms. It causes the evaporation of wa¬
ter, which in turn produces the earth’s
rainfall. This heat also creates the
earth’s winds. Although the entire bio¬
sphere may be considered one gigantic
biological system, it is actually a com¬
plex series of systems within systems.
Ecosystems as units of the biosphere.
Any stable environment in which living
and nonliving things interact, and in
which materials are used over and over
again, is called an ecosystem. An eco¬
system may be a jar with a fish, snail,
water plants, water, and sand; or it may
be a man in his spaceship. Forests,
rivers, coral reefs, and ponds are exam¬
ples of larger ecosystems.

The lake shown in Fig. 48-1 is an
ecosystem. Although the diagram does
not include all the organisms actually
found in a lake, it does show typical
ones. By becoming familiar with the
various organisms in and around such a
body of water, you will see how many
factors affect the lake and its surround¬
ing area.

The living organisms in an ecosys¬
tem are collectively referred to as the
biotic community. All the organisms
are not of equal importance in the com¬
munity. Hydra, for example, may be
plentiful on the water plants growing
near the shore, but their contribution to
the community would not be as signifi¬
cant as that of the crustaceans and in-

660

CHAPTER 48 INTRODUCTION TO ECOLOGY 661

✓ /
' t :

Willows

a '

> I

_f,y Caddisfly »/ “

larvae larvae..

Flatworms

't't A #s Algae
Roundworms^^Beetles

?Duck eating fish

~~~ tn'jt?**-.
y •*

! f j J/Se gmented worms „ ' Leeches"ro^ozoan^Diatc

l-*fc w

Flies ^.-

_ , iatoms

Snails Sn * 1 I?!?3" °S5^fih S^kieback

water lilies cattails / ^

Minnow feeding on plankton

Bass

eating

crayfish

■&«

Mm#

jUifi

48-1 A fresh-water lake is an example of an ecosystem, in which living and non¬
living things interact.

sects on which some of the large animals
feed.

Populations and their effects. A group
of organisms of the same or related spe¬
cies in a given ecosystem is called a pop¬
ulation. In discussing a population, we
must identify the kinds of individuals
and define their limits in the ecosystem.
In a lake, for instance, we may refer to
the plant-eating fish population in the
summer of 1965. Even a greater number
of different species would be involved if
we referred to the insect population of
the lake in the same summer.

The effects of various populations
on one another will have more meaning
if we consider the number of individuals.
Populations vary in numbers of individ¬
uals from season to season or from year
to year. An ecologist measures the
changing populations by counting and
recording individuals from a large area
at various times. This measure of popu¬

lation density , then, is expressed as num¬
bers of individuals in a definite area at
a specific time. Density studies pro¬
vide valuable information. For exam¬
ple, if we know the reasons for periodic
fluctuations in the numbers and kinds
of grasses on which cattle graze, we can
use natural pastures without destroying
them. All effective game laws are based
on population studies. Game and fish
populations are carefully harvested to
insure future generations of men the
pleasure of hunting and fishing.

How are density studies made?
Obviously, it would not be practical to
count all the clover plants in a field or
on a hillside, or to count all the bass in
a lake. Instead the ecologist selects
several areas which are large enough for
an accurate sample. Then he actually
counts the individuals in the selected
areas. Under natural conditions, or¬
ganisms are not distributed evenly.

662 UNIT 8 ECOLOGICAL RELATIONSHIPS

Such studies lead to new questions such
as these: What factors are responsible
for the uneven distribution? Why do
populations vary from year to year?

The population of a given organism
at any time depends on rates at which
individuals are dying, being produced,
or moving into or out of the area. If
the reproductive and death rates of a
trout population in a certain lake are
equal, the trout population stays at a
constant level. If studies over a period
of years show a decline in the total trout
population, there are reasons for the de¬
cline. If the ecologist can find the
reasons, it may be possible to restore
stability.

Levels of organization in the ecosystem.
The following arrangement of biolog¬
ical organization will help you to vis¬
ualize the relationships of the terms we
have been discussing and shall use
throughout our study of ecology.

Biosphere

Ecosystem

Biotic community

Populations

Organisms

Organ systems

Organs

Tissues

Cells

Molecules

Atoms

You can see that this list does not
apply to all organisms. If you were con¬
sidering the protozoans, for example,
you would omit tissues, organs, and or¬
gan systems.

In discussing the various parts of
the ecosystem, we have not mentioned
the nonliving component. This is called
the physical environment. As you will
see, it has an important influence on liv¬
ing things. The ecosystem, therefore,
can be more completely represented in
the following way:

Ecosystem^^^

Biotic Physical

community < * environment

Three kinds of relationships occur
in an ecosystem: 1. the interactions in
the biotic communitv; 2. the interac-
tion between the biotic community and
the physical environment; and 3. the
interactions among the physical factors
of the environment.

Interaction in the biotic communitv.

By examining the ecosystem of the lake,
you will be able to find many ways in
which organisms are dependent on one
another. How many organisms can you
identify that compete for food, light,
oxygen, or merely a place in which to
grow? Competition within any one
population is usually greater than that
between different populations, as organ¬
isms of the same species have almost
identical requirements. Male stickle¬
back fish, for example, not only compete
for food, but they also vie for a suitable
nesting site and for a female. Cattails
compete with one another for space,
soil, and a suitable depth for growth.
They would not compete as strongly
with duckweed, for example. This small
plant, floating on the surface of water,
does not have the same requirements.

CHAPTER 48 INTRODUCTION TO ECOLOGY 663

Interaction between the biotic commu¬
nity and the physical environment.
Sunlight, soil, and temperature are
some of the physical factors of the en¬
vironment which constantly interact
with the biotic community. As you
know, all green plants require sunlight
for photosynthesis. In a lake the plants
are limited by the depths to which sun¬
light can penetrate. This depth de¬
pends on the clearness of the water.
When this clearness is altered by a
heavy growth of algae, or by mud
stirred up from the bottom, the water
may become turbid. In the clearest of
lakes, light may penetrate to an average
depth of 75 feet for the year.

The remains of dead plants that
grow near the shore can alter the nature
of the bottom. Over a period of years
the area of a lake can be substantially
reduced by the accumulation of dead
shoreline plants. Organic acids and
pigments from decomposing shoreline
plants often affect the composition and
color of the water in a lake or stream.

Water temperature may not be a
significant factor to organisms living in

deep lakes. At great depths, tempera¬
tures vary only slightly. But variations
in daily and seasonal temperatures have
a very great effect on the organisms liv¬
ing in shallow lakes or along the shore.
These effects will be discussed further in
the next chapter.

Another important factor influenc¬
ing the distribution of plants and ani¬
mals in a lake is man. Although man
does not live in a lake, his influence
over its biotic and physical environment
should not be overlooked. Careless
pollution of the water by industrial
wastes or sewage can seriously alter both
the balance and distribution of living
things and the chemical nature of the
water. Man, in satisfying his own in¬
terests, has turned rivers and streams
into vast lakes, has turned rivers into
open sewers, and has even altered coast¬
lines. Many of the far-reaching effects
of man’s activities were not even con¬
sidered until vast aquatic wastelands
had been produced.

Interactions within the physical envi¬
ronment. The interactions within the
physical environment may be temporary,

48-2 This lake was created
as one result of an earth¬
quake in Yellowstone Na¬
tional Park in 1962. This
represents a permanent
change in the physical en¬
vironment. (U.S. Forest
Service)

664 UNIT 8 ECOLOGICAL RELATIONSHIPS

as when a cloud cover reduces the light
intensity. More permanent changes
occur when a flood carries debris into
a lake. Even the shape of the lake may
be slightly altered by deposited mate¬
rials. Some interactions in the physical
environment may be permanent, as
when an earthquake alters the course
of the stream so that it bypasses the lake
altogether. New springs may carry
chemicals into the lake to alter the
chemistry of the water. In many areas
soda and sulfur springs may affect the
chemical composition of the water.

The water cycle, a physical cycle
Closely associated with the interrela¬
tionships of the biotic and physical en¬
vironment, but exclusive to none, are
the chemical and physical cycles. You
have already learned much about the
role of water, oxygen, carbon, and nitro¬
gen in the maintenance of the living
condition. Now let us see how they are
returned to the atmosphere and become
available again to organisms.

The water cycle is a continuous
movement of water from the atmosphere
to the earth and from the earth back to

Evaporation

Precipitation!

Precipitation

Transpiration

1 | j|, Ml'.,! Ill,

jl'jjj1 Evaporates

!;| iilliij'i as it falls

M1 v"i

Evaporates

from surface

Enters topsoil

Absorbed
by roots

SurfacejgL* water

Ground water

Emerges as spring

To water table

Rises from water table

Water table

Stored surface water

48-3 Trace the steps in the water cycle as shown in this diagram.

CHAPTER 48 INTRODUCTION TO ECOLOGY 665

the atmosphere. The movement from
the atmosphere to the earth is called
precipitation; eventually this water re¬
turns to the atmosphere by evaporation
(Fig. 48-3) . When it rains, some of the
water evaporates while falling and some
evaporates quickly from the surface of
the ground. Much of the rainwater runs
along the surface of the ground and
travels from rivulet to stream to river.
This runoff water eventually reaches a
pond, lake, or the ocean. Water evapo¬
rates constantly from the surfaces of this
collection system. Large amounts of
precipitation normally enter the soil to
become ground water. This water may
reach a pond, lake, or ocean through
springs or underground streams; or it
may move upward through the soil dur¬
ing dry periods and pass again into the
atmosphere as water vapor. As warm
air containing water vapor rises through
the atmosphere, it cools. The vapor
then condenses as droplets of water,
forming clouds. The droplets collect
to form drops, which fall from the
clouds as rain. Snow is formed when
the vapor condenses at a temperature
below the freezing point of water.
Ground water and its movement. Top¬
soil acts as a sponge, receiving and hold¬
ing water from precipitation. Some of
this water moves downward into the
subsoil and fills the spaces around the
rock particles. The upper level of soil
that is saturated with water is called the
water table — the point at which water
is standing in the ground. If the rain¬
fall is heavy, most of the water may run
off the surface instead of penetrating
into the saturated topsoil.

The depth of the water table de¬
pends on the amount of precipitation,
the condition of the soil surface for re¬
ceiving water, the nature of the rock
layers under the soil, and the proximity

of large bodies of water. Where de¬
pressions occur, as in basins of lakes
and ponds, the water table may be above
the surface.

Plants do not wilt between rains be¬
cause water moves from the water table
up through the soil by capillary action.
Much of this water is absorbed by roots
and passed to the atmosphere during
transpiration. Some of it reaches the
surface of the soil, where it evaporates
into the atmosphere. This movement of
water upward from the water table is an
important part of the water cycle.
However, the largest proportion of the
water enters the atmosphere by evapora¬
tion from the oceans and smaller bodies
of water.

The role of living things in the water
cycle. To a certain extent, both plants
and animals are involved in the water
cycle. Plants take in water through
their roots by absorption, and give off
water vapor from their leaves in tran¬
spiration. Animals are involved to the
extent that they drink water and give
off a certain amount of water vapor in
exhalation. The percentage of total
water that cycles through living things
is small, however, compared to the
amount that cycles through bodies of
water, especially the oceans.

The carbon-oxygen cycle. Respiration
and photosynthesis are the two basic life
processes involved in the carbon-oxygen
cycle. Land-dwelling organisms take
oxygen directly from the atmosphere,
while aquatic and marine organisms use
oxygen that is dissolved in the water.
The oxygen that is chemically united
with hydrogen in water molecules is not
available for respiration because organ¬
isms lack the ability to decompose wa¬
ter for this purpose.

During respiration, compounds
containing carbon are oxidized to form

666 UNIT 8 ECOLOGICAL RELATIONSHIPS

carbon dioxide, and this gas is released
to the environment.

During photosynthesis, green plants
take water and carbon dioxide from the
environment. In this process, as you re¬
call, water molecules are decomposed so
that their hydrogen atoms may be com¬
bined with carbon dioxide to form car¬
bohydrates. Oxygen is released to the
environment as a byproduct of photo¬
synthesis. The atmosphere normally
contains about 21 percent oxygen and
0.04 percent carbon dioxide. These per¬

centages are fairly constant, indicating
the efficiency of living things in main¬
taining the carbon-oxygen cycle.

Another portion of the carbon-
oxygen cycle relates to the organic com¬
pounds synthesized by plants and
animals from the carbohydrates pro¬
duced in photosynthesis. Plants pro¬
duce proteins and other protoplasm¬
forming substances. Animals that eat
plants may synthesize other organic
substances, and carnivorous animals in
turn resynthesize these substances to

Oxygen

^ Carbon dioxide

■

■■ - ,

Carbon dioxide

Carbon compounds found
in weathering rock, and

48-4 The carbon-oxygen cycle. Carbon dioxide is used by green plants in photo¬
synthesis and oxygen is released. Oxygen is used by animals and plants in res¬
piration, and also in the burning of fuels. Carbon dioxide is released in both
these processes, as well as in the process of decay.

CHAPTER 48 INTRODUCTION TO ECOLOGY 667

suit their own needs. The carbon in
these compounds is retained in the bod¬
ies of organisms until they die. It is
then released as carbon dioxide when
their remains decompose after death.

Smaller amounts of oxygen and
carbon are involved in formation and
decomposition of rocks and of mineral
fuels such as coal and petroleum. These
two elements are also involved in chem¬
ical changes when fuels are burned and
when volcanoes erupt. However, the
major contributing factors to the car¬
bon-oxygen cycle are respiration and
photosynthesis.

The nitrogen cycle. The nitrogen cycle
involves green plants and several kinds
of bacteria. It may or may not involve
animals. As you read about the various
steps in the nitrogen cycle in the next
two paragraphs, follow them in the dia¬
gram shown in Fig. 48-5.

Let us begin with the green plant
and the formation of protein. Its roots
absorb nitrates , a group of soil minerals.
These compounds contain nitrogen in
chemical combination with oxygen and
usually with sodium or potassium. In
building proteins, the plant adds nitro¬
gen from nitrates to the carbon, hydro¬
gen, and oxygen which have been or¬
ganized in an organic compound during
photosynthesis. Sulfur and phosphorus
may be added from other soil minerals.

Proteins are used in forming plant
or animal protoplasm. This happens
when one animal eats a plant or an¬
other animal. The chemical compo¬
nents of protein might change from
plant to rabbit to fox, or from plant to
beef steer to you. Each time an animal
consumes protein in food, digestion sep¬
arates its amino acids, which are then
combined to form a protein character¬
istic of the organism in which it is
formed.

However, not all of the protein
consumed by an animal is converted
into protoplasm. Some of it may be
oxidized to produce energy. In this
event, nitrogen compounds may be
excreted by the animal. As you recall,
this is one of the common functions of
kidneys in higher animals. These ex¬
creted compounds may be further de¬
composed by bacterial action in the soil
or water.

When an organism dies, decay by
bacterial action begins. Nitrogen is re¬
leased from the decaying protein in com¬
bination with hydrogen as ammonia.
We refer to this part of the nitrogen
cycle as ammonification. Other kinds
of bacteria, living in the soil, oxidize
ammonia and form nitrites , which can¬
not be absorbed by plant roots. Further
oxidation by still other bacteria results
in the formation of nitrates, the mineral
compounds from which green plants re¬
ceive their necessary nitrogen. We refer
to the chemical process of nitrate forma¬
tion by bacteria as nitrification. Thus,
in starting with nitrates and ending with
nitrates, we complete the nitrogen cycle.
The role of the atmosphere in the nitro¬
gen cycle. As you probably know from
your previous science courses, the atmos¬
phere is composed of about 78 percent
nitrogen. Is this pure nitrogen involved
in any way in the nitrogen cycle? It is,
but in a rather roundabout way. Atmos¬
pheric nitrogen cannot be used by green
plants in their chemical activities. How¬
ever, two groups of bacteria can oxidize
free nitrogen and form nitrites and ni¬
trates in the soil. One of these groups
of bacteria lives in the soil. The other
lives on the roots of clover, alfalfa, and
other members of the legume family in
a close relationship with the plant. The
legume is the host, since the bacteria
live within its tissues. These remarkable

668 UNIT 8 ECOLOGICAL RELATIONSHIPS

Air is 78% nitrogen
but this free nitrogen
cannot be used
by green plants

NITROGEN
IN THE AIR

Man gets protein
from plants and animals

Animals get protein

Dead plants
and

excrement

Plants absorb nitrates
v v ligand use them in

Nodules on ^building proteins ^^)ecay bacteria

root of legume break down

NITRATES proteins and

IN SOIL release ammonia

^ i Nitrifying bacteria use ammonia "jm

Denitrifying
3 jm &

Jr f bacteria use
Jr nitrates and
release
nitrogen
to the air

and excrete nitrites. Nitrate

bacteria convert nitrites

Nitrogen-fixing
bacteria use

to nitrates

Ht!

nitrogen from

the air to produce
nitrates

;! , ;

■

■ p

HHI

48-5 The nitrogen cycle. In addition to the steps shown here, some free nitro¬
gen in the air is changed to nitrates by lightning and is carried to the soil by
rain or snow.

bacteria receive sugar from the host and
use it in oxidizing free nitrogen to ni¬
trates. The nitrates, formed within the
cells of the roots of the host, can be ab¬
sorbed and used in protein formation.
Thus, the bacteria are of great benefit to
the host plant. We refer to this im¬
portant process as nitrogen fixation.

Fortunately, legumes accumulate
more than enough nitrates to meet their
own requirements. The excess builds

up the nitrogen content of the soil.
When a farmer plants clover or alfalfa
in a field as part of a crop rotation
schedule, he knows he is building up his
soil from almost unlimited supplies of
atmospheric nitrogen. What he is actu¬
ally doing is raising nitrogen-fixing bac¬
teria. He will receive the greatest
benefit if he plows the clover back
into the soil at the end of the growing
season.

CHAPTER 48 INTRODUCTION TO ECOLOGY 669

One phase of the nitrogen cycle,
however, is unfavorable to agriculture.
Certain bacteria liberate nitrogen by
breaking down ammonia, nitrites, and
nitrates in a process called denitrifica¬
tion. In this way some nitrogen may be
lost from the soil. Fortunately, denitri¬
fication does not occur in well-drained,
cultivated soil, since denitrifying bac¬
teria are anaerobic. That is, they live in
an environment that has little or no
oxygen present. They thrive in soils
that are water-logged, or packed so
tightly that air cannot easily penetrate.
Biological balance in nature. The
chemical cycles illustrate the interrela¬
tion of the life processes in plants and
animals. Organisms are chemically as¬
sociated with one another in a living
society. Any biotic community is a

complex of societies. It is composed
of many kinds of plants and animals,
living together in a very close associa¬
tion. If you study any plant and animal
relationship closely, you can discover
numerous ways in which they are de¬
pendent on one another. Some of the
mutual dependence involves food sup¬
ply and environmental conditions. Of
great importance is the limiting effect
organisms have on one another. Some
forms of life may be more abundant
than others, but no one kind of or¬
ganism controls the community com¬
pletely.

Let us consider the events in an
open-field community. A meadow
mouse is running along a pathway
through the grasses in search of seeds.
One might expect that by eating seed

48-6 The nodules at the left, containing nitrogen-fixing bacteria, are found on
clover. The diagram at the right is a detail of the bacteria themselves ( Rhizo -
bium). Of what benefit are the clover and the bacteria to each other?

670 UNIT 8 ECOLOGICAL RELATIONSHIPS

48-7 The owl is a natural enemy of rodents,
as it depends on them for its food supply.
(Thornhill from National Audubon Society)

the mouse would reduce the next gen¬
eration of grasses and other open-field
plants. But since plants produce a far
greater number of seeds than is needed
for maintaining the plant population,
plant numbers are not necessarily re¬
duced. An owl swoops down and
catches the mouse. The owl is one of
the natural enemies of mice and other

small mammals. We speak of it as a
predator (pred- at-er) because it preys on
other animals. Suppose there were no
owls or other natural enemies of mice.
Soon the mice would overrun the field.
They might, then, actually reduce the
numbers of grass plants and starve by
virtue of their own numbers. This has
happened many times in areas where
animals have become too numerous.

In a forest, a field, a lowland marsh,
a rocky meadow on a mountaintop —
wherever life is found — there exists a
close relationship between plant and
animal and between prey and predator.
Natural enemies play a vital part in
maintaining the density of populations.

Keep in mind, however, that the
relationships between organisms are not
static. They may be affected by sea¬
sonal and annual fluctuations, or by dis¬
ease or man. If the owl population
should increase rapidly in a given area,
for example, the rodent population
would decrease. With the lowering of
the number of rodents the grasses might
increase in number and variety within
the community. Many owls, however,
would starve and the owl population
would decrease. After a year or two
the rodent population would build up
to where it could support more owls,
and the total animal-plant population
would return to proportions similar to
those existing before the “owl explo¬
sion.”

IN CONCLUSION

To understand biology, we must know about the interrelationships of plants
and animals and their environments. The study of these interrelationships is
called ecologv. The living world maintains a measure of stability by means
of cycles. Constructive processes utilize simple inorganic compounds supplied
by the soil and atmosphere. Equally important are the destructive processes
that reduce complex organic matter to the simple compounds essential to fu-

CHAPTER 48 INTRODUCTION TO ECOLOGY 671

ture generations of living things. Thus, cycles continue age after age, and life
continues.

Other conditions in nature have a direct and powerful influence on living
things. Light, water, and temperature are critical factors in the life of plants.
Plants in turn control the animal population. In the next chapter we shall
consider, in more detail, the factors of the environment and their effects upon
organisms.

BIOLOGICALLY SPEAKING

ammonification

biosphere

biotic community

carbon-oxygen cycle

denitrification

ecology

ecosystem

evaporation

ground water
natural enemies
nitrates
nitrification
nitrites

nitrogen cycle
nitrogen fixation
physical environment

population
population density
precipitation
predator
runoff water
water cycle
water table

QUESTIONS FOR REVIEW

1. What is the biosphere?

2. How would you go about making a density study?

3. What factors would you say determine the population of an organism at
any time?

4. Would the list of biological organization apply to a sponge?

5. What does an ecologist mean when he speaks of interactions in the eco¬
system?

6. What interactions occur between the biotic community and the physical
environment?

7. What interactions occur among the physical environmental factors?

8. Describe the water cycle and discuss its importance.

9. Summarize the carbon-oxygen cycle and its value to living things.

10. Why is the nitrogen cycle so important in biology?

APPLYING PRINCIPLES AND CONCEPTS

1. Describe an ecosystem near your home.

2. Biologists sometimes speak of a “closed ecosystem.” Explain why this
might not be an accurate term.

3. Discuss possible reasons for annual variations of any specific population.

4. Discuss the methods an ecologist uses in determining the density of popu¬
lations of organisms in an ecosystem.

5. Discuss the differences between the environment of the shore region of a
lake and that of the deep-water bottom region

CHAPTER 4 9

THE HABITAT

The address and occupation of an or¬
ganism. You are aware of the many
general conditions under which plants
and animals live. Conditions in a forest
are quite different from those in an open
field bordering it. The environment of
a ravine or valley is unlike that of a
hillside or mountaintop. Some of these
differences are obvious; others are not.
Still, each factor of an environment
has a critical influence on the plants and
animals that live there.

The habitat of an organism is the
place where it lives. There are many
habitats within the ecosystem of a lake.
The habitat of the bullfrog is quite dif¬
ferent from that of the bass, and yet
both contribute to the complex struc¬
ture of the ecosystem. In fact, a bass
may occasionally eat a bullfrog if their
habitats should happen to overlap.
However, you would not go out to the
middle of a lake to find a bullfrog.

The “address” of an organism is its
habitat, but its niche is what might be
called its “occupation.” Within a biotic

community there are many ways in
which organisms can “earn a living.” A
lake, for example, may contain tiny sus¬
pended organisms of many species,
called plankton. It is the niche of many
small fishes to feed on these organisms.
Larger fishes may occupy the same habi¬
tat as the small fishes, but are in a dif¬
ferent niche because they feed on the
small fishes. Different organisms that
occupy the same niche are in competi¬
tion with one another. For example,
where they occur together, wolves and
mountain lions occupy the same niche
and are in competition for the animals
on which they prey.

Limiting factors in the habitat. If an

organism is to live in a certain habitat,
it must be able to obtain the materials
it needs for growth and reproduction.
Anything that is essential to organisms
and for which there is competition is
called a limiting factor. Cattails grow¬
ing along the shore of a lake, for ex¬
ample, require a marshy condition
where the water is not too deep. In a
lake, therefore, the area where the bot¬
tom is soft and the water is shallow is
a limiting factor for cattails. They
compete within that area and cannot
live bevond it.

As you are probably becoming
aware, the presence and continuing suc¬
cess of an organism may depend on a
very complex and specific set of condi¬
tions. fust as a deficiency of any kind
may limit the survival of an organism,
so will an excess. An organism’s ability
to withstand a variety of environmental
conditions is called its tolerance. A
knowledge of the extremes of tolerance
of organisms to various conditions will
help us to understand why they live
where they do. Many organisms, for
example, live in estuaries where rivers
carry fresh water into oceans or bays

672

CHAPTER 49 THE HABITAT 673

of salt water. The periodic rise and fall
of tides and the variation in amounts
of fresh water in the river during storms
causes great fluctuation in the salt con¬
centration of the water. The many
kinds of worms, clams, oysters, fishes,
and barnacles that live in such an area
have a wide tolerance to water of vary¬
ing salt concentration. Some of the
deeper marine species of corals, sponges,
sea urchins, and fishes would perish un¬
der such conditions, as they have a
much narrower tolerance to changes in
salt concentration.

As you have already seen, the habi¬
tats of organisms are governed by two
sets of factors — physical and biotic.
In order for an organism to survive in a
particular habitat, both sets of factors
must be such that they allow the organ¬
ism to carry on its life processes. Thus
geographic distribution of a species is
governed by its limits of tolerance. We
would expect to find a species concen¬
trated in areas where the conditions are
best for it. In their efforts to learn more
about living things, biologists may de¬
liberately subject organisms to a variety
of conditions. Such tests reveal the lim¬
its of tolerance of organisms. Let’s
examine some of the important physical
factors that limit the habitats of plants
and animals.

Soil — a physical factor of the environ¬
ment. Soil is more than just dirt that
covers the earth. It is one of the most
important factors of an environment.
Careful examination shows that soil
varies greatly in different localities.
The plant and animal life it supports
varies accordingly. Some soils are com¬
pact because they are composed mostly
of clay, while others are loose because
they contain mostly sand. The particles
of silt are intermediate in size between
those of clay and sand. Loam is a mix¬

ture of clay, sand, and organic matter.
Sandy soils may support a pine forest
in Michigan, New Jersey, Georgia, or
eastern Texas. Heavy loam supports a
beech and maple forest in Ohio and In¬
diana. Waterlogged soils of bogs and
swamps provide ideal conditions for
larch, white cedar, and cypress forests.
The rocky, shallow soils of certain
mountain slopes produce luxuriant for¬
ests of redwood, yellow pine, and spruce
in our western states.

A sour soil is one that is acid; a
sweet soil is alkaline. The degree of
acidity or alkalinity in soil is an impor¬
tant factor in plant growth. For many
plants, a neutral point midway between
the acid and alkaline is best. Under
cultivation soil tends to become more
acid. To correct this condition, lime is
often worked into the soil. Such plants
as beets, spinach, lettuce, cauliflower,
onions, peas, alfalfa, and clover do not
grow well in acid soils. Plants like
rhododendrons, azaleas, and blueberries,
however, grow better in sour soil, and
lime is harmful to them.

Salty and alkaline conditions of soil
lower the productivity and value of
much agricultural land in the United
States. An estimated one fourth of our
29 million acres of irrigated land con¬
tains an excess of soluble salts of so¬
dium, calcium, and magnesium. High
concentrations of these elements reduce
the rate at which plants absorb water,
so that their growth is considerably re¬
tarded.

The character of a soil is always
changing. In some places rocks are
breaking down to form more soil. This
breaking down is caused by the action
of weather, by chemical disintegrations,
and by plants growing on the rocks. In
other places the mineral content of the
soil is being depleted because of the

674 UNIT 8 ECOLOGICAL RELATIONSHIPS

49-1 Temperature condi¬
tions in an alpine mead¬
ow such as this allow a
spring growth of lupine,
Indian paintbrush, and
other flowers after the
snows have melted.
(Sumner from National
Audubon Society)

quantities of salts removed by plants
through their roots. Certain soils may
be enriched through the decay of lay¬
ers of vegetation, while other soils are
becoming exhausted because of heavy
crop production and failure to replace
the lost minerals. Much useful farm
land is ruined by bad soil care. As soils
change, plants and animals must find
other suitable habitats.

Temperature — an important control¬
ling factor of the environment. In tem¬
perate regions of the earth, including
most of North America, temperatures
vary considerably. They range from nar¬
row fluctuations between day and night
to the much more extreme differences of
summer and winter. Many animals do
not maintain a constant body tempera¬
ture. These are the cold-blooded, or
poikilothermic ( poy-kil-oh-f /ler-mik ) ,
animals. Their body temperature fluc¬
tuates with that of their environment.
The birds and mammals maintain a
fairly constant body temperature regard¬
less of their surroundings. These are
the warm-blooded, or homoiothermic

(hoh-moy-uh-ther-mik) , animals. Thus,
as you might expect, the warm-blooded
animals can extend their habitats over
a wide range of temperatures.

On a cold morning a snake may
crawl slowly out on a flat rock and lie
in the sun. When its body temperature
has increased, the snake becomes more
active. Similarly, when the tempera¬
ture is low, a butterfly may fan its wings
for several minutes to warm up before
flying. A meadow mouse, living in the
same area as the snake and the butter¬
fly, however, can wake up on a cold
morning and dart about actively.

Any organism must be able to adjust
to the slight variations between day and
night. But seasonal variations between
winter and summer present a much
greater problem. Most trees and shrubs
in temperate regions flourish through
the fairly warm weather of spring, sum¬
mer, and fall. Then they enter a dor¬
mant, or inactive, period through the
colder months. Leaves may fall, and
sap may move to parts of the plant that
are not injured by freezing. The

CHAPTER 49 THE HABITAT 675

leaves of the pine, spruce, and other
evergreen trees remain throughout the
winter, even though most activity in the
plant has stopped. Nonwoody plants
may die to the ground and then reap¬
pear in the spring from dormant roots,
stems, or seeds.

Many birds of the Far North mi¬
grate into the northern regions of the
United States during the winter months.
Meanwhile, summer residents of these
same areas have migrated into southern
areas, even as far south as the tropics of
South America.

Other animals may seek a pro¬
tected cave or burrow under the ground
where they become inactive or sleep
during the cold of the winter. Some
desert animals remain in their burrows
during the heat of the day. A more
detailed discussion of the adjustments
of animals to seasonal and daily temper¬
ature variations will be found in the
next chapter.

Water is essential to life. Probably no
environmental factor is more important
to living things than water. The habi¬

tats of plants and animals vary from a
complete water environment to a sun-
parched desert. The ways in which
various plants and animals meet the
universal need for water is always of in¬
terest to the biologist.

Oceans, lakes, rivers, streams, and
ponds contain plants and animals that
need a water environment. Water¬
dwelling organisms are said to be aquat¬
ic. Those living only in salt water are
called marine. The bodies of such or¬
ganisms are adapted to perform all their
functions in water. Removed to land,
even in the wettest surroundings, aquatic
and marine organisms soon die. Land
plants require less water than aquatic or
marine forms. Rainfall is a major factor
in controlling the lives of these terres¬
trial , or land-living, plants.

Ecologists classify plants that grow
entirely or partially submerged in water
as hydrophytes. Included among these
plants are pond lilies, cattails, bulrushes,
eelgrass, and cranberries. Plants that
occupy neither extremely wet nor ex¬
tremely dry surroundings are classified

49-2 Xerophytes, such as
these cacti and other
plants, make up the flora
of a true desert. (Phoenix,
Arizona, Chamber of Com¬
merce)

676 UNIT 8 ECOLOGICAL RELATIONSHIPS

as mesophytes ( mez-oh-fvts ) . The trees
of the hardwood forests of the central
and eastern states are mesophytes, as
are most of the flowers and vegetables
we cultivate in our gardens. In general
mesophytes have well-developed roots
and extensive leaf areas.

The driest environments — semides¬
erts and true deserts — are occupied by
xerophytes ( zer-oh-fy ts ) . These plants
have extensive root systems for absorb¬
ing water and a greatly reduced leaf area
to cut evaporation to the minimum.
Cacti are examples of xerophytes whose
leaves are reduced to spines and whose
thick stems are adapted for water stor¬
age.

Light, an additional critical factor in the
environment. As you already know,
light is essential to all green plants in
food-making. But we find certain plants
and animals living normal lives in en¬
vironments of total darkness. Blind
fishes with undeveloped eyes live in un¬
derground streams and rivers in the
Mammoth Cave of Kentucky and other
dark places. Likewise deep-sea fishes
live at depths to which light cannot

49-3 Note that the branches on this tree ex¬
tend only in one direction. What factor is
responsible for this? (Ewing Galloway)

penetrate. Many bacteria live without
light, and are killed by long exposure to
direct sunlight. Careful study of these
organisms living in darkness, however,
shows that all but a few species of bac¬
teria depend indirectly on light for ex¬
istence. They all require food and its
stored energy for their activities. This
food can be traced to the green plant
and its food-making processes, which
are, of course, dependent on light.

Light conditions vary from place to
place. Deep valleys, the floor of a for¬
est, or the north side of a hill are places
where plants and animals with low light
requirements can thrive. Here we find
snails, toads, and salamanders as well as
ferns and mosses. Open fields, south¬
ern slopes, deserts, and other exposed
places offer ideal situations for plants
that need full sunlight. With these
plants we find rabbits, ground hogs,
coyotes, badgers, prairie dogs, ground
squirrels, horned toads, and many other
animals.

The atmosphere, a chemical storehouse.
The air around us has an important di¬
rect effect on living things. With the
exception of the anaerobic bacteria and
a few other organisms, all living things
must have free oxygen for life. This
oxygen may be taken directly from the
atmosphere or as a dissolved gas in
water.

Deep-sea life has a greater oxygen
problem than other forms of life. Since
water receives its supply of oxygen from
air, the oxygen content of water de¬
creases with depth. The ocean is over
35,000 feet deep in the Mindanao Deep
off the Philippine Islands. Yet deep-
sea fishes thrive there only down to a
mile below the surface.

Plants and animals that live in the
soil are most abundant near the surface.
The depth to which life can penetrate

CHAPTER 49 THE HABITAT 677

the soil is partly limited by food supply
and certainly by oxygen supply.

Air movement, caused by fluctua¬
tions in barometric pressure, also has a
direct influence on living things.
Storms may destroy plants and drive ani¬
mals to shelter. Air currents and winds
have a much greater effect on life than
most of us realize. Winds greatly in¬
crease the rate of evaporation of water.
Plants and animals whose habitats are
windy plains, prairies, and mountainous
regions must not only withstand the
wind, but must also survive the accom¬
panying loss of water by evaporation.
On mountains, high winds force trees
to grow close to the ground and to form
their branches only on the protected
side (Fig. 49-3). Winds also cause a
reduction in size of leaves and an in¬
crease in root systems.

The varied surroundings provided by
land formations. The physical features
of the earth, called topography , have a
great influence on living things.
Changes of the earth’s topography are
caused by such events as erosion, vol¬
canic eruptions, earthquakes, and flood¬
ing. As the earth changes, so environ¬
ments change. As a result, plants and
animals must migrate to new and more
favorable areas. They are then replaced
by other organisms more suited to the
conditions they left behind.

Nutritional relationships — important
biotic factors. The way living things
affect one another is equally as impor¬
tant as the effect of physical factors.
Many of the biotic relationships involve
food. The autotrophs, or “self-feeders,”
require only inorganic nutrients from
the environment to synthesize organic
compounds, so we call them food pro¬
ducers. In a lake these food producers
are of three kinds: the emergent, rooted
plants like cattails, and water lilies found

near the shore; the submerged plants,
like eelgrass and hornwort; and the sus¬
pended algae which form the phyto¬
plankton. Microscopic examination of
a few drops of pond or lake water usually
reveals hundreds of one-celled algae.

At certain times of the year, the phy¬
toplankton may increase so much that
the lake water turns to a dark green color.
At the time of such algal “blooms,” the
small crustaceans such as ostracods and
copepods feed well. Since these organ¬
isms feed on plants, they are called
herbivores (er-biv-ores) . They are
heterotrophs, or “other feeders,” and are
the first food consumers of the ecosys¬
tem. The energy synthesized and stored
by phytoplankton is transferred to the
protoplasm of the herbivores. The car¬
nivores (kar- niv-ores), or flesh-eating
animals, are sometimes divided into two
groups: the first-level carnivores, which
consume and use the energy of the her¬
bivores; and the second-level carnivores,

49-4 Small crustaceans, like this copepod,
are herbivores in a pond because they live
on the phytoplankton. (Walter Dawn)

678 UNIT 8 ECOLOGICAL RELATIONSHIPS

49-5 Scavengers such as snails occupy an
important place in food chains. (Ross
Hutchins)

which prey on the first-level carnivores.
The copepods and ostracods in a lake are
herbivores, while the minnows or the
young fish that eat the copepods and
ostracods are carnivores. Since young
fishes may eat small minnows and her¬
bivorous crustaceans, you can see that
young fishes may be carnivores either of
the first or second level, depending on
the availability of food.

The scavengers feed on dead organ¬
isms. They are important in the cy¬
cling of chemicals and in the transfer of
energy to the animals in the ecosystem
that feed on them. These “garbage col¬
lectors” are represented in the lake by
the crayfish and some snails. Many
fishes are partial scavengers. The bac¬
teria and yeasts are the decomposers in
a lake. They break the tissues and excre¬
tions of organisms into simpler sub¬
stances through the process of decay.
Other bacteria present in the mud bot¬
tom of the lake and in the soil convert
the simpler substances left by the de¬
composers into nitrogen compounds,
which are in turn used by the plants.

The bacteria that do this are called trans¬
formers. Together, then, the decom¬
posers and transformers return the nitro¬
gen, phosphates, and other substances
to the soil or water so that the plants
can begin the cycle again. If decom¬
posers did not exist, matter would not
be available for reuse in the ecosystem.
Food chains in an ecosystem. The
energy from the sun’s radiation is con¬
verted into stored energy by the green
plants. As the plants are consumed,
much of the stored energy is released,
but some of it is stored in the bodies of
herbivores. When a carnivore consumes
a herbivore, some of the stored energy
is used and some is stored in the carni¬
vore. Energy passes from the carni¬
vores to the scavengers, decomposers,*
and transformers. It is important to
remember, however, that not all the en¬
ergy stored by a herbivore is stored by
the carnivore. Much of it is used in
vital processes like metabolism, locomo¬
tion, and reproduction. Only the excess
energy is converted or stored.

We may examine the lake again for
examples of energy transfer. Energy
passes from the algae to the copepod to
the minnow to the sunfish to the bass.
If there is no predator of the bass, it is
the top carnivore and may furnish the
crayfish with food when it dies. The
transfer of the sun’s energy to a specific
herbivore to a first-level carnivore to a
second-level carnivore to a scavenger is
called a food chain. Food chains are
sometimes long and complex. The
chain in the lake could be extended bv
suggesting that a bullfrog may eat the
crayfish and a raccoon may eat the bull¬
frog. Other food chains may also op¬
erate within the lake. The frog might
eat a crayfish or a minnow and in turn
be swallowed by a snake. If the con¬
ditions are right, the bass could con-

CHAPTER 49 THE HABITAT 679

Pond water and dissolved minerals

49-6 A pyramid based on the number of in¬
dividuals in a pond.

ceivably consume the snake. Now it
appears that the biotic relationships are
complicated by the integration of the
food chains.

If you were to list every organism
in the lake and draw arrows to indicate
which organisms are used for food by
others, your diagram would represent
all the biotic relationships involving en¬
ergy transfer. Since there are so many
possibilities, this diagram would ac-
tuallv resemble a web more than a chain.
For this reason food chains are some¬
times called food webs.

Ecological pyramids. Food chains in an
ecosystem are often represented as a

pyramid, with the food producers form¬
ing the base and the top carnivore at the
apex. Such a food pyramid can be made
to represent an actual food chain by
counting the numbers of individuals in¬
volved in each step of the chain. One
such count of a bluegrass field revealed
the following average per acre: 5,842,424
producers to 707,624 herbivorous inver¬
tebrates to 354,904 ants, spiders, and
predatory beetles to 3 birds and moles.

Another way of studying food
chains is to determine the weight in
each step of the chain. This might
produce a clearer picture of the biotic
relationships, resulting in a more grad¬
ually sloping pyramid. Let’s consider a
sample pyramid involving a willow tree,
caterpillars feeding on it, birds feeding
on the caterpillars, and hawks feeding
on the birds. In this pyramid one tree
could support a number of caterpillars.
Drawn on the basis of numbers, our
pyramid would look like Fig. 49-7, left.
However, if we were to represent this
food chain by the weights of organisms,
our pyramid would somewhat resemble
Fig. 49-7, right.

Pyramids based on numbers of in¬
dividuals or by weight indicate a con¬
dition found in the ecosystem at any
particular moment. Remember, how-

■HHMgi

Hawks

■■■mi

r*y- '4*^

Birds

Caterpillars

Tree

49-7 Left: pyramid based on numbers; right: pyramid based on weight. The
latter shows truer energy relationships.

680 UNIT 8 ECOLOGICAL RELATIONSHIPS

ever, that conditions within the ecosys¬
tem are not static — they are always
changing. The number and weight of
a group of organisms does not depend
on the number and weight of their food
at any given time, but more on how
quickly the food is being replenished.

By having a knowledge of food
chains and pyramids, ecologists may be
able to predict future events. Returning
to our lake ecosystem, let us suppose
conditions in the spring are favorable
and an algal bloom results. The in¬
crease in the phytoplankton of the lake
would be favorable to the organisms
feeding on it. Many minnows, then,
would be able to survive on the abun¬
dant food, whereas in less favorable
years many would have died. These
favorable conditions would suggest a
larger gross weight for the bass popula¬
tion in the next year.

A knowledge of food pyramids may
aid us in solving a human social prob¬
lem. Can a population explosion such
as we have in the world todav continue

indefinitely without a critical food short¬
age? In crowded regions, as in the Far
East, people must live largely on vege¬
table diets. They cannot afford to use
the food substance necessary to raise
herbivores for meat, because the plant
level of the pvramid supplies far more
food from the available soil. Remem¬
ber that each level of a food chain uses
up some of the energy originally ob¬
tained from the sun. More food en¬
ergy is available closer to the base level
of the pyramid. By finding more ways
to get nearer to the base of the pyramid,
it may be possible to accommodate
larger human populations.

Special nutritional relationships. In our
study of food chains, most of the ani¬
mals we have discussed are bulk feeders.
That is, they consume tissues in bulk as

whole organisms or parts of them. Most
animals, including man, are bulk feeders.
In another nutritional relationship the
individuals live in direct association with
each other. The relationship is called
symbiosis , which means “living to¬
gether.” Biologists usually further di¬
vide symbiosis into three different kinds:
1. parasitism; 2. mutualism; and 3. com¬
mensalism. In parasitism , the parasite
lives in or on another organism, called
a host. The parasite benefits from this
association, while the host is harmed.
Tapeworms and lampreys are good ex¬
amples of animal parasites. Other para¬
sites include disease-causing bacteria,
mildews, rusts, and smuts.

It is to the advantage of the para¬
site to keep the host alive. Death of
the host results in loss of the parasite’s
habitat and food supply. Neither ticks,
fleas, mosquitos, nor the fungus that
causes athlete’s foot kills the host. They
take only enough nourishment to sus¬
tain themselves, to grow, and to repro¬
duce. The biologically successful para¬
site is often a degenerate organism.
However, it may possess structures lack¬
ing in free-living relatives. For instance,
special hooks may hold it to the intes¬
tinal wall to prevent it from being swept
away. It may have thick skin or cuticle
to protect it from the corrosive action
of the host’s digestive juices. Many
parasites have complicated methods of
dispersal, requiring two or more hosts
before reaching maturity.

Every free-living organism appears
to have its parasites, and many parasites
have parasites of their own. In terms
of numbers of individuals, there are
more parasites than there are free-living
organisms.

In mutualism , another form of sym¬
biosis, two different kinds of organisms
live together to the advantage of each.

CHAPTER 49 THE HABITAT 681

In some instances the two organisms
have become so dependent on each
other that neither can live alone. Ter¬
mites, for example, can chew and ingest
the cellulose in wood, but they cannot
digest it. The cellulose is digested by
protists living in the termite’s digestive
tract. The termite is provided with a
means of digestion, and the protist is
given a place to live. The association of
an alga and a fungus in lichens is an¬
other example of mutualism. The non¬
photosynthetic fungus provides moisture
and support for the alga, which in turn
synthesizes food for the fungus and it¬
self. Another well known example of
mutualism is the relationship between
the flowers that supply nectar to the
insects that pollinate them. Many
mammals are associated in a mutualistic
relationship with specific birds that pick
off and eat ticks.

In commensalism one of the part¬
ners is benefited while the other is
neither benefited nor harmed. A good
example of a commensal relationship
is the remora and the shark. The remora
is a small fish with a suction pad on top
of its head. It attaches this pad to the
lower side of a shark and feeds on scraps
of the shark’s food. The remora thus
benefits, although the shark does not.
But the shark is not harmed either. The
term commensalism means, literally,
“common table” or “messmates.”

Another group of heterotrophs ab¬
sorbs nutrients from dead tissues or prod¬
ucts of organisms. These are the sapro¬
phytes. In this group are the bacteria
that decompose plant and animal bod¬
ies; the molds that live on bread, fruit,
leather, and other organic materials; the
yeasts that ferment sugars; the fungi
that live on dead trees, and many other
organisms. While some saprophytes are
destructive, others are verv useful.

49-8 Here is a hermit crab with a colonial
coelenterate living on its shell. What do
biologists call such a relationship? (Walter
Dawn)

Passive protection. Individual animals
have some form of protection from bulk-
food eaters. Claws, teeth, spines,
stingers, pincers, and the ability to run
fast are all used in defense. But many
animals are able to survive because they
can hide in their surroundings. Sim¬
ilarly, a predator uses concealment in
hunting prey. Many animals would per¬
ish, either as prey or predator, if re¬
moved from their usual environment.
A green katydid is nearly impossible to
find among the green leaves of a lake¬
side tree, but if it should fall into the
lake, it would be immediately vulner¬
able to birds, fishes, other insects, or
frogs. The blending of an animal with
its surroundings is a kind of camouflage.
Many animals owe their existence to
this protection.

Animal camouflage involves several
principles. Sometimes the animal is
colored or marked like its surroundings.
We refer to such camouflage as protec¬
tive coloration. The orange background
and black stripes of the tiger blend al¬
most perfectly with the grasses and shad¬
ows of its environment. A covey of

682 UNIT 8 ECOLOGICAL RELATIONSHIPS

49-9 The yellow perch illustrates the prin¬
ciple of countershading. Its upper surface
blends with the colors on the bottom of the
lake or stream but its under surface is quite
light so it blends with the reflection of the
sky on water. (Chace from National Audu¬
bon Society)

quail crouched in a thicket goes unno¬
ticed until the individuals become fright¬
ened and fly into the air. The common
tree frog has irregular markings of brown
and ashy gray that blend with the bark
of a tree. As you can see, protective col¬
oration is a common adaptation for sur¬
vival. Some animals blend so perfectly
with their surroundings that only the
most careful observer can discover them.

Fishes illustrate a slightly different
principle. Darker colors on the upper
side of the fish fade into light colors on
the lower side. The coloring of the
largemouth black bass is a good example
of this principle, called countershading.
The upper region of the body is greenish,
blending with colors on the bottom of a
lake or stream. The bass is nearly white
on the lower side, so that it blends with
the reflection of sky in the water. Thus
the fish is protected from enemies above
and below. You doubtless know how
hard it is to see a fish in the water.

Still another principle of animal
camouflage is illustrated when an animal
resembles something in its environment.

This is called protective resemblance.
Several kinds of butterflies resemble
brown leaves when their wings are
folded. The walking stick, a relative of
the grasshopper, actually looks like a
stick with legs.

Mimicry is another type of protec¬
tive resemblance. In mimicry, however,
the animal looks like another animal
rather than a part of its environment.
Several kinds of defenseless flies resem¬
ble stinging insects. This gives them
nearly the same protection as if they had
stingers. Another example of mimicry
is found in two butterflies, the viceroy
and the monarch (Fig. 49-10). The
monarch is the common orange and
black butterfly seen around milkweed
plants. The viceroy looks almost ex¬
actly like the monarch. The monarch
has such an unpleasant taste that it is
avoided by birds. The more palatable
viceroy escapes because it looks so much
like the unpalatable monarch.

In studying animal camouflage, we
must keep in mind the principle of
cause and effect. The animal does not
blend with its surroundings in order to

49-10 Mimicry is represented by these but¬
terflies. The one at the top is the viceroy
while that on the bottom is a monarch.
(Walter Dawn)

CHAPTER 49 THE HABITAT 683

live. Rather, it survives because it
blends with its surroundings. Today,
we see the result of many years of sur¬
vival by those animals best adjusted to
their surroundings. Through slight var¬
iations in form and color, certain indi¬
viduals resembled their surroundings
more than others. They had a better

chance to survive and produce more
of their kind. Variations appear
slowly. It has taken many thousands
of years for the animals we have just
mentioned to develop. In the process,
countless millions perished because they
were not very well adapted to their en¬
vironment.

IN CONCLUSION

All living things are interrelated with others and with the physical environ¬
ment. Energy, originally from the sun, is transferred from one organism to
another, and chemicals are cycled in the ecosystem. Soil, temperature, water,
light, atmospheric conditions, and earth changes determine plant growth.

