Full text of "Biology; the story of living things"

MARINE BIOLOGICAL LABORATORY.

Received June 27* 1939
Accession No. 50458
Given by Amer loan Book Co*
Place, New York City

*,t*flo book OP pamphlet is to be removed ttom the Iiab-
oratopy tuithout the pepmission of the Trustees.

BIOLOGY

The Story of Living Things

GEORGE WILLIAM HUNTER

Lecturer in Methods of Science Teacliimj

Department of Education

Claremont Colleges

HERRERT EUGENE WALTER

Professor of Biology, Brown Unirersity

GEORGE WILLIAiVI HUNTER, III

Assistant Professor of Biology, Wesleyan Unirersity

"^1

AMERICAN BOOK COMPANY

NEW YORK CINCINNATI

BOSTON ATLANTA DALLAS

CHICAGO

SAN FRANCISCO

COLLEGE BIOLOGY, H. W. & H.
W. P. 2

MADE IN U. S. A.

This hook is gratefully dedicated to our wives, to whom
much of the credit and none of the blame is due.

1>11EFACE

Here are a few chips left over from the authors' workshop.

First of all we do not pretend to ha\'e presented herein the last
word in a field already overcrowded by worthy ri\'als. The "last
word" has an undesirable mortuary connotation quite out of keeping
in a book about living things.

The authors have been teaching biology for a total of ninety-four
academic years, in addition to over sixty seasons of strenuous service
in summer field work with classes at marine and fresh-water labora-
tories, and they can truthfully and enthusiastically say that they
have enjoyed this experience.

If what they would pass on to other students of biology appears
from the table of contents to bear the familiar marks of old stuff, the
reason is that it represents, in their minds at least, what remains
after many years of trial and elimination at the hands of an army of
different teachers and scholars. The fact that much material that has
been worked over before it was retained does not necessarily prevent,
it is hoped, some degree of freshness in its presentation. Any text-
book, the authors hold, should be somewhat like a dish of uncracked
nuts, accompanied by a good substantial nutcracker. It is desirable
that the reader should have some of the fun of wielding the nutcracker,
for no pedagogical cellophane can preserve nuts already shelled in an
entirely fresh and satisfactory condition for a very long time.

An inevitable handicap that the textbook method of presentation
of any subject is bound to suffer, is the fact that between the covers
of a book the whole banquet is set upon the table at once in a more or
less complete array. It is the part of the instructor to break up the
feast into courses and to serve them in digestible portions. Perhaps
the method of suspense employed in magazine serials woidd furnish a
better way of arriving at the desired end than presenting the matter
all at once in l^ook form, since sufficient time shoukl always be pro-
vided between the planting and harvesting of intellectual ideas to
allow for unforced sprouting and growth.

In the use of any textbook it is well to remember that the pages
may be turned backward as well as forward, and that it is no crime
either to skip or to reread.

Every studious and effective reader, moreover, is wary about

vi PREFACE

accepting witliout question whatever he may come across in print,
for even textbooks are often known to l)o incomplete and liable to
error.

Again, if the art of reading between the hnes has not been culti-
vated, it does not greatly avail simply to scan the printed hnes
themselves. Every opening that induces the reader to seek further
should be gratefully prized.

Goethe once said: "Wer nicht mit der Bewunderung anfangt,
werdet nie in das innere Heiligthum eindringen." Wonder is truly
the mother of wisdom, for once the capacity for wonder slips away,
one is prone to become blase, imcomfortably sophisticated, and
intellectually slothful.

With this explanation of the way it is hoped that this book will
be used, the authors unite in cordially inviting the reader to join
them in exploring the following pages.

ACKNOWLEDGMENTS

The authors wish to make grateful acknowledgment to all who
have aided them in the preparation of a college textbook in biology.

In particular, mention should be made of the members of the Biology
Department of Wesleyan University who so willingly collaborated in
trying out the ecological approach to a study of general biology for
several years prior to publication of this book. Grateful acknowledg-
ment is also made to them for innumerable suggestions as well as for
their willingness to include certain successful features of the course in
the text. Their help and advice has frequently been sought and
willingly given.

Special mention should also be made of the tireless effort and
willing help of Wanda S. Hunter and Alice Hall Walter, both of whom
read the manuscript and proof and contributed much to whatever
success this book may attain.

It is impossible here to enumerate all who have aided in the
production of this book, but the following names must be men-
tioned : Dr. E. C. Schneider, Shanklin Biological Laboratory,
Wesleyan University, for reading the entire manuscript ; Dr. Francis
R. Hunter, Rhode Island State College, for reading the entire proof ;
Dr. Aurel O. Foster, Gorgas Memorial Laboratory, Panama, for
reading section XII ; Dr. Hurbert B. Goodrich, Shanklin Biological
Laboratory, Wesleyan University, for reading sections XIX-XXIII ;
Dr. Frederick L. Hisaw, Biological Laboratories, Harvard University,
for reading section XVIII ; Dr. John A. AIcGeoch, Psychological
Laboratory, Wesleyan University, and Dr. Bernard C. Ewer, Depart-
ment of Psychology, Pomona College, for reading section XVH ;
Dr. Philip A. Munz, Department of Biology, Pomona College, for
reading the botanical portions of the book ; Messrs. Emil Kotcher
and Wilson C. Grant for aid in preparing the index.

Acknowledgment is also made to organizations and individuals
without whose co-operation it would have been impossible to secure
many of the instructive and attractive illustrations.

CONTENTS

PAGE
1

NATURAL HISTORY

CHAPTER

I. The Stage Settin"g (Ecology)

II. The Biological Conquest of the Would . . . 20

III. The Interdependence of Living Things — The Web of

Life 44

IV. Roll Call (33

FUNDAMENTALS OF STRUCTURE AND FUNCTION

V. Life and Protoplasm
VL Cells and Tissues

125

138

ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

VII. Beginnings: The Large Group of the Smallest Organlsms

VHI. The Development of Sexuality in Plants

IX. Division of Labor in Coelenterates

X. Being a Worm

XI. The Popular Insect Plan .

NIL The Art of Parasitism

XIII. Advantages of Being a Vertebrate

151
lf)8
17!i
187
199

233

THE MAINTENANCE OF THE INDIVIDU.VL

XIV. The Role of Green Plants

XV. The Metabolic Machinery of Animals

XVI. Support, Motion, and Sensation

XVII. The Display of Energy

XVIII. Chemical Regulators

237
274
32()
3C.4
390

THE MAINTENANCE OF SPECIES

XIX. Reproduction and Life Cycles
XX. The Great Relay Race

405
434

X CONTENTS

THE CHANGING WORLD

CHAPTER PAGL

XXI. Time Spent (Palaeontology) 473

XXII. The Epic of Evolution 483

XXIII. That Animal, Man (Anthropology) .... 530

MAN AS A CONQUEROR

XXIV. Man's Conquest of Nature ...... 567

XXV. Conservation and Its Meaning 589

XXVI. Man's Fight for Survival 608

XXVII. The Next Million Years 637

Index 645

NATURM. IIISToin

THE STAGE SETTI\(; (ECOLOGY)

Preview. Ecology of a typical region • How to study ocolog>' • IMaiit
and animal associations • Basic environments : water as a factor ; tempera-
ture ; light as a factor ; chemical factors ; gravity as a factor ; substratum ;
molar agencies ; biotic factors • Life in the water • Life in the air • Life on
land • Suggested readings.

PREVIEW

"My heart is fixed firm and stable in the belief that ultimately the sun-
shine and the summer, the flowers and the azure sky, shall Ijecome, as it
were, interwoven into man's existence. He shall take from all their beauty
and enjoy their glory." — Richard Jefferies : The Life of the Fields.

There is a lure in knowing something intimate about i)lant and
animal neighbors, their habits and the places where thoy live. A
trout fisherman finds almost as keen enjoyment in watching a king-
fisher make its catch as in having a trout take his own fly. Flic
banks, meadow\s, and woods along a trout stream are aliv'e with
interesting plants and animals. Even a slight acquaintance with
what may be expected along the path makes a hike througii the
forest and field immensely more worth while. An early morning
walk, if one knows a few permanent bird residents and can recognize
a migrant here and there, takes on an absorbing interest for tlie
observer. Such trips in the open are eventful experiences, the joy
of which is not easily forgotten. One may see the beauty of living
things, and enjoy the songs of birds and the gay colors of insects, or
get a thrill out of the sight of the first violet or bluebird, as he drinks
in the sweet odors of the flowery meadow. From the standpoint of
the more observant, another side than passive enjoyment of nature
is to be found. It is discovered in asking and trying to answer the
how and why of life aroimd us.

Charles Elton has called the science of ecology "scientific natural
history." This deals with the occurrences and behavior of organ-
isms in a given habitat or home. Anyone who feels a genuine response
to the call from the natural environment surrounding him cammf

2 NATURAL HISTORY

fail to find an interest in this approach to natural history. Why, for
example, do eertaiii kinds of animals live in the swift water of trout
streams, while different ones are associated with plants in a quiet
pond? Why are the types of life found along the seashore so unlike
those around the edge of an inland lake? Why do forest trees grow
tall in the dense woodland, more spreading in the open, and stunted
near the tops of mountains? These and hundreds of like questions
can be answered truthfully with the background afforded by the
science of ecology.

Ecology of a Typical Region ^

New England scenery is characterized by rounded granite hills,
often heavily wooded with second or even third growth. In the
hollows surrounded by these hills nestle little lakes, bodies of water
varying in area from a few hundred square feet of surface to many
scores or even square miles in extent.

A survey of the inhabitants of one of these smaller lakes, chosen as a
typical example, reveals relatively few fish and fewer plants in the
open water. Nearer shore are found unmistakable zoning of plants
and animals, depending on whether the shore is rocky, sandy, or
muddy. In sheltered bays having a bottom of soft mud are found
numbers of pond lilies and other aquatic plants, which give shelter
to pickerel, bass, and smaller fish, as well as a vast array of small
crustaceans, insect larvae, and microscopic plants and animals.

Part of the lake shore is a sandy beach, at one end of which a slug-
gish stream, after meandering through a meadow, empties into the
lake. This constitutes quite a typical environment and will yield
abundant material if searched carefully.

The edge of the lake bordering on the beach contains relatively
few plants and animals. It is exposed to the wind and consequently
to wavelets which cause more or less movement of the loose sand, thus
giving slight protection to living things. We find here almost no

' BOOKS USEFUL FOR FIELD WORK

Downing, Our Living World, Longmans, Green, 1924.

Johnson and Snook, Seashore Animals of the Pacific Coast, Maemillan, 1927.

Lntz, Field Book of Insects, Putnam, 1921.

Mann and Hastings, Out of Doors, Holt, 19.32.

Morgan, Field Book of Ponds and Streams, Putnam, 19.30.

Needhani and Needham, Guide to the Study of Fresh Water Biology, 3rd ed., Comstock Publ. Co.,

193.-).
Weaver and Clement, Plant Ecology, McGraw-Hill, 1929.

THE STAGE SETTING

./. N lOinkin. Jr.

A slow-ilowing stream presents a habitat for characteristic plants ami animals
adapted to this type of environment. Head i)afjes ."5-1.

plants and only occasional bass, pickerel, or minnows. A few
dragonfly nymphs live under the small stones in shallow water, while
ninnerous snails (Campeloma) are foimd buried in the sand or crawl-
ing along the bottom.^ It is possible to collect a few specimens of
plankton, which consists of minute free-swimming or floating organ-
isms, but, on the whole, it is a relatively inhospitable environment
inhabited by comparatively few organisms.

Within a few yards of this beach the stream flows gently over a
shallow sandbar, flanked by cattails and rushes. Here are nimierous
representatives of several groups of plants : in the water a \'ariety
of algae, Spirogyra (pond scum), streaming filaments of Ocdogonium,
Oscillatoria, and Cladophora, and iimiunerable unicellular organisms,
such as desmids and diatoms. Water cress, water plantain, water
smart-weed, and burr-weed grow along the banks, while in sheltered
bays the surface of the water may be covered with duckwec'd or per-
haps yellow and white water lilies. H(>re and there in boggy i)I;ic(>s
are dense masses of cattails, yellow flowering rushes, and numerons
sedges, while on the banks are fotmd grasses of se\'eral species.

'It is expected that the student will make free use of IV, ' Roll Call." for cencral idcntiBcation
and of the books of reference noted for more intimate and exact classification.

4 NATURAL IIISTOHY

buttercups, Jack-in-the-pulpit, bog arrow-grass, and a few shrubs
such as button-bush and willow. The vegetation shows a zonal
arrangement of, first, submerged or floating water plants, then emer-
gent forms, growing in the water and along the banks, while other
plants such as grasses and shrubs are found at a little distance from
the water. This zonal distribution is characteristic of shore associa-
tions of plants and animals.

In the slow-flowing stream live two species of sunfish, two or three
species of pickerel, bass, three species of frogs, bullfrogs, green frogs,
and pickerel frogs with their tadpoles, also an occasional painted
turtle and water snake. Of birds, the redwing blackbirds are numer-
ous, with occasional kingfishers, and more rarely a great blue heron.
Although no mammals are in sight, a telltale mound of sticks shows
that muskrats live there. Of the smaller organisms, the nymphs and
larvae of the dragonfly and Mayfly are the most abundant. The
water swarms with two species of water bugs and diving beetles, while
beetle larvae and the larvae of mosquitoes are numerous. Many
crustaceans, tiny amphipods and isopods, may be seen swimming or
feeding on the aquatic plants. The snails, Physa and Lymnaea, are
very abundant, while a few aquatic worms, Tuhifex, may be found in
the mud. Colonies of bryozoans may also be found, incrusting the
stems of water plants, as well as an occasional mass of fresh-water
sponge.

These two regions, the lake shore and the stream, although only a
few yards apart, present tremendous differences in populations.
Why these differences? At first sight, one might say it was due
entirely to abundance of food, but this is only begging the question.
Evidently many factors are at work. The fauna and flora of other
localities visited would show even greater changes. Across the
meadow and up into the nearby woods each locality would be found
to be inhabited by groups of living plants and animals differing in
many respects from those in neighboring localities. In each of these
localities there would be certain dominant organisms better fitted
than any others to live there. These become permanent species in
that locality.

How to Study Ecology

To understand much al)out ecology, one must be able to do much
more than simply study a book. The place to study the stage setting
is the stage. The place to learn about the relation of living things

THE STM^E SKTTINd

to their cnviroiuncnt is the luihitat. Kltoii ' in his iiiteivstiiijr intro-
duction to ecology cU^scribes the attack on a ceilain ccolofrical prob-
lem in these words :

"Suppose one is studying the factors limiting the distribution of animals
living in an estuary. One would need to know amongst other things what
the tides were (but not the theories as to how and why they occur in a par-
ticular way) ; the chemical composition of the water and how to estimate
the chloride content (but not tlie reasons why silver nitrate precipitates
sodium chloride) ; how the rainfall at different times of the year affected
the muddiness of the water; something about the physiology of sulphur
bacteria which prevent animals from living in certain parts of the estuary ;
the names of common plants growing in salt-marshes ; sometliing about the
periodicity of droughts (but not the reasons for their occurrence). One
would also have to learn how to talk politely to a fisherman or to the man
who catches prawns, how to stalk a bird witli field-glasses, and possibly how
to drive a car or sail a boat. Knowing all these things, and a great deal
more, the main part of one's work would still be the observation and coUeo
tion of animals with a view to finding out their distribution and habits."

This gives us our approach. Our own interests, our reading, and
the time involved must largely determine the extent to whicii we
solve the ecological problems
of our own environment.

Plant and Animal
Associations

In making an ecological
study of living communities
we notice that one kind of
plant or one kind of animal
is never found li^-ing entirely
alone. Plants, for example,
are associated together by
lack or abundance of water ;
those living under abundant
water conditions being called
hydrophytes ; those associated
in a condition of moderate

Water lilies, catta
(•haracteristi(

hulriislu's
pliyle."*.

1 From Elton, Charles. Animal Ecology, p. 35. By permission of The Macmillan Company.

publishers.

NATllRAr. HISTORY

Typical xerophytic plants of the desert areas.

Hau'oTtIt

water supply, mesophytes ; and those which associate in desert condi-
tions, xewphytes. Animals which live in the water are said to be aquatic,
those on land terrestrial, while those that live both on land and in
water are called amphibious. Animals and plants associated in still
water are quite different from those in running water, while different
types of plants and animals are found close to shore, in deep water, in
rapid water, on rocky shores or on sandy shores, in salt or in fresh
water, and in tidal pools or on the sand. Everywhere we find dif-
ferent associations of plants and animals. Many explanations are
given, but no one explains everything. One investigator, Merriman,
emphasizes temperature as an all-important factor ; Walker gives
atmospheric pressure ; Heilprin, food ; and Shelford, in recent experi-
ments, indicates that the conditions under which an animal breeds
may greatly influence its distribution. He experimented with tiger
beetles, using different soils such as clay, clay and humus, humus,
humus and sand, and pure sand. The beetles lay their eggs only in
moist soil, therefore this factor was constant with all the soils. In
this experiment the soils were also placed at a level and on slants.

THE STAGE SETTING

Eighty per cent of all the eggs were laid in steep elay, and <)S per
cent in sloping soil. Thus ho concludes that the egg-laying hahits
of these beetles determine tiieir habi-
tat, for if they could not get the kind
of soil and the slope needed, they
would not breed. In this case the
fluctuation and distribution of a spe-
cies would be dependent upon a single
factor. This may be true in the dis-
tribution of a great many plants and
animals.

Basic Environments

There are three states of matter,
gas, liquid, and solid. These are evi-
dent in the land, the water, and the
air in which living things are found.
Life is only found in conditions where
it is at least partially fitted or adapted
to live. These conditions, called factors of the en\iroinnent. are air
or its contained gases; water or moisture; temperature; light;
chemical constituents in soil, water, or foods; gravity; the presence
of a substratum on which the organism rests, such as soil, moving
objects in the water, or the sea bottom ; molar agencies, such as
wind, water currents, or any moving force in the environment ; and
finally, biotic factors which come through the interaction of other
organisms in the same environment.

A birch forest is composed of
typical me.sophyte.s.

Water as a Factor

Water is absolutely essential to life, from 40 to 95 per cent of all
living things being formed of this substance. It is generally true that
no growth or life process of either plants or animals can take place
without water. An example of this relationship of moisture to life is
shown in the story of the British Mu.seum snail related by Mr. Baird.'

" On the 25th of March 1846 two specimens of Helix desertorum, colloc-ted
by Charles Lamb, Esq., in Egypt some time previously, were fixed ui>on
tablets and placed in the collection among the other ^h)llusca of the .Musmnn.
There they remained fast gummed to the tal)let. About the loth of .Marcli
1850, having occasion to examine some shells in the same ca.se, Mr. Il'iird

1 Ann. Mag. Nat. Hisl. (2) vi. (1850). p. 68.
H. W. H, — 2

NATURAL HISTORY

w|i»4«jiK.

'-t**-».

WiiylU J'itrcc

These photographs were taken from the same spot on the Mohave desert floor.
The upper was made at the end of the rainy season, the lower about two months
later. What one factor causes this difference.!^

noticed a recently formed epiphragm over the mouth of one of these snails.
On removing the snails from the tablet and placing them in tepid water, one
of them came out of its shell, and the next day ate some cabbage leaf. A
month or two afterwards it began repairing the lip of its shell, which was
broken when it was first affixed to the tablet."

THE STAGE SETTING .,

The uses to which water is put by an organism are nianit'old. It
is necessary as a solvent for foods within the body. In HvIiik tissues
it becomes a medium of exchange between different parts of tlio body,
while in higher animals it carries off body heat, thus helping in tiic
regulation of their temperature. In air it causes humidity. In soil
it carries the raw food materials of green plants. In many alkali
lakes, such as Great Salt Lake, fish life is practically absent and the
numbers of insects and crustaceans inhabiting such water are greatly
reduced because of the high mineral content of the water. On the
other hand certain crustaceans, such as the brine shrimps, are only
found in water containing a high concentration of salts. Acid lakes
and streams contain only certain types of fish, and according to in-
vestigation by Jewell ^ are lacking in snails, possibly because of the
absence of lime from which snails build their shells.

Temperature

Differences in climate (which after all are largely differences in
temperature and water supply) are accompanied by changes in the
appearance and kinds, of plants and animals. The life processes of
organisms proceed between certain maximum and minimum limits
of temperature. Somewhere between these is an optimum temi^era-
ture at which the life processes function most normally. In i)lants
optimum temperatures vary greatly for different species, and are
largely instrumental in determining what plants will grow in a gi\-cn
locality. For example, apple-raising regions must have a mean
summer temperature of not more than 70° F. The optimum of
most tropical plants ranges over 90° F., while alpine species require
a temperature slightly above freezing. The temperature of plants
changes rapidly, depending on the amount of external heat they re-
ceive. This has an important bearing on horticulture. Lemons on
the trees, for example, freeze at a temperature of 28° F., and oranges
at 26° F. They are often kept from freezing by means of heaters.
Plant injuries caused by freezing are due to the rapid withdrawal of
water from the soft parts, therefore plants with a high water con-
tent are more easily injured. This accounts for the freezing of the
young tips of trees. Seeds which have a small water conteut are
capable of withstanding very low temi)eratures.

In animals, as in plants, the lif(> processes proceed best at oi)timuiii
temperatures which differ with the species. Mast p rotozo a divide

1 Jewell, •• The Fishes of an Acid Lake." Tran.. Amer. M \ol. XLIII, 1924. pp. 77-84.

NATURAL HISTORY

^g»

*"■ ■ ■>^---''^'>j*S3B||)iiN^:'-

nil wl^^^^^^^H

y^^:

(,?) ir. L. Macchtlin
During the freezing weather in January, 1937, in California, citrus groves
which were adequately protected by heaters lost relatively little fruit, while many
unprotected groves suffered a complete loss of fruit as well as some trees.

much more rapidly at warm than at cold temperatures, and this is true
of the reproduction of many animals. Many tropical animals may
withstand cold temperatures, but will not propagate at those tem-
peratures. H. B. Ward ^ has made observations on the sockeye
salmon which indicate that these fish in swimming up rivers to spawn
always take the river of slightly cooler temperature, a difference of
1° F. being sufficient to divert the fish. Seasonal cycles of activity
are largely influenced by temperature, this being particularly true of
reproductive activity, which plays a part in the migrations of birds,
the rapid multiplication of plankton and other forms. Some animals
respond to a cold temperature by going into a resting state or hiberna-
tion, while others go into a dormant condition because of unfavorable
conditions of heat and dryness. This latter state, aestivation, is often
seen in regions having marked periods of alternating rain and drought.

I Ward, H. B. " Some Responses of Sockeye Salmon to Environmental Influences during Fresh-
water Migration." Ann. and Mag. of Nat. Hist., Vol. VI, pp. 18-36.

THE STAGE SETTING

Animals are said to be warm-blooded or cold-blooded. The foriiK r
term means that they have a constant body temi)erature {honwio-
thermal), while the latter means that the body temperature varies
with the external temperature {poikilothcnnal) . Frogs can often
be frozen stiff and, when thawed out gradually, will live. This is
true of many animals and is an undoubted adaptation which enables
them to withstand great cold. Homoiothermal animals, however,
are more or less independent of the external temperature because
their internal body heat remains at a constant temperature regard-
less of outside fluctuations.

Animals are divided into two groups depending on whether they
can easily stand changes in external temperature, some being
restricted to a relatively narrow range of temperature changes {steno-
thermaV), while others have not only the ability to withstand a large
range of temperature, but also may become acclimated to new tem-
perature ranges if they are changed gradually from one environment
to another {eur y thermal) . A classic series of experiments by Dallinger
with protozoans showed that he could change their li\'ing conditions
from 15.6° to 70° C. without having the animals die. It is this ability
that gives us the plant and animal populations in some hot springs.

Light as a Factor

Light is a form of radiant energy. Passed througli a prism it is
broken up into the primary colors of the spectrum, each of which has

Left : A nasturtium plant exposed to ordinary Kreeniu...se lif^hl sin.e ^.vnuuMum.
Right: Same plant exposed to onr-sideddlunnnal.on for .SIX hours.

I^ettuce. a long-day plant.

Salvia, a short-day plant. These series of plants were grown experimentally
at the Boyce Thompson Institute for Plant Research, Yonkers, N. Y.

THE STAGE SETTING

i:i

a different wave length. In addition there is the non-visible radiant
energy of the ultra-spectrum. These different wave lengths ha\c
various effects on plants and animals. Chlorophyll, the gnnMi color-
ing matter of plants, which depends on the presence of light, absorbs
light waves only from the red and blue bands of the spectrum.
Whereas most of the radiant energy absorbed by a plant changes
to heat, a very small part of it, estimated at not more than 0.5 per
cent to 3 per cent, is used by the chlorophyll in the process of starch
making. As in the case of temperature, optimum light is necessary
for the best work of plants, some preferring shade and others living
at their best in bright sunlight.

Light causes movements in leaves and stems as well as changes
in the size and shape of these organs. Plants respond to light, tlie
leaves being placed so as to get the most light possible. The amount
of light largely determines the shape of the entire plant, trees in a thick
forest having a very different shape from similar trees in the open.

The length of daylight has an effect on plants. Some plants, like
the radish, spinach, and clover, require a long day to produce flowers
and fruit, while fall flowers, such as cosmos, dahlia, and ragweed,
require a short day in order to form flowers and fruit. It has been
shown experimentally that for each species there appears to be a
most favorable length of day for flowering, fruiting, tuber formation,
and other food-storing activities. This discovery is of great value to
agriculturists.

30 V

Legend

Diaptomus Lake Eaton

Holopfdium 8<Q Sirnon Pond

Cladocera LaVe Madeleine

Diaptomus

13 Noon 2

6 8 10 13 Noon

.V. Y. State Conscrralion Drpt.

Curves showing the variation in nmnbers over a period of 21 hours of scvcriil
species of plankton organisms from three Adirondack lakes. W hiil factors might
be expected to influence their dislribiilion?

NATURAL HISTORY

Animals definitely respond by movement to the stimulus of light,
but unlike green plants, some respond positively and others nega-
tively. The unicellular Ameba is killed by too much light. Earth-
worms and some other animals are definitely repelled by light. The
moth, on the other hand, is attracted to light. Although of great
importance, light may be injurious to some forms, for bacteria and
some animals are killed by long exposure to it. The dangers from
certain wave lengths of light are seen in a bad case of sunburn.

Light influences animals in other ways. Light stimulus coming
through the eyes of flounder is said to give rise to changes in the pig-
ment of the skin. Thus the surface of the skin takes on the general
color and markings of its background. Some animals in caves lack
pigment, and there seems to be a general relationship between light
and pigment in the skin. There is a day and night rhythm in the lives
of many animals. Land snails feed at night, while activities of most
birds are confined to the daytime. Bees go to flowers during day-
light. Migrations of plankton are influenced by light, many crusta-
ceans coming to the surface only at night and going deep down into
the water during the daytime.

A dry alkali lake.

Life is practically absent in such areas.
why this is so.'>

II riiilil I'll rcc

Can you explain

THE STAGE SETTING

Fishing boats at the mouth of the Klamath River in northern Cahfornia.
Salmon run in on the outgoing tide apparently in response to the fresh water
coming out through the narrow mouth of the river.

Chemical Factors

Under this heading are inckided all of the chemical factors in the
environment of living things. Such are soil, rocks, and the various
salts and chemical substances found in food and water. Experi-
mental evidence shows that certain mineral substances are needed
for plant growth, and that these minerals are found in the composi-
tion of living matter.

Alkali soils form a great problem of agriculture. In sixteen west-
ern states this is the greatest problem outside of the water supply.
In thirteen irrigated states there is enough alkali present to be harm-
ful to crops. Alkalies are chiefly harmful because their presence
causes the soil water to become permeated with these salts, thus
hindering absorption of water by the plant.

Acidity of the soil is another problem for the agriculturist. It is
produced by a number of factors, such as the removal of calciinn from
the soil, or the production of acids by certain bacteria or from decom-
position. Acid affects the plant growth by checking the multii)lica-
tion of useful bacteria and keeps earthworms and other useful animalN
out of the soil. However, some species of plants demand aciti soils.
Mountain laurel, rhododendron, blueberries, and cranberries are
examples, as are sphagnum mosses found in certain bogs.

NATURAL HISTORY

The distribution of fishes and other organisms in water depends
largely on whether these waters are neutral, acid, or alkaline. Brook
trout, for example, are usually found in acid and neutral waters,
while sunfish, bass, perch, and certain other fish are typically asso-
ciated with alkaline waters.

Carbon dioxide in the atmosphere is another factor which deter-
mines plant distribution, three parts to 10,000 being necessary if
plants are to make starch. Oxygen is essential for living things.
Certain so-called anaerobic bacteria and a few animals appear to be
able to live without oxygen. Some insect larvae, worms, and molluscs
live a part of the year in deep lakes where little or no free oxygen is
present, due to decomposition of the algae. Certainly one factor in
the distribution of aquatic animals appears to be the oxygen content
of the water.

Gravity as a Factor

The pull w^e call gravity brings about differences in pressure both of
air and of water. Plants and animals must adjust themselves to this
factor. In a general way gravity determines the size of organisms.
Insects and birds which move about swiftly in the air must be small,
otherwise gravity would bring them down. Gravity is important in

the growth and orienta-
tion of plants. It is a
stimulus for the direc-
tion taken by the plant
body, apparently caus-
ing the root to grow
downward and the stem
to grow upward, while
horizontal branches are
neutral to the pull of
gravity. This same
force acts upon sessile or
rooted animals, such as
hydroids and sponges.
Adaptations to offset
the force of gravity are seen in the air spaces of floating plants, oil
drops in eggs, spines and long hairs on the surfaces of aquatic plants
and animals, and the air spaces in bones and other tissues of birds,
and in the construction of feathers.

Successive positions, from photographs, showing
effect of gravity on a green plant {Impatiens glandii-
ligera). — After Pfeffer.

THE STAGE SETT1\(;

' 2^

'• r^'

ij^

.l/M/irs

Cypress trees have become adapted to live in swampy lands by developing
buttressed bases of the trunks and erect growths (knees) from the roots. Tliese
enable the tree to get sufficient air.

Substratum

Anything in which a plant grows or on which an animal comes to
rest is known as substratum. Types of soil differ from cold, dense,
clayey soils, which though they hold water do not readily give it up
to humus that is well aerated, has a high nitrogen content, hokls
water, and gives it up readily. The distribution of plants depends
to a considerable extent on the kind of soil found in a given locality.
For example, mosses and ferns grow in moist soil, while cacti are
found in sandy desert soils. Varying soil temperatures are brought
about by the kind of soil, whether coarse or fine ; by the pre.'^ence
of a blanket of living things over it ; by its color (dark soils absorb
heat more readily than light-colored soils) ; and by the water it will
hold (wet soils are cooler than dry). Great variations occur in the
air content of soils and this again determines the plants and animals
found in a given area. Water-soaked soil, for examj^le, contains
practically no air and does not ordinarily have a large jjiant or
animal population. In some cases a plant adapts itself to water-
soaked soil, as seen in th(> bald cypress.

NATURAL HISTORY

Animals also differ with different types of soil. This is particularly
true of the bottoms of lakes or streams. A different fauna is found
on the rocky stream bed from the soft mud of the pool below. Mud
contains more food, but it is also more difficult for organisms living
in it to carry on respiration. Soil is also the home of such burrowing
animals as nematode worms, earthworms, ants, beetles, digger wasps,
and the larvae of various insects.

Molar Agencies

Such are any moving agencies. Running water and winds erode ;
ice moves soil and rocks. Tides cause great differences in aggrega-
tions of plant and animal life, animals living between tides having
different problems to face from those below the tidal flow. Moving
air has a definite effect on vegetation, as is often seen in the wind-
blown trees on mountainsides or plains. Moving air acts upon seeds,
tumbleweeds, spores, and fruits, thus spreading plants over vast

American Museum uf Xatural History

Tidal shores, along the New England coast, show wide variations in habitat.
The flora and fauna of the intertidal zone differs greatly from that of the regions
above and below the tidal flow.

TUE STAGE SETTING

1«»

\y ritjlil /'it re,

The ell'ect of differences in environiiient upon the same s[)e(ies of tree (Piiins
ponderosa). Here molar agencies are largely responsible for Ihi; changed ai)pear-
ance of the tree.

areas, but it also fells much timber, breaks off branches, and destroys
crops. Winds may either help or hinder in the migration of insects.
The cotton boll weevil travels north more rapidly in the years when
more wind is recorded. Winds blow birds and insects out to sea,
thus destroying them, or they may land them in a new location where
they may multiply rapidly. Currents of air as well as water currents
distribute plants and animals. Many animal forms react to wind
and water currents. Fish head upstream, an adaptation favorable to
food-getting. The swiftness of the current not only determines tiic
distribution of fishes, but also of other forms, such as caddis fly
larvae and "water pennies."

Biotic Factors

These are factors arising from the presence of other li^•ing organ-
isms. One is concerned when studying ecology not only with the
environment of living things but also with how li\ing things react
on others in their immediate environment. There is competition
not only between plants and animals, but al.so between plants of the

20 NATURAL HISTORY

same and of different species for a place under tlie sun ; literallj'- under
the sun, for competition is caused by the limited amount of light that
will fall in a given area. Young plants often die because of the shad-
ing by the parent plant, and larger plants preempt areas of soil which
give little or no space for young growth. Feeding by animals, such

Effect of sheep grazing upon trees. Thousands of young trees are destroyed

every year in this way.

as rabbits or sheep, may change the entire flora of a region, while
parasitic organisms injure a vast number of the hosts on which they
live. There are also marked cases of partnership between organisms,
bacteria in the soil giving and taking from both plants and animals,
and helping to create a cycle of food substances which pass through
the bodies of both animals and plants. The feeding of animals is their
biggest business in life, and the presence of a food supply determines
very largely the presence of animals in a given locality. It is said
that the oak tree serves as food for over 500 species of insects, the
apple for 400, clover and corn, over 200 each. Thus man with his
tilling of the soil, destruction of the forest, and domestication of
plants and animals has changed the fauna and flora of the land.

Having discussed the effects of the factors of the environment on
living (jrganisms, let us now see how the interaction of these factors
affects life in the situations that living things are forced to meet.
Animals and plants must be adapted to live either in the water, in
air, or on land. The pages that follow show some of these adaptations.

THE STAGE SETTING

Life in the Water

Plants are adapted for lite in water hy a mucli reduced root system,
by leaves which either float, are ribbonlike, or are finely divided with
air passages and air spaces. The latter spaces help buoy up the plant
and also allow for an accumulation of oxyu-en and carbon (hoxidc
Green coloring matter is abundant, such plants being better fitted
for vegetative propagation than reproduction by flowers and fruits,
as is shown by their numerous horizontal and thickened stems. In
general, aquatic plants are restricted to relatively shallow water,
many species being found floating near the surface.

Animals, usually locomotor and having definite adaptations for
movement in the water, have a much wider ^•ertical range. The
bodies of most fishes are more or less streamlined, and protected by
mucus which covers the backward-pointing scales, their fins being
placed where they offer the least possible resistance to the medium.
In some animals, the limbs are transformed into flii)pers, while in
lower types, such as protozoa, threads of living matter, cilia, are
used as whiplike organs of locomotion. Since the oxygen content
of water is only about 1 per cent as against over 20 per cent in air,
we find special adaptations

for taking in oxygen. These
structures are usually in the
form of gills, delicate struc-
tures which will be discussed
more fully later.

The water forms an ideal
medium for vast numbers of
small, free-swimming, or float-
ing organisms, the plankton.
Oceans and lakes swarm with
them. Every small pool has
its plankton, and even rapidly
flowing waters will disclose
some of these tiny organisms.
In certain tested regions in
the Atlantic, plants form about 56 per cent and animals 44 jx-r cent
of the total plankton. The flora consists mostly of diatoms, bac-
teria, and many forms of algae, while the fauna includes numerous
dinoflagellates and other one-celled animals, eggs of fish, molluscs,

Diatoms have various forms aFid may !><•
colonial as well as unicellular. riicrc arc
probably I. "),()()(» species known.

NATURAL HISTORY

numerous crustaceans mostly copepods, jellyfish, and the larvae of
many crustaceans, molluscs, and fish. Some of the plankton, such
as small crustaceans, tunicates, medusae, small fishes, and larger
algae, may be visible to the naked eye, but most of it is microscopic.

LAKE

PLACID

UPPER
SARANAC

MIDDLE
SARANAC

LOWER
SARANAC

deptm\

t—

Q-
LJ

CO
C3
<

.1-
t—

a
<

CO
Q-

BLUE GREEN
ALGAE

1 M

■

BOTTOM

GREEN ALGAE :

1 M

„

■

BOT 1 OM

DIATOMS

1 M

■

BOTTOM

iV. y. Siaie Conseruation Dept.

Comparison of the distribution of nannoplankton (minute forms that will
pass through the meshes of a plankton net) at the surface and bottom of four
Adirondack lakes.

The larger pelagic organisms mostly found in the ocean, such as fish,
squid, whales, turtles, and seals, are called collectively nekton.

Currents, wind action, the shapes of bays and coasts, migrations
of various animals, all cause differences in the horizontal distribution
of plankton. Sometimes given forms, as Cladocera, will multiply very
rapidly, even coloring the water in a large area. The vertical dis-
tribution is much more regular with reference to plants, since algae
and other green plants depend upon sunlight. Plants get very little
light at a depth of 100 meters. At 75 meters' depth, only half as many
plants are found as at 50 meters, and careful investigation in various
areas shows that most of the plant plankton lives within a few feet
of the surface. On the other hand, animals exist at great depths.
Beebe reports jellyfish, shrimps, and other plankton at a depth of
over 1000 feet and the tunicate, Salpa, as well as fishes, at his greatest
depth of 3028 feet. Dredgings from the "Challenger" and other
expeditions reveal many living organisms, particularly protozoans,
in the abysmal depths.

Till': sr\(;|.: si:rnN(; o.,

Towins with a phnikton i.cl (a llun-mcshcd uv\ cf l,oltii,K ,-l„tl,)
near the surlacc of tlu> ocean on an early summer day would yield
a very different distribution of organisms from those collected "on ;,
fall or winter day. There is a seasonal variation in distrihutioii.
The eggs and larvae of animals ar(> abundant in the spring and early
summer, while great numbers of algae appear then which are Hot
found later. This rhythm of plant life is believed to be correlated
with a turn-over of the available phosphates and nitrates in the water.
In the winter, the (^ooler top layer of water sinks and pushes up the
water rich in the salts necessary for plant growth from underneath,
so that with the coming of warmer weather tlu^ life cycle goes on and
a seasonal rhythm of algae appears. This turnover of plant and
animal life is very great. The fishing industry on the Grand Banks
and in the North Sea is largely due to the occurrence of this great
seasonal rhythm of plankton.

There is also a considerable variation in the numbers of plankton
near the surface of the water during the day and night. Many crus-
taceans, for example, come to the surface at night and go down in
the daytime, while green algae are usually nearer the surface during
the day.

In oceans and lakes, there is a more or less distinct zoning of living
forms, depending on the depth of water, the type of shore, or the kind
of bottom. A very different fauna and flora exist on a rocky coast
from that along a sandy beach. The forms of botii plants and ani-
mals are different in salt and fresh water areas.

Life in the Air

Here life is more circumscribed. There are no true air plants unless
they be the so-called epiphytes of the tropical rain forest, some algae,
such as the Pleurococcus found on the bark of trees, or the lichens,
which encrust rocks and tree trunks. The reproducti\'e bodies of
plants, such as spores, seeds, and fruits, are furnished with adai)ta-
tions which enable them to pass long distances through the air, thus
allowing new areas to be populated. In animals where locomotion
is possible various special adaptations exist. Flying animals hiivv
their wings placed wdiere they will not onlv cause the liody to mo\e
forward, but also assist in balancing it. Instead of one ])ropeller
placed astern, as in fish, flying animals have two paired i)roi)ellers
placed forward at a greater breadth of beam. The body is not onl.\-
streamlined, but in higher forms special adaptations exist for protec-
H. w. H. — 3

21.

NATURAL HISTORY

Epiphytes in a semitropical forest. Note the aerial roots for securiiif? moisture

from the air.

tion against low temperatures and moisture. Oiled skin and feathers
of birds are examples. Bones are hollow and large air spaces are
found between muscles. In insects a special aerating system exists,
since in these heavier-than-air machines a very rapid oxidation of
fuel material must take place if the organism is to be efficient in
the medium.

Life on the Land

Adaptations in plants for life on the land are seen in the widely
branching root systems, the woody stiffened stems, the leaves placed
in positions where light may reach them, and in the various adaptive
movements which enable green plants to get a share of the much
needed light. In tropical rain forests, this relation to light is seen in
a vertical zoning where sun plants form long twining stems, making
their way up the tall trimks of trees to an upper zone where light is
available, while in the lower areas are found shade-loving plants which
prefer less sunlight. In animals, where movement is much more
evident, there are special adaptations in the form of legs, which
support the body off the ground and allow of various types of loco-
motion such as climbing, crawling, walking, running, and leaping.

THE STAGE SETTLNG

Various other types of movement are found as, for example, tlir
waves of muscular contraction in the foot of the slug ; the crawling
of earthworms where tiny setae are used as levers; the erawlinfr
of the snake with its definite use of scales as "ground grippers" ; the
adaptations for leaping in the grasshopper and the frog; adaptations
for climbing, such as the sucking disks on tiie toes of tree frogs {Ilyln)
and of some lizards, or the arrangement of the toes in climbing birds.
These and scores of other adaptations for obtaining food, for brcatii-
ing, and for protection may be recalled.

SUGGESTIONS FOR FURTHER READLXG

Borradaile, L. A., The Animal and Its Environment, Oxford University Press,

London, 1923.

A general book on the natural history of animals.
Elton, C., Animal Ecology, The IVIacmillan Co., 1927. Chs. I, II, III, I\', \'.

A fascinating book, written in a charming style. Accurate and authentic.
Jordan, D. S., and Kellogg, V. L., Animal Life, D. Appleton tV: Co., 1900.

Contains some valuable chapters fundamental to an understanding of

ecology.
Needham, J. C, and Lloyd, J. T., The Life of Inland Waters, Charles C.

Thomas, 1930. Chs. Ill and V.

Interesting aquatic natural history.
Pearse, A. S., Animal Ecology, McGraw-Hill Book Co., 192G. Chs. II and III.