The producers, consumers, scavengers, and decomposers all have an im¬
portant function in the transfer of energy and the cycling of inorganic com¬
pounds in a biotic community. On the basis of the manner in which organic
nutrients are received from the environment, heterotrophs can be classified as
symbionts, saprophytes, or bulk feeders.

In the next chapter we shall see the effect of periodic changes on or¬
ganisms.

BIOLOGICALLY SPEAKING

aquatic

homoiothermic

producers

bulk feeders

hydrophytes

protective coloration

carnivores

limiting factor

protective resemblance

commensalism

marine

saprophytes

consumers

mesophytes

scavengers

countershading

mimicry

symbiosis

decomposers

mutualism

terrestrial

dormant

niche

tolerance

food chain

parasitism

topography

food pyramid

plankton

transformers

habitat

poikilothermic

xerophytes

herbivores

QUESTIONS FOR REVIEW

1. Explain the importance of plankton.

2. How does the tolerance of an organism for an environmental factor limit
its distribution?

3. How does soil control the distribution of plants and animals?

4. Name several important controlling physical factors of the environment.

5. What is meant bv the terms “cold-blooded” and “warm-blooded”?

684 UNIT 8 ECOLOGICAL RELATIONSHIPS

6. How do deciduous plants adjust to the freezing temperatures of winter?

7. How are environments classified according to the available water?

8. In what ways are plants adapted to live in environments of varying water
content?

9. Although some organisms can live in total darkness, all are dependent on
light. Explain.

10. In what environments is oxygen a limiting factor for organisms?

11. How does topography affect the distribution of living things?

12. Starting with the food producers, name and define the various types of
organisms in a food chain.

APPLYING PRINCIPLES AND CONCEPTS

1. Choose some common organisms in your environment, and discuss possible
limiting factors for each.

2. Discuss the importance of research on the limiting factors of specific or¬
ganisms.

3. Does man’s tolerance to a wide variety of conditions enable him to extend
his habitat? What other factors are involved?

4. Discuss the survival value of the constant internal temperature condition
found in some animals.

5. Give some possible reason why evergreens do not freeze and die when
covered with snow during the freezing temperatures of winter.

6. Discuss the adaptations of plants growing in areas of reduced light.

7. Make a diagram of a food web that exists in your local area.

CHAPTER 50

PERIODIC
CHANGES IN
ENVIRONMENT

Alternating periods of activity. In the
springtime people living in rural areas
may waken to the sounds of hundreds of
birds announcing the arrival of a new
day. Attention is focused on the birds
because of their songs. On the ground
at the same time, snails, slugs, and sow-
bugs are making their way to protected
spots where they will remain through¬
out the daylight hours. Some distance
away, a meadow mouse narrowly escapes
a diving hawk that had spotted it while
soaring in the early morning sky above.
If all goes well for the mouse, it will be
safe for the remainder of the day. This
was not the first escape for the little
four-footed creature. Three hours ear¬
lier, in the dark of night, it had narrowly
escaped the sharp talons of a barn owl.
Each morning similar events occur wher¬
ever animals exist.

An organism that is active during
the day is said to be diurnal (dv-urn-l),
while one that is active at night is noc¬
turnal (nok-turn-1). Each diurnal or¬
ganism occupies a niche that may also
be occupied by a nocturnal organism.
The diurnal hawk’s predatory niche, for
example, is occupied by the owl during
the night. Alternating periods of ac¬
tivity are called periodicity , and when
the periodicity is regular, it is said to be
rhythmic. Thus, the early bird getting
its worm every morning shows rhythmic
behavior. Although biologists have ob¬
served and studied rhythmic behavior in

plants and animals for many years, there
are still many challenging unsolved ques
tions. Some animals have been de¬
scribed as having “internal clocks.’’
What causes migrations of certain in¬
sects, birds, or mammals, and what
causes hibernating animals to wake up?

Where rhythms have been observed
and studied, light seems to be an im¬
portant factor. It is not too surpris¬
ing, then, to find rhythms based on
daily, seasonal, and annual light varia¬
tions. Rhythms that coincide with the
changes in the moon phases have also
been studied. However, as you will see,
factors other than light are important
in the control of regular activity of or¬
ganisms. In the nocturnal white-footed
mouse, daily behavior is not a simple
matter of activity in the dark and rest
in the daylight. When the animal is
kept in constant darkness, its waking
and sleeping periods continue as if it
were exposed to normal periods of day¬
light and darkness. Tims, the animal
has the capacity to “remember” its
rhvthm and is not entirely dependent on
the external stimulus of light.

The daily rhythms. Similar environ¬
mental conditions are found in all for¬
ests, whether a lowland, hill top

685

686 UNIT 8 ECOLOGICAL RELATIONSHIPS

50-1 The white-footed mouse is strictly a
nocturnal animal and as such is a natural
prey for owls. (Walter Dawn)

woods, or a loftv mountain forest. The
tall trees provide areas of shade under
which the shrub layer of the forest floor

flourishes. In other areas, the sunny
spaces provide meadows of various sizes
where the grasses grow. Ferns cover the
banks of streams. A summer morning
may find the birds, chipmunks, and
ground squirrels searching for food as
the deer browse among the meadows.
Before noon, however, the activity of
the forest reduces.

As evening approaches, many of
the diurnal animals renew their activ¬
ity for a short time. Dragonflies and
bats dart over streams and ponds search¬
ing for both diurnal and nocturnal in¬
sects available in this transition period.
The sounds of the cicadas and birds
gradually fade as the chirping crickets
begin their activity. The foxes, rac¬
coons, skunks, owls, and mountain lions
begin their nocturnal search for food.

Although respiration in plants also
occurs during the daylight hours, the
photosynthetic manufacture of sugars
proceeds rapidly during that time. In
the darkness, when photosynthesis
ceases, the manufactured sugars con¬

tinue to be transported downward to
storage tissues, and only respiration oc¬
curs. When plants bloom, the flowers
may open and close at regular times of
the day. The petals of the poppy flower,
for example, open in the morning and
close at night for several days. Many
cacti bloom only at night and depend
on nocturnal insects for pollination.

The desert environment, with its
extreme temperature changes, brings
about a sharper division between day
and night activities. In the early morn¬
ing hours, birds feed on insects and
seeds, and jack rabbits come out of
their burrows to look for food. The
snakes, hawks, vultures, and ground
squirrels are awake and active. Since
the temperature may reach 170° F on

50-2 The night-blooming Cereus flowers only
in the evening or at night. (Schneider from
Black Star)

CHAPTER 50 PERIODIC CHANGES IN ENVIRONMENT 687

the desert surface, noontime finds most
of the desert creatures in the shade
at the base of cacti, sagebrush, or creo¬
sote bushes. Many of them take to their
cooler burrows, where the moisture in
the air may be more than twice that of
the atmosphere.

In the early evening many of these
creatures again come out for a brief
period. During the night, the poison¬
ous Gila monster and the rattlesnake
prey upon the many small nocturnal
desert mammals. The bobcat, coyote,
fox, and owl are some of the larger noc¬
turnal desert carnivores.

In contrast to the desert are the
polar regions, where the cold nights re¬
veal very little activity. During the
brief summer, most organisms are ac¬
tive throughout the prolonged daylight,
but in the winter, only a few warm¬
blooded animals are active.

Farther from the poles, however,
are the vast regions of treeless plains,
where the ground remains frozen most
of the year. For brief periods a shallow
layer of earth at the surface thaws
enough to allow lichens and some grasses
to grow there. Among this plant life,
a few warm-blooded animals such as
the caribou, Arctic hare, fox, lemming,
and ptarmigan spend the daylight hours
obtaining food.

In equatorial regions the length of
the day and night is nearly equal and
the temperature is less variable. Light
is, therefore, a more important factor
in determining the periods of activity of
the equatorial organisms. In the equa¬
torial forests, nocturnal and diurnal an¬
imals are both very numerous.

Day-night rhythms are also found
in the oceans, where periodic vertical
migrations have been studied. Vast
numbers of copepods and shrimp are
found at night near the surface, where

they are able to feed on the plankton.
During the day these herbivores sink to
a lower level and may be found 300
feet below the surface. Small carniv¬
orous fishes follow the daily excursions
of these plankton-feeders.

Seasonal community changes. Animals
meet the problems of seasonal tempera¬
ture changes in several ways. When the
cold winter brings snow and freezing
temperatures, an organism must be able
to adjust or move away. Otherwise, it
will die. The first freezing nights of
winter take a heavy toll of insect life.
Although the mourning cloak butterflies
find winter protection in a hollow tree
or crevice, most adult, insects have com¬
pleted their life cycle before the begin¬
ning of winter and are killed by the first
frosts. Species survival through the win¬
ter is insured by other means. Many
of the moths spend winter as pupae
within silk-insulated cocoons. Grass¬
hoppers, crickets, and cicadas lay eggs
within the ground or in the bark of
trees. Stoneflies, mayflies, and dragon¬
flies spend the winter as nymphs in the
water, sheltered beneath the frozen sur¬
face of a pond or stream.

The familiar honeybee finds pro¬
tection in numbers. During the winter
months, the bees feed on the honey
that had been stored in the spring and
summer. This supplies the bees with
energy as they remain active in their
hives during the winter months. The
temperature within the hive may be as
much as 75° F higher than that outside.

Animals like the eastern cottontail
rabbit, the white-tailed deer, the car¬
dinal, and the bluejay are permanent res¬
idents in their regions. During extremely
cold weather, they find protection in
woods and thickets; but when snow is
on the ground, food-getting becomes
a serious problem.

688 UNIT 8 ECOLOGICAL RELATIONSHIPS

50-3 During the winter months many ani¬
mals, like this chipmunk, hibernate. The
animal’s metabolism slows down markedly
and does not accelerate until spring ar¬
rives. (Annan Photo Features)

Ground squirrels, chipmunks, wood¬
chucks, and many reptiles and amphibi¬
ans undergo true hibernation during
cold weather. The rate of body metab¬
olism drops greatly. Heart action and
respiration decrease, and the animal loses
consciousness. Greatly reduced activity
lowers energy requirements to the mini¬
mum. An animal undergoing true hi¬
bernation cannot resume activity until
the temperature of the environment in¬
creases and the bodv processes speed up.

Some animals that remain in a re¬
gion during unfavorable periods may
enter a state of dormancy. That is, they
reduce their life activities to the mini¬
mum necessary for survival. The bear
finds a hollow log, a cave, or some other
protected location and lives on stored
fat during his winter sleep. Although
its activities slow down, a normal body
temperature is maintained, and the bear
can be awakened. It may even leave its
shelter on a mild winter dav. The

skunk, raccoon, and opossum undergo a
similar winter sleep.

During hot weather many animals
enter a period of dormancy sometimes
referred to as summer hibernation. The
biological term for this is estivation. A
frog may estivate in the cool mud at the
bottom of a pond. The box turtle often
escapes the heat by burying itself in a
pile of leaves. The period of estivation
may be several days or several weeks.
The gopher tortoise of the southeastern
states finds protection deep in a burrow
in the ground. The California ground
squirrel sleeps in its burrow, thus reduc¬
ing both its food and water needs.

Many animals migrate to warmer
regions when winter comes. We usually
associate migration with birds, but some
mammals also show the tendency to
migrate. These seasonal journeys may
cover thousands of miles. Some mam¬
mals make a migratory journey because
of seasonal changes in the food supply.
Others migrate to a more favorable cli¬
mate regardless of food supply. Still
others make seasonal journeys to regions
where they can produce their young
under the most favorable conditions.

The bighorn sheep spends its sum¬
mers in the high meadows near the
summits of the Rocky Mountains. As
winter approaches, it moves down into
the protection of the forests on the
mountain slopes. Through the summer
months, herds of Olympic elk browse
in the high altitude of the mountains
on the Olympic Peninsula. During the
winter these herds move to the more
protected mountain valleys and nearby
plains. With the coming of spring,
the herds move back up the slopes in
long, single-file processions.

Among the most remarkable mi¬
grations is that of the fur seal. During
the winter, females, young males, and

CHAPTER 50 PERIODIC CHANGES IN ENVIRONMENT 689

50-4 These monarch but¬
terflies spend the winter
in a pine grove in Cali¬
fornia after migrating
from Canada. (Shrop¬
shire Camera Exchange)

pups roam the waters of the Pacific
Ocean to as far south as California.
The older males winter in the cold
waters near Alaska and the Aleutian
Islands. With the approach of the
breeding season in the spring, the males
migrate to the Pribilof Islands, north of
the Aleutians. The males arrive sev¬
eral weeks before the females and battle
for a territory. The females and young
seals start their long journey of 3,000
miles or more to the Pribilofs in the
spring and arrive in June. A herd of
50 or more females gathers around each
male. Pups from the past year’s breed¬
ing are born almost immediately and
within a week, breeding occurs again.
After this the seals migrate southward.
A butterfly migration. The monarch, or
milkweed butterfly, one of the strongest
insect fliers, makes a remarkable seasonal
journey. In later summer these butter¬
flies gather by the thousands in north¬
ern Canada and begin a long flight
southward. Some of them travel to the
Gulf states to spend the winter, but

their habits there have not as yet been
studied. Others travel a southern route
along the Pacific Coast. Some time be¬
tween the middle of October and the
first of November, tens of thousands of
these insects arrive on the Monterey
Peninsula in the small town of Pacific
Grove, California. Here they seek shel¬
ter in a specific grove of pines. The
monarchs hang by their legs from the
branches and needles in such large num¬
bers that the trees appear to be solid
brown (Fig. 50-4). They stay there in
a state of semihibernation until the
winter is over. On warm sunny davs
throughout the fall and winter, many
will be seen flying about local gardens
gathering nectar.

In March the monarchs fly out over
Monterey Bay to begin their northward
flight. As they fly north, they lay eggs
on milkweed plants. It is unlikely that
any of the travelers ever reach their
northern home, because many die after
the eggs are laid. However, after the
eggs hatch, the larvae pupate, the young

690 UNIT 8 ECOLOGICAL RELATIONSHIPS

butterflies emerge, and the trip north¬
ward is continued. These new butter¬
flies also lay eggs on milkweed as they
progress.

By late summer the monarchs begin
amassing for their southward journey to
the same locality and the same trees
where their ancestors of two generations
past spent the winter. Although we at¬
tribute the monarch’s behavior to in¬
stinct, the factors causing the migrations
and the ability of these butterflies to
find their way are not well understood.
Bird migration. One of the most inter¬
esting instincts of birds is that which
controls their migration. Many birds
fly long distances in the spring, then
nest and raise their young in a new
home. They return to warmer climates
in the fall. Migration may be prompted
by food needs, climatic changes, or
breeding habits. It is difficult to de¬
termine why some species leave abun¬
dant food and warmth in the tropics to
migrate to breeding grounds in the Far
North. Much more easily explained is
the southward migration of insect-eaters
when cold weather kills their prey, and
the southward flight of water birds be¬
fore the ponds and lakes freeze over.
It is logical, too, that fruit- and seed-
eaters would tend to follow their food
supplies.

Some birds make their migratory
flights at night and some during the day,
depending on the species. Perhaps you
are familiar with the night flights of
geese during the spring and autumn,
when they become confused by the
lights of a city and circle about, honk¬
ing noisily. The daylight flights of
thousands of red-winged blackbirds and
grackles are familiar sights during spring
and fall.

While many birds migrate slowly,
feeding along the way and averaging

only 20 to 30 miles a day, others are
marvels of speed and endurance. The
ruddy turnstone travels each autumn
from Alaska to Hawaii in a single flight,
and the golden plover travels from Can¬
ada to South America, more than 8,000
miles as you can see from the map in
Fig. 50-5.

Migratory routes. We do not know
much about the instinct that governs
the time and route of migration. Any
given species follows the same route
year after year and may be expected
to arrive at a certain point within a
few weeks of the same time each sea¬
son, depending on the weather. As
though to vary the scenery, certain spe¬
cies travel northward along one route
and return by an entirely different route.
How do they know the way? Keen
sight may help, but not over water or
through dark nights and fogs. Even
the memory of old birds that have made
the flight before cannot account for the
unescorted flights of young birds. Bi¬
ologists have now determined that the
sun guides birds on their daylight flights.
There is also good indication that birds
are able to allow for the variations in
position of the sun in different seasons
and in different latitudes. This finding
does not infer that birds understand in¬
tellectually their position in relation to
the sun.

What about the migratory flights
of birds at night? Recent studies of
night flights reveal even more amazing
possibilities. Birds seem to be directed
by the position of the stars. Investiga¬
tors in Europe have used caged birds
in a planetarium in which star patterns
of different seasons and various places
in the earth can be projected on a dome
representing the heavens. They have
found that certain birds, including war¬
blers, make a definite response to the po-

CHAPTER 50 PERIODIC CHANGES IN ENVIRONMENT 691

50-5 The golden plover travels more than 8,000 miles in a single migratory
flight. It breeds in northern Canada during the summer, flying in the fall to
South America, where it spends the winter before returning north.

692 UNIT 8 ECOLOGICAL RELATIONSHIPS

sition of stars during the normal season
for migration. Under a fall star pat¬
tern, they face toward the winter mi¬
gratory home in their cages. Under a
spring pattern, they face toward the
summer home.

Seasonal bird study. Migration adds
much to the study of birds as a hobby,
for new species are arriving during the
seasons of the year. Each locality has
permanent residents which remain the
year around. Certain species may be
present in the winter only, moving far¬
ther north with the coming of spring.
These species are called winter residents.
The summer residents spend summers in
a given locality and migrate southward
in the fall. Many species are found
only at certain times in the spring and
fall. These are the migratory birds
which are passing through a given lo¬
cality on their journey between winter¬
ing areas farther south and breeding
areas farther north.

Lunar rhythms. There are many folk
tales about planting certain crops with
various phases of the moon. Although
these are interesting stories, there is no
experimental data, as yet, to support
them. The greatest effect of the moon
is seen along the coasts, because the
moon is mainly responsible for ocean
tides. Tidal extremes vary in different
parts of the earth, depending on the
shape of the coastline. In the Bay
of Fundv, located between Maine and
Nova Scotia, 50-foot tides have been
measured. On the coast of France, the
Bay of St. Malo has tides of 39 feet.
Although most regions do not have such
spectacular tides, the rhythmic rise and
fall of water level affects the lives of the
organisms living within this zone of
tidal influence.

Many marine biologists spend their
time studying the ways in which plants

and animals are able to adjust to the
changing conditions of such an environ¬
ment. The greatest problem is the dan¬
ger of drying out at low tide. The small
fishes and crustaceans find protection in
the waters of the tidepools left as the
water recedes. Many limpets and snails
resist drying out by clamping down
tightly on the rocks. Mussels and bar¬
nacles preserve moisture by closing their
shells tightly. Some sessile sponges and
tunicates live under rock ledges pro¬
tected by sea weed, which covers them
when the tide is out.

Drying is only one of the problems
the intertidal organisms face. In both
summer and winter, tides cause wide
variations in temperature. Fresh water
from rains alters the salt content of
tidepools. Thus, the organisms inhabit¬
ing the intertidal zone must adjust to
a great variety of changes.

On certain nights from March to
August, cars are parked bumper to bum¬
per on the coast highway of California.
Thousands of beach fires, added to the
light of the moon, create a spectacular
scene. The people are waiting for the
turning tide, at which time the beach
will be left shimmering with thousands
of fish. These fish, called grunion, are
caught and roasted over the fires. The
grunion’s behavior offers a precise ex¬
ample of lunar periodicity. Exactly at
the turning of the tide on the second,
third, and fourth nights of the highest
night tides, pairs of these fish swim up
the beach with the breaking waves. The
female digs into the sand and deposits
eggs about three inches below the sur¬
face. The male fertilizes the eggs, and
on the next wave, the pair slips back
into the sea. The eggs remain in the
sand until the next unusuallv high tides,
about ten days later, at which time they
are washed out of the sand. The eggs

CHAPTER 50 PERIODIC CHANGES IN ENVIRONMENT 693

50-6 The photograph on the left shows a tidepool at high tide, and the one on
the right at low tide. Note the barnacles and brown algae which are always ex¬
posed at such periods. (Kinne — Photo Researchers, Inc.)

hatch as soon as they are immersed, and
the tiny fish swim away. Several ma¬
rine annelid worms have also been ob¬
served to swarm and breed at definite
phases of the moon.

Annual rhythms. Many of the repro¬
ductive cycles of plants and animals are
associated with seasonal changes and
occur in a yearly rhythm. The female
bear has her young during the win¬
ter in the protection of her den. Birds
nest in the spring, thereby insuring
full growth of the young before winter.
Wildflowers bloom in the spring and
produce seeds for the development of
the next generation. Deciduous trees
lose their leaves in the fall. These are
all familiar expressions of annual cycles.
You will be able to name many more.

What is the value of periodicity? As

you are now well aware, an organism
must be able to meet environmental
changes in order to survive. You have
seen many ways in which various or¬
ganisms adjust to changes in the en¬
vironment. The establishment of a
svnchronized rhvthm for any entire
population is of value for the survival
of the species. The behavior of swarm¬
ing grunion insures the aggregation of
males and females so the eggs can be
fertilized. Male and female gametes of
the oyster are shed into the surrounding
water at the same time. Even though
a female oyster may produce 1 14,000,000
eggs, her efforts would be futile if a
neighboring male did not release sperm
at the same time. The search for fac-

694 UNIT 8 ECOLOGICAL RELATIONSHIPS

Pioneer stage — bacteria Flagellate stage

Colpoda stage

Paramecium — ameba stage

Climax stage — balanced

50-7 Succession in a jar of water.

tors causing synchronized behavior in
populations occupies the time and ef¬
forts of many biologists.

Changing biotic communities. We
have seen some of the daily and seasonal
changes occurring in various communi¬
ties. Plants and animals are continually
on the move, but since the changes in
plant populations of an area usually
occur slowly, they are not always easy to
identify. As plants change, animals find
new homes. This changing of commu¬
nities is called succession. If an area
within an ecosystem were completely
cleared of living things, the natural
changes bringing about its populations
would be more apparent. The gradual¬
ness of changes results in a relatively
stable community in equilibrium with
the local conditions.

Winds, fires, volcanic activity, and
other events in nature, as well as man’s
clearing, may destroy the organisms liv¬
ing within a natural area. Then, if the
area is left alone, succession starts.
Eventually a permanent community will

reclaim the region. From beginning to
climax may take as long as 100 years.

Succession can occur even in a jar
of water. If the culture medium is made
by boiling hay and then exposing it to
the air, it will soon be teeming with bac¬
teria. Since the bacteria are the first
organisms to enter the area, they are
called pioneers. If a few drops of pond
water containing several kinds of protists
are added to the bacterial culture, the
protists will multiply at varying rates.
The flagellate population thrives on the
bacteria, but as their numbers increase,
their food becomes scarce, and they be¬
gin to dwindle. The disappearance of
the flagellates is also speeded up by an
increase of Colpoda , a ciliate resembling
the paramecium. As the Colpoda feed
on the flagellates, the medium becomes
less acid and the Colpoda population
is replaced by the increasing numbers
of paramecia and amebas, which can ad¬
just to a more alkaline environment. As
the amebas begin to increase and con¬
sume more ciliates, the ciliate popula-

CHAPTER 50 PERIODIC CHANGES IN ENVIRONMENT 695

tion declines. This succession may take
several months, and at the end of this
stage, the organisms will all die unless
more nutrients are added. If we were
to add a few cells of green algae to the
jar, they would multiply and the addi¬
tion of a few drops of pond water might
allow • some rotifers and crustaceans to
develop. At this stage, there are a few
of all the organisms in the water, and
as long as the plants receive enough
light to manufacture food, the numbers
of each kind will remain about the same.
This stable, or balanced, system may re¬
main for months or even longer. The
balance would, then, be the climax con¬
dition of the ecosystem in the jar.
Natural succession in a forest. When¬
ever a tree falls in a forest, succession
begins. This type of succession is much
more complex than that occurring in a
hay infusion, and lasts for a great many
years. Whenever rocks are put in a bay
for a breakwater, or wooden or steel piles
are sunk for a pier, ecological succession
occurs. Wherever succession has been
studied, the sequence of organisms and
their time of appearance can be pre¬
dicted with surprising accuracy.

To examine a more complex series
in a succession, we shall start with a
section of bare soil in an open area.
It might be a region devastated by fire
or one cut for trees and not reseeded.
We shall locate it in a broad-leaved
forest in the eastern United States,
where beech and sugar-maple forests
once grew over much of the land. First,
the seeds of grasses and other open-field
plants that may have been dormant in
the soil or are carried in by animal or
wind find the environment satisfactory.
A meadow is produced by these pio¬
neers which may dominate and control
the region for several years. Next, the
seeds of elms, cottonwoods, and shrubs

find their way into the meadow, mark¬
ing the beginning of a forest. The
larger plants shade the shorter grasses
and field plants. Thus, the environ¬
ment of the once open field is changed
so that one might describe the area as
an open, low woods. This second stage
may soon become too shady even for
the seedlings of the grass and shrubs, so
the area will again begin to change.

The third stage may be the arrival
of seeds of oaks, ashes, and other trees
whose seedlings can grow well in a shady
environment. These trees grow among
the elms and cottonwoods and gradually
assume control by becoming the domi¬
nant vegetation. Finally a dense forest
begins to form. The ground becomes
moist and fertile, and beech and maple
seedlings outdistance all other species in
the competition for a place to live in the
forest. Eventually they crowd out most
of the other trees. Since the beech and
maple trees assume final control of the
region, we refer to them as the climax
species. If such a succession occurred
on a ridge, the climax species might
have been an oak and hickory forest.
Short grasses are the climax plants in the
Great Plains.

Succession in ponds and lakes. Ponds
and lakes are excellent for the study of
succession. As we learned earlier, the
cattails and water lilies around the edges
hold soil around their roots, building up
the soil over a period of many years.
When this occurs, the pond actually
grows smaller. By closely examining
the organisms from the pond’s edge out¬
ward to the terrestrial climax plants of
the region, we can then predict the suc¬
cession of plants and animals that will
gradually move inward until the pond
is obliterated. The size of the pond or
lake, the way in which its water is sup¬
plied, and its location also determine

696 UNIT 8 ECOLOGICAL RELATIONSHIPS

50-8 Schematic diagram of succession in a pond. A: pioneer, open-pond stage;
B: submerged vegetation stage; C: Cattail stage; D: sedge meadow stage;
E: climax forest stage.

CHAPTER 50 PERIODIC CHANGES IN ENVIRONMENT 697

the extent to which succession fills it
in. These conditions also determine the
time for succession to occur and whether
it will take a few years or hundreds of
years.

On August 17, 1959 an earthquake
occurred in the Madison River Canyon
of Gallatin National Forest in Mon¬
tana. A new lake was formed and much
land on the mountainside was cleared
when tons of earth, rocks, and trees fell
into the canyon. Biologists are cur¬
rently studying the changes occurring in
the new lake and clearings. A knowl¬
edge of ecological succession enables us
to predict the changes, but phenomena
such as this earthquake provide us with
opportunities to prove our theories. For

this reason the United States Forest
Service has set aside 37,800 acres of the
earthquake area as a preserve for public
enjoyment and scientific study.

The drama in nature goes on end¬
lessly. Conditions in any given area are
never permanent or static. Each so¬
ciety of plants that occupies the area
alters the environment, and often makes
it unsuitable for more of the same plants
but favorable for other kinds that
move in. Over a period of time these
changes in plant population prepare the
way for the climax vegetation. Since
the animal population is dependent on
the plant population, the kinds and
numbers of animals change as plant suc¬
cession occurs.

IN CONCLUSION

Periodic changes in communities are brought about by regular variations in
light, temperature, and climatic conditions. These physical factors may act
as stimuli or inhibitors, controlling the behavior of organisms. Rhythmic be¬
havior sometimes continues, however, even when an animal has been placed
in a laboratory under constant conditions. Therefore many biologists say that
some animals have an “internal clock.”

In cleared areas an orderly progression of living things occurs. The pio¬
neers make the environment right for other organisms, which in turn create
conditions allowing different plants and animals to move in. Gradually a bal¬
ance is achieved and the climax vegetation dominates the flora. Ecological
succession can be studied in an ecosystem as small as a jar of water or as large
as a forest.

The next chapter will present some theories to explain why animals and
plants may be found in certain areas of the world and not in others.

BIOLOGICALLY SPEAKING

climax species
diurnal
estivation
hibernation

migration

nocturnal

periodicity

pioneers

rhythmic
succession
winter sleep

698 UNIT 8 ECOLOGICAL RELATIONSHIPS

QUESTIONS FOR REVIEW

1. The owl and the hawk are both predators. Do they compete with one
another? Explain.

2. \\ hat evidence is available to indicate that daily rhythms are not merely
a matter of light and dark stimuli?

3. What factors make the equatorial environment favorable for growth of
many organisms?

4. In what ways do animals adjust to seasonal changes?

5. In what ways are hibernation and estivation similar? How are they differ¬
ent?

6. W hat explanation has been given to account for the migrations of the
monarch butterfly?

7. What environments are directly affected by lunar rhythms?

8. How does the grunion’s behavior demonstrate lunar periodicity?

9. Of what value is periodicity to survival of some species?

10. By what methods can succession be studied?

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss why man’s activities are not entirely governed by external rhythms.

2. Discuss the senses that are well developed in nocturnal animals.

3. Discuss the ways in which a desert and an arctic treeless plain are similar.

4. Review and discuss possible causes for migration in birds.

5. Name and identify the permanent residents, winter residents, and summer
residents of the bird population in your area.

6. What climax communities exist in the area where you live? Can you
identify any stages leading toward this climax?

CHAPTER 51

BIOGEOGRAPHY

The distribution of plants and animals.

Have you ever seen or felt cobwebs float¬
ing in the breezy air of a spring or fall
day? The young of many spiders spin
silken threads which, when caught by
the air currents, are able to support the
weight of the tiny creature. Why do
spiders travel in this way? Where are
they going?

Perhaps, while hiking, you have
stopped to pick out burrs and stickers
from your socks. Cats, dogs, and other
animals that roam are often covered
with such seeds or fruits of weeds and
other plants. As you learned in a previ¬
ous chapter, fruits and seeds may have
very intricate devices for getting from
one place to another. A milkweed seed,
for example, may travel through the air
for miles on its fluffy parachute. Sailors
have observed tiny airborne spiders more
than two hundred miles from the near¬
est land. Spiders have also been found
in the air at 10,000 feet altitude.

Methods of dispersal are of value
to a particular species because they al¬

low new environments to be inhabited.
The baby spider and the milkweed seed
are likely to float to areas where there
are not so many of their own kind. Of
course, if a spider lands in the ocean,
or if a milkweed seed lands in a lake
or on a granite cliff, they will die. How¬
ever, dispersal has much to do with
producing the succession about which
you learned in Chapter 50.

Biogeography is the study of the
distribution of plants and animals
throughout the various regions of the
earth. Natural methods of dispersal
may extend the range of a species, or
human beings may deliberately or ac¬
cidentally serve as dispersal agents. For
example, the Scotch broom and the
French broom are shrubs that have been
imported because of their delicate, al¬
most lacelike green foliage and bright
yellow flowers. In the Coast Ranges
and the Sierra Nevada foothills of Cali¬
fornia, the seeds produced by these gar¬
den plants found a favorable environ¬
ment. The plants have become so
numerous that entire hillsides may be
a vivid yellow during the blooming sea¬
son. The plants have “escaped’ 7 from
cultivated gardens and become locally
established almost as thoroughly as
though they were native.

Most people associate the pineapple
with Hawaii. Actually, the pineapple is
native to South America but was im¬
ported to the Hawaiian Islands where
its growth and production have played
a large part in the economy of this state.
The introduction of European starlings,
English sparrows, and Japanese beetles
to North America are other examples
of dispersal of animals by man.

Man actively extends his living area.
He fills in tidelands with rocks and then
soil. Entire communities, airports, shop¬
ping centers, and industrial sites may

699

700 UNIT 8 ECOLOGICAL RELATIONSHIPS

occupy areas that were once covered
with water. New designs and building
materials make it possible to build
houses on hillsides once considered far
too steep for human habitation. By ir¬
rigation man makes human life possible
in deserts. Man’s experiments in space
may become a method of dispersal for
man. Even now missiles being sent to
the moon are sterilized to prevent es¬
tablishment of microorganisms on our
nearest space neighbor.

Barriers to dispersal. A frog might be
transported across a large fresh water
lake on a log. It could jump into the
water periodically to keep its skin moist.
But if the log were floating in the ocean,
the salt water would cause the death of
the frog. Salt water, then, can act as a
barrier to the dispersal of frogs. High
mountains, deserts, lakes, rivers, and soil
conditions are other geographical barri¬
ers across which many plants and ani¬
mals cannot pass. Continents are bar¬
riers to marine organisms. For shallow-
water marine forms, deep water is a bar¬
rier. Many marine organisms are lim¬
ited in their dispersal by the lowering
of salt concentration where rivers flow
into the sea. Similarly, many brackish-
water organisms are limited to estuaries,
where fresh water and sea water mix.

Lack of food may act as a biotic
barrier , keeping animals from moving
into new areas. A zebra from Africa
might find a suitable habitat in Eurasia,
but it would not find enough food to
sustain itself while attempting to cross
the Sahara desert. For deer, squirrels,
and other forest animals, a desert would
be a climatic as well as a biotic barrier.

Climatic barriers prevent the spread
of many organisms. Although many
mammals would be physically able to
climb over a mountain range, the
weather conditions in the mountains

might keep them from doing so. A
desert, with its dry, hot climate by day
and near freezing temperatures at night,
will stop many transients.

Major climatic zones of North America.
Temperature ranges in the various re¬
gions of North America divide it into
climatic zones. Northern Canada, part
of Alaska, Greenland, and other land
masses of the polar regions lie in the
area of the polar climate. We com¬
monly speak of these areas as the arctic
region. Similar climatic conditions pre¬
vail above the timber line on high
mountains. Most of Canada and most
of the United States lie in the area of
mid-latitude climate. This is often re¬
ferred to as the temperate region. Flor¬
ida is in a semitropical region. Through
Mexico, the semitropics gradually be¬
come the tropical region.

As the climatic regions of the earth
vary from either the north or south poles
to the equator, so do the principal kinds
of living things. A journey from the Far
North, for example, would begin with a
region of ice and snow which would
gradually give way to low herbaceous
vegetation. Still farther south we find
the large coniferous forest belts. The
division between the low vegetation and
the coniferous forest, called the tree line,
is usually distinct. Next are the decidu¬
ous forests. Depending on the rainfall
of the area in which we are traveling, the
deciduous forest may give way to prairie
grassland and then desert or a tropical
forest.

In a few places all of the climatic
zones may be found within a small area.
Fig. 51-1 shows that the broad zones
from polar to tropical are duplicated on
a high mountain at the equator. Thus,
you can see how climatic barriers can
be produced by the topographv of the
land as well as by a change in latitude.

CHAPTER 51 BIOGEOGRAPHY 701

Pole Equator

LATITUDINAL GRADIENT

51-1 Horizontal climatic regions of the earth are similar to vertical climatic
regions. Hence, life zones on a high mountain can be compared to those found
while traveling from the equator to either of the poles.

Biomes — regions identified by climax
vegetation. The coniferous, deciduous,
and tropical forests found in various cli¬
matic zones are made up of the climax
species that result as living things in¬
teract with the climate and succession
occurs. A large geographical region iden¬
tified mainly by its climax vegetation is
called a biome. For example, a large
zone encircling the Arctic Ocean of the
Northern Hemisphere is a biome known
as the tundra (Fig. 51-2). Since there
is no large land mass at a corresponding
latitude of the Southern Hemisphere,
the area of southern tundra is very
small when compared to that of the
North. The climate of the tundra is
extremely cold, and the ground is per¬
manently frozen a few feet below the
surface. During the continuous day¬
light of summer, the surface thaw pro¬

duces saturated bogs, many streams,
and ponds.

Mosses and lichens form the prom¬
inent perennial vegetation, although
some dwarf birches, alders, willows, and
conifers may be found. The annual
plants have a rapid growing season, and
many produce large, brilliant flowers
even when interrupted by periods of
freezing temperatures. Most of the
birds are summer migrants, but the
ptarmigan is a permanent resident.
Many of the inhabitants of the tundra,
such as the Arctic hare, lemmings,
Arctic foxes, and polar bears have white
coats that act as protective coloration.
In the summer insects are very nu¬
merous, and the eggs they produce are
resistant to freezing. Herds of caribou
visit the tundra to graze on the moss
and lichens.

702 UNIT 8 ECOLOGICAL RELATIONSHIPS

51-2 The boulders shown in this tundra scene are covered with mosses and
lichens, the prominent perennial vegetation in this biome. (Walter Dawn)

The coniferous forest. Another biome
occurs in Europe, Asia, and North Amer¬
ica just south of the tundra. As with
southern tundra, there is no large cor¬
responding zone in the Southern Hemis¬
phere, as there are no large land masses
in these latitudes. Sometimes this large
coniferous forest is called the taiga ( ty -
gah). In this region the growing season
may be as long as six months, although
the winter temperatures may be as severe
as in the tundra. The tree line marking
the transition from the tundra to the
taiga may be quite noticeable. At this
point the most obvious tree is the
spruce. In the taiga, soil is shallow be¬
cause of glacial scraping years ago.

Farther south the broad coniferous
belt covers much of Canada where
alders, birches, and junipers may be
found in groves. Here, where fire has
destroyed large areas and where succes¬
sion has occurred, the pioneer grasses are

followed by aspens and birches. These
are eventually replaced by the spruces
and pines and firs that form the climax
community. Along the coastal ranges
of Washington, Oregon, and California
are magnificent stands of pine, spruce,
and redwoods. Here giants reach a
height of 200 feet or more. Rainfall
may be as heavy as 80 inches per vear,
and fogs blanket much of the area.

The prominent permanent residents
of the coniferous forests are manv.
Moose are plentiful in areas that have
not been excessivelv hunted or where
they are protected. Black bears roam
these forests. Martins, wolverines, and
lynxes may be found. Squirrels, voles,
chipmunks, rabbits, and mice may be
preyed upon by the bobcat, fox, and
wolf. Beavers and porcupines are found
in many of the coniferous forests. Dur¬
ing the summer months manv birds
breed in these forests, but in the fall

CHAPTER 51 BIOGEOGRAPHY 703

they migrate south. The numerous in¬
sects and other invertebrates found in
the coniferous biome during the sum¬
mer lie dormant during the cold winter
months.

The deciduous forest. In areas of the
temperate zone the growing season may
be six months or more. Rainfall aver¬
ages around 40 inches per year and,
where the soil is suitable, large decidu¬
ous forests occur. The eastern United
States, England, central Europe, and
parts of China and Siberia are or once
were covered with large stands of de¬
ciduous trees. Although a similar zone
occurs in South America, it is limited
in size bv inadequate rainfall.

Local conditions of soil, drainage,
and variations in climate throughout the
temperate zone provide conditions nec-
essarv for different climax communities.

In the United States the beech-maple
forests are found in the north central
regions while the oak-hickory forests are
common in the western and southern
regions. Although most of the native
chestnut trees were destroyed by blight,
oak-chestnut forests formerly covered
much of the Appalachian Mountain
chain. Other common deciduous trees
found in the temperate zone are syca¬
more, elm, poplar, willow, and cotton¬
wood. Although each of these specific
forests has its typical animal species,
many animals are characteristic of de¬
ciduous forests. Deer are the common
herbivores. Foxes, martins, raccoons,
and squirrels inhabit the area. Wolves
may wander between the taiga and the
deciduous forests. Woodpeckers and
other tree-nesting birds are plentiful.

The deciduous forest undergoes
great seasonal change. In late spring
and summer the trees are green, and
shrubs growing in their shelter produce
beautiful blooms. During the fall many

areas are turned into artistic splendor by
the multicolored leaves of trees prepar¬
ing for winter. In the winter the bare
branches contrast with a white blanket
of snow.

The deciduous forest biome is one
of the most important regions of the
world, as it is here that man has
achieved his greatest cultural and tech¬
nological development. Cultivated
plants have replaced many of the nat¬
urally occurring trees. Large forests
have been replaced by cities.

The grasslands. In vast areas where
rainfall is between 10 and 30 inches per
year, grasslands occur. The variable
rainfall is not enough to support large
trees, but is sufficient for many species
of grass. These natural pastures have
always been used by huge herds of graz¬
ing animals. However, improper use of
the land by man has turned thousands
of acres of grassland into bare wasteland

51-3 The oak-hickory woodland is a good
example of a deciduous forest. (U. S. For¬
est Service)

704 UNIT 8 ECOLOGICAL RELATIONSHIPS

51-4 When scattered trees are present, grassland areas are called savannahs.
Here, a Southern savannah has Spanish moss hanging from the live oak trees.
(Brunswick-Glynn County Chamber of Commerce)

by erosion of the topsoil. When the
grasses are destroyed, essential topsoil
is worn away by water and wind, with
tragic results.

In North America the grasslands oc¬
cur in the Great Plains east of the Rocky
Mountains. The tropical grasslands of
Africa, with their giraffe, zebra, antelope,
ostrich, and lion populations, are a famil¬
iar picture to most of us. A savannah is
a grassland with scattered trees. In the
West the oak-grass savannah is familiar.
South America also has large areas of
savannah. In Australia the grasslands
and savannahs support cattle and sheep
for food, grazing kangaroos, and burrow¬
ing animals. Wild dogs are found very
often and are prominent predators.

In America great herds of bison and
antelope once grazed on the grasses of
the plains. Burrowing mammals such

as hares, prairie dogs, ground squirrels,
and pocket gophers are still abundant
and form an important link in the food
chain, for they are eaten by weasels,
snakes, and hawks. Locusts and grass¬
hoppers are important members of the
insect population.

The desert biome. It may surprise you
to learn that there are hot and cold
deserts. Death Valley, where the creo¬
sote bush is the climax vegetation, is a
representative hot desert. In the typical
cold desert, occurring in the temperate
zone, the sagebrush is the dominant
shrub (Fig. 51-5). Cold deserts are
found in several of our northwestern
states. In both hot and cold deserts,
the plants are xerophytic; that is, they
are adapted for living in areas where the
rainfall may be only 10 inches per year.
The leaves are small, with thick, leath-

CHAPTER 51 BIOGEOGRAPHY 705

ery outer layers that help conserve the
plant’s water. Other desert plants, like
the cacti, have no typical leaves at all.
Spines are nonfunctioning leaf vestiges.

Desert animals also have special
adaptations. The reptiles, some insects,
and birds excrete nitrogenous wastes in
the form of uric acid. Uric acid can be
excreted in an almost dry form, thus con¬
serving valuable water. Mammals, how¬
ever, cannot do this. They excrete
nitrogenous waste in the form of urea
dissolved in water. This involves the
loss of water from the mammal’s body.
Most desert mammals burrow during the
day to conserve moisture. Some rodents
are able to live on the small amount
of water in the seeds and fruits they
eat. Others get water from the tissues
of cacti or other water-storing plants.

Desert herbs, grasses, and flowering
plants burst forth in growth and color
in a surprisingly short time after a rain.
Many of these plants are able to com¬
plete their cycle of growth, flowering,
and seed production within a few weeks.

Some of the larger perennial plants have
very long tap roots. Most desert plants
can hold water in the angles between
the leaves or in spongy tissue.

The rain forest. Although each rain
forest may have its particular flora and
fauna, the conditions and ecological
niches are similar. In areas of abundant
water supply and a long growing season,
life flourishes. There is a temperate
rain forest on the northwest Pacific
Coast in the Olympic Peninsula of
Washington. The tropical rain forests
are found on and near the equator, in¬
cluding most of Central America, north¬
ern South America, central Africa,
southern Asia, the East Indies, the
South Pacific Islands, and northeastern
Australia. The seasonal variation in
temperature is usually less than that of
the day and night.

The numbers of plants in a rain
forest produce a dense growth. Tall
trees have shorter trees growing beneath
them. The canopy produced may be
so dense that few plants can grow on

51-5 The cold desert occurs in northern latitudes. Note the sagebrush, which
is common in these areas. (Bureau of Land Management)

706 UNIT 8 ECOLOGICAL RELATIONSHIPS

51-6 A temperate rain forest is shown on the left while a tropical rain forest
appears on the right. (Left: U. S. Forest Service; right: Walter Dawn)

the ground. Even so, many smaller
plants have become adapted to life in
the tropical rain forest. Some have
evolved long vines that make it possible
to have roots in the moist ground and
leaves high up toward the davlight.
Other small plants grow high among
the trees so they can receive sufficient
light for photosynthesis. Most tropical
rain forest plants have very large leaves,
as the conservation of water is not a
problem for them. The critical factors
are finding a place to grow and obtain¬
ing sufficient light for photosynthesis.

Epiphytes (ep-i- fyts) are plants that
attach themselves to trees, sometimes
100 feet or more above the ground.
They have thick, porous roots adapted
to catching and holding rainfall. Many
epiphytes also have leaves arranged so
that they catch water, insects, falling
leaves, and other debris. As the insects
decompose, essential minerals are re¬
leased for the epiphyte’s use. Manv
species of orchids, mosses, ferns, li¬
chens, and members of the pineapple
family are epiphytes. Even though epi¬
phytes do not take nourishment from

the plants upon which they grow, they
may cause minor injury by shading the
leaves of the supporting plant or caus¬
ing limbs to break from the weight.
Spanish moss is an epiphyte (Fig. 51-4).

Although animal life in the rain
forest is plentiful, it may not be obvi¬
ous to the casual observer in the dav-
time because so much of it exists high
in the trees. Except for an occasional
bird call or chatter from monkeys, the
day in a rain forest is relatively quiet.
But toward evening everything seems to
come alive. The crickets and tree frogs
begin singing, and the birds make more
noise as they search for food. The tree¬
dwelling monkeys chatter excitedlv and
howl for the last time before settling
down for the night. Ants, beetles, ter¬
mites, and other insects are numerous,
supplying many animals with food.
The nocturnal carnivorous cats, such as
the jaguar in South America, the leop¬
ard in Africa, and the tiger in Asia, hunt
for monkeys, deer, and other animals.

Many people confuse jungle growth
with a rain forest. A typical rain forest
is climax vegetation. Jungle is ex-

CHAPTER 51 BIOGEOGRAPHY 707

tremely dense ground growth that oc¬
curs along the edges of rivers or on land
that was once cleared by man or by some
natural means like fire. If left alone,
most jungle eventually becomes rain
forest. Jungle is therefore a kind of im¬
mature rain forest.

The marine biome. The oceans cover
more than two thirds of the earth’s sur¬
face and support a surprising quantity
and variety of living material. The
marine biome may be divided into the
bottom, or benthic zone, and the ocean,
or pelagic zone (Fig. 51-7). The shore¬
line comprising the continental self is
the littoral zone. This is the most pro¬
ductive area of the marine biome. The
pelagic zone, including waters to a
depth of about 600 feet, allows the pene¬
tration of some light. It is in this zone,
then, that photosynthesis occurs in mi¬

croscopic suspended algae as well as in
the large drifting algae. Near the shore
the area periodically covered and un¬
covered by water is called the intertidal
zone. This is exposed at low tide but
covered at high tide.

Although plants in the ocean play
an important role in energy production,
they do not have as great a controlling
influence on the environment as do the
plants on land. Compared to the ter¬
restrial environment, the ocean provides
a stable condition for life. With the
exception of the intertidal zones, estu¬
aries, and changing currents, the tem¬
perature and salinity remain fairly con¬
stant.

The basic food of the pelagic zone
is the plankton. This is composed of
diatoms, dinoflagellates, unicellular al¬
gae, protozoans, and the larval forms

51-7 The ocean may be divided into several zones, each having characteristics
which determine the kinds of organisms able to live there.

708 UNIT 8 ECOLOGICAL RELATIONSHIPS

of many animals. Many copepods,
small shrimp, small jellyfish, and worms
are considered to be part of the plank¬
ton, even though they can swim poorly.
The copepods feed on the microscopic
diatoms and algae and in turn provide
the major food for the largest animal
of all — the whale. The food chains
of the ocean involve many carnivorous
fishes, squids, and sharks.

Animals that live beneath the
photosynthetic zone depend on sinking
plankton, dead animals, and the swim¬
ming organisms that pass between the
levels. Many of the forms that con¬
stantly inhabit the deeper dark regions
of the ocean are very unusual in appear¬
ance, often having luminescent organs.

Recent studies have revealed many
animals living on the deep-sea bottom.
For their nutrition, deep-sea scavengers
depend on the descent of dead animals
from above. Bacteria living in the soft
ooze on the bottom break up complex
organic molecules of dead organisms
that have settled there. However, min¬
eral exchange occurs in the ocean as on
the land. Currents cause upwellings of
the deeper waters and the minerals and
compounds essential to life are again
brought to the surface, where they can
be synthesized into organic compounds
by the phytoplankton and larger algae.
The process of upwelling brings colder
waters, rich in sedimentary materials, to
the surface. Upwelling is a very con¬
spicuous phenomenon near the coasts
of Morocco, southwest Africa, Califor¬
nia, and Peru. Since the rising waters
bring nutrients to the photosynthetic
zone, you can see why these areas are
very rich in phytoplankton. Associated
with the abundance of food producers
are the many consumers. These are
very popular fishing areas.

The intertidal area of the littoral

zone corresponds to the tropical rain
forest on land. Here, in spite of greater
variations in temperature, salinity, and
exposure to drying conditions, life is
abundant. Space for growing is at a
premium. Algae and many tiny colonial
animals attach to the shells of snails,
limpets, and kelp, as well as to rocks.
Epiphytic algae are found on most kelps.
Snails, periwinkles, and barnacles can
be found high on rocks where they are
exposed to the air for long periods of
time. They conserve water by clamping
down tightly when the tide is out.
Then, when covered by the tide, they
browse on the algae. Other herbivores
present in the intertidal zone are shrimp,
small fishes, and copepods. Clams,
mussels, oysters, and sponges filter many
forms of microscopic life from the wa¬
ter. The starfish, sea anemone, larger
fishes, octopus, and squid are carni¬
vores. Sea urchins '"graze” on algae.
Various worms, crabs, and hermit crabs
are familiar scavengers of the intertidal
zone. Other worms and bacteria play
an important role in breaking down
waste materials and dead organisms for
the recycling of essential elements.

The fresh-water biome. The fresh-water
biome includes bodies of standing water
such as lakes, ponds, and swamps, as
well as bodies of moving water such as
springs, streams, and rivers. Many of
the plankton organisms of the ponds
and lakes do not survive under condi¬
tions of running water. The strength
of the current and the type of bottom
are factors that determine the kinds of
life able to inhabit a stream. A bottom
of shifting sandy soil greatly limits the
life in a stream, but in streams with a
sluggish current and a sandy bottom,
many burrowing forms are found among
rooted vegetation. Stony streams pro¬
vide a habitat for activelv swimming or

CHAPTER 51 BIOGEOGRAPHY 709

51-8 The fresh-water biome has its peculiar plants and animals, each suited to
the environment found there. (American Forest Products Industries, Inc.)

clinging animals. The larvae of caddis
flies, mayflies, dragonflies, and dobson-
flies are common inhabitants of stony
brooks and streams, where they provide
part of the trout’s diet. Algae grow
attached to the rocks, stumps, or stones.
The plankton organisms of lakes have
already been discussed. These floating
organisms are found in streams only as
occasional transients.

Water temperature is important to
the organisms living in an aquatic en¬
vironment. To raise the temperature
of water requires a large amount of
heat. Also, when water evaporates,
much heat is absorbed from both the
water and the atmosphere. These prop¬
erties of water are well known to all
who enjoy diving into a pool on a hot
summer day. Upon coming out of the
pool, even a warm breeze may seem
chilling as the water evaporates.

Another property of water that is
very important to the inhabitants of a
lake is that it becomes less dense, or

lighter, as it freezes. Therefore, when
water freezes, it floats on the surface.
This prevents lakes from freezing solid,
but it interferes with light penetration
and oxygen exchange at the surface. In
many lakes living things can, and often
do, suffocate during the cold months of
the winter season.

Water is continually passing into
fresh-water organisms by osmosis. Thus,
as you have learned in earlier chapters,
fresh-water organisms must be able to
excrete large quantities of water in or¬
der to survive. This problem is solved
in protozoans by their contractile vac¬
uoles. The efficient kidneys of the fresh¬
water fishes excrete the excess water
that enters through the gill membranes.
The problem in salt-water fishes is just
the reverse. Thev live in water of a
higher salt concentration than that in
their own body fluids and are in danger
of dehvdration. This problem is solved
in marine fishes with the excretion of
salts by the gill membranes.

710 UNIT 8 ECOLOGICAL RELATIONSHIPS

IN CONCLUSION

The distribution of plants and animals depends on organic, climatic, and geo¬
graphical factors. Dispersal involves both a method of travel and favorable
environment once a new location is reached. You know some of the special
adaptations that allow organisms to live under various conditions.

The diverse forms of life have found niches in the different terrestrial,
marine, and fresh-water environments. The adaptations of many plants and
animals to their environments have been discussed.

The preservation of the great variety of living things for man’s future use
and enjoyment will be determined by his interest in and knowledge of ecology.
Wherever man alters his environment, other living things are affected. Soil
and water are of prime importance to the producers in man’s food chain. In
the next chapter we shall study the various methods of soil and water con¬
servation.

BIOLOGICALLY SPEAKING

arctic region

biogeography

biome

biotic barrier
climatic barrier
climatic zone
deciduous forest
desert

epiphyte

fresh-water biome
geographical barrier
grasslands
marine biome
pelagic zone
rain forest

savannah

semitropical region
taiga

temperate region
tree line
tropical region
tundra

QUESTIONS FOR REVIEW

1. How do methods of dispersal limit the distribution of plants and animals?

2. Name the major barriers to dispersal and give an example of each.

3. Name and define the major climatic zones of North America.

4. What two factors are important in determining the flora and fauna within
a climatic zone?

5. What flora and fauna are found in the taiga?

6. Where are the coniferous forests in the United States?

7. Where are the deciduous forests in the United States?

8. Where are the grasslands in the United States?

9. What are the differences between the hot and cold deserts?

10. What adaptations to desert climate are found in the mammals? in the
birds? in the reptiles?

11. What environmental factors are found in a rain forest?

12. In the rain forests the plants are competing with one another for space in
which to grow. How are the epiphytes adapted for living in such forests?

13. What extremes of environmental conditions occur in the intertidal zone?

CHAPTER 51 BIOGEOGRAPHY

711

14. What furnishes the food for organisms living in total darkness in the
depths of the ocean?

15. What prevents the organisms in a lake from being frozen solid in the
winter?

APPLYING PRINCIPLES AND CONCEPTS

1. What are some food chains found in coniferous forests?

2. Give some examples of organisms finding their way to a favorable new en¬
vironment in spite of barriers.

3. Give reasons to support the following statement: A high mountain on the
equator shows the same climatic regions as are observed from the equator
to either of the poles.

4. If you were an entomologist and discovered an unusual insect destroying
the pineapple plants in Hawaii, what might be a good plan to follow in
searching for a biotic method of control?

5. What environmental factors differ in a coniferous and a deciduous forest?

6. Discuss the adaptations of organisms in the intertidal zone.

7. How does excretion differ in salt-water and fresh-water organisms?

CHAPTER 52

SOIL AND WATER
CONSERVATION

The vital need for conservation. The
human population of the world is ex¬
pected to double by the end of this cen¬
tury or sooner. Supplying food for
these vastly increasing numbers is be¬
coming an urgent problem. Food pro¬
duction is dependent on the soil and its
water supply. Any misuse of soil and
water resources, such as we have had
continually in the past, eventually cuts
down on the food supply.

We must realize how short-sighted
and selfish it is to continue to squander
our natural resources. Perhaps people
before us could plead ignorance when
they turned forests and grasslands into
useless dust bowls. But today we have
a much better understanding of cause
and effect in ecology. When we do not
know the answers we should proceed
with caution, carefully observing the re¬
sults of the many changes we may make.
Unfortunately it often takes many years

to observe the results of changing an
environment. You are now well aware
of the importance of plants in deter¬
mining the ecological setting for the
animals in an area. In this chapter we
shall discuss the importance of con¬
serving soil and water in order to pre¬
serve the plant and animal populations.
Soil and its origin. Think of the
earth as a gigantic ball of rock. Soil
lies in a thin film on the surface of this
great ball. Season after season, running
water, freezing and thawing, wind, and
other forces of nature crumble the rocks
and form gravel, sand, or clay. These
materials become mineral soil, or sub¬
soil. In most regions the subsoil forms a
layer several feet thick, representing
thousands of years of the slow disintegra¬
tion of rock.

The organic part of the soil comes
from the slow decay of roots, stems,
leaves, and other vegetable materials,
and the remains of animals. We refer
to the organic remains of land plants as
humus, while sphagnum moss and other
aquatic plants form peat in lakes and
bogs.

A mixing of mineral matter from
the subsoil and organic matter combine
to form topsoil , or loam. Topsoil is the
most vital part of soil, the nutritional
zone of plants both large and small. It
forms very slowly, at a rate of about one
inch in 500 years.

Topsoil supports great numbers of
bacteria, molds, and other fungi, which
we refer to as the soil flora. Activities
of the many soil organisms are essen¬
tial to fertility of the soil. Decay,
ammonia production, nitrate formation,
and many other chemical processes con¬
dition topsoil for the growth of higher
plants.

If you examine a soil profile along
a bank or the side of a ditch, you can

712

CHAPTER 52 SOIL AND WATER CONSERVATION 713

52-1 Note the difference that soil can make
to a crop. The corn on the left was grown
in soil containing plenty of humus. That on
the right is low in humus. (USDA)

see the dark topsoil and the lighter sub¬
soil below. Under natural conditions
a small quantity of topsoil washes away
or is blown away each season. This is
replaced by additional topsoil formed
by decaying vegetation added to the
upper surface. Thus, topsoil formation
is a continuous process. Remember,
however, that it is an extremely slow one.

The original topsoil in America
averaged nine to ten inches in depth.
Today it averages about five inches.
When topsoil is gone, land becomes
barren. In other words we are now
within five inches of barrenness. The
conservationist refers to this situation
when he says we have lost half the bat¬
tle to save our soil. Conservation of
topsoil is an extremely serious problem,
but it can be solved.

Cover crops and row crops. In discuss¬
ing the various kinds of soil loss, we shall
refer to row crops and cover crops. As
the name indicates, row crops are
planted in rows in cultivated fields. The
soil lies exposed between the rows.
Corn, beans, tobacco, and tomatoes are

examples of row crops. Wheat, oats,
rye, clover, alfalfa, and various grasses
are grown as cover crops. These close¬
growing plants form a dense mat of roots
which bind the soil and an aerial cover
which protects the soil surface from
wind and water. The relation of row
crops and cover crops to the soil is very
important in soil conservation.
Depletion of soil minerals. A century
ago, abundant fertile land was still avail¬
able to anyone who would go West to
claim it. The soil contained a rich store
of minerals, accumulated through cen¬
turies of the growth and decay of native
vegetation. Year after year corn, cotton,
and other field crops were grown in the
same fields with little thought about the
condition of the soil. Overproduction
was the order of the day. The object
seemed to be to produce as much as pos¬
sible in as short a time as possible. After
a few seasons the loss of fertility began
to show in the form of reduced crop
yield. We refer to such mineral exhaus¬
tion as depletion. Rather than heeding
the danger signal, many farmers pushed
the land even harder. When the soil
finally became exhausted, the field was
abandoned for a new and more profitable
area.

A scientific view of the mineral deple¬
tion problem. It stands to reason that
agricultural crops with a high food value
draw heavily on soil minerals. The most
critical of these minerals are nitrates,
phosphates, and potash (potassium com¬
pounds). In a natural cycle plants
draw minerals from the soil and or¬
ganize them into the various parts of
the plant body. When the plant dies,
these are returned to the soil through
the process of decay. But a crop plant
is harvested for its food value. The
minerals contained in the crop are re¬
moved from the soil permanently. This

714 UNIT 8 ECOLOGICAL RELATIONSHIPS

removal can continue only a few sea¬
sons before the soil shows evidence of
depletion of the most heavily used min¬
erals.

Soil that is badly depleted of min¬
erals usually becomes acid, or sour. To
correct this condition the farmer uses
lime. Other minerals are restored by
heavy applications of commercial fer¬
tilizer, especially superphosphate and
bone meal.

Principles of crop rotation. In addition
to fertilizing, which is a rapid method
of restoring minerals, the scientific farm¬
er avoids depletion by practicing crop
rotation. Various crops differ in the
minerals they take from the soil. Many
farmers prevent mineral depletion as
well as serious erosion by rotating crops
in each growing area. Many rotation
plans follow a three-year cycle. The
first crop might be corn, followed by
wheat or oats, then by grass or clover.
Clover, alfalfa, cowpeas, lespedeza, and
other legumes are important in a rota¬
tion cycle because they support nitro¬
gen-fixing bacteria on their roots. As
we mentioned in the discussion of the
nitrogen cycle, these bacteria produce
nitrates from atmospheric nitrogen.

The problem of leaching. Before a field
is planted, it is plowed and disked, which
turns the weeds under the soil and con¬
ditions it for planting. Rains soak into
the pulverized soil easily. This process
is ideal for young plants, but it may
cause a serious problem, especially in
loose sandy loam. If the soil is properly
cared for, the topsoil should be rich in
minerals. These minerals must be dis¬
solved in water, or plants cannot absorb
them. As water runs through the top¬
soil, it dissolves minerals and carries
them down below the reach of the roots
of many crop plants. Bv such leaching
action, valuable fertility is lost.

We have always assumed that fre¬
quent cultivation is good for crops. But
this may not be true. Row crops espe¬
cially leave much of the soil exposed to
soaking rains. One method of reduc¬
ing leaching is minimum cultivation.
Another is the planting of cover crops
between row crops. Deep-rooted crops,
like alfalfa, absorb minerals from the
subsoil and bring them into the plant
body. If such a crop is plowed under
without harvesting it, the minerals are
thus returned to the topsoil.

Loss of organic matter. In a natural
environment, the organic matter pres¬
ent in topsoil decays slowly as it is acted
on by bacteria and other soil organisms.
In time it would disappear entirely were
it not for the leaves, roots, and other
plant parts that are added to the soil
each season. However, in an agricul¬
tural situation, when hay crops are
harvested or when stalks of wheat or
oats are used for straw, little organic
matter is left to return to the soil.
Sometimes weeds and native grasses are
even burned off fields before plowing.
This procedure is a waste of valuable
organic matter which should be plowed
into the soil.

If fields are cleared season after
season, the organic part of the topsoil
may disappear to the extent that much
of the soil flora dies out. When these
organisms are gone, many of the proc¬
esses necessary for maintaining soil fer¬
tility cease. There are several ways in
which organic matter can be added to
soil. One is the addition of manure
and decayed straw. An even better
method is sowing grass or clover in a
field, then plowing it under.

The suburban dweller may like the
smell of burning leaves, but he does not
recognize the value of the organic ma¬
terials he is destroying. It is much more

CHAPTER 52 SOIL AND WATER CONSERVATION 715

52-2 Severe rill erosion may continue to
such an extent that the rills become gullies.
(USDA)

costly to buy minerals and peat moss for
soil replenishment than it is to spade
the fallen leaves into the soil. Many
people save the leaves in a pile or heap,
alternating a layer of leaves with a layer
of soil. This compost is used to add
humus to the garden the following year.
Good conservation procedures are as
valuable to the homeowner as they are
to the nation’s farmers.

Erosion, the loss of topsoil. Of all
forms of soil loss, erosion is the most
advanced and the most destructive.
Precious topsoil from millions of acres
of our most productive land now lies in
riverbeds and ocean bottoms. Some
has been blown thousands of miles by
the wind in violent dust storms. This
destruction is the tragic result of man’s
carelessness and shortsightedness.

Some erosion has always occurred.
Before any land was cultivated, soil for¬
mation generally kept pace with this
slow blowing away or washing out of
the soil. But when land was stripped
of its natural vegetation and poor farm¬
ing methods exposed it to forces of wa¬
ter and wind, a far more dangerous, ac¬
celerated erosion began.

Much of the land that is badlv
eroded today was abandoned for agricul¬
ture when the minerals became de¬
pleted — a result of overcultivation.
With the rapid expansion of agricul¬
tural industry, more and more areas
were cultivated for crops. In eagerness
to make every inch of land pay, many
farmers began cultivating hillsides,
river bottoms, and all other available
locations rather than increasing yields
from land already cultivated.

In hilly forest lands, where oak
and hickory trees were thriving in shal¬
low topsoil, native vegetation was
cleared to make more tillable ground
available. Such lands were excellent
for forests but were unsuitable for crops.
Soon they were abandoned to the rav¬
ages of erosion or were left for families
to eke out an existence from a few
patches of dwarfed corn and vegetables.
Water erosion. One form of water ero¬
sion, known as sheet erosion , occurs
when water stands in a field during a
flood, then flows away gradually. The
water carries a thin layer of soil with
it as it flows away. When the water is
gone, the land is left at a level much
nearer the sterile subsoil. A few such
floods may leave the land worthless.

In rolling and hilly sections where
the rain falls on exposed soil, raindrops
carry with them particles of soil, form¬
ing tiny channels, or rills, as they trickle
down the slope. This process is the be¬
ginning of rill erosion. Each time water
flows down the slope, it follows the same
rills. These deepen and widen much as
a stream increases the size of its bed. A
more advanced stage known as gully ero¬
sion may follow. If the gully is not
checked, it may in time become a canyon
(Fig. 52-3) This can be avoided even
on cultivated hillsides when proper con¬
servation methods are used.

716 UNIT 8 ECOLOGICAL RELATIONSHIPS

52-3 This gully is about 150 feet deep and
the land it was formed from was once a pro¬
ductive crop area. (Soil Conservation Serv¬
ice)

Contour farming. When land is cul¬
tivated up and down a slope, the furrows
act as man-made rills. Each time it
rains, water pours down the furrows and
enlarges them. They can become gullies
in a short time.

The solution to this problem is logi¬
cal. Plow around the hill rather than
up and down, a method called contour
farming. When furrows are plowed
around the slope, each one serves as a
small dam to check the flow of water.
Water stands in each furrow and then
soaks into the ground. If this simple
practice had been followed long ago, our

lands today would be richer and our
rivers deeper and clearer.

Strip cropping. The extremely valuable
soil conservation practice of strip crop¬
ping frequently combines two important
measures. Broad strips are cultivated
on the contour of a slope for growing
row crops such as corn, cotton, potatoes,
or beans. These strips alternate with
strips in which cover crops such as
wheat, oats, clover, alfalfa, or grass, are
grown. These cover crops completely
cover the surface of the soil and hold it
securely. As water runs from the strip
of row crop, it is checked upon enter¬
ing the strip of cover crop.

Frequently clover is used as a cover
crop. Nitrogen-fixing bacteria, asso¬
ciated with roots of clover, alfalfa, and
other legumes, return the various nitro¬
gen compounds to the soil. Strips may
be alternated every few years with the
result that water erosion is checked con¬
tinually and fertility of the soil is main¬
tained.

Terracing. The practice of terracing is
used extensively to check the flow of
water on steeply sloping land. A long
slope is broken into numerous short ones
by forming a series of banks. A type of
machine, the terracing grader, is used to
form flat strips on the contour of the

52-4 Terraced hillsides such
as these in Idaho prevent
runoff and the resulting ero¬
sion. (Bureau of Land Man¬
agement)

CHAPTER 52 SOIL AND WATER CONSERVATION 717

slope. Each strip is divided from an¬
other by a bank. Drainage ditches at
the base of each bank conduct the wa¬
ter around the slope.

Gully control. When large gullies have
already formed, measures other than
those discussed previously must be used.
One of these is planting the slopes of
the gully with trees, grasses, or other
plants to act as soil binders and prevent
further widening. Deepening may be
prevented by building a series of small
dams. The dams slow the flow of water
and soil gradually fills the gully.

The problem of wind erosion. Wind
erosion is a critical factor in western
Texas, Kansas, and Oklahoma. The
prevailing strong winds blow from the
south, especially during the spring and
summer months. Originally, native
grasses and other plants bound the soil
firmly in place with their extensive, shal¬
low root systems. Much of this land
was extremely fertile and suitable for
growing cereal crops and, as a result,
extensive areas were plowed for agricul¬
ture. This practice began to take its
toll in the 1930’s.

During the spring and early sum¬
mer months, the soil was moist enough
to hold its place, but with the late sum¬
mer drought the strong hot winds blew
the dry topsoil away. Entire fields were
covered with fine particles of topsoil
carried in dense clouds during a dust
storm. Abandoned fields added to the
growing wasteland. The farmer who
was fortunate enough to hold his soil in
check was powerless to stop the tons of
soil that blew onto his land from other
areas. There was nothing left but to
abandon the homestead with its half-
buried houses and barns and join the
procession of landowners out of the
growing "dust bowl,” as the area came
to be called.

52-5 Planting rows of trees as windbreaks
helps prevent serious erosion of the soil due
to wind. (USDA)

Control of wind erosion. Wind erosion
is an especially difficult problem because
it involves such large areas. Any local
wind erosion control can be wiped out in
a single dust storm. Consequently these
projects must be undertaken on a very
large scale and with the aid of state and
national agencies. One such measure is
the planting of windbreaks, or shelter-
belts. Extensive experiments have been
conducted to find trees that can be
planted at intervals to break the force
of the wind. In addition to windbreaks,
plants are needed to act as soil binders.
Every inch of exposed land not used
regularly for crop production must be
anchored firmly by the roots of grasses
and other soil-binding plants.

When land is cultivated, furrows
should be plowed at right angles to the
prevailing wind. Thus the wind does
not blow down the furrows, but blows
across them. Each furrow helps to stop
the movement of soil. The effect is
similar to that of contour plowing for
water erosion. In sections where irriga¬
tion is possible, diversion of water into
the fields during dry periods will check

718 UNIT 8 ECOLOGICAL RELATIONSHIPS

wind erosion because moist soil does not
blow away.

Problems in administering soil conserva¬
tion. In 1935 the United States Soil
Conservation Service was established as
part of the Soil Conservation Act. This
agency became a permanent part of the
Department of Agriculture. This divi¬
sion of the Department of Agriculture
has embarked on an extensive program
of soil conservation. Expert agricul¬
tural engineers are investigating all
phases of the problem. They travel
throughout the country, studying vari¬
ous problems and offering aid where
needed.

Farmers have the opportunity to
examine demonstration farms where soil
conservation measures are in operation.
If the farmers of a community wish to
use these methods on their farms, they
must first form a local soil conservation
district under local control. Engineers
from the Soil Conservation Service will
then cooperate with the local district
in applying soil conservation methods
to the problem.

Soil problems and water problems — a
vicious cycle. Disastrous floods and

52-6 Irrigation ditches like this store water
and make it available for crops at the time
when it is most needed. (United States De¬
partment of Agriculture)

droughts, the two extremes in water
problems, are inevitable results of mis¬
use of soil and its plant cover. Rains
that should soak into the ground and
supply plant roots during drier periods
run off the surface of eroded land in
torrents. Streams flood with muddy
water from nearby fields. Then there
is too much water, but later in the
season, plants may die for want of
ground water that should have accumu¬
lated during the rainy period. During
floods water washes soil away, and dur¬
ing droughts the wind blows it away.
Soil erosion, floods, and droughts —
these three disasters form a vicious
cycle. Yet if we solve the soil prob¬
lems, we will help correct the water
problem. The wide flood plains of the
streams and rivers of the Mississippi
River drainage basin indicate that high
and low water stages have always oc¬
curred. Rivers of this enormous sys¬
tem drain the land from the Appalach¬
ian Mountains in the east to the Con¬
tinental Divide in the Rocky Moun¬
tains.

Prolonged droughts and desert con¬
ditions are normal in some sections of
the country, too, because of uneven
distribution of rainfall. But the past
few generations are responsible for the
mistakes that increased these natural
conditions to the proportions of major
disasters.

Depletion of ground water. The water
table is lowering dangerously in many
sections of the country, largely because
of loss of water as runoff water during
rainy periods and the increased use of
ground water in cities. Numerous wells,
necessary to supply drinking water,
draw heavily on the supply of ground
water. Industries require large quan¬
tities of water. Large cities must sup¬
plement their supply of ground water

CHAPTER 52 SOIL AND WATER CONSERVATION 719

with treated surface water taken from
rivers, lakes, and reservoirs.

In recent years cities have tapped
the supply of ground water heavily in
supplying cold water for air-condition¬
ing systems. After the water has been
used to remove the heat in buildings, it
enters the sewer system and adds to the
amount of surface water in rivers. It
is not returned to the ground from
which it was taken.

In an attempt to raise the level
of ground water, the people in many
areas of California have built dams to
hold back the rainwater in natural
basins. During the summer months the
water is released slowly from these reser¬
voirs and admitted to a series of ponds
and gravel stream beds where it seeps
into the ground. These percolation
beds allow the water to maintain the
water table level rather than flowing di¬
rectly into the bays or oceans imme¬
diately after rains. In many commu¬
nities of heavily populated Long Island,
New York, street drains empty into dug
basins, where the water seeps into the
ground rather than running into salt¬
water bays, as it formerly did.

Runoff water. The water problem in
America can never be solved until we
have succeeded in reducing the amount
of runoff water. To do this we must
correct costly mistakes of the past and
restore part of the original wilderness.

The following is what should hap¬
pen when it rains. Rain that falls in
level areas strikes the plant cover and
drips to the surface of the soil. Most
of the water that falls enters the soil
and becomes ground water. Only when
the soil is thoroughly soaked does it run
off the surface or collect in pools.

In hilly and mountainous regions
called watersheds , some of the water
soaks into the topsoil, protected by trees

and other vegetation. Much of it fol¬
lows rivulets that lead down the slope to
streams. Streams in turn carry the wa¬
ter to rivers. As rivers rise, water over¬
flows channels and spreads into flood
plains. Much flood water is received
by sloughs and backwaters. As the
crest of the flood passes, the flood plains
feed water back to the channel. With
the end of the rainy season, water is
maintained in the river by the sloughs
and backwaters and by numerous
springs that feed streams in the water¬
sheds. Thus the ground and natural
surface reservoirs receive excess water
during rainy periods and maintain the
water supply during dry periods.

This condition is altered somewhat
in the Great Plains where there are no
forests or tall grasses to help hold water
back during brief but heavy rainy peri¬
ods. Flash floods and some soil loss
cannot be avoided in these regions,
which explains why De Soto saw muddy
flood water in the Mississippi River.
Much of this mud came from the Great
Plains region. Farther east the original
tall grass prairies were dotted with
sloughs and marshes during the rainy
season.

But what does happen when it rains
today? The spongy topsoil is gone in
many regions. Rains pour onto hard
subsoil clay, rush through deep gullies,
and choke streams. In watersheds
where forests have been cut away, noth¬
ing stops the rush of water downhill
during flash floods. Rivers rise rapidly
and have no place to store the excess
water. Long ago we reclaimed their
backwaters and swamps and extended
our fields and cities to their very banks.
Storm warnings go out and people flee
from their lowland homes. Because
much of our water supply roars to the
sea, drought later in the season is in-

720 UNIT 8 ECOLOGICAL RELATIONSHIPS

52-7 Check dams such as this one in California hold back rainwater in natural
basins called percolation beds. (USDA)

evitable. River channels that were
swollen with flood waters during the
rainy period become narrow, winding
trickles when the rain stops. Crops
bake in the fields.

Water conservation. We can sum¬
marize the measures necessary to control
floods and prevent seasonal droughts.

1. Control soil erosion and restore top¬
soil.

2. Restore forests, especially in water¬
sheds.

3. Restore sloughs and backwaters along
the rivers.

4. Prohibit cultivation of lowlands and
flood plains of major rivers and re¬
store the forests of these areas.

5. Build dams and reservoirs to hold
back flood waters and store water for
dry periods.

6. Control the use of ground water.

7. Maintain dikes and levees along ma¬
jor rivers. This measure, although
the major flood control project in past

years, is probably the least effective
of all.

Dams and water power projects. As
another means of controlling water, the
government has constructed enormous
dams in several sections of the country.
These dams and the great reservoirs they
form are important in preventing floods.
In addition, they are the sites of hydro¬
electric plants, which use the water
power of the dams to turn turbines at¬
tached to generators. In this way wa¬
ter power is converted into electricity
for use in large areas of the nation. By
raising the water level in the rivers,
dams have made rivers navigable for
long distances. In the West, water
from reservoirs formed by dams is used
for irrigation, and has made many semi-
arid regions ideal for crop production.
Deep, clear lakes that lie above the
dams are considered ideal recreation
places for swimming, boating, and fish¬
ing and other aquatic sports.

CHAPTER 52 SOIL AND WATER CONSERVATION 721

IN CONCLUSION

A soaking rain is a welcome sight to a farmer whose crops are flourishing in
fertile fields. But to a family fleeing the flood waters of a river on a rampage,
the sight of more rain only adds to the misery. Man cannot destroy nature’s
balance without paying a terrible price, for almost every dust storm, advancing
gully, and disastrous flood is related somehow to man’s carelessness or indiffer¬
ence. The forces of nature are powerful. They can produce a rich harvest or
tremendous destruction. They can provide us with great wealth or great
poverty.

Conservation is everybody’s business. No nation can prosper on poor
soil. Our economy depends on agriculture, and agriculture is geared to the
water cycle. In the next chapter, we will consider a job for everybody — forest
and wildlife conservation.

BIOLOGICALLY SPEAKING

compost

contour farming
cover crop
crop rotation
depletion
gully erosion
humus

irrigation

leaching

peat

percolation bed
rill erosion
row crop
sheet erosion

shelterbelt

soil flora

strip cropping

subsoil

terracing

topsoil

watershed

QUESTIONS FOR REVIEW

1. What are the several forces in nature that crumble rock and form subsoil?

2. From what sources are humus and peat formed?

3. Give several examples of row crops and cover crops.

4. How did overproduction in past years lead to soil depletion?

5. How can deep plowing and cultivation cause leaching, especially in sandy
areas?

6. What are various methods of restoring organic matter to soil?

7. How can rill erosion lead to gully erosion?

8. Why are row crops alternated with cover crops in strip cropping?

9. In what kind of situation would terracing be used?

10. What are two methods of stopping the advance of a large gully?

11. What various methods can be used to prevent wind erosion? Can these
be used in all parts of the country, or are they adapted only to local situ¬
ations?

12. List seven or more measures to reduce seasonal floods and prevent severe
droughts.

722 UNIT 8 ECOLOGICAL RELATIONSHIPS

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the various parts of topsoil and their importance to plant life.

2. Outline a crop rotation program, and explain why each crop is included.

3. How can loss of soil organic matter lead to the loss of another vital part
of topsoil?

4. Discuss the combination of strip cropping, contour farming, and crop ro¬
tation in hilly agricultural areas.

5. Discuss various mistakes in past years which have led to disastrous floods
and droughts. How do you think they might have been avoided?

CHAPTER 53

FOREST AND
WILDLIFE
CONSERVATION

Upsetting the natural balance. When
the pioneers pushed westward through
the North American wilderness, they
found abundant wildlife at every turn.
Inland waters teemed with fish, birds,
and aquatic mammals. Large and small
game roamed the forest and grassland.
The woods bison, now extinct, lived in
the forests as far east as New York State.
Its western form, the plains bison, or
buffalo, thundered across the grasslands
in herds numbering tens of thousands.
The white-tailed deer thrived on millions
of acres of forest lands and supplied the
early settlers with both meat and buck¬
skin. The wildcat and cougar, or moun¬
tain lion, timber wolf and fox, beaver
and muskrat, weasel and mink were some
of the mammals of the wilderness.

As the settlers began their conquest
of the wilderness, wildlife began a slow

retreat. Man learned to live in new en¬
vironments, obtaining his needs from the
plentiful supply. Large areas of forests
were cleared and grasslands were plowed
to raise crops to feed the people in the
community. With the development of
better transportation methods, man be¬
gan to satisfy his wants by harvesting a
surplus to be shipped and sold to other
communities. As you have seen, the
balance of life within a community is
subject to the interactions of biotic and
physical factors. The natural balance
existing in many areas was further upset
by man as he developed better rifles,
more efficient lumbering machines and
techniques, and better trapping devices.

Whether our forests will continue
to meet our increasing demands and
whether our wildlife will flourish in the
last half of the 20th century will depend
on what we have learned from the past,
how well we understand ecological re¬
lationships, and our efforts to conserve
our natural resources for future use.
Some forest facts. The original forests
covered nearly half our land — a total of
more than 822 million acres. Forests
occupied the eastern and western parts
of our country. Prairies, plains, and arid
lands covered much of the large central
area.

The two great forest belts of the
East and West were in turn divided into
distinct types of forests. Then as now,
forests were greatly influenced by tem¬
perature, rainfall, soil, topography, and
other physical factors of environment.
Today all these same forests exist, al¬
though, for the most part, vigorous
young second-growth and third-growth
forests have replaced the original virgin
stands. The nation’s forest areas are
shown in Fig. 53-1.

Forests — their use and misuse. More
and more Americans are learning to re-

723

724 UNIT 8 ECOLOGICAL RELATIONSHIPS

Pacific Coastal forest
Rocky Mt. forest

Central hardwood forest
Northern forest

Southern forest
Tropical forest

53-1 This map of the principal forest regions of the United States shows that
almost one third of our land is occupied by forests.

gard forests as a crop. We are learning
to harvest trees when mature and to
leave seed sources to replace them as
they are used. Our forest lands could
have supplied all the timber required for
building America without their being
seriously depleted if they had been used
wisely. Instead, they were destroyed by
a rapidly growing nation that couldn’t
see the harm it was doing.

As early as 1905, officials in Wash¬
ington, among them President Theodore
Roosevelt, became alarmed about the
critical condition of the forests. Ac¬
cordingly Congress created the United
States Forest Service under the control
of the Department of Agriculture.
When Theodore Roosevelt signed the
act creating this agency on February 1,

1905, the forest conservation movement
in America was begun. Vast tracts of
timber, especially in the West, were set
aside as national forests.

The United States Forest Service
has built a splendid record through its
years of activity. State and local agen¬
cies of conservation have looked to this
agency for guidance and assistance in
carrying out forest conservation pro¬
grams. A staff of biologists highly
trained in various specialties works untir¬
ingly in research laboratories in an effort
to discover better conservation methods,
controls for forest diseases, more efficient
lumbering practices, and new uses for
timber products. Even more remark¬
able are the changes that have come on
privately owned forest lands. Since it

CHAPTER 53 FOREST AND WILDLIFE CONSERVATION 725

has become possible to manage forests
properly and still make a financial profit,
forestry has been spreading rapidly.
More professional foresters are hired by
forest industries today than by the fed¬
eral government. No small task in both
the administration of the national for¬
ests and the management of millions of
acres of private woodlands is the pro¬
tection of these timberlands from fire.
Fire — the forest’s worst enemy. In a
recent year, 83,392 fires burned 3,409,038
acres of forest in the continental United
States. This is a staggering loss to our
nation. Fire protection has been ex¬
tended to more and more forest areas in
recent years, but there are still areas
that lack organized fire protection.

Aside from the danger to human
life, a forest fire destroys standing tim¬
ber and consumes the seeds and young
trees of the future forest. A large fire
may even burn into the rich humus of
the forest floor. The large toll of animal
life cannot be determined. Disaster
continues as rains pour over the black¬
ened earth and debris, washing the re¬
maining humus into streams.

Causes of forest fires. According to re¬
ports of the United States Forest Service
in recent years, the following are the
most common causes of forest fires. The
causes are listed in order of frequency:

1. Incendiarists (those who set fires de¬
liberately) .

2. Debris burners (those who let brush
fires get out of control).

3. Smokers (especially those who throw
lighted cigarettes, cigars, and matches
from automobiles).

4. Lightning.

5. Campers (especially those who leave
live coals in a campfire).

6. Railroads.

7. Lumbering.

8. Miscellaneous.

It is shocking to learn that incendi¬
arists are the leading cause of forest
fires. Fires resulting from debris burn¬
ers and smokers could be avoided if
people were more careful and consider¬
ate of others. Lightning, the only
natural cause, is responsible for less than
14 percent of the forest fires per year
(Fig. 53-3).

Fire prevention. In protected forests,
fire towers are placed at strategic points.
Usually all areas of the forest are visible
from at least two towers. Trained rang¬
ers survey the forest from the towers and
report any evidence of fire to headquar¬
ters. A ranger cannot determine the
exact distance of a fire from his tower,
but he can report its direction. The
same fire, spotted from another tower,
will be reported from another angle.
The point where the two lines of direc¬
tion cross indicates the exact location of

53-2 Fire, whether caused by lightning or by
man, is the forest’s worst enemy. (U. S. For¬
est Service)

726 UNIT 8 ECOLOGICAL RELATIONSHIPS

53-3 Note that fires from lightning are only a small percentage of the total
causes of forest fires. (American Forest Products Industries, Inc.)

the fire. Fire fighters equipped with
trucks, water tanks, and chemical fire ex¬
tinguishers can often bring a fire under
control before it becomes extensive.
Fire lines, which resemble roads, pene¬
trate the forest at regular intervals.
They serve as avenues for reaching a fire
and provide gaps at which a fire can be

stopped. In some places, however, fires
can be reached only by dropping fire¬
fighting crews from airplanes by para¬
chute.

Our forests are protected from fire
at a risk of many lives and at a cost of
millions of dollars. It would seem a
simple matter for all of us to cooperate

CHAPTER 53 FOREST AND WILDLIFE CONSERVATION 727

in observing the following seven basic
rules :

1. Never throw away lighted tobacco or
matches.

2. Build campfires only in protected
areas, and put the fire out completely.

3. Watch for fire while driving through
a forest. If you see a small one, stop
and put it out. If the fire is too large
to extinguish by yourself, report it at
once.

4. Report any suspicious person at¬
tempting to start a fire.

5. Never burn a field or debris close to
a forest or without adequate help to
keep the fire from spreading.

6. Become acquainted with your state’s
forest-fire laws.

7. Get to know your nearest fire warden
or foresters.

Forest management. How valuable is
a virgin forest? You may be surprised
to learn that it is not as valuable as a
planted and second-growth, managed
forest. A virgin forest is always domi¬
nated by large, overripe trees. They have
occupied their places for centuries —
long past their period of rapid growth,
and many of them are dying at the top.
Some have been damaged by storms and
are liable to attack by disease and insect
pests. Much of the forest space is oc¬
cupied by trees that have no commercial
timber value. Other trees have grown
crooked in their race for light. The
floor of a virgin forest is littered with
fallen trees in various stages of decom¬
position.

Compare this forest with a managed
forest, operated on a sustained yield
basis. A forest so planned will yield
trees for cutting regularly. All the trees
are valuable timber species. Weed trees,
damaged trees, crowded trees, crooked
trees, and diseased trees are removed in
improvement cutting. As timber trees

mature, they are removed by selective
cutting. The forest is a source of con¬
stant revenue to the owner and yet is
never cut extensively. Every inch of
space produces good timber, and every
tree is a nearly perfect specimen.

Today all national forests are using
good cutting practices. Furthermore,
the private owners who belong to the
Tree Farm Program have pledged their
cooperation.

Block cutting is another kind of
lumbering used in stands of timber in
which all the trees are about thd same
age. In such lumbering it is desirable
to cut out a complete block and reseed
it or replant it. In block cutting, stands
of timber are left around the block for
natural reseeding and protection of the
exposed land. When small trees are es¬
tablished, another block can be cut.
This method of lumbering is used in
extensive stands of Douglas fir, and a
modified method called strip cutting is
used in harvesting of spruce.

Insect enemies. In our study of insects
we mentioned the damage some of them
cause to trees. The sawflies, bark
beetles, spruce budworm, woodboring
beetles, western pine beetle, white pine
weevil, gypsy moth, hemlock looper,
browntail moth, pine shoot moth, tent
caterpillar, and tussock moth are among
the worst forest pests. Sprays contain¬
ing DDT and other powerful insecticides
are effective against these pests. How¬
ever, it is both difficult and undesirable
to spray an extensive forest. As you
learned in Chapter 32, in some areas
where DDT has been used to kill mos¬
quitoes, the death of many insect-eat¬
ing fishes and birds has resulted. Studies
have also revealed that widespread use
of DDT has, by natural selection, been
responsible for an increase in the num¬
ber of insects resistant to the chemical.

728 UNIT 8 ECOLOGICAL RELATIONSHIPS

53-4 Replanting is an important part of forestry management. With tractor
and the tree planter it tows, two men can set up to ten times as many tree
seedlings in one day as they can by hand. (American Forest Products Indus¬
tries, Inc.)

We must depend principally on
control of the forest insects by natural
enemies. Birds are most valuable, al¬
though assistance is rendered by frogs,
toads, snakes, ichneumon flies, and
other insect destroyers.

Grazing and gnawing animals in forests.
In many of the deciduous forest states,
pasturing is the greatest forest enemy.
Pasturing of cattle, horses, sheep, or hogs
in wood lots will completely destroy the
trees in time. First the animals eat or
trample the young trees and reduce the
woods to an open grove. Later they
destroy the leaves on the lower limbs
and injure roots and trunks. Aside from
shelter, the animals receive very little
value from a woods pasture. Trees and
other plants of the forest are high in
cellulose but very low in protein — an

essential substance for the production
of flesh and milk.

Effect of weather conditions on forests.

Wind, ice, and snow are beyond our
power to control. But damage done to
forests by natural forces should receive
attention. Broken limbs open a tree to
attack by fungus diseases and insect
pests. Forest litter resulting from ice
storms creates a fire hazard.
Reforestation. The return of a forest
from an open area is a slow process. Re¬
forestation is a lengthy and costly proc¬
ess, but it is a vital part of the conser¬
vation program. Large sections, useless
for agriculture, have been cleared un¬
wisely in past years. These regions, as
well as eroded, heavily lumbered, and
burned-out areas, should be returned to
trees as rapidly as possible.

CHAPTER 53 FOREST AND WILDLIFE CONSERVATION 729

The various forest states, the United
States Forest Service, and many private
lumber and paper companies maintain
large nurseries where seedlings of tim¬
ber species are grown. Private land-
owners of nearly 48 million acres of
forest land in 45 states are growing trees
as a crop in the Tree Farm Program.
Some associations of wood-using indus¬
tries now grow their own seedlings and
give them free of charge to forest land-
owners in their operating areas. Pine
is being used in reforestation of many
hardwood forest regions because it ma¬
tures rapidly and yields valuable con¬
struction lumber.

Lumber, our chief forest product. The

greatest drain on the forests has been
the demand for construction lumber.
Once it was for log cabins and rail
fences. Later it was lumber for frame
buildings. Even in this age of brick
and stone buildings, four out of every
five houses are still made of wood. Re¬
gardless of the exterior building material,
the average house uses 10,000 board feet
of lumber for flooring, trim, window
casings, and other wooden construction
parts.

The evergreen forests supply most
of the construction lumber today be¬
cause of the fact that our commercial
forests have always contained more soft¬
woods than hardwoods and also because
of the ease with which the soft wood of
conifers can be worked. The most im¬
portant native timber trees supplying
lumber are in the table on page 730.
Newsprint for our daily paper. A single
daily newspaper published in New York
needs the paper produced from the pulp-
wood of 44 acres of timberland to print
a single edition! When you consider
the number of daily newspapers in the
nation, the timber required for a day’s
supply of newsprint is almost staggering.

53-5 This is a log pond of a large mill whose
output is devoted to paper. (American
Forest Products Industries, Inc.)

In addition to newsprint, high quality
book paper, stationery, packaging paper,
toweling, and a great many other kinds
of paper must be supplied daily.
Distillation products of lumber. Vari¬
ous hardwoods yield valuable products
as a result of distillation. When certain
kinds of wood are heated in closed iron
cylinders, various products are given off
as gases. By cooling, these gases are con¬
densed into a varietv of substances called

distillation products. The wood turns
black during distillation and becomes
carbon or charcoal. The following are
some of the products of hardwood dis¬
tillation: wood alcohol, used as a sol¬
vent; acetic acid; lampblack, used in
making certain inks; paints and var¬
nishes; oxalic acid, used for dyeing and
bleaching; and charcoal, used as a fuel
and in water purification.

Beech, maple, and birch are com¬
monly used in hardwood distillation.

730 UNIT 8 ECOLOGICAL RELATIONSHIPS

For this reason, the distillation-industry
centers are in Wisconsin, Michigan,
Ohio, Pennsylvania, and New York.
Pine products. The South as a region
leads the nation in the production of
pine products. The fast-growing pines
of the southeastern evergreen forest sup¬
ply nearly two fifths of our nation’s
lumber. Nearly half the pulpwood used
in making paper comes from this pro¬
ductive forest.

Turpentine, rosin, and pine tar are
products of southern pines. When cer¬
tain species of pine are tapped, they yield
a large quantity of resin, a thick gummy
sap. Resin is removed from the trees by

cutting diagonal gashes through the bark
into the wood, and is collected in jars
or pots. When the resin is boiled, tur¬
pentine comes off as a gas. It is con¬
densed to liquid form bv cooling. After
extraction of turpentine, tar and rosin
remain. Rosin has manv uses. You’ve
probably seen a baseball pitcher reach
for the rosin bag or a batter dust rosin
onto the bat handle.

The sap of the sugar maple tree. Maple
sugar is an important product of Ver¬
mont, New York, Ohio, and other states
of the northern region of the deciduous
forest. Maple sugar comes from the
sap of the sugar maple tree, which is col-

IMPORTANT NATIVE TIMBER TREES

Name of Tree

Forest Region

Southern pine <

loblolly

slash

shortleaf

Southern forest

Douglas fir
Ponderosa pine

longleaf

Pacific coastal forest

Rockv Mountain forest

Oak

Hemlock (eastern and

Central hardwood forest

Northern and Pacific coastal forest

western )

Eastern white pine

Northern forest

Red gum

White fir

Poplar

Maple

Redwood

Tupelo

Spruce

Cottonwood and aspen

Central hardwood and southern forest

Rocky Mountain and Pacific coastal forest

Central hardwood and southern forest

Northern forest and central hardwood forest

Pacific coastal forest

Southern forest and central hardwood forest
Northern forest

Northern forest and Rockv Mountain forest

Cedar

Sugar pine

Beech

Western larch

Idaho white pine

Pacific coastal, northern, and southern forest

Pacific coastal forest

Northern and central hardwood forest

Rocky Mountain and Pacific coastal forest

Rockv Mountain and Pacific coastal forest

Cypress

Birch

Lodgepole pine

Southern forest

Northern forest

Rockv Mountain forest

Balsam fir

Northern forest

CHAPTER 53 FOREST AND WILDLIFE CONSERVATION 731

53-6 This worker is using an acid gun to
stimulate resin flow from the new cut on the
face of a pine tree that is being prepared
for turpentining. (U. S. Forest Service)

lected in buckets attached to tapped
trees in the early spring. The sap is
boiled to remove the water until it be¬
comes a thick syrup.

Tanning materials from bark. The tan¬
ning industry depends on the forests of
the world for bark containing tannic
acid , which is used in the tanning of
hides. Chestnut wood, chestnut oak
bark, eastern hemlock bark, and tan-
bark oak are the chief American sources.
About 70 percent of the nation’s tanning
supply comes from foreign woods, how¬
ever. The tanning industry is centered
mostly in the lake states, Appalachian,
and northeast forest regions.

Control of the water supply. The forest
area acts like a sponge by absorbing the
rainfall in its layers of humus. The

leaves of trees keep the rain from falling
directly on the soil and washing it away.
The network of roots binds the soil, and
the deep layers of humus hold water, so
that there is a gradual runoff of soil wa¬
ter rather than a flash flood.

Forested land helps to control the
water supply in the following ways:

1. Prevents floods and causes steady
stream flow by reserve water held in
humus.

2. Prevents spring freshets by shading
snow so that it melts slowly.

3. Prevents drought by storing water in
the wet season.

4. Prevents washing of soil into rivers.
Benefits to soil. The early settlers re¬
garded the forests as an enemy to agri¬
culture because clearing had to be done
to make room for farms. But in a larg¬
er sense forests are a distinct benefit to
soil. Erosion is one of the worst ene¬
mies of agriculture. But it is prevented
by forests whose roots hold back the
earth and whose leaves protect the sur¬
face. Furthermore, the humus which
collects on the forest floor enriches the
soil. In some areas the forest performs
another function by preventing the
spread of wind-blown sand over fertile
areas, which are thus saved for use.
Effect of forests on climate. While the
effect of forests on climate may not
rank in importance with the two pre¬
ceding benefits, it is certain that by
their retention of moisture, forests mod¬
ify the climate over large areas and ap¬
parently influence rainfall. To a lesser
extent forests affect climate by giving
protection from wind and sun.

Wildlife conservation. The term wild¬
life includes all native animals. The
wildlife conservation program is con¬
cerned with those native animals that
have a direct food, fur, or sporting value.
After reading about the problems in-

732 UNIT 8 ECOLOGICAL RELATIONSHIPS

volved in wildlife conservation, you
should be able to appreciate the value
of scientifically based game laws.

Fish conservation. We often hear that
a stream or a lake is “fished out.”
When the biologist studies such a lake,
he is more likely to find that it is in real¬
ity overpopulated with thousands of
stunted fishes so overcrowded that few
can ever grow to a size worthwhile for
fishermen. More rigid laws regulating
fishing are not the solution; nor will re¬
stocking help in many cases for it has
been established that with the excep¬
tion of the trout species, most fishes
maintain a maximum population in
spite of man’s fishing pressure. Give
the fishes the proper environment and
they will hold their own and even in¬
crease in heavily fished streams and
lakes.

In the study of fish conservation we
must first find out the characteristics of
productive waters. Then we shall con¬
sider various practices that destroy fish¬
es and make waters unproductive. Fi¬
nally we shall discuss conservation
measures that can restore ideal habitats
and return the fishes to our waters.
Characteristics of a productive lake and
stream. A lake that supports a thriving
fish population must supply many dif¬
ferent environments suitable for large
species and smaller ones, frogs, insects,
and other vital parts of a complicated
society and food chain. Deep channels
and holes are necessary to protect fishes
during both the cold of winter and the
extreme heat of summer. The depth
at which fishes live varies greatly with
water temperature. During the spawn¬
ing season, sunfish, bass, and many other
species leave the deep water and enter
clear, shallow pools and backwaters.
Here the fry hatch and live protected
by water plants in shallow areas.

53-7 Fish ladders are built around large hy¬
droelectric dams to enable fish to travel up¬
stream during the spawning season. (Union
Pacific Railroad)

Protozoans, tiny crustaceans, and
larvae of various insects supply food
necessary for small fishes. As the small¬
er fishes leave the shallow water and
enter deep water, they become food for
predatory game fishes such as the bass,
pike, trout, and perch. Forage fishes,
including such species as shiners, chubs,
and other minnows, while they have no
sport or food value for man, are vital
as a food for the game fishes.

The productive stream has rapids
and riffles, shallows and depths, chan¬
nels and undercut banks. Larger rivers
are fed from productive backwaters.
Fallen logs and driftwood provide ideal
habitats for many species. Abundant
water plants are necessary to provide
food for vegetarian fishes and other
aquatic animals, and supply the organic
matter required by bacteria, protozoans,
and organisms essential in the food
chain.