Rather technical.
Shelford, V. E., Animcil Communities in Temperate America, University of

Chicago Press, 1913.

A pioneer work, but still reliable and usable.
Shelford, V. E., Laboratory and Field Ecology, The Williams & Wilkins

Co., 1929.

Very usable for field work.
Weaver, J. E., and Clements, F. E., Plant Ecology, McGraw-Hill Book Co.,

1929. Chs. IX, X, XI, XII, XIII, XV.

Authentic and well written. It should be of great value in the field.

THE BIOLOGICAL CONQUEST OF THE WORLD

Preview. A comparison of two forests • The why of distribution ;
barriers ; successions and their causes ; overpopulation and its results •
The shifting world of organisms • Ways of locomotion • Adaptability to
new conditions • Human interference • Life zones • Life Realms • Sug-
gested readings.

PREVIEW

The science of Ecology, or the distribution of animals and plants in
a given habitat, was considered in the preceding section. Chorology
attempts to determine the laws governing the distribution of animals
and plants over the surface of the earth.

So long as man accepted the naive assum]3tion that the earth was
originally populated by means of isolated creative acts, there was
no point in attempting to explain the distribution of living things.
They had all been put arbitrarily in the places where they occurred,
and that was all there was to it. With the rise of the belief which
culminated in Darwin's famous theory, that dissimilar species have
arisen by modification from other species, and that all organisms are
related, more or less distantly, to one another, the interpretation of
plant and animal distribution became a very interesting and challeng-
ing field for study.

How about the varied populations of living things in arctic, tem-
perate, and torrid climates ; the absence of animals and plants from
areas quite suited to their existence? Why is it that tapirs are
found only in South America and the East Indies, while certain
fishes, such as the pickerel, occur only in North America and north-
em Europe ? Equally difficult aspects of distribution cropped out,
notably in the Australian fauna and flora, which differ so greatly
from that of the rest of the world, while most perplexing of all,
probably, the habit of migration that makes certain animals, such
as birds, seals, salmon, and eels, change residence regularly from one
region to another. A gradual suspici(jn that two environments quite
similar in general appearance might nevertheless be populated by
species of plants and animals different from each other gave the clue

TfTE BIOLOGICAL CONQUEST OF Till: Would 27

to a scientific differoiitiution of specific distribution, AVWor///, Irom
.aionoral distribution, Choroloc/!/.

A Comparison of Two Forests

Two writers, Victor E. Shclford, the well-known ecologist. and
William Beebe, ornithologist and naturalist, have given two widely
different pictures, one, an accurate description of a hard-wood forest
in Illinois, and the other, a survey of life in a British Guiana jungle
forest.

A typical beech-maple forest, such as Dr. Shelford describes, can
be found anywhere in the vicinity of Chicago. A.ssociated with the
two dominant trees are ash, elm, walnut, linden, and a wealth of
smaller trees and shrubs forming a lower layer under the higher trees.
Wild cherry, sassafras, and dogwood are abundant, and in some of
the more northern forests, azalea and rhododendron form an inter-
mediate growth. The floor of the forest is covered with herbs and
flowering plants, large and small, which change with the season. In
spring, trilliums, violets, wild geraniums, anemones, phlox, and scores
of other plants are in bloom, succeeded in the fall by asters and other
composites, in areas having ample light. A relatively large number
of plants having spiny or hooked fruits occur, which aid in their
accidental distribution by wandering animals. A few large mammals,
deer, fox, and hares, are found occa.sionally, though are rarely seen.
The woodchuck is perhaps the mo.st numerous of the mammals, and
the red, gray, and fox squirrels are not uncommon. Of birds the
crested flycatcher, wood pewee, blue jay, scarlet tanager, wood
thrush, and red-eyed vireo nest in the lower trees, while the oven-bird
conceals its curious architecture on the ground. The wood frog,
red-backed salamander, and Pickering's tree frog are found, although
not always in evidence, and insects abound, especially those that live
on trees, such as borers of various sorts, beetles, millipeds, spiders,
and in.sect larvae. Inhabiting the lower layer of the forest are snails,
centipedes, sowbugs, and earthworms. This represents, \\\\]\ \aria-
tions, a typical association of life in a northern deciduous forest.

At first sight the jungle forest does not appear to be very difTerent
from the northern forest. Both contain large and small trees, the
larger ones in the jungle, such as mora and greatheart. towering to a
height of two hundred feet or more, but here the likeness stops.
There is an almost complete absence of large horizontal branches in
the tropical forest, the trunks of trees shooting straight up for si.xty

■m

NATURAL HISTORY

}\'iUiam Beebe American Museum of Xatural History

CouTtestj U. S. Forest Serrice

A comparison of two widely separated forests. The right-hand photograph is a
typical northern mesophyte beech-maple association, the left-hand photograph a
tropical rain forest of British Guiana. Note the superficial likenesses and dif-
ferences.

or seventy feet without a branch, festooned with long cHmbing hanas,
which in this way work from the forest floor into the upper zones.
Four general horizontal regions, or zones of life, are distinguishable,
namely, the forest floor, the lower jungle up to about twenty feet, the
mid-jungle up to seventy feet, and the tree-tops, towering a hundred
and fifty or two himdred feet high. Life at first seems almost absent
in the jungle to the casual observer, but if one stops, and simply
looks, the jungle wakes up and life appears everyu^here. The forest
floor is covered with the accumulated debris of ages, fallen trees in
different stages of decay, fungi, mosses, and lichens, with a generous
covering of brown leaves, for here the leaves fall all the year around,
instead of only in the autumn season as in northern regions. The
ground area is occupied by occasional deer, paca, and tapirs, with
agoutis and armadillos found more frequently. Partridge and the
strange tropical tinamou are seen here and there, as well as jungle
mice and rats, salamanders, frogs, a few snakes, innumerable scorpions,
beetles, grubs, worms, and rarely, the unique and interesting Peripatus.
In the low jungle are found manikins of several species, ant-birds,
with trumpeters and jungle-wrens, while at night opossums climb

THE BiULUCilCAL CONQUEST OF Till-: Would u)

about through the underbrush. During the daytime tiie wonderful
morphous butterflies, brilhant spots of blue, add a touch of col.,!- to
the picture.

The mid-jungle contains the most life. Here iimumeraljlc birds,
curassows, guans, pigeons, barbets, jacamars, trogons, and smaller
feathered species abound, in company witli ant-eaters, sloths, squir-
rels, bats, coatis, and small monkeys such as marmosets.

The upper jungle of the tree-tops is the mo.st difficult region to
know. Red howlers and be.som monkeys move about in the tree-
tops, and occasional glimpses may be had of toucans, macaws, and
great flocks of parakeets and parrots that live ther(\ Fierce ants
prevent tree-climbing, and the relatively great height and mass of
foliage make living things not easily acce.ssible to observers in this
upper layer of the tropical rain forest.

These two forests, the northern maple-birch and the jungle, by their
entirely dissimilar populations illustrate contrasts that might be
found in many parts of the world. Sometimes conditions in widely
separated areas may be almost similar, with diverse populations
inhabiting them, and again, localities close at hand may show remark-
able diversities in their living inhabitants. When regions far apart
have similar populations, which does not commonly happen, the
biologist is faced by a puzzling problem.

The Why of Distribution

Jordan and Kellogg ^ give three laws to account for the distril)u-
tion of organisms which they state as follows : E\ery species is found
everywhere that conditions are suitable for it unless (I) it was unable
to reach there in the first place, or (2) having reached there it was
unable to stay because it could not adapt itself to the new condi-
tions, or (3) having entered the new^ environment it became modified
into another species. It is not only the normal habitat that deter-
mines the presence of a given plant or animal, but its accessibility
from the place of origin.

Although every species originated historicaUy from some i)receding
species at some definite place, its present distribution results from
the working of two opposing factors, expansion and repression. The
factors of expansion will be mentioned later, lliose of n^pression
are, first, inadequate means of dispersal because slow-moving animals

1. Jordan, D. S.. and KellofiK. V. L., Animal Life. Appletou, 190().

NATURAL HISTORY

are necessarily limited in their distribution. A second means of
repression lies in the poor adaptability of organisms to new localities
which they have invaded. A round peg will not fit in a square hole,
nor a square peg in a round hole, but if the peg consists of a plastic
material it will adapt itself. The normal habitat for a species is the
place where the organism is most nearly in physiological equilibrium,
the geographic range being determined by the fluctuation of a factor,
or factors, which are necessary for the life of a species.

Barriers

Each species widens its range of distribution as far as possible and
tries to overcome obstacles which nature has put in its way. These
obstacles may be chemico-physical, geographical, or biological bar-
riers.

In general chemico-physical barriers are climatic in nature, such
as unfavorable conditions of moisture, soil, or temperature. Soil
deficiencies, salinity, the presence or absence of light, or character
of the surrounding medium might also be mentioned. These climatic

Friislur

Why might such a mountain barrier restrict the distribution of certain plants

and animals .3

THE BIOLOGICAL CONQUEST OK TIIL WolU.O

barriers may be in vertical zones, extending from tlie ocean level to
mountain tops, as well as horizontal, spreading out north and south
from the equator in zones of latitude.

Map showing ancient and modern ranges of the elephants and their ancestors.
The shaded area shows the former habitat of the maniniolh and mastodon,
ant^estor of the modern elephant. A land connection probal)Iy existed I)etween
Asia and North America. Note the restricted range of the present-day elephants
indicated by heavy shading. How can this be accounted for.^

Sometimes natural barriers occur, such as high mountain ranges
with eternal snow, deserts with unfavorable conditions of moisture,
or in the case of water-distributed animals such as fishes, high water-
falls may prevent them from moving up a stream beyond a cciiain
point. The barrier for one organism, however, might l)e a highway
for another. A desert would be an impassable barrier- to a squirn^l
but not to a camel.

Geographical barriers have not always been fixed. Geological
history reveals the fact that some land surfaces were once occujiied by
water and what is now water may have been land. The presence of
fo.ssil sea.shells in the Panama Canal area indicates that the Isthmus
was formerly submerged, and there is evidence that as late as Eocene
times there was a land connection acro.ss Bering Straits. As bar-
riers have changed so has the resulting distribution of organisms.
Distribution often indicates the geography of the i)ast. .Mnnbers
of the same genus may differ widely in certain isolated localities, as,
for example, the tapirs found in tropical America and the Malay

32 NATURAL HISTORY

Peninsula with its adjacent islands. In early geological times mem-
bers of this genus were widespread and abundant, whereas now, due
to the disappearance of former land connections, there are but two
widely isolated species in existence.

The distribution of animals is bound up in their food supply.
Hence carnivorous animals are restricted to areas wiiere the animals
on which they prey live. Often a biological barrier is created by the
presence of animals which are parasitic on a given form. The tsetse
fly, Glossina, which frequents the river bottoms and shores of lakes
in certain parts of Africa, prevents the ranging of other than native
cattle in these areas because of the fact that they transmit a blood
parasite fatal to such animals. Man himself is most active in both
creating and breaking down barriers. He introduces new animals
and plants either purposely or by chance into areas where they thrive
and replace other species, or by building dams, irrigating, deforesta-
tion, or accidentally burning over areas, he destroys one kind of life
perhaps never to replace it with another.

Successions and Their Causes

Succession means that in a given area organisms succeed one an-
other because of changes in the environment, migration taking place
so that they may reach conditions favorable to their development.
An example of plant succession may be seen in almost any pond that
is gradually drying up. In deep water there are a few submerged
aquatic plants ; in water from 6 to 8 feet deep floating plants such as
pond lilies are found ; in shallow water from 1 to 4 feet deep, cat-
tails and reeds are abundant ; while at the edge we find a meadow of
sedges and some bushy plants. As the pond becomes drier, these
plants slowly push outward until eventually it may be completely
filled with plants which build up soil, making first a swamp and
eventually a meadow, while around the edge of the former pond will
now be a forest of trees and bushes. In the tropical oceans different
corals succeed each other, growing on the skeletons of other species,
thus building their way into shallow and warmer water, or along the
ocean shore colonial diatoms may occur, to be followed by hydroids and
seaweeds, the latter becoming a dominant climax formation, a group of
species that are better fitted to survive in that habitat than any others.

Erosion, which carries away the original inhabitants, or a deposit
of new soil by running water, wind, or other agencies, gives oppor-
tunity for the establishment of new life in a region thus devastated.

THE BI()LO(;iC,\L CONQUEST ()\- nil] \\(,|u,|)

The question of how long seeds will survive, uiidci- whal condilions
they will germinate, and how fast they will grow is of g,vat inipor-
tance in the repopulation of areas after soil erosion oi- fire. Beale
reports an experiment where
ten out of twenty-two species
of seeds sprouted after hav-
ing been buried in open bot-
tles in moist sand at a depth
of three feet for over forty
years. After a coniferous for-
est has been devastated by
fire, an entirely new series of
plants spring up in the area ;
first herbs, such as fireweed
or wild mustard ; then trees
or bushes, the seeds of which
may be brought by birds, as
raspberry, blackberry, or wild
cherry ; later a stage of trees
having wind-blown or bird-
carried seeds, such as aspen,
cottonwoods, or birches. Still
later the forest may become
repeopled by its original in-
habitants, which becomes the
climax.

Conditions of wind, mois-
ture, sunlight, and weather, the sum total of which constitutes climate,
play a most important part in succession. If drought destroys life in
a given region, an entirely new group of plants may come to occujiy
that area, bringing with them a new group of animals. Migrations
of animals may be brought about by changing seasons.

The biotic conditions governing successions are many. Man,
through clearing forests, throwing wastes into ri\ers, or introducing
new plants or animals which may compete with existing species, often
completely upsets the balance of life and causes succe.'^sioiis. Indus-
trial pollution may completely depopulate streams of fish life, bac-
terial growth replacing the original plants and animals. Sometimes
new organisms add so many competing mouths to feed in a gix'cn terri-
tory that it becomes necessary for some to break away if any are to li\('.

Wriijlu I'itrct

A lypiciil undt'CKruwtti succession after a
I'orcsl (ire.

34 NATURAL HISTORY

Overpopulation and Its Results

One of the I'aclors in dcteniiiiiiug tlie .spread and distribution of
organisms is overpopulation. An annual plant, for example, pro-
ducing only two seeds a year, which is far below the actual number,
and always developing these into mature plants, in only twenty-one
years would have 1,048,576 descendants. A pair of common house-
flies which usually produces eggs six times a year, each batch con-
taining 150 to 200 eggs, with the young flies beginning in turn to
lay eggs in about fourteen days after hatching and repeating the life
cycle, might, it is calculated, beginning to breed in April, if all the
eggs were hatched and no individuals died, give rise to 191,010,000,-
000,000,000,000 descendants by the end of August. However, each
species, year in and year out, tends to remain about stationary in
number. Indeed, many species are actually disappearing. The
reasons for this check of potential populations are found in lack of
adequate food supply, lack of favorable breeding conditions, and in
the fact that many animals and plants become food for others.

The Shifting World of Organisms

There is no doubt that desire for food furnishes the greatest urge
to locomotion and exploration in animals. Dr. Crothers once said
in one of his essays that the "haps and mis-haps of the hungry make
up natural history." Indirectly there is the same necessity for food
on the part of plants, but here the urge is expressed not so much in
locomotion as in a struggle for position with reference to light, which
is essential to every green plant in the manufacture of its own food.

Changing environmental conditions may force the movements of
organisms and produce faunal and floral repopulations. For example,
it is known that drifting coconuts frequently float long distances and
grow into trees upon some distant shore. A recent cataclysm of
nature has given us an opportunity to see the repopulation of a
devastated area taking place. In 1883, the volcanic island of Kra-
katao was literally blown to pieces by a series of terrific explosions
that destroyed every living thing on the island. Less than three
years after the volcano became quiescent, a Dutch botanist visiting
the island found the ash which covered its surface completely car-
peted with a layer of bacteria, diatoms, and primitive blue-green algae.
Here and there ferns were found, along with several kinds of mosses.
There were even a few flowering plants, but no trees or shrubs. In

THE BIOLOGICAL CONQUEST OF THE Would r,

that short time, the naked land had been partially ropoijulatcd with
these several low forms of life, by spores or seeds blown througli tho
air, or floated in water from the nearest islands, wliich wore aboiil
fifteen miles away. Twenty-three years after the explosion, I'rofessor
Ernst visited Krakatao and reijorted a forest of cocoinit pahns and
figs growing near the shore line, a luxuriant jungle in the interior, and
considerable animal life, represented by species that could either fly
or drift to the island on floating wood. Ernst estimated that within
another fifty years this island would differ in no respect from its
neighbors, a prediction, however, wliich seems doomed to failure of
confirmation because the volcano has again gone on a rampage.

Variations in temperature, brought about by the changing seasons,
are a factor in the movements of animals. This is particularly true
in the case of the annual migrations of such animals as crabs, lobsters,
and squid, which go into deep water in winter, returning to shallow
shore-water in spring. Movements apparently dependent to some
extent upon temperature occur in the case of many marine fishes, and
birds, certain butterflies, and bats, that go north and south according
to the season. Many animals mo\T up and down mountain slojjes
probably for the same reason.

Sometimes other factors than scarcity of food, em'ironmental
changes, or seasonal differences cause migration. I^emmings. for
instance, small rodents living in the mountainous districts of Scan-
dinavia, at intervals of from five to twenty years suddenly mo\'e
forth in vast numbers, with no apparent Pied Pi])er of Hamelin to
lead them, but always in the same general direction, swimming rivers
and lakes, overcoming all sorts of obstacles, and eventually ending
the mysterious trek in the ocean. Although they feed on the way
and consume enormous amounts of food material, the search for food
is not sufficient to explain their fatal pilgrimages.

The relation of different degrees of salinity to th(> breeding habits
of food-fishes jjrobably influences their distribution also, by det(>r-
mining the character of organisms in their feeding grounds. Tetters-
son found that herring only enter the Baltic when the .salinity g<'ts
to a certain degree, whereas Galtsoff found that in America tli(> mi-
gration of mackerel is due not so much to salinity as to temperature.
Thus, different factors appear to influence different species in deter-
mining their movements.

Birds, becau.se of their ability to fly, are better aide to seek out :i
favorable place for abode than most animals. Many tlifferent reasons

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NATURAL HISTORY

have been given to account for the long-distance migrations of ducks,
geese, the Arctic tern, golden plover, and other remarkable feathered
travelers. Food cannot be the deciding factor, jjecause many birds
leave for the south while food is still abundant. Neither can tempera-
ture be the only cause, because a
majority of migrating birds go
south when the weather is still
warm, while robins and other
lairds often stay behind and win-
ter successfully in cold climates.
Humidity, atmospheric pressure,
winds, have all been considered
as playing a part in migration,
but it is more likely that some-
thing within the bird rather than
any external environmental factor
is the impelling cause for this
impressive phenomenon. For in-
stance, among the hormones pro-
duced by the ductless glands, are
sex hormones which may stimu-
late the bird to the extraordinary
activity that results in long mi-
gratory flights. How to account
for the direction and exactness of
these migratory flights is another
matter, even more difficult to
explain.

Changing climatic conditions
probably influence plants more
directly than animals, because the
latter are more capable of move-
ment, and, consequently, better able to escape from unfavorable
surroundings. Nevertheless, living things make up a world of
shifting organisms, always on the move.

Ways of Locomotion

Much of the delight that the naturalist experiences comes from
observing and interpreting the ways and devices by which the move-
ments of organisms are brought about.

WINTER HOME

The annual migration routes of the
Arctic tern. It covers about 22,000
miles in its yearly round trip from its
winter range in the Antarctic to the
summer breeding range in the Arctic.
Note the different routes taken going
and coming.

THE BTOLOGICAI. CONQUEST OF 'll||.; Woiuh

.'.'»

II riijhl I'll rci

The Russian thistle {Salsolu) introduced into this country in 1!571. Today it
covers the entire country. What adaptations have enabled this pesi to do tliis?

In the world of attached animals, like sea-anemones and corals, that
apparently are doomed to remain in one place, the free-swimming
larvae seize the opportunity to break away from the maternal apron-
strings before settling down for life, just as stationary plants by means
of spores, seeds, and chmbing or trailing vegetative parts are enabled
to shift about and occupy new territory. Seeds of orchids and certain
spores of fungi, mosses, and ferns, for example, are light as dust and
may be wafted hundreds of miles in the air before settling down to
germinate on some distant soil. Seeds of dandelions and other plants,
such as milkweed, willow, and cottonwood, have feathery paracluite-
like structures, which support them in the air for some time, e\-en in a
wind blowing only two miles an hour. Insects, ballooning spiders, and
birds make use of air currents, sometimes being carried long distances,
particularly by heavy winds. Whole plants, like the Russian lliistl(\
and the "resurrection plants" of desert regions, may dry uj) and
break loose from their anchoring roots, and roll along the ground or
ride the breeze scattering their seeds, thus taking root in newly invatknl
regions.

H. w. H. — 4

NATURAL HISTORY

Estimate ok Seeds Produced by a Single LARtiB WE>;n

Dandelion .
Cockle-bur .
Oxeye daisy
Prickly lettuce
Beggar's ticks
Ragweed

1,700

9,700

9,750

10.000

10,500

23,000

Crabgrass .
Russian thistle
Pigweed
Purslane (large)
Tumble mustard
Lamb's-quarters

89,600

150,000

305,000

1,250,000

1,500,000

1,600,000

Some fruits, like those of violets and the witch-hazel, explode, send-
ing their seeds to a distance. Even gravity may sometimes be re-
sponsible for spreading plants by means of soil-slides, while animals
in such accidentally disturbed soil may be carried considerable dis-
tances to a new situation.

Birds inadvertently scatter fruits and seeds by first swallowing
and then depositing them elsewhere with their droppings. As a
result cherry bushes and poison-ivy vines may often be seen growing
along fences where birds have roosted.

Adaptability to New Conditions

The fact that some organisms do not invariably adapt themselves
to new localities which they have invaded is a great deterrent to
their permanent spread. Successful invaders that gain a new foothold
as pioneers, and retain it as settlers, are conspicuous enough to be
discovered and remembered, but unsuccessful ones, reaching the
Promised Land but unable to establish themselves there, escape atten-
tion. Indian corn, for example, seems unable to reproduce and main-
tain itself if allowed to run wild. The yellow-fever mosquito has a
certain dead-line, north of which it cannot successfully continue to live.

Just as in economic life, so in communities of plants and animals,
undesirable individuals frequently appear, bumming their way into
places where they are not wanted. Weecis are notorious plant-
hoboes that are pre-eminently successful on their own part, but are
unwanted by man, and reckoned as outlaws with a bad reputation,
because they rob other plants which man favors, of food, moisture, and
sunlight. Having great natural vitality, they are successful because
they usually grow even in unfavorable conditions which would kill
competing plants, and produce enormous numbers of seed. Their
persistence and varied means of seed dispersal are easily realized by
anyone who has tried to pick "beggar's ticks," and "sticktights,"
and burrs from his clothes after a ramble in the autumn woods.

THE BlULOCilCAL CONQUEST OF THE WOULD n

Human Interference

Man is often the unwitting cau.se of sliifts, .sometimes with serious
results, of animal and ])lant ixjpulations. Tlie Russian thistle,
already mentioned, was introdueed into South Dakota in 1S74 with
flax-seed from Europe. By 1888, it was reported as a troublesome
weed in both the Dakotas. By 1898, it had covered all the area east
of the Rocky Mountains from the Gulf of Saskatchewan, and today
ranges over the whole country.

There are many curious cases of the accidental transport by human
agency of animals and plants to regions far from their point of origin.
Recently a tropical boa landed in Middletown. Connecticut, with a
bunch of bananas. Tropical tarantulas, too, are known to be carried
over long distances in the shipment of this fruit. Such instances as
these, however, usually have no lasting effect on the general spread
of organisms, yet they emphasize the fact that unanticipated develop-
ments in distribution are quite jiossible from very insignificant and
unsuspected beginnings. Man's interferences with the distribution
of organisms have by no means always been unfortunate or disastrous.
In many instances his rearrangements of plant and animal popula-
tions have been eminently successful. The planting of various
species of trout in new streams has proved to be a wise move, \\hile
the introduction of reindeer into Alaska and Labrador is of incal-
culable benefit to both man and beast. The list of cases where man
has lifted the lid of Pandora's box and set free plants and animals
for weal or woe into new localities could be extended indefinitely.

Life Zones

Reference has already been made to a zonal distribution of i)hints
and animals in a pond. A similar condition is easily seen in climbing
any high mountain. Life zones are often rather sharply marked, but
usually show transitional areas between them. A region which has
been carefully studied and which shows this zonal distribution in a
marked way is the San Francisco mountain region in north Arizona.
Here, a mountain nearly 13,000 feet in height rises out of a desert
plain. This mountain shows successively two tyj^es of desert zone,
a lower and upper, each with its own desert fauna and flora, cacti,
sagebrush, a few birds, mice, lizards, and snakes. Then a r(>gion at
between 6000 and 7000 feet of pinon pines and red cedars, inhabited
by more birds and a small number of mammals. Between 7000 and

NATURAL HISTORY

Zonal distribution of flora on a moun-
tain peak rising from a desert area.
How would you account for these differ-
ent life zones ?

8200 feet there are forests of Douglas and balsam fir, with such mam-
mals as meadow mice, chipmunks, deer, lynx, and puma. Higher still
between 8200 and 9500 feet, is a typical Canadian vegetation, timber

pine, Douglas and balsam fir,
and aspens, while the wood-
chuck, porcupine, rabbit, mar-
ten, fox, wolf, and other northern
forms are found. From 9500 to
11,500 feet we find a fauna and
flora almost like that of northern
Canada and called Hudsonian.
Stunted spruce and pine exist
up to the timber line with a
few typical mountain mammals
such as the marmot, and pika
or mountain hare. Above this
area lies the rocky Alpine zone,
snow-clad for half the year even
in this warm, sunny climate.
Lichens on the rocks and a few
stunted herbs are the only plant life visible, while a limited number
of insects and an occasional mammal from the Hudsonian zone are
the only signs of animal life.

The facts that the chorologist has discovered concerning life zones
have been put to practical use by the Biological Survey of the United
States Department of Agriculture. A life zone map has been pre-
pared so that the settler going into a new region will know at once
the kind of plants and animals best adapted to live there. In addi-
tion, information is available about the character of the soil, the
rainfall, temperature range, and the particular cereals, fruits, and
vegetables that can be grown in the region.

Life Realms

Different parts of the world, each with its several life zones, have
been separated into life regions, or realms. If we plot the distribu-
tion of a given family of animals or plants, we often find that species
within the group have a wide distribution, in some instances covering
more than a single continent. Australia has long been set aside as
a distinct realm because its peculiar fauna and flora differ from those
in other parts of the earth and so is called the Australian Realm.

THE BIOLOGICAL CONQUEST OF Till: WOULD

\:\

"L,_ Hblarctic p,.

3 /

^^^^^4i/^•

Ethiopian

Auslralia-ri ^:;»
"Rsalin

Map showing life realms.

Similarly there are the South American, or Xcotroi)i('al, Etliioi)iaii,
Oriental, and Holarctic realms, the latter comprising most of the
land surface of the Tropic of Cancer. Each of these regions has
animals and plants peculiar to itself, although resemblances are often
found between inhabitants in different realms.

SUGGESTED READINGS

Beebe, C. W., Hartley, G., Howes, P. G., Tropical Wild Life in Britif^h

Guiana, New York Zool. See, 1917. Ch. VI.

Contains an interesting description of a tropical rain-forest.
Borradaile, L. A., The Animal and Its Environment, O.xford University Press.

London, 1923. Chs. VII, VIII, X, XI, XIII.

Excellent for general reading.
Elton, C, Animal Ecology, The Macmillan Co., 1927. Chs. Ill, V, X.

Fascinating reading.
Jordan, D. S., Kellogg, V. L., and Heath, H., Animals, D. Applcton ct Co.,

1909. Chs. VII, XVI.

Old but reliable.
Pearse, A. S., A7iimal Ecology, McGraw-Hill Rook Co., 192G. Ch. IV.

Rather a book of reference than a reading book.
Roule, L., Fishes, Their Journeys and Migrations, W. ^^■. Norton & Co., 1933.

All of this book makes interesting reading.
Walter, H. E., Biology of the Vertebrates, The Macmillan Co., 192S. Cli. III.

Interesting and reliable.
Weaver, J. E., and Clements, F. E., Plant Ecology, McGraw-Hill Book Co.,

1929. Chs. IV, V, VII, XVIII.

Very' scientific and yet interesting.

II J

THE INTERDEPENDENCE OF LIVING THINGS —
THE WEB OF LIFE

Preview. Relations between members of the same species; care of
eggs by parents; care of young • Relations of mutual aid • Animal can-
nibalism • Relations of competition • Relation of members of different
species ■ Adaptations for food-getting in animals • Scavengers • Food-
getting in plants ; carnivorous plants • Symbiosis • Commensalism • Par-
asitism • The chemical relationship of plants and animals • Life habits of
bacteria • Relation of bacteria to free nitrogen • Rotation of crops • The
relations between insects and flowers • Suggested readings.

PREVIEW

Those who have been fortunate enough to be in California or Flor-
ida when the oranges are in bloom will never forget their odor ; nor
will they, when examining the grove, fail to notice the large number
of bees vi-siting the flowers. The bees are after nectar and pollen,
yet without these winged agents, the crop of oranges for the follow-
ing year would probably be small. This interrelationship between
insects and flowers was noticed by Charles Darwin, who pointed out
that the size of the clover crop in England depended upon the num-
ber of cats in a given region. His friend Huxley, who knew better
than Darwin how to popularize science, immediately went him one
better and added that the size of the clover crop depended upon the
number of old maids. When asked to explain, he gave this logical se-
quence of events. Old maids keep cats ; cats prey upon field mice ;
mice provide nesting places for bumblebees ; bumblebees pollinate
clover, upon which pollination the next year's crop depends. So he
had a perfectly logical chain of events. Throughout nature there is
this give and take between different organisms which we call the web of
life. When man interrupts or displaces a link in the chain of interre-
lationships, the web is broken and the whole fauna or flora of a region
may be changed, as in the case of the Englishman who took a bit of
water cress to Australia, planting some in a nearby stream to remind
him of home. This foreign plant, having no enemies and finding
conditions favorable for its growth, literally overran the waterways
until today the rivers of Australia are choked with water cress. Look-

THE INTERDEPENDENCE OF LIVING THINGS i:,

ing over the world of plants and animals an unescajDabie dcixMidenro
of one form of life upon another is found in the food relationship
by which green plants supply animals with food and in the shelter
relationship, by which animals find safety in the protection given
by plants. Reducing this search for food and shelter to its ultimate,
we find that all animals are dependent upon green plants.

But does the green plant get anything from the animal ? At first
sight it would seem as though it were all give and no take. As we
study the situation more closely, however, we find that food-making
is dependent upon certain raw materials, some of which, such as
nitrogenous wastes, can only be supplied from the dead bodies of
organisms or their excreta. Moreover, another important raw
material, carbon dioxide, used by green plants in starch-making, is
given off as a respiratory by-product by animals, and in this same
process oxygen is released.

All of these facts suggest certain problems. Why, for example,
when some animals produce enormous numbers of eggs and others
only a few, do not the former outnumber the latter? Of what
significance is the mutual aid so frequently observed in nature?
What is symbiosis and why is it significant? What is the \'alue of
pollination by insects as compared with pollination by other means?
What part do bacteria play in the fives of plants and animals?
What is the reason for parasitism ? Can the oft-repeated statement
that green plants make food for the world be proved ? A start on the
answers to some of these questions will be made in the pages that follow.

Relations between Members of the Same Species

Many examples of helpful relationships can be .seen between ani-
mals of the same species, especially in the care of young. Although
in low forms, such as sponges, coelenterat(>s. echinoderms, and a good
many fishes, large numbers of eggs are laid and given little or no
parental care, the production by the male of immense numbers of
sperm cells in the vicinity of the eggs insures chance fertilization and
continuity of the species. For example, Norman ' reports that a cod
w^hich weighed 21^ pounds produced over 6,650,000 eggs. At tiie
time of egg laying each male of the above .species throws billions of
sperm cells into the water near the eggs. Higher in the animal scale
we find greater provision for care of the young correlatcnl with a re-
duction in the number of eggs laid. Many insects lay their eggs on

1 Norman, J. R., A History of Fishes. Stokes, 1931.

NATURAL HISTORY

Bruinu II

Ichneumon fly {Ophion macnirum)
laying eggs in the cocoon of a Cecropia
moth.

plants which will become food for the larvae or caterpillars. Others
lay their eggs either in the ground where they are protected, or in

dead bodies of animals on which
the larvae may feed, as in the case
of certain beetles, or in a ball of
dung, as in the case of the dung
beetle. Certain ichneumon flies
bore deep into tree trunks in order
to lay their eggs in the larvae of
wood-boring insects. Some w^asps
paralyze caterpillars or spiders,
laying eggs in the still living victim
so that when the eggs hatch the
young larvae will have food. In
many animals, food is provided in
the yolk of the egg, the eggs of
fish and birds being examples.
Spiders and earthworms form
cocoons, which in the case of the
earthworm are usually filled with a nutritive fluid on which the young
feed after they are hatched, while in the cocoon of the spider the
young feed upon each other, the strongest of the group surviving.

Care of Eggs by Parents

Some of us as youngsters have angled for sunfish and will always
remember the thrill that came when a brightly colored male dashed
at the bait dangled over the hollowed nest containing eggs which he
was guarding. From the simple nest of sunfish and salmon through
the more complicated nests of the stickleback or lake catfish we come
to the more elaborate nesting habits of birds. Some birds, as terns,
sandpipers, or gulls, simply make shallow holes in the sand, as does
the sand ostrich. Grebes and rails make nests of floating decaying
vegetation. Nuthatches and woodpeckers make nests in holes in
trees where the young are protected. At the top of the ladder are
more elaborate nests such as those of the oriole and oven-bird of our
latitude or the tailor bird and weaver bird of the tropics.

Care of Young

Sir Arthur Newsholme has said that the most dangerous work in
the world is that of being a baby. If the young of plants and ani-

THE INTKHDEI'llNDENCE OF MV|\(; ril|\(;s

A. 1'. .sV((/i CtinserrniUin iJcpl

Stickleback and nest. Of what advantage would this be to the species?

mals survive this dangerous stage, their chances of growing to adults
are very considerable. Although parental care is not associated
with plants, nevertheless in low forms of plant lif(> locomotor stages
occur, called zoospores or swarm spores, by means of which the plants
gain footholds in new areas. Many devices have already been men-
tioned by means of which seeds are scattered far from the parent
plant. In higher plants, hard shells, spiny coverings, or inedible pulp
protect seeds within the mature fruit, thus giving greater ojjpor-
tunity for the scattering and germination of seeds.

Adaptations for the protection of young are more evident among
animals. In crustaceans, the larvae of which form the chief food
for great numbers of fish, there are not a few protective adaptations.
In some instances crustaceans have brood pouches in which the young
are kept, or, as in the case of crayfish and lobster, the developing
eggs are cemented to the abdominal appendages of the mother and
carried around by her. The male bullhead .swims arountl with and
broods over his young, while the male sea horse has a brood pouch in
which the young are held. In some worms and crustaceans, the eggs
may be retained in the burrow of the parent, or they may be held
in the mantle cavity or a space similar to it, as in the fresh-water
mussels, barnacles, and tunicates. Some spiders, notably the wolf
spiders, carry the egg cocoon about with them and when the yoimg
are hatched, they are carried on the backs and legs of the female

NATURAL HISTORY

Huijh Spiricer

A spider with its egg cocoon.

until large enough to care for themselves. The male of the so-called

midwife toad (Alytes) carries the eggs entangled around the legs.

The male Surinam toad places the eggs on the back of the female,

where each sinks into a tiny pouch as it develops.

Animals that lay eggs which hatch outside of the mother's body

are said to be oviparous. A modified form of this procedure is seen

in some nematodes, arthropods,
fish, amphibia, and reptiles. Here
the eggs remain in the oviduct or
uterus of the mother until they
are almost ready to hatch, the
body of the mother acting as an
incubator. Such forms are said
to be ovoviparous. Most of the
mammals which retain the eggs in
the body until the young are born
are said to be viviparous. Here
the young are held as embryos
within the body of the mother

and nourished by means of an organ called the placenta. The young

of mammals are suckled at the breasts of the mother until they

are able to eat solid food.

Relations of Mutual Aid

A certain amount of protection is afforded plants from their habit
of living in communities. Examples are the aggregations of cacti in
our western deserts or the acacia and "thorn bush" communities of
Australia. The animal world, too, shows many examples of protec-
tion among gregarious forms. The schooling of fishes not only is a
defense for the group from larger fish, but it also enables small fish,
working concertedly, to prey on organisms much larger than them-
selves. The driver ants in Africa, traveling in great swarms, often
overcome and devour animals hundreds of times larger than them-
selves. Wolves hunt in packs, several of them rushing together to
bring down their larger prey. Deer and other herbivorous animals
move in herds for mutual protection.

Another relation of mutual aid results from the development of
division of labor among certain animals. Although social division
of labor is well seen in the human species, there are many examples
in the insect world, particularly among the social bees and wasps,

THE INTERDEPENDENCE OF LIV1\(, Tl||\f;s

such as tho division of the colony into castes thai include nuih-s
(drones), fertile females ((lueens), and infertile females (workers).
Castes are even more mmierous among ants, there being winged and
wingless females, intermediates between females and workers, soldiers,
several groups of workers, and winged and wingless males. Not all of
these forms, however, are found in any one species. By means of such
division of labor, life in the colony goes on at a very efficient level.

Animal Cannibalism

Most of us have had the experience of having some pet destroy
her young when they were in danger, or of having laboratory-bred
rats or mice eat their newborn young. This is probably a perverted
instinct, but nevertheless animal cannibalism is .seen rather fre-
quently. The destruction of a wounded member of a pack of wolves
when hunting is usual. The female spider usually kills the male
after fertilization of the eggs, this habit being common to some
other forms. Similarly the eggs may be destroyed by the male,
as in the case of the mole cricket and centipede, w^hich eat the eggs
shortly after they are laid, the mothers resorting to numerous pro-
tective devices in order to thwart the cannibalistic fathers. Many
fish eat the eggs of their own species. Even the domestic hen at
times will eat her own eggs.

Relations of Competition

Evidences of competition in the plant world are numerous. Be-
cause of their sessile habit, older plants may overshadow and crowd
out the young ones, or one group of plants may prevent the growth
of other plants in the vicinity. Weeds and plants in general pro-
duce enormous quantities of seed, which are kept from germinat-
ing by the rapid growth of the older plants. Many grasses and some
shrubs grow rapidly by means of underground shoots, in this way
securing territory which might be used by other plants. Thus plants
with favorable adaptations may completely pre-empt new territory for
themselves at the expense of others le.ss able to use the environment.

In animals, competition between individuals of a species is almost
universal. Males fight each other for the possession of females, or
sometimes just for the sake of fighting. There is a contituial struggle
for food, for water, and for a place to live. I>arger animals, as we
have seen, prey on smaller ones and in general those best fitted to
compete in the battle of \Uo, survive.

NATURAL HISTORY

Wright Pierce

This desert weed, rabbit brush (Chrysothamnus nauseosus) has pre-empted newly
cleared areas along the border of the Mohave Desert. How would you account
for its rapid spread ?

Relation of Members of Dififerent Species

No one who has carefully watched the life that goes on in a grove
or forest can escape seeing there the enactment of a drama that repre-
sents the larger picture of relationships between living things the
world over. Insects are flying through the air, crawling along the
ground, or burrowing into decaying logs and the ground. Spiders
and ground beetles may occasionally be observed making off with a
victim, while here and there birds such as woodpeckers, flycatchers,
and warblers may be seen feeding on adult insects or their larvae,
while a hawk may be watching to pounce upon some one of the
insect-eating birds. If we were able to make a prolonged study of
the area we would find that squirrels, rabbits, and wood mice are food
for larger flesh-eating animals or carnivores, such as foxes. In such
an area we might also find a series of herbivorous animals ranging
from plant lice (aphids) living on the leaves of trees to occasional deer
which browse on the leaves of the same plants.

zSn

plants rrxike
the fooct fbr-
tha- worloC

THE INTERDEPENDENCE OF L1VT.N(; TIIIN(JS :,i

It will be noted in the illustrations given that animals almost
mvanably feed upon others smaller than themselves. The same
relationship is seen in lakes or oceans where microscopic plants and
animals (plankton) form the food of other larger organisms, especially
fish. These living things form
definite "food chains" in
which larger animals feed on
smaller and smaller ones until
ultimately the lowest forms
subsist on tiny green plants
or bacteria. For example, in
a small pond we may find
billions of diatoms, unicellular
algae, and protozoa and feed-
ing on them millions of small
crustaceans. With them are
thousands of insect larvae,
hundreds of small fish, and a
few large fish, such as bass,
pickerel, or perch, which are
dependent upon all the other
forms of life. In this case a
few large animals are depend-
ent for food upon the development of myriads of smaller organisms,
the basis of this food being very simple plants. Take away any link
in the food chain and life in the pond becomes disorganized, with the
ensuing death of many of the inhabitants.

Since smaller animals reproduce more rapidly than larger ones,
the food supply for those "on the top of the heap" remains fairly
constant. It should be borne in mind, however, that the larger
animals require a range of sufficient size to support them.

Adaptations for Food-getting in Animals

Protozoans, if ameboid, engulf their food, but in other members of
this group, food passes into the cell through a definite opening or
through the plasma membrane. Sponges and many molluscs pick
up microscopic food as it comes to them in water currents. Some
molluscs bore holes through the hard shells of bivalves, in that way
securing the soft parts of the animal for food. Insects have biting,
chewing, or sucking mouthparts, each type being fitted to utilize a

drassViopptrs-
° Gat gi-ass

\\ hy will a break in the food chain often
cause disorganization of life in that locality ?

NATURAL HISTORY

Wright Pierce

Adaptations of beaks of birds for
food -getting.

different kind of food. Carnivorous
mammals have sharp teeth fitted for
tearing and holding prey ; herbivorous
mammals have flat, corrugated teeth ;
rodents, gnawing or chisel-like teeth ;
while snakes, which swallow their prey
whole, have pointed, needlelike teeth
to hold their food securely. More
striking adaptations for food-getting
are found in birds whose beaks and feet
both give clues to their food habits.
The flesh-eating birds have hooked
beaks and curved claws ; aquatic
birds have feet shaped like paddles
and scooplike bills for straining out
small organisms from the water ; wad-
ing birds display a remarkable variety
of highly specialized beaks and feet ;
and the smaller land birds show
equally interesting adaptations for se-
curing food. Bizarre adaptations for
procuring food characterize the giraffe,
with its long neck that enables it to
reach up to feed on branches of trees
fifteen feet from the ground, the ant-
eater, with its sticky tongue, and the
walrus, which digs bivalves with its
tusks.