Destruction of aquatic habitats. Many
practices, some careless and others de-

CHAPTER 53 FOREST AND WILDLIFE CONSERVATION 733

liberate, have destroyed aquatic habi¬
tats. In regions where the water table
has been lowered by removal of vege¬
tation, soil erosion, and other misuse of
the land and water resources, the level
of lakes and ponds has fallen. Small
ponds may dry up completely during
the summer. The water in larger lakes
may recede to the point that shallow
areas and backwaters are left dry, thus
destroying both spawning areas and re¬
gions where much of the food supply
develops. Many backwaters and marsh¬
es have been drained deliberately be¬
cause they were thought to be useless.

The channels of many rivers have
been dredged and straightened to in¬
crease the flow of water. Such open
channels may be good from the stand¬
point of flood control, but they are not
good surroundings for fishes. Fishes
thrive better in a river with natural
bends, rapids, and deep pools.

Dams across rivers interfere with
fish migrations unless fish ladders are
provided. A fish ladder is a channel
around a dam through which fish can
travel upstream. The fast flow of water
is broken by a series of staggered plates
projecting into the water from the sides.
In other fish ladders a long slope is
broken into a series of steps like terraces,
which can be leaped by fish traveling up¬
stream.

Serious floods are tragic, not only
to man but also to fishes and aquatic
animals. When flood waters overflow
the lowlands, many fishes follow the ris¬
ing water. Large numbers are left
stranded in isolated pools when the wa¬
ters recede.

Water unfit for fish. Many game fishes
cannot survive in water containing a
large amount of mud and silt. Mud
coats the surface of the gill filaments
and prevents oxygen from reaching the

JruSp? * f

- *

♦

53-8 The shad shown here could not survive because of the pollution of their
stream by sewage. (U. S. Fish & Wildlife Service)

734 UNIT 8 ECOLOGICAL RELATIONSHIPS

blood stream. This type of suffocation
is common in species with a high oxy¬
gen requirement.

Sewage, garbage, cannery waste,
and other organic refuse dumped into
water kill fishes for another reason. Or¬
ganic matter decays in water as a result
of bacterial action. Oxygen is used up
during this process, and fish die for
want of it in the water.

Industrial and chemical wastes
poured into streams poison fishes and
other aquatic animals. Cyanide, acids,
alkalies, and other industrial wastes may
affect the stream’s inhabitants for many
miles downstream.

Indiscriminate stocking. One of the
most difficult problems facing fishery
biologists today is the result of intro¬
ducing new species to waters in times
past. Often the new species crowded
out the more desirable native fishes. In
the Northeast many beautiful trout
lakes have been completely taken over
by stunted yellow perch and golden
shiners that were introduced 50 years or
more ago.

Where possible lakes that have
been overrun by undesirable fishes are
being completely drained for a period
of time to kill the fishes. Other lakes
are poisoned. Then these reclaimed
lakes are restocked with the species that
were present before man attempted to
improve them. Thus many waters have
been returned to the public.

Hatchery programs. Both state and fed¬
eral fish hatcheries produce large num¬
bers of game fishes in their rearing
ponds. The hatcheries maintain a
stock of adult breeders kept in special
spawning pools. After spawning, the
breeder fishes are removed to prevent de¬
struction of the small fishes. Fry hatched
in late spring reach stocking size by
midsummer or early fall. Many hatch¬

eries use special trucks equipped with
aerated tanks for transporting small
fishes to distant streams and lakes.

Artificial propagation and stocking
are essential parts of fish conservation
programs. However, it is useless to
stock polluted waters, or those that lack
conditions necessary to supply adequate
food, or those that are already overpop¬
ulated.

Private rearing ponds and lakes. Many
farmers have constructed earthen ponds
in low corners of fields by building up
banks. These farm ponds are stocked
with crappies, bluegills, catfish, and
other species. The pond provides rec¬
reation, conserves water, helps prevent
soil erosion, provides a reserve for fire
protection, and supplies many strings of
fish for the dinner table.

Farm pond fish populations must
be managed just as carefully as the cat¬
tle, pigs, or chickens reared on the farm.
To be continually productive, such a
pond must be periodically fertilized and
adequately harvested. Otherwise the
fish population will almost certainly
eventually deteriorate to thousands of
fishes too small to be eaten.

Great numbers of artificial lakes
have been made by building dams and
backing up water in valleys. Lakes of
this type are ideal for fishing and boat¬
ing.

Useless destruction of valuable allies,
the birds. The advancing tide of civi¬
lization. has made dangerous inroads
into the populations of songbirds and
game birds. Much of this destruction
is useless and avoidable.

The cutting of forests, clearing of
underbrush, and burning of fields have
removed vast areas of bird habitats.
Unnecessary drainage of marshes and
lowering of the water in ponds and
lakes have deprived water birds and

CHAPTER 53 FOREST AND WILDLIFE CONSERVATION 735

wading birds of both food and nesting
sites.

In former years many thousands of
birds were slaughtered for flesh or feath¬
ers. Fortunately such market hunting
is forbidden now, both by federal and
state laws. But conservation measures
came too late to save some species that
were once common but are now extinct.

In the early 1800’s Audubon de¬
scribed flocks of passenger pigeons so
large they darkened the sky. When
such flocks settled in trees to roost for
the night, their enormous weight bent
the branches and sent many crashing to
the ground. We have reports of pi¬
geon hunters who climbed the trees and
knocked thousands of pigeons from
their perches with clubs. Thousands
of others were gathered in sacks to be
sold for a few cents each or left on the
ground as food for hogs. The last sur¬
vivor of what is estimated to have been
over two billion passenger pigeons died

in the Cincinnati zoo in 1914. What
caused the complete extermination of
this valuable bird in less than 100 years?
No simple explanation can be found.
Market hunting took a heavy toll, as
did the removal of large areas of the
birds’ food supply. But these alone
could not have accounted for the ex¬
termination of such an abundant spe¬
cies. The introduction of an epidemic
disease may have been a contributing
factor. Certainly no thought was given
to conservation until too late.

Rare birds we may still save. Bird soci¬
eties are watching several other species
which are nearing extermination but
which may still be saved.

Only a few ivory-billed woodpeck¬
ers remain in the forests of river bot¬
toms in the South. Anyone seeing one
of these rare birds should report it at
once to the National Audubon Society.

The whooping crane is disappear¬
ing from the region of the Great Plains.

53-9 The California condor on the left and the northern bald eagle on the right
are both nearing extinction. (Left: Kofard from National Audubon Society; right:
Ambler from National Audubon Society)

736 UNIT 8 ECOLOGICAL RELATIONSHIPS

53-10 The ring-necked pheas¬
ant is a game bird that needs
to be protected. (USDA)

In a recent count, only 26 adult whoop¬
ing cranes were found. These were in
the Aransas, Texas, National Wildlife
Refuge, their wintering grounds.

The prairie chicken was hunted
widely for market in earlier times.
More recently its numbers have been
further reduced by the plowing of na¬
tive grasslands in the prairie states.

The California condor, a type of
vulture, is a rare species of the moun¬
tainous regions of the Far West. Less
than 100 of these great birds are left.
The condor, with a body three feet long
and wing spread of ten feet or more,
lives on decaying flesh. Lack of food
and the fact that it lays only one or two
eggs each season may bring about its
extinction.

The eastern bluebird, which is
sometimes seen around settled areas
and farms, has been declining in num¬
bers for years. This pretty little song¬
bird may disappear in your lifetime.
Problems relating to game birds. Mi¬
gratory ducks and geese have been fav¬
orite game birds for hunters. But the
decline in their populations during the
past 50 years has been due only partially

to hunters. The draining of marshes,
which supply natural food such as wild
rice as well as nesting places and cover,
has presented a serious problem in con¬
serving these birds. Large numbers of
ducks die of alkali poisoning on ponds
and lakes of the western prairies when
the water is low and salts become con¬
centrated. In some regions thousands
of ducks and geese have died of botu¬
lism when they consumed the deadh'
toxin while probing for food in the mud
of ponds and lakes at low water.

Upland game birds, including the
quail, partridge, grouse, wild turkey,
and pheasant, are widely hunted by
sportsmen. They can survive regulated
hunting only if adequate food and cov¬
er are provided, especially during winter
months. Many farmers allow trees,
shrubs, and tall grass to grow along
their fence rows and roadsides as cover.
A few rows of grain left at the margin
of fields provides winter food for these
and other birds. Boy Scouts, 4-H
Clubs, sportsmens’ clubs, and many in¬
dividuals place grain in woods and fields
to help game birds get through the diffi¬
cult winter months.

CHAPTER 53 FOREST AND WILDLIFE CONSERVATION 737

Private agencies work closely with
the Fish and Wildlife Service, a branch
of the United States Department of the
Interior. This important bureau has
charge of the conservation of birds and
other animals, controls the national
wildlife reservations, administers laws
regarding commerce in game, and pub¬
lishes many educational bulletins and
other information.

Destruction of fur-bearing animals.

The first drain on the mammal popu¬
lation of North America occurred early
in our history. Long before the pioneers
began their journey westward, trappers
had explored the wilderness in search of
fur-bearing animals, and fur traders were
buying pelts from the Indians. As long
ago as 150 years, trappers and fur traders
were penetrating the forests of the Pa¬
cific Northwest. Their prize was the
beaver, highly valued in the eastern and
European markets. “Empire builder”
is an appropriate name for this valuable
fur-bearer, which played a major role in
the settlement of this vast wilderness

country. Fortunes were made in the
fur business during these times. The
pelts of beaver, mink, otter, muskrat,
fox, skunk, and other fur-bearers
brought an annual revenue of over
$100,000,000. Early trappers were very
ruthless hunters. They gave no
thought to conservation of the animals
they sought. The numbers of fur-bear¬
ing animals decreased rapidly. Later
the settlers began clearing and cultivat¬
ing the land. Only in the most remote
wilderness areas were the fur-bearers
able to survive unmolested.
Destruction of larger mammals. The
slaughter of the plains bison is a story of
useless waste. In the middle of the last
century herds of bison numbering many
thousands thundered across the Great
Plains. For centuries this noble animal
had supplied both flesh and hides to
plains Indians. But as calves replaced
the mature animals taken from the
herds by hunting, both the Indians and
the bison flourished. Then came the
buffalo hunters. Bison were slaughtered

53-11 The mink on the left and the gray fox on the right are both fur-bearing
mammals which are so rare in nature that they are raised in great numbers on
fur farms. (Left: Morton from National Audubon Society; right: Maslowski from
National Audubon Society)

738 UNIT 8 ECOLOGICAL RELATIONSHIPS

53-12 The American bison is now protected on large game preserves and its
numbers are slowly increasing. (Bureau of Land Management)

in tremendous numbers as food for
workmen building railroads through the
West or for pure sport. At one time
passengers on trains amused themselves
by firing at bison from open platforms.
Thousands were killed for buffalo robes
and the carcasses were left to rot. It is
shocking to learn that great numbers of
bison were killed deliberately to starve
the plains Indians into submission.
Within 50 years bison herds were re¬
duced from tens of thousands to mere
remnants. At one time it appeared
likely that the bison might die out en¬
tirely.

The white-tailed deer and black
bear vanished from the remaining for¬
ests of most of the central and eastern
agricultural states. Elk, mule deer, and
antelope were slaughtered in the West.

What did the destruction of so
much of the game resources profit
America? It supported a rich fur trade
for a time, and it provided food and
clothing to pioneer families. But this
could have been accomplished bv sac¬
rificing only a small portion of wildlife
population. Much of the destruction
of wildlife was useless, careless waste.

Wildlife restoration. Most small mam¬
mal and big-game mammal species have
survived in greatly reduced numbers in
the remaining wilderness regions. Can
they return with our help to at least a
portion of their former ranges? Can
we make room for them among our
manv farms and ranches with their

enormous population of domestic ani¬
mals? This will require careful plan¬
ning and scientific management. But
it can be done.

There are already some dramatic
examples of wildlife restoration. Be¬
cause of wise management and the
adaptability of the animal, the white¬
tailed deer population is now larger
than it was before the arrival of civi¬
lized man. In Pennsvivania after the

turn of the century, there were so few
deer that stock was brought in from
other states. Today the Pennsylvania

J J

deer herd is so large that thousands die
of starvation each winter — in spite of
tremendous hunting pressure.

The wild turkey is a similar exam¬
ple of restoration through effective man¬
agement. Once common throughout
the Northeast, wild turkeys became

CHAPTER 53 FOREST AND WILDLIFE CONSERVATION 739

nearly extinct in all but a few southern
states. Today turkeys have been re¬
stored to much of their former habitat.
The greatest problem in restoring tur¬
keys was obtaining stocks of truly wild
birds, for wild turkeys will not mate in
captivity. The big breakthrough came
when the manager of a Pennsylvania
game farm discovered that if wing-
clipped hen turkeys were kept in a
fenced area, the wild toms would fly in
to mate and would then leave. Sys¬
tematic harvesting of eggs then became
a relatively simple matter.

National parks and national forests.
The 29 national parks with their six
million acres of forest are to be preserved
forever. They are part of preserves and

recreational areas dedicated to those who
seek undisturbed and unspoiled natural
beauty.

In addition there are 149 national
forests covering more than 181 million
acres. The national forests supply tim¬
ber for about 12 percent of the nation’s
forest products. In addition they sup¬
ply homes for wildlife and aid greatly in
restoring birds and animals that have
been reduced in number in past years.
During summer months, when drought
strikes the Great Plains, they are opened
to cattle and sheep as grazing areas un¬
der supervision. In addition, they pro¬
vide recreational areas attracting camp¬
ers and sportsmen who fish in their lakes
and streams.

IN CONCLUSION

The forests and wildlife of America belong to you. What you do to preserve
them will be your contribution to future generations. If you live on a farm
or a ranch, think of the birds and animals along your fence rows or in the brush
and woodland that you have not cultivated. If you are one of the many city
dwellers, protect the squirrels in the park and the rabbits in the vacant lot.
Do not leave litter around. If you fish and hunt, respect the laws and limits.
Be sure your campfire is really out. The fewer signs of human presence you
leave behind you in a natural area, the more you are doing to maintain the
beauty of America.

Have you thought of joining the Audubon Society or the Wilson Club?
Have you heard of the Isaak Walton League? These organizations would wel¬
come you if you are interested in wildlife and conservation. Ducks Unlimited,
The National Wildlife Federation, and The American Wildlife Institute are
other civilian agencies carrying on important work. They deserve your full
support and cooperation.

BIOLOGICALLY SPEAKING

distillation products
farm pond
fish ladder
forage fish
fry

game bird
game fish
maple sugar
pulpwood
rosin

sustained yield
tannic acid
turpentine
wildlife

740 UNIT 8 ECOLOGICAL RELATIONSHIPS

QUESTIONS FOR REVIEW

1. What are the various causes of forest fires? Indicate which ones are de¬
liberate, the result of carelessness, or unavoidable. How can forest fires be
prevented?

2. How can two forest rangers, viewing a fire from towers, pinpoint the loca¬
tion of the fire?

3. List several of the worst insect enemies of forest trees.

4. What types of trees are removed from a managed forest during improve¬
ment cutting?

5. Under what conditions are whole sections of a forest cleared by block
cutting?

6. Name several products of the distillation of hardwoods.

7. List the principal products of pine resin. What forest region supplies
nearly all of these products?

8. What are some of the trees that supply bark for the tanning industry?

9. What are some features of a productive river or stream?

10. How has lowering of the water table caused serious loss of fish even in
bodies of water that do not dry out?

11. Describe the operation of a fish ladder around a dam.

12. How are farm ponds made?

13. Make a list of bird species that are now very rare or nearly extinct.

APPLYING PRINCIPLES AND CONCEPTS

1. Discuss the scientifically managed forest operated on a sustained yield
basis, and show how this forest is more productive than a virgin forest.

2. Discuss the role of sportsmen in the wildlife conservation program.

3. List various problems in restoring big-game mammals in the agricultural
states.

4. Why are predatory birds and animals essential to proper game manage¬
ment?

RELATED READING

Books

Archer, Sellers. Rain, Rivers and Res¬
ervoirs: The Challenge of Running
Water. Coward-McCann Inc.,
New York. 1963

Bates, Marston. Animal Worlds. Ran¬
dom House, Inc., New York. 1963

Bates, Marston. Man in Nature.
Prentice-Hall, Inc., Englewood
Cliffs, N. J. 1961

Bates, Marston. The Forest and the
Sea. Mentor Books, New York.
1959

Bear, Firman. Earth: The Stuff of Life.
University of Oklahoma Press,
Norman, Okla. 1961
Berrill, Jacqueline. Wonders of the
Woods and Deserts at Night.
Dodd, Mead and Co., Inc., New
York. 1963

CHAPTER 53 FOREST AND WILDLIFE CONSERVATION

741

Brown, Vinson and others. Common
Wildlife of the Southwest Deserts.
Naturegraph Co., Healdsburg,
Calif. 1964

Buchsbaum, Ralph and Buchsbaum,
Mildred. Basic Ecology. Box¬
wood Press, Pittsburgh, Pa. 1957
Carthy, J. D. Animal Navigation.
Charles Scribner’s Sons, New York.

1963

Christian, Garth. While Some Trees
Stand: Wildlife in Our Vanishing
Countryside. Transatlantic Arts,
Holly wood-By-The-Sea, Fla. 1964
Dasmann, Raymond F. The Last Hori¬
zon. The Macmillan Co., Chi¬
cago. 1963

Farb, Peter and the Editors of Life.
Ecology. Time, Inc., Chicago.

1964

Leopold, A. Starker and Editors of Life.
The Desert. Time, Inc., New
York. 1961

Milne, Lorus J. and Milne, Margery.
The Balance of Nature. Alfred A.
Knopf, Inc., New York. 1960
Odum, Eugene P. and Odum, How¬
ard T. Fundamentals of Ecology ,
2nd Ed. W. B. Sjimders Co.,
Philadelphia. 1959

Peterson, Roger Tory and Fisher,
James. Wild America. Houghton
Mifflin Co., Boston. 1960
Sobers, Allen. Ours Is the Earth: Ap¬
praising Natural Resources and
Conservation. Holt, Rinehart and
Winston, Inc., New York. 1963
Udall, Stewart L. The Quiet Crisis.
Holt, Rinehart and Winston, Inc.,
New York. 1964

Farb, Peter and the Editors of Life.
The Forest. Time, Inc., New
York. 1961

Friendly, Natalie. Wildlife Teams.
Prentice-Hall, Inc., Englewood
Cliffs, N. J. 1963

Gleason, Henry A. and Cronquist, Ar¬
thur. Plant Dominions , U.S.A.
Columbia University Press, New
York. 1964

Gleason, Henry A. and Cronquist, Ar¬
thur. The Natural Geography
of Plants. Columbia University
Press, New York. 1964
Hitch, Allen S. and Sorenson, Marian.
Conservation and You. D. Van
Nostrand Co., Inc., New York.
1964

Idyll, C. P. Abyss: The Deep Sea and
the Creatures Who Live in It.
Thomas Y. Crowell Co., New
York. 1964

Articles

Deevey, E. W. “Life in the Depths of
a Pond.” Scientific American.
October, 1951

Dietz, Robert S. “The Sea’s Deep Scat¬
tering Layers.” Scientific Ameri¬
can. August, 1962

Llano, George A. “The Terrestrial Life
of the Antarctic.” Scientific
American. September, 1962

Murphy, Robert Cushman. “The
Oceanic Life of the Antarctic.”
Scientific American. September,
1962

Opik, Ernest J. “Climate and the
Changing Sun.” Scientific Ameri¬
can. June, 1958

Woodwell, George M. “The Ecologi¬
cal Effects of Radiation.” Scien¬
tific American. June, 1963

APPENDIX

SOME SPECIALIZED BRANCHES OF BIOLOGY

ANATOMY

BACTERIOLOGY

Study of the gross structure of plant and animal organs

Study of microscopic nongreen protists, some of which
cause disease

CYTOLOGY

ECOLOGY

Study of the structure and functions of cells

Study of the environmental relations and distribution of
organisms

EMBRYOLOGY

ENTOMOLOGY

Study of the early development of organisms

Study of insects

EUGENICS

Branch of genetics dealing with human heredity

GENETICS

Study of heredity

HERPETOLOGY

HISTOLOGY

ICHTHYOLOGY

Study of reptiles

Study of the structure of tissues

Study of fish

MORPHOLOGY

Study of total structure of organisms

MYCOLOGY

ORNITHOLOGY

PALEONTOLOGY

PARASITOLOGY

Study of fungi

Study of birds

Study of the life of past geological periods

Study of organisms which live on or in the bodies of other
organisms and derive their nourishment from that organism

PATHOLOGY

PHYCOLOGY

PHYSIOLOGY

PROTOZOOLOGY

SPACE BIOLOGY

TAXONOMY

VIROLOGY

Study of diseases of organisms

Study of algae

Study of the functions of organisms

Study of protozoans

Study of survival problems of living things in outer space
Naming, grouping, and classifying of organisms

Study of viruses

744 APPENDIX

A MODERN CLASSIFICATION OF ORGANISMS

KINGDOM PROTISTA

Organisms having a simple structure; many unicellular, others colonial or multi¬
cellular, but lacking in specialized tissue; both heterotrophic and autotrophic;
neither distinctly plant nor animal.

PHYLUM SCHIZOMYCOPHYTA (SCHIZOPHYTA)

Mostly parasitic or saprophytic organisms; cells lacking an organized nucleus,
with nucleoproteins in contact with cytoplasm; reproduction by fission, certain
forms producing endospores: bacteria, Rickettsiae, actinomycetes, spirochetes
[placed in Kingdom MONERA in certain classifications].

PHYLUM CYANOPHYTA

Cells containing chlorophyll and other pigments not localized in plastids;
cells lacking an organized nucleus, with nucleoproteins in contact with cytoplasm;
reproduction by fission and spores: blue-green algae ( Nostoc , Anabaena, Gloeocapsa,
Oscillatoria ) [placed in Kingdom MONERA in certain classifications].

PHYLUM CHLOROPHYTA

Cells containing chlorophyll and other pigments localized in plastids; food
stored as starch; cells with organized nucleus; unicellular, colonial, and filamentous
forms; motile, free-floating, and sessile: green algae ( Spirogyra , Protococcus,
Chlorella, desmids, Ulothrix, Oedogonium) .

PHYLUM CHRYSOPHYTA

Cells containing chlorophyll and other pigments localized in plastids; cells
often yellow-green, golden-brown, or brown in color; food stored as oil and com¬
plex carbohydrates; cell walls often containing silicon; unicellular, colonial, and
filamentous forms; motile and free-floating: yellow-green algae, diatoms.

PHYLUM PYRROPHYTA

Cells containing chlorophyll and other pigments localized in plastids; cells
often yellow-green or golden-brown; food stored as starch or oil; unicellular flagel¬
lates with two flagella, one lateral and one longitudinal; mostly marine organisms:
dinoflagellates, cryptomonads.

PHYLUM PHAEOPHYTA

Cells containing chlorophyll usually masked by a brown pigment, localized
in plastids; food stored as oil and complex carbohydrates; multicellular; nonmotile;
plant body usually large, complex, and sessile; mostly marine organisms living in
shallow water: brown algae ( Fucus , Sargassum, N ereocystis) .

APPENDIX 745

PHYLUM RHODOPHYTA

Cells containing chlorophyll usually masked by a red pigment, localized in
plastids; food stored as a carbohydrate related to starch; multicellular; nonmotile;
plant body complex, usually sessile; mostly marine, deep-water organisms: red
algae ( Chondrus , Gelidium, Polysiphonia) .

PHYLUM MYCOPHYTA ( EUMYCOPHYTA )

Organisms lacking chlorophyll; parasitic and saprophytic: true fungi.

Class Phycomycetes : Algalike fungi: black molds (Rhizopus), water mold
(Saprolegnia) , white “rust,” downy mildews.

Class Ascomycetes: Sac fungi, usually producing eight ascospores in an ascus;
many forms producing conidiospores: blue and green molds ( Penicillium ,
Aspergillus) , morels, yeasts, cup fungi, powdery mildews.

Class Basidiomycetes : Basidium (club) fungi: rusts, smuts, mushrooms, puff¬
balls, bracket fungi.

Class Deuteromycetes: Imperfect fungi: ringworm fungi; thrush (Candida) ,
athlete’s foot fungus.

PHYLUM MYXOMYCOPHYTA

Amorphous slimy growths consisting of a naked protoplasmic mass creeping
slowly by a flowing, amoeboid motion; mostly saprophytes; spores produced in
sporangia: slime fungi (slime molds).

PHYLUM SARCODINA

Organisms forming pseudopodia; pellicle at cell surface lacking; reproduction
principally by fission; fresh-water and marine: amoeboid organisms (A meba,
Endameba, Arcella), foraminifers, radiolarians.

PHYLUM MASTIGOPHORA

Organisms that propel themselves with one or more flagella; pellicle usually
present; fission longitudinal; flagellates ( Euglena , Trypanosoma , Vo/vox, Leish-
mania ) .

PHYLUM CILIOPHORA (CILIATA)

Locomotion by means of cilia; pellicle present; many forms with macronucleus
and micronucleus: ciliates ( Paramecium , Vorticella , Stentor , Stylonychia) .

PHYLUM SPOROZOA

No structures for locomotion; spore-forming; all parasitic: sporozoans (Plas¬
modium).

KINGDOM PLANTAE

Multicellular plants having tissues and organs; cell walls containing cellulose;
chlorophyll a and b present and localized in plastids; food stored as starch; cell
walls containing cellulose; sex organs multicellular; autotrophic.

746 APPENDIX

PHYLUM BRYOPHYTA

Multicellular green plants living on land, usually in moist situations; alter¬
nation of generations with the gametophyte the conspicuous generation; vascular
tissues lacking; reproduction by spores and gametes.

Class Hepaticae: Gametophyte leafy or thalluslike, usually prostrate: liver¬
worts ( Marcantia , Riccia).

Class Anthocerotae: Gametophyte thalluslike, sporophyte elongated and
cylindrical: horn worts.

Class Musci: Gametophyte usually an erect leafy shoot, sporophyte incon¬
spicuous and parasitic on the gametophyte: true mosses ( Polytrichium ,
Sphagnum) .

PHYLUM TRACHEOPHYTA

Plants with vascular tissues; sporophyte plant body prominent; highly spe¬
cialized roots, stems, leaves, and reproductive organs in most forms.

subphylum psilopsida: Leaves usually absent, if present small and simple;
roots absent; mostly fossils (only four living species): Psilotum, Tmesipteris.
subphylum lycopsida: Leaves simple and usually small, spirally arranged on
stem: club mosses ( Lycopodium , Selaginella) .

subphylum sphenopsida: Leaves small and simple and arranged in whorls;
mostly fossils: horsetails ( Equisetum ).

subphylum pteropsida: Leaves usually large and complex; plant body often
large.

Class Filicineae: Sporophyte producing a leafy frond, usually bearing spo¬
rangia; rhizome usually creeping; gametophyte plant body a small prothallium:
ferns and tree ferns.

Class Gymnospermae: Seeds not enclosed in an ovary; mostly large, woody
plants; many evergreen.

Order Cycadales: Primitive fernlike gymnosperms: cycads or sago palms
( Cycas , Dioon, Z amia).

Order Ginkgoales: Large trees with two kinds of branches, one bearing
most of the wedge-shaped leaves in clusters; mostly fossils (one genus and
species remaining) : Ginkgo biloba.

Order Coniferales : Cone-bearing gymnosperms, mostly evergreen; leaves
in the form of needles or scales: pines, cedars, spruces, firs, larches, yews.
Order Gnetales: Possible forerunners of the flowering plants; two seed
leaves on the embryo; wood containing vessels; mostly fossils (only three
remaining genera): Ephedra, Welwitschia, Gnetum.

Class Angiospermae: Flowering plants; seeds enclosed in an ovary which ripens
into the fruit.

Subclass Monocotyledonae : Embryo with one cotyledon; fibrovascular tis¬
sues scattered through the stem tissues; flower parts in 3’s and 6’s; leaves
parallel-veined: grasses, sedges, lilies, irises, orchids, palms (including about
9 orders).

APPENDIX 747

Subclass Dicotyledonae: Embryo with two cotyledons; fibrovascular tissues
in a zone around a central pith tissue in the stem; flower parts in 4’s or 5’s;
leaves with netted veins: buttercups; roses, apples, elms (including about
35 orders).

KINGDOM ANIMALIA

Multicellular animals having tissues and, in many, organs and organ systems; pass
through embryonic or larval stages in development; heterotrophic.

PHYLUM PORIFERA

Body in two cell layers, penetrated by numerous pores; “skeleton” formed by
silicious or calcareous spicules or horny spongin; marine and fresh-water animals:
sponges.

Class Calcispongiae: Simple sponges of shallow waters; calcareous spicules
forming “skeleton”: ascon and sycon sponges (Grantia) .

Class Hylospongiae: Deep-water sponges; “skeleton” composed of silicious
spicules in open framework: Venus’s flower basket.

Class Demospongiae: Large sponges; often brilliantly colored; “skeleton” of
spongin or a combination of spongin and silicious material; fresh-water and
marine: bath sponge, finger sponge; crumb-of-bread sponge.

PHYLUM COELENTERATA

Usually free-swimming animals with a baglike body of two cell layers with a
noncellular substance between them; gastrovascular cavity with one opening lead¬
ing to the outside; many with tentacles and all with stinging capsules; solitary or
colonial forms; marine and fresh-water: jellyfish.

Class Hydrozoa: Solitary or colonial; fresh-water and marine; reproduction by
asexual buds and gametes; alternation of generations in many forms: Hydra ,
Obelia, Gonionemus , Physalia.

Class Scyphozoa: Exclusively marine; most have mesenteries; polyp stage
usually absent: Aurelia, Cyanea.

Class Anthozoa: Marine forms; solitary or colonial; without alternation of
generations; body cavity with mesenteries; numerous tentacles: sea anemones,
corals, sea fans.

PHYLUM CTENOPHORA

Marine animals resembling jellyfish; hermaphroditic; definite digestive system
with anal pore; biradially symmetrical: comb jellies.

PHYLUM PLATYHELMINTHES

Body flat and ribbonlike, without true segments; no body cavity, skeletal,
circulatory, or respiratory systems; head provided with sense organs; nervous
system composed of two longitudinal nerve cords: flatworms.

Class Turbellaria: Mostly free-living aquatic or terrestrial forms; many with
cilia on the epidermis: Planaria.

748 APPENDIX

Class Trematoda: Parasitic forms with mouth at anterior end; intestine present;
no cilia on adults: human liver fluke, sheep liver fluke.

Class Cestoda: Parasitic forms; body a series of proglottids; intestine lacking;
hooked scolex adapted for attachment to intestine of host: tapeworms.

PHYLUM NEMERTEA

Body elongated and flattened; long proboscis extending through mouth open¬
ing at anterior end; circulatory system present; bilaterally symmetrical; mostly
marine: proboscis worms.

PHYLUM NEMATODA

Body slender and elongated; unsegmented; body wall in three lavers; body
cavity present; bilaterally symmetrical; free-living and parasitic forms: roundworms
(Ascaris, Trichinella , pin worm, hookworm, vinegar eel).

PHYLUM NEMATOMORPHA

Body slender and elongated, resembling a hair; larvae parasitic in insects,
adults free-living in fresh water; mouth often lacking in adults: horsehair worms
(“horsehair snakes”).

PHYLUM ACANTHOCEPHALA

Body elongated; digestive tract lacking; anterior proboscis armed with many
recurved hooks; parasitic in vertebrates: spiny-headed worms.

PHYLUM TROCHELMINTHES ( ROTIFERA )

“Wheel animals” with rows of cilia around the mouth which beat with a
motion suggesting the rotation of a wheel; chitinlike jaws and a well-developed
digestive system; body usually cylindrical, ending in a forked grasping foot: rotifers.

PHYLUM BRYOZOA

Microscopic organisms forming branching colonies; row of ciliated tentacles
at anterior end; usually marine: brvozoans (sea mosses).

PHYLUM BRACHIOPODA

Body enclosed in dorsal and ventral shells resembling those of a clam; two
spirally coiled arms within shell bearing a row of ciliated tentacles; simple cir¬
culatory' system; marine animals; mostly fossil forms: brachiopods, lampshells.

PHYLUM PHORONIDEA

Wormlike animals; sedentary and tube-dwelling; spirally coiled arm with
ciliated tentacles at anterior end; marine: Phoronis.

PHYLUM CHAETOGNATHA

Free-swimming, transparent, slender animals resembling arrows; mouth lined
with curved bristles; body divided into head, trunk, and tail with finlike projections;
marine: arrow worms.

APPENDIX 749

PHYLUM MOLLUSCA

Soft-bodied animals without segments or jointed appendages; most forms
secrete a valve, or calcareous shell, from a mantle; muscular foot usually present;
terrestrial, fresh-water, and marine animals: mollusks.

Class Amphineura : Elongated body and reduced head, without tentacles;
many forms with a shell composed of eight plates: chiton.

Class Pelecypoda: Axe-footed with bivalve shell; gills in mantle cavity; head,
eyes, and tentacles lacking: clam, oyster, scallop.

Class Gastropoda: Flat-footed, with or without coiled shell; head, distinct
eyes, and tentacles present: snail, slug, whelk.

Class Scaphopoda: Body elongated and enclosed in a tubular shell, open at
both ends; gills lacking; marine animals: tooth shells.

Class Cephalopoda: Head large; foot modified into grasping tentacles; marine
animals: squid, octopus, chambered nautilis, cuttlefish.

PHYLUM ANNELIDA

Segmented worms with the body cavity separated from the digestive tube;
brain dorsal and nerve cord ventral; body wall containing circular and longitudinal
muscles: segmented worms.

Class Polychaeta: Fleshy outgrowths, or parapodia extending from segments;
marine animals: sandworm (Nereis).

Class Archiannelida: Similar to Polychaeta but without parapodia and with
two rows of cilia: Polygordius.

Class Oligochaeta: Head not well developed; setae on body wall; terrestrial
and fresh-water forms: earthworm, Tubifex , Chaetogaster.

Class Hirudinea: Body flattened from top to bottom; no setae on body; suckers
at both ends; mostly fresh-water forms, but may occur as terrestrial or marine
organisms: leeches.

PHYLUM ARTHROPODA

Animals with segmented bodies, the segments bearing jointed appendages;
chitinous exoskeleton; aerial, terrestrial, and aquatic forms: arthropods.

Class Crustacea: Head and thorax joined in a cephalothorax; two pairs of
antennae; mostly aquatic; gills for respiration; many with calcareous deposits
in exoskeleton: crayfish, lobster, crab, shrimp, water flea, sowbug, barnacle.
Class Chilopoda: Body flattened and consisting of 15 to 170 or more seg¬
ments; one pair of legs attached to each segment; maxillipeds developed into
poison claws: centipedes.

Class Diplopoda: Body more or less cylindrical and composed of 25 to 100
or more segments; most segments bearing two pairs of legs: millepedes.

Class Arachnida: Head and thorax usually fused into a cephalothorax; antennae
lacking; four pairs of legs; lung books and tracheae for respiration: spiders,
scorpions, ticks, mites.

750 APPENDIX

Class lnsecta: Head, thorax, and abdomen separate; three pairs of legs; one
pair of antennae; usually two pairs of wings; tracheae for respiration: insects.

Order Thysanura: Wingless; chewing mouthparts; no metamorphosis;
primitive insects: silverfish.

Order Orthoptera: Two pairs of wings, the outer pair straight and leathery;
chewing mouthparts; incomplete metamorphosis: grasshoppers, cock¬
roaches, walking stick, mantis, crickets.

Order Isoptera : Some forms wingless, others with two pairs of long, nar¬
row wings lying flat on back; chewing mouthparts; incomplete meta¬
morphosis; social insects: termites.

Order Neuroptera: Four membranous wings of equal size, netted with
many veins; chewing mouthparts; complete metamorphosis; larvae of
some forms aquatic: dobson fly (hellgrammite), aphis lion.

Order Ephemerida: Two pairs of membranous wings, the front pair larger
than the hind pair; mouthparts nonfunctioning in adults; metamorphosis
incomplete; adults short-lived: Mayfly.

Order Lepidoptera : Four wings covered with colored scales; mouthparts
modified into a coiled, sucking proboscis; complete metamorphosis: but¬
terflies, moths, skippers.

Order Hymenoptera: Wingless or with two pairs of membranous wings,
the fore wings larger; fore wings and hind wings hooked together; chew¬
ing and sucking mouthparts; complete metamorphosis; many members
living in social colonies: bees, ants, wasps, hornets, ichneumon fly.

Order Odonata : Two pairs of strong, membranous wings, the hind pair as
large or larger than the fore pair; chewing mouthparts; incomplete meta¬
morphosis, compound eyes very large; larvae aquatic: dragonflies, damsel
flies.

Order Mallophaga: Wings absent; chewing mouthparts; incomplete met¬
amorphosis: chicken lice.

Order Anoplura: Wingless; piercing and sucking mouthparts; no meta¬
morphosis; external parasites on mammals: human body louse.

Order Coleoptera : Four wings, the front pair hard and shell-like, the
second pair folded and membranous; chewing mouthparts; complete
metamorphosis: beetles, ladybugs, firefly, boll weevil.

Order Hemiptera : Wingless, or with fore wings leathery at the base and
folded over the hind wings; piercing and sucking mouthparts; incomplete
metamorphosis: true bugs, water bug, water strider, water boatman, back
swimmer, bedbug, squash bug, stink bug.

Order Homoptera : Wingless, or with two pairs of wings held in a sloping
position like the sides of a roof; piercing and sucking mouthparts; incom¬
plete metamorphosis: cicada, aphids, leaf hopper, tree hopper, scale
insects.

Order Diptera : Fore wings membranous, hind wings reduced to knobbed
threads; mouthparts for piercing, rasping, and sucking; metamorphosis
complete: housefly, bot fly, blowfly, midge, mosquitoes, crane fly, gall gnat.

APPENDIX 751

Order Siphonaptera: Wingless; piercing and sucking mouthparts; com¬
plete metamorphosis; legs adapted for leaping; external parasites on
mammals: fleas.

PHYLUM ECHINODERMATA

Radially symmetrical; spiny exoskeleton composed, in some cases, of calcareous
plates; most forms with tube feet for locomotion; marine animals: echinoderms.

Class Crinoidea: Five branched rays and pinnules; tube feet without suckers;
most forms with stalk for attachment; many fossil forms: sea lily.

Class Asteroidea: Body usually with five rays and double rows of tube feet
in each ray; eyespot: starfish.

Class Ophiuroidea: Usually with five slender arms or rays: brittle stars.

Class Echinoidea : Body spherical, oval, or disk-shaped; rays lacking; tube feet
with suckers: sea urchin, sand dollar.

Class Holothurioidea: Elongated, thickened body with tentacles around the
mouth; no rays or spines: sea cucumber.

PHYLUM CHORDATA

Notochord present at some time, disappearing early in many forms; paired
gill slits temporary or permanent; dorsal nerve cord.

subphylum hemichordata: Wormlike chordates; body in three regions with
a proboscis, collar, and trunk: acorn worm (tongue worm).
subphylum tunicata ( urochordata ) : Marine animals with saclike body in
adult; free-swimming or attached: sea squirts and other tunicates.
subphylum cephalochordata: Fishlike animals with a permanent notochord:
lancelet ( Amphioxus ).

subphylum vertebrata: Chordates in which most of the notochord is re¬
placed by a spinal column composed of vertebrae and encasing the dorsal
nerve cord: vertebrates.

Class Cyclostomata ( Agnatha ): Fresh-water or marine eel-like forms without
true jaws, scales, or fins; cartilaginous skeleton: lamprey, hagfish.

Class Chondrichthyes (Elasmobranchii) : Fishlike forms with true jaws and
fins; gills present but not free and opening through gill slits; no air bladder;
cartilaginous skeleton: sharks, rays, skates.

Class Osteichthyes (Pisces): Fresh-water and marine fishes with gills free
and attached to gill arches; one gill opening on each side of body; true jaws
and fins; bony skeleton: bony fishes.

Subclass Ganoidei: Mostly extinct forms with armored body; heterocercal
tail; air bladder with duct: sturgeon, garpike, amia, fossil armored fish.
Subclass Teleostomi: Tail rarely heterocercal; air bladder present or absent,
with or without duct: perch, bass, trout, salmon, eel, catfish, shiner, flounder,
cod, haddock.

Subclass Dipnoi: Air bladder connected with throat and used as a rudi¬
mentary lung: lungfish.

752 APPENDIX

Class Amphibia: Fresh-water or terrestrial forms; gills present at some stage;
skin slimy and lacking protective outgrowths; limbs without claws; numerous
eggs, usually laid in water; metamorphosis: amphibians.

Order Apoda: Wormlike amphibians with tail short or lacking; without
limbs or limb girdles; small scales embedded in the skin in some forms:
caecilians.

Order Caudata: Body elongated and with a tail throughout life; scales
lacking; most forms with two pairs of limbs: salamanders, newts, sirens.
Order Salientia: Body short and tailless in adult stage; two pairs of limbs,
the hind limbs adapted for leaping; gills in larval stage, lungs in adult
stage: frogs, toads, tree frogs.

Class Reptilia : Terrestrial or semiaquatic vertebrates; breathe by lungs at all
stages; body scale-covered; feet, if present, provided with claws; eggs provided
with a leathery, protective shell; fertilization internal; oviparous or ovovivipa-
rous reptiles.

Order Testudinata ( Chelonia ): Body enclosed between two bony shields
or shells, usually covered with large scales; toothless: turtles, terrapins,
tortoises.

Order Rhynchocephalia: Skeletal characteristics of the oldest fossil rep¬
tiles; lizardlike in form; parietal eye in roof of cranium: tuatara ( Sphe -
nodon).

Order Squamata: Body elongated; with or without limbs (vestigial in
snakes); body covered with scales which are molted with outer skin at
regular intervals: snakes, lizards.

Order Crocodilia : Large, heavy-scaled body with strong, muscular tail;
heart approaching four-chambered structure: alligators, crocodiles, cay¬
mans, gavials.

Class Aves: Body covered with feathers; forelimbs modified into wings; four
chambered heart and double circulation; bones containing air cavities; lung
breathing throughout life: birds.

Order Gaviiformes: Loons: common loon.

Order Columbiformes: Grebes: pied-billed grebe.

Order Pelecaniformes: Tropical birds: white pelican, brown pelican, cor¬
morant.

Order Ciconiiformes: Long-legged wading birds: heron, bittern, ibis,
spoonbill, flamingo.

Order Anseriformes: Short-legged gooselike birds: duck, goose, swan.
Order Falconiformes: Large birds of prey: hawk, falcon, eagle, kite, vul¬
ture, buzzard, condor.

Order Galliformes : Fowl-like birds: pheasant, turkey, quail, partridge,
grouse, ptarmigan.

Order Gruiformes : Cranelike birds: crane, coot, gallinule, rail, limpkin.

APPENDIX 753

Order Charadriiformes : Shore birds: snipe, sandpiper, plover, gull, tern,
auk, puffin.

Order Columbiformes: Pigeons and doves: mourning dove, white-winged
dove.

Order Psittaciform.es: Parrots and parrotlike birds: parrots, parakeet,
macaws.

Order Cuculiformes : Cuckoos: cuckoo, roadrunner.

Order Strigiformes : Nocturnal birds of prey: owls.

Order Caprimulgiformes: Goatsuckers: whippoorwill, chuck-will’s widow,
nighthawk.

Order Apodiformes : Swifts: chimney swift, hummingbird.

Order Coraciiformes: Fishing birds: kingfisher.

Order Piciformes : Woodpeckers: woodpecker, sapsucker, flicker.

Order Passeriformes : Perching birds: robin, bluebirds, sparrow, warbler,
thrush.

Class Mammalia: Body more or less covered with hair; warm-blooded with
four-chambered heart; mammary glands; diaphragm; central nervous system
highly developed; viviparous except in one order.

Order Monotremata: Egg-laying mammals.: duckbilled platypus, spiny
anteater.

Order Marsupialia : Pouched mammels: opossum, kangaroo, Koala bear.
Order Insectivora: Insect-eating mammals: mole, shrew.

Order Chiroptera : Flying, or hand-winged mammals: bat, vampire.
Order Edentata : Toothless mammals: armadillo, sloth, great anteater.
Order Rodentia: Gnawing mammals: squirrel, woodchuck, prairie dog,
chipmunk, mouse, rat, muskrat.

Order Lagomorpha : Rodentlike mammals: rabbits, hare, pika.

Order Cetacea : Marine mammals: whale, porpoise, dolphin.

Order Sirenia : Aquatic mammals: sea cow.

Order Proboscidea : Trunk-nosed mammals: elephant, fossil mammoth,
fossil mastodon.

Order Carnivora: Flesh-eating mammals: bear, raccoon, ring-tailed cat,
weasel, mink, otter, skunk, lion, tiger, cat, dog, fox, wolf.

Order Ungulata: Hoofed mammals: odd-toed ( Order Perissodactyla in
some classifications) — horse, tapir, rhinoceros; even-toed (Order Artrio-
dactyla in some classifications) — bison, cow, goat, sheep, deer, antelope,
camel, llama, pig, hippopotamus.

Order Primates: Erect mammals: monkey, lemur, marmoset, gibbon,
orangutan, gorilla, chimpanzee, man.

754 APPENDIX

A TRADITIONAL CLASSIFICATION OF ORGANISMS

KINGDOM PLANT AE

PHYLUM THALLOPHYTA

SUBPHYLUM ALGAE

Class Cyanophyceae: blue-green algae
Class Chlorophyceae: green algae

Class Chry sophy ceae: yellow-green algae, golden-brown algae, diatoms
Class Phaeophyceae: brown algae
Class Rhodophyceae: red algae

SUBPHYLUM FUNGI

Class Schizomycetes: bacteria, rickettsiae, actinomycetes, spirochetes

Class Myxomycetes: slime fungi

Class Phycomycetes: algalike fungi

Class Ascomycetes: sac fungi

Class Basidiomycetes: basidium fungi

Class Deuteromycetes: imperfect fungi

PHYLUM BRYOPHYTA

Class Hepaticae: liverworts, hornworts
Class Musci: mosses

PHYLUM PTERIDOPHYTA

Class Filicineae: ferns
Class Equisetineae: horsetails
Class Lycopodiaceae: club mosses

PHYLUM SPERMATOPHYTA

Class Gymnospermae : gymnosperms

Order Cycadales : cycads or sago palms
Order Ginkgoales: Ginkgo
Order Coniferales: pine, spruce, fir, cedar, larch
Order Gnetales: Ephedra , W elwitschia, Gnetum

Class Angiospermae: flowering plants
Subclass Monocotyledonae
Subclass Dicotyledonae

KINGDOM ANIMALIA

PHYLUM PROTOZOA

Class Sarcodina: amoeboid organisms
Class Mastigophora: flagellates
Class Sporozoa: sporozoans
Class Ciliata: ciliates

(The additional phyla in the Traditional Classification are the same as those listed
in the Modern Classification.)

GLOSSARY

abdomen, the body region posterior to the
thorax.

abscission layer, the two rows of cells near
the base of a leaf peticle that are
involved in the natural falling of the
leaf.

absorption, the process by which water and
dissolved substances pass into cells,
acromegaly, abnormal growth, especially of
the bones of the face and extremities, as¬
sociated with malfunctioning of the an¬
terior lobe of the pituitary gland.

ACTH, a hormone secretion of the anterior
lobe of the pituitary gland which stimu¬
lates the cortex of the adrenal glands,
active transport, the passage of a substance
through a cell membrane by means of
energy.

adaptation, an adjustment to conditions in
an environment.

adaptive radiation, a branching out of a pop¬
ulation through variation and adaptation
to occupy many environments,
addiction, the body’s need for a drug which
has been developed by the use of the
drug.

adductor muscles, those in bivalves that con¬
trol the opening and closing of the valves,
adenoid, a mass of lymph tissue that grows
from the back wall of the nasopharynx,
behind the internal nares.

ADP (adenosine diphosphate), a low energy
compound found in cells which functions
in energy storage and transfer,
adrenal gland, a ductless gland located above
the kidney, often referred to as the
“gland of emergency.”

adventitious root, one which develops from
the node of a stem or from a leaf,
aeciospore, a spore produced in an aecium
cup on a leaf of common barberry in the
life cycle of wheat rust,
aerial roots and stems, those which do not
enter the ground.

aerobic, requiring free atmospheric oxygen
for normal activity.

agglutinin, an immune substance in the blood
which causes specific substances, includ¬
ing bacteria, to clump.

agglutinogen, a protein substance on a cor¬
puscle surface which is responsible for
blood types.

air bladder, a thin-walled, elliptical sac found
in fish which allows the animal to main¬
tain a level in the water,
air sacs, in insects, the enlarged spaces in
which the tracheae terminate; in birds,
cavities extending from the lungs; in man,
thin- walled divisions of the lungs,
albumen, a protein substance that surrounds
the yolk of a bird’s egg.
alimentary canal, those organs composing the
food tube in animals and man.
allantois, an extraembryonic membrane. In
birds and reptiles it serves as an em¬
bryonic lung.

allele, one of a pair of genes for contrasting
traits.

allergy, an abnormal reaction in some people
to certain foods, drugs, and pollens,
alternation of generations, a type of fife cycle
in which the asexual reproductive stage
alternates with the sexual reproductive
stage.

alveoli, microscopic protrusions in the lungs
in which the exchange of gases takes
place.

amebocytes, amebalike cells in sponges which
function in circulation and excretion,
amino acids, substances from which organisms
build protein; the end-products of pro¬
tein digestion.

ammonification, the release of ammonia from
decaying protein by means of bacterial
action.

amnion, the innermost fetal membrane, form¬
ing the sac that encloses the fetus and
forms a sheath for the umbilical cord,
amniote egg, an egg having an amnion, a
membrane that can be seen beneath the
shell of a bird’s egg.

amylase, an enzyme or the pancreatic fluid
which changes starch to maltose,
anabolism, the constructive phase of metabo¬
lism.

anaerobic, deriving oxygen for fife activ¬
ity from chemical changes and, in some
organisms, being unable to five actively
in free oxygen.

anal pore, an opening in the pellicle of the
paramecium by which wastes leave the
animal.

analogous organs, those which are similar in
function.

755

756 GLOSSARY

anaphase, a stage of mitosis during which
chromosomes migrate from the equator
to opposite poles.

anatomy, the study of the structure of living
things.

annual, a plant which lives for only one sea¬
son.

annual ring, a circle in the stem of a plant
marking a season’s growth of wood,
annulus, the ring on the stipe of a mushroom
marking the point where the rim of the
cap and stipe were joined,
antenna, a large “feeler” in insects and cer¬
tain other animals.

antennule, a small “feeler” in the crayfish and
certain other animals,
anterior, head, or front, end.
anther, that part of the stamen which bears
pollen grains.

antheridium, a sperm-producing structure,
anthocyanin, a red, blue, or purple pigment
dissolved in cell sap.

anthropology, the study of man and the so¬
cieties in which he groups himself,
antibiotic, a germ-killing substance produced
by a bacterium, mold, or other fungus
plant.

antibody, an immune substance in the blood
and body fluids.

antigen, a substance, usually a protein, which
when introduced into the body stimu¬
lates the formation of antibodies,
antipodals, three nuclei found in the embryo
sac at the end farthest from the micro-
pyle.

antitoxin, a substance in the blood which
counteracts a specific toxin,
antivenin, a serum used against snakebite,
anus, the opening at the posterior end of the
intestine.

aorta, the great artery leading from the heart
to the body (arising from the left ventri¬
cle in the bird and mammal),
aortic arch, an arching curve in the aorta,
near the heart.

apical cell, the terminal, or tip, cell of a
growing plant.

apical dominance, the influence of the termi¬
nal bud by its production of auxin which
inhibits the growth of lateral buds,
appendage, an outgrowth of the body of an
animal, such as a leg, fin, or antenna,
applied science, practical use of knowledge
gained from pure science,
aquatic, living in water.

aqueous humor, the watery fluid filling the
cavity between the cornea and lens of the
eye.

arachnoid mater, the middle of the three
membranes of the brain and spinal cord,
archegonia, an egg-producing structure,
arteriole, a tiny artery which eventually
branches to become capillaries,
artery, a large, muscular vessel that carries
blood away from the heart,
artificial pollination, controlled pollination to
produce a hybrid plant from selected
parents.

artificial respiration, a method of artificially

forcing the lungs to inspire and expire
rhythmically.

ascus, in Ascomycetes, the saclike structure
which contains the spores,
asexual reproduction, reproduction without
eggs and sperm.

association fibers, nerve processes connecting
different parts of the cerebral cortex,
asters, the fibrils that form and radiate from
the centriolelike rays from a star during
cell division.

asymmetrical, having no definite shape,
atom, the smallest unit into which an element
can be broken without losing its identity,
atomic mass, the sum of the protons and neu¬
trons in the atom of an element,
atomic number, the number of protons in the
nucleus of an atom. No two elements
have atoms with the same number of
protons.

ATP ( adenosine triphosphate ) , a high-energy
compound found in cells which func¬
tions in energy storage and transfer,
atrioventricular node, the structure in the
heart which relays the beat to the mus¬
cles of the lower heart chamber,
atrioventricular valves, the heart valves lo¬
cated between the auricles and ventri¬
cles.

atrium, a thin-walled upper chamber of the
heart.

auditory nerve, the nerve leading from the
inner ear to the brain.

autonomic nervous system, a division of
the nervous system which regulates the
vital internal organs in an involuntary
manner.

autosome, any paired chromosome other than
the sex chromosomes.

autotrophs, organisms capable of organizing
organic molecules from inorganic mole¬
cules.

axil, the angle between a leaf stalk and a
stem.

axillary bud, a lateral bud produced in a leaf
axil.

axon, a nerve process which carries an im¬
pulse away from the nerve body.

bacillus, a rod-shaped bacterium,
bacteria, a group of microscopic, one-celled
protists.

bacteriology, the study of bacteria,
bacteriolysin, a blood antibody which causes
a specific kind of bacteria to dissolve,
bacteriophage, one of several kinds of viruses
that can destroy bacteria,
barb, a tiny ray in the vane of a feather,
barbule, one of the divisions of the barb of a
feather.

bark, the outer region of a woody stem, com¬
posed of several kinds of tissue,
barrier, anything that prevents the spread of
organisms to a new environment,
basal disk, the structure that secretes a slimy
material by which certain coelenterates
are attached to the substrate,
basal metabolism, the activities required to
maintain the body and to supply the en-

GLOSSARY 757

ergy necessary to support the basic life
processes.

basidiospore, a reproductive structure found
in the basidiomycetes, or club fungi,
basidium, a club-shaped structure found
in the club fungi that bears spores,
bast fiber, a tough, thick-walled plant fiber
that serves as a supporting structure in
the phloem region,
belly, the body of a striated muscle,
beta particles, high-speed electrons emitted
by radioactive materials,
biennial, a plant that lives two seasons,
bilateral symmetry, the type exhibited by or¬
ganism that may be divided into two
equal parts by only one plane,
bile, a brownish-green emulsifying fluid se¬
creted by the fiver and stored in the gall
bladder.

binary fission, the division of cells into two
approximately equal parts,
binomial nomenclature, the system of giving
an organism a two-part name,
biogenesis, the biological principle that life
arises from fife.

biogeography, the study of the distribution of
plants and animals throughout the vari¬
ous regions of the earth,
biology, the science of fife,
biome, a large geographical region identified
mainly by its climax vegetation,
biosphere, the area in which fife is possible
on our planet.

biosynthesis, the organization of organic mole¬
cules by living organisms,
biotic community, all the living organisms in
an ecosystem.

biotic factors, the living surroundings of an
organism.

bivalve, a mollusk possessing two valves, or
shells.

blastocoel, the space between the ectoderm
and endoderm in the early stages of em-
bryological development,
blastula, an early stage in the development of
an embryo, in which cells have divided
to produce a hollow sphere,
blood, the fluid tissue of the body,
bony layer, the hard region of a bone be¬
tween the periosteum and the mar¬
row.

botulism, a severe type of food poisoning
caused by a bacillus.

Bowman’s capsule, the tuft-shaped structure
forming one end of the tubules in the
nephron of the kidney.

brain stem, an enlargement at the base of the
brain where it connects with the spinal
cord.

branchial arteries, those that lead to and from
the gills of the fish.

breathing, the mechanical process of getting
air into and out of the body,
bronchial tube, a subdivision of a bronchus
within a lung.

bronchiole, one of numerous subdivisions of
the bronchial tubes within a lung,
bronchus, a division of the lower end of the
trachea, leading to a lung.

bud, an undeveloped shoot of a plant, often
covered by scales.

budding, the uniting of a bud with a stock;
a form of asexual reproduction in yeasts
and hydra.

bud-mutant, the offspring of a plant in which
a mutation has occurred,
bud scales, small leaflike structures which
completely enclose the tender growing
point to protect the delicate tissues in¬
side from drying and mechanical dam¬
age.

bud-scale scar, a mark at intervals along a
twig which shows where the bud scales
of a terminal bud were fastened during
a previous season.

bulb, a large underground bud protected by
scales.

bulbus arteriosus, a muscular bulblike struc¬
ture attached to the ventricle in the fish
heart.

Calorie (large), the amount of heat required
to raise the temperature of 1,000 grams
of water one degree centigrade,
cambium, the tissue in roots and stems respon¬
sible for growth in diameter,
capillary, a tiny blood vessel through which ex¬
change of gases, foods, and wastes takes
place between the blood and tissue fluid,
catabolism, the destructive phase of metabo¬
lism.

catalyst, a substance that accelerates a chem¬
ical reaction without itself being per¬
manently altered chemically,
cell wall, the outer, nonliving, cellulose wall
secreted around plant cells,
cellulose, a carbohydrate substance present in
the walls of plant cells,
cementum, the covering of the root of a
tooth.

central cylinder, the central core of a root,
where conduction occurs,
central nervous system, the brain and spinal
cord.

central neurons, those in the brain and spinal
cord that connect motor nerves to sen¬
sory nerves.

centriole, a cytoplasmic body lying just out¬
side the nucleus in animal cells. Its divi¬
sion is perhaps the earliest event in cell
division.

centromere, a single granule that, during cell
division, attaches after replication and
just before separation.

cephalothorax, a body region in crustaceans
and certain other animals consisting of
the head and thorax.

cerebellum, the brain region between the
cerebrum and medulla, concerned with
equilibrium and muscular coordination,
cerebrospinal fluid, a clear fluid in the brain
ventricles and surrounding the spinal
cord.

cerebrum, the largest region of the human
brain, considered to be the seat of emo¬
tions, intelligence, and other nervous ac¬
tivities.

cervix, the neck of the uterus.

758 GLOSSARY

chemical change, matter changing from one
substance to another as a result of a
chemical reaction.

chemosynthesis, the organization of carbohy¬
drates by organisms by means of energy
from inorganic chemical reactions in¬
stead of energy from light,
chitin, a material present in the exoskeleton
of insects and other arthropods,
chlorophyll, green pigments essential to food
manufacture in plants.

chloroplast, a plastid containing chlorophyll,
chondriosome, a structure in the cell whose
exact function is unknown,
chorion, a membrane that forms early during
development and attaches to the uterine
wall.

chorionic villi, small, fingerlike projections
attaching the chorion to the uterine
wall.

choroid layer, the second and innermost layer
of the eyeball.

chromatid, during cell division, each part of
a double chromosome after duplication,
chromatophores, structures containing pig¬
ments in the skin of fish and other an¬
imals.

chromoplasts, plastids containing pigments
other than chlorophyll,
chromosome, a rod-shaped gene-bearing body
in the cell nucleus. It is composed of
DNA joined to protein molecules,
chrysalis, a hard case containing the pupal
stage of a butterfly.

chyme, partly digested, acidic food as it
leaves the stomach.

cilia, tiny hairlike projections of cytoplasm,
ciliary muscles, those which control the shape
of the lens.

cleavage, the division of the cytoplasm into
two approximately equal parts during
cell division.

cleavage furrow, an indentation that appears
during the telophase of dividing animal
cells.

climax plant, a species that assumes final
prominence in a region,
clitellum, a swelling on the earthworm in¬
volved in reproduction,
cloaca, a chamber below the large intestine
in certain vertebrates into which the ali¬
mentary canal, ureters, bladder, and re¬
productive organs empty,
coccus, a sphere-shaped bacterium,
cochlea, the hearing apparatus of the inner
ear.

cocoon, a silken case containing the pupal
stage of a moth.

coeolom, the space between the mesodermal
layers that forms the body cavity of the
animal.

cohesion, the clinging together of molecules,
as in a column of liquid,
collar cells, flagellated cells in sponges which
set up water currents.

colloid, a gelatinous substance such as proto¬
plasm or egg albumen in which one or
more solids are dispersed through a
liquid.

colon, the large intestine.

colonial, living in a group, with each indi¬
vidual living independently but in close
association with the others,
comb, the structure in the beehive composed
of six-sided cells made of wax.
commensalism, one organism living in or on
another, with only one of the two bene¬
fiting.

companion cells, long, narrow, nucleated cells
bordering sieve tubes in phloem tissue,
complete metamorphosis, four stages of de¬
velopment of certain insects — egg, larva,
pupa, and adult.

compost, a mixture of leaves and soil used
for fertilizing and conditioning land,
compound, two or more elements combined
chemically.

compound eye, an eye composed of numerous
lenses and containing separate nerve end¬
ings, as in insects and crustaceans,
compound leaf, a leaf in which the blade is
divided into leaflets.

conditioned reaction, a behavior pattern in
which a particular response continually
follows a specific stimulus,
cone, a reproductive part of a conifer; also a
color-sensitive nerve ending in the retina
of the eye.

conifer, a cone-bearing gymnosperm.
conjugation, a primitive form of sexual repro¬
duction in Spirogyra and certain other
algae and fungi in which the content of
two cells unite; also an exchange of nu¬
clear substance in the paramecium, re¬
sulting in rejuvenation of the cells,
connective tissue, a type of tissue that lies be¬
tween groups of nerve, gland, and mus¬
cle cells and beneath epithelial cells; also
includes bone, cartilage, blood, and
lymph.

conservation, the preservation and wise use of
natural resources.

consumers, the heterotrophs in an environ¬
ment.

contact infection, a disease spread through
direct contact with an infected person,
contour farming, the practice of following the
contour around a slope in plowing,
contour feathers, those that cover a bird’s
body and give it a smooth outline,
contractile vacuole, a large cavity in proto¬
zoans associated with the discharge of
water from the cell and the regulating of
osmotic pressure.

contraction (muscle), the shortening of a
striated muscle.

conus arteriosus, a large vessel lying against
the front side of the frog’s heart and
leading from the ventricle,
convergent evolution, the type in which or¬
ganisms of entirely different origin evolve
in a manner that results in certain simi¬
larities.

convolution, one of many irregular, rounded
ridges on the surface of the brain,
cork, a tissue formed by the cork cambium
which replaces the epidermis in woody
stems and roots.

GLOSSARY 759

cork cambium, a layer of cells in the outer
bark which produces new cork,
corm, a shortened underground stem in which
the leaves are reduced to thin scales,
cornea, a transparent bulge of the sclerotic
layer of the eye in front of the iris,
through which light rays pass,
corolla, the petals of a flower, collectively,
coronary, pertaining to the heart,
corpus luteum, refers to the follicle after the
ovum is discharged.

corridor, a pathway that allows the spread of
a population to new areas,
cortex, in roots and stems, a storage tissue; in
organs such as the kidney and brain, the
outer region.

cortin, a hormone complex secreted by the
cortex of the adrenal glands,
cotyledon, a seed leaf present in the embryo
plant that serves as a food reservoir,
countershading, a form of protective colora¬
tion in which darker colors on the upper
side of the animal fade into lighter colors
on the lower side.

covalent bond, a force holding atoms together
that results from the sharing of pairs of
electrons.

cover crop, a crop such as wheat and oats in
which the plants grow close together and
bind the soil with their closely mingled
roots.

coverts, the small feathers that cover the
lower portions of the quill feathers in
birds.

Cowper’s gland, located near the upper end
of the male urethra. It secretes a fluid
which is added to the sperm,
coxa, a joint of the leg of the grasshopper
which, with the trochanter, acts like a
ball and socket.

cranial cavity, the cavity in the skull contain¬
ing the brain.

cranial nerves, the twelve pairs of nerves con¬
nected to the human brain,
cretinism, a stunted condition resulting from
lack of thyroid secretion,
crop, an organ of the alimentary canal of the
earthworm, bird, and certain other ani¬
mals which serves for food storage,
crop rotation, alternation in the planting of
crops that use nitrates with those that
replace nitrates.

cross-pollination, the transfer of pollen from
the anther of one plant to the stigma of
another.

crossing-over, the exchange of segments of
two chromosomes, and the genes in the
segments, when the two chromosomes
He side by side during reduction division,
crown, the exposed portion of a tooth above
the gum line.

culture medium, a nutrient mixture used for
growing bacteria, molds, and other fungi,
cuticle, a waxy, transparent layer covering tne
upper epidermis of certain leaves; the
outer covering of an earthworm,
cyst, in lower animals and plants, a spore
with a capsule covering constituting a
resting stage.

cytoplasm, the protoplasm lying outside the
nucleus of a cell.

dark reaction, the second stage of photosyn¬
thesis during which carbon is fixed in
a series of chemical reactions, none of
which require light.

daughter cell, a newly formed cell resulting
from the division of a previously existing
cell called a mother cell. The two
daughter cells receive nuclear materials
that are identical.

decay, the reduction of the substances of a
plant or animal body to simple com¬
pounds by the action, usually, of bac¬
teria.

deciduous, woody plants that shed their leaves
seasonally.

decomposers, organisms that break the tissues
and excretions of other organisms into
simpler substances through the process
of decay.

deficiency disease, a condition resulting from
the lack of one or more vitamins,
degeneration, the loss of a system or struc¬
ture by an animal.

dehiscent, a class of fruits that open and dis¬
charge seeds.

dehydration, loss of water from body tissues,
deliquescent, a type of branching in which
the trunk divides into several main
branches, resulting in a wide, spreading
crown.

dendrite, a branching nerve process which
carries an impulse toward the nerve cell
body.

denitrification, the process carried on by deni¬
trifying bacteria of breaking down am¬
monia, nitrites, and liberating free nitro¬
gen.

dentine, a substance that is relatively softer
than enamel, forming the bulk of the
tooth.

deoxygenation, the process during which oxy¬
gen is removed from the blood or tissues,
depletion, mineral exhaustion of the soil
through continued planting of agricul¬
tural crops without proper fertilizing,
dermis, the skin layer beneath the epidermis,
diabetes mellitus, a condition resulting from
lack of insulin so that the body cannot
store or oxidize sugar efficiently,
diaphragm, a muscular partition separating
the thoracic cavity from the abdominal
cavity.

diastase, an enzyme in green plants that
changes starch to sugar,
diastolic blood pressure, arterial blood pres¬
sure maintained between heartbeats,
dicotyledon, a seed plant with two seed
leaves, or cotyledons.

differentially permeable membrane, one which
is permeable to different substances to
different degrees.

diffuse root system, one composed of spread¬
ing roots of similar size,
diffusion, the spreading out of molecules in a
given space.

diffusion pressure, the force resulting from

760 GLOSSARY

differences in molecular concentration,
temperature, and pressure,
digestion, the process during which foods are
chemically simplified and made soluble
so that they can be used by the cells,
dihybrid, an offspring having genes for two
contrasting characters.

diploid number, the full set of chromosomes
in a nucleus, with both members of each
pair present.

diurnal, active during the day.
division of labor, specialization of cell func¬
tions resulting in interdependence,
division plate, a wall of cellulose which forms
across the dividing plant cell forming a
common boundary between daughter
cells.

DNA (deoxyribonucleic acid), a super mole¬
cule consisting of alternating units of de-
oxyribose sugar, phosphates, and organic
bases. DNA transmits hereditary infor¬
mation and controls cellular activities,
dominant, in genetics, refers to a trait that ap¬
pears in a hybrid,
dormancy, a period of inactivity,
dorsal, pertaining to the upper surface of an
animal.

double-cross, one in which four pure-line par¬
ents are mixed in two crosses,
down feathers, those that form the plumage
of newly hatched birds,
drone, the male bee.

droplet infection, a disease spread through
cough or sneeze droplets bearing micro¬
organisms from the respiratory tract or
mouth.

ductus arteriosus, a connection between the
pulmonary artery and the aorta during
fetal life. It closes at birth,
duodenum, the region of the small intestine
immediately following the stomach; in
man, about 10 inches long,
dura mater, the outer of the three membranes
of the brain and spinal cord.

ecology, the study of the relationship of liv¬
ing things to their surroundings,
ecosystem, a unit of the biosphere in which
living and nonliving things interact, and
in which materials are used over and
over again.

ectoderm, the outer layer of cells of a simple
animal body; in vertebrates, the layer of
cells from which the skin and nervous
system develop.

ectoplasm, the outer layer of thin, clear cy¬
toplasm, as in the ameba.
elongation region, the area behind the em¬
bryonic region of a root or stem in which
cells grow in length,
embryo, a developing organism,
embryo sac, the tissue in a plant ovule that
contains the egg, the antipodals, the
polar nuclei, and the synergids.
embryonic region, the area near the tip of
a root or stem in which cells are formed
by division.

enamel, the hard covering of the crown of
a tooth.

endocrine gland, a ductless gland which se¬
cretes hormones directly into the blood,
endoderm, the inner layer of cells of a sim¬
ple animal body; in vertebrates, the
layer of cells from which the lining of
the digestive system, the liver, the lungs,
etc. develop.

endodermis, a single layer of cells located
at the inner edge of the cortex of a root,
endoplasm, the inner layer of cytoplasm, as
in the ameba.

endoplasmic reticulum, a complex system of
membranes which tend to fie parallel to
one in the cytoplasm.

endosperm, the tissue in some seeds contain¬
ing stored food,
enzyme, an organic catalyst,
epidermis, the outer tissue of a young root or
stem, a leaf, and other plant parts; the
outer layer of skin.

epithelial tissue, that composing the covering
of various body organs,
erepsin, a digestive enzyme of the intestinal
fluid which changes peptides to amino
acids.

erosion, the loss of soil by the action of water
or wind.

esophagus, the food tube, or gullet, which
connects the mouth and the stomach,
estivation, a period of summer inactivity in
certain animals.

estrogen, a female hormone secreted by the
ovaries.

eugenics, the science of human heredity.
Eustachian tube, a tube connecting the phar¬
ynx with the middle ear.
evaporation, movement of water from the
earth to the atmosphere,
evolution, the slow process of change by
which organisms have acquired their dis¬
tinguishing characteristics,
excretion, the process by which waste mate¬
rials are removed from living cells or
from the body.

excurrent, a form of branching in which a
single stem extends through a plant as
a shaft.

excurrent pore, in sponges, the osculum.
excurrent siphon, the structure in the clam
through which water passes out of the
body.

exoskeleton, the hard, outer covering or skele¬
ton of certain animals, especially arthro¬
pods.

exotoxin, a soluble toxin excreted by certain
bacteria and absorbed by the tissues of
the host.

expiration, the discharge of air from the lungs,
extensor, a muscle which straightens a joint,
external respiration, the exchange of gases
between the atmosphere and the blood,
extinct, no longer in existence,
extraembryonic membrane, a membrane which
functions during development of a mam¬
mal, but does not become part of the
embryo.

eyespot, the sensory structure in the euglena
and planaria that is believed to perceive
fight and dark.

GLOSSARY 761

facultative anaerobes, organisms that grow
best as aerobes but may grow, at least
to some extent, as anaerobes.

Fallopian tube, oviduct in the mammal,
fang, a hollow tooth of a poisonous snake
through which venom is ejected,
farm ponds, artificial bodies of water which
farmers may make and stock with some
species of fish.

fat bodies, structures in the frog that store
fat.

fats, a class of foods which supplies energy
to the body.

fatty acids, one of the end-products of fat
digestion.

feces, solid intestinal waste material,
femur, the long bone of the upper leg.
fermentation, glucose oxidation that is anaero¬
bic and in which lactic acid or alcohol is
formed.

fertilization, the union of sperm and egg.
fetus, mammalian embryo after the main
body features are apparent,
fibrin, a substance formed during blood clot¬
ting by the union of thrombin and fibrin¬
ogen.

fibrinogen, a blood protein present in the
plasma, involved in clotting,
fibrous root, a small, slender secondary root
that is generally very much branched,
fibrovascular bundle, a strand containing xy-
lem and phloem tissues in higher plants,
filament, a stalk of a stamen, bearing the an¬
ther at its tip; in algae, a threadlike
group of cells.

filoplume, the slender, hairlike feathers in
birds, having a tuft at the end.
filterable virus, a virus that passes through
the extremely small pores of unglazed
porcelain filters used in separating bac¬
teria from fluids.

fin, a membranous appendage of a fish and
certain other aquatic animals,
fire line, a lane cut through a forest to pre¬
vent a possible fire from spreading,
fish ladder, channel around a dam through
which fish can travel upstream,
flagellate, an organism bearing one or more
whiplike appendages, or flagella,
flagellum, a whiplike projection of cytoplasm
used in locomotion by certain simple or¬
ganisms.

fleshy root, an enlarged root which serves as
a reservoir of food for the plant,
flexor, a muscle which bends a joint,
flower, an .organ of a flowering plant spe¬
cialized for reproduction,
follicle, an indentation in the skin from which
hair grows; a mass of ovarian cells which
produces an ovum.

food, any substance absorbed into the body
which yields material for energy, growth,
and repair of tissue and regulation of the
fife processes, without harming the or¬
ganism.

food-borne infection, any infection caused by
contaminated food.

food chain, the transfer of the sun’s energy
as organisms feed on one another.

food infection, the introduction of infectious
organisms into the body by means of
food.

food poisoning, a condition resulting from
the action of preformed toxins present
in food.

food pyramid, a quantitative representation of
a food chain with the food producers
forming the base and the top carnivore
at the apex.

forage fishes, species that have no sport value
but are vital as a food for game fishes,
forebrain, that part of the brain composed of
the cerebrum.

fovea, a small, sensitive spot on the retina of
the eye where cones are specially abun¬
dant.

fraternal twins, those that are produced from
two separately fertilized eggs,
frond, the leaf of a fern,
fruit, a ripened ovary, with or without as¬
sociated parts.

FSH (follicle-stimulating hormone), a hor¬
mone produced by the anterior lobe of
the pituitary.

fungus, a protist lacking chlorophyll and
therefore deriving nourishment from an
organic source.

gallbladder, a sac in which bile from the fiver
is stored and concentrated,
game birds, those that have sport value for
hunters.

game fishes, those that have sport value,
gamete, a male or female reproductive cell,
or germ cell.

gametophyte, the stage that produces gam¬
etes in an organism having alternation
of generations.

gamma globulin, a blood protein sometimes
used to give temporary immunity to
polio.

ganglion, a mass of nerve cells lying outside
of the central nervous system,
gastric caeca, structures in the digestive sys¬
tem of the grasshopper,
gastrocoel, the central cavity occurring in the
early stages of embryonic development,
gastrovascular cavity, the central cavity of
Coelenterata.

gastrula, a stage in embryonic development
during which the primary germ layers are
formed.

gemmule, a coated cell mass produced by the
parent sponge and capable of growing
into an adult sponge.

gene, that portion of a DNA molecule that
is genetically active and produces a trait,
gene frequency, the extent to which a gene
occurs in a population,
gene linkage, the assemblage of genes in a
linear arrangement on a chromosome,
gene popl, all the genes present in a given
population.

generative nucleus, the nucleus in a pollen
grain that divides to form two sperm,
genetic code, the sequential arrangement of
the bases in the DNA molecule which
controls the traits of an organism.

762 GLOSSARY

genetics, the science of heredity,
genotype, the hereditary make-up of an or¬
ganism.

genus, a group of closely related species,
geologist, a scientist who studies the earth,
geotropism, the response of plants to gravity,
germination, growth of the seed when favor¬
able conditions occur.

gestation period, the period between fertiliza¬
tion and birth of a mammal,
gill, an organ modified for absorbing dissolved
oxyen from water; in mushrooms, a plate¬
like structure bearing the reproductive
organs.

gill arch, a cartilaginous structure in fish to
which the gill filaments are attached,
gill filament, one of many threadlike projec¬
tions forming the gills in fish,
gill raker, fingerlike projections of the gill
arches in fish.

gizzard, an organ in the digestive system of
the earthworm and birds modified for
grinding food.

glomerulus, the knob of capillaries in a
Bowman’s capsule.

glottis, the upper opening of the trachea in
land vertebrates.

glucose, a simple sugar, or monosaccharide,
which is a product of photosynthesis and
which is also an end-product of digestion,
glycerin, one of the end-products of fat di¬
gestion.

glycogen, animal starch, formed in the liver
and muscles.

goiter (simple), an enlarged condition of
the thyroid gland, resulting from iodine
deficiency.

Golgi bodies (Golgi apparatus), small groups
of parallel membranes in the cytoplasm
near the nucleus, especially prominent
in secretory cells. Their function is
not known.

gonadotropic hormone, a hormone of the an¬
terior lobe of the pituitary gland which
influences activity of the reproductive
organs.

gonads, the male and female reproductive or¬
gans.

grafting, the union of the cambium layers of
two woody stems, one the stock and the
other the scion.

green gland, an excretory organ of crusta¬
ceans.

ground water, that which enters the soil fol¬
lowing precipitation.

guard cell, one of the two epidermal cells sur¬
rounding a stoma.

gullet, the passageway to a food vacuole in
paramecia; the food tube or esophagus,
gully erosion, an advanced stage of water
erosion following rill erosion.

habitat, place where an organism lives,
half-life, time required for one-half of a
iven amount of radioactive substance to
isintegrate.

haploid number, half the number of chromo¬
somes that are ordinarily present in the
nucleus; occurs during reduction division.

haustoria, short, branching hyphae that ab¬
sorb nourishment in certain fungi.
Haversian canals, numerous channels pene¬
trating the ebony layer of a bone,
heartwood, inner, inactive wood usually
darker in color than sap wood,
hemoglobin, an iron-containing protein com¬
pound giving red corpuscles their color;
combines easily with oxygen,
hemophilia, an inherited disease in which the
blood does not clot properly,
hemotoxin, a poison that destroys red blood
cells and breaks down the walls of small
blood vessels.

hepatic portal vein, a vessel carrying blood
to the liver before the blood returns to
the heart.

herbaceous, an annual stem with little woody
tissue.

herbivores, plant-eating animals,
heredity, the transmission of traits from parent
to offspring.

hermaphroditic, having the organs of both

sexes.

heterocysts, in Nostoc, empty cells with thick
walls which enable the filaments to break
into shorter pieces.

heterogametes, male and female gametes
which are unlike in appearance and struc¬
ture.

heterotrophs, organisms that are unable to
synthesize organic molecules from inor¬
ganic molecules, i.e., nutritionally de¬
pendent.

heterozygous, refers to an organism in which
the paired genes for a particular trait
are different.

hexose, the saccharide, or sugar unit, of which
all sugars are composed,
hibernate, to spend the winter months in an
inactive condition.

hilum, the scar on a seed where it was at¬
tached to the ovary wall,
hindbrain, that part of the brain which is
composed of the cerebellum, the pons,
and the medulla.

holdfast, the special cell at the base of cer¬
tain algae that anchors them to the sub¬
strate.

homeostasis, a steady state which an organ¬
ism maintains by self -regulating adjust¬
ments.

homoiothermic (“warm-blooded”), refers to
animals whose internal temperature re¬
mains relatively constant regardless of
the environmental temperature,
homologous chromosomes, the corresponding
chromosomes of a pair,
homologous organs, those similar in origin and
structure but not necessarily in func¬
tion.

homologue, a single chromosome of a homolo¬
gous pair.

homozygous, refers to an organism in which
the paired genes for a particular trait are
identical.

hormone, the chemical secretion of a duct¬
less gland producing a definite physiolog¬
ical effect.

GLOSSARY 763

homy layer, the outer layer of the epidermis;
in bivalves, the outermost layer of the
shell.

host, in a parasitic relationship, the organism
from which the parasite derives its food
supply.

humus, organic matter in the soil formed by
the decomposition of plant and animal
remains.

hybrid, an offspring from a cross between
parents differing in one or more traits,
hybrid vigor, having desirable characteristics
lacking in both parents,
hybridization, the crossing of two different
varieties to produce a new one.
hydrolysis, the breakdown of fats into fatty
acids and glycerol by combination with
water.

hydrophytes, plants that grow in water or
partially submerged in water in very
wet surroundings.

hydrotropism, the response of roots to water,
hyperthyroidism, overactivity of the thyroid
gland and its attendant symptoms,
hypha, a threadlike filament of the vegeta¬
tive body of a fungus.

hypocotyl, that part of a plant embryo from
whose lower end the root develops,
hypothesis, a scientific idea or working theory,
hypothyroidism, underactivity of the thyroid
gland and its attendant symptoms.

identical twins, those which develop from the
fertilization of one egg which later splits
into two organisms.

ileum, the third and longest region of the
small intestine.

immune therapy, the assistance and stimula¬
tion of the natural body defenses in pre¬
venting infectious disease,
immunity, the ability of the body to resist a
disease by natural or artificial means,
improvement cutting, the removal of diseased
or injured trees from a managed forest,
inbreeding, line breeding,
incisor, a tooth in the front of the jaw; highly
developed for gnawing in rodents,
incomplete dominance, the equal appearance
of two unlike characteristics in the off¬
spring, resulting from a cross of these
characteristics.

incomplete metamorphosis, the life stages of
certain insects consisting of the egg, sev¬
eral nymph stages, and the adult,
incubation, the providing of ideal conditions
for growth and development, as in the
incubation of eggs or the growth of bac¬
teria.

incurrent pore, one of many holes in the
sponge through which water passes into
the animal.

incurrent siphon, the structure in a clam
through which water passes into the
body.

indehiscent, a class of fruits which do not
open to discharge seeds,
individual characteristics, traits that are in¬
herited but that make an organism dif¬
ferent from all others.

innate behavior, inborn behavior,
inoculation, voluntary addition of germs or
viruses to a culture medium or to a living
organism.

inorganic, refers to a compound not contain¬
ing carbon, with the exception of CO2.
insecticide, a chemical used to destroy insects,
insertion, the attachment of a muscle at its
movable end.

inspiration, the intake of air into the lungs,
instinct, a natural urge, or drive, not depend¬
ing on experience or intelligence,
insulin, a hormone secretion of the islet cells
of the pancreas which regulates the oxi¬
dation of sugar in the tissues,
integument, one of the two layers of the walls
of an ovule.

interdependence, the dependence of cells on
other cells for complete functioning, or
of organisms on the activities of other or¬
ganisms.

internal respiration, the exchange of gases
between the blood and body tissues,
internode, the space between two nodes,
interphase, the period of growth of a cell that
occurs before and after mitosis,
intestine, the portion or portions of the ali¬
mentary canal extending beyond the
stomach.

invertebrate, an animal lacking a backbone,
involuntary muscle, one which cannot be con¬
trolled at will.

ionic bonds, forces holding atoms together
resulting from differences in electrical
charges.

ions, charged atoms.

iris, the muscular, colored portion of the eye,
behind the cornea and surrounding the
pupil.

irrigation, of soil, diversion of water into an
area during dry periods,
irritability, the ability to respond to a stimu¬
lus.

islets of Langerhans, the special cells in the
pancreas that secrete insulin,
isogametes, male and female gametes that
look alike.

isolation, the confinement of a population to
a certain location because of barriers,
isotopes, different forms of an element result¬
ing from varying numbers of neutrons.

jejunum, a section of the small intestine ly¬
ing between the duodenum and the il¬
eum.

joint, the point at which two separate bones
are joined by ligaments.

kidney, a glandular organ which excretes
urine.

Koch postules, the steps in Robert Koch’s
procedure in the investigation of anthrax.

labium, the lower portion or “lip” of an in¬
sect’s mouth.

labrum, the two-lobed upper portion of an in¬
sect’s mouth.

lactase, a digestive enzyme of the intestinal
fluid which changes lactose to glucose.

764 GLOSSARY

lacteal, a lymph vessel which absorbs di¬
gested fat from the intestinal wall,
lactic acid, a product of anaerobic respira¬
tion.

larva, the stage that follows the egg in the
development of certain animals,
larynx, the voice box; also called the “Adam’s
apple.”

lateral bud, a bud that develops at a point
other than at the end of a stem,
lateral line, a row of pitted scales along each
side of the fish, functioning as a sense
organ.

leaching, the loss of soluble soil minerals
as a result of the movement of ground
water.

leaf, the photosynthetic organ of the plant,
leaf scar, a mark on a twig left at the point
of attachment of a leaf stalk of a pre¬
vious growing season,
leaflet, a division of a compound leaf,
lens, the transparent disk by means of which
light rays are directed to the retina of
the eye.

lenticel, a small pore in the epidermis or bark
of a young stem through which gases are
exchanged.

lethal gene, one which bears a characteristic
that is usually fatal to the organism,
leucocytes, phagocytic white blood cells,
leucoplast, a colorless plastid serving as a
food reservoir in certain plant cells.

LH (luteinizing hormone), a substance pro¬
duced by the pituitary gland which con¬
trols the development of the corpus lu-
teum.

life span, the period of existence of an or¬
ganism.

ligament, a tough strand of connective tis¬
sue which holds bones together at a
joint.

light reaction, the first stage of photosynthesis
in which light energy excites chlorophyll
molecules and causes water molecules to
be split.

limiting factor, any factor that is essential to
organisms and for which there is compe¬
tition.

line breeding, the process of allowing plants
to self-pollinate over several generations,
removing offspring with undesirable
traits in each generation; inbreeding in
plants and animals.

lipase, a digestive enzyme of the pancreatic
fluid that changes fats to glycerin and
fatty acids.

liver, the largest organ in the human body,
associated with several vital activities in¬
cluding digestion and sugar metabolism,
locomotion, the spontaneous movement of an
organism from one place to another,
lung, an organ for aerial breathing,
lung books, the respiratory organs of spiders,
lymph, the clear, liquid part of blood which
enters the tissue spaces and lymph ves¬
sels.

lysogenic phage, a virus that invades a bac¬
terial cell without causing immediate de¬
struction. It is passed along to daughter

bacteria and becomes destructive at a
later time.

lysozyme, an enzyme which dissolves the
cell walls of many bacteria,
lytic cycle, the disintegration of a cell as a
result of invasion by a phage.

macronucleus, the large nucleus of the para-
mecium and certain other protozoans,
maggot, the larval stage of a fly.

Malpighian tubules, the structures in the di¬
gestive system of the grasshopper that
collect wastes from the blood,
maltase, a digestive enzyme of the intestinal
fluid which changes maltose to glucose,
mammary glands, those found in female
mammals that secrete milk and give the
class its name.

mandible, a strong cutting mouthpart of
arthropods; a jaw, as in the beak of a
bird or the bony structure of a mammal,
mantle, the tissue covering the safe parts of
a mollusk.

marine, living in salt water,
marrow, the soft tissue in the central cavity
of a larger bone.

mass selection, the picking of ideal plants or
animals from a large number to serve
as parents for further breeding,
matrix, a gelatinous secretion of cells of
Nostoc and certain other blue-green al¬
gae.

maturation region, the area of a root or stem
where embryonic cells mature into tis¬
sues.

maxilla, a mouthpart of an arthropod; the
upper jaw of vertebrates,
maxilliped, a “jaw foot,” or first thoracic ap¬
pendage, of the crayfish and other ar¬
thropods.

medulla, in the kidney, the inner portion con¬
taining pyramids which in turn contain
numerous tubules; ir. the adrenal gland,
the inner portion, which secretes epineph¬
rine.

medulla oblongata, the enlargement at the
upper end of the spinal cord, at the base
of the brain.

medusa, the bell-shaped, free-swimming form
in the jellyfish.

megaspores, four cells formed from the meg¬
aspore mother cell, three of which dis¬
integrate and one of which develops
into the embryo sac.

megaspore mother cells, diploid cells in the
plant ovary which divide twice, forming
four haploid magaspores.
meiosis, the type of cell division in which
there is reduction of chromosomes to the
haploid number during oogenesis and
spermatogenesis.

meninges, the three membranes covering the
brain and spinal cord,
menstruation, the periodic breakdown and
discharge of sort uterine tissues that oc¬
curs in the absence of fertilization,
meristematic tissue, small, actively dividing
cells that produce growth in plants,
mesentery, a folded membrane which con-

GLOSSARY 765

nects to the intestines and the dorsal body
wall of vertebrates.

mesoderm, the middle layer of cells in an
embryo.

mesoglea, a jellylike material between the
two cell layers composing the body of
a coelenterate.

mesophytes, plants which occupy neither ex¬
tremely wet nor extremely dry surround¬
ings.

mesothorax, the middle portion of the thorax
of an insect, bearing the second pair of
legs and usually a pair of wings,
messenger RNA, the type of RNA that is
thought to receive a code for a specific
protein from the DNA in the nucleus
and to act as a template for protein syn¬
thesis on the ribosome,
metabolism, the sum of the chemical process
of the body.

metamorphosis, a marked change in structure
of an animal during its growth, as the
change from larva to pupa and adult in
insects.

metaphase, the stage of mitosis in which the
chromosomes line up at the equator,
metathorax, the posterior portion of the thorax
of an insect, bearing the third pair of
legs and the second pair of wings,
micronucleus, a small nucleus found in the
paramecium and certain other proto¬
zoans.

micropyle, the opening in the ovule wall
through which the pollen tube enters,
microspore mother cells, diploid cells in the
anther which divide twice, forming four
haploid microspores.

microspores, four cells, formed from the mi¬
crospore mother cell, which develop into
pollen grains.

midbrain, that part of the brain which is
composed of nerve fibers connecting the
forebrain to the hindbrain,
middle lamella, a portion of the plant cell
wall that forms a common boundary be¬
tween two touching cells,
midrib, the large, central vein of a pinnately
veined leaf.

migration, seasonal movement of animals
from one place to another,
mimicry, a form of protective coloration in
which an animal closely resembles an¬
other kind of animal or an object in its
environment.

mitochondria, rod-shaped bodies in the cyto¬
plasm known to be centers of cellular
respiration.

mitosis, the division of chromosomes pre¬
ceding the division of cytoplasm,
mixed nerve, one which consists of both sen¬
sory and motor nerve fibers,
mixture, two or more substances that inter¬
mingle without chemical combination,
molar, a large tooth for grinding, highly de¬
veloped in herbivores.

molecule, the smallest portion of a substance
that keeps the properties shown by that
substance in large quantities,
molting, shedding of the outer layer of exo¬

skeleton or arthropods, or of a scale
layer of reptiles, or of plumage of birds,
monocotyledon, a flowering plant that de¬
velops a single seed leaf or cotyledon,
monoecious, bearing staminate and pistillate
flowers on different parts of the same
plant.

monohybrid, an offspring from a cross be¬
tween parents differing in one trait,
mother cell, a cell that has undergone growth
and is ready to divide,
motor end plate, the terminus of the axon of
a motor nerve in a muscle,
motor nerve, one containing only motor fibers,
motor neuron, one that carries impulses from
the brain or spinal cord to a muscle or
gland.

mucous membrane, a form of epithelial tissue
which lines the body openings and di¬
gestive tract and secretes mucus,
mucus, a slimy secretion of mucous glands,
mulch, a substance placed on or in the soil to
retard water loss or to improve soil tex¬
ture.

multicellular, having many cells,
multiple alleles, one of two or more pairs of
genes that act together to produce a spe¬
cific trait.

mutant, an offspring possessing a character¬
istic that was not inherited,
mutation, a change in genetic make-up re¬
sulting in a new characteristic which may
be passed on to offspring,
mutualism, a form of symbiosis in which two
organisms live together to the advantage
of each.

mycelium, the vegetative body of molds and
other fungi, composed of hyphae.
mycologist, a scientist who studies fungi,
mycorhiza, a fungus that lives in a symbiotic
relationship with the roots of trees and
other higher plants.

narcotics, a group of drugs which result in
addiction when used without medical di¬
rection.

natural immunity, one which is present in
the individual at birth and is not artifi¬
cially acquired.

natural selection, the result of survival in the
struggle for existence among organisms
possessing those characteristics that give
them an advantage.

nematocyst, a stinging cell in coelenterates.
nephridia, the excretory structures in worms,
mollusks, and certain arthropods,
nephron, one of the numerous excretory struc¬
tures in the kidney, including the Bow¬
man’s capsule, glomerulus, and tubules,
nerve, a cordlike structure containing bun¬
dles of nerve fibers, or processes,
nerve cell body, the body of a neuron con¬
taining cytoplasm and a nucleus,
nerve net, the network of sensory cells in the
hydra.

neuron, a nerve cell body and its processes,
neurotoxin, a poison that affects the parts of
the nervous system that control breathing
and heart action.

766 GLOSSARY

neutron, a particle in the nucleus of an atom
which carries no electrical charge,
niche, the wa^ in which an organism “earns
its living. >

nictitating membrane, a thin, transparent cov¬
ering, or lid, associated with the eyes of
certain vertebrates; a third eyelid,
nitrification, the action of a group of soil bac¬
teria on ammonia, producing nitrates,
nitrogen fixation, the process by which cer¬
tain bacteria in soil or on the roots of
leguminous plants convert free nitrogen
into nitrogen compounds which the plants
can use.

nocturnal, active during the night,
node, a growing region of a stem from which
leaves, branches, or flowers develop,
nondisjunction, abnormal segregation of chro¬
mosomes.

notochord, a rod of cartilage running longi¬
tudinally along the dorsal side of lower
chordates and always present in the early
embryological stages of vertebrates,
nuclear membrane, a living membrane sur¬
rounding the nucleus.

nucleolus, a small, spherical body within the
nucleus.

nucleoplasm, the dense, gelatinous living con¬
tent of the nucleus.

nucleus, the part of the cell that contains
chromosomes; the central mass of an
atom, containing protons and neutrons,
nymph, a stage between egg and adult in an
insect having incomplete metamorphosis.

obligate aerobes, organisms that require at¬
mospheric oxygen for respiration,
obligate anaerobes, organisms that cannot
grow in the presence of atmospheric oxy¬
gen.

olfactory lobe, the region of the brain regis¬
tering smell.

olfactory nerve, the nerve leading from the
olfactory receptor endings to the olfac¬
tory lobe of the brain.

oogonium, an egg-producing cell in certain
thallophytes.

ootid, a cell that matures into an egg.
open system, a circulatory system in which
the blood is not confined in a continuous
system of vessels,
operculum, the gill cover in fish,
opsonin, a blood antibody which prepares
bacteria for ingestion by white cor¬
puscles.

optic lobe, the region of the brain registering
sight.

optic nerve, the nerve leading from the retina
of the eye to the optic lobe of the brain,
oral groove, a deep cavity along one side of
the paramecium and similar protozoans,
organ, different tissues grouped together to
perform a function or functions,
organelles, specialized structures present in
the cytoplasm.

organic, refers to a compound containing car¬
bon, with the exception of CO2.
organic nutrients, the three classes of foods;
carbohydrates, fats, and proteins.

organism, a complete and entire living thing,
origin, the attachment of a muscle at its im¬
movable end.

ornithology, the study of birds,
osculum, an opening in the central cavity of
sponges through which water leaves the
animal; the excurrent pore,
osmosis, the diffusion of water through a
semipermeable membrane from a region
of greater concentration of water to a
region of lesser concentration,
ossification, the process by which cartilage
cells of childhood are replaced by bone
cells, resulting in a hardening of the body
framework as the organism grows,
ostia, pairs of openings through which blood
enters the crayfish heart,
outbreeding, hybridization,
ovary, the basal part of the pistil containing
the ovules; a female reproductive organ,
oviduct, a tube in a female through which
eggs travel from an ovary,
oviparous, egg-laying animals,
ovipositor, an egg-laying organ in insects,
ovoviviparous, bringing forth the young alive
after they have developed without pla¬
cental connection in the mother,
ovule, a structure in the ovary of a flower
which, when fertilized, can become a
seed.

oxidation, the union of any substance with
oxygen and the resulting release of en¬
ergy.

oxygen debt, the condition that may occur at
times of muscular exertion when enough
oxygen may not be supplied to the body
and lactic acid accumulates. During a
rest period following, some lactic acid is
oxidized and some is converted to gly¬
cogen.

oxygenation, the process whereby the blood
is supplied with oxygen from the lungs,
oxyhemoglobin, hemoglobin with which oxy¬
gen has been combined,
oxytocin, a hormone secreted by the posterior
lobe of the pituitary gland which helps
regulate blood pressure.

palisade layer, a dense tissue in green leaves
and twigs consisting of elongated cells,
palmate venation, leaf veining in which the
main vains radiate from a central point,
palpus, an appendage of a mouthpart of an
arthropod.

pancreas, a gland located between the stom¬
ach and intestine which is both endocrine
and digestive.

parallel venation, a vein pattern characteris¬
tic of the leaves of a monocot.
parasite, an organism that obtains its food en¬
tirely from another living thing,
parasympathetic nervous system, a division
of the autonomic nervous system,
parathormone, the hormone secreted by the
parathyroid glands.

parathyroid, one of the four small ductless
glands embedded in the thyroid,
parenchyma, the thin-walled, soft tissue in
plants forming cortex and pith.

GLOSSARY 767

parietal eye, an organ in the top of the head
of the tuatara’s head. It does not func¬
tion as a sense organ.

parotid, one of the pair of salivary glands near
the ear.

parthenogenesis, the development of an egg
without fertilization.

passive transport, the passage through a cell
membrane without the expenditure of
energy.

pasteurization, the process of killing and/or
retarding the growth of bacteria in milk
and other beverages by heating,
pathogenic, disease-causing,
pearly layer, the inner layer of the shell of
bivalves.

peat, a substance formed by the decomposi¬
tion of plants in the presence of water,
pectin substances, jellylike material in the
middle lamella of plant cells,
pedicel, the stalk that supports a single flower,
pedipalps, the second pair of head append¬
ages in spiders.

pellicle, a thickened membrane surrounding
the cell of a paramecium.
pelvis, the hip girdle, in man consisting of the
ilium, ischium, and pubis bones; the cen¬
tral portion of a kidney,
penicillin, an antibiotic produced by a mold;

also made synthetically,
pepsin, a digestive enzyme of the gastric juice
which changes proteins to peptones and
proteoses.

peptone, a stage in protein digestion prior to
the formation of amino acids,
percolation beds, gravel stream beds in which
water is allowed to flow slowly by re¬
leasing it from reservoirs during the sum¬
mer. They aid in maintaining the water
table.

perennials, plants that grow more than two
growing seasons.

pericardial cavity, the area in which the heart
lies

pericardial sinus, a cavity surrounding the
crayfish heart.