Scavengers

Some forms of life are not only om-
nivorous in their diet, but are actually
scavengers, living on dead organic ma-
terials. The bacteria,^ smallest of all
plants, feed upon or destroy millions
of tons of organic wastes which other-
wise would make life on earth impossi-
ble. Think of a world without decay.
Land and water would soon become

' See pages 165-166.

THi: INTEHDKPKNDKNCE OF I.IVINC T|||\,;s ;,.•,

roverod witli the dead bodi(>s of plants and animals. TUr i)acteriu of
decay are very numerous in rich, damp soils containing large amounts
of organic material. They decompose organic materials, changing
them to compounds that can be absorbed by plants to be used ii,
building protoplasm. Without decay life would be impossible. f„r
green plants would otherwise be unable to get the raw food materials
to make food and living matter.

In general all plants, both colorless and green, may be said to play
a part in ridding the earth of organic wastes. The fungi, or colorless
plants, get their nourishment from the dead bodies of plants and
animals, while the green plants take organic wastes from the soil
to be used in the manufacture of foods.

Many animals also take part in scavenging. Some of the food of the
protozoa is made up of decaying unicellular material and the bacteria
which cause its decay. Certain forms, especially insects, feed upon and
lay their eggs in decaying flesh, while myriads of insects and their
larvae help to break down decaying wood in a forest. These are
only a few instances of this important function.

Food-getting in Plants

Although green plants make foods and use raw food materials ' from
their environment to do this, there are some that destroy foods.
Fungi, such as bacteria, molds, smuts, and rusts, ruin billions of dollars'
worth of food plants and plant jiroducts each year. This is seen in
damage to crops, fruits, stored foods, and animals used as food by
man.

Carnivorous Plants

A curious exception to ordinary green plant nutrition exists in
carnivorous plants, which also illustrates a different interrelationshij)
between plants and animals. Carnivorous plants add to their nitro-
gen requirement in several ways. The fresh-water aquatic plants
known as bladderworts {Utricidaria) catch water fleius and other
small crustaceans in hving bladderlike traps. Just what lure urges
the crustaceans to destruction is hard to say, but the fact that they
are caught in numbers is verified by their decomposed remains found
in the bladders. Other animal-eating forms are the various pitch(>r
plants (Sarracema sp.), some of which are found in our northern
swamps. Insects are apparently lured to the urn-shaped leave^'

> See pages 253-262.

NATURAL HISTORY

by a trail of sweet nectar secreted just outside the mouth of the
pitcher. Once inside, a shppery surface and incurving hairs prevent
egress, and the insect is soon digested by the enzymes in the fluid
contained inside the pitcher. Still another leaf modification with a
similar function is seen in the sundew (Drosera sp.). Here the leaves
are covered on one surface by sticky glandular hairs, which close

The leaf of a bladder wort {Uiricu-
laria vulgaris). Many of its numerous
divisions bear bladders (6), especially
near the place of attachment to the
main leaf axis (a). Note the aper-
tures of the bladders (p) into which
small aquatic animals may crawl or
swim.

The modified leaf of a sundew {Drosera
rotund if olia) showing the conspicuous
glandular hairs (g) covering the upper
surface, the hairs at the right having
caught an insect. Note that the hairs
are tipped by a drop of secreted liquid
{d), which attracts insects to the leaf and
also entangles them. — After Kerner.

over the insect, hold it fast, and ultimately digest it and absorb its
juices. In the Venus's-flytrap (Dionaea sp.), another carnivorous
plant found in some parts of this country, the leaves have two sensi-
tive lobes provided with marginal hairs. If an insect lights on a
leaf, the two lobes close over it and the insect is trapped. After its
prey is digested, the lobes of the leaf open up and the plant is ready
for action again.

Symbiosis

The process of living together for mutual advantage is called
symbiosis. Plants may join forces as may animals, or in some
instances, plants with animals. Lichens, for example, illustrate this

THE INTERDEPENDENCE OF LIVI\(. TIIIN(;S

W I III III I'll rci

An encnistiiif^ licht-n. Why docs lliis pl;inl suc-
ceed in such an unfavorable cm ironiucul i'

mutual partnership in

an interesting way. A

lichen is composed of

two kinds of plants, a

green alga and a fungus,

one of which at least

may live alone. The

two plants form a part-
nership for life, the alga

making the food and

nourishing the fungus,

while the latter gives the

alga raw food materials,

protects it, and keeps

it from dying when the

humidity of the air is low.

Other examples are bac-
teria and the mycelial

filaments of fungi {my-

corhiza) which live sym-

biotically on the roots of certain plants, taking food from the plants,

but giving them nitrogen in a usable form in return.

A common example of symbiosis between plants
and animals is the green Hydra {Chlorohydra viri-
dissima), which holds in its body wall a unicellular
alga known as Zoochlorella. These plants contain
chlorophyll, using the sun to make food. In this
partnership, the algae get carbon dioxide and ni-
trogenous wastes from the animal, to which, in
turn, they give food and the oxygen set free in the
process of starch-making. There are numerous
examples of this kind of symbiosis in tlio animal
world, as is seen in many of the protozoa, sjionges,
A root' tip of the coelenterates, flatworms, molluscs, and sea urchins.

European beech xhe symbiotic relationship of animals to each

{Fagus sylvalica), ^^^ -^ ^j^^^^.^^ y ^j^^ ^j, protozoans iixing in

showing ectotrophic '^ „ . i •. *

mycorhiza, the fun- the digestive tracts of termites or white ants.

gal hyphae forming These Httlc animals act as digestive cells for the
:nS:the?tLtLt termites, making it possible for them to use
-After Frank. wood fibers on which they live. In return th(>

H. W. H. — 5

NATURAL HISTORY

a shark-

s-cccker-

protozoans receive food and are protected by their hosts. A some-
what similar situation prevails in the large intestine of man, where
certain types of useful bacteria are found. These forms help keep
down putrefying bacteria, receiving in return a home, food, and
a favorable temperature in which to live. Certain species of ants
protect and feed aphids, in turn feeding upon the sweet fluid secreted
by the aphid.

Commensalism

Some associations are not obviously to the advantage of either
organism, the two feeding together as messmates. Animals like

the small crabs that live
in the water canals of
certain sponges, or the
tiny fishes that live in
the lower part of the
body of a "trepang," a
sea cucumber, are ex-
amples. The young of
some species of rudder-
fish (Stromateidae) ac-
The shark sucker (Remora brachypiera, Lowe) company large jellyfish,
showing sucking disk and its method of attach- geekina; shelter under
ment to the shark. The Remora gets free trans- ,, ■ +• • j. 4. i

portation and makes sudden forays after food as their stmgmg tentacles
well as sharing the "left-overs" of the shark's food, when chased by larger
But it seems doubtful if the shark gains anything ggj^ while another fish
from the association. . ^ .

{Nomeus) lives in con-
stant association with the beautiful coelenterate known as the
Portuguese man-of-war.

Parasitism

Not all life is give and take. Some plants and animals live at the
expense of others, giving nothing and taking all. These are known
as parasites, the organism which entertains them being called the
host. From the lowest to the highest forms in the plant and animal
kingdom there are few which are not attacked by parasites at some
stage of their existence.

Parasitism implies plenty of food, shelter, and a relatively protected
life for the parasite, but it also usually spells degradation in structure
and loss of activity. It may mean only inconvenience, but more
Ukely a shorter and disturbed life for the host, especially if the parasite

THE INTERDEPENDENCE OF LIVING THINGS

causes disease. In some instances, the complicated life history is so
bound up with more than one host that if one of the hosts is absent
a hnk in the chain of life is broken, the life cycle cannot be completed'
and the parasite dies. The black-stem grain rust, which ref,uires

recC spore
blov/n to
anotber^ stem

recC or
Sxtmreierc
rixst on
•wheat stem

barlserrv rtcst
spore in?ectintf
ths cells of -^heat
stem ira
spring-

"bocrbei^ry
leaves

mfscts stem

through

breo-^hing"

?«""«. red rust syjrecuil^ ,
from stem to stem/
cCixriijg' Sixmme'P

blaclc or
^vinter rust
lives on

straw thrcuflh ,
winter- * "

infection form',
barberry rust
onborberrjlea.^
a cluster cup'

The life history of black stem grain rust.

controlled.

r, ■!, black spora

inject ing^
bocfics, sporicCa

a spoT~id.ium

infects the

Cells of a

barberry leaf

Explain how this rust may he

both the barberry plant and the wheat to complete its life history ;
the pine tree blister, which lives on the currant or gooseberry at one
stage of its life history, and on the pine at another ; and the parasite
causing malaria, which requires both the anopheline mosquito and the
blood of man to complete its cycle, are examples.

The Chemical Relationship of Plants and Animals

The study of plant and animal ecology may be said to be analogous
to the study of human economics. Social conditions among men,
animals, and plants are all determined by the environmental factors
present, but chiefly by the availability and abundance of food. The
world's food supply in the long run depends upon the chemical ele-
ments making up the environment and energy derived from the sun.

NATURAL HISTORY

Plants and animals are made out of the same chemical elements.
Burn some beans or a piece of beefsteak, a piece of wood or a bit of
living bone, an entire green plant or a dead mouse, and the chemist
would tell us that the same chemical elements are present in animals
and plants ; that certain of these elements passed off in the smoke,
others into the air as colorless gases, leaving still others as a
whitish ash. All living things are composed mainly of carbon,
oxygen, hydrogen, nitrogen, with about twelve other chemical ele-
ments found in very minute quantities. These elements are all
present in the immediate environment of plants and animals, air,
water, and soil.

How they get from the basic environment into living things can
be briefly stated. Carbon, which is contained in all organic foods
and in this condition is taken into the animal body, can only be
absorbed in the form of carbon dioxide by food-making green
plants. This gas, which is present in the atmosphere to the average
amount of about 0.03 per cent, gets there as a result of oxidative
processes taking place in plants and animals, as well as by the com-
bustion of organic substances. Factories and volcanoes alike form
their quota of carbon dioxide to diffuse out into the atmosphere.
The cycle of the passage of carbon from plants to animals and from
animals back to plants is shown in the accompanying figure.

Hydrogen, another component part
of living things, cannot be used in
its pure state by either plants or
animals. In water (H2O) , it becomes
an important part of the food of
animals, and as water vapor it is
used in starch-making by green
plants.

Oxygen is freely available to both

plants and animals. As a gas, making

up over 20 per cent of the air, capable

of being dissolved in water for aquatic

plants and animals, it is used by all

living things in respiration. Green

plants add this gas to the air during the process of starch-making.

Nitrogen is one of the most important elements found in living

things. Making up 79 per cent of the air, it is not usable in the

form of a gas except by the nitrogen-fixing bacteria.

The carbon and oxygen cycles in a
balanced aquarium. Trace the pas-
sage of an atom of carbon from a
green plant back to the plant.

THE INTERDEPENDENCE OK LIVING THINGS

:><>

The other mmeral components of living matter, of wliich sulpliur
phosphorus, calcium, potassium, and iron are among tlie most impor-
tant, are all found either in water, soil, or both. How the plant makes
use of them and turns them over for the use of animals is an interesting
story to be told later. But enough has been said to show that foods
made by the green plants form the supply on which all animals live.

carbohydrates^,, .^^^^ carbon iiox.ae^^ /'0>^yg«?'v

proteins^ X -^-ureot /^ >\ n'til«ts

slltsZI^ AnimaU (Gr^enPlantC r^f"'

>.^ter- \^ y-sctlts V y^-^^'■-1^<.^

The food relationships between green plants and animals.

Life Habits of Bacteria

In this web we call life, bacteria play a most important part. Since
bacteria contain no chlorophyll, they are unable to make carbo-
hydrate food, and must obtain their foods from decaying organic
matter. In order to absorb such food it must be made soluble so
that it will pass into their bodies. This they do by digesting food
substances by means of enzymes ^ which they secrete. Bacteria
that grow or thrive in the presence of oxygen are called aerobic, while
those which live without free oxygen are called anaerobic. The latter
need oxygen, like other living things, obtaining it by breaking down
the foods on which they live, and utilizing oxygen freed in this process.

Relation of Bacteria to Free Nitrogen

It has been known since the time of the Romans that the growth of
clover, peas, beans, and other legumes causes soil to become more
favorable for the growth of other plants, but the reason for this was
not discovered until modern times. On the roots of the plants
mentioned are found little nodules, or tubercles, in each of which
are millions of nitrogen-fixing bacteria {Rhizobium leguminosarum) ,
that take nitrogen gas from the air between the soil particles and
build it into nitrites which arc tlu>n converted by otluM- bacteria
(Nitrobacter) into nitrates. In this form it can be used by plants.
Nitrogen-fixing bacteria live in a symbiotic relationship with the
plants on which they form tubercles, their hosts pro\iding them with
organic food.

J See pages 127-128.

NATURAL HISTORY

fjring'
JoactcHee

Bacteria also act upon ammonia formed from plant and animal
wastes, one kind (Nitrosomonas) producing nitrites, or nitrate salts,
and others (Nitrobader) converting the nitrites into the more stable
nitrates. Thus all of the compounds of nitrogen are used over and
over, first by plants, then as food by animals, eventually returning to
the soil, or in part being released as free nitrogen. This process is

called the nitrogen cycle.
Although free nitrogen is
fixed for use by means of
electrical discharges dur-
ing thunderstorms, by
man-made machines, by
ultraviolet light (which
is estimated to return
100,000,000 tons a year
to the earth's surface),
and from other sources,
yet these means give an
almost negligible amount
of usable nitrogen to the
soil, compared with what
is used in crop produc-
tion, especially since so much nitrogen is lost from the soil in various
ways. The nitrogen-fixing bacteria supply the deficiency, thus form-
ing one of the most important inter-relationships between plants and
animals because of their direct relationship to the production of the
food of the world.

Rotation of Crops

Plants that are hosts for the nitrogen-fixing bacteria are raised
early in the season, then plowed under and a second crop of a differ-
ent kind is planted. The latter grows quickly and luxuriantly be-
cause of the nitrates left in the soil by the bacteria which lived with
the first crop. For this reason, clover is often grown on land used
later for corn, or cowpeas will be followed by a crop of potatoes.
On well-managed farms, different crops are planted in succession
in a given field in different years so that one crop may replace some
of the elements taken from the soil by the previous crop. This is
known as rotation of crops. ^

' Crop rotation is not only a process to conserve the fertility of the soil, but also a sanitary meas-
ure to prevent infection of the soil.

The nitrogen cycle. What additions could
made to this diagram?

THE INTERDEPENDENCE OF LIVINt; Tll|\(;s

<)l

Wriilhl I'Urcc

Tiger Swallow-tail (Papilio turnns) on rose.

The Relations between Insects and Flowers

One of the most interesting symbiotic relationships is that which

exists between msects and flowers. Flowering plants produce .seed.s

and fruits, and from

these come new gen-
erations of plants,

but if it were not for

the visits of insects,

many plants would

not produce seeds.

Insects visit flowers

in order to obtain

nectar, a sugary sub-
stance formed by the

nectar glands, and

pollen. The glands

which produce the

nectar are usually so

placed that an insect

has to push its way past the stamens and pistil of the flower in order

to reach the desired food. In
doing this, pollen grains may
adhere to the hairy covering of
the insect and be transferred to
the sticky surface of the upper
end of the pistil (stigma). Inside
the pollen grains are the male re-
productive cells (sperms), while
in the ovary of the pistil are
held the female reproductive cells
(eggs). In order to ha\e develop-
ment of a new plant, it is essen-
tial for a sperm cell to unite with
an egg cell. Pollen grains on the
stigma are stimulated to send out
hairlike tubes, wiiich j)enetrate
the stalk (style) of the j)istil and
eventually reach the ovary. The
pollen tube carries one or more

•derminatino:

anther*

•filameriti

ovulell''

. .-finicropyle-'

A longitudinal section of the repro-
ductive organs of a flower showing the
penetration of a pollen tube through
the opening in the pistil called the
micropyle, and the growth of the pollen
tube to the ovule.

62 NATURAL HISTORY

sperm cells, which are thus enabled to unite, each with a single egg
cell, in the ovule of the pistil. The union of the sperm nucleus with
its egg nucleus is called fertilization. As a result of this process the
fertilized egg develops into an embryo or young plant which is held
in the seed. When favorable conditions arise, this embryo m.ay
develop into a plant.

Bees are the chief pollinizing agents, although butterflies, moths,
flies, and a few other insects perform this service as well. Hum-
mingbirds often pollinate tubular flowers, while other small birds,
snails, and even bats are agents in the pollination of certain forms.
Man and animals may accidentally pollinate flowers in brushing past
them through the fields. The value of cross-pollination is obvious
and is an example of the close weaving of life in which man, animals,
and plants are all inescapably entangled.

SUGGESTED READINGS

Borradaile, L. A., The Animal and Its Environment, Oxford University Press,

London, 1923. Chs. IV, V, XIV.

Excellent for reference.
Elton, C, Animal Ecology, The Macmillan Co., 1935. Chs. V, VI, VIII.

Particularly valuable on the animal community and the relationship of

animals to a food supply.
Needham, J. C, and Lloyd, J. T., Life of Inland Waters, 2nd ed., Charles C.

Thomas, 1930. Ch. V.

Interrelationships among fresh-water organisms.
Pearse, A. S., Animal Ecologij, McGraw-Hill Book Co., 1926. Chs. VIII, X.

A wealth of material on interrelationships.
Rau, Phil., Jungle Bees and Was-ps of Barro Colorado Island, privately printed,

Kirkwood, St. Louis, 1933.

An ecological study of a tropical environment.
Wallace, A. R., The Geographical Distribution of Animals, 1876. Books I

and II. Ch. IV, especially.

This book forms the basis for most of the modern work in distribution.

All of Part III, Books I and II, is extremely interesting.
Weaver, J. E., and Clements, F. E., Plant Ecology, McGraw-Hill Book Co.,

1935. Ch. XVI.

An interesting chapter on relations between plants and animals, with

especial emphasis on insect pollination.
Wells, H. G., Huxley, J. S., and Wells, C. P., The Science of Life, Doubleday,

Doran & Co., 1931. Book 6, Chs. IV and V.

A fascinating book for general reading.

ROLL CALL

Preview. Earh^ contributions to classification • Binomial nomencla-
ture ■ Law of priority • What is a species? • A classification of plants and
animals • Classification of the plant kingdom • Classification of the animal
kingdom ■ Glossary of terms occurring in the Roll Call.

PREVIEW

It is hoped that this section will be freely used by the student.
It is not expected that the classification of plants and animals will be
learned by rote, but rather used for reference from time to time as
new forms are seen. By this means the diagnostic characteristics
of different phyla and classes will gradually be learned as needed, and
the relationship of one group to another become more apparent.

In order to enjoy hikes or longer trips, the student should be able
to recognize the larger groups of the plant and animal kingdoms.
Fortunately there are museums, botanical gardens, and zoological
parks to which one may refer, all the more intelligently of course if he
has himself first discovered living animals and plants.

Identifying plants and animals correctly becomes more of a plea.s-
ure than a task, if the principles of scientific, as well as common,
nomenclature are understood. Both scientific and common names
will be encountered. The former are written in the dead, unchanging
Latin language, and are of more universal usefulness, since the latter
are frequently misleading and confu.sing, as more than one common
name may be applied in different countries, or in different parts of the
same country, to a single plant or animal. For example, the common
"chain pickerel" is listed under the scientific name of Esox, indicating
the larger or generic group to which the fish belongs, and niger, which
is its specific name, but it has at least twenty-two connnon names in
different parts of this country. Here are a few of them: black
pickerel, pike, common eastern pickerel, duck-bill pickerel, green p'lkv,
little pickerel, and lake pickerel. The terms pike, pickerel, aii.l lake
pickerel are also quite commonly used in some parts of the country
to designate another fish, the great northern pike, Esox lucius. In
still other localities "pike" refers to an entirely different group, the
pike-perches, belonging to the genus Stizostedion. This examph^ will

64 NATURAL HISTORY

serve to indicate the necessity for the use of Latin scientific names in
classification. There may be other members of the genus Esox, but
there is only one niger, although varieties of the same are possible in
different environments. The terms of genus and species were intro-
duced to the scientific world in the middle of the 18th century by
Carl von Linn^ (1707-1777), of Sweden.

The study of classification is called Taxonomy and is subdivided
into zoological taxonomy, or Systematic Zoology, and botanical taxon-
omy, or Systematic Botany.

Early Contributions to Classification

In order to secure an idea of the development of taxonomy it is
necessary to go back several hundred years to some of the earlier
biologists and glance at a few of the contributions of these students.
Obviously such an excursion can hope to touch upon only a few of
the more important workers. Logically, one should go all the way
back to Aristotle's time, but lack of space forbids such an interesting
excursion. Consequently we must confine ourselves to the immediate
forerunners of Linne, or Linnaeus as he came to be called, who intro-
duced the concept of binomial nomenclature and with it a more ade-
quate idea of genus and species.

In 1576, Matthias de TObel published an important work on plants.
This was an attempt to arrange plants according to their structure.
He took the shape of the leaf as the basis for this classification, and it
led him to put such things as ferns in the same group with trees because
the fronds of the fern bore a superficial resemblance to the needles
of the hemlock. Another botanist was the Swiss, Kasper Bauhin
(1560-1624), who described in order 6000 species of plants, beginning
with the ones he considered most primitive. He approached the
concept of genus and species, because he grouped together plants
which resembled one another externally.

John Ray (1627-1705) deserves recognition along with Linnaeus
as the founder of the science of systematic biology. This enthusiast
published a catalogue of British plants in 1670 and later works (1703)
in which he introduced and explained the groups of Monocotyledons
and Dicotyledons. He also made less extensive contributions to the
classification of animals. Some of these he published with his good
friend Willughby (1635-1672). Ray gave evidence in his work that
he realized the fundamental differences between genus and species ;
furthermore, he had the keenness to group together both related plants

ROLL CALL 63

and animals. Ray also advanced the idea that fossils are extinct
species.

Linnaeus was born in 1707, the son of a Swedish clergyman. Ho
would have been destined to become a cobbler had it not been for the
influence of a physician who recognizcnl the lad's abilities. To make a
long story short, he finally secured his medical degree, aided in no
small amount by the contributions of his fiancee, and eventually
became a professor of natural history at Upsala. It seems that Lin-
naeus had a passion for natural history and for classifying everything
which came to hand. He initiated several changes in the study of
systematic biology, many of which are still in use today.

Binomial Nomenclature

The most important contributions of Linnaeus center about (1) brief,
clear, and concise diagnoses ; (2) sharper divisions between groups ;
and (3) a definite, clear-cut system of scientific terminology, known
as hinomial nomenclature. These innovations appeared in the 1753
edition of Species Plantarum and the 1758, or tenth, edition of his great
work, the Systema Naturae. The tenth edition of this latter work is
taken as the starting point of zoological nomenclature. Linnaeus
divided the plant and animal kingdoms into Classes, Orders, Genera,
and Species. This was a great step over the use of popular common
descriptive terms, as you can now appreciate if you refer back to the
example of the pickerel. However, a big mistake made by Linnaeus
was his concept of fixity of species.

In 1898 the International Congress of Zoology appointed an inter-
national commission which drew up a set of rules ajjplying to the
divisions of the animal kingdom. Thus classification today is really
an expansion of the Linnaean system which now includes in the case
of the animal kingdom, for example, the following :

Animal Kingdom — is made up of

Phyla — each of which is composed of
Classes — in turn made up of
Orders — then

Families — and finally
Genera — and
Species.

In the plant kingdom a comparable arrangement is utilized, beginning
with Divisions (= phyla).

66 NATURAL HISTORY

Law of Priority

In describing species it sometimes happened that more than one
person described the same form, giving it different names. In such
cases the name assigned by the one who first described it is used, the
second being considered a synonym. This is the reason for writing
the describer's name and date of pubHcation after the specific name.
Ordinarily the date and frequently the describer's name is omitted.
Thus the true daisy is properly Bellis perennis, Linn. 1758, or the
English sparrow, Passer domesticus, Linn.

What Is a Species?

We have taken a glimpse at the contributions of some of the con-
temporaries and near contemporaries of Linnaeus and have gained a
sHght concept of the problems these early workers faced in defining
and describing a species. Biological scientists of today are still
working on this problem. The principle involved is readily under-
stood when we look at a sheep, a cat, and a dog. One can easily sepa-
rate them from each other, various cats being put in one group and
diverse dogs in another. All domestic cats, whether they be the alley
variety or pet Persians, and all dogs, whether they be a "dog in the
manger" or "man's best friend," fall into well-marked and easily
separable groups, known as species. To continue further, one finds
in looking over representative mammals that many other species such
as the jaguars, ocelots, jaguarundis, and cougars, all have certain char-
acteristics in common with our domestic cats. These characteristics
are size, build, shape of head, nature of claws, teeth, and fur. The
zoological systematist, therefore, places them in one larger group or
genus which is called Felis, a relationship expressed below.

Kingdom. Animal . Plarzb

'Phylum - Onovdcdxx . Arthropod a. 'Koiltx£r<i.a.. eh=>.
Gla55 - "rlocmiTacxIioD , Pisces .Pept/ilia, Aves.cstc-.
Order"- Carpi vonx . 'R'ocCe-aticc.Chiroptera,ete-
Kimil/- PeUcLcce^ . <Zo.n idoe ."LCr^icCoca.etc.

Genzxs . " feli^ . Lumhricus, eto.

-SpecieS- domesticcc.leo.tigTjs.eu^.

Tndi vidical . lorn , ,Dick .Harry, etc.

ROLL CALL 57

However, species have otlicr characteristics besides oxtornal or
morphological similarities. They breed true, that is, cats produce
cats, and dogs produce dogs. Usually diflferent species cannot be
crossed. There are exceptions, for sometimes one species crossed with
another may yield a sterile hybrid. Thus a horse crossed with an ass
produces a mule. But on the whole the preceding statement holds
true.

Two criteria have been used in classifying organisms, first, struc-
tural differences, or appearance, which really means comparative mor-
phology, checked physiologically and genetically by the cross-breeding
of species, and second, the approach through a study of the early
development and the life cycle, emhnjology, and the distribution
of the organism, ecology. The latter leads to a consideration of
varieties, subspecies, and races, which through mtergradations often
complicate the problem of determining species.

Such a study may be made either more complicated or facilitated
according to whether a so-called natural classification or artificial
classification is utilized. Thus bats and birds might be artificially
classified together, simply because they both fly, just as whales and
fishes are placed together by the ignorant, because both inhabit
the water. A careful study of the anatomy and development of these
animals would indicate that if one is trying to show relationships,
which is what a classification should do, bats and whales would both
have to be put in the mammalian group.

Determination of the type of symmetry present is useful in clas-
sification. Some organisms possess a universal symmetry, as the
protozoan Volvox. In such cases the organism is divided into equal
halves by any plane that passes through the center. Starfish and
hydra, on the other hand, are well-known examples of radial sytn-
metry. In such forms there is a single axis, as may be seen in a
cylinder, and a number of planes through such an axis would di\-id(^
the organism into symmetrical halves. Most of the more highly
developed forms possess bilateral symmetry, which is characterized
by similar halves on either side of a main axis. Other secondarv'
planes occur in bilaterally symmetrical animals, resulting in anterior-
posterior, and in dorso-ventral differentiation. Sometimes segmenta-
tion, or metamerism, is apparent, as in the case of the earthworm and
many of the Arthropods.

If one attempts a classification that is based primarily upon struc-
ture, it is necessary to differentiate between homology and analogy.

68 NATURAL HISTORY

The former refers to similarity of structure and the latter to similarity
of function. Thus the f orelimbs of a bat, bird, cat, and turtle are ail
homologous, while the wings of a bat or a bird are analogous to the
wings of a butterfly, but they are not homologous since they differ in
structure.

A Classification of Plants and Animals

As stated earlier, the appended scheme of classification is simply a
tool to be used by the student. Remember that a scheme of classi-
fication is not only the "who's who" of the plant and animal world
but it shows relationships as well, indicating what we know at present
in this field. Classification involves a knowledge of the occurrence,
distribution, development, and structure of the form studied, and
so is much more than simply applying a scientific name to an ani-
mal or plant. The use of scientific names cannot readily be avoided,
as will be realized from a study of these pages. If one really desires
to excel in biological work, he must set out cheerfully and with
determination to acquire an understanding of the use of these tools as
an indispensable aid to a comprehension of the interrelationship of
organisms to one another.

In the first place it is hoped that diagrams which accompany this
classification are detailed enough to give the student some concept of
the more common or important kinds of representative organisms
occurring in each group. It is unfortunately impossible in these
drawings to represent the different animals according to scale. The
student hardly needs, however, to be reminded that whales and
protozoans should be interpreted as decidedly different in size. In
most cases, the classification will be carried only as far as the class,
although in a few groups, as with the Arthropoda and Tetrapoda,
it is necessary to go to the orders. In some instances attempts have
been made to simplify the classification in order to avoid unnecessary
scientific terminology. It should be added that the classification
here presented is only one of many that may be encountered in various
books, differing in details but agreeing in essential particulars.

It is impossible to designate readily all of the characteristics which
are utilized in the separation of the larger plant and animal groups.
However, it is of importance to know (1) whether we are dealing with
a one- or many-celled form (uni- or multi-cellular) ; (2) the number
of germ layers present in the organism, diplohlastic — • two (ecto-
derm and endoderm) ; triplohlastic — three (ectoderm, endoderm, and

ROLL CALL

mesoderm) ; (3) the nature of the body — usually divisible into tubes
within tubes or sacs ; (4) the symmetry — radial or bilateral ; (5) the
nature of the appendages — if present, whether jointed or non-jointed,
paired or unpaired ; (6) whether the organism is segmented or non-
segmented ; (7) which organ systems or organs are present, in what
form they occur and how they function ; (8) type of skeleton, ab-
sent, exo- or endoskeleton ; (9) the presence or absence of a noto-
chord ; (10) the presence or absence of special organs ; (11) the type
of tissues present, as bark, phloem, muscular, or circulatory.

Inasmuch as some of these and other terms appearing in the scheme
of classification are new, a short glossary is included. This is designed
to elucidate terms used in the appended classification. Other words
are defined as they are first used in the text and may be found by
reference to the index.

CLASSIFICATION OF THE PLANT KINGDOM

(mainly after Sinnott)

All members of the plant kingdom are characteristically ses.sile ; typically
possess chlorophyll; usually take food in inorganic form; cell walls of cellulose
or hydrocarbon.

DIVISION I — THALLOPHYTA — Thallus Plants (algae,' fungi, bacteria).
Chabacteristics : Small, often minute, little differentiated plants some-
times possessing chlorophyll; se.x organs, when present, typically one
celled ; spore-bearing organs are single celled ; 80,000 species.

Subdivision A — Algae — Composed mostly of blue-green, green, brown, or red

algae.
Characteristics : Chlorophyll frequently associated with other pigments ;
manufactures own food.

Class I — Cyanophyceae — Blue-green algae {Gloeocapsa, Nostoc, Oscilla-
tor ia).
Characteristics : Simplest and lowliest of green plants ; body consists of a
single cell with nucleus ; sap cavity and chloroplasts absent ; often tend-
ing to adhere in colonies; usually in threadlike rows ( filaments); cyto-
plasm homogeneous, pigment evenly dispersed, or a colored outer and a
colorless inner zone may be distinguishable ; blue-green color probably due
to chlorophyll mixed with blue pigment (phycocyanin) .

Class II - Chlorophyceae - Green algae (Cartena, Ulva, Ulothrix, Oedogo-
nium, Vaucheria, Spirogyra).

1 Genera in boldface type indicates that the form is illustrated in this unit.

NATURAL HISTORY

Class I

Cyanophyacaofi/

blua-ghsen olgcus

5UBOIV(5ION A

ALGAE

l.OsciUatoi-ioc

1. d ^-^^ — ^v.

LaTY^inarig Txjccus wiopfccryx

Class 12"

Vhaeaphyceoiz

brov/n algae

Class 3Zr

Kicxtomoceae/

cCicctoms

S.FragillariCL

2'Pe.nticula

Cl?ciropby<2<2a.<s

sborje/Nx/'ocr-t/S

CYiar-a

Staphylo Coitus

Olcxss "SC

"RbocCop^^yeeoa

THALLOPMYTA

ovass I.
bojcte-r-ict

Class is:
'Phycomycetes

ol^-like fitnga.

astreptococcTxs 4.E»cu:ilIuS 5.Spirillurai.$cipi'o1egmgt^^i:ggi3^

Class IT
SacAhca-omyceLejs

yeast/S

*spor<3

z^;'

//^y

2. Cbmatr ic"ha
l.Kemitrichia s.Trichompboro

Class HE.
slime "Yucn^i

, 2,

-1. .,
£>coa5Cir^

Class "C.
Ascomycetes

gac ■{ ^UT7^i

wheat

2
smuts ^"puff balls

SUBDIVISION B
FU-NGI

3ctsicCiomycetes
smutts ondrtxstls

ROLL CALL 71

Chakacteristics : Chlorophyll associated with carotin and xantlKjphyll;
marine or fresh water organisms, or inhabitants of moist hind; nucleus
and one or more chloroplasts present; starch synthesizeil in pi/rentmls;
plant composed of single cells, colony, filament, or plate of cells; most
species produce motile i-eproductive cells (zoospores) ; botli equal {iso-) and
different sized (hetero-) gametes present.

Class III — Charophyceae — Stonewarts {Chara and Nitella).

Characteristics: Vegetative body consisting of long, jointed stems with
whorls of short branches arising at joints {nodes) ; asexual spores absent ;
more complicated antheridia and oogonia than found in Thallophytes borne
along branches.

Class IV — Phaeophyceae — Brown algae, kelps, rockweeds, sargassum {Lami-
naria, Fucus, Ulopteryx).
Characteristics: Multicellulate; exclusively marine ; brown color (due to
one or more brown pigments associated with chlorophyll) ; normally found
in intertidal zone.

Class V — Diatomaceae — Diatoms (Meridion, Diatoma, Denticula Fragillaria).
Characteristics : Large group of unicellular algae ; related in color to
brown algae ; common as plankton organisms in both fresh and salt water ;
siliceous walls.

Class VI — Rhodophyceae — Red algae {Nemalion, Polysiphonia, Phyllophora,
Corallopsis) .
Characteristics : Mostly marine ; characteristically reddish in color ;
branched, vegetative body filamentous and delicate; grow entirely sub-
mersed; cell wall often thick, gelatinous; color due to pigment, phyco-
erythrin ; no motile cells ; sexual reproduction highly specialized.

Subdivision B — Fungi — Fungi, bacteria, and molds.

Characteristics : Chlorophyll lacking; exist as parasites or saprophj-tes.

Class I — Schizomycetes — Bacteria (Diplococcus, Staphylococcus, Streptococ-
cus, Bacillus, Bacterium, Spirillum).
Characteristics : Unicellular plants, usually without pigment, dividing in
one, two, or three planes; apparently structureless, but probably con-
taining a diffuse nucleus.

Class II — Saccharomycetes — Yeasts (Saccharoimjces).

Characteristics: Sometimes regarded as reduced Ascomycetes; single
cells with definite nucleus ; cytoplasm and sap cavity ; buds a.sexua!ly ;
under unfavorable conditions forms four spores, in a modified ascus.
Class III — Myxomycetes — Slime fungi, slime molds (Hemitrichia, Coma-
tricha, Trichamphora) .
Characteristics: Border-Hne plants; spores borne by fruiting bodies,
germinating into small, naked mass of protoplasm without a wall ; indi-
vidual cells fuse, forming a Plasmodium.
Class IV — Phycomycetes — Algalike fungi, molds, and bliglits {Saprolegnia,
Mucor).

H. w. h. — 6

NATURAL HISTORY

Hcpa'ticoce

1 L vei^-NVortS

BRYOPHyfA

, iverwor ts , mosses

r^^^-Sporxs capsule

CD 6'phcc^nixxn
peoct moss

•e^C

Ccctbcarinia

acomrrion moss

arche^nium Qnt"hei4diuTn
of corartiorL moss

Olo-SS IC

ROLL CALL

Characteristics : Resemble algae ; plant body consists of filaments {hyphae)
which are not divided into cells by cross walls ; multinucleate.

Class V — Ascomycetes — Sac fungi {Morchella, Exoascus, Microsphaera).
Characteristics: Includes over 20,000 species, mostly saprophytes or
parasites; body consists of branching mycdium throughout substratum
and a definite fruiting body at surface ; produce spore sacs {asci) contain-
ing eight spores (ascospores) ; group of asci embedded in sterile hyphae
may or may not be surrounded by protective envelope.

Class VI — Basidiomycetes — Basidia fungi, smuts and rusts, wheat rust
(Puccinia), puff balls.
Characteristics: Large and varied group; specialized reproductive struc-
ture (basidium) is swollen terminal cell of hypha, in mushrooms the ija-
sidium usually bears four basidios pores, each carried on a delicate stalk
{sterigma) ; sexual reproduction rare; lichens — composite plants in which
algal cells are entangled in mycelium. Usually regarded as a parasitism
of algal member rather than an example of symbiosis.

DIVISION II — BRYOPHYTA — Liverworts and Mosses.

Characteristics : Alternation of generations in which sexual (gametophyfic)
stage dominates; asexual (sporophyiic) stage typically parasitic upon
the gametophyte ; archegonium and nmlticcUulate antheridium pre.sent ;
gametophyte contains x number of chromosomes while the 2 x number
occurs in the sporophyte; careful study of archegonium reveals typical
flask shape, with sterile cells (neck and venter) surrounding the egg and
associated cells ; antheridium more or less stalked and consisting of layer
of jacket cells surrounding cuboidal sperm mother cells.

Class I — Hepaticae — Liverworts (Marchantia, Riccia).

Characteristics: Intermediate between green algae and higher plants;
thaUus flattened and attached to soil by rhizoids; growth by repeated
division of single large apical cell.

Class II — Musci — Mosses (Sphagnum, Polytrichum, Catherinia).

Characteristics: In every habitat except salt water; very common in
alpine and arctic regions; gametophyte erect, consisting of stalk with
spirally arranged leaves ; attachment by rhizoids.

NATURAL HISTORY

Subdivision A
pi"! mi live,
vasculctr plants

WTJVjynia
Devonian plant ^

RsilophyCalcs ^-"^
-.i <'..^ .(2)p5notum

(3)Tmesipteris

$UBDIVI5I0M B
Lycopsicta

club mosses

sporopVr/te y,-

Lycopodium.

:5UBD I VISION C
ophejiopsido.

TI?ACH£OPHYTA

vascular' plants

Subdivision!)
Pberopsida

ferns . seed plants,

polkw

<j'gametopViyt<2

tipof

pollen

tube

^''\ cells '

txtbe

@,^/C<3ll

^mtoikr

>>^ cell i

eicxssi

Filicineae

arc

Gymr205perma<2

dicotyledon ii20i20Ccit/kfGn

AndioSpermae

oclVc, iTioLple . elm , ccc .

ROLL CALL

<■>

DIVISION III - TRACHEOPHYTA - Vascular Plants.

Chakacteristics : Fibro- vascular system for transportati..,i „f raw mate-
rials up and food down; separation of specialized cl.l..njphyll-lK.ari..K
tissue ; adaptation to absorption of water from soil.

Subdivision A — Primitive Vascular Plants — {Psilotum and Tmesipteris, Rhynia).
Characteristics : Fossil primitive vascular plants giving rise in tliree lines
to Lycopsida, Sphenopsida, and Pteropsida.

Subdivision B — Lycopsida — Club mosses, ground pines (Lycopodium, Selagi-
nella).

Characteristics: Stem clothed with small, numerous, spirally arranged
leaves; sporangia borne on upper surface of spowphyll; latter usually
grouped into terminal cones.

Subdivision C — Sphenopsida — Horsetails (Equisetum).

Characteristics : Hollow, typically jointed stems, bearing small leaves at
joints (nodes); stems ribbed; diaphragms often across stem at nodes;
sporangia borne in groups on stalked shield-shaped structures forming
terminal cones ; ribs opposite fibro-vascular bundles which are associated
with small air-filled canal; abundant in Paleozoic age; now only about
35 species.

Subdivision D — Pteropsida — Ferns and seed-bearing plants.

Characteristics: Typically large leaves; sporophytic generation domi-
nates ; sporangia relatively large.

Class I — Filicineae — Ferns.

Characteristics : Small, herbaceous plants with typical pinnately com-
pound leaves (fronds) ; stem relatively weak and inconspicuous ; roots
numerous but do not form an extensive system; small sporangia borne
on lower surface of leaf in groups usually protected by membrane (iiulu-
simu) ; spore germinates, forming small, thin gametophyte (prothallus),
which in turn bears antheridial and archegonial structures. About 15,000
species, some of which reach a height of 30 feet. From forms like the
ferns evolved the higher vascular plants whic-h dominate the earth's
surface today.

Class II — Gymnosperil\e — Evergreens, pines, hemlocks, spruces, junipers.
Characteristics: Seeds freely exposed to air; usually nondeciduous
types; megaspore retained within megasporangium where it germinates
producing female gametophyte; integument, a new structure, enclo.ses a
sporangium and embryo sac; reduced male gametophyte transferred
directly to vicinity of female ; male obtains access to female gametophyte
by new structure (pollen tube); young sporophyte develops in contact
with and at expense of parental sporophyte; gametophyte with haploid
(x) number of chromosomes entirely parasitic upon sporophyte. Mem-
bers of this group are phylogenetically ancient; only about 450 living
species.

Class III — Angiospermae — Deciduous trees and plants. Dicotyledons, oak,
maple, beech ; Monocotyledons, corn.

NATURAL HISTORY

Class I

SarcocCina

(2)
ATCella

C3) "'-J
"Radiolarla

Clctss IE
Kastigbphorec

PROTOZOA

one cellecC animals

c'C®) \^

•«oY^

^^-^ ctsexuctl \^^9©
Cycle in. /

mosquito

Bisali-

PlasmocCium.