pericardium, the membrane around the heart,
pericycle, the tissue in roots from which sec¬
ondary roots arise.

periderm, the corky layer forming the outer
edge of a root after secondary thicken¬
ing^

periodicity, alternating periods of activity,
periodontal membrane, the fibrous structure
that anchors the root of the tooth in the
jaw socket.

periosteum, the tough membrane covering the
outside of a bone.

peripheral nervous system, the nerves com¬
municating the central nervous system
and other parts of the body,
permeable membrane, one that allows sub¬
stances to pass through it.
petal, one of the colored parts of the flower.
(In some flowers the sepals are also
colored. )

petiole, the stalk of a leaf,
phage, a bacteriophage, or a virus that repro¬
duces in a bacterium.

phagocytic cells, those that engulf bacteria
and digest them with enzymes, including
lysozymes.

pharynx, the muscular throat cavity, extend¬
ing up over the soft palate and to the
nasal cavity.

phenotype, the outward appearance of an or¬
ganism as the result of gene action,
phloem, the tissue in roots and stems that
conducts dissolved food substances down¬
ward.

photoperiodism, the dependence of some
plants on the relation between the length
of light and the length of darkness in a
given day.

photoreceptor, an organ that is sensitive to
light.

photosynthesis, the process by which certain
living plant cells combine carbon diox¬
ide and water, in the presence of chloro1
phyll and light energy, to form PGAL
and release oxygen as a waste product,
phototropism, the response of plants to light,
phycocyanin, the blue pigment found in cer¬
tain algae.

phylum, one of the large divisions in the clas¬
sification of plants and animals,
physical change, one in which no change oc-.

curs in the chemical composition,
physical factors, the nonbiological elements
in the environment.

physiology, the study of the functions, or life
activities, of living things,
pia mater, the inner of the three membranes
of the brain and spinal cord,
pineal body, a gland of undetermined func¬
tion that lies between the cerebral hemi¬
spheres and near the pituitary gland,
pinnate venation, a vein pattern in which the
leaves have a single large vein extend¬
ing through the center of the blade,
pioneers, the first organisms to enter an area
that is void of living things,
pistil, the part of the flower bearing the ovary
. at its base.

pith, a tissue of roots and stems consisting of
thin-walled cells and used for food stor¬
age.

pith rays, thin cellular layers leading from the
pith to the bark in stems of dicotyle¬
dons.

pituitary gland, a ductless gland composed of
two lobes, located beneath the cerebrum,
placenta, a large thin membrane in the uterus,
in the area of the chorionic villi, which
transports substances between the mother
and developing young by means of the
umbilical cord.

plankton, the organisms suspended near the
surface in a body of water,
plasma, the liquid portion of blood tissue,
plasma membrane, a thin living membrane
located at the outer edge of the cyto¬
plasm.

plasmolysis, the collapse of cell protoplasm
due to loss of water.

plastids, living bodies in the cytoplasm of
plant cells.

plastron, the lower shell of the turtle.

768 GLOSSARY

platelet, the smallest of the solid components
in the blood, releasing thromboplastin in
clotting.

pleural membrane, one of two membranes
surrounding each lung,
plexus, a mass of nerve cell bodies,
plumule, that part of a plant embryo from
which the shoot develops,
poikilothermic (“cold-blooded”), refers to
animals whose internal temperature fluc¬
tuates with that of the environment,
polar nuclei, the two structures in the em¬
bryo sac in flowers that fuse to form the
endosperm nucleus.

pollen grain, the male reproductive structure
of flowering plants.

pollen sacs, structures in the anther contain¬
ing pollen grains.

pollen tube, the tube formed by a pollen
grain when it grows down the style,
pollination, the transfer of pollen from anther
to stigma.

polyp, one of the stages in the life cycle of
coelenterates.

polyploidy, the condition of a plant having
some multiple of the diploid number of
chromosomes.

pome, an applelike fruit consisting of a rip¬
ened receptable surrounding the ovary,
pons, a part of the hindbrain located in the
brain stem.

portal circulation, an extensive system of
veins which lead from the stomach, pan¬
creas, small intestine, and colon, unite
and enter the liver.

posterior, tail, or rear end, of an animal,
potassium-argon clock, a method used to date
fossils by measuring the amount of
argon— 40 present in a rock sample,
population density, the number of individuals
of a species in a definite area at a specific
time.

population genetics, the study of the fre¬
quency of genetic traits in populations
precipitation, the movement of water from
the atmosphere to the earth,
precipitin, a blood antibody which causes
bacteria to settle out.

predator, any animal which preys on other
animals.

premolars, large teeth for grinding,
primaries, in birds, those quill feathers which
grow from the end section of the wing,
primary germ layers, the ectoderm, endo-
derm, and mesoderm.

primary oocyte, the structure in the female
gonads that divides to form the second¬
ary oocyte and first polar body,
primary root, the first root of the plant com¬
ing from the seed.

prismatic layer, the middle of the shell of bi¬
valves.

proboscis, a tubular mouthpart in certain in¬
sects; the trunk of an elephant,
producers, the autotrophs in an environ¬
ment.

proglottid, a segment of a tapeworm’s body,
prolegs, tne extra pairs of fleshy legs at the
end of the abdomen in caterpillars.

propagation, the multiplication of plants by
vegetative parts.

prophase, the stage of mitosis in which chro¬
mosomes shorten and split longitudinally,
propolis, a brown substance gathered by bees
from the sticky leaf buds of some plants,
prostate, a gland located near the upper end
of the urethra in the male. It secretes a
fluid which is added to the sperm,
prostomium, a kind of upper lip in the earth¬
worm.

protective coloration, the blending of an or¬
ganism with the color of its surroundings,
protective resemblance, similarity in shape to
something in the environment,
protein synthesis, a universal phase of cell
anabolism whereby protein molecules are
built up from amino acid molecules,
proteins, a complex tissue-building class of
foods containing not only C, H, and O,
but also N, S, and usually P.
proteoses, a stage in protein digestion prior to
the formation of amino acids,
prothallus, the tiny heart-shaped structure
that develops from the spore of the fern,
prothorax, the first segment of an insect’s
thorax, to which the head and first pair
of legs are attached.

prothrombin, an enzyme produced in the
liver; it is an inactive part of blood
plasma except during clotting,
proton, a particle in the nucleus of an atom
that bears a positive electrical charge,
protonema, a filamentous structure produced
by a spore in mosses.

protoplasm, organized complex system of sub¬
stances found in living organisms. It is
a colloidal suspension in water,
priming, the cutting off of surplus branches of
trees and shrubs.

pseudopodium, a “false foot” of the ameba or
amebalike cells.

ptyalin, a digestive enzyme of saliva which
changes starch to maltose; also called
salivary amylase.

puberty, the age at which the secondary sex
characteristics appear.

pulmocutaneous arteries, those in the frog
that branch to the lungs, skin, and mouth
membrane.

pulmonary, pertaining to the lungs,
pulp cavity, the area within the dentine of a
tooth containing nerves and blood ves¬
sels.

pulpwood, timber used for making paper,
pulse, regular expansion of the artery walls
caused by the beating of the heart.
Punnett square, a grid system used in com¬
puting possible combinations of gametes,
pupa, the stage in an insect having complete
metamorphosis following the larva stage,
pupil, the opening in the front of the eyeball,
the size of which is controlled by the iris,
pure science, research conducted for the sake
of knowledge itself.

pus, collection of dead bacteria and white
corpuscles at the site of an infection,
pyloric caeca, pouches extending from the in¬
testine in fish.

GLOSSARY 769

pylorus, a muscular valve situated at the junc¬
tion of the stomach and intestine in ver¬
tebrates.

pyramids, conical projections of the medulla
of the kidney.

pyrenoid, a small protein body surrounded
by starch on a chloroplast of Spirogyra
and certain other algae.

quadrate bone, the bone in the snake’s skull
to which the lower jaw is attached,
quarantine, isolation of plants or animals to
prevent the spread of suspected infec¬
tion.

queen, the egg-laying female bee in a hive,
quill feathers, the large stiff feathers in the
wing or tail of a bird.

rachis, the axis of quill feathers of a bird,
radial symmetry, the type exhibited by an
organism that may be divided into two
equal parts by any plane which passes
through the diameter of the disk and the
central axis.

radioactive, refers to an element that spon¬
taneously gives off particles,
radula, a tonguelike structure in snails which
acts as a scraper.

reaction time, the elapsed time between the
moment a stimulus is received and a re¬
sponse occurs.

receptacle, the end of the flower stalk bearing
the reproductive structures,
receptor, a cell or group of cells that receive
a stimulus.

recessive, refers to a gene or character that is
masked when a dominant allele is pres¬
ent.

rectum, the posterior portion of the large in¬
testine, above the anus,
red corpuscles, the cells in blood containing
hemoglobin.

reduction division, the reduction of chromo¬
somes during meiosis from the diploid
number to the haploid number,
reflex action, a nervous reaction in which a
stimulus causes the passage of a sensory
nerve impulse to the spinal cord or
brain, from which, involuntarily, a motor
impulse is transmitted to a muscle or
gland.

reforestation, the planting of forest trees in
an open area from which previous trees
have been removed.

regeneration, the ability of organisms to form
new parts.

renal, relating to the kidneys,
rennin, an enzyme in the gastric fluid of some
mammals which coagulates casein,
replication, self-duplication, or the process
whereby a DNA molecule makes an ex¬
act duplicate of itself.

reproduction, the process during which plants
and animals produce new organisms of
their kind.

respiration, the exchange of gases between
cells and their surroundings and accom¬
panying oxidation and energy release,
response, the reaction to a stimulus.

retina, the inner layer of the eyeball, formed
from the expanded end of the optic
nerve.

Rh factor, any one of six or more protein sub¬
stances in the blood of certain people,
rhizoid, a rootlike growth which carries on
absorption.

rhizome, a rootlike horizontal underground
stem often enlarged for storage,
rhythmic, refers to regular periodicity,
rickettsiae, a group of organisms midway be¬
tween the viruses and bacteria in size,
that cause disease.

ribosomal RNA, the RNA that is contained
within the ribosomes. Its function is
not known.

ribosomes, tiny dense granules attached to
the endoplasmic reticulum and lying be¬
tween its folds. They contain RNA and
protein-synthesizing enzymes,
rill erosion, the formation of tiny channels as
a result of rain water carrying particles
of soil down a hill.

rind, the outer covering of a monocot stem,
composed of thick- walled hard cells,
ring canals, those that lead through each ray
in the starfish to a circular canal in the
central disk.

RNA, a nucleic acid in which the sugar is
ribose. It is a product of DNA and
serves in controlling certain cell activi¬
ties.

rod, a cell of the retina of the eye that re¬
ceives impulses from light rays and which
is sensitive to shades but not to colors,
root cap, a tissue at the tip of a root protect¬
ing the tissues behind it.
root hair, a projection of an epidermal cell of
a young root.

root pressure, that which is built up in roots
due to water intake and resulting turgor,
rostrum, a protective area which is an exten¬
sion of the carapace in crustaceans,
row crop, that grown in rows with soil ex¬
posed between them.

rumen, the first of the four stomach divisions
in the ruminant mammals,
ruminant, a cud-chewing ungulate,
runoff water, rain water that runs off the sur¬
face of the ground and enters the drain¬
age system.

saliva, a fluid secreted into the mouth by the
salivary glands.

saprophyte, an organism that fives on non¬
living organic matter.

sapwood, active tissue in the outer area of
wood in a stem.

savannah, a grassland with scattered trees,
scales, the epidermal plates forming the outer
covering in fish and reptiles,
scavengers, animals that feed on dead or¬
ganisms.

scion, the portion of a twig grafted on to a
rooted stock.

sclerotic layer, the outer layer of the wall of
the eyeball.

scrotum, pouch outside the body that contains
the testes.

770 GLOSSARY

scutes, the broad scales on the lower side of
the snake’s body.

secondaries, in birds, those quill feathers that
grow from the modified forearm section
of the wing.

secondary oocyte, the cell that results from
reduction division and develops into the
ootid.

secondary root, a branch root developing
from the pericycle of another root,
secretion, formation of essential chemical sub¬
stances by cells.

seed, a complete embryo plant protected by
one or more seed coats,
selective cutting, cutting timber trees from a
managed forest only when they are ma¬
ture.

self-pollination, the transfer of pollen from
another to stigma in the same flower or
another flower of the same plant,
self-preservation, a basic instinct possessed by
all animals to stay alive,
semen, fertilizing fluid consisting of sperm
and fluids from the seminal vesicle, pros¬
tate gland, and Cowper’s gland,
semicircular canals, the three curved passages
in the inner ear that are associated with
the sense of balance.

semilunar valves, the heart valves located at
the base of the arteries where they join
the auricles.

seminal receptacles, structures that receive
sperm cells in certain animals,
seminal vesicles, structures that store sperm
cells in certain animals,
seminiferous tubules, a mass of highly coiled
tubes within the testes,
sensory nerve, a nerve composed only of the
fibers of sensory neurons,
sensory neurons, those that carry impulses
from a receptor to the spinal cord or
brain.

sepal, the outermost part of a flower, usually
green and not involved in the reproduc¬
tive process.

septum, a wall separating two cavities or
masses of tissues, as the nasal or heart
septum.

serum, a substance, usually an extract of blood
containing antibodies, used in treating a
disease after it has struck and to produce
immediate passive immunity,
serum albumin, a blood protein necessary for
absorption.

serum globulin, a blood protein that contains
antibodies.

setae, bristles on the earthworm used in lo¬
comotion.

sex chromosomes, the chromosomes that deter¬
mine the sex of an offspring,
sex-influenced character, a characteristic that
is dominant in one sex but recessive in
the other, as baldness.

sex-limited character, a characteristic that de¬
velops only in the presence of sex hor¬
mones, as the beard.

sex-linked character, a recessive character car¬
ried on the X, or sex, chromosomes, as
color blindness.

sexual reproduction, that involving the union
of a female gamete, or egg, and a male
gamete, or sperm.

sheath, a capsule surrounding an entire fila¬
ment of bacteria.

sheet erosion, the loss of a thin layer of soil
due to standing water.

shelterbelts, rows of trees planted at intervals
to break the force of the wind,
shoot, the first part of the plant visible above
ground, formed from the plumule,
sieve plate, in the starfish, the opening of the
water-vascular system to the outside,
sieve tube, a conducting tube of the phloem,
silk scar, a tiny point seen near the top of a
corn kernel which marks the position
where the style was attached,
simple leaf, one in which the blade is in one

{)iece.

e sugar, a monosaccharide, one which
can be absorbed by the body without
further simplification, as glucose,
sinoauricular node, a small mass of tissue on
top of the heart in which the automatic
beat originates.

sinus venosus, a thin-walled sac which is an
enlargement of the cardinal vein of the
fish and frog, and which lies at the en¬
trance to the heart.

skeletal muscle, that which is striated and
voluntary.

slime layer, that which surrounds a bacte¬
rium.

smooth muscle, that which is involuntary and
found lining the walls of the intestine,
stomach, and arteries.

soil, a mass of rock particles and humus from
which plants obtain essential materials,
solar plexus, the large nerve ganglion of the
sympathetic nervous system located in
the abdomen.

solution, a homogeneous mixture of two or
more substances.

somatotropic hormone, the hormone secreted
by the anterior lobe of the pituitary
gland that regulates the growth of the
skeleton.

song box, the part of the larynx of birds in
which the voice is produced; also called
a syrinx.

son, small clusters of sporangia which appear
on fern leaves when they are mature,
species, a group of plants or animals exhibit¬
ing the same characteristics and freely
interbreeding.

species characteristic, one possessed by all
members of a species,
sperm, a male reproductive cell,
sperm nuclei, the two structures formed from
the generative nucleus in the pollen grain
which function in double fertilization,
spermatid, a structure formed from second¬
ary spermatocytes, which mature into
sperm.

spermatocyte, a structure formed by reduc¬
tion division from a spermatogonial
cell.

spherical symmetry, the type exhibited by an
organism that may be divided into two

GLOSSARY 771

equal parts at any point passing through
the diameter of the body,
sphincter muscle, a ringlike muscle which
closes an opening or a tube,
spicule, the material forming the skeleton of
certain sponges.

spinal cord, the main dorsal nerve of the cen¬
tral nervous system in vertebrates, ex¬
tending down the back from the medulla,
spindle, the numerous fine threads formed be¬
tween the poles of the nucleus during
mitosis.

spiracles, external openings of the insect’s
tracheal tubes on the thorax and abdo¬
men.

spirillum, spiral-shaped bacterium,
spirochete, a group of spiral-shaped, one-
celled organisms resembling both proto¬
zoans and bacteria.

spongin, fibers comprising the skeleton of cer¬
tain sponges.

spongocoel, the central cavity in sponges,
spongy layer, a term applied to loosely con¬
structed tissue with many spaces,
spontaneous generation, a disproved belief
that certain nonliving or dead materials
could be transformed into living organ¬
isms.

sporangiophore, in molds, an ascending hypha
bearing sporangia.

sporangium, a structure that produces spores,
spore, an asexual reproductive cell,
sporophyte, the stage that produces spores in
an organism having alternation of gen¬
erations,
sport, a mutant.

spring wood, wood containing many large ves¬
sels mingled with tracheids and fibers,
stamen, a part of the flower bearing an an¬
ther at its tip.

statocyst, the balancing organ of the crayfish,
stigma, the part of the pistil that receives pol¬
len grains.

stimulus, a factor or environmental change
capable of producing activity in proto¬
plasm.

stipe, the stalk of a mushroom,
stock, the plant on which a scion has been
grafted; a line of descent; to supply with
seed, plants, eggs, or animals,
stolon, a transverse hypha of a mold,
stomata, pores regulating the passage of air
and water vapor to and from the leaf,
streaming, the movement of cytoplasm in a
cell.

strip cropping, the alternation of strips of row
crops and cover crops,
style, the stalk of the pistil,
subclavian vessels, large arteries and veins in
the arm area in vertebrates,
sublingual, one of the pair of salivary glands
lying under the tongue,
submaxillary, one of the pair of salivary
glands lying in the angle of the lower
jaw.

subsoil, soil that lies below topsoil and that is
usually poor in plant nutrients,
succession, the changing plant and animal
populations of a given area.

successive osmosis, the cell-to-cell diffusion
of water.

sucrase, a digestive enzyme of the intestinal
fluid which changes sucrose to glu¬
cose.

summer wood, wood containing few vessels
and a large number of fibers,
suspension, a mixture formed by particles that
are larger than ions or molecules,
sustained yield, a forest so managed as to
give regular crops for cutting,
sweepstakes dispersal, the movement of or¬
ganisms into new areas despite strong
barriers.

swimmerets, appendages on the abdomen of
a crustacean.

symbiosis, the relationship in which two or¬
ganisms live together for the mutual ad¬
vantage of each.

symmetrical, have a definite shape,
sympathetic nervous system, a division of the
autonomic nervous system,
synapse, the space between nerve endings,
synergid, one of two structures formed on
either side of the egg in the embryo sac
of flowers.

synovial fluid, a secretion of cartilage that
lubricates a joint.

systemic circulation, the body circulation
as distinct from the pulmonary, hepatic,
renal, and coronary circulations,
systems, groups of organs performing similar
functions.

systolic blood pressure, arterial pressure pro¬
duced when the ventricles contract.

tadpole, the larval stage of a frog or toad,
taproot, the main root of a plant, often serv¬
ing as a food reservoir.

tarsus, the foot of the grasshopper; a bone of
the human foot.

taste buds, flask-shaped structures in the
tongue containing nerve endings that are
stimulated by taste.

taxonomy, the branch of biology which groups
and names living things,
teliospore, a two-celled, black, winter spore
of wheat rust.

telophase, the last stage of mitosis, during
which two daughter cells are formed,
telson, the posterior segment of the abdomen
of certain Crustacea.

tendon, a strong band of connective tissue in
which the fleshy portion of a muscle
terminates.

tendril, a part of a plant modified for climb¬
ing.

tentacle, a long appendage or “feeler” of cer¬
tain invertebrates.

terminal bud, the terminal growing point of
the stem.

terracing, the checking of water on sloping
land by building level areas to prevent
soil erosion,
terrestrial, land-living,
testa, the outer seed coat,
testes, male reproductive organs of higher
animals.

testosterone, a male hormone produced in the

772 GLOSSARY

testes and in the cortex of the adrenal
glands in both males and females,
thoracic, pertaining to the chest cavity,
thoracic duct, a vessel carrying lymph and
emptying into the left subclavian vein,
thorax, the middle region of the body of an
insect between the head and abdomen;
the chest region of mammals,
thrombin, a substance formed in blood clot¬
ting as a result of the reaction of pro¬
thrombin, thromboplastin, and calcium,
thromboplastin, a substance essential to blood
clotting formed by the disintegration of
blood platelets.

thymus, one of the ductless glands, situated
near the breastbone, which begins to
atrophy at puberty, and whose function
is undetermined.

thyroid, a ductless gland, located in the neck
on either side of the larynx, which regu¬
lates metabolism.

thyroid hormone, the secretion of the thyroid
gland.

tibia, one of the long bones of the lower leg.
tissue, a group of cells similar in structure and
function.

tissue destruction, the destruction of cells by
pathogenic organisms.

tissue fluid, that which bathes the cells of
the body and is called lymph when con¬
tained in vessels.

tolerance, an organism’s ability to withstand
an environmental condition,
tone (muscle), the condition in which flexor
and extensor muscles oppose each other,
resulting in a continuous state of slight
contraction.

topography, the physical features of the earth,
topsoil, that top part of the soil consisting of
mineral matter combined with organic
matter.

toxin-antitoxin, a mixture of diphtheria anti¬
toxin and toxin, formerly used to develop
immunity.

toxoid, toxin weakened by mixing with form¬
aldehyde or salt solution, used exten¬
sively to develop immunity to diphtheria,
scarlet fever, and tetanus,
trachea, an air tube in insects and spiders;
the windpipe in air-breathing verte¬
brates.

tracheids, thick-walled conducting tubes that
strengthen woody tissue,
transfer RNA, the type of molecule thought
to deliver amino acids to the template
formed by messenger RNA on the ribo¬
somes.

transformation, in pneumococcus, the change
from a noncapsulated to a capsulated
form, brought about by the transfer of
DNA.

transformers, bacteria that change the sim¬
pler substances left by the decomposers
into nitrogen compounds which are used
by plants.

translocation, the movement of dissolved
foods in plants.

transpiration, the loss of water from plants,
transpiration pull, one of the forces involved

in the rise of water in a stem. As cells
lose water to the atmosphere, water enters
them from adjacent cells, resulting in
upward movement of water,
tree line, the division between low vegetation
and the coniferous forest,
trichinosis, the infestation of muscle by en¬
cysted trichina worms.

trichocysts, sensitive protoplasmic threads in
the paramecium, concerned with protec¬
tion.

trochanter, a joint in the appendages of the
grasshopper which with the coxa forms a
ball and socket.

trochophore, a larval form of mollusks.
tropism, an involuntary growth response of
an organism to a stimulus,
trypsin, an enzyme of the pancreatic juice
which converts protein to peptones and
proteoses

tube feet, movable suction discs on the rays
of most echinoderms.

tube nucleus, one of the two nuclei present in
a pollen grain.

tuber, an enlarged tip of a rhizome swollen
with stored food.

tundra, large biome encircling the Arctic
Ocean of the Northern Hemisphere,
turbinate, one of three layers of bones in the
nasal passages.

turgor, the stiffness of plant cells due to the
presence of water,
tympanic membrane, the eardrum,
tympanum, a membrane in certain arthro¬
pods, serving a vibratory function.

umbilical cord, that in female mammals lead¬
ing from the placenta to the embryo,
urea, a nitrogenous waste substance found
chiefly in the urine of mammals but
formed in the liver from broken-down
proteins.

uredospore, a one-celled red, summer spore
of wheat rust.

ureter, a tube leading from a kidney to the
bladder or cloaca.

urethra, the tube leading from the urinary
bladder to an external opening in the
body.

uric acid, a waste product of cell activity,
urinary bladder, the sac at the base of the
ureters which stores urine,
urine, the liquid waste filtered from the blood
in the kidney and excreted by the blad¬
der.

uropod, a flipper, or developed swimmeret, at
the posterior end of the crayfish,
uterus, the organ in which young mammals
are nourished until they are born,
uvula, the extension of the soft palate.

vaccination, producing immunity by inocu¬
lating with a vaccine.

vaccine, a substance used to produce immu¬
nity.

vacuolar membrane, a membrane surrounding
a vacuole in a cell and regulating the
movement of materials in and out of the
vacuole.

GLOSSARY 773

vacuole, one of the spaces scattered through
the cytoplasm of a cell and containing
e fluid.

vagina, cavity of the female immediately out¬
side and surrounding the cervix of the
uterus.

vagus nerve, the principal nerve of the para¬
sympathetic nervous system,
vane, part of a quill feather of a bird,
variations, the differences that occur within
the offspring of a given species,
vasa efferentia, tiny tubes in the reproductive
system of the frog through which sperm
pass into the kidney.

vascular bundles, strands of phloem and xy-
lem tissue found in roots, stems, and
leaves of higher plants,
vasopressin, a hormone secreted by the poste¬
rior lobe of the pituitary gland which
stimulates smooth muscles,
vegetative organs, the parts of a plant that
perform all the processes necessary for
life except the formation of seeds,
vegetative reproduction, a common method
of asexual reproduction in higher plants
whereby pieces of the plant tissue are
capable of growing into a complete or¬
ganism.

veins, strengthening and conducting struc¬
tures in leaves; vessels carrying blood to
the heart.

vena cava, a large collecting vein found in
many vertebrates.

venation, the arrangement of veins through
the leaf blade.

venom, the poison secreted by glands of poi¬
sonous snakes or other animals,
ventral, front, or lower (abdominal), surface
of animals.

ventricle, a muscular chamber of the heart; a
space in the brain,
venules, small branches of veins,
vermiform appendix, a fingerlike outgrowth
of the intestinal caecum,
vertebra, a bone of the spinal column of ver¬
tebrates.

vestigial organs, those which are poorly de¬
veloped and not functioning,
viability, the ability of seeds to germinate
after dormancy.

villi, microscopic projections of the wall of
the small intestine which increase the ab¬
sorbing surface.

virulent phage, a bacteriophage that produces
a lytic cycle of destruction,
viruses, particles that are noncellular and
have no nucleus, no cytoplasm, and no
surrounding membrane. They may re¬
produce in living tissue,
visceral hump, the area in bivalves contain¬
ing the principal digestive organs,
visual purple, the chemical in the rods of the
eye necessary for their proper function¬
ing in reduced light.

vitamin, an organic substance, though not a
food, that is essential for normal body
activity.

vitreous humor, a transparent substance that
fills the interior of the eyeball.

viviparous, bearing the young alive, and
nourishing them before birth by means
of the placenta.

vocal cords, those structures within the larynx
which vibrate to produce speech,
vocal sacs, membranous sacs between the ear
and shoulder of certain male frogs which
serve as resonators and increase the vol¬
ume of sound.

voluntary muscle, that controlled by the will
of the organism.

vomerine teeth, those in the roof of the
mouth of the frog which aid in holding
prey.

water-borne infections, those produced by
certain pathogenic organisms present in
water.

water cycle, the continuous movement of wa¬
ter from the atmosphere to the earth and
from the earth to the atmosphere,
watershed, a hilly region, usually extending
over a large area, which conducts surface
water to streams.

watertable, the level at which water is stand¬
ing in the ground.

water-vascular system, the circulatory system
of certain echinoderms.
web membrane, the flexible structure lying
between the toes of frogs,
white corpuscles, colorless cells of the blood,
whorled, in stems, having three or more
leaves at each node,
wigglers, the larvae of the mosquito,
wildlife, all native animals,
withdrawal symptoms, nervous reactions and
hallucinations resulting from the lack of a
drug to which the victim is addicted,
woody stem, one containing conducting and
supporting tissue that forms layers which
are added to year after year,
worker bee, an infertile female bee.

X chromosome, a sex-determining chromosome
present singly in human males and as
a pair in females.

xanthophyll, a yellow pigment found in cer¬
tain chromoplasts.

xerophyte, a plant that requires very little
water to five.

xylem, the woody tissue of a root or stem that
conducts water and dissolved minerals
upward.

Y chromosome, a sex chromosome found only
in males.

yolk, the part of the bird’s egg from which
the egg cell obtains its nourishment,
yolk sac, an extraembryonic membrane pro¬
viding food for the embryo.

zoospores, in Ulothrix, the flagellated cells
that leave the mother cell and later de¬
velop into new organisms,
zymase, the enzyme system in yeast cells that
acts on sugar to produce carbon dioxide
and alcohol.

Page references for illustrations are printed in italics.

INDEX

Abalones, 402

Abdomen, Crustacea, 412;
grasshopper, 427 -428; insect,
423

Abdominal cavity, 552, 576
Abert squirrel, 192, 193
Abiogenesis, 19, 22-25
Abscission layer, leaf, 346, 347
Absorption, of alcohol in body,
630; cellular, 57, 72-79; in
intestines, 578—579; by roots,
320—322; see also Diffusion
Acetic acid, 4
Acetobacter, 232
Acetylcholine, 613
Acorn worms, 453
Acromegaly, 642
ACTH, 642
“Adam’s apple,” 601
Adaptation, 30-31; to environ¬
ment, 190-191, 192
Adder, 498

Addison’s disease, 643
Adenine, 50, 96, 142
Adenoids, 593
Adolescence, 641, 644-646
ADP, in photosynthesis, 87, 89;

and respiration, 100-101
Adrenaline, 643

Adult, in insect metamorphosis,
424, 434

Adventitious roots, 318, 319-320
Aeciospores, 271
Aedes mosquito, 443
Aerial roots, 318
Aerobes, 228

Afferent branchial artery, 467
African elephant, 530
Afterbirth, 656
Agar-agar, 287-288
Age, chronological and mental,
166

Agglutination, of red blood
cells, 586, 587
Agglutinins, 244
Agglutinogens, 161-162
Agnatha, 459

Air, and bird flight, 509;
breathing and, 603-605;
gases in, 40-41; and malaria,
3; movement of, 672—677
Air bladder 468
Air sacs, or bird, 516; human,
601, 602

Albino, 166; corn, 147; fawn,
187

Albumen, 518, 569
Alcohol, 77; body and, 630-
633; fermentation and, 100,
222-223, 228-229, 270
Alcoholics Anonymous, 632
Alcoholism, 631—633
Alfalfa, 667, 668, 713, 714, 716
Algae, 90, 277-290, 285; cellu¬
lar division, 107-108; and
giant clam, 402; and space
travel, 289, 610

Alimentary canal, 479, 571, 575,
576

Alkalinity, 673-674
Allantoic duct, 655
Allantois, 654

Alleles, 122, 127, 139, 148;
multiple, 161-162, 165; re¬
combination of, 189; and Rh
factor, 162
Alligators, 501, 502
Alpha particles, 36
Alternation of generations, 286,
359

Altitude, climate and, 700, 701;

and respiration, 609-610
Alveoli, 601, 602
Ambergris, 529

Ameba, 28, 255 -256, 694, (Ta¬
ble) 261
Amebocytes, 376
Ameboid movement, 254
American hellbender, 474
Amino acids, 47— 48, 57, 96-98;
in body, 568; gastric fluid
and, 577; and genetic code,
142-143; intestinal fluid and,
577-578; in Neurospora, ISO-
152

Amino group, 47
Ammonia, and nitrogen cycle,
667, 669

Ammonification, 667
Amnion, of birds, 518; human,
654, 655, 656; of reptile egg,
489

Amniote egg, of reptiles, 489
Amphibians, 455, 472-486
Amphioxus, 453
Amylase, 367, 577
Anabaena, 278, 279
Anabolism, 94
Anaerobes, 228

Anaerobic, bacteria, 232, 669,
676; respiration, 100, 228,
229, 270, 607

Anal, fin, 465; opening, human,
576, 579; pore, of parame-
cium, 258

Anaphase, mitosis, 104, 106;
meiosis, 112

Anatomy, comparative, 413;
definition, 12; of frog, 477,
488; human, 584—585
Anemia, and hookworms, 391
Anesthetic effect on cells, 77
Angiosperms, 304, 306-311,

(Tables) 308, 309; reproduc¬
tion in, 352-369
Animals, breeding of, 170, 178-
180; cold-blooded, 485, 491,
496, 674-675; in coniferous
forest, 702; decay of, 232; in
deciduous forest, 703; in des¬
ert, 705; diurnal and noc¬
turnal, 685-687; estivating,
485, 688; flesh-eating, 530-
531, 677—678; fur-bearing,

737; grassland, 704; her¬
maphroditic, 387; hibernat¬
ing, 688; marine, 707-708;
migration of, 190-191, 688-
692; night vision of, 627; in
rain forest, 706; and seed
dispersal, 361; soil and, 231;
tundra, 701; vertebrate and
invertebrate, 374—375, 452,
453; warm-blooded, 507,
512, 674-675

Annelids, 393-396, 400, 417,
(Table) 410

Annual, rings, 330; rhythms,
693

Annuals, 310, 311, 325; seeds
of, 366

Annulus, of mushroom, 274
Anole, 500
Anoxia, 610

Anteaters, 523, 524, 528
Antelope, 531, 532
Antennae, of crayfish, 412, 414;
of grasshopper, 429; of spiny
lobster, 416

Antennules, of crayfish, 412
Anterior horns, 616—617
Anther, 352, 356; and pollen
formation, 354— 355
Antheridium, fern, 300-301;
moss, 295—297; in Oedo-
gonium, 284

Anthocyanins, 62—63; in leaves,
346

Anthrax, immunization against,
246; Koch and, 238—239
Antibiotics, 3, 250- 252
Antibodies, 243-244, 453;

against polio, 6-7; Rh factor
and, 587

Antigens, 243—244, 587
Antipodals, 356

Antitoxins, 244; diphtheria and,
248- 249
Ants, 438—439

Anus, earthworm, 393; fish, 466;
grasshopper, 428; mollusk,
400, 401; roundworm, 391;
starfish, 406

Aorta, fish, 466; grasshopper,
429; human, 589, 590, 591,
593, 595

Aortic arches, 395, 480
Apes, 532
Apical cell, 280
Apical dominance, 328, 329
Apoda, 474

Appendages, arthropods, 410;
crayfish, 412^113, 416; in¬
sect, 432; jointed, 432; spi¬
der, 420

Appendicitis, 576
Aqueous humor, 625, 626
Arachnida, 419-421
Arachnoid mater, 613
Archaeopteryx, 506, 507
Archegonium, 295, 300-301

INDEX 775

Archeozoic era, 222
Arctic region, 700, 701
Arginine, 97, 151-153
Argon-40, 544
Aristotle, 199
Armadillos, 528
Arsenic, syphilis and, 249
Artemia, 417

Arteries, 590, 591; coronary,
590, 593; fish, 466, 467-468;
frog, 480—481; pulmonary,

589, 591, 593, 602, 603;
renal, 481, 593, 595-596

Arterioles, 591, 595-596
Arthropods, 410-421
Ascaris, 131, 390, 391
Ascomycetes, 266, 269, 270-271
Ascus, 266, 270
Aspergillus, 268
Assimilation, 27
Association areas, of cerebrum,
615

Asters, 105, 107
Asymmetrical organisms, 395
Atmosphere, carbon-oxygen cy¬
cle in, 665—667, 666; and life,
676-677; nitrogen cycle and,
667-669, 668; oxygen in,

665-667, 666; water cycle
in, 664— 665
Atoll, 382

Atomic, mass, 35, 36-37; num¬
ber, 35, 36; particles, 35;
weight, 36
Atoms, 34, 35-39
ATP, 561, 566; in photosyn¬
thesis, 87, 89; and respira¬
tion, 99, 100-101
Atrioventricular, node, 563;

valves, 588-589, 590
Atrium, bird, 517; fish, 466;
frog, 480, 481; human, 588,

590, 593

Auditory, canal, 622; nerve,
623, 624
Audubon, 735
Aurelia, 381
Auricles, 466, 480
Australia, marsupials in, 196,
524

Australoid type, 547- 549
Australopithecus, 544
Autonomic nervous system, 612,
618, 619

Autosomes, 136, 137, 141
Autotrophs, 91, 94, 677; eu-
glena as, 260-261
Auxins, 322, 328, 358
Averages, breeding ratios as,
127-128

Avery, Oswald T., 144
Avocado pear, 355
Avoiding reaction, of parame-
cium, 258
Axil, 326
Axillary buds, 326
Axolotls, 475
Axons, 612, 613

Backbone, 374—375, 453
Bacteria, 3, 28, 222-235, 238;

anthrax, 238-239, 245; cellu¬
lar division, 107-108; chem-
osynthetic, 91-92; diphtheria,
248; in lymph, 594; and ni¬
trogen cycle, 667, 668-669;
as pioneers, 694; related or¬
ganisms, 235-236; sapro¬
phytic, 681; as transformers,
678

Bacteriochlorophyll, 84, 228
Bacteriology, 2, 222-223, 238-
239

Balance, biological, 669-670; in
endocrine glands, 646; ho¬
meostatic, 70-71; sense of,
615, 623-624; upset by man,
723

Bald eagle, 735
Baldness, 139, 164
Ballooning, 420
Barberry, 271, 272
Barbiturates, 635
Barbs, of feather, 509
Barbules, of feather, 509
Bark, 330—332; tannic acid
from, 731
Barnacles, 417-418
Barriers, to dispersal, 700; ev¬
olution and, 192-193
Basal, disks, 378; metabolic
rate, 608; metabolism, 608
Base triplets, 96-98
Bases, DNA, 50-51, 95-96,
142-143; RNA, 96
Basidia, of mushroom, 274
Basidiomycetes, 271-275
Basidiospores, 271, 273, 274
Basidium, 271, 273

Beadle, George W., 150, 151-
153

Beak, bird, 506, 512, 514, 515;
turtle, 503

Bean, embryo, 369; plant, 308;

seed, 362-363; as food, 569
Bean seedling experiment, 10-
11

Beards, 164-165
Bears, 200, 531; winter and,
688

Beavers, 525, 526
Beebread, 437
Beetles, 440

Behavior, vertebrate, 455-457
Benthic zone, ocean, 707
Beta particles, 36-37
Bicuspids, 574
Biennials, 310, 311
Big Tree of Tule, 306
Bilateral, larva, 404, 405; sym¬
metry, 385- 386
Bile, bird, 516; frog, 479, 481;

human, 575, 577, 598
Binomial nomenclature, 200
Biochemical similarity, classi¬
fication and, 204
Biogenesis, 18, 19, 25
Biogeography, defined, 699
Biology, definition, 2; devel¬
opment, 2-5; specialized
branches, 12

Biomes, coniferous, 702-703;
deciduous, 703; desert, 704-
705; fresh-water, 708-709,
grassland, 703 -704; marine,
707- 708; rain forest, 705-
707, 706; tundra, 701 -702
Biosphere, 660
Biosynthesis, 44
Birds, 506-520, 508, 513; con¬
servation of, 734-737; migra¬
tion of, 690-692, 691; and
seed dispersal, 362
Birth, human, 655-656; Rh fac¬
tor and, 587
Bison, 532, 723, 737 -738
Bivalve mollusks, 401-^402
Bladder, urinary, 482, 595, 596
Blade, leaf, 341-343
Blastocoel, 653
Blastula, 653
Blind spot, 626
Blinking, 455
Block cutting, 727
Blood, at birth, 656; circula¬
tion, 2; in crayfish, 414; in
earthworm, 394-395; in fish,
466-468; in frog, 480-481;
and gamma globulin research,
6-7; human, 581-594; leeches
and, 396; in lungs, 602; mos¬
quito and, 442; oxygenated,
467, 468, 480, 535, 582, 593,
602; and pneumonia, 144;
serum from, 249
Blood type, 586; inheritance
and, 160-162, (Table) 163
Blood vessels, in earthworm,
395; human, 591-592; in
small intestine, 579
Bloodsuckers, 396
Blue babies, 656
BMR, 608

Body cavity; see Coelom
Bonds, 38; covalent, 38-39, 40;
chemical, 40, 99; double, 44;
ionic, 39; triple, 44
Bones, 455; of birds, 510; hu¬
man, 556-560, 557; in mid¬
dle ear, 623

Bony, fishes, 462-470; layer,
557, 560; skeleton, 462
Botulism, 228, 240
Boveri, Theodor, 131
Bowman’s capsules, 595, 596
Brace roots, 318
Bracket fungi, 274—275
Braford cattle, 180
Brahma poultry, 178
Brahman cattle, 179-180
Brain, 6; alcohol and, 631, 632;
bird, 536; fish, 468-469, 535,
536; frog, 482, 536; grass¬
hopper, 429; human, 584—
585, 613, 614—616; insect,
432; mammal, 535-536; of
primates, 532; reptile, 536;
vertebrate, 468—469, 535, 536
Branching, of stems, 328-329
Brangus cattle, 179
Breastbone, of birds, 510
Breathing, of aquatic mammals,

776 INDEX

528—529; of birds, 516; of
crocodilians, 501; of frog,
480; human, 600—610
Breeding, animal, 170, 178—180;

plant, 170—178
Breeding ratios, 127-128
Bridge, turtle shell, 504
Bridges, C. B., 139-140
Bridges, Calvin, 134
Bronchi, 516, 601, 603
Bronchial tubes, 601
Bronchioles, 601, 602
Bronchitis, smoking and, 637
Brood comb, of bee, 437
Brownian movement, 226
Bruises, 586
Bryophyta, 294
Bud scales, 325
Bud-scale scars, 326
Budding, 108; and grafting,
337; in hydra, 380; in
sponges, 377; in yeasts, 269-
270

Bud-mutants, 176
Buds, on plant stems, 325-326,
337

Bugs, 423, 426, 440
Bulb, of potato, 335
Bulbus arteriosus, of fish, 466
Bulk feeders, 680, 681
Bundle scars, 325
Burbank, Luther, 110-171, 173,
174-176

Bureau of Entomology and
Plant Quarantine, 443
Bursa, 561
Bushmaster, 498
Butter, 568; bacteria and, 233
Buttercup root, 316
Butterflies, 43 3—434, 682, (Ta¬
ble) 435; migration of, 689-
690; pollination and, 356

Cabbage, 177, 178
Cacti, 675, 676
Caecilians, 474
Caecum, 576
Caimans, 501, 502
Calcium, and blood clotting,
583; in body, 566; com¬
pounds, in bones, 556; para¬
thormone and, 641, 646
Calcium-40, 544
Calluses, 596
Calories, 566
Calorimeter, 608
Calyx, of flower, 352, 359
Cambium, cork, 331; layer,
308; vascular, 327, 332
Camels, 190-191, 531, 532
Camera, eye and, 627
Camouflage, 463, 681—683
Cancer, smoking and, 637; vi¬
ruses and 219—220
Canine tooth, 573
Canning, of foods, 234
Cap, of mushroom, 274; root,
314

Capillaries, fish, 466, 467-468;
human, 590, 591, 593, 596,
602

Capillarity, 337 -338
Capillary action, of ground
water, 665

Capsule, of bacterial cells, 224;

mosses, 295—297
Carapace, crayfish, 415; Crus¬
tacea, 412; turtle shell, 504
Carbhemoglobin, 582
Carbohydrates, 63; chemosyn-
thesis and, 91, 92; chloro-
plasts and, 61; as food, 565,
566—568; nature of, 44^46;
synthesis, 94; see also Pho¬
tosynthesis

Carbon, 43-^44; atom, 35, 36,
39; and photosynthesis, 86,
88-89

Carbon dioxide, 43, 665, 666—
667; acceptor, 88, 89 and
breathing, 602; and fermenta¬
tion, 100, 229, 270; forma¬
tion, 39 — 40; in leaves, 346;
molecule, 39; and photosyn¬
thesis, 84, 86, 88-89, 90, 346,
347; poisoning, 609; and
respiration, 99—100; and
sponges, 376
Carbon-14, 85, 86, 543
Carbon-oxygen cycle, 665-667,
666

Carbonic acid, roots and, 314
Carboniferous period, 298, 304;

reptiles in, 489
Carboxyl group, 47
Cardiac muscle, 562, 563
Caribou, 531, 532
Carnivores, 530— 531, 677-678,
687

Carotenes, 61; in leaves, 345—
346

Carotid arches, of frog, 480
Carrel, Dr. Alexis, 28 -29
Carriers, of disease, 240-241
Carrion eaters, 512, 513
Carrot root, 317
Cartilage, 556
Casein, 569
Cassowary, 51 1
Cat family, 530
Catabolism, 94, 98
Catalysts, 48; chlorophylls as,
83—84; enzymes as, 95; vita¬
mins as, 57l

Caterpillar movement, of snake,
495

Caterpillars, 424, 434, 439
Cattails, 672

Cattle, breeding of, 178-180
Caucasoid type, 547
Caudal fin, 465, 483
Caudata, 474
Cavities, brain, 614
Cell sap, 62

Cells, 5; defenses against di¬
sease, 243; division (human),
652-653; division of labor,
374; environment and, 70—79;
growth and reproduction,
103-112; in human body,
551; interdependence, 374;
and living organisms, 26; me¬

tabolism, 94-101; multicel¬
lular organisms, 374; parts of,
57-65, 59, 61; processes of,
56—57; red blood, 582, 586-
587; somatic, 134, 136, 141;
specialization, 65—66, 71,

374, 394; viruses and, 214,
215-216; white blood, 582,
583

Cellular organization, 26; clas¬
sification and, 204
Cellulose, 45-46, 63; in diges¬
tive system, 568; molecules,
in telophase, 107; termites
and, 439—440, 681; in trees,
728

Cementum, 574

Cenozoic Era, birds in, 506—507
lobed fins and, 472; mammals
in, 195, 522; ungulates in,
532

Centipedes, 410, 418- 419
Central, cylinder, root, 316;
nervous system, 612; neurons,
612, 618

Centrifuges, 56, 64
Centrioles, 105
Centromeres, 106
Cephalization, 387
Cephalopods, 403—404
Cephalothorax, 412
Cereals, 566

Cerebellum, bird, 517; fish, 468;
frog, 482; human, 615, 624;
mammal, 536

Cerebral hemispheres, of ver¬
tebrates, 535—536
Cerebrospinal fluid, 614
Cerebrum, 455; alcohol and,
631; bird, 517; fish, 468-
469; frog, 482; human, 614-
615, 624, 625; vertebrate,
535-536

Cervix, 650, 655
Cestoda, 386, 389-390
Cetaceans, 528—529
Chambered nautilus, 403-404
Chameleon, 500
Characteristics, acquired, inher¬
itance of, 185; chromosomes
and, 132-133; inherited, 116,
131; see also Inheritance;
Traits

Cheese, 568, 569; bacteria and,
233; molds and, 268
Chelipeds, 413, 416
Chemical, bonds, 40, 99;

change, 33, 40; control of
insects, 445^446; cycles, 232-
233, 664-669; defenses

against disease, body’s, 243-
244; energy, 34, 94, 98-101;
equation, 39; food preserva¬
tives, 234; organization, of
living things, 52; phase of di¬
gestion, 576-578; reactions,
34, 37, 39-40, 49, 84-89,
94, 347; symbols, 34
Chemistry, biology and, 3
Chemosynthesis, 91-92
Chemosynthetic bacteria, 228

INDEX 111

Chemotherapy, 3, 249
Chemotropic bacteria, 228
Chick embryos, incubation tem¬
perature, 8

Chicken heart muscle experi¬
ment, 28 —29

Chickens, 518; see also Poultry
Chigger, 421
Chilopoda, 418, 419
Chimpanzee, 532
Chiropterans, 525
Chitin, 410, 414, 426
Chlorella, 85, 281; photosyn¬
thesis and, 85; and space
travel, 289, 610
Chloroform, 77

Chlorophylls, 61, 62, 91; in
algae 277, 279, 286-287; in
leaves, 343, 345—346; and
photosynthesis, 82, 83-84, 87
Chlorophyta, 280
Chloroplasts, 61, 62; in euglena,
260; in leaves, 343, 344; of
Oedogonium, 283; and photo¬
synthesis, 82- 83, 84, 85, 87,
88-89; of Spirogyra, 281-
282

Cholera, 228
Cholesterol, 47
Cholinesterase, 613
Chondrichthyes, 456, 461— 462
Chordata, 452—453
Chorion, 654
Choroid layer, 624
Chromatids, 105, 110, 112,

141

Chromatophores of fishes, 463
Chromoplasts, 61-62
Chromosomal aberrations, 146
Chromosome number, 109-110,
176-177; in algae, 284, 286;
human reproduction, 648,
649, 652; in mosses, 296—297;
in plants, 354, 355, 356, 358
Chromosomes, 49, 57, 58; of
fruit fly, 136, 155-156; genes
and, 120, 126, 132-134; ho¬
mologous, 110—112; human,
137, 146, 155 -156; mitosis
and, 104, 105, 106-107, 112;
and Mongolian idiocy, 167-
168; mutations and, 146, 189;
recombination of, 189
Chrysalis, of butterfly or moth,
434

Chrysophyta, 286
Cicada, 440—441
Cilia, 387, 388; in human tra¬
chea, 601; of mollusks, 401;
of paramecium, 257; of
trochophore larva, 400
Ciliary muscles, 624
Ciliates, 694
Ciliophora, 257

Circular muscles, of earth¬
worms, 394

Circulatory system, bird, 517;
in crayfish, 414; of fish, 466-
467; of frog, 480-481; of
grasshopper, 429; human,
581, 589-598, 592; in mam¬

mals, 534-535; vertebrate,
454

Cirrhosis, 631
Citrulline, 151—153
Citrus fruits, mold on, 268
Clams, 399, 401, 402; starfish
and, 405-406

Classes as Linnaean grouping,
204

Classification, 199-207, (Ta¬
bles) 202-203; phylogenetic
tree and, 406-608; verte¬
brates, 374-375
Claws, in carnivores, 530
Clay, 673

Cleavage, of cytoplasm, 104,
107; furrow, 107
Climate, forests and, 731
Climatic, barriers, 700; zones,
700, 701-101

Clitellum, of earthworm, 394,
396

Cloaca, bird, 517; frog, 479,
482, 483; turtle, 503
Clotting of blood, 566, 581,
583-586, (Table) 586
Clover, 667-669, 713, 716
Club mosses, 301
Coal, 299
Coca plant, 634
Cocaine, 634

Cocoon, of butterfly or moth,
434; of earthworm, 396; of
spider, 420
Codeine, 634

Coelenterates, 377—382, 385,

386

Coelocanth, 472
Coelom, 407-408; of human
embryo, 653
Coenzymes, 49, 87, 91
Cohesion, and water rise, 338
Colchicine, 177, 178
Cold, deserts, 704, 705; recep¬
tors, 619-620
Coleoptera, 440
Collar cells, 376
Collarbone, of birds, 510
Collecting tubule, 595
Colloids, 42, 72, 577
Colon, frog, 479; grasshopper,
428; human, 576, 579,

598

Colonial organisms, 65, 66
Colonies, algae, 277, 278
Color blindness, 139, 162 -163,
164

Coloration, of fishes, 463; of
moths, 190; protective, 681;
of toad, 476
Comb, of bee, 437
Combustion, 98
Communities, biotic, 693—697
Companion cells, bark, 332
Competition, 186-187, 230,

662

Complemental air, 605
Compost, 715

Compound, eyes of crayfish,
412, 414; leaf, 342, 343; mi¬
croscope, 13— 15

Compounds, 38-40; inorganic,
43; mineral, 43; organic, 43-
48

Computers, 10
Concave cell, algae, 280
Concentration, molecular, 73,
75

Conches, 402
Concussion, 614
Conditioned reactions, 457
Condor, 735, 736
Cones, of eye, 625-627; pine,
304, 305-306

Conifers, 305, 306, (Table)
^ 306

Conjugation, in bread mold,
267-268; in paramecium,
259; in Spirogyra, 281, 282
Conservation, bird, 734-737;
fish, 732-734; forest, 732-
734; importance of, 712;
mammal, 737-739; soil, 712-
718; water, 718-720
Constrictors, 493 -494, 497
Consumers, food, 677-678
Contour, farming, 716; feathers,
508-509

Contraction, of heart muscle,
590; muscle, 561, 562, 615;
of stomach, 575
Control, in experiment, 9, 10-
12; of insects, 443—448, 727-
728

Conus arteriosus, of frog, 480
Convolutions, of cerebrum, 614
Copepods, 418, 708
Coracoid bones, of birds, 510
Corals, 376, 378, 381-382
Cork, 308, 330-331
Corm, 335

Corn, as food, 569; gene muta¬
tions in, 188; hybrid, 172,
173, 174, 175; seed (kernel),
363-364, 366, 368, 369; smut,
272, 273

Cornea, 624 625, 626
Cornish poultry, 178
Corolla, of flower, 352
Corpus luteum, 651
Correns, 132
Corridor, land, 190
Cortex, of adrenal glands, 642-
643, 644; bark, 331; of

cerebrum, 614—615, 631; kid¬
ney, 595; plant, 308; root,
315; stem, 334
Cortin, 642-643
Cortisone, 643

Cotyledons, 306, 362; of bean
seed, 362, 363, 368-369; corn
kernel, 363-364, 366, 368—
369

Cougar, 200, 530
Countershading, 463, 682
Cow, 531, 532
Cowper’s gland, 648—649
Cowpox, 245

Coxa, of grasshopper, 427
Coyote, 530
Crabs, 410, 416
Cranial cavity, of bird, 517; of

778 INDEX

fish, 468; human, 552; of
mammals, 536, 622, 625
Crayfish, 410, 412^416, 413,
414

Cretaceous period, birds in,
596; crocodilians in, 501;
marsupials in, 524
Cretinism, 640
Crick, Francis H. C., 49, 50
Cricket, 430
Cristae, 60

Crocodilians, 501— 502
Crop, of bird, 513-516; of
earthworm, 394; of grasshop¬
per, 428

Crops, cover and row, 713, 714;

disease-resistant, 171, 172
Cross-pollination, 173-174, 356;

of pea plants, 117
Crosses, blood types and, 162;
diagramming, 122; dihybrid,
124-126, 125; of garden peas,
118-120; monohybrid, 122-
124, 123, 162; of plants, 173-
176, 175

Crossing over, 141
Crown, of tooth, 574
Crustaceans, 412-418, 677, 678
Cryptomonads, 287
Cud, 532

Culex mosquito, 262, 443
Cup fungus, 270
Cuspid tooth, 574
Cuticle, of earthworm, 395; of
flukes, 388; of leaf, 343; of
parasitic worms, 388
Cuttings, 177
Cyanophyta, 279
Cycads, 304

Cycles, chemical, 232-233,
663-669; lysogenic, 219; lyt¬
ic, 216-217; ovarian and
uterine, 650—651; in photo¬
synthesis, 89; see also
Rhythms

Cyclostomata, 459
Cylinder, central of root, 316
Cysts, of parasitic worms, 389,
390, 382
Cytolysins, 244

Cytoplasm, 57-63, 83, 94, 96;
centrioles in, 105; cleavage
of, 104

Cytosine, 50, 96, 142

Dairy industry, bacteria and,

234

Dams, 719, 720; fish and,

733

Dark reaction, of photosynthe¬
sis, 86, 88—89
Dart, Dr. Raymond, 544
Darters, 468

Darwin, Charles, 185—187, 188,
504, 542-543
Dating, fossil, 543-544
Daughter cells, 103—104, 107;
in bacteria, 225; in parame-
cium, 259- in plant ovule,
356; in pollen sac, 354-355;
in yeast, 269-270

DDT, 445-446, 727
Deamination, 569
Decay, 232, 667
Decomposers, 678
Deer, 531, 532; restoration,
738

Defenses against disease, 242-
244

Degeneration, of parasitic
worms, 388

Dehydration, 565; of food, 234
Dendrites, 612, 621
Denitrification, 669
Density studies, of population,
661-662
Dentine, 574

Deoxyribonucleic acid, 50-52;
see also DNA

Deoxyribose sugar, 50, 51;

units, 142

Depletion, of ground water,
718-719; of soil minerals,
713-714

Depressant, alcohol as, 631
Dermis, human skin, 596
Descent of Man, The, 542
Desert region, 686-687, 700,
704 -705
Desmids, 286
Deuterium, 36
Deuteromycetes, 275
Devonian period, 454, 472;

fishes in, 462-463
DeVries, Hugo, 132, 187, 188
Dextrose, and nutrition, 568
d’Herelle, F. H., 216
Diabetes, insipidus, 642; mel-
litus, 166, 643—644
Diaphragm, human, 552, 601,
603, 604
Diastole, 590
Diatomaceous earth, 287
Diatomic molecules, 38
Diatoms, 286-287, 376
Dicots, (Table) 309; flowers
of, 354; leaves of, 341; seeds
of, 362, (Table) 364; stems,
333-334, 335

Diet, carbohydrates in, 566-
568; vitamins and, 571
Differentially permeable mem¬
brane, 72, 75, 76
Diffusion, 72, 73-78, 74, 75;
successive osmosis and, 320-
321

Digested foods, in blood, 581
Digestion, in ameba, 255; cel¬
lular, 57; in crayfish, 414; in
grasshopper, 428; in humans,
571-579, (Table) 578; in
hydra, 379-380; in parame-
cium, 258; in planarians,
387; in roundworms, 390-
391; in sponges, 377; in star¬
fish, 405-406

Digestive enzymes, 57, 568,
571, 576-578; of bacteria,
227-228

Digestive system of bird, 513-
516; of earthworms, 394-
395; of fishes, 465-466; of

frog, 479; of roundworms,
390-391; vertebrate, 454
Dihybrids 125-126, 125
Dinoflagellates, 287
Dinosaurs, 490— 491
Diphtheria, 228, 247-249
Diploid chromosome number,
110, 112, 120, 176; human,
648, 652; mosses, 29; plants,
356, 358; and sex chromo¬
somes, 136
Diplopoda, 418, 419
Diptera, 441^443
Disaccharides, 45, 89; and nu¬
trition, 568

Disease-resistant crops, 171,

172

Diseases, alcoholism, 631-633;
diphtheria, 228; fungi and,
271-273, 275; housefly and,
441; infectious, 5, 238—252;
inherited, 166; protozoans
and, 262-263; smoking and,
637; viruses and, 212, 213-
214, 215, 219-220; vitamin-
deficiency, 569, 631
Dispersal, of animals and plants,
699-700; barriers to, 700;
man and, 699—700; sweep-

"J QQ

Distillation products, 729-730
Diurnal organisms, 685-687
Division, cell, 103-112; in bac¬
teria, 229—230, 231; rate of,
107

Division of labor, among bees,
436-437; cellular, 66, 374;
among insects, 432
Division plate, 107
DNA, 57, 566; bacteria, 225;
bases in, 50-51, 95-96,

142-143; and cancer, 220;
control of cell activities, 52,
58, 63; genes and, 142-146;
phage, 216, 217-219; proof
of, 143-144; and protein syn¬
thesis, 95-98; replication, 51-
52, 104-105; structure of,

50-52, 51; in virus core, 214
Dodo, 511
Dog family, 530-531
Dolphins, 528, 529
Domagk, Dr. Gerhard, 249
Dominance, incomplete, 126—
127, 128; Mendel’s law of,
120, 135; and nondisjunc¬

tion, 139-141; principle of,
117-120

Dominant characteristics, 120,
122, 123-124; genes, 157-
159

Dormancy, 688; in seeds, 366
Dormant plants, 674
Drainage, mosquitoes and, 447
Drone bee, 110, 436, 437
Droplet infection, 241
Drosophila, 134, 135, 140-142,
149; see also Fruit fly
Droughts, 718—719
Drugs, antibiotics, 250- 252;
barbiturate, 635; colchicine,

INDEX 779

176, 177; narcotic, 634-635;
prontosil, 249; salvarsan,
249; sulfa, 250; thiouracil,
640

“D. T.’s,” 632

Duckbilled platypus, 523, 524
Ducks, 510, 512, 518
Ducts, bile, 576; and digestion,
571-572, 573; lymphatic,

593, 594; pancreatic, 576
Ductus arteriosus, 656
Dujardin, 55
Duodenum, 575, 576
Dura mater, 614
Dwarfism, 640-641, 642
Dysentery, amebic, 263

Eardrum, of frog, 478; human,
622, 623-624

Ears, bird, 517; fish, 464, 469;
frog, 478; human, 622-624,
623; snake, 493, 496
Earth, age of, 182; shape of,
3-4

Earthquakes, 663, 664, 697
Earthworms, 393-396, 394
Echinoderms, 404-406; sym¬
metry, 385
E. coli, 231

Ecology, 660—711; definition,
12, 660; see also Environ¬
ment

Ecosystems, 660 —661, 662, 672,
^ 677-680; in jar, 694-695
Ectoderm, of human embryo,
653; of hydra, 378, 380; in
worms, 386

Ectoplasm, of ameba, 254
Edentates, 528
frft rprl 474

Egg (ovum), 109, 110, Hi¬
ll 2, 131, 132; of amphibians,
537; bee, 436, 437; bird,
518-520, 519; crayfish, 415;
of duck-billed platypus, 523;
earthworm, 396; fish, 469—
470, 537; of flowering plant,
356, 357; frog, 482, 483;
grunion, 692-693; human,
132, 159, 649- 650, 651-652;
in insect metamorphosis, 424,
434; of Oedogonium, 284; of
pea plants, 117, 120; reptile,
489, 537; of spiny anteater,
523; turtle, 503
Ehrlich, Paul, 2, 249
Elasmobranchii, 461
Electrical charge, and ion pene¬
tration of membrane, 77
Electrolytes, 41
Electron beam, 15—16
Electron cloud, 37
Electron microscope, 4—5, 13,
15- 16, 56; viruses and, 212-
213

Electrons, 35, 37—38, 87; trans¬
fer, 38-40

Elements, 35—39; heavy, 85,
86; in living organisms, 42-
43, (Table) 42

Elimination, bird, 517; crayfish,

414; earthworm, 394; fluke
eggs and, 388; grasshopper,
428; hydra, 380; planarian,
387; in sponges, 377; in star¬
fish, 406; and tapeworms,
389-390

Elk, 521, 532, 688
Elongation region, root, 314,
" 315

Embryo sac, flower, 356-358
Embryology, 182-184
Embryos, bird, 518, 519; com¬
parison, 182-184, 186; corn,
365-366, 369; of fish, 469;
human, 652-655, 654; of
mammals, 537; marsupial,
524; plant, 358-359, 362-
364, 369; skeleton of human,
556

Emphysema, smoking and, 637
Emu, 511
Emulsion, 577
Enamel, of tooth, 574
Encephalitis, 443
Enders, Dr. John F., 6
Endocrine system, vertebrate,
^ 454

Endoderm, of human embryo,
653; of hydra, 378, 379, 380;
in worms, 386
Endodermis, root, 315
Endoplasm, of ameba, 254-255
Endoplasmic reticulum, 57, 60,
63

Endoskeleton, human, 552—556,
545, 555; of vertebrates, 452,

^ 454

Endosperm, of com seed, 362,
364; of plant, 358-359; nu¬
cleus, of plants, 358
Endospores, bacterial, 230- 231
Endotoxins, 242
Energy, 33-34, 40, 73, 94;
body, 566, 568; from chemo-
synthesis, 91-92; food chains
and, 678—680; as fuel for fife,
81—82; levels, of electrons,
38, 87; of plants, 348; re¬
lease in body, 607-608; re¬
quirement of living things,
26; respiration and, 98-101;
transfer through photosyn¬
thesis 82—90
Entomology, 423
Environment, 12, 30; adapta¬
tions to, 190; and algae re¬
production, 286; balance in,
669—670 biosphere and, 660;
birds and, 507; carbon-oxy¬
gen cycle in, 665—667, 666;
cells and, 70—79; changes in,
685—697 ; and convergent evo¬
lution, 197; crustaceans and,
417 — 418; ecology and, 660;
ecosystems as, 660 -661; and
evolution, 190; heredity and,
116; insects and, 432; inter¬
actions in, 662—664; isolated,
192—194; levels of organiza¬
tion in, 662; and life, 29-31,
70-79; light and, 81-82, 676;

marsupials and, 195-196; ni¬
trogen cycle in, 666-669,
668; organisms in, 187-188;
physical, 662, 663, 673-677;
populations in, 190, 661-662;
root responses to, 322-323;
and speciation, 194-195; wa¬
ter and, 663-665, 664, 676-
677, 718-721

Enzymes, 48; amylase, 368; of
bacteria, 227-228; digestive,
57, 568, 571, 576-578; ex¬
tracellular, 95; and genes,
153, 168; intracellular, 95;
lysozymes, 242; in mitochon¬
dria, 60; in Neurospora, ISO-
152; and phenylpyruvic idi¬
ocy, 168; photosynthesis and,
84; respiratory, 98, 99-100;
RNA control of, 58—63
Eocene period, bats in, 525; sea
cows in, 529

Epicotyl, of bean seed, 363,
369; corn kernel, 364, 369; in
germinating seed, 368, 369
Epidermis, or earthworm, 395;
human skin, 596; of leaf, 343;
plants, 308; root, 315; sponge,
376; stem, 334; twig, 330
Epididymis, 648
Epiglottis, human, 601
Epinephrine, 643
Epiphytes, 706
Epithelial cells, human, 596
Epithelial tissue, 551, 552, (Ta¬
ble ) 553

Epithelium, of brain, 614
Equator, cellular, 106, 107
Equatorial plate, 106, 110, 112
Equatorial regions, 687
Equilibrium, 615, 624; molecu¬
lar, 73

Equisetums, 300, 301
Erepsin, 577
Ergosterol, 47
Erosion, 715—718
Erythrocytes, 582
Erythromycin, 251
Eskimos, 158

Esophagus, of bird, 513; of
crayfish, 414; of earthworm,
394; of fish, 466; of grass¬
hopper, 428; human, 574
Estivation, 485, 688
Estrogen, 644, 650
Estuaries, 672—673
Ether, 77; diffusion of, 73
Euglena, 259 -260, (Table) 261
Euglenoid movement, 260
Eustachian tube, of bird, 517;
tube, of frog, 478, 479; hu¬
man, 622

Evaporation, 565, 665; human
skin and, 597; reptile skin
and, 489; wind and, 677
Evolution, 504; adaptive radia¬
tion and, 195—196; conver¬
gent, 197; Darwin and, 185-
187, 188; definition, 182; and
environment, 190; evidence
for, 182-184; Lamarck’s

780 INDEX

theory of, 184-185, 187-188;
migration and, 190-191; mu¬
tations and, 187, 188—189,
190-191; natural selection
and, 185-187, 188-191; phy¬
logenetic tree and, 406-408;
recombination and, 189;
speciation and, 193-195; of
vertebrates, 453—454
Excretion, 72; alcohol and, 630;
in birds, 516—517; cellular,
57; in crayfish, 414; in earth¬
worm, 395; in frog, 481—
482; in grasshopper, 428; hu¬
man, 594-597; see also Elim¬
ination

Exoskeleton, 410, 411-412,

415-416

Exotoxins, 241—242, 244
Experiments, controlled, 8—11
Expiration, 604
Extensors, 562

Extinction, of species, 30, 735
Eyepiece, of microscope, 14
Eyes, of bird, 517; color, 165—
166; compound, 429; of fish,
464, 469; of frog, 478; hu¬
man, 624— 627, 625, 626; of
land snails, 403; of potato,
335; simple, 429; of snakes,
493; of tuatara, 492; of turtle,
504

Eyespot, of euglena, 260; in
planarian, 387

Fi generation, 119, 120; of di¬
hybrids, 124-126; of fruit
flies, 135

F2 generation, 119, 120; of di¬
hybrids, 125; of fruit flies,
135

Fallopian tubes, 650, 651, 654
Family, as Linnaean grouping,
204

Fangs, carnivore, 533; snake,
494, 499

Farming, contour, 716
Fat bodies, of frog. 485
Fatigue, 607

Fats, 46- 47; as food, 565, 568;

lipase and, 578
Fatty acids, 46, 568, 577-578
Fatty liver, 631
Feathers, 508— 509, 510—511
Feces, human, 579; and tape¬
worms, 389

Feedback, and endocrine
glands, 646

Feet, of birds, 512, 514, 515;
claw, 413; frog, 476, 477,
478; of starfish, 404-405
Femur, of grasshopper, 427 ; hu¬
man, 556, 557, 560
Fer-de-lance, 498
Fermentation, 100; bacteria
and, 222-223, 228-229; Pas¬
teur’s studies of, 23-25, 222-
223; sauerkraut and, 233; of
silage, 233-234; vinegar and,
233; yeast and, 270
Ferns, 298-301, 299

Fertility, plant breeding and,
177-178

Fertilization, of fish eggs, 469,
537; of flowers, 357-358; of
frog’s eggs, 483, 537; of gam¬
etes, 109, 112; of human
ovum, 649— 650, 651—652; of
mammals’ eggs, 537; mem¬
brane, 651

Fertilizers, algae as, 287
Fetus, human, 653, 656
Fever, 243

Fibers, muscle, 561-563; nerve,
612, 615, 616; phloem, 308,
331-332; xylem, 308, 332
Fibrils, aster and spindle, 105
Fibrin, 586
Fibrinogen, 581, 586
Fibrous roots, 313—314
Fibrovascular bundles, 334, 346
Filaments, algae, 277, 278; of
flower, 352
Filoplumes, 508
Filtration, urine and, 596
Finches, 512

Fingers, of primates, 532, 533
Fins, 463, 464-465; lobed, 472-
473

Fire, forests and, 725-727, 726
Fish ladder, 732, 733
Fish and Wildlife Service, 737
Fisher, R. A., 191
Fishes, 19, 456, 455-470; blind,
676; conservation of, 732-
734; countershading of, 463,
682; as food, 566; forage,
732; game, 732; stocking of,
734

Fission, 387; in algae, 279, 281;
in ameba, 256; and bacterial
reproduction, 229— 230; cellu¬
lar, 103-112; in paramecium,
259; in spirochetes, 236
Flagella, of bacteria, 225-226,
227; of collar cells, 377; of
euglena, 260; human sperm,
649; of water molds, 268
Flagellates, 694
Flatworms, 386—390
Flax, retting of, 232
Fleming, Alexander, 3
Fleming, Sir, Arthur, 250
Flesh-eating mammals, 530-
531, 677-678
Flexors, 562

Flight, birds and, 506, 507-
508, 510-511; mammals and,
525

Floods, 718-720
Florey, Dr. Howard, 250
Flowering plants, 303, 306—311;

reproduction in, 352—369
Flowers, 307, 352—358; of pea
plants, 117
Flukes, 388—389
Follicle, ovarian, 650-651
Food, (Table) 567; algae as,
287, 288, 289; in blood, 581;
cell metabolism and, 94;
chains, 678-681; definition,
565; infection, 240; mole¬

cules, 57; poisoning, 240;
preservation, 234-235, 568-
569; pyramids, 679— 680;
spoilage, 234; webs, 679; see
also Nutrition
Foraminifers, 263
Ford, E. B., 191
Forebrain, 535

Forest Service, 725, 726, 729
Forests, areas of in U.S., 723,
724; coniferous, 700, 701,
702-703; conservation of,
723-731; deciduous, 700,
701, 703; rain, 705—707,

706; succession in, 695
Formula, chemical, 39
Fossils, bird, 506, 507; dating,
543-544; of mammals, 522;
of mollusks, 399; plant, 304;
reptile, 489; vertebrate, 453-
454

Four-o’clocks, 126, 127
Fovea, 626, 627
Foxes, 531

Fractures, bone, 556, 557
Fragmentation, of algae cells,
278

Freezing, of foods, 234
Frogs, 19, 455, 456, 475-488
Frond, of fern, 300
Frontal lobes, 615—616
Fructose, 44, 45; intestinal fluid
and, 578; and nutrition, 568;
PGAL and, 89

Fruit, 359-363; of flowering
plants, 307; as food, 566;
maple, 195

Fruit fly, 134 —136, 137- 141,
140; mutations in, 149
Fry, fishes, 732
FSH, 650

Fuel, 94; body, 566; for respira¬
tion, 99; see also, Nutrition;
Photosynthesis

Functions, of living things, 12
Fungi, 265-275
Funk, Dr. Casimir, 569
Fur-bearing animals, 531; de¬
struction of, 737-738
Furrows, erosion and, 716, 717

Galactose, 45; intestinal fluid
and, 578; and nutrition, 568

Gallatin National Forest, 695-
697

Gallbladder, bird, 516; frog,
479; human, 575, 597

Game, birds, 736- 737; fishes,
732; laws, 661

Gametes, 109; of bread mold,
267; chromosome number in,
110; human, 648—650, 651-
652; and law of segregation,
120; mosses, 295-297; pla¬
narian, 387 ; in Punnett
square, 122; recombination
and, 189; of Ulothrix, 283,
284-286

Gametophyte generation, algae,
286; ferns, 300—301; in flow-

INDEX'" 781

ering plants, 359; mosses,
295-297

Gamma globulin, 6—7, 9
Gamma rays, 37
Ganglia, of bivalve mollusks,
401; of earthworms, 396; of
grasshopper, 429; of retina,
625; in spinal nerves, 617;
sympathetic, 618
Gases, 33; diffusion of, 73;

methane, 229; mixture, 40—41
Gastric, caeca, of grasshopper,
428; fluid, 575, 577; glands,
479, 574, 577; protease, 577
Gastritis, 631
Gastrocoel, 653
Gastropods, 402-403
Gastrovascular cavity, of hydra,
380; of medusa, 381
Gastrula, 653
Gavials, 501, 502
Geckos, 500

Gel phase, 42, 58; in prophase,
105; in telophase, 107
Gemmules, 377

Gene frequencies, human, 157—
159

Gene linkage, 134, 141
Gene pools, 157-159, 195
Genes, 27, 104, 110, 120-124,
648; action of, 144, 146; be¬
havior and, 455—457; and
chromosomes, 132-134; and
enzymes, 152, 168; human,
144, 146, 155, 163-167; re¬
combination of, 189; Sutton’s
hypothesis of, 132-133
Genetic code, 142; DNA and,
52, 95, 104; errors in (muta¬
tions), 146-149; RNA and,
58

Genetics, breeding and, 170;
chromosome number and,
110; definition, 12, 116; dia¬
gramming crosses in, 122; hu¬
man, 155-168; meiosis and,
112; Mendel and, 116—128,
131-132; molds and, 150-
153; population, 156-159,
190; sex linkage in, 137—139;
see also Heredity; Inheritance
Genital pore, in flukes, 388
Genotypes, 120-122; of dihy¬
brids, 125

Genus, as Linnaean grouping,
204; characteristics, 195
Geological timetable, 182, 183
Geotropism, 323
Germ, layers, primary, 653,
(Table) 655; mutations, 146
Germ Theory of Disease, 238,
239

Germination, bean seed, 367,
369; corn kernel, 368, 369;
seed, 366-369, 367, 368
Germinative layer, human skin,
596

Gestation periods, of mammals,
(Table) 537
Giants, 642
Gibbon, 532

Gila monster, 500-501
Gills, 455, 463, 464, 466, 467-
468, 534; arch, 467, 468;
cover, 464; in crayfish, 413,
415; in Crustacea, 412; fila¬
ments, 467; of mollusks, 400,
401; of mushroom, 274; rak¬
ers, 467; slits in chordate,
453, 455

Ginkgo tree, 304, 305
Giraffe, 531-532
Gizzard, bird, 516; earthworms,
394; grasshopper, 428
Glands, endocrine, 639—646,
640, (Table) 645; gastric,
479, 574, 577; intestinal, 575;
mammary, 523, 537; mucous,
of frog, 479; pancreas, 643-
644; prostate, 648—649; sa¬
livary, 572, 517; tear, 627
Glomerulus, 596
Glottis, of frog, 479, 480; of
snake, 495

Glucose, 44, 45; intestinal fluid
and, 578; light and, 85; liver
and, 577; molecules, 62; and
nutrition, 568; oxidized in
respiration, 99-101; PGAL
and, 89; photosynthesis and,
82, 84, 85, 89, 94, 349; and
plant respiration, 347-348
Gluten, 569

Glycerin, 568; lipase and, 577;

villi and, 579
Glycerol, 46
Glycine, 98

Glycogen, 45; in body, 568,
607; liver and, 577
Gnawing mammals, 525—526
Goat, 531
Goiter, 640

Golden plover, 690, 691
Golgi, Camillo, 60
Golgi bodies, 57, 60-61, 64
Gonadotropic hormones, 641
Gonads, 469, 648
Gopher, 525; tortoise, 504
Gorilla, 532, 546
Grafting, 177, 336—337
Grana, 61, 82- 83, 84, 85, 87
Grand Canyon, 192
Granules, 84
Grasshopper, 426-430
Grasslands, 700, 703 —704
Gray matter, 614—615, 616,

617

Gray wolf, 530
Grebe, 513
Green glands, 414
Griffith, Frederick 143—144
Gross, Dr. Ludwik, 219
Grouse, 518

Growth, of ameba, 256; bac¬
teria, 226-229; cellular, 103;
of embryo, 652-655, 654; of
fungi, 274; hormone, 426,
641; of living organisms, 26-
29; root, 322—323; of seed¬
ling, 368-369; of stems, 326-
328; of tapeworms, 390
Grub, 424

Grunion, 692, 693
Guanin crystals, of fishes, 463
Guanine, 50, 96, 142
Guard cells, leaf, 343, 346-347,
350

Guinea, pigs, dominant and re¬
cessive genes in, 123-124;
worm, 390
Gull, 456

Gullet, of euglena, 260; of frog,
479; of paramecium, 257; of
snake, 495

Gymnosperms, 303, 304-306

Habitats, 672-683
Hairs, root, 315; in statocyst,
414-415
Half-life, 543
Hammer bone, 622
Haploid chromosome number,
110, 112, 120, 136; human,
648, 649; in mosses, 296;
plants, 355, 356, 358; of
Ulothrix, 284
Haploidy, 189
Hares, 526, 528
Harvestman, 421
Harvey, William, 2
Hatchery programs, 734
Haversian canals, 557, 560
Hawks, 512, 513, 520, 685
Healing of wounds, 583-586
Hearing, of bird, 517; human,
623-624

Heart, of amphibians, 535; of
bird, 455, 517; dorsal, 410;
fish, 455, 466, 534-535; frog,
455, 480-48 1; of grasshop¬
per, 429; of mammals, 455,
534- 535; of reptiles, 535;

criQ vp 4^n

Heart (human), 587-593, 588
Heartwood, 330
Heat, body, 565, 566, 597; re¬
ceptors, 619—620; from respi¬
ration, 100; from sun, 660
Heine, Dr. Jacob, 6, 8
Hemiptera, 440
Hemlocks, 305

Hemoglobin, 95, 582, 602, 607,
609

Hemophilia, 139, 163-164, 165
Hemotoxin, 499
Hen, 518
Henle’s loop, 595
Hepatic portal vein, of frog, 481
Herbaceous stems, 325, 333-
336

Herbivores, 426—427, 532, 677
Heredity, and environment,
116; human, 155-168; laws
of, 119—128; Mendel’s ex¬
periments with, 116—119; sci¬
ence of, 3, 116; see also In-

npritfliipp

Hereford cattle, 178, 180
Heroin, 634
Heterocysts, 279
Heterogametes, 109; of algae,
278; mosses, 296; of Oedo-
gonium, 284

782 INDEX

Heterotrophs, 91, 349, 677, 681;

euglena as, 260-261
Heterozygous organisms, 125,
126, 127, 135; human, 157-
158, 162, 163; and incom¬
plete dominance, 126; in
monohybrid crosses, 122-
124; mutations and, 147,
188; recombination and, 189;
and sex linkage, 137-138
Hexose, 89, 568
Hibernation, 485, 688
Hilum, of bean seed, 362
Hindbrain, 535
Hippopotamus, 532
Histidine, 98
Hive, bee, 437
Holdfast cell, 282, 283
Holstein-Friesian cow, 178
Homeostasis, 70-71; in leaves,
346

Hominidae, 205

Homoiothermic animals, 674-
675

Homologous, chromosomes,
110-112, 141; organs, 182,
413, 427, 434
Homoptera, 440-441
Homozygous organisms, 122-
124, 127; gene mutations

and, 188-189; human, 157-
158, 162, 163; and incom¬
plete dominance, 126-127; in
monohybrid crosses, 122-124
Honeybees, 435—438, 436 ; pol¬
lination and, 356 —357; win¬
ter and, 687
Hooke, Robert, 55
Hookworms, 390, 391-392
Hormones, 95, 639—644; estro¬
gen, 644, 650; FSH, 650; and
insect metamorphosis, 426;
LH, 651; progesterone, 644,
651; and root growth, 322;
and sex-limited traits, 164-
165

Hornets, 439

Horns, of gray matter, 616
Horny layer, human skin, 596
Horny outer layer (shell), 401
Horse, 522, 531, 532
Horsetail rushes, 300, 301
Host, for fungi, 265, 267, 268,
269; of infectious organisms,
241; parasites and, 680; of

{>arasitic bacteria, 227; of sea
amprey, 459— 460
Housefly, 441— 442, foot, 441
Hummingbirds, 507, 512; and
pollination, 357
Humus, 712, 731
Hybrid, com, 172, 173, 174,
175; vigor, 172, 179
Hybridization, 172
Hybrids, 120, 122— 126, 135;
and incomplete dominance,
126- 127, 128
Hydra, 378- 382, 379
Hydrochloric acid, 574, 577
Hydrogen, acceptor, 87; atom,
35, 38-39; isotopes, 36; and

photosynthesis, 87, 88; and
plants respiration, 347-348;
and respiration, 99—100
Hydrogenation, 46
Hydrolysis, 46; of starch, 577
Hydrophobia, 246—247
Hydrophytes, 675
Hydrotropism, 323
Hymenoptera, 435-439
Hyperthyroidism, 639—640
Hypha, of bracket fungi, 274-
275; of bread mold, 267, 268;
of fungi, 265; of mushroom,
274; of wheat rust, 271
Hypocotyl, of bean seed, 363,
368—369; corn kernel, 364,
366, 368-369
Hypoglycemia, 644
Hypothesis, 9-11
Hypothyroidism, 640

Ichneumon fly, 439
Idiocy, 167-168
Iguanas, 500
Ileum, 575

Image, magnification of, 14-15;

resolution of, 14—15
Immunity, 11; acquired, 244;
active, 244-245; after virus
infections, 219; natural, 244;
passive, 244-245
Immunization, 244—249; an¬
thrax, 246; diphtheria, 247-
249; polio, 7-8; rabies, 247;
smallpox, 245—246
Impulse, nerve, 613, 616— 617
Inbreeding, 173, 174
Incisors, human, 573
Incubation, of bird eggs, 518,
519, 520; temperature, of
chick embryos, 8
Indians, 159

Industrial melanism, 192
Infantile paralysis, 6; see also
Polio

Infections, phage, 216- 219,
218; virus, 214, 215, 219-
220; see also Diseases
Inflammation, and disease, 243
Infusion, definition, 23; hay,
694-695

Inheritance, of acquired charac¬
teristics, 185; chromosome
theory of, 131-132; human,
155-168

Inherited characteristics, 116,
131; chromosomes and, 133-
134

Insecticides, 445-446, 727
Insectivores, 525
Insects, 410, 411, 423—430,

432-448; birds and, 512, 513;
and disease, 235-236, 241,
262-263; forests and, 727-
728; frogs and, 476, 477, 478;
grassland, 704; as pollinators,
356—357; rain forest, 706; and
rickettsiae, 235-236; taiga,
702; tundra, 701; winter and,
687

Insertion, skeletal muscles, 562

Inspiration, breathing and, 604
Instincts, 457
Insulin, 643-644
Integumentary system, verte¬
brate, 454

Intelligence, in carnivores, 530;
inheritance of, 166-167; in
twins, 160, 167; quotient, 166
Interbreeding, barriers to, 190-
191, 192-193; migration, and,
190-191

Intercellular layer, 57, 63
Interdependence, cellular, 374
Interface, 58

Internode, plant stem, 326
Interphase, 104

Intestines, of earthworms, 394;
of fish, 466; of grasshopper,
428; human, 575; of planar-
ians, 387; parasites in, 390,
391-392; protozoans in, 263
Invertebrates, definition, 374-
375; characteristics, 452; and
vertebrates, 452, 453
Iodine, 566; and larva; and sala¬
manders, 475; in thyroid
gland, 639—641

Ions, bonds and pairs, 39; most
common, 41; and plasma
membrane, 71-73, 77; and
root absorption, 321
I.Q., 166

Iris, 176; human eye, 624—625
Iron compounds, 566
Irrigation, 717 -718, 720
Irritability, 29
Islets of Langerhans, 643
Isogametes, 109; of algae, 278;

of Ulothrix, 284
Isolation, evolution and, 192-
193

Isopods, 418
Isoptera, 439—440
Isotopes, 36— 37; and photosyn¬
thesis, 85; radioactive, 56, 64,
543-544

Isthmus, of fish head, 464

Jacobson’s organs, 493
Jaguars, 530
Jaundice, 575

Jaws, arthropod, 410; in car¬
nivores, 530; crayfish, 412;
grasshopper, 426—427 ; hu¬
man, 574; snake, 494
Jejunum, 575-576
Jellyfish, 377, 380- 381
Jenner, Edward, 2, 5, 213, 245-
246

Joints, human, 560-561
Jungle, rain forest and, 706—707
Jurassic period, 454; birds in,
506; toads and frogs and,
475-476; turtles in, 503

Kaibab squirrel, 192, 193
Kangaroo, 524, 525
Katydid, 430

Kettlewell, H. B. D., 191-192
Kidneys, bird, 516—517; fish,
466; frog, 479, 481 — 482; hu-

INDEX 783

man, 565, 568, 594-A96, 595;
of mammals, 534
Killdeer, 520

Kingdoms, as Linnaean group¬
ing, 204
Kingfisher, 513
Knee, jerk, 617; joint, 560
Knees, cypress, 318
Knipling, Dr. Edward, 447-448
Koala bear, 196
Koch, Robert, 2, 238-239
Komodo dragon, 501

Labium, of bee, 437; of butter¬
fly or moth, 433; of grasshop¬
per, 427

Labrum, of grasshopper, 426
Lacewing, 433, 441
Lactase, 577
Lacteals, 579

Lactic acid, 100; and fatigue,
607; fermentation and, 223,
229; and sauerkraut, 232; and
silage, 232
Lactobacillus, 232
Lactogenic hormone, 641
Lactose, 45; intestinal fluid and,
578; and nutrition, 568
Ladybugs, 440
Lagomorphs, 528
Lakes, as ecosystems, 660 -661,
662-663, 672, 677-680; fish
population of, 732; succes¬
sion in, 695—697, 696
Lamarck, Jean Baptiste, 184—
185, 187-188
Land formations, 677
Landsteiner, Dr. Karl, 6, 8,
160-161

Larva, bilateral, 404; of cray¬
fish, 415; of echinoderms,
404, 405; of flukes, 388-389;
insect, (Table) 426; in insect
metamorphosis, 424, 434; of
housefly, 441-442; of mos¬
quito, 442; of Odonata, 440;
of queen bee, 436; sea lam¬
prey, 460, 461; trochophore
(mollusk), 399, 400
Larval forms, 407
Larynx, human, 601
Lateral, buds, 325-326, 329;
line, of fishes, 465; undula-
tory movement, 495; ventri¬
cles, of brain, 614
Latin, classification and, 199
Latitude, climate and, 700, 701
Laws of heredity, Mendel s,
119-128
Layering, 336

Layers, of cell wall, 57, 63, 65;
primary germ, 653, (Table)
655

Leaching, 714
Leaf scars, 325, 326
Leaflet, 342, 343
Leakey, Dr. L. S. B., 544
Leather, tanning, bacteria and,
232

Leaves, 341-350, 342; as com¬

post, 714—715; flowering
plant, 307; node and, 326
Leeches, 396
Leghorns, 178
Legumes, 668
Lemurs, 532

Lens, human eye, 624, 625; of
microscope, 13-14; of snake
eyes, 493
Lenticels, 326
Leopard, 456
Lepidoptera, 433-435
Leucocytes, 243, 582
Leucoplasts, 62
Leukemia, viruses and, 220
LH, 651

Lichens^289-290, 701
Life, biochemical state of, 3;
characteristics of, 25—31;
chemical basis of, 33-52;
competition for, 186-187,
662; DNA and, 51-52; en¬
vironment and, 29-31, 70-
79; structural basis of, 55-68
Life cycle (history), Aurelia,
381, butterflies and moths,
434—435; corn smut, 272,
273; fern, 299, 300-301; frog,
484; moss, 295-297, 296;
Oedogonium, 283-284; para¬
sitic worms, 388 -392, 389;
Plasmodium, 262; Ulothrix,
282-283; wheat rust, 271,
272

Life span, of fruit fly, 135; hu¬
man, 155; of living organ¬
isms, 27-28

Ligaments, 554, 555, 560
Light, and bean seedling, 10-
11; and environment, 81-82,
676; importance of, 81-82;
leaves and, 341, 344— 345;
microscope, 4, 14—15, 56;

photosynthesis and, 82—88;
rays, 89; reaction of photo¬
synthesis, 86—88, 87; rhyth¬
mic behavior and, 685, 692-
693; stem growth and, 328;
visible spectrum, 89—90
Lignin, 63

Lille, University of, 222
Limb modifications, in mam¬
mals, 533

Lime, 381-382, 412, 426, 674
Lime-producing glands, of
birds, 518

Limiting factors, 672 -673
Linnaeus, Carolus, 199—201,
204

Lions, 530
Lipase, 577

Littoral zone, 707, 708
Liver, alcohol and, 631; bird,
516; fish, 466; frog, 479, 481;
human, 568, 575, 577, 581,
597; insulin and, 643
Liverworts, 294, 297, 298
Lizards, 500 -501; salamanders
and, 474

Llama, 190, 531, 532

Loam, 673, 712

Lobes, of human cerebrum,
615- 616; of pituitary gland,
641-642

Lobsters, 410; spiny, 416
Locker, Joseph D., 8
Locomotion, anteater, 528; of
aquatic mammals 528; grass¬
hopper, 427; snake, 495; see
also Movement
Locusts, 430
Loon, 513

Lumbering, 724, 727, 728-730
Lung books, 420
Lungfishes, 472-^173
Lungs, bird, 516; frog, 479,
480; human, 597, 601-610,
603; in lungfish, 472-473; of
mammals, 534—535; smoking
and, 637; snake, 496
Luteinizing hormone, 651
Lymph, 593—594; and disease
243; nodes, 593; vessels, 579,
593 -594

Lymphatics, 593 -594
Lymphocytes, 646
Lynx, 530

Lysogenic cycle, 219; phages,
218, 219
Lysozymes, 242
Lytic cycle, 216, 217

MacLeod, C. M., 144
McCarthy, M., 144
McIntosh apple, 176
Macronucleus, of paramecium,
258-259

Madison River Canyon, 695-
697

Maggots, 19, 20, 424, 441-442
Magnesium, 566
Magnification, of image, 14-15
Maidenhair tree, 304
Malaria, 3, 262-263, 443
Malpighi, Marcello, 2-3
Malpighian tubules, of grass¬
hopper, 428
Maltase, 577

Maltose, 45; and nutrition, 568
Mammals, 454, 455, 522-538;
man and, 542; placental, 195,
196, 197, 524, 537; pouched,
1 95- 1 96, 524
Mammoths, 522, 530
Man, Australophithecus and,

544, 548; Cro-Magnon, 546;
and deciduous forest biome,
703; development of, 542-
543, 544-549, 548; and grass¬
lands, 703; Homo habilis,
544; Homo sapiens, 546-549,
548; Java, 545; Neanderthal,

545, 548; and other mam¬
mals, 542; Peking, 545; and
physical environment, 663;
primitive, 543-546; racial
types, 548-549; Zinjanthro-
pus and, 554, 545, 547

Manatee, 529-530
Mandibles, of crayfish, 412,
414; of grasshopper, 426

784 INDEX

Mantle, of mollusks, 400, 401;

cavity, 400, 401
Maples, speciation in, 194— 195
March of Dimes, 5
Marijuana, 634-635
Marine, biome, 707-708; or¬
ganisms, 675
Marmosets, 532
Marrow, 557-560
Marsupials, 195 -196, 524
Mass, 33; selection, 171-172,
174, 178

Mastigophora, 260
Mastodons, 522, 523, 530
Matrix, of Nostoc algae, 279
Matter, 33—34

Maturation region, root, 314,
315

Maxilla, of bee, 437; of cray¬
fish, 412, 414; of grasshop¬
per, 426

Maxillary teeth, of frog, 478
Maxillipeds, of crayfish, 412,
415

Medin, Dr., 6, 9

Medulla, 6; adrenal, 642, 643;

of bird, 517; kidney 595
Medulla oblongata, of fish, 468;
of frog, 482; human, 616,
617, 619; of mammals, 536
Medusa, 380—381
Megaspores, 356
Meiosis, 111-112, 133-134; in
Ascaris, 131; and crossing
over, 141; of megaspore
mother cell, 356; of micro¬
spore mother cell, 354; in
mosses, 296; of Ulothrix, 286
Membranes, of brain, 613-614;
diffusion through, 74—78; ex-
traembryonic, 654; fertiliza¬
tion, 651; mucous, 242; nicti¬
tating, 478, 479; nuclear, 57,
60; periosteum, 557; perme¬
able, 72-73; placenta, 655-
656; plasma, 58-60, 71-73,
103, 225; pleural, 602; stom¬
ach fining, 575; tympanic,
478, 504; uterus lining, 650,
651—652; vacuolar, 60, 62;
web, 478

Mendel, Gregor, 2, 116-128,
117, 131-133, 134-136
Meninges, 613, 614
Menstruation, 651
Mental disorders, inherited,
167-168

Meristematic, regions, growth
from, 327; root, 314- 315; tis¬
sue, 308

Mesentery, of frog, 479
Mesoderm, of human embryo,
653; in worms, 386
Mesoglea, 378, 380
Mesophyll, of leaf, 343, 346
Mesophytes, 676
Mesothorax, of grasshopper,
427

Mesozoic era, 306- lobed fins
and, 472; reptiles in, 490,
491, 500, 501

Metabolic wastes, of bacteria,
230; in mammals, 533
Metabolism, in birds, 512; of
cells, 94-101; rate of human,
607-608; in mammals, 533;
thyroid and, 639—641
Metamorphosis, amphibian,
473; frog, 483-485, 484; in¬
sect, 424-426, 425, 434, 440;
peeper, 476

Metaphase, 104, 106, 110
Metathorax, of grasshopper, 427
Methane, 229
Methyl cellulose, 257
Microbiology, 13, 211
Micro gr aphia, 55
Micronucleus, of paramecium,
259

Micropyle, 355, 356, 357; of
bean seed, 362

Microscopes, 3, 4-5, 13-16, 22,
56

Microspore mother cells, 354
Microspores, 355
Midbrain, 535

Middle, ear, of frog, 478;
human, 622—624; lamella,
57, 63