Class JSL

5porq3oa

Vorticella

(2) ^^

Stentor^

(3)

5tyionych i cc

Class IST

ly^ftcsoricc

ROLL CALL

Characteristics: Seeds enclosed by a case (ovary), so that pollen Rrain
does not reach the ovule but rests on surface of carpel ; closure to form
case probably arose by folding together of edges of megasporophyll {carpd) ;
pollen received on special organ (stigma) at tip of ovary. Members of
this group probably were derived from gymnosperm stock ; now number
135,000 species and are subdivided into dicotyledons and monocotyledons
which may be separated by the following ciiaracteristics :

Dicotyledons

Monocotyledons

Number of cotyledons
of embryo . . .

two

one

Vascular bundles . .

arrange to form vas-cylinder
enclosing pith

scattered

Leaves

open venation, veinlets end-
ing freely in margin, which
is often toothed or lobed

closed venation (i.e. parallel)
margin therefore entire

Flowers

in sets of four or five

in sets of three

CLASSIFICATION OF THE ANIMAL KINGDOM

(mainly after Hegner)

All members of the animal kingdom are characteristically free-moving organ-
isms; generally capable of assimilating organic foods; rarely possessing chloro-
phyll ; cell membranes composed of protoplasm or proteins.

PHYLUM I — PROTOZOA — One-celled animals.

Characteristics : Single cells or colonies of loosely aggregated unspecial-
ized cells ; rarely differentiated into germ cells ; 8500 species.

Class I — Sarcodixa — Naked protozoa (Ameba,^ Arcella, Radiolaria).
Characteristics : Locomotion by means of pseudopodia.

Class II — Mastigophora — Flagellate protozoa (Euglena, Trypanosoma).
Characteristics : Locomotion by means of flagella.

Class III — Sporozoa — Parasitic protozoa (Lankesieria, Myxosporidia, Plas-
modium).
Characteristics : Xo organs of locomotion in adults ; endo-parasites repnn
ducing by schizogony and spore formation.

Class IV — Infusoria — Ciliate protozoa (Vorticella, Stentor, Stylorjychia,
Paramecium).
Characteristics : Locomotion by means of cilia.

1 See footnote at beginning of classification of Plant Kingiioni.

NATURAL HISTORY

Class T

Calcarea

KexactiY^ell ioCa

Grantia

(1^
Euplectella

PORIFERA

sponges

Spongilla

fresh- wcxter-Spon^e

ELc5pong"ia

ROLL GALL 79

PHYLUM II — PORIFERA — Sponges.

Characteristics : Usually considered as diploblastic animals ; body con-
sists of a perforated (inhalent pores) cylinder, leading to central canal
opening to outside through exhalent pore; [peculiar flagellate, collared
cells (choanocyfes) typically present; body structure frequently compli-
cated by budding ; 2500 species.

Class I — Calcarea — (Grantia).

Characteristics : Small marine sponges possessing one-, two-, or four-rayed
calcareous spicules.

Class II — Hexactinellida — Deep-sea sponges (Euplectella).
Characteristics : Sponges with six-rayed siliceous spicules.

Class III — Desmospongia — Finger sponge, bath sponge (Chalina, Spongilla,
Euspongia) .
Characteristics: Diverse groups of sponges possessing spicules of silicon,
not six-rayed, with spongin, or a combination of spicules and spongin.

NATURAL HISTORY

Olass I

H/cCro3oa;

(1^
Otoe-lia

C3)

PhyBcclicc
Portuguese mar?- of -^/ar

COELENTERATA

(IV

Aurelia

Class IE

5c/pbo3)Oa

Secc anerrzor^e

Astra n^ioc

Class HE
Antbo^oa

ROLL CALL

PHYLUM III — COELENTERATA — Jellyfishes and corals.

Characteristics: Mostly marine; radially syinniotrioal ; diploblastic- ani-
mals with a noncellular layer of niesoglea lying between; po.ssL's.sing
tentacles, armed with nematocysts; body composed of a single gastro-
vascular cavity ; 4500 species.

Class I — Hydrozoa — Fresh-water polyps, jellyfishes, and a few stony corals
{Hydra, Obelia, Physalia).
Characteristics : Mostly marine ; usually hydroid and jellyfish forms
occur in the same life cycle; the jellyfish (medusae) po.ssess a shelflike
velum extending inward from the margin toward the mouth (manubrium) ;
a few species like Hydra possess no medusoid stage; the stony coral,
Millepora, represents a colony with a coral-like skeleton of calcium car-
bonate.

Cl.\ss II — Scyphozoa — (Amelia).

Characteristics: Entirely marine, with the medusoid stage dominating;
produced from subordinate polyp by terminal budding (strobilalion) ;
velum usually absent; lobate, typically eight-notched.

Class III — Anthozoa — Sea-anemones, sea-pens, and stony corals (Metridium,
Pennatula, Astrangia, Sagartia).
Characteristics : Entirely marine with medusoid stage suppres.sed ; organ-
isms characterized by an introverted ectodermal mouth (sto7nodaeum) anti
vertical radiating mesenteries extending inward from the body wall; one,
two, or more rarely three cihated gullet grooves (siphonoglijphs) carry
a stream of oxygenated water to interior. Corals produce islands and
reefs; in addition they sometimes protect a shore from wave action.

NATUIIAL HISTORY

CTENOPHORA

ytonna iphorroc

Comb i<2-^Vy "'^i^^'S^^w^'i^pM

C2)

Venas* gxindLie

ROLL C^LL 8:{

PHYLUM IV — CTEXOPHORA — Sea-walnuts {Cestus, Hormiphora, Mnemi-
opsis) .
Characteristics : Eight radially arranged rows of comhjlike plates typi-
cally present; fundamentally bilaterally syninictrical; with a distinct
mesodermal layer (therefore triploblastic); no nematocysts : 100 species.

NATURAL HISTORY

Class I
Tarbellaria

#'G>'

■■^

'Planoria ^' Microsbmum

Clccss IT
TrematooCa

twoflUKZf

Yrom,
Turtles
mouth

'PnGUtTiono®<ie5
frog lun^ fluke/'

PLATYnaKINTHES

(a^Taa.nioe

Cb J^s bicerC'Lcs

uterus

J?^^

:••• Cv55

ovary- ^telloricx.
2(aO proglottv

Class HI

CsstodUx

Diphyllobothrium

brocccC tapeworm

of mctn_

cionorc^is
liver fluke
of -man.

•prohoscis..

naphridia

lo«^. naPVB J

ovary -

hrodn
Jnoufh.

Olsons

ofcx

inemertine

Class ISL

ROLL CALL ^^.

PHYLUM V — PLATYHELMLXTHES — Flatworms.

Characteristics : Dorso-ventrally flattened, soft bodies, bilaterally sym-
metrical, animals lacking true segmentation and blood vascular system;
no anus; excretory system of flame-cell type; only Class I free-living, all
others parasitic ; 4600 species.

Class I — Turbellaria — Free-living flatworms {Planaria, Microstomum,
Bdelloura).

Characteristics: Typically free-living, possessing a ciliated ectoderm;
some ectodermal cells secrete mucus, or produce rodlike bodies (rhnMite.s) ;
classification into orders depends upon nature of intestine.

Cl.\ss II — Trematoda — Flukes (Polystoma, Pneumonoeces, Clonorchis).

Characteristics: Parasitic flatworms with non-ciliated ectoderm in the
adult, possessing one or more suckers; highly specialized for parasitic
existence; many are internal parasites having complicated life cycle,
occupying as many as four hosts during development ; digestive system
present.

Class III — Cestoda — Tapeworms {Taenia, Diphyllobothrium).

Characteristics : Members of this group are completely parasitic, living
as adults in the alimentary canal of vertebrates; digestive tract absent;
body typically divided into a chain of segments (proglottids), except for
Cestodaria, budded from neck, gradually increasing in diameter towards
posterior end; the head (scolex) typically bearing organs of adhesion in
the form of hooks and suckers.

Class IV — Nemertinea — Nemertines (Micrura, Cephalothrix, Cerebralidus).
Characteristics : Members of this gnnij) because of uncertain systematic
position not always placed with the flatworms ; characteristically found in
moist earth or fresh water, most forms being marine; characterized by
possessing alimentary canal with mouth and anus, definite blood-va.scular
system, and a long proboscis enclosed in a proboscis sheath.

NATURAL HISTORY

CAccss I

KematooCa

^^^ , ■ ' ^" (2) (5) ^

Trichi^Gllo: spiralis Trichuris ovis 'NecatDr onnericaTiiK

■pork TDundvorra NK-'J^ip '•v^orm yiooy<:\i/or-m

NmAtnELMINTHES

•rotxncC-wox^ms

;^'<^'t,C:iife^^i^^^^'-

ClotSS IE

Gordiacsa

Leptorty-nchoicLes £hecatus

Clots s HE
Aj:iar2thoc<2.pbala

ROLL CALL 37

PHYLUM VI — NEMATHELMINTHES — Roundworms.

Characteristics: Bilaterally symmetrical ; cylindrical, unsegmented, long
and slender worms ; usually a distinct alimentary canal with mouth and
anus ; primitive body cavity present ; papillae or spines at anterior tip of
body.

Class I — Nematoda — Threadworms (TrichineUa, Trichuris, Necator, Oxyuris).
Characteristics : Members of this group art; l)oth free-living and parasitic
on plants and animals; mouth usually terminal and alimentary canal
composes a relatively straight tube with anal opening near posterior end
of body ; body cavity not lined by epithelium but bounded directly by
muscles of the body ; four thickenings of the ectoderm, one dorsal, one
ventral, and two lateral, produce ridges containing excretory canals and
nervous system ; sexes separate.

Class II — Gordiacea — Hairworms {Gordius, Paragordius).

Characteristics : Long, slender, and hairlike ; free-living adults in water ;
larvae usually parasitize aquatic insect larvae (often Mayflies) ; asually
reach a second host, as beetle or grasshopper, in which development con-
tinues ; escape to water made by breaking through body wall ; no lateral
lines present; body cavity hned by distinct peritoneal epithelium derived
from mesoderm ; eggs discharged into body cavity instead of to outside.

Class III — Acanthocephala — Spiny-headed worms (Leptorhynchoides, Neo-
echinorhynchus, Macracanthorhynchus) .
Characteristics: Protrusible proboscis armed with hooks; alimentary
canal absent; reproductive sy.stem complex; entirely parasitic, larval
stage in Arthropods.

h. w. h. — 7

NATURAL HISTUllY

TROCHEUMINTHES

(1^

'Philodina
^iKSisa. animalcule

Clccss I

li^otifera

(1)
ChoQXandtus

OlccSS 31

Gastrotricboc

ROLL CALL

PHYLUM VII — TROCHELMINTHES — Rotifers, Gastrotricha.

Characteristics : Small, frequently microscopic, identifiable by cilia around
the mouth region ; about 1300 species.

Class I — Rotifera — Wheel animalcules (Philodina, Notommata, Trocho-
sphaera).
Characteristics : Mostly free-living, inhabiting fresh water ; distinct nerv-
ous system ; universally characterized by presence of jaws inside pharynx
(mastax) ; usually a foot.

Class II — Gastrotricha — (Chaetonotus).

Char.acteristics : Microscopic organisms reaching maximum length of
about 0.5 mm. ; animal divided into indistinct head, neck, and body ; oral
bristles on side of head ; often a forked tail containing cement glands ;
locomotion by ciliary bands or by long bristles.

NATURAL HISTORY

Class I
Br/ojoa

'Pec:*tir?atella
fresh -water hr/oysan

M0LLU5CpiDEA

moss aniYnals ana. lamp svjetis

ejdsrnol viev

(1) lyTageWania

Class IE

BrachiopocCa

Phoronie

Class IL

Phor-onidea

ROLL CALL

PHYLUM VIII — MOLLUSCOIDEA — Moss animals and lamp-shells.

Characteristics : Unsegmented, sessile, typicull}' marine, bilaterally S3'm-
metrical animals possessing a ridge (lophophore) bearing ciliated tentacles
which surrounds the mouth ; 5700 species, including fossils.

Class I — Bryozoa — Moss animals (Electro, Pectinatella, Iletniseptella, Bugula,
Plumatella).
Characteristics: Colonial, sessile, free-living animals; mostly marine;
lophophore usually horseshoe-shaped ; alimentary canal L'-shaped ; divi-
sion into subclasses depends upon whether anus opens within or without
lophophore.

Class II — Brachiopoda — Lamp-shells (Magellania).

Characteristics : Marine organisms possessing characteristic lophophore ;
body covered by calcareous, dorso-ventrally arranged bivalve shell, usu-
ally attached by a stalk (peduncle).

Class III — Phoronidea — (Phoronis).

Characteristics : Small, marine, sedentary animals living in tubes ; unseg-
mented adults are hermaphroditic, possessing a body cavity as well as
characteristic horseshoe-shaped lophophore; two excretory organs and a
vascular system.

NATURAL HISTORY

Class T

Arc>2ia]f7r?elicCa

Polygordlius

cMass "K
Cl2aetopocCa

Nsreis Chaetoptert£5

c\ccro..^»/'or-m. tube vorm. (^3)

ANNELIDA

segmentecC "^orms

KirucCo
•■medicinal leech

Class HL

Hirudinaa

Lambricud

Garthvorm 1

fhaecolosoTQa

arrow vorm

Class sr

Csrephyrea

Cla&S"y

Chaetognatha

ROLL CALL y ,

PHYLUM I X — ANNELIDA — Segmented worms.

Characteristics : Segmented animals bearing distinct head, digestive tube,
coelom, and sometimes nonjointed appendages; frequently supplied with
chitinous bristles (setae) ; 6500 species.

Class I — Archiannelida — (Polygordius).

Characteristics : Marine worms lacking setae or parapodia ; trochophore
larvae present.

Class II — Chaetopoda — Clam worms, tube worms, earthworms (Nereis,
Glycera, Chaetopierus, Lumbricus).
Characteristics : Members of this class marine, terrestrial, or fresh water ;
paired setae characteristically arranged in integumentary pits or upon
parapodia ; further subdivision based upon number of setae present :
Oligochaeta, a few ; Polychaeta, many.

Class III — Hirudinea — Leeches (Hirudo, Glossiphonia).

Characteristics : Hermaphroditic, dorso-ventrally flattened annelids with
32 body segments, two suckers, one surrounding mouth, the other the
posterior end ; setae and parapodia absent ; growth of mesenchyniatous
cells reduces coelom.

Class IV — Gephyrea — Sipunculid worms (Phascolosoma).

Characteristics : Non-segmented when adult, without setae or parapodia ;
characterized by a large coelom and trochophore larvae.

Class V — Chaetognatha — Arrow worms (Sagitta).

Characteristics: Small, transparent, marine invertebrates with well-
developed body cavity, alimentary canal, nervous system, two eyes;
lobes on sides of mouth armed with bristles which aid in capturing food.

NATURAL HISTORY

Class I

Asteroidea

Clccss X
Ophijiroidea

OphioglypVja
brittle -star-

ECHINODERMATA

starfishes, etc

m'w

■'-i'lyfi"'-:.-

K'#^- Her)

Arbcxoicx

sect urcHin

i\»

Thj/one.
sea: - cucuiTzber

EcVjinarachniuS
sccncC dCollocr

Class is:

Holothuroidea

Class IE

Echirzoidea

l^er^tacrmus

Class ^

Crinoidea

ROLL CALL 95

PHYLUM X — ECHINODERMATA — Starfishes, sea-urchins, sea-curumbors.

Characteristics : Adults radially symmetrical (pentamerous) ; marine ;

tube-feet, water vascular system, distinct alimentary canal, large body

cavity usually present ; frequently a spiny skeleton of calcareous plates ;

larvae bilaterally symmetrical ; 4800 species.

Class I — Asteroidea — Starfishes (Asterias, Mediaster).

Characteristics : Typically five rays or arms not marked off from central
disk ; each ray possessing ventral ambulacral groove through which numer-
ous tube-feet extend ; gastric pouches and hepatic caeca extend into rays ;
blunt spines and pedicellariae present; respiration by dermal branchiae.

Class II — Ophiuroidea — Brittle-stars (Ophiopholis, Ophiothrix, Ophioglypha,
Ophioderma).
Characteristics: Typically pentamerous with arms sharply marked off
from disk ; no ambulacral groove ; hepatic caeca and anal opening lacking.

Class III — Echinoidea — Sea-urchins, sand-dollars, spatangoids (Arbacia,
Strong ylocentrotus, Echinarachnius, Spatangus, Moira).
Characteristics : Typically pentamerous without arms or free rays ; test
of calcareous plates bears movable spines; i)ediceilariae usually three-
jawed ; mouth with five conspicuous teeth constituting part of Aristotle's
lantern.

Class IV — Holothuroidea — Sea-cucumbers {Holothuria. Thyone, Leptosy-
napta).
Characteristics : Long, ovoid, soft-bodied cchinoderms ; tentacles about
mouth; body wall muscular ; skeleton greatly reduced.

Class V — Crinoidea — Sea-lilies or feather-stars {Antcdon, Halhromelra, Co-
rnadinia, Pentacrinus).
Characteristics : Usually five branched arms, possessing featherlike divi-
sions (pinnules) ; aboral pole sometimes possessing cirri but more gener-
ally a stalk for temporary or permanent attachment ; a few modern types,
most forms known as fossils.

NATURAL HISTORY

Class I

AiTiphineura

Class I
GastropocCa

Class HE

ScaphopocCa

1 5cb r2och itoio.
chiton

Helix
Iccnd. snccil

marine

Snail

C5).

Limccx

tootlri snocil

MOLLUSC A

clams , Snccils, etc

soctllop

Class 3Sr

PslecypocCa

ra^or-shell Cicom

(2)

OCtopLCS

Class ^

Cephalopoda

ROLL CXLL 97

PHYLUM XI — MOLLUSCA — Snails, clams, and oysters.

Characteristics : Unsegmented, bilatorally synunotiical, triijloblastic ani-
mals bearing a shell, muscular foot, and mantle; four main pairs of nerv-
ous ganglia ; 70,000 species.

Class I — Amphineura — Chitons (Chaetopleura, Ischnochiton) .

Characteristics: Bilaterally symmetrical; shell typically composed of
eight transverse calcareous plates with many pairs of gill filaments.

Class II — Gastropoda — Snails, slugs, whelks {Umax, Physa, Helix, Lymnaea,
Campelotna, Busy con).
Characteristics : Asymmetrical animals with well-developed head ; spi-
rally-coiled shell.

Class III — Scaphopoda — Elephant's-tusk shells (Dentalium, Siphonodenta-
lium).
Characteristics: Both shell and mantle tubular; protrusible foot ; rudi-
mentary head.

Class IV — Pelecypoda — Clams, mussels, oysters, and scallops {Ensis, Ano-
donta, Venus, Teredo, Ostrea, Pecten).
Characteristics: Usually bivalved shells with two-lobed mantle; no
head ; body laterally compressed ; bilaterally symmetrical.

Class V — Cephalopoda — Squids, cuttlefishes, octopus, nautilus (Loligo,
Polypus, Dosidieus).
Characteristics: Bilaterally symmetrical; with foot divided into siphon
and arms provided with suckers; well-developed nervous system con-
centrated in head; mouth possesses strong jaws.

/.<^

NATURAL HISTORY

ClctSS I

Cmstacea

Olci-ss -jn
Oiiychopbortt

PecLiculus /^TXi-jh

Class IS"

liasecta

"Po-pilio .

ciccss"sr
Aractiiaoidea

ROLL CALL ^,j

PHYLUM XII - ARTHROPODA - Lobsters, crabs, spider., millir>odes
insects. ' * '

Characteristics: External evidence of segmentation, body at least beine
divisible into a well-defined head, thorax, and abdomen; jointed append-
ages ; chitinous exo-skeleton ; nervous system of ladder f vpo witl, tondcnry
toward concentration in head region; main longitudinal blood vessel
with heart dorsal to alimentary canal; coelom reduced; body cavity
filled with blood (hemocele) ; 640,000 species.

Class I — Crustacea — Crayfish, crabs, water fleas, barnacles, sowbugs (Cam-
barus, Callinectes, Gammarus, Asellus, Trior thrus).
Characteristics: Mostly aquatic; usually bearing gills; with two pairs
of antennae (feelers) ; chitinous exo-skeleton ; body divided into head,
thorax, and abdomen ; head and thorax sometimes fused {cephalolhorax) ;
further subdivision depending largely upon characteristics of carapace.

Class II — Onychophora — Annelidlike arthropods (Peripatus).

Characteristics: Tropical, primitive, wormlike tyi)os j)resumably inter-
mediate between the segmented worms and the arthropods; excretory
system of annelid type (nephridial) ; respiratory organ resembles tracheae
of insect group ; external appendages ringed, suggesting segmentation of
arthropods.

Class III — Myriapoda — Centipedes and millipedes {Scolopendra, Spirobolus).
Characteristics : Body relatively long and definitely nietamcric ; one
pair of antennae ; appendages segmented ; legs similar ; respiratory sys-
tem of tracheal type ; in millipedes there are two pairs of legs per somit«,
in centipedes one.

Class IV — Insecta — Insects, as butterflies, grasshoppers, beetles, bees.

Characteristics: L^sually possess wings; one pair of antennae; tracheal
respiratory system ; segmented legs.
Order 1 — Thysanura — Bristletails, Silverfish (Lepistna, Campodea, Thermobia).
Characteristics : Wingless arthropods ; primitive ; probably derived from
wingless ancestors ; 11 abdominal segments; chewing mouth parts; usu-
ally two or three long, threadlike, segmented caudal appendages; less
than 20 species in the United States ; no metamorphosis.
Order 2 — Collembola — Springtails (Archorules).

Characteristics : Primitive wingless insects with chewing or sucking mouth
parts; four segmented antennae; usually no tracheae; six abdominal
segments; a springing organ (furcida) present on ventral side of fourth
abdominal segment in most species ; no metamorphosis.
Order 3 — Orthoptera — Grasshoppers, cockroaches, walking sticks {Melanoplus,
Periplaneta, Diapheromera).
Characteristics: Members of this order are characterized by two pairs
of wings (sometimes greatly reduced) ; the fore wings usually thickened.
sometimes leathery ; hind wings folded fanlike beneath fore wings ; biting
mouth parts ; gradual or simple metamorphosis.

100 NATURAL HISTORY

Order 4 — Isoptera — Termites or white ants {Reticulitermes) .

Characteristics : Four similar wings lying flat on back when at rest ;
workers are wingless; chewing mouth parts; abdomen joined directly to
thorax ; gradual or simple metamorphosis.

Orders — Neuroptera — Dobson flies, alder flies, lacewings, ant-lions {Corydalis,
Chrysopa, Myrmeleon).
Characteristics : Four membranous wings with many veins ; chewing
mouth parts ; larvae carnivorous ; tracheal gills usually present on aquatic
larvae; the larvae of the horned Corydalis known as hellgrammites are
used by fishermen as bait ; complete metamorphosis.

Order 6 — Ephemerida — Mayflies (Ephemera).

Characteristics : Mouth parts of adult vestigial ; two pairs of membra-
nous, more or less triangular, wings ; fore wings larger than hind wings ;
caudal filaments and cerci very long; aquatic larvae breathe by tracheal
gills, usually located on either side of abdomen ; adult's span of life short ;
mouth parts poorly developed, probably making organism incapable of
taking food; nymph remains one to three years in water; adults moult
within 24 hours after acquiring wings, therefore called sub-imagos ; gradual
or simple metamorphosis.

Order 7 — Odonata — Dragonflies and damsel flies (Macromia, Agrion).

Characteristics : Chewing mouth parts ; two pairs of membranous veined
wings; characteristic joint (nodus) on anterior margin of each wing; eyes
large, compound ; nymphs are aquatic ; gradual or simple metamorphosis.
When at rest dragonflies hold their wings horizontally and at right angles
to body, while damsel flies maintain theirs vei-tically.

Order 8 — Plecoptera — Stone flies (Allocapnia, Taeniopteryx).

Characteristics : Chewing mouth parts often poorly developed in adults ;
two pairs of wings; hind wings usually larger and folded beneath fore
wings ; nymphs aquatic, bearing filamentous tracheal gills ; usually be-
neath stones in flowing water; gradual or simple metamorphosis. The
salmon fly, Taeniopteryx pacifica, is a dangerous pest in the State of
Washington because it destroys buds.

Order 9 — Corrodentia — Book- and bark-lice (Trodes).

Characteristics : Either wingless, or two pairs of membranous wings char-
acterized by a few prominent veins; fore wings larger than hind wings;
when at rest held over body like sides of a roof; chewing mouth parts;
gradual metamorphosis. Book-lice often eat paper and bindings of old
books.

Order 10 — Mallophaga — Chewing lice or bird-lice (Menopon, Trichodectes).

Characteristics : Chewing mouth parts ; wings absent ; eyes degenerate ;
metamorphosis gradual or wanting. Members of this group are ecto-
parasitic upon hair and scales of birds and mammals.

Order 11 — Embiidina — Emhiids {Emhia).

Characteristics : Chewing mouth parts ; wingless or possessing two pairs
of delicate membranous wings with few veins ; cerci present on two seg-
ments ; males usually winged, females wingless ; gradual metamorphosis.
These organisms live under stones, etc., in tunnels formed of silk produced
in tarsal glands.

ROLL CALL 101

Order 12— Thysanoptera — Thrips (Thnps, Franklinella, Crypiolhnps).

Characteristics: Piercing mouth parts; either wingless or with two pairn
of long, narrow membranous wings, practically veinless; large, free pro-
thorax; feet clawless but possessing small protrusible membranous sacs
for clinging; manj^ parthenogenotic ; gradual metamorphosis.

Order 13 — Anoplura — Sucking lice {Pediculus, I'hthirins).

Characteristics: Wingless ectoparasitic lice with piercing and sucking
mouth parts; eyes poorly developed or absent; parasitic on bodies of
mammals ; gradual metamorphosis. At least two species, the head louse
and crab louse, occur on man.

Order 14 — Hemiptera — True bugs {Artocorixa, Lethocercus).

Characteristics : Either wingless, or with two })airs of wings ; in such cases
fore wings are thickened at base ; mouth parts adapted for piercing and
sucking; gradual or simple metamorphosis. Members of this group con-
tain many interesting and sometimes economically important forms. The
water-boatmen (Corixidae) have long, flat, fringed metathoracic legs which
are adapted for swimming. These peculiar forms carry a film of air about
body when under w^ater. The leaf bugs (Xeridae) are frequently numer-
ous and injurious to plants. Bedbugs (Cimicidae) have been accused of
transmitting various diseases. The cabbage bug does damage to garden
vegetables.

Order 15 — Homoptera — Cicadas, aphids, leaf-hoppers, and scales {Euscclis,
Empoasca, Rhopalosiphum) .
Characteristics : Mouth parts adapted for piercing and sucking ; two pairs
of wings of uniform thickness held over back like sides of a roof. The cica-
das (Cicadidae) are better known as the "seventeen-year locust." Plant-
lice (Aphididae) are mostly small green insects that suck juices from
plants and have a gradual metamorphosis.

Order 16 — Dermaptera — Earwigs (Anisolabis, Labia).

Characteristics : Either wingless, or possessing one or two pairs of wings ;
in such cases fore wings are small and leathery, meeting in straight line
along back; chewing mouth parts ; gradual metamorphosis. Earwigs are
nocturnal and feed principally upon vegetation.

Order 17 — Coleoptera — Beetles and weevils {Hydrous, Dytiscits, Photinus,
Anthonomus).
Characteristics : Either wingless or with two pairs of wings, fore wings
being hard and sheathlike {elytra); hind wings membranous and are
folded two ways under elytra; large movable prothorax; chewing mouth
parts; complete metamorphosis. Many forms are found in this group.
as the tiger beetles, fireflies, click beetles, whirligig, ladybird, and leaf
beetles.

Order 18 — Strepsiptera — Stylopeds {Xenos).

Characteristics: Mouth parts reduced or wanting; nutrition by absorp-
tion; males possessing club-shaped fore wings and large membranous
hind wings ; females wingless and legless ; life cycle complex ; para.sitic on
bees, wasps, and homopterous bugs.

102 NATURAL HISTORY

Order 19 — Mecoptera — Scorpion-flies {Panorpa, Bittacus).

Characteristics : Members of this group are wingless or characterized by
two pairs of long membranous wings containing many veins; head pro-
longed into beak; antennae long and slender; mouth parts adapted for
chewing ; males with olasping-organ on caudal extremity resembling sting
of a scorpion ; metamorphosis complete.

Order 20 — Trichoptera — Caddis flies {Phryganea, Molanna).

Characteristics : Adults with vestigial mouth parts ; two pairs of mem-
branous wings obscurely colored by long silky hairs and narrow scales;
antennae long and slender; metamorphosis complete; larvae and pupae
aquatic, constructing portable cases of sand grains or vegetable debris
fastened together with silk from modified salivary glands.

Order 21 — Lepidoptera — Butterflies and moths {Tinea, Alsophila, Papilio).

Characteristics : Wingless, or with two pairs of membranous wings cov-
ered with overlapping scales; sucking mouth parts coiled beneath head
consist of two maxillae fastened to form a tube; metamorphosis com-
plete ; larvae known as caterpillars ; many species known.

Order 22 — Diptera — Flies and mosquitoes (Tipula, Culex, Prosimulium, Musca,
Drosophila).
Characteristics : One pair of membranous fore wings on mesothorax, or
wingless ; knobbed threads (halteres) on metathorax represents hind wings ;
mouth adapted for piercing and sucking, forming proboscis ; larvae known
as maggots ; complete metamorphosis.

Order 23 — Siphonaptera — Fleas (Ctenocephalus, Pulex).

Characteristics : Wingless insects with laterally compressed body ; head
small ; no compound eyes ; mouth adapted for piercing and sucking, legs
for leaping; metamorphosis complete; ectoparasites of mammals and
more rarely birds.

Order 24 — Hymenoptera — Saw flies, ichneumon flies, ants, wasps, and bees
(Cladius, Ophion, Formica, Vespa, Apis).
Characteristics : Wingless or with two pairs of membranous wings ; fore
wings usually larger ; venation reduced ; wings held together on each side
by hooks (hamuli); mouth parts adapted for chewing or sucking; first
abdominal segment fused with thorax ; complete metamorphosis.

Class V — Arachnoidea — Spiders, scorpions, ticks, mites, and king crabs
{Caddo, Lycosa, Phalangium, Buthus, Argas, Sarcoptes, Limulus).
Characteristics : No antennae nor true jaws ; two of six pairs of jointed
appendages modified for mouth parts; respiration by lung-books or
tracheae; first pair of appendages usually contain poison glands, second
pair used as jaws ; terminal portions as sensory organs ; body usually
divided into anterior cephalothorax and posterior abdomen ; former bears
four pairs of legs for locomotion.

THE ANIMAL KINGDOM >

Phvllm

Chordata

Arthropoda

Mollusca
Echinodermata

Annelida (Annulata)
Molluscoidea
Platyhelminthes
Nemathelminthes

Troehelmi n thes
Coelenterata

Porifera
Protozoa

Claj;

Mammalia

Aves

Reptilia

Amphibia
Pisces
Miiior
Cl asses

Onychophora
Crustacea

Myriapoda

Insecta

Arachnoidea

Examples

Kmtimatek

iNl-MMEU (IK I.IVIM.
.Si'EciEM DkhciiiiiEU

VERlEliUATES

Man, cat, horse, bat, whale
liirds, fowls

Turtles, snakes, lizards, alli-
gators
I'rofis, toads, salamanders
I'ishos

Tunicates, Balanoglossus, etc.
Total Chordata

INVERTEBRATES

Crayfish, crabs, water fleas,

barnacles, sowliugs
Centipedes, millipedes, etc.
All true insects
Spiders, scorpions, ticks,

mites, and king crabs

Total .\rthropoda

Snails, slugs, clams, oysters

Starfish, sand dollar, sea-
urchin

Earthworm, leeches

Bryozoa, Ijrachiopods

P'latworms, flukes, tapeworms

Roundworms, Trichinclla,
Filaria

Rotifers, wheel animalcules

Jelly-fishes, coral animals.
Hydra

Sponges

Ameba, Paramecium,

Euglena, malarial organ-
isms, trypanosomes

Grand total

:{.7.')()
l.l.."j(t()

•4.000

1.7.">0

lIl.jOO

1,500

20,000

2,430

625,000

27.500

38.000

fi75,000

S0.(JO0

5,000
5,000
2,5(X)
G.500

.3.500
1.500

5.000
3.000

15,000

S40,000

' Modified from Metf-alf and Flint, Destmrtirp and Useful Insects. By pprmis.xjon r)f the McOrnw-
Hill Book Company, publishers. The discrepancies between this table and the tt'Xt illusirato the
pragmatic nature of taxonomy.

H. W. H. — 8

104

NATURAL HISTORY

Sub -phylum I

HEKIC«ORT>ATA
half - CL ' cViorcC

Sub • pliylum H

Urockorbata

chor*<jC - in- t<xil

CI)
Tunicate.

CI.)

sute-pViylvcm, uc

CEPHALOCKORDOTA

cViorcC-in- Vj«acC

(1^ ri'^^i

Arophioxus '
Iccnc-elet

ammals '*\/it*h a -notocVjorcC

(A) SUPERCLASS AGNATHA

(ii fossil Ostracoderm

'Pbsrichthys

(2 ) Cyclostoma.to.

^etronwjjon. . lamprey

CB) SUPERCLASS PI5CE6

(1) ^

A-mphibia
frog-

(3)

Aves
bird

"ReptiLia
turtle

sub-phylura IS"

VERTEBRATA

'->vit/Vi "toccc-Vctoones

"Mammalia

ROLL CALL jqs

PHYLUM XIII — CHORD ATA — Animals with notochord.

Characteristics: All possess a dorsal supporting rod or notochord and
pharyngeal gill clefts at some stage in life cycle; tubular nerve cord
dorsal to digestive tract ; 36,000 species.

Sub-Phylum I — Hemichordata — (Balanoglossus).

Characteristics : Wormlike marine oiganisms of doubtful relationship that
burrow in sand and resemble the larval echinoderms in development ; head-
end with proboscis and collar; with or without a notochord.

Sub-Phylum II — Urochordata — Tunicates and ascidians.

Characteristics : Marine organisms with saclike covering {tunic) ; larvae
resemble tadpoles, possessing notochord in tail; gill slits and endostyle
present in pharynx.

Sub- Phylum III — Cephalochordata — Lancelots (Amphioxus).

Characteristics : Segmented primitive chordates, burrowing in sand ; lat-
erally compressed ; notochord extending from anterior tip to tail.

Sub-Phylum IV — Vertebrata (or Craniata) — Vertebrates.

Characteristics : Animals with definite head, sense organs, closed circula-
tory system, and axial notochord at some period in life cycle; skull and
vertebral column present either in cartilaginous or bony stage.

Super-Class A — Agnatha — Fossil, armored Ostracoderms, lampreys and hag-
fishes (Cyclostomata). Primitive fishlike forms (Pterichthys, Petromy-
zon).
Characteristics: Animals w'ithout jaws; sucking mouth and primitive
brain present.

Super-Class B — Pisces — True fishes.

Characteristics : Organisms with true jaws ; typically scaled ; charac-
teristically aquatic; appendages developed into fins; two-chambered
heart.

106

NATURAL HISTORY

Clocss I

Ela5mobranchn

Class IE
Holocepholi

(1^

Chimaera
spook fislT.

(B) Superclass PISCES

FISHES PROPER.

1£>

Protopterus

lungfish

Class 12:

'Pe^rccc

class "JT

Tsleostei

l\OLL CALL jy^

Class I — Elasmobranchii — Gristle-fishes {Sgualus, Raia).

Characteristics: Cold-blooded fishlike vertebrates witli jaws; charac-
terized by a cartilaginous skeleton, i)(>rsistent notochord and placoid scales;
upper jaw suspended to ci-aniuin indirectly by means of ligaments and
cartilages (hyostylic).

Class II — Holocephali -^ Elephant-fishes (Chimaera).

Characteristics : Immovable upper jaw fused with cranium (autostyiic)
resembling higher forms; gill slits covered by flap (operculum); tail
heterocercal.

Class III — Ganoidei — Enamel-scaled fishes (Acipenser, Lepisosteus, Polyp-
terus).
Characteristics : More or less armored fish ; remnants of group dominant
in Devonian seas; degenerating spiral valve in intestine associated with
presence of pyloric caeca; scales usually rhomboidal, fitting together
rather than overlapping; dorsal fin usually close to caudal fin.

Class IV — Dipnoi — Lung-fishes (Neoceratodus, Lepidosiren, Protopterus).

Characteristics: Semitropical fishes, passing dry season by aestivating in
slimy cocoon ; during period of active life use gills, and while aestivating
breathe air, the modified swim bladder acting as a lung; cycloid scales;
auricle of heart partially divided.

Class V — Teleostei — Bony fishes (Ctenolabriis, Perca, Gadus, Microptcrtis).
Characteristics : Bony fishes, breathing primarily by gills ; well-develoiM-d
operculate bones, cycloid or ctenoid scales ; tail homocercal. These fishes
constitute about 90 per cent of all known varieties.

108

NATURAL HISTORY

OrcCe^r- (i)

OrdiQr (2")

jg^css amphibia

Rftstorations
Steg'ooepViali

(1"^

Cccecilicc
"blincC" ^vo^mUke
amphibian.

'RAPODA

IBIA

(1^

Triturus
spottccC incvt

Necturzxs

Order (3)

UrodLela . . . .,
gtnpbibitt wttn taits

OncCer- (4-)

Anurcc

taillC96 amphibia

ROLL CALL ,„y

Super-Class C — Tetrapoda — Four-footed vertebrates.

Characteristics : Well-defined limbs witli hands and feet typically con-
structed on plan of five digits; stapes or coluniolla present in ear; Rirdles
adapted to bear weight on land; body divisible into neck and trunk, tail
present.

Class I — Amphibia — Frogs and salamander.'?.

Characteristics: Cold-blooded, naked vertebrates undergoing a meta-
morphosis ; usually with five-fingered limbs (pentadactylous) ; young
u.sually aquatic, breathing by gills; adults using lungs and skin, u-suallj'
air breathers.
Order 1 — Stegoccphalia — Extinct fossil amphibians (Erynps, Loxomma).

Ch.^racteristics : Fossil forms resembling amphibia, flourishing in car-
boniferous age ; probably earliest four-footed air breathers.
Order 2 — Apoda — Legless amphibia (Herpeles, Siphonops, Caecilia).

Characteristics : Small, tropical, wormlike, often blind amphibia, burrow-
ing in ground.
Order 3 — Urodela — Salamanders {Desmognathus, Necturus, Cryptobranchus,
Triturus).
Characteristics: Tadpole-like tail retained throughout life; some never
emerge from water; a few retain external gills in adult stage.
Order 4 — Anura — Frogs and toads {Rana, Bufo, Ilyla).

Characteristics : Tailless upon completing their metamorphosis ; capable
of singing ; characterized by the possession of movable eyelids.

NATURAL HISTORY

OrcCeJr, 1 , ^.

RViy n cho ctepnou loc

'tJhe. old, t-imer-^''

Spherzodon

OrcL©3~ 2 •

Cro<iociilicc

Crocodilea . aUigators

Alligator

Soft iheileat turtle

"i5o>: turtle^

OrcCer -3

Chelonia

turtles, tortoises

^ 'osoxers
fish -like reptile^

sub ordter Soci^ria
lixccr-cCs

Plesiosaurs

Pberodoc'tyls

./(2) -^^.^^.ffla^ (4)

'' sub order Serper^teS Hiriosaurs
SriccKe.S ^lant reptiles

Ordei^ 4r

5c|uamata
snokss , li3ards

Orders 5-8 fossil rejjtiles
Ichthyosouria ,Plesio5auria
Ptcrc?ctactylia,T)inoscturia

UOLL CALL ijj

Class II — Reptilia — Turtles, snakes, alligators, and lizards.

Characteristics: Cold-blooded; usually covered with scales and fre-
quently bony plates ; air breathers.

Order 1 — Rhynchocephalia — "The old-timers," Sphenodon.

Characteristics : Biconcave vertebrae often containing remnants of noto-
chord; quadrate bone immovable; parietal eye present. This group i.«
represented by one genus of lizards, Sphenodon, found only in New Zea-
land.

Order 2 — Crocodilia — Crocodiles and alligators {Crocodiliis, Alligator).

Characteristics : Anterior appendages bearing five digits, jiosterior four
with trace of fifth ; longitudinal slit constitutes cloacal opening; vertebrae
procoelous.

Order 3 — Chelonia — Turtles and tortoises (Amyda, Eretmochelye, Terrapene,
Testudo, Chelonia).
Characteristics : Body surrounded by bony case forming a carapace and
plastron; toothless jaws ; immovable quadrate bone; appendages typi-
cally with five digits.

Order 4 — Squamata — Snakes and lizards (Phrynosoma, Heloderma, Tham-
nophis).
Characteristics: Usually with horny epidermal scales or plates; movable
quadrate bone; vertebrae usually procoelous; ril)s with single heads.
This order is usually subdivided into two sub-orders: lizards (Sauria) ;
and snakes (Serpentes).

Orders 5-8 — /)z/;o.s<7;/m — Fossil reptiles (Ichthyosaurs, Plesiosaurs, Pterodac-
tyls, Dinosaurs).

In these groups belong such forms as the fishlike reptiles (Ichthyosaurs) ;
the long-necked reptiles (Plesiosaurs) ; the flying reptiles (Pterodactyls) ;
and the giant reptiles (Dinosaurs).

112

NATURAL HISTORY

Subcbss A - Arcbaeomithes
r| fossil reptile-like "bircCs

SUPERCLASS TETRAPO"DA

ClassI AVE5 BIRDS

Kestserornithifbrmes ^,
^5sil toothe^blrasl

Aptery^iforme?
« Ca5i:arii|brTOes Kivi^^

^Caseovarie^ ^ Ciconiifomes

C)Icbtbxor^^i7orm<25 ^ /« stork -like bircfs

Grui formes
rails at2ct coots

(10)

cViarBucCrji^nTMs
glover, 5nipi« ,^115

11^ ^ . ^

m.^s??^ Cuculi formes

stratHorjiformes |f^^ M-^WxV Talcomfl^nes
Afncan oitrich jV ]j col^'mbilbrmcs falcon-like-binis
(-r) <?4rV;;^V-- l-oons and Grcbas /^

i)moTnitbi(t>rmes ^"^^W ^^^'^S^l ^^^^ -"^ssi^*^
Moots ^-r-'^'^/ j'^^'^,.// Coraciiformas

extinct :^^/ <^^f< — ^'^

(8) (^^^ ^ Gcclhformss

=^ elep hant birds aibatrossa.s .petrels

RheifoT^m©© .
American ostrioh

Subclass D -Neorrzitbes

(2/)

fixsseriformes
percViing- birds

HOI.L CALL ,lj

Class III — Aves — Birds.