Midgets, 642
Midrib, leaf, 341
Migration, 30—31; animal, 688—
689; bird, 690-692, 691; but¬
terfly, 689- 690; and specia¬
tion, 194; variations and, 190—
191

Mildews, 269

Milk, 566, 569; pasteurization,
234

Milkweed pod, 362
Millepede, 410, 418- 419
Milt, 469
Mimicry, 682
Mindanao Deep, 676
Minerals, in blood, 581; root
absorption, 321-322; in soil,
712, 713-714
Mink, 531
Mites, 235, 421
Mitochondria, 57, 60, 63; respi¬
ration and, 98—99, 100-101
Mitosis, 104-107, 105, 106, 110,
133; in ameba, 256; of hu¬
man zygote, 652; of mega¬
spores, 356-357; in Ulothrix
algae, 286

Mixtures, 40—41, solutions and
suspensions, 41-42
Moas, 511

Molars, human, 574
Molds, 265-269, 266; mutations
and, 149-152; reproduction,
108, 266

Molecules, 5, 38, 39; cell pene¬
tration by, 78-79; DNA,
142-143; metabolism and,
94; organic, 44-52; and
plasma membrane, 71-73; in
solution, 41
Moles, 525

Mollusks, 399-404, 400
Molting, in birds, 509—510; of

caterpillar, 434; in crayfish,
414; of exoskeleton, 412
Monera, 206-207
Mongolian idiocy, 141, 167-168
Mongoloid type, 546—547
Monkey kidney tissue, 6, 7
Monkeys, 532

Monocots, (Table) 308; flowers
of, 354; seeds of, 362, (Ta¬
ble) 364; stems, 334, 335
Monosaccharides, 45, 89; and
nutrition, 568
Monotremes, 523
Moose, 531, 532
Morel, 270, 271

Morgan, Thomas Hunt, 134—
136, 135, 137-139
Morphine, 634
Morphology, 399
Mosquitoes, 441, 442-443; ma¬
laria and, 262-263
Mosses, 294-297, 295, 296,

701

Mother cell, 103-104, 107;

megaspore, 355—356; micro¬
spore, 354; of yeast, 269-270
Moths, 433 -434, (Table) 435;
peppered, 191 -192; pollina¬
tion and, 356
Motion sickness, 624
Motor, neurons, 612— 613; units,
561

Motor areas, of cerebrum, 614-
615

Motor end plate, 613
Mountain lion, 530
Mountains, climate and, 700,
701

Mouth, butterfly or moth, 433-
434; earthworm, 394; fish,
466, 468; frog, 478 -479, 480;
grasshopper, 426- 427; hu¬
man, 571, 572—574; mos¬
quito, 422; roundworm, 391;
snake, 493

Mouth cavity, of paramecium,
257

Movement, of air, 677; ame¬
boid, 254; of bird wings, 509,
510, 511; Brownian, 226;
cellular, 57; euglenoid, 260;
flagella and, 225-226; of food
in stem, 338; of paramecium,
256-257; of snake, 495; of
water in stems, 337-338
Mucous, feeders, 401; glands,
frog stomach, 479; mem¬
branes, as barrier to disease,
242

Mucus, digestion and, 572; in
gastric fluid, 575, 577; fining
of uterus, 651
Mulch, 297
Mule, 179

Muller, H. J., 134-135, 148-
149

Multicellular organisms, 65;

374-375, 377
Mumps, 572

Muscles, bee, 435—436; bird,
510, 511; in bivalve mollusks,

INDEX 785

401; brain areas and, 614-
615, 616; earthworm, 394;
eye, 625, 627; fish, 465; hu¬
man, 558, 559, 561-563, 562;
tissue of, 551, (Table) 553,
561; used in breathing, 604
Mushrooms, 273— 274
Muskrat, 525
Mussels, 402

Mutations, 146—152; bud, 176;
evolution and, 187, 188-189,
190-194; and speciation, 194
Mutualism, 680—681
Mycelium, 265; bread mold,
267; mushroom, 273-274
Mycophyta, 265
Myofibrils, 561
Myriapoda, 418
Myxedema, 640
Myxomycophyta, 265

Narcotics, 630, 633—635
Nasal, cavities, of fish, 464; of
snake, 493; passages, human,
600, 622

National forests, 724, 739
National Foundation for Infan¬
tile Paralysis, 5, 7
National parks, 739
Natural selection, and insecti¬
cides, 445, 727; theory of,
185-187, 188-191
Navel, 656
Neck, of tooth, 574
Nectar, of flowers, 356-357,
433, 437
Necturus, 474
Needham, John, 22-23
Needles, of conifers, 305
Negative, charges, of electrons,
35; response, root, 322
Negroid type, 547
Nemathelminthes, 390—392
Nematocysts, 378, 379
Nematoda, 390—392
Nephrons, 595-596
Nerve, net, 380; tissue, 535
Nerves, auditory, 623; of fish,
468; human, 612—617; longi¬
tudinal, 387; muscles and,
561; olfactory, 622; optic,
625; skin endings (recep¬
tors), 619-6 20; tissue, 551,
(Table) 553; tranverse, 387
Nervous system, of arthropods,
410; of bird, 517; of bivalve
mollusks, 401; of crayfish,
4 1 4 — 4 1 5 ; of earthworms,
395-396; of fish, 468-469; of
flukes, 388; of frog, 482; of
grasshopper, 429; of mam¬
mals, 535-536; of planarians,
387; ventral, 410, 412; verte¬
brate, 455

Nervous system (human), 612-
627, 620; alcohol and, 631;
and endocrine glands, 646;
muscles and, 561
Neurons, 612— 613, 614, 617—
618

Neuroptera, 441

N eurospora crassa, 150- 152
Neurotoxin, 499
Neutrons, 35, 36
New Jersey Agricultural Ex¬
periment Station, 251-252
New Zealand, tuataras and, 492
Newt, 474—475
Niche, 672; of hawks, 685
Nitrates, 667, 668
Nitrification, 667
Nitrogen, in DNA, 49; and in¬
sectivorous plants, 345; cycle,
667-669, 668; fixation, 668
Nobel prize, 6, 134, 153, 214
Nocturnal organisms, 685-687
Node, plant stem, 326
Nondisjunction, 139-141, 140,
146, 189- human, 167- 168
Nonelectrolytes, 41
Nose, human, 600, 622
Nostrils, of crocodilians, 501; of
frog, 478, 479, 480; of snake,
493; of turtle, 504
Notochord, 453

Nucleic acids, 49-52; in virus
core, 214

Nucleolus (i), 57, 58
Nucleoplasm, 58
Nucleotides, DNA, bases in, 51-
52, 95-96, 97-98; and inter¬
phase, 104

Nucleus, atomic, 35; of cell, 48-
52, 57-58, 62, 96; mitosis
and, 104-107; of egg and
sperm cells, 131; of mega¬
spore, 356; of microspore and
megaspore mother cells, 354-
355; of paramecium, 258; of
somatic cells, 136
Nutrients, organic, 566; see also
Food

Nutrition, of bacteria, 227-228;
cellular, 57, 81-92; human,
565-571

Nutritional relationships, 677-
681

Nymph, in insect metamorpho¬
sis, 424

Objectives, microscope, 14
Occipital lobes, 614, 625
Oceanographers, 4
Oceans, day-night rhythms in,
685-687; tides, 692 -693;

zones in, 707— 708
Odonata, 440

Oil, on bird feathers, 509;

whale, 529; as food, 568
Olduvai Gorge, 544
Olfactory lobes, 468-469, 482,
517, 535, 536

Olfactory nerves, human, 622;

of bird, 517; of snake, 493
Olympic elk, 688
One-celled organisms, 28
Oocytes, 112
Oogonial cell, 111-112
Oogonium, in Oedogonium,
284

Ootid, 112

Open system of blood circula¬
tion, 414

Operculum, 464, 468
Opium, 634
Opossum, 522, 524
Opsonins, 244

Optic lobes, 429, 468, 469, 482,
517, 535

Optic nerve, of bird, 517; hu¬
man, 625

Optic region, of mammals, 536
Oral, groove, of paramecium,
257; vaccine, polio, 8, 10
Orangutan, 532
Orb weavers, 419
Orders, as Linnaean grouping,
204; of living reptiles, (Ta¬
ble) 491; of mammals, (Ta¬
ble) 526, 527
Ordovician period, 459
Organ systems, 68
Organelles, 60, 76
Organic, catalyst, 48; com¬
pounds, 43—48; matter in soil,
712, 714-715; nutrients, 566
Organs, 66—68; analogous, 413—
414; homologous, 413, 427,
434

Origin, skeletal muscles, 562
Origin of Species by Natural
Selection, 185
Ornithine, 151-153
Ordovician period, 453
Orthoptera, 426-430
Osmosis, 75-76; and leaf, 347;

in root, 320-321
Osmotic pressure, 320—321, 337
Ossification, 556
Osteichthyes, 462
Ostracods, 418
Ostrich, 507, 511, 518
Outbreeding, 172
Oval window, 623
Ovaries, 110, 112; of birds,
517-518; of fish, 469; of
flower, 352, 353, 355, 359; of
frog, 482; of grasshopper,
429; hormone secretions, 641,
644-646; human, 650-651; in
hydra, 380
Overproduction, 186
Oviduct, of bird, 517—518; of
frog, 479, 482
Oviparous snakes, 496
Ovipositors, 430, 437
Ovoviviparous snakes, 496, 497
Ovulation, 651

Ovules, 353, 358; of bean seed,
362; formation, 355— 358;
seeds and, 359

Ovum, 109; human, 649-6 50,
651-652; see also egg
Owls, 513, 517, 520
Ox, 531, 532

Oxidation, and alcohol in body,
630; in body, 565, 566, 607-
608; combustion and, 98, 99-
101; in snake, 496; thyroid
gland and, 639-640
Oxygen, atom, 35, 36; and bac¬
teria respiration, 228; and

786 INDEX

breathing, 602, 605—607; and
carbon-oxygen cycle, 665-
667, 666; debt, 605 -607;
heavy, 85; and life, 676-677;
molecules, 38-39; and photo¬
synthesis, 84, 85, 86, 88; and
respiration, 99-100; in water,
39

Oxyhemoglobin, 582
Oxytocin, 642
Owls, 670, 685

Oysters, 401, 402, 693; starfish
and, 405-406

Pain, eye and, 627; receptor,
619-620
Palate, 571-572
Paleozoic era, 277; amphibians
in, 473; lobed fins and, 472
Palisade cells, leaf, 343, 344,
346

Palpus, of grasshopper, 426
Pancreas, bird, 516; frog, 479;
hormone secretions, 643—
644; human, 575—576, 577-
578

Pancreatic, duct, 576; fluid,
479, 575-576, 577
Paralysis, 618; due to polio, 6
Paramecium, 256-259, (Table)
261

Parasites, bacteria, 227-228;
fungi, 265, 268, 269, 271-
273, 274-275; worms, 238,
287-392

Parasitism, 680—681
Parasympathetic nervous sys¬
tem, 619

Parathormone, 641, 646
Parenchyma, 308; cells, of leaf,
343; cells, root, 315; spongy,
343

Parental care, in birds, 519—
520, 537; in mammals, 537-
538

Parietal eye, 492
Parthenogenesis, 110
Passenger pigeons, 735
Pasteur, Louis, 2, 4, 5, 243; and
anthrax vaccine, 245; and
bacteriology, 222-223; and
rabies, 213, 246-247; and
spontaneous generation, 23-
25, 24

Pasteurization, of milk, 233
Pasturing, forests and, 728
Pathogenic organisms, 238-252
Pearls, 401

Pearly layer (shell), 401
Peas, 1 19; Mendel’s experiments
with, 117-120, 118, 121, 131,
134

Peat, 297, 712; moss, 297
Pectin, 63
Pectoral fins, 465
Pedicel, of flower, 352, 353
Pedipalps, of spiders, 420
Pelagic zone, 707
Pelecypoda, 401
Pelican, 513

Pellicle, 257

Pelvis, human, 560; of kidney,
595, 596

Penguin, 511, 512
Penicillin, 3, 250, 251, 268-269
Pepsin, 577
Peptides, 577
Peptones, 577
Perch 464-465
Percolation beds, 719, 720
Perennials, 311, 325; seeds of,
366

Pericardial cavity of fish, 466
Pericardium, 587-588
Pericycle, root, 316
Periodicity, 685—693
Periosteum, 557

Peripheral nervous system, 612,
616

Permian period, amphibians in,
473; reptiles in, 489-490
Personality, in twins, 159-160
Perspiration, 565
Pests, insect, control of, 443—
448

Petals, of flower, 352, 354
Petiole, leaf, 341, 344, 346, 347
PGA in photosynthesis, 88—89
PGAL, 91, 94, 349; in photo¬
synthesis, 88—89
Phaeophyta, 287
Phages, 216- 219, 217
Phagocytic cells, 243, 244
Pharynx, of earthworm, 394; of
fish, 466, 468; of flukes, 388;
human, 572, 600; of planar-
ians, 387

Phenotypes, 191; of dihybrids,
126; in monohybrid crosses,
122-124

Phenylalanine, 168
Phenylpyruvic idiocy, 168
Phenylthiocarbamide, 157-158
Phipps, James, 245, 246
Phloem, 308; bark, 331-332;
fibers, 332; parenchyma, 332;
root, 316, 317; stem, 334
Phosphates, in DNA, 50, 51;
and respiration, 100-101;
units, 142

Phosphoglyceraldehyde, 88
Phosphoglyceric acid, 88
Phosphorus, in body, 566
Photo phase, 86—88, 87
Photolysis, 86
Photoreceptors, 625
Photosensitivity, 387
Photosynthesis, 82—91, 94, 663,
666, 667; in algae, 277, 280,
282, 288, 289; in euglena,
260; leaves and, 307, 341,
346-347, 348-349; and plant
respiration, 348-349, (Table)
348

Phototropism, 328, 344, 387
Phycocyanin, 279
Phycomycetes, 266; bread mold,
266-268, 267

Phyla, animal, 374-376; as Lin-
naean grouping, 204
Phylogenetic tree, 406 -408

Physalia, 382

Physical change, of matter, 33
Physiology, definition, 12
Phytoplankton, 677
Pia mater, 613
Pig, 531, 532

Pigment granules, of fishes, 463
Pigments, 83, 84, 95; in fungi,
265; in green algae, 280; in
leaves, 345-346; vacuolar,
62-63

Pikas, 526, 528
Pillbugs, 418
Pineal body, 646
Pineapples, 699
Pines, 305; products of, 730
Pinocytosis, 78- 79
Pinworm, 390
Pioneers, bacteria as, 694
Pistil, of flower, 352, 354, 355,
356, 357; of pea plants, 117
Pit vipers, 493, 498-500
Pitcher plant, 345
Pith, plant, 308; rays, 330; root,
316; stem, 329, 332
Pits, tracheid, 332
PKU, 168

Placenta, 524, 655—656
Placental mammals, 195, 196,
197, 524

Planarians, 386-387
Plankton, 672, 707-708
Plant eaters, 532
Plants, breeding of, 170-178;
dormant, 674; flowering, 303,
307-3 1 1 ; 352-369 ; herba¬

ceous, 311; insectivorous, 345;
reproduction in, 352-369;
respiration in, 347-348;
seed, 303—311; woody, 311
Planula (e), 381
Plaques, 216

Plasma, blood, 565, 581-582,
587

Plasma membrane, 57, 58-60,
103; of bacteria, 225; diffu¬
sion through, 75—78; penetra¬
tion, 71-73, 75-78, (Table)
72; structure, 71, 78-79
Plasmodium, 262-263, 443
Plasmolysis, 76-77
Plastids, 57, 61-62
Plastron, turtle shell, 504
Platelets, 582, 583
Platyhelminthes, 386—390
Pleistocene era, bears in, 531
Pleural membrane, 602
Plumage, bird, 164
Plymouth Rock, 178
Pneumococcus, 224; transfor¬
mation in, 143-144, 145
Pneumonia organisms, 144, 145,
224

Pod fruits, 360

Poison, of Gila monster, 500-
501; insecticides, 445-446;
sea lampreys and, 461; snake,
494, 499

Polar, bodies (meiosis), 112;
nucleus, ovule, 356, 358; re¬
gions, 687, 700, 701

INDEX 787

Polio, 5-8, 9, 10, 11; vaccine,
7-8

Pollen, of pea plants, 117; for¬
mation, 354-355; sacs, 354;
tube, 357

Pollination, 356-357, 443;

breeding and, 173, 174; of
oea plants, 117
Pollution, 733, 734
Polyploidy, 176-177, 189
Polyps, coelenterate, 380, 381;
coral, 318-382

Polysaccharides, 45; and nutri¬
tion, 568

Ponds, farm, 734; succession in,
694, 695-697, 696
Pons, 616

Popper, Dr. Erwin, 6, 8
Poppies, opium and, 634
Populations, 157-159, 190, 661—
662

Pore, genital, 388; incurrent
and excurrent, 376
Porifera, 376

Pork, trichinosis and, 392
Porpoises, 528, 529
Portal circulation, 593
Portuguese man-of-war, 378,
382

Posterior, horns, 616—617; lobe,
pituitary gland, 642
Postulates, Koch’s 239
Posture, 542

Potassium-argon dating method,
544

Potassium compounds, 566
Potatoes, 335; Burbank, 170,
171, 173

Pouched mammals, 195-196,
524

Poultry, breeding of, 177, 178
Prairie dog, 525
Praying mantis, 430
Precipitation, 665
Precipitins, 244

Predators, 192; crocodilians,
501; definition, 670; and fish
eggs, 537; sea lampreys, 459-
461; sharks, 456, 461- 462;
turtles, 503
Premolars, 574

Pressure, diastolic, 591; diffu¬
sion, 74, 75-76, 78; in mid¬
dle ear, 622-623; osmotic,
320-321, 337; root, 320-321,
337; systolic, 591; turgor, 75-
77, 321

Pressure receptors, 619—620
Primary, germ layers, 653, (Ta¬
ble) 655; oocycte, 112; quill
feathers, 510-511; root, 313;
spermatocyte, 112; wall, 57,
63

Primates, 202, 532, 533; man,
542, 543

Primroses, evening, 187
Prismatic layer (shell) 401
Proboscideans, 530
Proboscis, of butterfly or moth,
433

Producers, food, 677

Progesterone, 644, 651
Proglottids, 389-390
Prontosil, 249
Prop roots, 318

Propagation, by stems, 336-337;

vegetative, 146, 173
Prophase, 104, 105, 106, 110; of
meiosis, 112
Prostomium, 393—394
Protection, animal, 681-683
Protein filaments, 561
Protein synthesis, 97, 142; cell
growth and, 103
Proteins, 47— 49; antibodies,
244; antigens, 243-244; in
chromosomes, 49; erepsin
and, 578; as food, 565, 568-
569; functions and organiza¬
tion, 95; and nitrogen cycle,
667; pepsin and, 577; pep¬
tides and, 577; ribosomes
and, 95-98; synthesis, 94—98;
in trees, 728
Proteoses, 577
Proterozoic rock, 277
Prothallium, fern, 300
Prothorax, of grasshopper, 427
Prothrombin, 583, 586
Protista, 207, 223, 235, 254
Protococcus, 280- 281
Protonema, mosses, 296
Protonephridia, 407
Protonephron, 407
Protons, 35, 36, 37
Protoplasm, 42, 55; character¬
istics of, 25-26

Protozoans, 235, 238, 254—263,
255, 374, 376, 439-440; cel¬
lular division, 107-108
Pruning, 337
Pseudocoel, 408
Pseudopodia, 254, 255
Ptarmigan, 509
PTC, 157-158
Pterodactyls, 506
Ptyalin, 576
Puberty, 644—646
Public Health Service, 636
Puffballs, 275

Pulmocutaneous arches, of frog,
480-481

Pulmonary, artery, 589, 591,
593, 602, 603; circulation,
593; veins, 481, 589, 593,
602, 603
Pulp cavity, 574
Pulpwood, 729
Pulse, human, 591
Puma, 200, 530
Punnett square, 122, 125
Pupa, in insect metamorphosis,
424, 434

Pupil, human eye, 624, 625,
626; snake eye, 493
Purines, 50-51, 104, 105
Pus, 243; white corpuscles and,
583

Pygmy, 159

Pyloric caeca, of fish, 466
Pyloric valve, 479, 575, 577
Pylorus, of frog, 479

Pyramids, food, 679-680; kid¬
ney, 595

Pyrenoids, of Oedogonium, 283;

of Spiro gyra, 281-282
Pyrimidines, 50-51, 104, 105
Pyrrophyta, 287
Pyruvic acid, 99-100, 607; fer¬
mentation and, 229

Q fever, 235
Quadrate bone, 494
Quail, 518

Quarantine, insect, 443-445
Queen Victoria, hemophilia and,
164 -165

Quill, of feather, 509

Rabbits, 526, 528, 529
Rabies, 213; Pasteur and, 246-
247

Raccoon, 531

Races, 205, 546-549; blood
types, (Table) 163
Rachis, of feather, 509
Radiation, adaptive, 195-197;
and food preservation, 234;
of isotopes, 36-37; medical
use, 37; and mutations, 148-
150, 151-153; from sun,

660

Radicles, 363, 364
Radioactive iodine, 37, 640
Radioautography, 64-65
Radiolarians, 263, 385
Radula, 403
Rainfall, 718-721
Rays, fishers, 462; light, 15;
pith, 330; of starfish, 404-
405

RDP, in photosynthesis, 88, 89
Reactions, nervous, 617, 618-
619

Receptacle, of flower, 352, 353
Receptors, 387; of crayfish, 414;

human skin, 619, 620
Recessive characteristics, 120,
122, 123-124; and nondis¬
junction, 139-141
Recombination, 190; and evolu¬
tion, 189

Rectum, 428, 576, 579
Red corpuscles, 95, 560, 566,
582, 586-587, 591; malaria
and, 263

Red-green color vision, 162-
163, 164
Red Sea, 279
Red spores, 271
Redi, Francesco, 20—21
Reduction division, 111- 112;
131-132; and law of segrega¬
tion, 120, 122
Redwoods, 305
Reefs, coral, 382
Reflex actions, 617—618; iris,
625

Reforestation, 728—729
Refrigeration, of foods, 234
Regeneration, in amphibians,
485; in crayfish, 416; of pla-

788 INDEX

narians, 387, 388; of starfish,
406

Reindeer moss, 290

Remoras, 681

Renal, arteries, of frog, 481; cir¬
culation, 593, 595, 596; veins,
of frog, 481

Replication, 51—52, 105, 142;
in meiosis, 112

Reproduction, 29; of algae, 278,
279, 280, 281; 282-283, 284-
286; in ameba, 256; asexual,
107-109 261, 265, 275, 648;
in Aurelia, 381; of bacteria,
229- 230, 231; in birds, 517—
520, 518, 519; cellular, 57,
103—112; in crayfish, 415;
DNA, 52; in earthworm, 395-
396; in euglena, 261; in ferns,
298-301; in fish, 469-470; in
flowering plants, 352-369; of
flukes, 389; in frog, 482—483;
fungi, 265, 275; in grasshop¬
per, 429—430; in housefly,
441-442, (Table) 442; in
hydra, 380; in mammals,
536-537; in mosquitoes, 442;
in mosses, 295-297; in para-
mecium, 258 —259; of planar-
ians, 387; roots and, 320; of
roundworms, 391-392; in
seed plants, 117; sexual, 109-
112, 131, 231, 265, 267-268,
648; in snakes, 496—497; in
spiders, 420; in sponges, 376-
377; in spore-forming proto¬
zoans, 261-262; in tape¬
worms, 389-390; vegetative,
108; of viruses, 214; yeasts,
269-270

Reproduction (human), 648-
656

Reproductive system, verte¬
brate, 455

Reptiles, 489—504; birds and,
506

Reservoir, of euglena, 260

Residual air, 605

Resolution, of image, 14-15

Respiration, algae and, 277,
287; anaerobic, 100, 228, 229,
270, 607; artificial, 605, 606;
in bacteria, 228-229; in
birds, 516; cellular, 57, 98—
101; in crayfish, 415; in
Crustacea, 412; in earth¬
worms, 395; in fish, 467-468;
in flowering plants, 307; forms
of, 99-100; in frog, 480; fuel
for, 99; in grasshopper, 427-
428; human, 600—610; in
mammals, 534-535; in para-
mecium, 258; phosphate
groups and, 100-101; com¬
parison with photosynthesis,
348-349, (Table) 348; plant,
347-349; in spiders, 420

Response, cellular, 57; of living
organisms, 29—31; of roots,
322-323; of vertebrates, 455-
457

Reticuloendothelial system, 243
Retina, 625-626
Retting of flax, bacteria and,
232

Rh factor, 162, 587
Rheas, 511

Rhesus monkey, 162, 587
Rhinoceros, 532
Rhizoids, 267; moss, 294-297
Rhizomes, 334-335; of tree
ferns, 299
Rhizopus, 266-268
Rhodophyta, 287
Rhynchocephalia, 491 -492
Rhythms, 685; annual, 693;
daily, 685—687; lunar, 685,
692—693; seasonal, 687-692;
tidal, 692-693 *

Riboflavin, 270

Ribonucleic acid, 51, 52; see
also RNA
Ribose, 51

Ribosomes, 57, 60, 96, 143;

protein synthesis and, 95- 98
Ribs, human, 560; and breath¬
ing, 604; of snake, 495, 496
Ricketts, Dr. Howard T., 234
Rickettsiae, 234 -235, 238
Rill erosion, 715
Ring canal, of starfish, 405
Ringworm fungi, 275
RNA, 51, 52, 57, 566; bases in,
96; control of cellular en¬
zymes, 58; formation of, 142;
messenger, 96, 142, 144;

phage, 216, 219; ribosomes
and, 60, 96; transfer, 97—98;
in virus core, 214
Roach, 430
Roan offspring, 1.27
Robins, 513, 519
Robbins, Dr. Frederick C., 6
Rockefeller Institute for Med¬
ical Research, 28, 49, 144
Rocky Mountain spotted fever,
235—236

Rodents, 525-526, 528, 670
Rods, 625-627
Roosevelt, Franklin D., 5
Roosevelt, Theodore, 724
Roots, 313—323; of flowering
plant, 307; fungi and, 275;
pressure, 320-321, 337; of
tooth, 574; of tree ferns,
299

Roquefort cheese, 268
Roses, 176
Rosin, 730

Rostrum, of Crustacea, 412
Rotation, of crops, 714
Roughage, 568
Roundworms, 390—392
Royal jelly, 436
Rumen, of ungulates, 532
Ruminants, 532
Rust fungi, 271-273

Sabin, Dr. Albert, 8
Sabin oral vaccine, 10
Sac fungi, 266

Salamanders, 456, 474 -475; liz¬
ards and, 474
Salientia, 473
Saliva, 572, 573, 577
Salivary amylase, 576
Salivary glands, of grasshopper,
428; human, 572, 577
Salk, Dr. Jonas E., 7; test, 10;

vaccine, 7-8, 9
Salmon, 193, 457
Salt, table, 41; see Sodium
Chloride; curing, of foods,
234; as food, 566
Salvarsan, 249
Samaras, 195

Sampling, human genetics and,
157—159

Sand, 673; dollar, 404
Sandworm, 393
Sap, 338

Sapwood, 329-330
Sarcode, 55
Sarcodina, 254
Sauerkraut, bacteria, 232
Savannah, 704
Scabs, 586

Saprophytes, 227, 681; fungi,
265, 268, 269, 271-273, 274-
275

Scales, birds, 507; bud, 325; of
butterfly or moth, 433; of
conifers, 305; fish, 463;
snakes, 493
Scallops, 402
Scarlet fever, 228
Scavengers, 401-416, 678; in¬
sects, 440, 441, 443; worms,
387

Schizomycophyta, 223
Schleiden, Matthias, 55
Schwann, Theodor, 55
Science, of classification, 199;
development of, 2-5; interre¬
lations, 3—4; limitations of,
5; pure and applied, 12
Scientific methods, 3, 8-12
Scion, 336, 337
Sclerotic layer, 624, 625
Scorpions, 421
Scotch broom, 699
Screwworm fly, 446, 447^448
Scrotum, human, 648
Scutes, 493, 495
Sea, cow, 529—530; cucumber,
405, 406; fans, 378; lamprey,
453, 456, 459-461, 460; lions,
531; snakes, 498; squirts,
453; turtles, 503; urchin, 404,
405, 406; water, 90
Sea anemones, 378; and radial
symmetry, 385

Seals, 531; migration of, 688-
689

Seasonal rhythms, 687—692; mi¬
grations and, 688—692
Secondary, oocyte, 112; phloem,
317; quill feathers, 510—511;
roots, 313; sex characteris¬
tics, 644-646; spermatocytes,
112; thickening, root, 316-
317;^ wall, 57, 63; xylem, 317

INDEX 789

Secretions, acid in stomach,
242; bile, 576, 577; cellular,
57; digestive enzymes, 571;
of ductless glands, 639, 646,
(Table) 645; in gastric
glands, 575, 577; mucus, 573;
royal jelly, 436; of snail, 403;
sweat, 565, 597; synovial
fluid, 561
Sedges, 362
Sedums, 345

Seedling, growth, 367—369; tim¬
ber, 729

Seeds, 359—368; of angiosperms,
303, 304, 306, 307; birds
and, 512, 513; of conifers,
305; of gymnosperm, 303,
304

Segmented, body of arthropods,
410, 412; worms, 393—394,
^ 400

Segregation, abnormal, 139-
141, 140; law of, 120, 121,
122, 133; and sex linkage,
138

Self-pollination, 356; and in-
breeding, 173, 174; of pea
plants, 117, 119
Semen, 649

Semicircular canals, 623, 624
Semilunar valves, 589, 590
Seminal receptacles, of earth¬
worm, 396; of grasshopper,
429; of spiders, 420
Seminal vesicles, of earthworm,
396; of frog, 482; human,
648-649

Seminiferous tubules, 648
Semitropical region, 700, 702-
703

Sensations, of fish, 469; skin,
619-621

Sense organs, of bird, 517; of
grasshopper, 429; human,
619-627; of insects, 432; in
primates, 532

Sensitivity, in ameba, 256; in
paramecium, 258
Sensory, areas, of cerebrum,
614-615; nerves, 621; neu¬
rons, 612, 613, 617
Sepals, of flower, 352
Septum, 588, 600
Serum, definition, 7, 249; al¬
bumin, 581; globulin, 581
Sessile, 376
Setae, 394

Sewage, insects and, 447
Sex, chromosomes, 1 36-137,
138-139, 140-141, 149-150;
determination, 136—137, (Ta¬
ble) 137; hormones, 641,
644-646; linkage, 137-139;
138, 162-164
Sex-influenced traits, 164
Sex-limited traits, 164-165
Sharks, 456, 461—462, 681
Shasta daisy, 170, 174
Sheath, bacteria, 224
Sheep, 531, 532
Sheep-liver fluke, 388 -389

Sheet erosion, 715
Shellac. 440, 443
Shellfish, 399

Shells, of bird eggs, 518; of bi¬
valve mollusks, 400, 401, 402;
cephalopods, 404; diatoms,
287; of electrons, 38; gastro-'
pods, 402-403; of reptile egg,
489; turtle, 503, 504; of virus,
214

Shelterbelts, 717
Shields, turtle shell, 504
Shock, 586-587
Shortenings, 568
Shorthorn cattle, 178, 179
Shoulder, of bird, 510
Shrews, 525
Shrimps, 416-417
Side winding, 495
Sieve, plate, of starfish, 405;
tubes 308, 331

Sight, human, 625-627, 626;

in primates, 532
Sigmoid colon, 576
Silage, bacteria and, 232-233
Silk scar, of com kernel, 364
Silt, 673

Silurian period, 277
Simple, eye, of grasshopper,
429

Sinoatrial node, 563
Sinus venosus, 466, 481
Siphons, in mollusks, 400, 401
Sirenians, 529, 530
Skate, 456, 462

Skeletal, muscles, 562; system,
vertebrate, 464

Skeletons, of arthropods (exo¬
skeleton), 410, 411-412;

bony, 462; cartilage, 453,
454, 455, 461; coral, 381-
382; crayfish, 415—416; en-
doskeleton, 452, 454; human,
552, 554, 555, 556; starfish,
405; vertebrate, 452, 453,
454

Skin, as barrier to disease, 241,
242; color, 165-166; frog,
477; and frog respiration,
480; human, 596-597; rep¬
tiles, 489; respiration and,
535; sensations, 619—621
Skinks, 500
S-l valves, 589

Sleeping, pills, 635; sickness,
263, 441, 443

Slime, body of fish, 463; layer,
of bacterial cells, 224; molds,
265; ring, of earthworm,
396

Sloths, 528
Slugs, 403

Smallpox, 2, 5, 213; vaccina¬
tion, 245 —246

Smell, 621, 622; in fish, 469; in
primates, 532
Smith, Dr. J. L. B., 472
Smoking, 635—637, 636; and
forest fires, 725, 726, 727
Smooth muscle, 561, 562, 575
Smuts, 273

Snails, 399, 402, 403; and fluke
larvae, 388-389
Snakes, 456, 492-500
Social insects, 432, 435^439
Sodium chloride, 39, 41, 566;
ionization in water solution
(equation) 41

Soil, algae and, 287; bacteria,
231-233; characteristics of,
673-674; composition of,
712-713; conservation, 714-
718; flora, 712; forests and,
731; ground water and, 665;
organisms, streptomycin and,
251; roots and, 314, 321,
^ 323

Soil Conservation Service, 718
Sol phase, 42, 58; in prophase,
105; in telophase, 107
Solar plexus, 618
Solids, 33; diffusion of, 73-74
Solute, 41, 42; concentration,
active transport and, 78
Solutions, 41-42; and plasma
membrane, 71-72, 77; sat¬
urated, 41

Solvents, 41, 42; chlorophylls
removed by, 84; and. pene¬
tration of plasma membrane,
77

Somatotropic hormone, 641,
^ 642

Song box, 516
Sori, fern, 299
Sound, ear and, 623, 624
Sowbugs, 418

Space flights, algae and, 289;

and respiration, 610
Spallanzani, Lazzaro, 22-23
Spanish moss, 704
Sparrow, 519

Spawning, of fish, 469—470; of
sea lamprey, 460
Specialization, cellular, 65-66,
71, 374, 394; in Crustacea,
4 1 2 — 4 1 3 ; in insect, 423-424,
432; in mammals, 533, 535;
of protozoans, (Table) 261
Speciation, 194-195
Species, characteristics, 116;
classification and, 199, 200,
201, 204-205, 207; climax,
695, 701; definition, of, 207;
development of, 193—195; as
Linnaean grouping, 204;
preservation, 457
Spectrum, visible, 89
Speech, tongue and, 574
Sperm, 109, 110, 112, 131; bee,
437; bird, 517, 518; crayfish,
415; earthworm, 396; fern,
299-301; of fish, 469, 470,
537; of frog, 482, 483; hu¬
man, 132, 648-650, 649, 651;
mosses, 295-297; of Oedo-
gonium, 284; of pea plants,
117, 120

Sperm nuclei, of flower, 357
Spermatids, 112
Spermatocytes, 112
Sphagnum, 297

790 INDEX

Spicules, 376, 377 Stickleback, 469, 470

Spiders, 410, 419-421, 420 Stigma, of flower, 352, 356, 357
Spinal bulb, 6 Stimulus, 29—30, 455, 457, 617,

Spinal cord, 6, 453; of bird, 619-620

517; of fish, 468; of frog, Sting, of bee, 436, 437; ray,
482; human, 614, 616, 617; 462

of mammals, 536 Stingers, of scorpion, 421

Spinal nerves, of frog, 482; hu- Stipe, of mushroom, 274
man, 616-617; of mammals. Stirrup bone, 622
536 Stock, grafting on, 336, 337

Spindle fibrils, 105, 107 Stocking, of fish, 734

Spine, 453; of dorsal fin, 465; Stolons, 267

of snake, 495 Stomach, acid as barrier to

Spinnerets, of spiders, 420 disease, 242; of bird, 516; of

Spiracles, of grasshopper, 428 fish, 466; of frog, 479; of
Spirochetes, 235, 238 grasshopper, 428; human,

Sponges, 375- 377; reproduc- 574-575; lining, inflamma¬

tion, 108, 377
Spongin, 376—377
Spongy cells, leaf, 343, 344,
346

Spontaneous generation, defini

tion, 631; poisons (insecti¬
cides), 445; of snake, 495;
of ungulates, 532
Stomates, leaf, 343— 344, 346,
350

tion, 19; Redi’s experiment,'" Streaming, of cell protoplasm,
20 —21; Spallanzani’s experi- 58, 348

Streptococcus, 224, 240, 241
Streptomycin, 251
Strip, cropping, 716; cutting,
727

Structural, proteins, 95; similar¬
ities, classification and, 199,
200, 201, 204

ments, 22-23
Sporangia, fern, 299
Sporangiophore, 267
Sporangium, of bread mold,

267-268

Spore formation, bacterial, 230-
231

Spore-forming protozoans, 261- Sturtevant, A. H., 134

263 Style, of flower, 352, 353, 357

Spore production, 108, 109 Subarachnoid space, 614
Spores, of algae, 278, 284, 286; Subclavian veins, 593-594
fern, 299-300; of fungi, 265, Subsoil, 665, 712
270-275; mosses, 295-297 Succession, 694-696, 697

Sporophyte generation, algae, Sucker, of flukes, 388; of
284—285; ferns, 299—301; leeches, 396

flowering plants, 359; mosses, Suckling instinct, 457
295-297; seed plants, 303 Sucrase, 577
Sporozoa, 261 Sucrose, 45, 89; intestinal fluid

Sport, rose, 176 and, 578; and nutrition, 568

Sprouting of seeds, 366-369, Suffocation, tissue, 609
367, 368 Sugar units, 142

Spruces, 305 Sugars, 44- 46, 45, 88, 89; deox-

Squid, 403, 404 yribose, 49; in germinating

Squirrels, 525; isolation and, seed, 368; insulin and, 643-

644; maple, 730-731; and
nutrition, 568
Sulfa drugs, 250

192 193

Stalk, of flower, 352, 353;

mosses, 295—297

Stamen, of flower, 352, 354, Sulfanilamide, 249
357; of pea plants, 117 Sundew plant, 343

Stanley, Dr. Wendell, 213—214 Sunfish, 469, 470
Staphylococcus, 216, 224, 250 Superior vena cava, 589, 591,
Starch, 45 -46, 72, amylase and, 593

578; in bean seed, 362; in Supplemental air, 605
corn kernel, 364; in germinat- Suprarenals, 642-643
ing seed, 368; molecule, 62; Survival of the fittest, 187, 190
and nutrition, 568; ptyalin Suspension, 41-42
and, 576 Sustained yield, of forest, 727

Starfish, 404 -406 Sutton, Walter S., 132-134, 133

Statocyst, of crayfish, 414-415 Swallowing, 573; by snakes,
Steady state, 70 494—495

Stems, 325-338; brain, 616; of Swarming, of bees, 436
fern, 300; of flowering plants, Sweat, 565, 597
307; of tree ferns, 298 Sweepstakes dispersal, 191

Sterility, of mule, 179; plant Swimmerets, 413, 415
breeding and, 178 Symbiosis, 290, 680-681

Sterilization, and insect control. Symbol, of elements, 34
447-448 Symmetry, 385— 387

Sympathetic nervous system,
618-619
Synapse, 612
Synapsis, 112
Synergids, 356
Syngamv, 109
Synovial fluid, 561
Synthesis, of carbohydrates, see
Photosynthesis; cellular, 57;
protein, 94—98, 143
Synthetic, phase, photosynthe¬
sis, 88—89; vitamins, 569
Syphilis, 3, 235; cure, 249
Systemic circulation, 593
Systems, of earthworms, 394;

human, 551; organs and, 68
Systole, 590

Tadpoles, 455, 476, 483-^85;

toad, 476
Taiga, 702

Tail, of fishes, 463, 464—465; of
tadpole, 483, 485
Tanning, 232, 731
Tapeworms, 389— 390
Tapir, 532
Taproots, 313
Tarantula, 420
Tarsus, of grasshopper, 427
Taste, buds, 373, 620; as in¬
herited characteristic, 157—
158; sense of, 620, 622
Tatum, Edward L., 150, 151-
153

Taxonomy, 12, 199
Tears, as barrier to disease, 242
Technical method, 11-12
Teeth, in carnivores, 530, 533;
of fishes, 464; human, 574; in
rodents and rodent-like mam¬
mals, 526, 528; of snake,
493, 494; in ungulates, 532;
in walruses, 531
Teleostomi, 462
Teliospores, 271
Telophase, 104, 107
Telson, of crayfish, 413
Temperate region, 700, 703
Temperature, and bacterial
growth, 226; of cold-blooded
animals, 485; diffusion and,
74; and environment, 674-
675; fungi and, 265-266; in
mammals, 533; and muta¬
tions, 148; and oxidation, 98;
and photosynthesis, 90; rise
in body, 243; and seed ger¬
mination, 367
Templates, 96, 97-98
Tendons, 562
Tendrils, 345

Tentacles, of hydra, 378, 380;
of snail, 403

Termites, 263, 435, 439—440,
681

Terracing, 716
Terramycin, 251
Terrapins, 502—503
Tertiary period, aquatic mam¬
mals in, 528; armadillos in,

INDEX 791

525; carnivores in, 530;
snakes in, 492
Testa, of bean seed, 362
Testes, 10; of bird, 517; of
fish, 469; of frog, 482; of
grasshopper, 429; hormone
secretions, 641, 644—646; hu¬
man, 648; in hydra, 380
Testosterone, 644
Tetanus, 228, 241-242; spores,
231

Tetracycline, 252
Tetrad, 112, 141, 355; bacteria,
224

Tetraploid chromosome num¬
ber, 112; in plants, 177, 178
Thallophyta, 223
Thiouracil, 640

Thoracic cavity, human, 552
Throat, of frog, 479; of snake,
494; see also Pharynx
Thorax, of Crustacea, 412; of
insects, 423
Thorns, 345
Thrombin, 583-586
Thrombocytes, 582
Thromboplastin, 583, 586
Thrush, 275
Thumb, opposed, 542
Thymine, 50, 51, 96, 142
Thymus, 646

Thyroid, deficiency, and larval
salamanders, 475; extract,
639, 640; gland, 639 —641
Thyrotropic hormone, 641
Thyroxine, 639
Tibia, of grasshopper, 427
Ticks, 235, 421
Tidal air, 604, 605
Tigers, 530

Timber, 305, 329, 724, 727,
729; types of, (Table) 730
Tissue, fluid, 581, 591, 593-
594; suffocation, 609
Tissues, 65—66, 67; damage to,
107, 637; of earthworms, 394;
of flowering plants, 307-310;
human, 551, (Table) 553;
leaf, 343; in mammals, 533;
monkey kidney, and polio re¬
search, 6, 7; nerve, 535; root,
315-317, 316, (Table) 318;
vascular, 294; of woody stem,
330-333, (Table) 333
Toads, 475-476
“Toadstool,” 274
Tobacco, 630, 635-637; bac¬
teria and, 233; mosaic virus,
213, 214, 215

Toes, of primates, 532, 533; of
ungulates, 532
Tolerance, 672—673
Tone, muscle, 563, 615
Tongue, of bee, 437; frog, 476,
478; human, 572—573; roll¬
ing, 157, 158; of snake, 493;
taste and, 620
Tonsils, 593

Tools, man and, 542, 543
Topography, 677; climate and,
700 -701

Topsoil, 665, 712-713
Tortoises, 502, 503, 504
Touch-me-not, 361
Tourniquets, and snakebite, 500
Toxins, antitoxins and, 244,
248—249; diphtheria, 247-
249; endotoxins, 242; exotox¬
ins, 241-242
Toxoids, 249

TPN, in photosynthesis, 87
TPNH2, 87, 88
Tracer elements, 37, 56
Trachea, of bird, 516; of frog,
480; of grasshopper, 428; hu¬
man, 600—601, 603; of in¬
sects, 412, 420, 423; of snake,
494-495

Tracheids, plant, 308; stem, 332
Tracheophyta, 294, 298, 303
Traits, of garden peas, 117 -118;
genetic sampling of, 157-159;
sex-influenced, 164; sex-lim¬
ited, 164—165

Transformation, in pneumococ¬
cus, 143-144, 145
Transformers, 678
Translocation, 189, 338, 349
Transpiration, of flowering
plant, 307; 349-350; pull,
338

Transport, blood as medium of,
583, (Table) 586, 593; body
systems, 581-598; passive
and active, 72-73, 77-78
Transverse, colon, human, 576;
fission, of spirochetes, 235;
nerves, 387

Tree Farm Program, 727, 729
Tree, frogs, 476; line, 700
Trees, bracket fungi on, 274-
275; branching, 328—329;
conifers, 304—306, 305, (Ta¬
ble) 306; succession and,
695; timber, (Table), 730;
as windbreakers, 717; see also
Forests, Woody plants,
Woody stems
Trematoda, 386, 388-389
Trench fever, 235
Trepang, 406

Triassic period, amphibians in,
473

Triceps, 562, 563
Trichina worm, 390, 392
Trichinosis, 392
Trichocysts, 258
Triose phosphate, 88
Triple fusion, 358
Triplets, base, 96-98, 142
Triploid chromosome number,
358

Tritium, 36

Trochanter, of grasshopper, 427
Trochophore larva, 399, 400
Tropical, forest, 700, 701, 705—
706; region, 700
Tropisms, root, 322-323
Trunk, of fishes, 463, 464-465
Trunk-nosed mammals, 530
Trypsin, 577-578
Tsetse fly, 263, 441

Tuatara, 491 -492
Tube feet, of starfish, 404-405
Tube nucleus, 355, 357
Tuber, of potato, 335
Tuberculosis, 166, 228, 240;
streptomycin and, 251; tissue
destruction by, 241
Tubule, from Bowman’s cap¬
sules, 595, 596
Tundra, 701, 702
Tunicates, 453
Turbellaria, 386-387
Turbinates, 622
Turgor pressure, 75-77, 321
Turkeys, 178, 518, 738-739
Turtles, 456, 502 -504
Tusks, in walruses, 531
Twigs, 325-326; grafting and,
336-337

Twins, 159-160, 167
Twort, F. W., 216
Tympanic membrane, 478, 504,
622

Tympanum, of grasshopper,
429

Typhoid, 228, 240
Typhus fever, 235
Tyrosine, 168

Umbilical cord, 655, 656
Ungulates, 531-532
Unicellular organisms, 65, 66
Univalve mollusks, 402
University of Lille, 222
Uracil, 51, 96, 98
Urea, 569, 577 582, 594
Uredospores, 271
Ureters, of frog, 479, 482; hu¬
man, 595, 596

Urethra, 596; male human, 648,
649

Uric acid, 516-517, 594
Urine, of frog, 482; human,
569, 596

Uropod, of crayfish, 413
Urothrombin, 581
Uterus, embryo in, 652-655,
654; in flukes, 388; of frog,
483; human, 650-655
Uvula, 572

Vaccination, 2, 5; smallpox, 213
Vaccine, anthrax, 245-246; def¬
inition, 249; rabies, 247;
Sabin oral, 8, 10; Salk, 7-8;
smallpox, 245- 246
Vacuolar membrane, 57, 60, 62;

pigments, 62-63
Vacuoles, 57, 62-63, 79; in
ameba, 255—256; contractile,
76, 255-256, 258; of parame-
cium, 257-258
Vagina, 650, 655
Vagus nerve, 619
Valves, a-v, 588-589, 590;

(shells), 401; s— 1, 589, 590
Vampires, 525
Van Helmont, Jean, 19, 25
Van Leeuwenhoek, Anton, 13
Vane, of feather, 509

792 INDEX

Variations, 30-31, 52; among
bees, 436-437; animal breed¬
ing and, 178-180; of birds,
507; and evolution, 187;
floral, 352, 354; genetic, 52,
112, 135-136, 137-139,

190, 231, 648; industrial

melanism and, 192; in mam¬
mals, 522, 533; migration
and, 190-191; mutations and,
188-189, 190, 191; plant,

171, 176-178; recombination
and, 189; and speciation,
194—195; as Linnaean group¬
ing, 204; see also Adaptations
Vas deferens, 648
Vasa efferentia, of frog, 482
Vascular bundles, leaf, 341;

stem, 333—334
Vasopressin, 642
Vegetables, 566

Vegetative, organs, of flowering
plants, 306-307; propagation,
147, 173; reproduction, 108;
by roots, 319

Veins, of fish, 466; of frog,
481; hepatic, 593; human,
590, 591-596, 602, 603; leaf,
341

Venae cavae, of frog, 481; in¬
ferior and superior, 589, 591,
593, 596

Venation, leaf, 341
Venom, 494, 497-498, 499-500,
501

Ventilation, 608—609
Ventral, aorta, of fish, 466;
blood vessel, of earthworm,
395; nerve cord, of earth¬
worm, 396; nervous system,
410, 412; siphon, of mollusks,
400; surface, 386
Ventricles, 588, 591; of bird,
517; of brain, 615; of fish,
466; of frog, 480
Venules, 591
Venus’s-flytrap, 345, 346
Vergil, 19

Vermiform appendix, 576
Vertebrae, 453, 560
Vertebrates, brain, 535— 536;
characteristics, 452, 453,

454-455; classification, 374—
375; shape, 386
Vesalius, Andreas, 2
Vessels, plant, 310; stem, 332
Vestigial organs, 182, 184
Veterans Administration Hospi¬
tal, 219

Viability, seed, 366
Vibration; of eardrum, 623
Villi, chorionic, 654—655; in
small intestine, 579
Vinegar, bacteria and, 232
Vipers, 498—499
Virulence, of viruses, 215
Viruses, 212-220, 214; polio, 6,
8; rabies, 246-247

Visceral hump, of mollusks,
400, 403

Visual purple, 627
Vitamin deficiency, alcohol and,
631

Vitamins, 270, 556-557, 565,
569-571, (Table) 570, 581
Vitreous humor, 625
Viviparous, 497

Vocal, cords, 601; sacs, of frog,
479

Volcanoes, 666
Volvox, 385

Vomerine teeth, 478, 479
Von Behring, Emil, 248
Von Tschermak, 132
Vultures, 513

Wak-Wak tree, 20

Waksman, Dr. Selman, 251

Walking stick, 430

Wall, cell, 57, 63, 65, 224-225

Walruses, 531

Warblers, 512, 519, 690

Wasps, 439

Wastes, 57; nitrogenous, 582,
594, 630; undigested; see
Elimination

Water, absorption in colon, 579;
conservation of, 718-720; cy¬
cle, 664- 665; dog, 474; dif¬
fusion into cell, 75-77; and
environment, 675-676; ero¬
sion, 715-718; evaporation
of, 565, 597, 665, 676-

677; fleas, 418; as food, 565—
566; and living organisms,
43; molds, 268, 269; molecu¬
lar structure, 38-39; mole¬
cules split in photosynthesis,
85—88; pollution, 733, 734;
power projects, 720; table,
665, 719

Water-borne infections, 240-
241

Water- vascular system, 404—405
Watersheds, 719-720
Watson, James D., 49, 50
Watusi, 159
Wax, 46, 443
Weasel family, 531
Weather, forests and, 728, 731
Web, spider, 419
Web membrane, 478
Webbed feet, of birds, 512;
frog, 476, 478; in water-living
carnivores, 531
Weevils, 440
Weismann, August, 131
Weller, Dr. Thomas H., 6
Whales, 528, 529, 708
Wheat, 569; breeding and, 171,
178; rust, 271, 272
Whipworm, 390
White corpuscles, 560, 582, 583,
591, 594; matter, 614—615,
616, 617; and pneumnoia, 144

Whooping crane, 735-736
Wickman, Dr., 6, 9
Wiggler, 442

Wildlife, conservation of, 731—
739

Wilting, 76, 350
Wind erosion, 717-718
Wind-pollinated plants, 357
Windbreaks, 717
Windpipe of frog, 480; human,
600-601

Winds, 677; and seed dispersal,
363

Wings, of bee, 435-436; bird,
509, 510— 511; of butterfly or
moth, 433; of grasshopper,
426, 427; of housefly, 441; of
Hymenoptera, 435; of Isop-
tera, 439-440

Withdrawal symptoms, 633, 635
Wolverine, 531
Wolves, 530

Wood, stem types, 329-330;

tissues in, 332; xylem as, 316
Woodchuck, 525
Woodcock, 512

Woodpeckers, 53; ivory-billed,
735

Woody, plants, 310; stems, 325—
333, (Table) 333
Worms, 385-396; parasitic, 238
Wound infection, 241
Wounds, healing, 583—586

X chromosomes, 1 36-1 37, 138-
139, 140-141, 149
X rays, and insect control; 447,
448; and mutations, 148, 149,
151

Xanthophylls, 61; in leaves,
345-346

Xerophytes, 675, 676, 704
Xylem, 308; fibers, 332; paren¬
chyma, 332; root, 316, 317

Y chromosomes, 136—137, 138—
139, 140-141, 149
Yeasts, 24, 269— 270; and fer¬
mentation, 223, 270; repro¬
duction, 108; vinegar and,
233; vitamin B2 and, 270
Yolk sac, of fish, 469; human,
654

Yolks, of birds, 518

Zoospores, of Oedogonium,
283-284; of Ulothrix, 283
Zones, climatic, 700, 701- 707;
ocean, 707-708

Zygotes, 109, 110, 112; of algae,
278; of bread mold, 267; of
fern, 300; formation, 648;
human, 652—653; in hydra,
380; in medusa, 381; of Oe¬
dogonium, 284; of plant, 358;
Spirogyra, 282; of Ulothrix,
283, 284-286

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OTTO JAMES H JAMES HOWARD
MODERN BIOLOGY

NL 39128058 CURR HIST

Q H 307 M818 1965 C. 2

Otto | James Howard.

Modern biology.

002 637 OB CUkR

239141 1

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