Characteristics: Typically featliered and toothless; \varm-l)Ioo(lo(l.

Subclass A — Archaeornithes — Fossil birds (Archaeopteryx).

Characteristics: Ancient re[)tilelike fossil birds; only three specimens of
a single genus (Archaeopteryx) are known.

Subclass B — Neornithes — Recent birds.

Characteristics: Mostly composed of birds which are represented by
living forms; 21 orders.
Order 1 — Hesperornithiformes — (Hesperornis).

Characteristics : Fossil, toothed birds from America ; teetli set in a groove.
Order 2 — Ichthyornithiformes — {I chthyornis) .

Characteristics : Fossil, toothed birds from America, whose teeth are set
in sockets.
Order 3 — Struthioniformes — Ostriches (Stridhio).

Characteristics : Naked head, neck, and legs ; flightless, terrestrial forms ;
feet with two toes; no keel on breastbone {sternum).
Order 4 — Rheiformes — Rheas (Rhea).

Characteristics : Distinguished from preceding order by a partially feath-
ered head and neck; flightless terrestrial birds, with three-toed feet;
feathers without aftershaft.
Order 5 — Casuariiformes — Cassowaries and emus (Dromalus).

Characteristics: Terrestrial, flightless birds, possessing small wings;
feathers with large aftershaft.
Order 6 — Crypturiformes — Tinamous (Rhynchotus) .

Characteristics : Flying, terrestrial birds, with a short tail ; no pygostyle.
Order 7 — Dinornithiformes — Moas (Palapteryx).

Characteristics : Recently extinct, flightless, terrestrial birds, with large
hind limbs ; wing bones absent.
Order 8 — Aepyornithiformes — Elephant birds (Aepyornis).

Characteristics: Extinct terrestrial flightless birds with large hind limbs;
small sternum and wings ; large eggs.
Order 9 — Apterygiformes — Kiwis (Apteryx).

Characteristics: Small flightless terrestrial birds; hairlike feathers with-
out aftershaft.
Order 10 — Sphenisciform.es — Penguins (Eudyptes).

Characteristics: Marine antarctic birds, incapable of flight, with small
scalelike feathers; wings modified as paddles for swimming.
Order 11 — Colymbiformes — Loons and grebes (Gavia, Podiceps).

Characteristics : Aquatic birds with feet far back with webbed or lobed toes.
Order 12 — Procellariiformes — Albatrosses and petrels (Diomedea, Hydrobates).
Characteristics : Marine birds with great powers of flight ; webbed toes ;
bill sheath of several pieces.
Order 13 — Ciconiiformes — Storks, birds, pelicans, cormorants, snake-birds,
herons, ibises, and flamingos (Phalacrocorax, Ardea, Phoetncoptcru.f).
Characteristics: Long-legged aquatic marsh birds with feet adapted for
wading.

114 NATUllAL lilSTORY

Order 14 — Anseriformes — Swans, geese, and ducks (Mergus, Anas, Cygnus).
Characteristics : Aquatic birds whose beak is covered by soft sensitive
membrane edged with horny lamellae.
Order 15 — Falconiformes — Falcons, vultures, eagles, hawks, and secretary-birds
{Cathartes, Gymnogyps, Sagittarius, Falco).
Characteristics : Carnivorous birds with curved, hooked beak ; feet
adapted for perching and provided with sharp, strong claws.
Order 16 — GalUformes — Tui'keys, fowls, quails, and pheasants ; also the
hoactzin (Meleagris, Colinus, Bonasa).
Characteristics : Arboreal or terrestrial birds ; feet adapted for perching.
Order 17 — Gruiform.es — Rails and cranes {Rallus, Gallinula, Fulica).

Characteristics : Mostly marsh birds.
Order 18 — Charadriiform.es — Plovers, snipes, gulls, terns, auks, and pigeons
{Jacana, Larus, Rhynchops).
Characteristics : Marine, arboreal, or terrestrial forms.
Order 19 — Cuculiformes — Cuckoos and parrots {Conuropsis, Coccyzus).

Characteristics : Arboreal birds, first and fourth toes directed backwards ;
the latter may be reversible.
Order 20 — Coraciiformes — Kingfishers, owls, hummingbirds, swifts, and wood-
peckers (Streptoceryle, Antrostomus) .
Characteristics : Tree-inhabiting forms with short legs.
Order 21 — Passeriformes — Perching birds (Passer, Sayornis, Tyrannus).

Characteristics : More than half of all known birds belong in this order.
In America representatives of 25 families are found. A few of these are
the flycatchers, larks, thrushes, thrashers, wrens, warblers, swallows,
shrikes, nuthatches, crows, orioles, finches, and creepers.

ROLL CALL ,,-

Class IV — Mammalia — Mammals.

Characteristics: Members of this class are readily distiiiKuisl)0(l by a
covering of hair at some time in their existence; the females pos.seKs
mammary glands which secret(> milk for nourishment of young.

Subclass A — Prototheria — Monotremes {Echidna and Ornithorhynchus).
Characteristics: Egg-laying mammals; in case of Ecliidim the egg is
placed in a temporary pouch and incubated until hatched.

Subclass B — Metatheria — Marsupials {Didelphys, Petrogale, Macropus).
Characteristics : Carry young in marsupium or pouch ; allantoic placenta
typically absent.

Subclass C — Eutheria — Viviparous mammals.

Characteristics : Bring forth their young alive ; young never carried in
pouch ; nourished before birth by placenta.
section a — unguiculata — Clawed mammals.
Order 1 — Insectivora — Insect-eaters, moles, and European hedgehogs.

Characteristics : Small terrestrial clawed mammals with typically planti-
grade feet ; molar teeth enameled, rooted, and tuberculate.
Order 2 — Dermaptera — Flying lemurs.

Characteristics: Members of this group resemble the insectivores in the
structure of the skull and the canine teeth ; only two genera are known,
which inhabit the forests of Malaysia and the Philippines.
Order S — Chiroptera — Insectivorous bats, fruit-bats, and blood-sucking vam-
pires.
Characteristics : Mammals with claws whose fore limbs are modified for
flight.
Order 4 — Carnivora — Flesh-eating mammals, hyenas, raccoons, dogs, cats,
weasels, bears, sea-lions, seals, and walruses. «

Characteristics : Carnivorous mammals with claws and large projecting
canine teeth; incisors small; premolars adapted for flesh-cutting.
Order 5 — Rodentia — Gnawing animals, hares, rats, mice, squirrels, beavers,
porcupines, guinea pigs.
Characteristics : Members of this group are usually separated into two
suborders depending upon the possession of one or two pairs of incisors
in upper jaw.
Order 6 — Edentata — So-called toothless mammals, three-toed sloth, armadillo,
and pangolin.
Characteristics: Clawed mammals; teeth entirely absent or mi.-^sing
from anterior part of jaw ; teeth usually without enamel ; tongue often
long and protractile.
section b — primates.

Order 7 — Primates — M&mmaAs with nails; tarsiers, lemurs, monkeys, apes,
man.
Characteristics: Toe or thumb usually is opposable to other digits;
dentition rather primitive ; eye orbits directed forward ; posture usujilly
semierect.

116

NATURAL HISTORY

Subclass A PROTOTHERIA
MONOTREMES

®gig-layin^ mamnrzals

OrnitlQorbyr2c"bu4
cUxckbill

SubcktssB NETATHEQIA

HAR5(JP)AL5

mammals >vitl3

torOQgt pOLCCVx

Macropas
l<ocng"a.-roo

Cla$$ 32^ MAMMALIA.

Section a

UNGUICULATA

clawea mammals

SubclassC EUTHERIA

viviparous mamrcKxis

Cccmivora

Order 1 . Tnolss.e.tc.

Insectivora

S|>^

OrcCai- v5

RocCent-icc

gnaviog mammals

Dermaptera

flying ]emtxi~5

OroCer 6
Edentata

OrcCejT 8

Artiodoctyla

even- toed. \^

Section C/

UNGULATA

hoof«gcC mammali

OrcLai- 9 '

PerissododMct
octd-toedL "^

OroCer lO
Probosrcixfea

trttnk ondC tusl<S

ordterll secccovs
Siren ice

ROLL CALL jj-

SECTION c — UNGULATA — Hoofed mammals.

Order 8 — Artiodactyla — Even-toed ungulates; hippopotamus, camel, deer,
moose, domestic cattle, giraflfe.
Characteristics; An even number of digits, axis of symmetry passing
between digits three and four.
Order 9 — Perissodactyla — Odd-toed ungulates, horse, zebra, tapir, rhinoceros.
Characteristics : An uneven number of digits, axis of symmetry passing
through digit three
Order 10 — Proboscidea — Elephants.

Characteristics : Ungulates characterized by long, prehensile proboscis ;
incisors developed to form tusks ; broad molars.
Order 11 — Sirenia — Sea-cows, dugong, manatee.

Characteristics: Aquatic ungulate-type Eutheria; fore limbs finlike ; hind
limbs absent ; tail with horizontal fin.
Order 12 — Hyracoidea — Hyrax and coneys.

Characteristics : Small rodent-like mammals with reduced tail and short
ears ; four digits on fore limbs ; three digits on hind limbs.
section d — cetacea — Whales and dolphins.

Characteristics : Aquatic mammals ; probably derived from the Ungui-
culata or Ungulata.
Order 13 — Odontoceti — Toothed whales, dolphin, porpoise, grampus.

Characteristics : Cetacea with teeth (at least on lower jaw) ; no whale-
bone.
Order 14 — Mystacoceii — Whalebone whales, fin whale, right whale.

Characteristics: Cetacea without teeth in adult; mouth provided witli
plates of whalebone.

118 NATURAL HISTORY

GLOSSARY OF TERMS OCCURRING IN ROLL CALL

Abdomen — the posterior region of the body, behind the thorax of an insect ; the
region of the body below the chest in man.

Aestivating — passing the summer in a torpor.

Aftershaft — an accessory plume arising from the posterior side of the shaft of
the feathers of many birds.

Alimentary canal — food tube of animal, beginning with mouth and ending with

anus.
Allantoic — pertaining to a respiratory sac which in early fetal life grows out from

the hind-gut of an embryo.

Ambulacral groove — groove in which tube-feet are located.

Antennae — paired appendages, which are sensory in function, on the head of an

insect or crustacean.
Antheridium — organ or receptacle in which male sex cells of ferns are produced.
Anus — posterior opening of alimentary canal.

Appendage — an organ or part attached to a body, as a leg, arm, fin, or tail.
Arboreal — pertaining to forms frequenting trees.

Archegonium — a female organ in which the young plant begins development.
Aristotle's lantern — masticating apparatus of sea-urchin.
Ascomycetes — sac fungi.

Ascospore — one of a set of spores contained in a special sac or ascus.
Ascus — a membranous spore sac of fungi.
Asexual — having no sex.

Axial — pertaining to the fundamental central line of a structure.
Basidiospore — a spore formed on a basidium.

Basidium — the spore-producing organ of certain of the higher fungi.
Bilaterally symmetrical — having two symmetrical sides about an axis.
Bill sheath — protective covering for bill.
Bivalve — consisting of two shells or valves.
Body cavity — space in which the viscera lie.
Branchiae — gills.
Calcareous — containing lime or calcium, chalky.

Canine tooth — a pointed tooth situated between an incisor and a bicuspid or
premolar tooth.

Carapace — a bony or chitinous case covering an animal's back, as in the crayfish.

Carotin — yellow pigment of plants ; associated with chlorophyll and xantho-
phyll.

Carpel — a pistil, or one of the members composing a compound pistil or seed-
vessel.

Cartilaginous — gristly substance forming part of the skeleton.

Caudal — of, or pertaining to, the tail.

Cerci — bristlelike structures.

Chitin — a carbohydrate derivative forming the skeletal substance in arthropods.

Chlorophyll — green coloring matter found in plants and some animals.

Chloroplasts — small bodies of protoplasm containing chlorophyll.

Chromosome — a deeply staining body in the nucleus of a cell, supposed to carry

the determiners of hereditary characters.
Cirri — slender extensions found on bodies and appendages of many forms,

which are used for various functions.

HULL CALL Hy

Clasping organ — specialized holdfast structure of certain males used in conula-

tion. '

Coelenteron — internal cavity of a coelenterate, which servos as a diKcstivc
tract as well as body cavity.

Coelom — body cavity.

Columella — rodhke bone of middle ear of anura formed from hyomandihular

bone.

Compound eye — made up of several simple eyes.

Cotyledon — embryonic leaf, in a seed.

Ctenoid scales — scales with a comblike or serrate margin.

Cuticle — an outer layer of the skin.

Cycloid scales — scales with evenly curved free border.

Cytoplasm — the living substance of the cell outside of the nucleus and inside
the cell membrane.

Deciduous — falling off at maturity.

Dentition — number, arrangement, and kind of teeth.

Diaphragm — (Bot.) a septum or membranous layer.

Dicotyledon — a plant that bears seeds having two cotyledons.

Digits — terminal divisions of limb in any vertebrate above fishes.

Diploblastic — having two distinct germ layers.

Direct development — no metamorphosis, i.e., the young when hatched closely
resemble adult except for size.

Dorsal — pertaining to the back or top side of (as of a leaf).

Dorsoventral — pertaining to structures which extend from dorsal to ventral
side.

Ectoderm — the outer embryonic layer in a multicellular animal.

Ectoparasite — a parasite that lives on the exterior of an organism.

Elytra — the anterior wings of beetles, hard and caselikc.

Embryo sac — the megaspore in plants.

Endoparasite — a parasite which lives within the body of its host.

Endostyle — ciliated groove whose cells secrete mucus. Foimd in urochordate,s

and cephalochordates.
Epithelium — cellular tissue covering a free surface or lining a tube or cavity.
Excretory — pertaining to organs of elimination.
Exo-skeleton — an outside skeleton such as the shell of a lobster.
Fibrovascular bundles — collections of tubular cells, supported by woody cells,

which conduct fluids in plants.
Filamentous — composed of long, threadlike structures.
Flagella — threadlike projections of cells, which are used for locomotion.
Flame cell — the terminal cells of l)ranches of excretory system in flatwornis,

with cavity continuous with lumen of duct, and containing a ciliuni or l)uncli

of cilia, the motions of which give a flickering appearance similar to that of

a flame.
Foot — thick muscular locomotor organ of molluscs.
Furcula — a forked process or structure.
Gamete — a mature sex cell.
Gametophyte — a stage in the life history of a moss or fern in which sex cells

are produced.
Ganglion — a group of nerve cells situated outside of the brain or spinal .•oluinn.
Gastric — pertaining to or in region of stomach.
H. w. H. — 9

120 NATURAL HISTORY

Gastrovascular — serving both digestive and circulatory purposes.

Germ cell — sex cell.

Gill cleft — a branchial opening formed on the side of the pharynx.

Gill filaments — the soft filamentous structures on the respiratory organs (gills)
of aquatic animals.

Gland — an organ which secretes material to be used in, or excreted from, the

body.
Halteres — a pair of small capitate bodies representing rudimentary wings in

flies, used as balancers.

Haploid — having the number of chromosomes characteristic of mature germ-
cells for the organism in question.

Hemocoele — an expanded portion of the blood system which takes the place of
a true coelom.

Hepatic caecum — blind pouch or diverticulum of or in region of liver.

Hermaphroditic — pertaining to an organism with both male and female repro-
ductive organs.

Heterocercal — having vertebral column terminating in upper lobe of fin, which
is usually larger than the lower.

Homocercal — with equal or nearly equal lobes, and axis ending near middle of
base.

Hypha — one of the filaments composing the mycelium of a fungus.

Incisors — front chisel-like teeth of either jaw.

Incubate — ■ to keep warm and under other favorable conditions for hatching.

Indirect development — undergoing metamorphosis, i.e., showing a decided change
in form and appearance from time of hatching until maturity.

Integument — a covering or protective layer; skin.

Keel — ridgelike process.

Lamellae — thin platelike structures.

Larvae — young stages in the development of some forms of animals, which be-
come self-sustaining but which do not have the characteristics marking adults.

Lateral line — longitudinal line at each side of body of certain aquatic animals
marking position of sensory cells.

Laterally compressed — narrow from side to side.

Ligament — a band of connective tissue binding one bone to another.

Lobate — divided into lobes.

Lophophore — ridge bearing tentacles.

Lung-books — respiratory organs formed like a purse with numerous compart-
ments or a book with edges of leaves exposed.

Mammary glands — milk-secreting glands.

Mantle — the soft outer fold of skin in molluscs which secretes the outer shell.

Medusoid — like a jellyfish or medusa.

Megasporangium — a macrospore-producing sporangium in plants.

Megaspore — larger spore of heterosporous plants, regarded as female; embryo-
sac cell of seed plant.

Membranous — resembling or consisting of a membrane ; pliable and semi-
transparent.

Mesenchymatous — pertaining to mass of tissue intermediate between ectoderm

and endoderm, derived from mesoderm.
Mesenteries — peritoneal folds serving as a bridge for blood vessels and for

holding organs to body wall.
Mesoderm — the middle layer of tissue in a young animal embryo.

ROLL CALL ,o,

Mesoglea — an intermediate non-cellular layer in sponges and coeleiiterate.s.

Metamorphosis — change in form or structure of an animal in its devclotimerif

from embryo to adult.
Gradual or simple metamorphosis — young resemble adults at hatcliing cxccDt

for absence of wmgs or color, shape, and structure of some appendages '
Complete metamorphosis — young differ from adults in appearance habitat

etc., and undergo several changes in form such as larvae, pupae, and adult. '
Metathorax — posterior segment of insect's thorax.
Molars — grinding teeth.

Monocotyledon — a plant that bears seeds having but one cotyledon.
Mother cell — primary cell before division occurs.

Mycelium — the threadlike body of a mold, or other fungus ; made up of indi-
vidual threads called hyphae.

Nematocyst — a stinging cell.

Nervure — one of riblike structures which support membranous wing of insect.

Notochord — a rod of cells forming the supporting axis of lower chorda tes ; found
in early stages of development in all vertebrates.

Nucleus — the center of activity in the living cell.

Nymph — larva of aquatic forms which undergo gradual or simple metamor-
phosis.

Oogonia — female reproductive organs in certain Thallophytes ; the mother egg
cells.

Operculum — a lid or cover.

Oral — pertaining to the mouth ; side on which mouth lies.
Ovary — (Bot.) the base of a pistil, containing the ovules.
(Zool.) — the egg-containing organ.

Ovule — egglike cell of a plant.

Papillae — any small nipplelike projections or parts.

Parapodia — paired appendages used in locomotion, attached to body segments

of some marine worms.
Parasites — animals or plants which live at the expense of other organisms.
Parietal eye — rudimentary eye ari-sing as an evagination on the median dorsiil

surface of the brain.
Parthenogenesis — reproduction without fertilization by a male element.
Pedicellariae — minute pincerlike structures studding the surface of some of the

echinoderms.
Pentamerous — made up of five parts.

Peritoneum — membrane which lines the abdominal walls and invests the con-
tained viscera.
Phylogenesis — history of evolution of species.
Placenta — organ through which the mammalian embryo is nourished by the

mother.
Placoid scales — embedded scales and dermal teeth of elasmobranchs.
Plantigrade — walking with sole of foot touching the ground.
Plasmodium — a single mass of living material which contains many nuclei.
Plastid — small bodies of .specialized protoplasm lying in cytoplasm of some

cells — especially plants and certain protozoans.
Plastron — ventral bony shield of tortoises and turtles.

Poison gland — gland which secretes poison, used for protection or food-getting.
Pollen tube — a tubular process developed from pollen grains after attachnu-nt

to stigma.

122 NATURAL HISTORY

Polyp — a separate zooid of a colonial animal.
Prehensile — adapted for holding.

Premolars — bicuspid teeth between canine and molar teeth.
Proboscis — any of various tubular prolongations of the head of animals ; mus-
cular protrusible part of the alimentary canal in certain worms.
Procoelus — with concave anterior face.

Prothallus — a small, thin, gametophytic mass of tissue developed from spores
of ferns.

Prothorax — anterior thoracic segment of arthropods.

Protractile — capable of being thrust out.

Pseudopodia — protrusions of protoplasm (false feet) serving for locomotion and
prehension in protozoa.

Pygostyle — an upturned compressed bone at end of vertebral column of birds,
formed by fusion of caudal vertebrae.

Pyloric caecum — a blind diverticulum or pouch in the pyloric region.

Pylorus — the aperture between the stomach and the small intestine.

Pyrenoid — a colorless plastid of lower plants, a center of starch formation.

Quadrate bone — the bone with which the lower jaw articulates with the cranium
in some forms.

Radially symmetrical — having similar parts arranged on either side of a central

axis.

Reproduction — the process by which organisms produce off .spring. In asexual
reproduction a new organism is formed by the separation of a cell or cells
from a single parent; in sexual reproduction two cells from two plants or
two animals of different sexes join together to form a new individual.

Rhizoids — rootlike organs.

Rhomboidal — shaped more or less like an equilateral parallelogram, having its
angles oblique.

Rodent — animal with a habit of gnawing or nibbling.

Sap cavity — a vacuole, filled with water and dissolved substances in mature,
live plant cells.

Saprophytes — organisms which live on dead and decaying organic matter.

Scale — a flat, small, platelike external structure, dermal or epidermal.

Schizogony — repeated division of the nucleus without immediate cell division.

Sedentary — not free-living; animals attached by a base to some substratum.

Segmentation — the division or splitting into segments or portions ; cleavage of
an ovum.

Sessile — stationary or attached, opposite of free-living or motile.

Setae — bristlelike structures.

Siphon — a tube through which water may pass into and out from the mantle

cavity of a mollusc.
Sperm — male sex-cell.

Spicules — siliceous or calcareous secreted skeletal structures of sponges.
Spiral valve — a spiral infolding of intestinal wall.
Spongin — material of skeletal fibers of certain sponges.
Sporangium — a sac containing spores.
Spore — a type of reproductive cell, usually asexual, with a protective covering

enabling it to survive unfavorable environmental conditions.
Sporophyll — a sporangium-bearing leaf of ferns.
Sporophjrte — spore-bearing stage in the life cycle of a plant.
Stapes — stirrup-shaped innermost bone of middle ear of mammals.

ROLL CALL 12:j

Sterigma — a slender filament arising from basidium, giving rise to spores by
abstriction.

Stigma — the part of a pistil which receives the pollen grains.

Tarsal — pertaining to the tarsus or last region of the leg of an insect ; the ankle
bones of vertebrates.

Tentacles — flexible organs at the oral region of an animal, used for feeling,
grasping, etc.

Thallus — a simple plant body not differentiated into root, stem, and leaf.

Thorax — the part of the body between the head and abdomen.

Tracheae — respiratory tubes of insects.

Tracheal gills — small winglike respiratory outgrowths from the abdomen of
aquatic larvae of insects.

Triploblastic — with three primary germinal layers.

Trochophore — free-living pelagic, ciliated larval stage.

Tube-feet — organs of locomotion of echinoderms.

Tuberculate — resembling or having root-swellings or nodules.

Ungulates — hoofed animals.

Vascular — consisting of or containing vessels adapted for transmission or cir-
culation of fluid. •*;

Vein — branched vessel which carries blood to heart; rib or nervure of insect
wing.

Velum — a membranous partition likened to a veil or curtain.

Ventral — pertaining to the belly surface or under side.

Vertebrae — bones of the vertebral column (backbone).

Vestigial — small and imperfectly developed ; rudimentary.

Whorl — (Bot.) circle of flowers or parts of a flower arising from one point.
{Zool.) spiral turn of univalve shell.

Xanthophyll — a yellow pigment invariably associated with chlorophyll in higher
plants.

Zoospore — a motile spore of either plants or animals.

FUNDAMENTALS OF STIU CTURE AND Fl NC'lioN

LIFE AND PROTOPLASM

Preview. What is being alive? • Metabolism • Some signs of mani-
festation of life • The production and use of enzymes associated with living
things ■ Structure of protoplasm • Protoplasm and the cell • Chemical or-
ganization of living matter • Protoplasm a complex mixture • Protoplasm
a colloidal mixture • Diffusion ■ Osmosis and its significance to living cells •
Suggested readings.

PREVIEW

Being alive is something that we all know a little about. Liveliness
is associated with those of one group who are "up and coming,"
those who are active, both mentall}^ and physically. If living things
are thought about a little more closely, certain things are attributed
to them : they move, feed, grow, are sensitive, and they reproduce their
kind. The scientist goes a step further and compares the living thing.s
with those which do not possess this mysterious something we call
life. He says life is a manifestation of forces, like a flame, or elec-
tricity. He goes beyond superficial observation and asks himself
a good many questions about the make-up and action of the living
things which fill his environment. Some of the problems with which
one is confronted are relatively simple and may be solved with a little
close observation, even without the aid of a microscope, but other
problems are speculative and may never be answered in full.

If the problems were to be assembled with a view to attempting
their solution, some of the more important might be the following :
What is being alive? What differentiates living stuff from non-
living? What is known about the ultimate composition of the living
stuff ? Is it different for animals and for plants ? And what common
characteristics can be found for i)lants and animals?

It is obvious that our problems resolve themselves into two gr()Ui)s.
those which are more or less speculative and those which depend on
the knowledge provided by the physicist, chemi.st, and biologist.
The newer knowledge of chemistry and physics and the u.se of the
refinements of the compound microscope have made possible mudi
that was undreamed of a i'o^^■ decades ago. It is only 260 years since

12.')

126 FUNDAMENTALS OF STRUCTURE AND FUNCTION

the first simple microscopes of the Dutchman, Antony van Leeuwen-
hoek, but enormous advances have been made in this period. The
most rapid advances in chemistry, physics, and biology have come
in the last two or three decades, but we are still far from the solution
of the great riddle of the universe — What is life, and from whence
did it come ?

What Is Being Alive?

The chemist or the biologist weighs the food an organism eats and
thus finds out that much of the energy locked up in food is trans-
formed within the body of the organism, ultimately to be released
in another form, either in heat production or in work of some kind.
Not only do living things release energy but they also grow and are
able to repair parts that are wasted or lost. Think of the athlete,
hale and hearty, winning points for his team ; losing weight in a
football game, making it up after the game at the training table, or
imagine the same athlete recovering from a severe illness, or with
his leg in a cast after an accident. One may feel fairly sure that he
will soon be well again. The living stuff of which he is made will
not only use the food to release energy for his normal processes, but
will also rebuild the expended body material and rid itself of such
wastes as result from the process. Put in another way, this living
stuff of which an athlete is composed has the ability to take in food,
to use this food for the release of the energy stored up in it, or, under
certain conditions, to make some of the food over into living ma-
terial. Living things thus have the capacity for growth, for waste
and repair, and, like man-made machines, have the abihty to use food
fuel, and to release energy from it.

Metabolism

The sum total of all the processes involved in the business of being
alive is called metabolism. This series of processes is twofold : first,
constructive metabolism, or anabolism, in which the food material
becomes a part of the living organism, the energy being held there in
a potential form ; and second, by destructive metabolism or katabo-
lism, in which the body material is broken down to release energy,
and in which, as a result, there is the production of work and a
passing off of waste products. This text is concerned, by and large,
with the various phases of metabolism which will be considered in
greater detail later.

LIFE AND PROTOl'LASM joy

Some Signs of Manifestation of Life
One sign of life is the release of cnersy, which is a rosult of respir-i-
tion. It occurs in all living things, ho it a tree, a frog, or a man
when oxygen is taken into the body, whore it combines with oxidiz-
able materials to release energy. The by-products are carbon dioxide
and water, which are given off by the organism.

Living things are sensitive to and respond to various stimuli in
their environment. The plant in the window, the earthworm in
the ground, the fish in the water, and the bird in the tree are all
sensitive to and respond in different ways to the stimulus of light.
Temperature, chemical substances, gravity, electricity, radiations,
and mechanical factors, all are stimuli which affect living things in
different ways. It is this characteristic of li\ing things that we call
irritability or sensitivity.

One direct outcome of the ability of living things to respond to
stimuli in their environment is their adaptiveness. Thus li\ing
organisms ha\e the capacity to adjust themseh-es to changes in con-
ditions. Some plants throw out new roots, or suckers, or trailing
stems, by means of which they can get a foothold in slightly different
environments from those in which they are accustomed to live.
Certain low forms of plants have even become adapted to li\e in the
hot springs as those in the Yellowstone National Park, in a habitat
many degrees warmer than that of their near relatives found in
adjoining pools. Fishes, and certain small crustaceans, may similarly
adapt themselves to life in water containing a high concentration of
salts. This power of adaptation is a quality of the organism as a
whole, and results in adjustment between the external environment
and the internal body material.

The Production and Use of Enzymes by Living Things

In recent years a good deal of work has been done by physiologists
to see how the cell is able to perform the cycl(\ in which material
is taken into the organism as food or is made into food as in the case
of green plants. Food is changed into a j^oluble form so that it may
pass through the delicate living membrane of every coll. Meantime
each cell is using oxygen, which also has to be taken in through the
cell membrane, while wastes are given off by the same road. Physi-
ologists seem agreed that those living processes, called digestion,
absorption, respiration, and excretion, are made possible by the

128

FUNDAMENTALS OF STRUCTURE AND FUNCTION

presence of substances called enzymes, which act as catalyzing agents,
thus hastening by their presence the performance of such functions.
(See pages 279-280.) Enzymes are manufactured in certain cells
and it is believed that every cell, even an egg cell, contains enzymes
which are capable of digesting food substances, as well as those which
aid in oxidation within the cell.

In plants, enzymes seem to be made in almost any cell that is active
and these enzymes usually have a reversible action. For example,
certain insoluble foods may be broken down or hydrolyzed in the
cells of the leaf, so that they are soluble, then they will pass in that
condition to the stem, the roots, or the fruit, where a reverse action
takes place and the food is stored in an insoluble condition. In
animals, the hydrolyzing enzymes which make digestion possible are
usually formed by groups of cells forming glands.

Structure of Protoplasm

This living material, known as protoplasm, has been called by the
biologist Huxley "the physical basis of life." It is this stuff that is

always present in things
that are ahve. In our
present state of knowl-
edge we may liken it to
the albumen or white
of egg, a nearly colorless
and translucent sub-
stance, like soft jelly.
This substance seen un-
der the compound microscope has many granules floating in it.
It is more or less elastic, although in some cases it will flow like
a dense liquid. Seen under a high magnification it may be almost
homogeneous in structure or may appear foamy or spongelike, or
even fibrillar in appearance. A study of living cells shows that it
is obviously quite different in structure at different times and in
different animals and plants.

^^C^.

gretntjtlav
"structure

Stritctvtre

ctlveolecr
structure.

Three states of protoplasm. 2 and 3 have much
higher magnification than 1.

Protoplasm and the Cell

Although cells were first described in 1665 by Robert Hooke, it
was not until the nineteenth century that the cell theory came into
the spotlight. The knowledge that all organisms, plant and animal,
are composed of fimdamentally identical protoplasmic units, or cells,

LIFE AND PROTOPLASM

120

forms ono of the most important corner stones in th.. roun.JMli.,,, of
biology.

Wiiile plant and animal colls possess some rather striking cJifT.T-
ences m organization, they ar(> fnndamentally similar. Prac-tic-iUy
every cell that is microscopically visible possesses several difTerent
kmds of structures located within its borders. Some of these struc-
tures are alive, some lifeless. In the first group
may be placed the -plastiih of plant cells, the
mitochondria or chrondrio somes, some of which
probably give rise to plastids, fibers of various
kinds, the Golgi bodies and the centrosomes, the
latter of importance in animal cell division. In
the second group may be placed such inclusions
as yolk, or other food substances, fatty droplets,
granules of pigment or of secretions (as in gland
cells), and crystals of various kinds, such as
calcium oxalate in plant cells. To this list may
also be added vacuoles, which in plant cells often
occupy the major space within the cell membrane.
All of these structures are confined to the cyto-
plastn or part of the protoplasm outside the
nucleus. In Elodea, the cells present a green t^.
appearance, due to the presence of many tmy pi.yii cell of a K-af;
ovoid bodies, the chloroplasts, which are plastids <"• ihl()ro[)last ; n, nu-
containing chlorophyll. Careful obser^•ation of
a single cell shows that the chloroplasts move
slowly down one side of the cell, across one end, and uj) the other side,
keeping rather close to the outer edge of the cell during the process.
This is due to the movement of the cytoplasm. In the cells of the
hairlike stamen of Tradescantia, the movement of the cytopla.'^m is
also evident. Here it can be seen actively streaming in currents
within the cell, carrying along within it tiny crystals of inorganic
origin, as well as colorless plastids and granules. The latter term is
usually applied to inert materials, such as granules of stored food in the
form of starch grains (in plants), fat or yolk granules, or pigment
granules which frequently occur scattered througiiout |)r()toi)lasm.
Between the strands of (ytoplasm are spaces or vaeuolrs filled
with a watery fluid, called cell sap. In young jilant cells, the
vacuoles are small and the cytoplasm occupies the greater part
of the cell, but in mature plant cells the cytoplasm is found clo.se

{'k'us ; r
u\ cell wal

\ a<n(il('

130

FUNDAMENTALS OF STRUCTURE AND FUNCTION

Xell membrane

Cytoplasm

..Cenfrosomes
...Nucleolus

Nucleus
Plasiid

Vacuole

Diagram of a typical animal cell.

to the outer part of the cell, while the vacuoles form large sap cavities
within the cell. Although Golgi bodies appear much less stable and
more changeable in form than plastids, they are found in many kinds
of plant and animal cells. Fibrils of various kinds, such as those

seen in a muscle cell, are frequently
found. In plants the cell wall, a
delicate but rigid, secreted cellulose
covering, is lined with a delicate
living membrane which separates
the living stuff from the cell wall.
At one point can be found a slightly
denser jellylike part of the proto-
plasm called the nucleus. Both
the vacuoles and the nucleus are
separated from the cytoplasm by
delicate membranes. In many but
not all nuclei, dense, dark-staining
nucleoli appear. While their func-
tion is not clearly understood, they
generally break up and disappear
during cell division. The nucleus proper is a vital, definite part of
every living cell and is of great importance in cell division which
must take place if a many-celled organism is to grow in size, for
growth takes place by an increase in the number of cells, not in the
size of the cells. The nucleus is filled with nuclear sap, in which
is found a network of linin fibers. On these fibers are scattered
numerous granules of chromatin. This material, which as we will
see later forms chromosomes, is of the greatest importance, as through
it plants and animals are able to pass on to successive generations
their inheritable qualities.

Chemical Organization of Living Matter

A dozen or more of the ninety-odd elements recognized by the
chemist are found in living protoplasm, — carbon, hydrogen, oxy-
gen, and nitrogen comprising the greatest bulk. These elements also
form the basis of our so-called organic foodstuffs, which are called
proteins, carbohydrates, and fats. The two latter groups of sub-
stances are made up of carbon, hydrogen, and oxygen, while the pro-
teins have the element nitrogen added to their constitution, along
with sulphur, phosphorus and sometimes iron. In a simple carbo-

LIFE AND PROTOPLASM , ,,

hydrate, such as ghicose, for example, the chemist writes a fornuihi
representing a molecule of the substance. In such a simple nujlecule
the atoms of hydrogen and oxygen are usually united in the same
proportion as in water and the empirical formula is written CcHi-Oa.
This water proportion (H,.0) is maintained in other more complex
carbohydrates such as starch, but here the chemist writes an x after
the empirical formula (CeHioOa)^. This means that the molecular
formula is not exactly known but in the case of starch the x should
probably be about 200, which makes the molecule very much larger
than that of the simple sugar. The simple sugars with their small
molecules are ea.sily soluble in water, while the complex molecule of
the starch is not .so soluble. In fatty substances, oxygen is present in
a much smaller proportion than in the carbohydrates. An example
might be oleic acid, one of the components of butter fat, (Ci8H,3402).
Proteins have still more complex molecules. In the first place tliey
are built up of simpler substances, called amino acids, and in some
cases other radicals are added to them. For example, in the cell
nuclei the protein is combined with nucleic acid, which has the aston-
ishingly complex formula C38H49029Xi5P4, which really means very
little except to the student of chemistry.

Protoplasm a Complex Mixture

Living stuff, having the same elements as the complex foodstuffs
for a ba.sis, is even more intricate. No chemical compounds in nature
are quite as complicated in composition, for protoplasm not only
is made up of the foodstuffs but it also consists largely of water.
One estimate by weight gives 80 per cent water, 15 per cent proteins,
3 per cent fats, 1 per cent carbohydrates and other organic substances,
and 1 per cent inorganic salts. It has been determined that carbon,
nitrogen, hydrogen, oxygen, and phosphorus are alwaj's present in
protoplasm and are called the primary elements. Magnesium, pota.^-
sium, iron, and sulphur appear equally nece.s.sary for life. Sodium
and chlorine are always found in animal but only infrequently in jilant
tissues, and calcium appears necessary for life in the higher forms.
Other elements, bromine, fluorine, iodine, silicon, boron, manganese,
and even copper, zinc, and aluminum, are found in some organisms.

While some of these elements are solids and others gases, none of
them, except oxygen, typically occurs to any marked extent free
in the organism. Nor are th(>y found free in the foods or waste
products, but rather as various kinds of chemical compounds which

132

FUNDAMENTALS OF STRUCTURE AND FUNCTION

may be further subdivided into inorganic and organic compounds.
The former comprise most of the non-Hving compounds such as soil
and rocks and their decomposition products. However, in proto-
plasm, inorganic compounds are usually present as water, salts, or
gases. Water is important not only because it comprises 70-98 per
cent of protoplasm by weight, but also because it dissolves so many
different substances. Furthermore, water is an important factor in
promoting the dissociation of many salts into their constituent ions.
The inorganic salts which occur in marine organi.sms, for example,
are usually those commonly found in sea water. Some, such as ni-
trates and nitrites, occur chiefly in plants, while compounds con-
taining sodium and chlorides are characteristic of animal tissues.
Only three gases are found in varying amounts in the living cell, —
free oxygen, carbon dioxide, and ammonia.

Protoplasm a Colloidal Mixture

Matter exists in three states, gaseous, liquid, and solid. Frequently
it passes from one state to another, as when ice melts under the in-
fluence of heat, turn-
ing to steam as the
water boils away.
That protoplasm at
different times and
under different con-
ditions varies in ap-
pearance is probably
due to the fact that
A B c it is a colloid and as

Colloidal constitutions. The continuous phase in a such can change from
being fluid; in a jell (C) solid; while in the "sol " or liouid tO

sol (A

intermediate phase (B) the solid forms a net through

which the fluid is continuous.

a "gel," or solid state
and then, under cer-
tain conditions, back again. The scientist examines protoplasm
under the ultramicroscope and finds tiny dancing particles which
are invisible under the ordinary illumination of the microscopic field
(Brownian movement). This condition is known as a dispersion,
the dispersed particles being carried in the dispersion medium,
in this case water. A fog composed of tiny droplets of water is
an example of dispersion in nature. If the particles in a disper-
sion are small, the substance is called a crystalloid, when large it

LIFE AND PROTOPLASM

i:{3

is called a colloid. Xow these terms are not api)lie(l t(j fixed sub-
stances but to states of matt(M-. Gelatin passes from a li{|uid to ii
solid state on being heated or cooled. A study of the diap;ram shows
how this might be possible. In the left-hand diagram the solid i)ar-
ticles are floating freely in the fluid of the medium ; in the middle
diagram the solid portion is becoming a loose mesh; while in the
right-hand diagram the mesh has become a solid mass, including the
liquid within it . The protoplasm within the cells of plants and animals
probably behaves in a similar manner, under some conditions assum-
ing the "sol," and at others the "gel" state. Remembering that
protoplasm is not a single protein substance, but rather a mi.xture
of proteins, fats, carbohydrates, and sometimes even other sub-
stances, it is clear why there are many slightly different protoplasms
depending on the part of the animal or plant examined. This fact
may help us to see why the living matter of a muscle, the blood, or
the brain differs visibly in structure. For one thing, the water con-
tent differs greatly. Living bone is said to be 25 per cent water,
muscles about 75 per cent, the jellyfish almost 99 per cent, and some
fruits as high as 98 per cent water.

Diffusion

We have spoken of the work of the enzymes in making food sub-
stances soluble. Let us now see why .'^olul)ility is necessary for the
life processes of cells. The physical phe-
nomenon of diffusion is easily demonstrated
by the slow spread of red ink when a droj)
is put into a glass of water. Brownian
movement of dancing particles visible under
the high power of the microscope is a mani-
festation of molecular kinetic energy caused
by the water molecules bombarding these
particles. It is a similar movement of
molecules that occurs when diffusion takes
place. Molecules of any substance are
always in motion. If this substance is
soluble (the solute) in another substance
(the solvent), there is always a tendency
for these molecules to move from the
place of their greatest concentration to i^laces where they are not
so highly concentrated, until an equilibrium is reached and there

Lonfjit udiiial sect ion
through a tumbler of watt-r
containing soluble crystal.
sliowiiif; by arrows tlic direc-
tion of (lillusion. ami by
(lotted circles the lines of
equal concentration.

134

FUNDAMENTALS OF STRUCTURE AND FUNCTION

are just as many molecules of the solute in one part of the solvent
as in another. In the case of the diffusion of red ink in water,
the eosin (which is the coloring material used) was more concen-
trated in the drop than in the water, so the molecules of eosin began
moving away from this place of high concentration until they were
equally dispersed throughout the water. As a general rule we may
say that, if other conditions are equal, the diffusion rate between
two points is proportional to the differences in concentration of the
substances at these two points. One thing which affects the diffusion
rate is the nature of the medium, w^hether it be a gel, emulsion, or
some sort of semisolid (porous). Gelatin, for example, which is a
gel, offers no effective resistance to the diffusion of molecules of a
crystalloid nature through its meshes, but, upon the other hand,
this network may serve to block effectively the passage of colloidal
substances.

Suppose a membrane were stretched crosswise in a jar where
diffusion was taking place. Could the molecules of the diffusing
substance pass through the membrane? This depends on whether
the membrane is permeable to the diffusing substance. In some
membranes the ultramicroscopic "pores" are believed to be quite
large, thus letting through molecules of larger sizes, while in other
membranes the "pores" through which substances can diffuse are

very small. Other substances
penetrate in proportion to their
lipoid solubility. Thus some
membranes allow certain sub-
stances to pass through, while
they keep out others. Such mem-
branes are said to be selectively
'permeable. An ordinary parch-
ment membrane will allow the

'^l

ff^

" . • - •'•. ^«.-- ■',■ ■' i'ly. l,v-i .'•:~-r

'■f.^:.:^-.

■■"iPV^i^-

e p "'"-"-^f

Diagram of an imaginary section . i •. t-> - -u

through the cell wall and protoplast to eosm to pass through it. But the

show a, outer water ; iv, cell wall ; c, ecto- cell membrane does not act in the

plast or cytoplasmic membrane next ^^^^ manner, as it is a vldsma
to the cell wall; p, general cytoplasm; i i j.- i

/. tonoplast or inner cytoplasmic mem- membrane, and selectively per-

brane next to the water, thus forming meable.

a continuous pathway which carries ^j^^ plasma membranes sur-
solutes irom (a) to (?)) ; i\ vacuole. ,. ,. . , ,. ,

rounding living cells are believed

to be colloidal in nature, made of a combination of fatty and protein

substances. Careful experiments have demonstrated them to be

LIFE AND PROTOPLASM

i.r,

selectively permeable. Most living cells allow oxygen and carhon
dioxide to pass freely through their mcmijranes, while diss(jlved
sugars and digested proteins in the form of amino acids dilTu.sc
through more slowly. Water of course passes through, acting as a
vehicle for other substances. Such membranes are impernicablc to
certain salts and not to others. The permeability of living cells
to dissolved substances differs with the cell, and naturally with tlu;
organism. Salt- and fresh-water fishes are examples of types, the
cells of whose gills exhibit different permeabilities. Dead cell mem-
branes are usually permeable to crystalloid solutes, while living
cell membranes permit but few salts to enter. In general, cells are
not permeable to colloids, because of the large size of the particles
constituting the colloid.

Osmosis and Its Significance to Living Cells

We have already seen that if a membrane is sc^lectiveiy permeable,
then some substances, such as water or certain solutes, will pass
through readily, but other
solutes may not. because
their molecules are too large
to pound their way through
the ultramicroscopic "pores"
of the membrane. The
process by which substances
diffuse through membranes
is known as osmosis. It is
of the greatest importance
to living cells, as it is by this
means that dissolved gases,
such as oxygen, and dis-
solved food substances get
into the cell, as well as the
process by which waste ma-

- sugar

^littion...

lJ» selectively
lJ.pcrroeab\^

-'-'4*

molcciclc

^ • •wat«r'mo\eculti =^ °'TS^^

-penTKoBe

=1. "WCCt&V

vater-

.XLi»

Diagram to explain osmotic pressure. Sii>:ar
solution is of equal density in ea<h tube.
Explain rise of solution in left hand tube.

terials pass out. Perhaps a further word of oxplana .on ,s u, orcler.
Other things being equal, if two soh.tions «' '''«""": ™"''™'7,;"
are separated by a permeable membrane, the chffus.on ^v,ll ^fll ,
in the direetion of the greater to the lesser ™';'-"- "' ; V^
it a sugar solution be .separated by a pern,eabIo men,b,a,.o
another more dilute sugar solution, diffusion -l'/^' ;;■',"
the more coneentrated. If, however, we separate «at,. from ..

H. W. H. — 10

136

FUNDAMENTALS OF STRUCTURE AND FUNCTION

sugar solution by a selectively permeable membrane, the water mole-
cules tend to pass through the membrane (since it is permeable to
water) from the water, to the sugar solution where the water is in
less concentration. Actually it is a question of the water molecules
of the solvent reaching an equilibrium.

Osmotic pressure, in living cells, is one of the factors that accounts
for the rise of water in roots and up the stems of plants. Its effects
can easily be demonstrated experimentally in the laboratory by plac-
ing, for example, living cells of Spirogyra in a 10 per cent solution of
salt and water. The water from within the cell (where it is in greater
concentration) passes out through the cell membrane to enter the salt
solution (where water is in less concentration than in the cell). The
result is that the cell body shrinks away from the cell w^all and the
shrunken cell is said to be plasmolyzed. A solution which contains

a greater number of mole-
cules of the substance in
solution (solute) per vol-
ume than the interior of
the cell is said to be hy-
perosmotic; if it has less
concentration than the in-
terior of the cell it is
hyposmotic; and if it has
the same number of solute
molecules per unit volume
as the interior of the cell
the solution is isosmotic to
the cell.

When a cell is placed in
a hyposmotic solution it
will tend to swell up, be-
cause water is diffusing
more rapidly inward, and so, unless the cell is surrounded by
heavy walls as in the case of plants, the cell will tend to burst.
When this happens it is called cytolysis. This may be demon-
strated when human red blood corpuscles are placed in distilled
water. It is evident, therefore, that osmotic pressure differs greatly
in the cells of different organisms, possibly depending on whether
they live in fresh or salt water and the consequent concentration of
salts present. As a matter of fact, fresh- water organisms live in a

pelliole —

.cytoplasm. .

YlllClsZJiS.

..ceU\/cdl

Plasmolysis in a plant and an animal cell.
Note how the cytoplasm has shrunk away from
the wall in the case of the plant cell and the
pellicle in the case of Paramecium. Why has
this occurred ?

LIFE AND PROTOPJASM I37

hyposmotic solution. In plants, the cell walls prevent the cells from
swelling up, while in animals there are special ways of ridding the
body cells of excess water.

SUGGESTED READINGS

Calkins, G. N., Biology of the Protozoa, Lea and Febiger, 192(). Cli. L

A full and scientific approach to the cell.
Plunkett, C. R., Outlines of Modern Biology, Henry Holt & Co., 1930. Chs.

I and IV.

A chemical and physical approach to the study of protoplasm.
Singer, C. J., The Story of Limng Things, Harper & Bros., 193 L Chs. IV

and IX.

An interesting history of biology. Chapters IV and IX deal with the

historical approach to the cell theory.
Wilson, E. B., The Cell in Development and Heredity, The Macmillan Co.,

1925.

A classic authority on the ceU.

CELLS AND TISSUES

Preview. Living things composed of cells • Plant and animal cells
differ in size, shape, and structure • Why cells divide • How plant cells
divide • How animal cells divide • Tissues • The tissues in plants; the
meristematic tissues ; the protective tissues ; the fundamental tissues ; the
conducting tissues ; the tissues in animals ; the epithelial tissues ; the sup-
porting tissues ; the muscular tissues ; circulatory tissue ; the nervous tissues ;
reproductive tissues • Why are living organisms so called? • Suggested
readings.

PREVIEW

One characteristic of living things is that they are organized into
tiny units of Uving matter which have been called, rather inaptly,
"cells," because an Englishman, Robert Hooke, as early as 1665,
described the construction of cork which he saw under a lens as
"little boxes or cells distinguished from one another." He cut
cross sections with a penknife and saw that they were "all cellular or
porous in the manner of a honeycomb, but not so regular." What
Hooke saw was the woody walls enclosing spaces which in younger
plants would be filled with living matter.

From a comparison with the simplest organisms, it is evident that
the more complex forms are built up of cells, and that, although each
cell can function as an organic whole, far more efficient results are
obtained when groups of cells organized into tissues do the work. The
consideration of groups of cells, according to their structure and func-
tion, constitutes in itself a major chapter in biological study, called
Histology. The study of individual cells, which make up the sub-
ject matter of Cytology, is absolutely indispensable to a proper
understanding of the organism as a whole.

The problems for reading and further study are so numerous that
we might spend the major part of our available time in discussing
them. Why and how do cells divide? What are the differences
between plant and animal cells? What are the reasons for having
tissues and organs? How did many-celled organisms come into
existence, and why? The pages which follow will enable the student
to make at least a start on some of these interesting questions.

138

CELLS AND TISSUES ,.,,,

Living Things Composed of Cells

A very small proportion of living plants on the earth :uc uniccliui-ir
but accordnig to Hegncr, tiio number of species of protozoa or single-
celled ammals must be nearly, if not quite, as great as all the other
species of animals put together. He bases his estimate upon the fa<-t
that practically every kind of animal has its own species of parasitic
protozoa living upon or within it. Nevertheless the mctazoa, as the
many-celled animals are called, make up most of the living animals
that we know about on earth today, just as the many-celled plants
make up the visible and familiar plant life.

Just how the many-celled forms of life evolved from the unicellular
forms is a matter of conjecture. Two theories of origin in animals
have arisen, one of which, the colonial theory, postulates many-celled
organisms evolving as colonies of cells, which hold together after fi.ssion
to form plants or animals, instead of separating into individual isf>-
lated cells. As these cell masses evohcd, they became more and
more complex, different systems of organs appearing in more highly
organized forms. In the animal series shown on ])age 146, this
theory seems to be pretty well substantiated. But another theory,
the organismal theory, considers the living thing as a whole, being
divided into units of structure in the many-celled organism. Accord-
ing to such a theory unicellular organisms would become first much
chfferentiated within their own bodies, as is .seen in many of the
protozoa. These theories need not concern us further at present.
Both have many facts to support them, substantiatetl by the devel-
opment and structure of various types of organisms.

Plant and Animal Cells Differ in Size, Shape, and Structure

An examination of the figure on page 140, will sliow that cells
are far from uniform in size and shape. They differ in size from the
smallest bacteria which can just be distinguished with an ultra-micro-
scope that magnifies 3000 diameters, to cells that can be seen with the
naked eye. The egg-cell of the chick, for example, includes the con-
spicuous yolk, while certain cells in the human spinal cord, altluiu^h
microscopic in size, may have prolongations reaching down irito the
muscles of the fingers or toes. Cells are not of n(>cessity lar-jcr in
large animals or plants, some of our largest cells being found li\ing
isolated and alone. But under normal conditions a cell of a given
size and shape always reproduces the same kind of cell as itself.

140

FUNDAMENTALS OF STRUCTURE AND FUNCTION

idL "bouiillus/

Anthro^ bacillus^

As to shapes, their name is legion. A typical cell might be thought
of as a spherical or ovoid body, but we find them cubical, flat, thread-
like, spindle-shaped, columnar, or irregular in outline. They are often

modified by being com-
pressed by other cells,
but frequently if given
opportunity will resume
their original form when
released from pressure.

Structural differences
exist between plant and
animal cells, the chief of
which is the cellulose wall,
characteristic of plants,
which gives such cells
the rigidity and yet the
flexibility found in woody
stems. Other physiolog-
ical differences will be
discussed in the following
chapters.

red. corynxsc^s^
.of f nog.

a.no5om.<a

r<Ed corp
of )inoa7

Cugle:

no.

Spex-m ofTnarj.

human
livei- cell

Comparative size of cells. The anthrax bacillus
shown is among the largest of the bacteria, while
the human liver cell is not large as cells go.
(After Wells, Huxley, & Wells.)

Why Cells Divide

Every cell has its limits
of size and when that
size is reached, if food is
sufficient and conditions
favorable, it will divide. In both plant and animal cells, the mech-
anism and the. end results reached by cell division are similar, in
that the chromatin from within the nucleus is redistributed so that
the daughter cells have approximately the same amount of chromatin
and eventually the same size as the parent cell from which they
came. Cell division is a universal phenomenon and seems to be a
part of the normal life of cells. Theories advanced to account for
cell division are (1) colloidal changes in the protoplasm of which
they are composed, (2) electrical changes within the cell, (3) oxidative
changes within the cell, and (4) changes in surface tension. The
latter can be experimentally proven by treating unfertilized eggs with
certain chemicals which cause a change in surface tension and initiate
subsequent cell division.

CELLS AND TISSUES
How Plant Cells Divide

III

Both plant and animal cells are said to divide by a proc-ss c,f n-ll
division called mitosis. In plants, the resting cell has a nuclcM.. wl,i,-l.
contains a network of linin fibers, on the strands of which are f,.,n„l
irregular chromatin granules. When the cell is activated to divide
these granules assume the form of a thickened, irregularly coiled thread'
called a spireme. This thread splits lengthwise into two thr(.-„is
which remam so close together that for some time they appear as
one, finally splitting crosswise into a number of chromo.sonus that

resLind
cell ^

prophets e. metapWe

anaphase telophase

Mitosis in plant cells. Read the text and ('\|)laiii the (li;i;:r;iiii

celll

are constant in number in all cells of a given species. While this proc-
ess is going on there has appeared in the cytoplasm on oj)j")osite sides
of the nucleus two caplike masses of delicate fibers, which later will
give rise to the so-called spindle fibers. Now the iiuchar membrane
disappears and the fibers grow into the center of the nucleus, where
some become attached to the chromosomes while others join with
fibers from the opposite side or pole. This series of changes is
known as the prophase. These two cone-shapetl ma.s.ses of fibers
form the spindle, while the split chromo.somes arrange themselves

142 FUNDAMENTALS OF STRUCTURE AND FUNCTION

in a plane in the middle, or equator, of the spindle, this being
known as the meta phase. Next the half or split chromosomes
appear to be pulled apart by the spindle fibers so that an equal
number move toward each pole, where they come to rest. These
changes are called the anaphase.

Here the spindle fibers which extended from one pole to the
other begin to thicken at the equator. The swellings grow larger,
fuse, and spread out to form a delicate plate, which eventually extends
clear across the mother cell. This cell plate is in the nature of a
plasma membrane which splits into two, forming the new cell wall
between the two new cells. The fibers of the spindle now disappear
and cell division is completed. Meantime the recently split chromo-
somes lose their identity and again take on the netlike appearance as
in the original resting cell. The last series of changes comprises
the telophase.

How Animal Cells Divide

The resting animal cell undergoes a similar process in division.
However, in the animal cell a new structure is found, called the
centrosphere, which is a small body lying in the cytoplasm near the
nucleus. A central granule, called the centrosome or centriole, is
found within this centrosphere. The centriole usually divides to
form two of these granules at the beginning of mitosis. The initial
stages of cell division, collectively called the prophase, occur when the
particles of chromatin scattered throughout the nucleus take the form
of the spireme or tangled thread. This thread thickens and shortens
and then breaks up into the individual chromosomes. The number
of chromosomes for the body cells of the individual of a species is always
constant. Among plants, for example, in the pea there are always
14, in the onion 16, and in the lily 24 ; while examples taken at
random among animals show 4 for certain roundworms, 8 for the fruit
fly, Drosophila, of which you will hear more later, 32 in one of the
common earthworms, 200 in one of the crayfishes, 24 Mn a common
locust, 24 in one of the frogs, and 48 in man.

During the formation of the spireme the threads of the future spindle
are growing out from radiations, called asters, which appear around the
centrioles. (See figure on page 143.) As the process continues the two

' This is not quite exact, for it has been found that in some animals at the time when the chromo-
somes are reduced in number in the process of maturation (see page 429) , there is an even number
in the female sex cells but an odd number in the male sex cells or vice versa.

CELLS AND TISSUES ,,,

centrioles move farther apart, the spindle fibers elongate, the n.icl.-ir
membrane disappears, some spindle fibers api)ear to attach to tlic
chromosomes, and gradually the longitudinally-split chromosomes
collect at the equator of the spindle. The next step in mitosis, known
as the metaphase, is the arrangement of the chromosc^mes. with each
split body on opposite sides of the equator of the spindle. Then the
two sets of chromosomes begin to move toward the opposite poles of
the spindle, the fibers which are attached to them getting shorter tunl
shorter. At this time comes the first external appearance of cell

6enLrosphere
- (tentrosome
^ chromatin

spireme

\nwi\ear-
fnamhrexna.
resting cell

oCiso.ppeo.'ns
-spindle thread

.CentroSome

linVm - pi'ophase

spireme sViortens
anoL thickens

and of prophase
nuclecLr membi*ane
<A-isccppe<ir-s

metxx-phaSe

arjapbase

end of anaphase

te\ophasa
nuclear membranes

ctoir^ter Cells

restivxT stage.

Mitosis in animal cells. Compare this diagram with that on page i 11.

division, a slight constriction appearing in the cell body. The
constriction in the cell becomes more evident and, as the i)rocess
continues, the chromosomes become grouped so as to form the new-
nuclei of the two daughter cells. These progressive changes are
collectively known as the anaphase. In the final stage, or Idophasc,
the two sets of chromosomes gradually lo.-;e their individuality and
become Httle masses of chromatin grouped on linin fibers in the new
nuclei around which a nuclear membrane is formed. .Meantime the
constriction in the cell has gone far enougii to form two daughter cells,
the new separating partition appearing along the line nf the (-(luator

144 FUNDAMENTALS OF STRUCTURE AND FUNCTION

of the spindle. The centriole in many daughter cells divides imme-
diately into two, although in some cells it remains as a single body
until a new mitosis begins.

Tissues

Cells form aggregates called tissues, examples of which may be seen
in the woody cells making up the greater part of the stem of a plant ;
the elongated cells in this same stem which form the conducting
tissue; the flat protective cells covering the outside of the leaf, called
collectively the epidermis; and the large columnar cells filled with
green chloroplasts that form the parenchyma layer directly under the
epidermal cells. In our own body, we find numerous examples of famil-
iar tissues set apart for doing some particular work, such as the epithe-
lial, or protective, tissues ; the connective tissues, which serve to bind
the various groups of cells together ; the muscular tissues, of several
kinds ; the supporting tissue cells, which help to build the bones ;
glandular tissues ; the nervous tissues of several kinds ; and the blood,
which, though fluid, yet contains cells, and is classed as a circulating
tissue.

The Tissues in Plants

It is a difficult matter to make a classification of tissues that will
fit all plants and yet be simple enough to use at this stage of our
biological knowledge. But the following will give us a general survey
which can later be expanded by the student of botany.

The Meristematic Tissues. These cells in general are small,
thin walled, and rich in protoplasm. They are found in the rapidly
growing parts of plants, the buds, the tips of the roots, and in growing
layers. They represent the primitive and embryonic tissues.

The Protective Tissues. Such are the epidermal cells covering
leaves. These are often waterproofed with a waxy material called
cutin. Such layers are found on the outside of the stem, root, and
even the fruit, forming a protective covering. In the stem and the
root, the epidermis is often replaced by a layer of corky cells, while on
leaves, stems, and flowers the epidermal cells frequently develop
hairs or scales, which sometimes secrete sticky substances.

The Fundamental Tissues. These groups of cells form the great
mass of plant tissue, such as the soft green parts of the leaf, the pith
or cortex of plant stems, the soft parts of flowers and fruits. These
cells differ greatly in size and shape in different parts of the plant,

CELLS AND TISSlJi:s

II.-)

but in general they are alive and act as storage cells. Some of the
parenchyma cells, called collectively collnichynw, become f liickcncd
at the corners, as seen in a cross .section, and serve as strengthcnii.n

'menStQWatiC P'^'^^'^.X"'-^ colknch^roa sderenchv^xt
tissues "tancCameatial tissues

oooo

■ Sedition

epjdermis

"plant- -, I", y, ...

... i:icxir^ :jcylem M '.phloem

prouective t-issues <:tondxxctin<^ tissues

Types of plant tissue cells.

units in the outer part of the stem. The walls of the other funda-
mental tissue cells become much thickened and are called sclcrcrichyma
cells, which may become fibrous, helping to .support the stem, while
others form stone cells making up the covering of nuts antl other
hardened parts.

The Conducting Tissues. In the liigher plants, woody bundles
of elongated cells act as tubes for the conduction of water and food
substances. The water-conducting tissues are collectivelj' known
as the xylem and consist largely of supporting dead cells (trachcids)
impregnated with a strengthening substance called lignin, and long
tubular cells (vessels) which have lost their cross walls. Scattered
amongst them are various other types of cells, including jiarenchynia.
The tissues which conduct food materials down the stem from the
leaves, where food is made, are known collectively as the phlonn.
The characteristic conducting cells of the j^hloem are known as sifir
tubes, which have perforations in the end or .side walls known as the
sieve plates. Long threads of cytoplasm pa.ss through these holes.

146

FUNDAMENTALS OP^ STRUCTURE AND FUNCTION

connecting cell with cell and making a pathway for the food sub-
stances. Small companion cells are attached to the sieve tubes.
The phloem is also provided with parenchyma and fibrous cells,
which give strength to the tubular bundle.

The Tissues in Animals

Although the histologist makes a much more detailed classification
of tissues, a convenient grouping for animals is the following :

The Epithelial Tissues. Not only do these cells form the outer
layer of the animal body, but they also are responsible for the forma-
tion of such protective body structures as the calcareous shells of

reticular odiposs

emooth.

grlonduP

^ , fibrous Z,^ ..^ -- ^ ° ^' w^- -' .^^^l?

stratifM ^.,^.:.:-:,^,:mm\ Striated:

^m^mmSiim

Columnar

epithelial
ti55ue6

vv ;.:>: -v: :. ;;,U^ loom.

Yiyoline Cartilage

supporting

■tissues

red.
© <!orpuscles

circulcctorv

r<2/procCuclive

tissu:<2^

Types of animal tissue cells. Into how many groups may they be classified ?

clams and oysters, the chitinous covering of the insect, or the outer
covering of the crayfish. These tissues line all body surfaces as
well as the digestive tract and other inpocketings of the outer body
covering. They are of the utmost importance because they also
form the glands of the body, structures which secrete, for example.

CELLS AND TISSUES , 1^

digestive enzymes or the waste products of metabolism, such as
perspiration. They also form a largo portion of many of the sense
organs of the body. In shape, the cells of epithelial tissues as they
lie side by side may be fiat, cuboidal, columnar, or even ovoid.

The Supporting Tissues. These tissues .serve to bind together or
support the various parts of the body. They include bone, cartilage,
and connective tissue, and they differ from other ti.ssues in that it is
the material formed by the cells, rather than the cells themselves, that
is of functional importance. In bone or cartilage, for example, the
supporting portion or matrix is produced by the cytoplasm of cells
and surrounds it. Fat cells are connective ti.ssue cells in which the
body of the cell becomes a storehouse for a drop of fat, the living part
of the cell being much reduced. Pigment cells are branched irregular
structures of a somewhat similar nature. Most characteristic of
true connective tissues are the white non-elastic fibers that make a
network in certain parts of the body, or form the glistening cords or
tendons \\\nQh. connect bones with muscle, or ligaments, which connect
bones with bones. Other forms of connecti\e ti.ssue that might be
mentioned are the areolar, which forms an elastic padding underneath
the skin; and the yellow elastic fibers found in the air tubes of the
lungs and the walls of arteries.

The Muscular Tissues. Motion of certain cells is produced by
ameboid movement, or by the lashing of tiny threads of protoplasm,
that is, flagella or cilia. But in higher animals movement is brought
about by the muscle cells in which the propert}' of contractility is
greatly developed. In higher animals, muscles are groups of highly
specialized cells bound together by connective tissues. There are
three kinds of muscle cells, namely, smooth, striated, and cardiac.
Smooth muscle cells are long with an outer contractile fibrillar layer
surrounding a central area of semifluid protoplasm containing a
nucleus. In vertebrate animals, smooth muscle is found i^articularly
in the walls of the blood vessels and the walls of the digesti\-e tract.
Striated muscle fibers in higher animals are groups of cells slu)wing n(»
cell boundaries and held together by connective ti.ssue. They s1k)w
curious cross striations and on the whole in man are under control (»f
" the will," hence are called voluntary muscles. A third type o(
muscle, the cardiac, is striated, but involuntary in action, making
up the tireless muscles of the heart.

Circulatory Tissue. Although the blood, lymph, and other
fluids that serve to transport foods and wastes in the body are <-on-

148 FUNDAMENTALS OF STRUCTURE AND FUNCTION

stantly in motion, we must classify them as tissues, for they contain
living cells or corpuscles of various kinds, carried about in a fluid
matrix or plasma. These tissues are of the utmost importance to
animals, as it is only by means of them that the living cells of the body
receive nourishment and oxygen, and get rid of their wastes.

The Nervous Tissues. Even in its simplest form we have seen
that protoplasm is sensitive and responds to stimuli. In higher
animals this sensitivity and conductivity of sensations is taken over
by the nervous tissues. The unit of structure is the neuron, or nerve
cell. The elongated fibers from these cells are bound together into
nerves or conducting ])athways for nerve impulses. All parts of the
vertebrate body, with the exception of the cartilages and epidermal
derivatives, are supplied with nervous tissue, which may be said to
be the master tissue of the body.

Reproductive Tissues. These cells which, as one author puts
it, are "within the body though perhaps not of the body," form tis-
sues, eggs and sperms, that have to do with the futures of all animals.

Why Are Living Organisms So Called?

In the preceding pages, we have referred to living things as organ-
isms. The anatomist calls collections of tissues, which do specific
kinds of work, organs. The hand is an example of an organ which is a
collection of tissues. Muscles are attached to the hones by means of
tendons and bones are joined together by ligaments. The skm,
composed of several different kinds of tissue cells, is supplied with
blood and nervous tissues, while the whole organ is interlaced through
and through with other connective tissues. Living things are made up
of organs, and we call them organisms. The living world about us,
plant and animal, is a collection of organisms, some very simple,
others aggregates of simple cells, still others formed of untold billions
of differentiated cells, grouped into tissues forming an organism,
such as an insect, a fish, a tree, or a man. Yet all these different and
complex entities basically are made of the living stuff called proto-
plasm. In animals, this grouping of organs which are united in the
performance of some general function gives us a number of organ-
systems. There is, for example, the integumentary system, or outer
body covering ; the supporting system, which forms the body frame ;
the systems which have to do with the nutrition of the body, the
digestive, respiratory, circulatory, and excretory systems ; the nervous
system, which controls the activity of the body ; and the reproduc-

CELLS AND J ISSUES 1 j.,

live system, which has to do witli the contiiiuaiice of Hfc It is on
the structural development of these systems, ({('velojx'd to ;i greater
or lesser extent in all of the many-celled animals, that the various
groups of the metazoa are classified.

SUGGESTED READINGS

Dahlgren, U., and Kepner, W. A., Textbook of the Principles of Animal
Histology, The Macmillan Co., 1908. Chs. I, II. and \'.

Holmau, K. M., and Robbins, W. W., Elements of Jiotany, 2iul cd., John
Wiley & Sons, Inc., 1928. Ch. III.

Maximow, A. A., Textbook of Histology, edited by W. Bloom, W. B. Saunders
Co., 1930.
Rather technical. Chapters I and II useful.

Stohr, Philip, A Textbook of Histology, .')th ctl. (arranged by J. L. Bremer),
P. Blakiston's Son & Co., 193(3.
Chapters I and II make excellent reading.

Wilson, E. B., The Cell in Development and Heredity, 3rd cd., The Mac-
millan Co., 1925.

The most authoritative text on the cell. Rather advanced, but with
excellent figures. Chapters I and II especially us(>ful.

ORGANISMS ILLUSTRATING BlOL()(;i(;\|

PRLNCIPLES

BEGINNINGS: THE LARGE CROIP OF THE
SMALLEST OHGA.MSMS

Preview. Some forms found in a drop of fresh water: Ainoba. an
animal cell ; Euglena ; Paramecium ; Diatoms : Desmids ; Bacteria ■ Func-
tional differences between plant and animal cell • Suggested readings.

PREVIEW

Over two hundred and sixty years ago, when the Dutchman, Antony
van Leeuwenhoek, examined what he called "little animals" under
his homemade microscopes, he made the first real exploration of a
drop of water ever attempted. His microscopes were simple affairs,
consisting of a single lens. They had no tube or mirror such as our
microscopes of today have. When objects were examined they had
to be brought into position and focus through the use of rather coarse
screws.

Besides being the first person actually to see the capillary circulation
of the blood (a thing that Harvey knew must be so, but which he wa.s
unable to prove), van Leeuwenhoek made numerous other llhJ^sio-
logical and anatomical observations which gave him the title of
"founder of histology." One thinks of him most often as the first
man who saw protozoa, unicellular plants, and own bacteria in
standing water.

Let us read his own description and judge for ourseh-es a.s to what
he saw. The following extract is taken from a letter written on
October 9, 1676, to Henry Oldenburg, First Secretary of the Koyal
Society of London. It describes the finding of "little animals" in a
drop of rain water.

"Of the first sort that I discovered in the said water, I saw, after divers
observations, that the bodies consisted of 5, 6, 7, or 8 ver>' clear globules,
but without being able to discern any membrane or skin that held these
globules together, or in which they were inclosed. When these aninuilcules
bestirred 'emselves, they sometimes stuck out two little hnrns, which were
continually moved, after tlie fashion of a horse's ears. The i)art l)etween

H. W. H. — 11 131

152 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

these little horns was flat, their body else being roundish, save only that it
ran somewhat to a point at the hind end ; at which pointed end it had a
tail, near four times as long as the whole body, and looking as thick, when
viewed through my microscope, as a spider's web. At the end of this tail
there was a pellet, of the bigness of one of the globules of the body ; and
this tail I could not perceive to be used by them for their movements in
very clear water. . . .

"I also discovered a second sort of animalcules, whose figure was an oval ;
and I imagined that their head was placed at the pointed end. These were
a little bit bigger than the animalcules first mentioned. Their belly is flat,
provided with divers incredibly thin little feet, or little legs, which were
moved very nimbly, and which I was able to discover only after sundry
great efforts, and wherewith they brought off incredibly quick motions.
The upper part of their body was round, and furnished inside with 8, 10, or
12 globules : otherwise these animalcules were very clear. These little ani-
mals would change their body into a perfect round, but mostly when they
came to lie high and dry. Their body was also very yielding : for if they so
much as brushed against a tiny filament, their body bent in, which bend also
presently sprang out again ; just as if you stuck your finger into a bladder
full of water, and then, on removing the finger, the inpitting went away."

His description of the cause of movement in his little creatures is
amusing, yet it shows that he saw cilia plainly and estimated their
size quite clearly.

"But many of the things we imagine, and the natural objects that we
inquire into, are very insignificant; and especially so, when we see those
little living animals whose paws we can distinguish, and estimate that they
are more than ten thousand times thinner than a hair of our beard ; but I
see, besides these, other living animalcules which are yet more than ten
thousand times than a hair of our beard ; but I see, besides, these other
living animalcules which are yet more than a hundred times less, and on
which I can make out no paws, though from their structure and the motion
of their body I am persuaded that they too are furnished with paws withal :
and if their paws be proportioned to their body, like those of the bigger
creatures, upon which I can see the paws, then, taking their measure at but
a hundred times less, it follows that a million of their paws together make
up but the thickness of a hair of my beard ; while these paws, besides their
organs for motion, must also be furnished with vessels whereby nourishment
must pass through them." '

Van Leeuwenhoek was made a member of the Royal Society for his
clear reports of what he saw and at his death he had sent the Society a

1 Dobell, C, Antony van Leeuwenhoek and his "Little Animah," pp. 118 and 180, Harcourt,
Brace and Co. By permission of the publishers.

THE LARGE GROUP OF THE SMALLEST ()IU;\NISMS m

case containing 26 of his microscopes, a gift which was later lost ( )„,.
of the few remaniing of the 419 lenses put up at auction after van
Leeuwenhoek's death was recently examined by an expert who
reported that the biconcave lens that he inspected "was very good
indeed" and proved that its maker had attained "a very high degree
of proficiency in grinding extremely small glasses."

With the modern microscope of the college laboratory, infinitely
better work can be done than with this old pioneer. The best of \an
Leeuwenhoek's lenses are said to have magnified not more than 270
diameters, while the " high dry " power of the average modern micro-
scope gives a magnification of about 440 diameters, so that the college
freshman today has a far better physical equipment than did this
famous Dutchman. He also has much more. In the years that have
intervened between the time of van Leeuwenhoek and the present,
patient observations of minute forms of life ha\-c been made by
hundreds of scientists whose results may be found in these pages and
in other books suggested for collateral reading. With this intro-
duction the student might begin the study of simple organisms in
some such way as Antony van Leeuwenhoek did, by examining a
drop of pond water.

Some Forms Found in a Drop of Fresh Water

The pages that follow^ will serve to give us a slight acquaintance
with some of the simplest plant and animal forms that are likely to be
met in the examination of a drop of pond water or water from a
laboratory aquarium. Li addition to the unicellular organisms,
scores of other higher forms are likely to be seen. Countless protozoa,
including the many tiny species of monads, dart across the field of the
microscope ; others many times larger, with their highly specialized
cell parts, as Euplotcs or Stylonychia, may be found browsing on tiny
plants. Frequently one also encounters threads of the filamentous
algae, Zygneyna or Sjpirogyra, while debris, consisting of tiny bits of
wood, sand grains, and the glasslikc cases of diatoms and desmids.
may abound.

Many tiny crustaceans, water fleas, and cojx'pods are usually
present, and in addition one finds the easily recognizable rotifers,
with their whirling wheels of cilia, their prominent grinding organ
or mastax, and their slender toelike posterior foot by means of which
they often become attached to .solid objects. Sometimes a small
roundworm may be found working its way through the dt^bris. while

154 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

hyaline cap..

pseuctopodiam

\
ptemagsl.m o _,

many types of insect larvae and pupae may also be seen. This brief
list includes only a few of the many new acquaintances to be found
in a drop of water.

AmebOj an Animal Cell

Ameba is the classic representative of a single-celled animal which
illustrates the action of living protoplasm. Found in ooze taken
from the bottom of small ponds or sluggish streams, it is seen to be

an irregular and almost
transparent cell. When
in motion the protoplasm
of its body apparently
flows out into newly
formed bulging projec-
tions of the body called
pseudopodia (Gk. pseu-
dos, false; pous, foot).
The cell body consists of
two substances, an inner,
more fluid, granular por-
tion, the endoplasm and
a more viscous area, the
ectoplasm, on the outside.
The whole Ameba is
surrounded by a deli-
-foodvacuole cate plasma membrane.
When the animal moves,
the protoplasm appears
to flow into the pseudo-
podia. According to S.
O. Mast of the Johns
Hopkins University,
when an Ameba is mov-
ing in a given direction the endoplasm sol pushes out in a pseudopo-
dium and becomes changed to a gel, the "gel" at the other end of the
cell becoming a "sol" that moves into the cefl body. This illustrates
a characteristic of protoplasm mentioned earlier.

This cell, like others of its kind, has a nucleus containing chromatin.
Certain vacuoles are present, some of which are filled with a watery
fluid, others hold food in different states of digestion, while a single

---■nuclexcs

li.L.-fooct vacuole.

Contractile,
vacuole—

Ameba proteus. The direction of progress of
the cell is shown by arrows. What happens to
the protoplasm in the extreme anterior end during
movement. (After Mast.)

THE LARGE GROUP OF THE SMALLEST OI\(;\Ms\is i:..-,

vacuole, called the contractile vacuole, rhythmically collects and expels
fluid. The function of the contractile vacuole may he to eliminate
wastes from the cell, or it may have a hydrostatic function, that is,
it may control the amount of water contained in the ccjl. Food
particles are actually ingested or taken into the cell by the proto-
plasm which flows around the food, engulfs it, and then surrounds it
with digestive fluids in a food vacuole.

A recent series of observations by Mast and Hanliart ' indicate
that the Ameba selects certain kinds of food, ])referring, for instance,
Chilomonas to Monas, although both are flagellates of about the
same size, form, and activity. It was further sliown that Monas
was not digested in the food vacuoles, while Chilajnonas was, and
also, some organisms, such as mold spores, certain algae, and other
flagellates, might be eaten but were not digested.

The process of constructive and destructive metabolism may take
place in a single cell. Indigestible waste materials are pa.s.sed out
any^vhere from the surface
of the cell body, while
respiration takes place by
means of an osmotic ex-
change of the gases, oxy-
gen and carbon dioxide,
through the cell mem-
brane.

As a result of the taking
of food, the cell gradually
increases in size and then
divides by a process known
as binary fission. Accord-
ing to a recent study by
Chalkley and Daniel - the
division of the nucleus
shows the typical stages
of mitotic division, the

entire process lasting, under normal temperature conditions, about
half an hour. During the process the Ameba is quiescent and the

late prophorse.

mid- anaphase

Gccriy anaphase

•metaphose

Mitotic division in the nucleus of Vruflia. i, After
GliiilklcN ;uni Daniel. '

iMast, S. 0., and Hanhart, W. L., " Feedins, Digestion,
(Leidy)." Phusiol. -Zool.. Vol. 8, lO.'?."). Pp. 2,5,5-272.

-C
and the
Pp. 592-619.

and StarviiliiiM in .■inwrbii pmlrw

Iv)." Ph„.no}. 'Zool.. Vol 8. \9^r,. Pp. 2,5,5-272. ,i„„,vll

Chalkley, H. W.. and Daniel. G. E.. "The Relation between the lorn, of he I. v .« r 1
the Nuclear Pha.ses of Division in Amoeba protcu.. (Leidy). Phmol. looU \o\. rt. !..,«•«.

156 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

pspiidopodia are relatively small. After the nucleus divides, the cell
body separates into two equal parts, each of which grows into a
full-sized individual.

Euglena

Although Ameba is usually looked upon as the simplest of all animal
cells, there is another group of organisms containing equally simple

forms, making up a large

flagsllum
cytostome/.

stigma .^^<a.

I^lagsllcu- granule

rs-Ser-vDirr

basal granule

Contractile,
vacuoles -•'-

nuole-us —

Central bcx^

cbromatopbore
pyrenoid

striation$ —

lent

Euglena viridis. Read your text and give
the functions of each of the structures shown.
Note that the drawing makes the cell appear
flat whereas in cross section it is oval. What
evidence of holophytic nutrition is seen in
this diagram ? (After Hegner.)

proportion of the microscopic
plankton of the ocean and
bodies of fresh water. This
grotip, w^hich comprises one
of the classes of the Phylum
Protozoa, includes the Masti- .
gophora, or flagellates, cells
that move by means of one or
more long, whiplash threads
of protoplasm. Certain of
the Mastigophora bear a close
relationship to plants, and the
organism Euglena, selected as
a representative of the group,
is often claimed as a plant cell
by botanists. Euglena may
be found in shallow and some-
times temporary freshwater
ponds, where it often grows
with such rapidity as to give
a dull greenish color to the
water. When unfavorable
conditions set in, the organism
settles to the bottom, becomes
covered with a resistant coat

or cyst, and is only recalled
to active life by a recurrence of favorable environmental conditions.
Some species of Euglena have conspicuous spiral markings on the
surface of the body, which is roughly ovoid, with a depression at the
anterior end, called the gullet. A single flagellum has its origin near
the base of the gullet, in the form of a long axial filament anchored
in the protoplasm, that gives the filament free movement. By

THE LARGE GROUP OF rilE SMALLEST OUCXMSMS ir,7

means of a rotary movement of the Hagelluni, the cell is pulled forward
on a spiral course, which is caused partly by the way the flaRellum
moves and partly by the irregular shape of the cell. At the same
time a current of water is swept into the gullet, bearing with it par-
ticles of potential food. The niembranous covering of the bod\-
allows the shape of the cell to change, often moving by what is known
as euglenoid motion, that is, by a wave of contraction over tlie whole
body, thus causing a slow movement like that characteristic of Ameha.

Although some species of Euglena appear to ingest the food i)ar-
ticles that are swept into the gullet, the ordinary nutrition is tiie same
as that of a green plant. The imuM- protoplasm of the cell is filletl
with chloroplasts (chromatophores) by means of which the raw mate-
rials, water, carbon dioxide, and mineral salts, are synthesized into
food, thus storing the energy of sunlight. Different species of Euglena
are sensitive to different degrees of sunlight and are found to turn
towards a source of light, the anterior part of the cell, which contains
a red "eyespot," being most sensitive to the light stimulus. When
they are exposed to strong sunlight, they change their direction,
coming to rest in an area of moderate or "optimum light." Respira-
tion is carried on as in any other unicellular form by exchange of
gases through the membrane covering the body. During the period
when starch is being made in the sunlight enough oxygen is released
within the cell body to supply its needs. Excretion of waste products
appears to be taken care of by a number of very small contractile
vacuoles, that collect fluids from the cell, eliminating them period-
ically into a small reservoir which empties into the gullet. The
individual cell in some respects acts hke a plant, and in others like an
animal. It is a borderline representative, and as such must be
regarded as a very primitive organism.

Reproduction takes place as in other sim])le forms by fission, the
free-swimming cell splitting lengthwise*. The si)Iit begins at the
anterior end, the two new cells finally having the same structures as
are found in the parent cell. In some species of Euglena that encyst,
the cell divides by fission during the quiescent period, so that two
or more cells are eventually released from the cyst. In some instances
as many as 32 have been released from a single cyst.

Paramecium

Although protozoa are single cells, some representatives ..f the
phylum are much more highly specialized than tiie snnple Ameha. o.

158 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

Euglena. These living cells may often be seen with the naked eye as
whitish specks, moving slowly near the surface of a laboratory hay
infusion that has been standing for some time. There are several
different species commonly found, some larger than others, although

in a drop of infusion
much variation in size
within the same species
may be found. The
class, Infusoria, con-
tains a large number of
forms, one of which,
longitudinal fiber Paramecium, or the
— -trichocvst "slipper animalcule," is

.ei\

IOC

^basal i^TamAs.

1-^- - thread of attachment

Diagram showing structure of the pelHcle in Para-
nicciurri nnillimicronucleata. Under high power
of the microscope the peUicle is seen to form minute
hexagonal areas, from the center of each of which a
cilium protrudes. The cilia arise from basal gran-
ules (microsomes) which are located on strands of
protoplasm (longitudinal fibers). Where do the
t richocysts lie with reference to the cilia .•* Where
are the openings through which the trichocysts are
discharged? (After Lund.)

very common. It has a
somewhat flat, elliptical
body with the anterior
thinner end more blunt
and the broader poste-
rior end more pointed.
The cell body of Para-
mecium is almost trans-
parent and is made up
of an outer, non-granu-
lar layer, the ecto'plasm,
and an inner semifluid, granular layer, the endoplasm. The ectoplasm
is covered with a delicate, elastic, but lifeless covering called the
pellicle. Under it is the living cell membrane and through the pellicle
project numerous threads of protoplasm, the cilia, which are distrib-
uted over the surface of the body in regular rows. The cilia are
quite uniform in size except at the posterior end of the cell, where
they are a little longer. It is by means of a lashing movement of
these cilia that locomotion takes place. Embedded in the clear ecto-
plasm are also fotmd nimierous defensive structures, called trichocysts.
Under certain conditions, delicate filaments or threads are discharged
from them which serve as organs of offense and defense. It is be-
lieved that they may contain minute quantities of poison which
paralyzes other protozoa.

On one side a depression, the oral groove, runs diagonally from the
anterior end of the body to about the middle. This oral groove ends
in a gullet, which in turn leads to the interior of the cell. The

THE LARGE GROUP OF THE SiMVLLEST ()IU;\MsM<

!■)<»

diagonally beating cilia which cover the body cause the rotation of
the Paramecium on the longitudinal axis. Since the cilia in the oral
groove are longer and capable of more vigorous motion, th(> b<,<lv
tends to swerve toward the left. As the water passes down the <,ral
groove towards the gullet, the waving undulatinq mnnhranv f<,r,ne,l
of ciha fused together, guides particles of potential food down the
gullet by means of its wavelike motion. At the inner e.id f..od
vacuoles are formed within the body. The food vacuoles an<l other
granular inclu.sions shift about in a definite course within the cndo-
plasm of the cell. Gradually the food particles within a given vacuole
are digested by means of
enzymes formed in the endo-
plasm and released into the
food vacuole. The digested
food material is absorbed
into the protoplasm, there
to build up living matter or
to be used later in the release
of energy. Food wastes are
passed out of the cell through
the anal spot. Excretion of
wastes may also take place
through the cell-membrane
by diffusion, or through two
contractile vacuoles, one at
each end of the cell, which
consist of a central cavity
with canals radiating out
from it into the endoplasm.
Many experiments have
been made to test the sensi-
tiveness of Paramecium to
various stimuli. As in other
living cells responsive to
stimuli, factors of the envi-
ronment have a distinct in-
fluence upon its movements.
Paramecium swims in a spi-
ral course partly as a result of its shape and the arrangement and
diagonal beating of the cilia, and partly on account of the anteriorly

anLericfT end.

i pellicle-

ccxTial

■^^CXOLCole

— ectoplccSm.
•--endoplasm

--oral droavQ,
- -moLcth

;- gullet..

....■Cood,\roc\jio\Q.

arxxl ^pot

Contractile ,,

^_^ -.tricHocy^t

.;\^,,<o^..„ cilicx.

'"'^<:'.. posterior ond.

Internal structure of Pdrdnirritirn attula-
lurn. The cilia cover the cut ire surface of
the cell and are somewhat Ioiiltit .iI I In" jmis-
terior em].

160 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

pointed groove which turns the cell to the left as it progresses through
the water. When moving into an unfavorable environment or hitting
against a solid object, Paramecium reverses the direction of its ciliary
lashings, backs away, and goes forward again in a slightly different
course, repeating the performance until the obstacle is eventually
avoided. Other reactions take place with reference to light, gravity,
heat, dissolved chemicals, electricity, and water currents, all of which,
whether positive or negative, are co-ordinated by means of a so-called
neuromotor mechanism within the cell that enables it to adjust itself
to its environment. Under careful methods of staining a number of
very minute fibrils may be found in the cell which arise in a central

I I

Binary division of Paramecium caiidaiiim. Note the position and structure of
micro- and niacronucleus in I. Follow these structures to the formation of the
daughter cells (IV). Do both micro- and macronuclei divide by mitosis? What
other changes take place in the cells .3 (After Hegner.)

body near the nucleus and radiate out to the bases of the cilia. This
apparatus apparently aids in co-ordinating the action of different
parts of the cell.

Occupying a central area in the cell are two denser bodies, the
larger, knowni as the macronucleus, has to do with the metabolic
activities of the cell, while the smaller, or micronucleus, contains the
chromatic material which is associated with heredity.

In a hay infusion Paramecia may be found dividing by simple
fission. In this process both macro- and micronucleus elongate, and
then divide. ' A new gullet buds off from the original one, two new
contractile vacuoles appear, and the cell, which has been constricting
in the middle, pulls apart to form two new cells. This process may
continue for a good many generations where food is plentiful and
conditions of life favorable. Woodruff has kept one culture of Par-

THE LARGE GROl P OF THE SMALLEST ()U(;.\M.sM>

amecia in his laboratory at Yalo University lor thirty yoars and (hir-
ing that period over twelve thousand generations vv.t,. I,,-,,! I.y fissi,,,,
It has been observed in these cultures, however, (haf after 4() or inor.-
divisions have occurred, a i)rocess called cndomixis takes plac.-. in
which the old active niacronucleus is replaced by a new one made

12-

Endomixis in Paramecium aurelia. The normal condition of Parainociiiin is
shown in I showing niacronucleus and two niicronuclei. Follow throufrli the
series pictured. What happens to the niacronucleus? How many iniiTonuclci
are formed? What, happens next? Note in IV that only one daughler cell is
shown. How does this cell obtain the normal number of niicronuclei? Where
does the new niacronucleus come from? This rhythm of cell actixity seems to
occur with considerable regularity every 10 to .^0 generations and it gi\ es the new
macronucleus chromatin from the reserve sujiply held in the micronucleiis. This
process does not appear in all ciliates and is not beliexed to be necessary for
normal growth. (After Hegner.)

from chromatin of the reserve micronucleus. This process is similar
in many respects to conjugation, except that no foreign chromatin
is added.

Under normal conditions, another process known as amphimixis or
conjugation takes place somewhat resembling the sexual procc^^ses of
higher animals. Two cells come to lie with their gullet surfaces next
to each other and a bridge of protoplasm forms between them, \\hile
this is going on the micronucleus in each cell moves away from the

162 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

macronucleiis, elongates and divides twice in rapid succession. Three
of the micronuclei thus formed in each cell disappear, but the fourth
one divides again. In this last division two irregular masses of chro-
matin are formed. This process has been likened to a similar division

inr

"Xcr

Conjugation in Paramecium caudafiim. Shortly after the conjugating pair
come together with their ventral surfaces opposed (I) a protoplasmic bridge is
formed, the macronucleus breaks down (II) and each micronudeus divides a
second time (III). What happens to three of the four micronuclei? Compare
this stage with the figure on page 429 (maturation). Next the micronuclei
remaining in the cell divide into two, the smaller (migratory) micronucleus passing
over by the protoplasmic bridge into the opposite cell, there to unite with the
larger (stationary) nucleus (VI). Trace the subsequent divisions of the fused
micronucleus (VII, IX). How do we get back to the original cell condition?
(X-XIV). (After Hegner.)

that takes place in the eggs of animals, at the period known as matu-
ration, when the sex cells are losing part of their chromatic material
in preparation for fertilization of the egg by the sperm cell. The
smaller mass is thought to correspond to a sperm cell of the many-
celled animals, while the larger one corresponds to the egg cell. In
any event, each of the smaller micronuclei migrates reciprocally over

THE LARGE GROUP OF THE SMALLEST ()H(;.\MSMs

i«.:{

the protoplasmic bridge, and unites with the larger niicroiiuclcus of
the cell left behind. The two conjugating cells now separate, and the
newly fused nucleus, composed of a male and female microinu'lcus,
is left in each cell. Then a series of divisions of this nucleus takes
place until eight nuclei are formed, four of which become macro- and
four micronuclei. Three of the micronuclei next disintegrate, leaving
the cell with four macro- and one niicionucleus. The latter divides
again and with it the cell, so that two cells result, each witli a inicro-
and two macronuclei. A second division leaves the daughter cells
each with a single macro- and micronucleus, which, thus rejuvenateil.
start off on a series of several hundred cell divisions until another
period of old age comes on, when conjugation or endomixis is repetited.

Diatoms

These beautiful microscopic plants, sometimes called "jewels of the
plant world," are among the most numerous of the one-celled plants.
Over 2000 species have been identified and named. They form one
of the most abundant components of plankton in
both fresh and salt water, and are also found in
damp earth and on moist rocks, where they may
occur singly or massed together in groujis. Certain
species stick together because of a gelatinous ma-
terial which they secrete. Some diatoms move
with a slow gliding motion when they are in con-
tact with solid objects, although lacking visil)le
organs of locomotion. They secrete a glasslike
shell exquisitely marked by tiny ridges and rows
of extremely minute holes.

Diatoms have been, and still are, among the
most abundant of li\ing organisms. So abundant
were they in past ages that large deposits of their
shells exist in the form of diatomaceous earth.
In California, there are deposits of diatomaceous
earth lying hundreds of feet thick over an area of
many square miles, while the floor of the ocean is
covered with ooze made up of skeletons of diatoms,
which after death sink to the bottom of the water.
This diatomaceous material is used as a basis for i^ohshnig lu.wders
in the manufacture of bacteriological filters, and of certain kinds o\
porcelains and glass.

The (lialoiii \(i-
vinild («) Niilxt-sidf,
{h^ ;:inilc side. sIkiw-
inf.' IIk' rcliilion of
tlic\;iK('s. ThiMiii-
clfiis iiiid the two
rililmiilikc chlorn-
pliislsitrc iiol sliowii.
(After Plil/«T.i

164 ORGANISMS ILLUSTRATING RIOLOGICAL PRINCIPLES

One of the most common diatoms found in pond water is Navicula.
In this form the cell wall consists of two valves, one of which fits into
the other. The part that fits over the inner valve is called the girdle.
The cell appears quite different in structure when seen from the valve
side or the girdle edge. In the latter view, a bridgelike mass of pro-
toplasm containing a nucleus appears, while in a valve view a line
running down the center, called the raphe, is seen, that shows three
tiny spots, one in the middle and one at each end. A mucilaginous
material exudes through a series of pores which form the base of the
raphe. Navicula has two chloroplasts, colored yellowish-brown by a
pigment called carotin. These can be seen best when the cell is
viewed from the flat side. At the time of cell division, the chloro-
plasts first increase in size, pushing the two valves apart so that they
barely touch. Then the nucleus, chloroplasts, and cytoplasm of the
cell divide, an inner valve forming for each cell. Each of the new
cells thus formed is much smaller than the parent cell.

Desmids

Another one-celled form common in fresh water is the bright
green desmid, Closterium. Like diatoms, desmids are of various
shapes and sizes. They are beautiful symmetrical structures with

large, bright green chloro-
plasts, which may be lobed,
starshaped, or platelike. The
cell wall is thin and transpar-
ent, the granular protoplasm
within being obscured by
chlorophyll, but the nucleus,
in the center of the cell, may
be easily recognized.

Desmids divide by a simple
transverse splitting, forming
two cells, each new desmid
consisting of half of an old
cell from which an entire cell
is formed. In addition, a process of conjugation takes place, in
which two cells come together, each sending out a protoplasmic
protuberance that forms a connecting canal. The contents of the
two cells meet in this tube, fuse, and form a single cell which grows a
thick wall, whereupon it remains as a dormant spore or zygote until

Closterium.

Two cells undergoing conjuga-
. tion.

THE LARGE GROUP OF THE SMALLEST OlUiVMsMs |r,.-.

conditions are favorable for germination. When the zyjrotr .l,„..s
germinate, two new individuals come direetly from it.

Many other forms of algae may be found in fresh and .s.h w.-.i.t.
Some, like Scenedcsmus, occur in colonies, their end cells being ..ftcn
provided with characteristic spines. Another colony of gr,.,.,, cells
Pediastrium, made up of a flat plate of sixteen cells, is also frecpiently
seen. These are only a few of the many forms of green algae that
may be found in a drop of water debris tak(Mi from a (iniet poml
bottom.

Bacteria

Various kinds of bacteria are common in a drop of i)ond water or
hay infusion. They are sometimes seen moving through the water,
but more often are massed together in a scum covering the surface

""a^o?. O^tfJ /f#^^'

cocci

op oo oo

«■& S?

QO QO ^

ig^Hfl GO .. ao

rmcrococcA diplococci staphylococci streptococci
Forms of l);icleria.

of the water. Three large groups of bacteria ha\-e been established
according to their shape, coccus, baccillus, and spirillum. The coccus
or spherical-shaped bacteria may live singly, as micrococci. Anotlier
form, the diplococci, divides and remains attaclied s(j as to form
pairs ; a third, streptococci, reproduces to form chains ; while a fourth.
staphylococci, forms irregular groups of eight cells or more, resem-
bling a bunch of grapes; Sarcina divides in three directions to pro-
duce cubical packets. The rod-shaped bacteria, or bacilli, \i\ry a
good deal in size and shape, as well as in tiieir ability to form spores,
some being very short, others many times longer than wide. The
third type, comprising the spirilla, are cur\e(l or twisted in shape,
and move through the water rapidly by spiral movement. This
form can often be seen hi a droj) of pond water or hay infusion.
BacilH and spirilla move by means o( Jlagdla, protoplasmic threads

166 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

which are difficult to see except under the highest power of the
microscope.

The cell wall of a bacterium is usually considered as a selectively
permeable membrane, very delicate, and secreted by the cytoplasm.
A gelatinous capsule may be formed by some bacteria, so that groujis
of them clump together in masses. Although pigments are often
present, bacteria contain no chlorophyll, and consequently most of
them are dependent on other organisms for their food. They feed
both on living and dead organisms, using not only organic foodstuffs,
such as starches, sugars, and proteins, but even leather or wood.
Since their food must be liquid in order to be absorbed, they form
digestive enzymes within the cell which exude to digest the food out-
side of the cell body.

In addition to these foods, bacteria need certain mineral salts that
are found in protoplasm, water, and nitrogen in a usable form. Not
all bacteria are capable of nitrogen fixing, but many obtain their
supply of nitrogen for tissue building as green plants do, in the form of
compounds of ammonia or nitric acid.

The chromatin material is scattered through the cell, there being
no distinct nucleus in most bacteria. Bacteria need moisture, a favor-
able temperature, and food, in order to grow. Under favorable con-
ditions they multiply with great rapidity by simple fission. Under
unfavorable conditions, many bacterial cells can contract, lose con-
siderable water, and form resistant coats, thus making spores, which
can stand extreme conditions of dryness and temperature. While
bacteria are usually killed by heating to 100° C, some spores can
withstand this temperature for long periods.

Functional Differences between Plant and Animal Cells

A comparison of the several types of unicellular organisms described
might seem at first to show hard and fast distinctions between plant
and animal cells. Although chlorophyll is associated with plants, it is
sometimes found in borderline animals, while many plants, such as
the fungi and bacteria, lack chlorophyll. Locomotion is not exclu-
sively an animal characteristic. Some animal cells, as Vorticella, are
fixed during a part of their life history, while many unicellular plants
move freely through the water. Other plants, although fixed for part
of their lives, produce sex cells that are motile in water. The greatest
difference exists in methods of nutrition. In the green plant cell, for
instance, food substances are made inside the cell in the presence of

THE LARGE GROUP OF THE SMALLEST ORGANISMS 167

sunlight while in animal cells, food is made outside and has to be
absorbed before it can be used. The method of nutrition used by the
green plant is called holophytic, and that of the animal cells, holozoic.
The differences between these two types of nutrition are summed up
in the table below.

Animal Cell

Plant Cell

No chlorophyll

Chlorophyll present

Cannot make organic foods

Can synthesize organic foods out of law

food materials

Only source of energy is organic food

Source of energy is the sun

Ingests solid food

Cannot ingest solid food

Usually moves about after food, therefore

Does not ordinarily move about, and uses

greater destructive metabolism

sun's energy, therefore greater construc-

tive metabolism

Depends on other organisms for food

Supplies other organisms with food

SUGGESTED READINGS

Calkins, G. N., Biology of the Protozoa, Lea & Febiger, 1926. Chs. I, III,

and IV, especially.
Dobell, C., Antony van Leeuwenhoek and his "Little Animals," Harcourt, Brace

and Co., 1932.

The entire book, which contains excellent translations of most of the

original letters of van Leeuwenhoek, is well worth reading. It is a

most authentic picture of this interesting Dutchman and his times.
Giltner, W., Textbook of General Microbiology, P. Blakiston's Son & Co., 1928.

Ch. III.
Holman, R. M., and Robbins, W. W., Elements of Botany, 3rd cd., John

Wiley & Sons, Inc., 193(3. Ch. X.
Locy, W. A., Biology and Its Makers, Henry Holt & Co., 1908. Ch. V.

An excellent historical survey.
Needham, J. G., and Lloyd, J. T., Life of Inland Waters, 2nd ed. Charles C.

Thomas, 1930. Ch. IV.

Excellent descriptions and illustrations of the life found in pond water.
Singer, C. J., The Story of Living Things, Harper & Bros., 1931. Ch. IV.

An interesting and authentic history of biology.
Ward, H. B., and Whipple, G. C, Fresh-Water Biology, John Wiley & Sons,

Inc., 1918.

This book is invaluable for reference. Chapters VI, IX, and XVII arc

especially useful.

H. w. H.

VIII

THE DEVELOPMENT OF SEXUALITY IN PLANTS

Preview. The beginnings of sex in the algae • Oedogonium • A repre-
sentative fungus • Alternation of generations in the plant kingdom • Sug-
gested readings.

PREVIEW

The one unescapable fact that stands out in the observation of
plants and animals in the world about us is the remarkable variety
among living things. They range from tiny forms too small to be seen
with the unaided eye to huge organisms such as elephants or trees.

The biologist is not satisfied with random looking. He looks for
certain things, tries to interpret what he sees, but as Thoreau once
said, "We must look a long time before we can see." One of the
striking facts already noted in the Roll Call of forms of life is that
both plants and animals may be placed in groups having similar
characters, and that these groups arrange themselves in a series of
gradually increasing intricacy of structure, which goes hand in hand
with an ever increasing complexity in functions. Simple plants or
animals do things simply. Almost any part of the one-celled Ameba
can do any part of the work of the cell although lacking organs found
in higher forms. More refined ways of doing things, and a more'
efficient division of work, come with increasing complexity of organic
structure. The true investigator is ever alert to find forms that
illustrate this increasing division of labor, and is always asking why
and how such things come about. Biologists have picked out certain
representative forms that clearly suggest certain facts and principles
that are worth knowing. It is possible, for example, through the
study of some simple forms of organisms, such as the Thallophytes,
to discover the beginnings of sexuality in plants.

The Thallophytes include most of the simplest plants and are
divided into two great groups, algae and fungi, the latter containing
no chlorophyll. Wliile there are six classes of algae, four, namely,
the blue-green, the green, the brown, and the red, are classified
largely on color. All of the four groups are essentially water-
loving plants, showing in many ways that they are simple and
rather primitive organisms. In size they range from tiny uni-

168

THE DEVELOPMENT OF SEXUALITY IN PLANTS

169

cellular forms to some of the great brown seaweeds, or kelps of
the California coast which may be several hundred feet in length.
Ascending the scale of increasing complexity in structure, we find the
appearance first of sex cells and later of sex organs evolved to form
and protect these sex cells.

By selecting other representatives from the higher plant groups,
such as mosses, ferns, and flowering plants, we can follow this evolu-
tion of sex through the entire plant kingdom. The pages that follow
will at least give us a start on the answer to the question : How and
where does sex originate in plants and what is its meaning ?

The Beginnings of Sex in the Algae

Pleurococcus, or Protococcus as it is sometimes
called, is one of the simplest of all living plants,
familiar to most of us as the green "moss"
usually seen on the north side of trees. Indians
used it to find their direction through the forest,
as persons lost in the woods do today. Its
habitat suggests that the life of the plant has
direct relation to moisture, temperature, and
light. It would be injured by the direct rays
of the sun, because some rays such as those of
ultraviolet light are injurious to unprotected
protoplasm.

The cell of Pleurococcus is very simple as seen
under a microscope. It is found single, in twos,
threes, fours, or flat colonies of several cells
hanging together. Examination of a single
cell discloses the presence of a thin wall sur-
rounding a mass of green protoplasm, the protoplast, which almost
completely fills the cell. If a drop of iodine solution is placed under
the coverslip, the detailed structure of the cell becomes more evident.
The nucleus is completely surrounded by one large, spherical chloro-
plast. The cell is a complete entity, in spite of the fact that it is
often attached to other cells. Physiologically it is able to carry on
all the functions of a living green plant, making food, and digesting
it as well as absorbing food and water. It grows to a certain size
and then reproduces by simple fission, part of the mother cell going
into one daughter cell and part into the other. Theoretically the

Reproduction in Pleu-
rococcus. Each cell is
considered as an indi-
vidual, although colonies
(seen above) may be
formed. The protoplasm
of the cell body is not
shown, the single chloro-
plast being surrounded
by protoplasm in active
cells.

170 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

protoplasm of the pleurococcus is immortal, since it
passes from cell to cell by means of cell division.

Spirogyra is one of the multicellular green algae.
It is a slimy thread, called "pond scum," found near
the surface of a pond, often buoyed up by bubbles of
gas which it forms. The filamentous plant body
consists of several cells joined end to end, each with a
characteristic spirally-banded chloroplast.

Examination of a single cell shows a colorless cell
wall, the cytoplasm of the cell mostly adhering to its
inner surface. Strands of cytoplasm radiate from a
central colorless nucleus, which is suspended in a large
vacuole or sap cavity. The most characteristic fea-
ture is the large twisted chloroplast, on which are
scattered many pyrenoids, bodies which contain some
of the starch manufactured by the chloroplast. In-
dividuals grow in size by forming, through transverse
division of the cells, longer or shorter filaments, de-

A Spirogyra pending upon the environmental conditions.

cell showing the
spiral chloro-
plast containing
pyrenoids, and
the nucleus.

At certain times in the year, the plants form resting
spores called zygospores. Two adjoining filaments
come to lie parallel, the cells opposite to each other
sending out bulging outgrowths which meet to form a
connecting tube. Meantime, owing to the dissolving of the cell wall
at the end of the outgrowths, water gets inside of the cells, so that
they show signs of plasmolysis, rounding up into ovoid masses.
Curiously, however, the cells of one filament remain stationary,
while the cell contents from the other filament move over through

Conjugation of Spirogyra. Explain what happens. (After Coulter.)

THE DEVELOPMENT OF SEXUALITY IN PLANTS

171

the tube and fuse with th(^ quiescent cells. When this fusion takes
place, the nuclei unite so that a single resting cell is formed, called the
zygote, which develops a thick wall, very resistant to drought and cold.
The zygote is heavy enough to sink to the bottom of the pond when
the rest of the filament dies, and under favorable conditions will
germinate, giving rise to a new filament.

Since these cells from different filaments join or fuse, somewhat
after the manner of conjugation in Paramecium, we think of them as
sex cells, or gametes. Although the two cells are of the same size,
yet one is active and the other passive. In higher plants and animals,
the active cell is referred to as the male gamete, or spei^m, and the
non-active cell as
the female gamete,
or egg. A compari-
son of Spirogyra
with higher forms
suggests a very sim-
ple type of sexual
reproduction, known
as conjugation.

In another fila-
mentous form, Ulo~
thrix, certain cells
are modified to be-
come free-swimming
zoospores, provided
with four cilia which
may swim about for
as long as an hour
before settling
down. It is obvious
that such a free-
swimming cell may plant a new individual at some distance from
the original filament. Gametes of Vlothrix are also formed as free-
swimming cells, all alike, having two cilia instead of four. These
gametes fuse by conjugation and produce a zygote, which, like that
of Spirogyra, has a thick resistant wall, and is capable of developing
even after exposure to very unfavorable conditions.

In the formation of the conjugating gametes of both Vlothrix and
Spirogyra a significant thing happens to the nuclei of the cells before

Ulothrix: a. base of filament with holdfast; b, fila-
ment producing; zoospores or gametes; c. young filament
developed from zoospore; d. filament discharging zoo-
spores and gametes ; e. an escaped zoospore ; /, escaped
and pairing gametes ; g, zygospores ; h, zygospore pro-
ducing zoospores by reduction division, (a-f/. After
Coulter; //, after Dodel-Port.)

172 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

fusion

they conjugate. By a
series of divisions, such
as is shown in the dia-
gram, the number of
chromosomes in the nu-
clei of the zygote, result-
ing from the union of the
two gametes, is reduced
. . to half this number. If
\ spelling disappear g^j^^g g^^j^ dcvice as this

' ^ / \ were not used, every time

sex cells united, the num-
ber of chromosomes
would be doubled. How-
ever, by this so-called
reduction division during
the formation of the ga-
Diagram to show how reduction division takes metes, which OCCUrs in
place in the zygote of Spirogyra. both plants and animals,

the number of chromosomes is halved. We speak of the single number
of chromosomes as haploid and the double number, which comes with
the union of the two gametes, as diploid.

■2.y^U

first division,
reduction
of ci-ji-TomoSomcS
from ?T-; to ri ,
mat.u.ra.t,ion

/second
cLivision,
TTJitjDSiS

Oedogoniiim

In another of the filamentous algae,
Oedogonium, there is the first appearance
of two kinds of sex cells. This alga repro-
duces by zoospores and in addition forms
two sex organs, structures called anther-
idia, which produce a number of ciliated
sper7n cells and oogonia, the latter holding
a single egg cell. The sperm cells swim
through the water from the antheridia,
one uniting with the egg cell, and almost
immediately a thick wall is formed about
the fertilized egg. This oospore does not
produce a new plant directly, but gives
rise to zoospores, which in turn eventu-
ally become new plants.

Life history of Oedogonium.

THE DEVELOPMENT OF SEXUALITY IN PLANTS 173

Another form of Oedogonium forms antheridia and oogonia on
separate filaments, the male filament being much smaller than the
female filament. Thus the filamentous algae illustrate three big
ideas, namely, division of labor, development of sex, and reduction of
chromosomes.

In the simplest plants all cells tend to do the same work, but in the
more specialized algae there is a differentiation of work and an
accompanying differentiation of cells to accomplish it. In the
development of sex and of structures to take care of the sex cells, as
found in the forms described, the contribution of the sex cells seems
to be to provide a greater vigor to the offspring, especially when the sex
cells come from different individuals. Most important of all is the
fact that cells which fuse, as in the case of the sex cells, must have
some way of reducing the number of their chromosomes, else they
would be doubled each time two sex cells united. This is accom-
plished by the reduction division referred to above, by which process
the number of chromosomes, doubled at the time of fertilization, is
halved. This reduction process occurs in both plants and animals,
and although in plants it occupies a different place in the life cycle,
its ultimate effect is the same in both cases.

A Representative Fungus

Bread mold, Rhizopus nigricans, one of the most common of the
fungi, may easily be grown in the laboratory by exposing a moist piece
of bread to the air for a few moments. Mold spores are so numerous
everywhere that under ordinary conditions a growth of mold will be
evident within one or two days, first appearing as a white, fluffy
growth that rapidly covers the surface of the bread. This is the
mycelium, which consists of branching tubelike filaments, or hyphae,
containing many nuclei, but without cross walls. The absence of
chlorophyll shows the inability of the mold to make its own foods
and explains why the mycelium sends down into the bread, root-
like branches called rhizoids, that secrete enzymes, by means of
which the food substances in the bread are digested. Some of the
hyphae form long branches called stolons, which run along the sur-
face of the bread, forming new plants. At points where rhizoids
are developed, there arise later numbers of erect branches, or spo-
rangiophores, on the tips of which are developed sporangia, or spore-
bearing organs.

174 ORGANISMS ILLUSTRATING RIOLOGICAL PRINCIPLES

Great numbers of tiny spores are produced by division of the dense
terminal portions of the sporangiophores. As a sporangium becomes
mature an outer wall is formed and the spores turn black in color.

When this outer wall
breaks, the minute spores
are scattered far and wide
by air currents.

Molds also reproduce
sexually, by means of con-
jugation. Rhizopus has
two different strains of
mycelia, one of which is
called a plus ( + ) and the
other a minus ( — ) strain.
If hyphae of two such
strains come in contact
with each other, zygo-
spores are formed. Short,
club-shaped branches are
developed from the hy-
phae, the dense proto-
plasmic tips are cut off from the end of each by cell walls, and these
"cells," each of which contains several nuclei, unite to form a
zygote. The zygote with the hyphae which develop from it proba-
bly represents the diploid stage of chromosome in the life cycle,
the haploid stage being reached when the spores on the sporangium
germinate.

The fungi are of even more interest by reason of their method of
nutrition. They are typically neither holozoic nor holophytic, since
they live as saprophytes on dead organic materials. This means that
they must absorb food materials which are supplied to them from
outside sources after digesting them by means of enzymes, when
absorption takes place through the plasma membrane of the cell.

Alternation of Generations in the Plant Kingdom

The most important difference in the life cycle between the Bryo-
phytes or Mosses and lower forms, aside from a greater differentiation
of the plant body, is the alternation of an asexual with that of a sexual
generation in the hfe cycle. The asexual generation, which produces
spores, is called the sporophyte, while the sexual generation, which

Reproduction in bread mold (Rhizopus nigri-
cans). Read the text and then explain the
diagrEun.

THE DEVELOPMENT OF SEXUALITY IN PLANTS 175

gives rise to gametes of two different sexes, is known as the gameto-
phyte. The latter generation is the conspicuous green plant that
manufactures food and serves as host for the sporophyte generation
which is permanently attached to it.

The gametophyte of the simple moss, Funaria hygrometrica, is a
short upright stalk bearing usually three spiral rows of simple leaves,

sperm

embryo

Sfertili^ect egg

rontheridiunz.

mum.

dbcmetophone ,

bud

protonemol
threocC

ycrunS gb-metophyt'

The life cycle of Funaria, a moss. Which stage is more prominent,
gametophyte or sporophyte?

each containing numerous chloroplasts. At the lower end, a group of
small brown rkizoids furnish the means of attachment to the sub-
stratum. The moss plant is dioecious, having separate sex organs
on different plants. The male gametophytes are shorter than the
female gametophytes and bear at the upper tip a cluster of structures
known as antheridia. Each mature antheridium looks like a tiny
club with a wall formed of rather large, thin cells, which forms a recep-
tacle for numerous motile sperm cells. The female gametophyte
bears at the apex of the short stem, although in the mature plant

176 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

hidden by leaves, a cluster of flask-shaped structures called archegonia,
at the bottom of each of which is a single rather large egg cell.

Fertilization of the egg can take place only when the antheridia and
archegonia are wet from rain or dew. In such an event the sperm
cells ooze out in a mucilaginous substance secreted from the walls of
the antheridium and pass in drops of water to the necks of the
flask-shaped archegonia. Here they are chemically attracted by a
substance exuded from the inside of the archegonium and swim down
the tubular neck until one meets the egg cell, when fertilization takes
place. The gametophytic phase of the moss is the haploid stage of the
chromosomes, fertilization of the egg restoring the diploid number
characteristic of the sporophyte. This generation begins with the
cell division which follows the fertilization of the egg in the archego-
nium and results in the growth of a tiny stalk, bearing at its upper end
a capsule, that in the adult sporophyte is filled with asexual spores.
During the formation of the spores within the capsule, the formative
tissues produce a number of large, rounded spore mother cells, from
each of which by nuclear divisions tetrads, or groups of four spores,
are formed. During this tetrad formation, a reduction division
takes place so that the spores contain only the haploid number of
chromosomes.

The moss capsule is quite a complex structure with a cap, or oper-
culum, that covers an urn-shaped affair bearing at its upper end a
circle of teethlike structures collectively called the peristome. As the
sporophyte ripens it dries up and the numerous ripe spores are scat-
tered by the action of the peristome teeth, the latter being very
hygroscopic, or sensitive to moisture. When the weather is humid or
wet, the teeth of the peristome curl up and when dry they straighten
out, thus expelling the spores, which may then be scattered by the
wind. The germinating spore does not grow directly into a leafy
plant, but first forms a protonema or algalike filament from which
upright stalks later arise, while rhizoids grow downwards from it,
thus forming again the moss plant. This life cycle with its alterna-
tion of gametophytic and sporophytic stages is characteristic of the
life cycle of mosses and liverworts, as well as the higher group of the
ferns (Filicinae).

In the flowering plants (Angiospermae), one finds an almost com-
plete suppression of the gametophytic generation, the sex cells or
gametes being produced in modified leaflike parts of the flower. The
floral parts — sepals, petals, stamens, and carpels — are thought of

THE DEVELOPMENT OF SEXUALITY IN PLANTS

177

as leaves which have become metamorphosed from their vegetative
form and function to hold the sex structures. The stamens and
pistil (carpel) contain spore-forming tissues which, by means of
reduction division, produce pollen grains containing microspores
(sperms), while ovules produce a female gametophyte and its egg.
The sperm cells are formed in the pollen grains, while the egg cells

germinating poller

tube

osUs of
anLher

form pollen grains with— ^ sperm 2,-^ ^;

tuba nuclau^

TRe embr/o sac contains a dividimg nuclaus ^ v '/ bolUn
eight cxne jlnally formed, tuJo forn-i fusion rnAcleu^ /^tL-ubo

Development of male and female gametophyte in the flowering plants. Only
the cells which actually form these structure are shown. The parts of the sporo-
phyte upon which the gametophyte is parasitic are omitted for the sake of clarity.
Read the text carefully and then use the diagrams.

are held within the ovary of the pistil as has been previously stated.
In the angiosperms or flowering plants the male gametophyte is so
much reduced that it consists of only three cells, a tube nucleus and
two generative cells (see figure). Just previous to the formation of
the pollen grains (male gametophyte) reduction di\ision takes place
so that its cells contain the haploid number of chromosomes. The
female gametophyte is also greatly reduced. After reduction divi-
sion, the megaspore divides (see figure) one nucleus migrating to each
end of the etnbryo sac (female gametophyte). The nuclei continue
to divide until eight are formed in two groups at opposite ends of
the embryo sac. From each group a single nucleus then unites with
the other to form a fusion nucleus (see figure). At this stage the egg
nucleus is ready for fertilization by the sperm nucleus. A double ferti-
lization now takes place, the sperm nucleus fuses with the egg nucleus
and the second sperm nucleus unites with the fusion nucleus. The

178 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

former gives rise to the young plant, the latter to its food supply, the
endosperm. The transfer of pollen in flowers of the same species
may result in the fertilization of the egg and subsequent growth of

Division I

Thallopbyta

algcxe. "^

Division I

Bryophyta

Division BE Tr-ccchsopViyta.- vasculcti- T=lcL"ts

subdivision A*B;vc ^abdVvi/ion D PteropsicCcc

ferns ^^rnnospe4*ms angiospcrrrjs

primiuve plants

Lvcopsida ,Sl*enopJic£a

^ener-cction

Diagram showing relation of sporophyte and gametophyte generations in the

plant kingdom,

the plant body (sporophyte generation). The evolution of sporo-
phytic and gametophytic generations in the plant kingdom is shown
in the above chart.

SUGGESTED READINGS

Coulter, J. M., Barnes, C. R., and Cowles, H. C., A Textbook of Botany,

Vol. I, American Book Co., 1930.

This text gives an excellent foundation for the understanding of sexu-
ality in plants.
Gager, C. S., General Botany, P. Blakiston's Son & Co., 1926.

A general botany which gives much information on economic questions,

as well as sex development in simple plants.
Robbins, W. J., and Rickett, H. W., Botamj, D. Van Nostrand Co., 1929.

Chs. XV-XXIV.

Excellent diagrams help in the understanding of the development of sex.
Sinnott, E. W., Botany, Principles and Problems, 3rd ed., McGraw-Hill Book

Company, 1935. Chs. XI and XIV-XXIII.

A thoroughly up-to-date treatment of the subject.
Wilson, C. L., and Haber, J. N., Plant Life, Henry Holt & Co., 1935.

An interesting and well-written elementary text.

DIVISION OF LABOR IN THE COELENTERATES

Preview. The Hydra, a representative of the phylum Coelcnterata ; the
ectoderm and its functions ; the endoderm and its functions ; reactions to
stimuli ; reproduction ; regeneration ■ Hydroids • Suggested readings.

PREVIEW

It has already been shown that unicellular animals may exhibit
considerable complexity of structure, and that associated with this
complexity, there is a separation of functions in different parts of the
cell, but we have not traced this division of labor into the many-
celled animals or metazoa. The colonial forms, such as Pandorina,
Eudorina, and Volvox, claimed by both botanists and zoologists, are
interesting exam]iles of aggregations of many cells showing little
evidence of organization or division of labor. Even in the colony of
Volvox, most of the cells have common functions, only the reproduc-
tive cells being set off from the others.

The Hydra, a tiny animal little higher in the scale of life, gives every
evidence in its structure of being a simple organism and not just a
collection, or colony, of cells. It shows, in a convincing manner, how
a simple, many-celled organism lives. It answers the question of
how division of labor might arise among the cells of a simple organism,
For this reason it is chosen as a type in most courses in biology and
so has a place in this text.

The Hydra, a Representative of the Phylum Coelenterata

Hydras are quite abundant in many ponds or slow-moving streams,
where they may be collected on the stems and leaves of aquatic plants.
In an aquarium, they often leave these plants and become attached
to the glass walls of the aquarium, where they appear as tiny brown
or green cylinders one-half of an inch or more in length. At the free
or so-called oral end, a circle of tentacles surrounds a conelike area,
the hypostome, in which the mouth is found. The opposite, or aboral,
end forms a disklike structure which is provided with mucous cells
that aid it in sticking to a surface. Hydras are able to move slowly
by a looping motion of the body. The green ones, which are much more

179

180 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

active than the brown ones, frequently change their position if food
is not abundant. They respond to chemical stimuli of food, to light,
and to unfavorable temperatures, food being the chief factor in their
environment. The color of green hydras is due to the presence of

Hydra is able to change its position both by turning "handsprings" as shown
in the diagram and also by contraction and expansion of the basal portion of the
body.

minute green algae, called Zoochlorellae, that live in a symbiotic
relationship within the endodermal cells.

The term, Coelenterata, which is the name of the phylum to which
the common Hydra vulgaris belongs, comes from the Greek words
koilos, hollow, and enteron, intestine, which may be translated "hav-
ing an internal digestive cavity," an apt title, since a Hydra is really
a hollow, double-walled bag.

The Ectoderm and Its Functions

The bulk of the outer layer of cells (ectoderm) is made up of large
epitheUo-muscular cells, having a layer of muscle fibers placed lon-
gitudinally at their bases, that enable the animal to lengthen or
shorten its body. A similar layer of fibers on the inner layer of cells
which run circularly around the body allows it to expand or contract
in diameter. Between the epithelio-muscular cells and near the inner
margin of the ectoderm are found numerous smaller interstitial cells
from which are derived numerous other cells, including the cnido-
blasts. Nerve cells are likewise scattered throughout the ectoderm,
forming a nerve Jiet at the base of the epithelial cells.

Cnidoblasts are most abundant on the tentacles, although they are
found on all parts of the body exclusive of the basal disk. They hold
four kinds of stinging capsules, nematocysts, by means of which the
animal paralyzes living prey that comes in contact with its tentacles.
The nematocysts are capsules containing a hollow inverted thread
which under certain conditions can be thrown out, together with a
poisonous substance, hypnotoxin, that has the power to paralyze any
other small animal which it touches. The nematocyst reacts to cer-

DIVISION OF LABOR IN THE COELENTERATES IJil

tain chemical stimuli that apparently cause a change of osmotic pres-
sure within the cell, thus forcing out its threadlike portion. After a
nematocyst is protruded, the cnidoblast dies and is soon replaced by
another.

stinSina

"nerve
cell

"muscular

absorbing
cell ^

.flagellum

-sensory
cell -^

cell ®

cxxnthmd
cell

The Endoderm and Its Functions

By cutting a section through the body of a Hydra its similarity to a
two-walled sac is evident. Between the ectoderm and the inner layer
of cells (endoderni) a thin, structureless layer called the mesoglea
forms as a secretion
from the cells of the «^toclerm j e«dod^m

inner and outer layers.
Mesoglea forms much
of the bulk of other
coelenterates like the
jellyfishes. The endo-
derm consists principally
of large vacuolated cells
that have flagella at the
free or inner end, al-
though they are also
capable of developing
pseudopodia at this
end. Circular contrac-
tile fibers are developed
at their basal end. Thus

they are endothelial-muscular cells. In the third of the body nearest
the basal end, gland cells develop, which secrete digestive enzymes.
Nerve and sensory cells are also found in the endoderm.

For a simple animal, the Hydra seems to have many kinds of cells.
What is the use of so many ? The answer is found in the way it gets
food, ingests it, and finally absorbs it into the body cells. By watch-
ing a hydra in the aquarium it will be seen that its tentacles are con-
stantly moving as if seeking food. If a tiny bit of raw beef is placed
within reach, the animal will bend over and carry the meat to the
mouth, the edges of which soon close around it, forcing it inside. If
the piece is too large to be taken in, the Hydra actually turns inside
out in an attempt, usually successful, to put the meat inside the
gastrovascidar cavity. Once inside the cavity, digestive enzymes from
the glandular cells act upon the food, gradually breaking it down into

Sections through the body wall of hydra showing
the two layers of cells separated by the striated
lamella secreted by the basal parts of the ecto-
dermal and endodermal cells.

182 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

smaller and smaller fragments. Digestion appears to be aided by
the churning movements caused by expansion and contraction of
the body wall. Ultimately some of the food is reduced to a soluble
state, and absorbed into the endodermal cells. Meanwhile some of
the large vacuolated cells put out pseudopodia and engulf some of
the undigested food particles, finishing the digestive process inside
their own cell-bodies. Thus Hydra has two types of digestion, one
intracellular, like that found in all unicellular animals and, there-
fore, more primitive ; the other, extracellular, that is, taking place in
the digestive cavity. Most of the food of the Hydra is digested in
the latter way, the cells lining the cavity absorbing the digested food
before passing it along to the cells of the ectoderm. According to
Hegner, part of the absorbed food is in the form of oil globules which
are passed over to the cells of the ectoderm and stored there for future
use. Unusable or undigested material is thrown out of the digestive
cavity by a sudden contraction, there being no other way of eliminat-
ing such wastes except through the surface of the body, as in lower
forms. Hydra like other animals uses oxygen to release energy.
Respiration probably takes place through the surface of the entire
body, the cells receiving oxygen and giving off carbon dioxide by
diffusion through the cell membranes.

Reactions to Stimuli

Hydra show very definite reactions to certain stimuli, most of
which have to do with obtaining food. Hungry Hydra are much
more active than well-fed ones, and respond to various chemical
stimuli besides reacting to mechanical stimuli, to heat, to light, and
to electricity, all of which indicates the possession of some sort of
simple nervous system, since the movements made are more or less
co-ordinated. If touched lightly on a tentacle with a needle, only
the tentacle contracts, but with increased stimulation, the other
tentacles contract, until finally, the whole animal draws down into
a little ball. Its physiological condition, according to Jennings,^
determines whether it " shall creep upward to the surface and toward
the light, or sink to the bottom ; how it shall react to chemicals and
to solid objects ; whether it shall remain quiet in a certain position,
or reverse this position and undertake a laborious tour of exploration."

The nervous system of Hydra forms a nerve net. It consists of a
concentration of primitive nerve cells about the base of the hypostome

1 Jennings, Behavior of the Lower Organisms. Columbia Univ. Press, 1915, p. 231.

DIVISION OF LABOR IN THE COELENTERATES

183

and the foot. This network of cells lies in the ectodermal layer of
the animal, and receives impulses from sensory cells as well as trans-
mitting them to the muscle fibrils. The sensory cells of the ectoderm
.vary in their location ; one type occurs on the tentacles, one on the
hypostome, and a third on the foot
(base). Neuro-sensory cells which
are located in the mid-body area
.also resemble nerve cells, except that
they send processes to muscle fibrils
and so become intermediate between
those receiving stimulation and those
making the response. Some nerve
cells appear in the endodermal layer
but are not, so far as can be deter-
mined, connected with the ecto-
dermal nerve net.

Reproduction

Probably the most important
function of the interstitial cells is
their growth into sex cells. Most
Hydras are hermaphroditic, that is,
have both kinds of sex cells present
in the same individual, but since the
sperm cells and ova ripen at different
times, fertilization is accomplished
by sex cells from different indi-
viduals. Sperm cells are produced
by the mitotic division of interstitial
cells, each of which first produces a
number of parent male cells, contain-
ing the somatic number of chromosomes.

The nerve net in a young hydra
as seen with an intravitani methylen-
blue stain. Note the ringlike ar-
rangement in hypostome and foot.
What effect might such an arrange-
ment have on movement? (After
J. Ilodzi.)

These cells divide four
times and in the process a reduction division takes place, leaving the
sperm cells with just half as many chromosomes as the body cells.
A somewhat similar process takes place in the formation of the ova.
One interstitial cell becomes larger than the others, rounds into a
sphere, and is surrounded by other interstitial cells, which serve as an
ovary for the growing egg. The latter continues to grow in size, form-
ing yolk from the surrounding cells. Just before the egg becomes
mature, the process of maturation takes place (see page 429), dur-
H. w. H. — 13

184 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

ing which the number of chromosomes is reduced to half the body-
number. Spermaries and ovaries can be seen in the Hving Hydra as
little lumps on the ectoderm. The spermaries are always found near

the free end of the body, the
ovaries, when present, being
nearer the base. The egg
is fertilized while still at-
tached to the parent and
develops into an embryo
surrounded by a protective
chitinous case, in which
stage it sinks to the bottom
of the pond for a resting
period before emerging as
an adult.

Asexual development
also takes place. A small
bulging area, formed by
the interstitial cells, ap-
pears on the side of the
body, which more or less
rapidly grows into a short
column surrounded by
tentacles, depending on the
food supply available for
the parent Hydra. When

fully developed the bud
may separate from the
parent and lead a separate
existence. A Hydra fre-
quently produces more than
one bud on a single animal.

young
hixd

sperrr?
— cells
forming

ec.todJ2.rm
endocferm-

jonriing"

Longitudinal section through the body of a
Hydra, showing both sexual and asexual repro-
ductive structures.

Regeneration

Although regeneration takes place in other groups of animals it is
best seen in the phylum, Coelenterata. The primitiveness of Hydra
is shown by the fact that it can regenerate or replace lost parts by
growth of the body cells. It may be cut lengthwise or crosswise, or
even into small pieces, and the fragments will, under favorable con-
ditions, give rise to complete individuals.

DIVISION OF LABOR IN THE C0ELENTE1\ATES

185

Hydroids

Hydra vulgaris is a fresh water form, but many more representatives
of the Coelenterate group are found in salt water, the most famiUar
being the hydroids found attached to the piles of wharfs and other
submerged objects. Among the most common hydroids are members

hydranth.

-gbnobVzsca

^9 ^onacC

medusa /;<^^^»v^^",'^^
^^^ (^..fertiTe

asexual *~-^-Viyctrorhi3a. - y

Stage /

^..blastula

^T-planula

Life cycle of Obelia — showinff alternation of generations.' Compare with text
pages 18.5-186 for explanation of diagram.

of the genus Obelia. These animals form colonies, in which the indi-
viduals, called polyps, or zooids, are attached to each other by means
of hollow stalks, covered with a chitinous, cellophanelike perisarc.
At the tip of each branch, the covering expands into a cuplike hydro-
theca, which surrounds the living polyp. As in Hydra, each individual
polyp of Obelia is hollow and two layered, with a circle of tentacles
about the raised hypostome, in. which the mouth is located. The
tentacles are provided with nematocysts that act in the same manner
as in the Hydra. The food cavity, however, extends down each stalk-
like branch or individual and is continuous with that of the other
polyps, thus forming a common gastrovascular cavity in which food

186 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

is digested. There are also cells, as in Hydra, which perform intra-
cellular digestion.

Obelia gives rise to another type of polyp than the nutritive individ-
ual just described. This is the reproductive polyp, or gonangium that
grows out as a bud, expands into a knoblike central axis known as
the hlastostylc within a chitinous, closed vase, called the gonotheca.
On the sides of the blastostyle budlike structures, called medusa buds,
develop. These break off and swim away as tiny bisexual jellyfish,
or medusae, representing the sexual stage in the life history. A sperm
cell from one of these medusae fertilizes an egg from another, which,
after a developmental period, becomes a free-swimming ciliated larva,
called a planula. After a short time the planula settles down and
produces a new asexual colony of Obelia. Other related forms as the
jellyfish, Aurelia, possess a predominating free-swimming stage, while
the sessile, non-sexual generation is reduced.

This life cycle is reminiscent of a similar condition in plants, which
also have an alternation of generations. During the maturation of the
sperm and egg cells, reduction division takes place in which the chro-
mosomes of the sex cells are reduced to half the body number. In
alternation of generations of plants, all the cells of the gametophytic
generation are haploid, but as in animals only the mature sex cells are
haploid, the body cells having the same number of chromosomes as
the body cells of the sexual generation. The end result accomplished
in both plants and animals is the same.

SUGGESTED READINGS

Curtis, W. C., and Guthrie, M. J., Textbook of General Zoology, 2nd ed.,

John Wiley & Sons, Inc., 1933, pp. 278-301.
Guyer, M. F., Animal Biology, Harper & Bros., 1931, pp. 197-206.
Hegner, R. W., College Zoology, The Macmillan Co., 1936. Ch. X.

An authentic description of Hydra and its activities.

BEING A WORM

Preview. A typical worm ; external structure of the earthworm {Lum-
bricus terrestris) ; the digestive tract and its functions ; how blood circulates,
the blood and its functions ; organs of excretion ; the muscles and their work ;
reactions to stimuli ; the nervous system and its functions ; the reproductive
system and reproduction • Regeneration • Suggested readings.

PREVIEW

Passing from the simple two-layered development of the Hydra, in
which division of labor among the cells is slight, we come to the earth-
worm, another lowly animal, but one which represents the big idea
of a typical three-layered, segmented form.

In Hydra, the egg develops into an adult form having two layers,
namely, edodertn and endoderm, but in the earthworm, a third
layer, the mesoderm, appears, w^iich is characteristic of all the higher
animals. These three germ layers are of great significance in the
study of animals, for all of the complex tissues of the body are derived
from them.

Another reason why the earthworm is chosen for study is because it
represents a very simple type of segmented or metameric animal of
which a great variety is found not only among worms but also among
insects and crustaceans. Judging by the insects, segmented animals
are the most abundant and successful of all animals, since they out-
number all other species. The pages that follow will concern them-
selves chiefly with the "hows and whys" of the activity of the
common ''night crawler," some of which are: How far has division
of labor progressed ? What organ systems are well developed ? How
does co-ordinated movement take place, and how do worms become
aware of their surroundings ?

A Typical Worm
External Structure of the Earthworm (Lumbricus terrestris)

The body of the earthworm is divided into segmented parts, or
metameres, which in adult worms may number over one hundred.
The body tapers bluntly at each end, the anterior end being easily

187

188 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

distinguished by the rounded mouth which is just ventral to or under
a small protuberance, the 'prostomium, while the anus, or posterior end
of the digestive tract, is a tiny slit in the last segment. The posterior
end is also flattened, and between segments 32 to 37, not counting
the prostomium enclosing the mouth as the first, there is found a
swollen region, called the clitellum, important in reproduction.

The upper or dorsal side may be distinguished by its darker color,
while the ventral side is slightly flattened and contains four double
rows of tiny projections called setae, which give the worm a grip on
the ground when in locomotion. The dorsal side is devoid of any

The common earthworm, Lunibricus ierresiris.

Wright Pierce

Note the swollen area, or clitellum.

openings except some very minute dorsal pores that communicate
with the body cavity, or coelom, but the ventral side has several
paired openings, difficult to find, which lead to the reproductive and
excretory organs. The surface of the body is covered with a delicate
iridescent cuticle, secreted by the living epithelial cells of the skin, but
which is itself dead. Its iridescence is caused by the presence of
numerous grooves (striae), and its surface is pierced with small holes,
which are openings for the mucous gland cells of the skin. The coelom
or body cavity is cut up into small compartments by partition walls,
or septa, that are absent or incomplete in the extreme anterior region,
between the 18th and 19th segments, and in the region posterior to
the reproductive organs. The coelom in the living worm is filled

BEING A WORM

189

Septum
Vnuscle,---

hear-ts

also 3,4-.S

seroinal —
receptacle

with fluid which passes from one segment to another through single
perforations in each of the septa. The fluid contains ameboid cells,
that probably serve as scavengers, and it acts as blood, bathing and
nourishing the tissues and carrying away wastes.

The Digestive Tract and Its Functions

The food of earthworms, bits of animal or vegetable matter mixed
with soil, is taken into the mouth by means of suction. A muscular
pharynx, previously moistened by the fluid poured out from small
glands in its wall, is able to
pull the material into the
esophagus, a thin-walled part
of the tube which extends from
the 6th to the 15th segment,
beside whose walls, between
segments 10 to 12, there are
embedded three pairs of whit-
ish structures, the calciferous
glands. These glands produce
a limy secretion supposed to
neutralize the food materials.
The esophagus leads into a
thin-walled crop, occupying
the 15th and 16th segments,
which opens into a thick-
walled, muscular gizzard ex-
tending over segments 17 and
18. The latter organ has an
internal chitinous wall, and is
probably used to macerate bits
of undigested food by means
of muscular contraction. The
remainder of the food tube, ex-
tending from the 19th segment
to the anus, is called the intestine. Its inner surface is increased by a
fold on the dorsal side (typhlosoJc) , while surrounding it there is a layer
of yellow-brown tissue cMorogogen cells, which are thought to aid in
excretion and possibly digestion of food. The wall of the intestine
contains gland cells that secrete at least three kinds of enzymes, which
digest starches, fats, and proteins. The digested food is absorbed

Seminal
vesicle.—.

cConsal
vessel

.pViarxnx
.<?5c>p'hag"tc5

.Caldfe.r<3US
glancfs

.crop
intestine

t/pbloSole
rzerve CorcC

ventral
vessel

three ofhen-.-r^-l
vessels *■

The earthworm {Lnmhricns ierrestris)
opened from dorsal side to show internal
structure. (After Sedgwick and \\ ilson.)

190 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

through the walls of the intestine, most of it passing into the blood
and directly into the coelomic fluid, where it may continue to the
muscular wall outside the coelom. Unusable material, mostly earth,
is passed off by muscular contraction through the anus, and may often
be seen on lawns as little piles of "castings."

How Blood Circulates

Since in the earthworms there is a very different arrangement than
in Hydra, where food is directly available to all the cells, we would
expect to find some means of distributing it to the tissues where it

may be used. This is accomplished by
means of a closed system of blood vessels.
Some idea of the circulation may be derived
by a study of the accompanying diagrams.
Five large blood vessels run lengthwise
through the body, one dorsal vessel, close
to the food tube, into the walls of which
it sends two pairs of lateral vessels in each
segment ; another, the ventral vessel, runs
just ventral to the digestive tract and also
sends lateral branches into its wall. There
are also three others, the paired lateral
neural vessels and the suhneural vessel,
which run longitudinally, the latter directly
under the nerve cord, and two other smaller
ones lying parallel one on each side and
above the nerve cord. Five "hearts," so
called because of their frequent contrac-
tions, encircle the esophagus in the region
of the 7th to the 11th segments, connecting
the dorsal with the ventral vessel. Blood
passes into the dorsal vessel especially
from a long typhlosolar vessel which helps
drain absorbed foods from the intestinal
walls, flowing forward until it reaches the
" hearts." Its forward movement is caused
by slow, regular contractions of the dorsal
blood vessel. The blood passes posteriorly through the "hearts"
and then flows into the ventral blood vessel. Here it passes poste-
riorly, although some of it moves from the hearts toward the

nerves

buccal cavity
Esuprcicsophatfeol ,
tiixurnssoiohigeal
Sub esopho^ol

.„psai,
vessel

lateral
vessel

■esof>hogus

..ventral

VGSjel

- - (trop

.nsrve ccnet
■with. IntM-al
neurai vesssis

intastina.

The circulatory system of the
earthworm.

BEING A WORM

191

anterior end of the body. Blood also passes tliroush two intestino-
integumentary vessels which pass off at the 10th segment to supply
the walls of the esophagus and the skin, and to nephridia of that
region. Parietal vessels connect the dorsal and subneural vessels,

cross Section of typyosolar vessel

'>_Jat^rccl-y2eu:ral vessel

V nerve
CorcL

The '"hearts" of the earthworm. How do they function in circulation.^

that branch from the ventral vessel to supply the body muscle walls
and nephridia. Blood also passes from the ventral vessel to the body
walls, and to nephridia, and returns to flow, after passing through
capillaries, into the lateral neural trunks. In the subneural vessel,
the blood flows posteriorly and thence up by way of the parietal
vessels into the dorsal vessel. Both dorsal and ventral vessels supply
the anterior part of the worm.

The Blood and Its Functions

The blood of the earthworm consists of a liquid plasma, carrying
colorless corpuscles which are flattened spindle-shaped bodies. The
red color is due to hemoglobin, the same oxygen-carrying substance
found in the blood of man. But in the earthworm the plasma is
colored rather than the corpuscles. The exchange of food and
oxygen, which the blood picks up in the intestine and body walls,
respectively, occurs in the tiny lymph spaces around the individual
cells. Respiration takes place through the moist outer membrane

192 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

of the skin, where the oxygen is picked up and combined with the
hemoglobin, to be later released in the cells of the body where work
is done. Carbon dioxide and wastes are here taken up by the blood
and carried back to the skin and to the nephridia or excretory organs.
One can easily demonstrate the network of tiny capillaries in the skin
where this exchange takes place.

Organs of Excretion

The paired nephridia are essentially coiled tubular organs, made
up of a ciliated funnel or nephrostome that opens into the coelom,

a thin ciliated glandular

^-^epttcin

like region

tube, that loops on itself
about three times, and a
pore, the ncphridiopore,
through which the excre-
tory products pass to the
exterior. Some excretory
materials are probably
taken directly from the
coelomic fluid by means
of the currents caused
by the cilia, while other
wastes may be taken
directly from the blood-
capillaries which cover
the surface of the glan-
dular tubules. One characteristic feature of the nephridium is that it
always passes through the septum separating two segments.

A nephridium of an earthworm. Trace the
passage of fluid from the coelom to the exterior of
the worm. Note the ciliated surface of the neph-
rostome. What is its function .!^ (After Wolcott.)

The Muscles and Their Work

Movement is brought about by muscular contraction. As an
earthworm crawls, a wave of contraction from the posterior toward
the anterior appears to move up the body of the worm. A careful
examination shows that movement is brought about by the contrac-
tion and relaxation of two opposing groups of muscle fibers and by
the movement of the rows of setae on the ventral surface. The
muscles are arranged in two layers just under the skin, an outer
circular layer running around the body and an inner longitudinal
layer. When the worm lengthens, the longitudinal muscles relax

BEING A WORM

193

and the circular muscles contract, while a shortening of the worm
results from a contraction of the longitudinal muscles and a relaxing
of the circular muscles. Each stiff seta is placed in a little sac,
from which it extends out beyond the surface of the body. Inside
the sac, attached to the seta and to the outer body wall, are two pairs

endocterra ^
•muscle.-,

peritoneum ^^^
TOphridium

^Cuticle ectoderm Circtxlar

^."peritoneum.

muscle

^nephricCiTopore
<-Seta

/ verztro-l vessel
lataml vessel

Cross section through earthworm. Compare this with cross section of Hydra.
What advances in complexity of structure flo you find:' In the earthworm the
most noticeable difl'erence is seen in the coelom. which is formed by a sphtting of
the mesodermal bands in the embryo (seen on page 197). Note that the coeiom
is completely lined by a delicate membrane, the peritoneum. Notice also the
longitudinal fold or typhlosole which gives more surface to the inner wall of the
intestine. What is its function .^ In the diagram, the funnels of the nephridia
are not shown. Explain why this is so.

of muscles by means of which the seta can be directed forwards or
backwards, depending on the direction the worm is traveling. When
the worm is moving forward, the anterior end is extended, the setae,
that are pointed backward, are set into the ground, serving as an-
chors, while the posterior end of the worm is pulled forward by means
of the contraction of the longitudinal muscles.

194 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

Reactions to Stimuli

Earthworms live in soil and make burrows which extend from a few
inches to several feet under ground. They are nocturnal and lie in
their burrows not far from the surface during the day time, coming
out at night to forage for food. In winter, they go below the frost
line, remaining there inactive. In hot and dry weather, they go as
far down as possible into the earth, while a heavy rain will bring them
out of their burrows in great numbers. Earthworms react positively
to mechanical stimuli. A vibration on the earth will send them down
into their burrows. They are positively attracted to surfaces of solid
objects, as can be seen if worms are placed on moist blotting paper in
a covered pan. They will soon be found lying along the edges of the
pan, where two surfaces are in contact with the body. This response
to contact apparently keeps them quite constantly in their burrows.
They react positively to certain chemical substances, like foods, and
move away from others. A match that has been dipped in ammonia
and placed near the anterior end of an earthworm will demonstrate
this reaction. They respond positively to moderate moisture, which
is needed for respiration through the body covering, and to different
intensities of light, by withdrawing from bright areas and moving
toward weak illuminations. Like Hydra, however, reactions to
stimuli depend largely on the "physiological condition" of the worm,
that is, upon internal rather than upon external factors.

The Nervous System and Its Functions

The earthworm has a simple type of central nervous system con-
sisting of a ventral nerve cord, with thickenings, called ganglia, in
each segment, a dorsal "brain" or supraesophageal ganglion, made up
of two ganglia, and a "ring" of nervous tissue, called the circum-
esophageal connectives, which extends around the esophagus, connect-
ing the "brain" with the ventral nerve cord. Lateral nerves, which
leave the "brain" and cord to end in muscles, skin, and other organs,
form a peripheral nervous system. The worm does not have visible
organs of sensation, but the skin, especially at the anterior and
posterior ends, is dotted with groups of tiny sensory cells. Some of
these are sensitive to light, and still others probably to odor. Stimuli
received by these cells are transmitted to the central nervous system
by means of nerve fibers. Those which lead from the sensory cells
to the central nervous system are known as afferent fibers, while out-

BEING A WOIIM

195

going fibers which originate in nerve cells within the cord are known
as efferent or motor fibers, since they end in muscle cells and stimulate
them to contract, thus causing motion. The unit over which these
impulses travel is called a neuron, which is the term given to the nerve
cell and its prolongations. (See page 340.) In the earthworm sensory

anterior-

SerjSory c<=ll5{r<2cepto«)
epidermis.':

■muscle cells
;e|^fecton$)

, ^—Septu.rrL \j

•postsrior

The nerve cord of the earthworm showing neurons concerned in the reflex
arc. Explain how adjustment to an unfavorable condition might be affected.
How might movement in another segment of the worm be co-ordinated with the
one shown in the diagram.'' (After Curtis and Guthrie.)

impulses are passed longitudinally, both anteriorly and posteriorly,
by means of the peripheral nervous system, and these impulses are
modified by means of adjustor neurons in the central nervous system.
This accounts for the co-ordination between segments as the worm
crawls toward a desirable object or suddenly withdraws from a harm-
ful situation.

The Reproductive System and Reproduction

Earthworms have both testes and ovaries in the same animal,
and are therefore hermaphroditic, but they are not capable of self-
fertilization. Two pairs of testes lie attached to the anterior walls of

196 ORGANISMS ILLUSTRATING RIOLOGICAL PRINCIPLES

segments 10 and 11, and are enclosed by the ventral unpouched
portion of two of the three seminal vesicles. Dorsally the three pairs
of large pouches of the seminal vesicles in segments 9, 11, and 12 are
light-colored structures easily seen in a dissection. Immature sperm
cells are passed from the testes to complete their development in the
seminal vesicles. Two pairs of vasa efferentia in somites 10 and 11
fuse to form the paired vas deferens that carry the sperm to the
exterior through the male openings on segment 15. A pair of tiny
ovaries are attached to the anterior septum of segment 13, the eggs

i^..-^ Semirzal rsceptocle

^^..)... seminal vesicle

ovctr^

ovicCuct
spsrm. duct^

Reproductive organs of the earthworm. The seminal vesicles are cut away on
one side to show the funnels of the sperm ducts. Read your text carefully and
explain how reproduction takes place.

passing from this into the oviducts which open to the surface on seg-
ment 14. Fertilization of the eggs is accomplished by the process of
copulation in which two worms, placing themselves in opposite
directions, become "glued" together on their ventral surfaces by
means of mucus secreted from the glands of the clitellum region.
While they are thus placed a mutual transfer of sperm cells from the
seminal vesicles of one worm to the seminal receptacles of the other
takes place, rhythmic muscular contractions of the body helping to
force the sperms along. Then the worms separate. Later, when the
eggs are to be laid, a cocoonlike band of mucus is formed by the clitel-.
lum, which is forced forward by movements of the worm, and as it
passes by the oviducal pores, receives the ripe eggs. When it passes
over the opening of the seminal receptacles on the ventral surface of

BEING A WORM

191

sperm

...mesooCarm.

mesoderm

onus
V.

segments 9 and 10, it receives sperm cells from the other worm that
have been stored there. The girdle is passed down over the anterior
end of the worm, slipped off, forming a closed case which contains the
eggs, sperms, and a nutritive fluid. These capsules may be found in
late spring under stones,
boards, logs, or in manure
heaps. After fertilization,
the egg of the earthworm
divides first into two, then
four, then eight cells, and
so on, continuing until a
hollow ball of cells, called a
hlastula, is formed. These
cells are not all the same
size, larger cells appearing
on the lower pole of the
sphere, which begins to
flatten and show a depres-
sion, forming eventually
a hollow cuplike affair,
called the gastrula. This
process known as gastrula-
tion places the larger cells
of the lower pole on the in-
side of the cup where they
become the endoderm,
leaving the outer cells of
the sphere to form the
ectoderm. Meantime a
third layer of cells which
lies between the other two
layers buds off and be-
comes the mesoderm. This latter layer gives rise to the musculature,
blood vessels, and most of the excretory and reproductive tissues ; the
endoderm forms the food tube and much of the glandular material con-
nected with it ; the ectoderm gives rise to the epiderms, the nervous
system and sense organs, and the outer portions of the nephridia, repro-
ductive ducts, and digestive tracts. The young worms remain in th(^
egg case until they are about an inch in length. When first hatched
they have no clitellum, since this organ appears only in mature worms.

gostrola

Stages in development of earthworm. Fig-
ures II-V. Segmentation of egg and formation
of blastula. Figures VI-VIII. Sections, show-
ing formation of mesoderm as a band of cells.
IX. Late stage of gastrula, showing coelomic
spaces in mesoderm bands. X. Longitudinal
section of young worm showing food tube, mouth
and anus. (After Sedgwick and Wilson.)

198 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

Regeneration

Earthworms, like other members of the lower phyla of the animal
kingdom, have the ability, under certain conditions, to grow new
parts. Experiments have been made by Hazen, Morgan, and others
that show if a sufficient number of segments are present a worm may
regenerate a new posterior end, or even a new anterior end. Earth-
worms have even been successfully grafted end to end.

SUGGESTED READINGS

Curtis, W. C, and Guthrie, M. T., Textbook of General Zoology, 2nd ed., John

Wiley & Sons, Inc., 1933.

Excellent chapter on the Annulata, pp. 350 to 375.
Darwin, Ch., Formation of Vegetable Mould, D. Appleton & Co.

An easily read classic which ought to be known to every student of

biology.
Hegner, R. W., College Zoology, 4th ed., The Macmillan Co., 1936.

Chapter XV is a well-written and authentic chapter on the Annulata.

THE POPULAR INSECT PLAN

Preview. The insect body plan; the head and its appendages; the
thorax and its appendages ; honey manufacture ; digestion ; circulation,
respiration, and excretion ; the nervous system • Reproduction and life his-
tory • The life in the hive • Suggested readings.

PREVIEW

It would seem right in a text on biology that a representative of the
largest and most successful group of animals should be described and
that more than a passing glance be given to this enormous group,
which contains far more than half of all living animals. We are
always meeting insects, because they are so plentiful rather than
from choice. They annoy us when we are in the woods, they bite
us when we are lolling on the beach at the seashore, they get into our
foods and render them unfit for use, or they eat our stored clothes.
Worse than this, they defoliate trees, and sometimes destroy forests,
and take their tithe of the nation's food crops. A good many have
been implicated in the transfer of disease and some have actually
rendered regions uninhabitable by man.

Biologists have a good reason for a study of representatives of the
great phylum, Arthropoda, because the arthropod plan of structure is
the one employed by the majority of the species of the animal king-
dom. In its simplest form, it represents an organism made up of
segments, each body segment bearing a pair of jointed appendages.
The head always bears at least one pair of jointed antennae or feelers,
jointed mouth parts, and usually compound eyes. The body is pro-
tected by an exoskeleton composed of chitin secreted by the cells
beneath. A digestive tract passes straight through the body and
there is a nervous system such as we saw in the Annelids, consisting
of a ventral nerve cord, a dorsal "brain," and a nerve ring about the
esophagus. Dorsal to the food tube is an elongated heart, there
being no closed system of blood vessels. Such a simple arthropod
would be difficult to find for laboratory purposes, so we have to use
other more specialized forms.

From the strictly biological point of view there is another reason
for the study of an insect. It offers an example of a segmented ani-
H. w. H. — 14 199

200 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

mal that has gone in for specialization in a big way. The insects
are a subdivision of the Arthropods, animals that have jointed legs
and jointed bodies, and as such show definite repetition of similar
parts, or metamerism, a phenomenon previously noted in the Annelids.
As a group they have become differentiated to such an extent from
their not so distant relatives that, like the man on the flying tra-
peze, they ''fly through the air with the greatest of ease." In no
other group except the birds has this ability been so exploited. In
addition some forms, such as the bees, ants, and wasps, show an
astonishingly complex social life.

As a successful group insects show numerous adaptations, not only
in structure but in life habits. They are not only active but often
so inconspicuous as to pass unnoticed by their enemies. Insects are
characterized by a rapidly growing larval period associated with an
abundance of food. The protected pupa is characterized by internal
changes fitting the organism for the active reproductive life of an
adult. They deserve our careful consideration as a type for study.

The Insect Body Plan

Adult insects are readily identified because the body is made up of
three parts, an anterior head, a mid region or thorax, and a posterior
region, the abdomen. The body may be further subdivided into

Wright Pierce

The large vagrant grasshopper {Schistocerca vaga Scudder) normal size. A typical
insect. Give all the distinguishing marks of an insect as shown in this photograph.

THE POPULAR INSECT PLAN

201

segments and has three pairs of jointed thoracic legs. These charac-
ters distinguish any insect. If you will refer to the "Roll Call"
you will see that the various orders of insects are distinguished by still
other characters, such as the presence or absence of different kinds of
wings, or differences in the structure of the mouth parts, which may be
modified for various purposes. All insects breathe through tracheal
tubes and have a body

.^./Vclypeus

upper Up

mandibla

rTncuciUotry pcdptxs

hypoph<xrynyc

palpife

COL-rdc).

maxiilcc

^ TTjcxxilla

-Subment-um.

lotbium

Mouth parts of the locust.

armor of chitin, a protein
substance something like
cow's horn.

Many zoologists like to
use a locust or "grass-
hopper" as a laboratory
type for study. This is
because the body parts
are easy to see and be-
cause it is a form ha^'ing
relatively simple mouth
parts. It is provided
with two pairs of jaws, a
forklike pair, the 7?iax-
illae, and a pair of hard
toothed jaws, the mandi-
bles. These parts when
not in use are covered by two flaps, the upper and lower lips (labrum
and labiujn). Such mouth parts are found in the bee, although some-
what modified from the more primitive type seen here. Moreover,
the locust is a more typical insect because it has three distinct thoracic
segments, known as the pro-, meso-, and metathorax, and it also has
a more nearly typical number of abdominal segments, which in most
insects is ten or eleven. The bee, although not such a typical insect,
shows so many adaptations, and in addition has so complex a social
life, that it is selected as a representative of the class Insecta.

The honey bee {Apis mellifica) forms colonies which include three
kinds of individuals ; first, workers, bees with undeveloped female sex
organs, which form by far the largest number in the colony ; second,
drones, or males ; and third, a queen, or fertile female. An average-
sized colony of bees may contain from 35,000 to 50,000 workers,
several hundred drones, and one adult queen. In the following
description the worker bee is used, unless otherwise specified.

202 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

A study of the accompanying illustration indicates that the bee,
like other insects, has three body divisions — head, thorax, and abdo-
men, but instead of the usual three thoracic parts, there are four,
since one segment from the abdomen becomes fused with the thorax,

ocellus or-

Simple^ eye
Compound eye.

ctntcnna.

•momcCible

Tnaxilla
ccndi othei^
mouth ports--

Sting

Worker bee, lateral view, hairs removed, showing parts of body and appendages

on left side. (After Snodgrass.)

leaving only six visible segments in the abdomen. The head bears
a pair of jointed antennae, or "feelers," large compound eyes, and
mouth parts much modified from the plan shown by the locust.
Three pairs of jointed legs and two pairs of membranous wings are
attached to the thorax, the wings growing out of the meso- and meta-
thorax. At the posterior end of the abdomen an ovipositor in the
female is modified in the worker into a sting, which is withdrawn
inside its sheath within the body when not in use. The body is
covered with a horny three-layered coat made up of an outer chitiii-
ous cuticula that covers the entire body except at the joints, where
it becomes membranous, thus allowing movement ; a middle layer
of cells called the hypodermis ; and an inner delicate basement mem-
brane.

Protruding from the chitinous covering are many hairs and bristles,
outgrowths formed by the hypodermis, in which there are several
kinds of cells, some forming the chitinous coat, others the hairs, and
still others gland or sensory cells. In some cases the hairs are hollow
and contain sensory nerve endings. We must picture these animals
covered with heavy armor, through which sensation is impossible

THE POPULAR INSECT PLAN

2o:{

except where sensory nerve endings penetrate the armored surface,
ending in various sense organs such as compound eyes, antennae,
and sensory hairs.

.epicCar misl cuticulo.

5P
cte

ermis

structure- of bocCj^ woJl
(.. hair

chitin..

.Cell cf h/podermis
basement membrane

msmbrone •/*^^^

•chit 11

cuticula

g^^hypodermis «o Chitin in fSlcts or at joints

some, celts form bains

The body wall and its modifications. The epidermal portion of the body wall
is composed of a horny substance called chitin, the dermal portion having a some-
what different chemical nature, like cellulose. In places where movement is
necessary the chitin is replaced by a flexible membrane. Several types of hairs
are found, some solid, others hollow, all outgrowths of the exoskeleton. (After
Snodgrass. )

The Head and Its Appendages

According to the observations of embryologists the head of the
bee is made up of six segments that are fused together in the adult.
This statement is based on the well-estabhshed fact that every seg-
ment in its embryonic condition bears a pair of appendages. Two
compound eyes, which are very large in the drones, are placed on each
side of the head, while between them in a triangle on the top and front
of the head are three simple eyes, or ocelli. Below and between the
compound eyes are the jointed antennae. The mouth parts consist
of lahrimi and labium, the latter a complicated structure which con-
tains the long, flexible Ugula or tongue with a spoonlike labellum used
by the bee in withdrawing nectar from flowers. Attached to each
side of the ligula are two jointed labial palps. The base of the labium
consists of two pieces, the submcntum and mentum. The upper jaws
or mandibles are on each side of the labrum, while the lower jaws or
maxillae, with their tiny palps, fit closely and laterally over the men-
tum. The liquid food, nectar, is first collected by means of the hairs
on the ligula, the maxillae and labial palps being formed into a tube
through which the ligula works up and down with a kind of pumping

204 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

motion although the entire labium aids in the process. While feed-
ing, the flap of the labrum or epipharynx is lowered, making a pas-
sageway for the nectar to pass into the mouth. Thus the mouth
parts, which are all present in the locust as separate structures,

here form a sort of pro-
boscis, that when not in
use is folded back under-
neath the head.

Bees also feed on solids
such as pollen and "bee
sugar," which they mois-
ten with regurgitated
honey and saliva before
swallowing. The mandi-
bles and maxillae are both
used in feeding on solids,
but the chief uses of the
mandibles are in building
honeycomb.

Bees are well provided
with sensory structures.
Experiments by Mclndoo
and Von Frisch indicate
that bees can distinguish
between different-tasting
substances, for some of
which they show strong
preferences. But whether
they can actually taste or
whether they distinguish substances by means of a sense of smell
is difficult to prove. Several experiments have been made that
prove the presence of a well-developed perception of odor. Among
the most convincing experiments were those in which Von Frisch
trained bees to select certain odors, such as oil of orange peel, out
of 43 other odors. He concludes that not only can bees discover
feeding places through a sense of smell but they tell other bees of the
existence of food supplies by means of a "round dance" in which the
successful bee probably holds the odor of the particular flowers on
which she has been feeding and disseminates it to the bees that crowd
around her in the hive.

Wright Pierce

Head of worker bee. Anterior view. Com-
pare this with the accompanying hne drawing
and identify as many structures as you can.

THE POPULAR INSECT PLAN

205

Experiments in which the antennae were removed, together with
evidence from microscopic examinations of the antennae, indicate
that they hold many of the sense organs which perceive odors. Small
pits, in which these sensory cells are located, are found on the surface

A.'--

-;-simple. eyes

compcurjct eye

labrum.

n^andible.

maxilla

maxillary
palp

Simpla eyes
Compound.- eye^

clypeu?
■labram

^a-ndiWe

palp

ma^cilla

labium.
..labial palp

■■prob^

oseis-

labium.

labial palp

Ij.... tongue^ (glossal
i*»-labellu.ra X

L Head of worker, lateral view, mouth part labeled. H. Head of worker,
lower view, lower part of proboscis cut away. Compare these mouth parts with
those of the locust. Which shows the more primitive condition!'

of the antennae. The queen has about 1600 of these pits on each
antenna, the workers about 2400, and the drones about 37,800. This
large number probably makes it possible for the drones to find the
queen during her nuptial flight, at which time sperm cells are placed
within her body so as to insure fertilization of the eggs as they are laid.
The eyes of the bee, as well as those of other insects and crustaceans,
are compound. This means that they are composed of individual
units called ommatidia. Each onmiatidium consists of the retinula,
a group of elongated sensory cells, which encloses a rodlike rhahdom,
the latter made up of the sensory edges of the retina] cells. At the
outer edge is a corneal lens, under which is formed a crystalline cone.
The retinal cells are connected with the optic nerve fibers, the entire
apparatus being covered with a layer of i)igment cells, so that each
ommatidium is a unit, and according to experimental evidence, is

206 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

-crystalline lens

used as a single eye, in conjunction with the several hundred others in
the compound eye. Such eyes are not very efficient. It is probable

that they do not have any sharp vision
for distant objects and not very clear
vision for near objects. Bees have
been conditioned to visit boxes of
different-colored flowers in order to get
honey, but recent experiments by Lutz
and others indicate that they are
guided to flowers by odor rather than
by color.

Bees also have a tactile sense which
comes through tactile hairs on various

oCistol retir?alar nicclexcs P^rts of the body, these hairs being

most numerous on the antennae.

-Cr/stalline cone

.outer piginent. Cell

jcorneal pignQenL cell
-T-hctbcCom

.i_retinalcti-- cell

-Outer pigment/ oall

The Thorax and Its Appendages

The entire body of the bee is covered
with hairs, which indirectly play an
important part in pollen collection and
cross pollination, for the bee in rubbing
against the stamens of a flower gets a
good deal of pollen on the head and
back. The thorax is armored and thus
serves well its purpose as a base for
the attachment of legs and wings. The
delicate membranous wings, with their
ramifying veins and veinlets serving as
supporting structures, are outgrowths

appendages. A wing in flight describes
a figure eight course, its rapid move-
ments being caused by four pairs of
muscles.

The legs have most interesting special adaptations for the several
trades which the worker bee carries on. It is a typical insect leg, of
five divisions consisting of a heavy basal coxa, a short piece called
the trochanter, a long femur which with the adjoining tihia is pro-
vided with long hairs, and a five-jointed tarsus. The tarsus is
provided at the tip with a pair of strong claws, between which

♦..ne-rve^

Detail of an ommatidium.

THE POPULAR INSECT PLAN

207

is found an adliesive organ that enables the animal to hold fast to
slippery surfaces.

Each pair of legs bears different structures which are of use in pollen
gathering and the making of wax. The anterior pair of legs has along
the anterior margin of the tibia a fringe of short, stiff hairs, eye brushes,
used for cleaning pollen or other materials from the compound eyes.

femsxr:.,/^

coxa

Spina of the
cl«aner

front ^
of vorker
'hDneyb<?e-

g.... tibia

eyebmsbes

> tarsus

^-articularis

■poiten
cojtib

middle leg'
of worker
honey beer-

„f)dten Ijasket
vnelatorsxTS

hind leg"
o^-workei"
Money bee

-^lanta.

inner .surf a<ie
ofmatatarsus
of hincC le^

These appendages are used for more purposes than locomotion. Find all the
adaptations shown and give the use of each adaptation to the bee.

The first joint of the tarsus is provided with long hairs which form a
pollen brush. This is used to collect pollen grains scattered over the
hairs of the body. At the base of the first joint of the tarsus is a
semicircular notch lined with short, stiff bristles, while a flat spur
projects from the distal end of the tibia. This apparatus is the
antennae cleaner. To accomplish this function the front leg is ex-
tended with the notch placed at the base of the antenna, which
when drawn backward through the notch is effectively cleaned of
pollen.

The middle pair of legs is not so highly specialized as the anterior
pair. There is a large spine near the outer end of the tibia which is
used as a pick for removing flakes of wax secreted from the wax pockets
located under the abdomen. The flattened basal segment of the
tarsus is called the planta. Its hairy surface is used for brushing
pollen from the body hairs.

The hind legs are larger and broader tlian the two anterior pairs.
They carry most of the pollen gathered from flowers to the hive.
The slightly concave outer surface of the tibia, called the pollen
basket, is lined by long outward-curving hairs, and may often be seen

208 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

filled with pollen in bees returning to the hive. The inner surface
of the first tarsal segment, or metatarsus, is covered with rows of stiff
bristles forming the pollen comb, while the lower edge of the tibia ends
in a row of spines, called the pecten. The pecten of one leg is scraped
over the pollen comb of the opposite leg, the pollen thus obtained
being pushed up into the pollen basket by means of a Uttle projection
on the upper edge of the metatarsus.

Honey Manufacture

Although bees make honey, which is a good energy-releasing food,
they do not live entirely upon it because of its lack of proteins needed
for building up the body. Both adults and larvae use pollen mois-
tened with saliva and honey, which forms "bee bread." Bees suck
up nectar from flowers, pass it through the esophagus into a thin-
walled crop, or honey stomach. This organ is an extensile sac which
when filled holds only a drop or two of fluid so that numerous trips
to and from the hive are necessary to fill a single cell of honeycomb.
The gathered nectar remains in the honey stomach until the bee
returns to the hive, when it is regurgitated and placed in the cells of
the honeycomb. As honey, it is still too watery, so some of the
workers, by a rapid vibration of their wings, cause enough water to
evaporate to bring it to the right consistency. Just before the honey
is capped in the comb, the worker places a minute amount of formic
acid from its poison glands in the cell. This aids in the preservation
of the honey. Bees store somewhat over two pounds of honey a day
for the average hive. This is in addition to what the adults eat and
what is fed the young. Honey storage, of course, varies with the
weather. Bees, like human outdoor laborers, do not work on rainy
days.

Dr. L. Armbruster of Berlin made some interesting computations
on the number of visits of bees to flowers necessary to store up about
two and one half pounds of honey. He found that bees have to visit
at least 6,000,000 clover heads, as clover honey seems to require the
most work. Peas, at the bottom of the scale, called for as low as
80,000 visits from the bees, and other honey-producing plants fell
within these two limits. Among the most important honey-produc-
ing plants are white clover, buckwheat, and fruit trees in the East
and North ; alfalfa, sweet clover, and a few trees, as the tulip tree, in
the Central West ; the citrus fruits, palmettos, and mangrove in the
South ; and alfalfa, sages, citrus and other fruit trees in the far West.

THE POPULAR INSECT PLAN

209

-Salivary glarjcCS
esophogors

Digestion, Circulation, Respiration, and Excretion

The digestive tract posterior to the crop has to do with the digestion
of food. The stomach, a large cyhndrical structure, has a valvelike
arrangement between it and the crop to prevent nectar not used as
food from going further. It leads into a small intestine, which in
turn expands to form the
rectinn at the posterior •

end of the body. - (^'W'^'^ ■ - pl^^'^yn&al glands

Attached to the an- x% '^-^A<a#-P°stc«nlbml%iands

terior end of the intestine
is a circle of Malpighian
tubules, about one hun-
dred in number, named
after their discoverer,

Marcello Malpighi, who '^V^^^^^^'^^^-'J^f
first pictured them in
his Anatomy of the Silk-
worm published in 1669.
The tubules are excre-
tory in nature, as is
proven by the fact that
small crystals of nitrog-
enous wastes are formed
in them.

In the insects and
crustaceans, there is no
closed system of blood vessels as was found in the earthworm, but
in the former there is a well-developed, dorsally placed, tubular
heart, located in the abdomen and perforated by paired openings,
or ostia, through which blood enters. Blood is forced out of the
anterior end into spaces, or sinuses, which in the insects are found
throughout the body cavity and take the place of blood vessels.
The heart acts somewhat like a rubber bulb syringe in a pail of
water, serving, along with the muscular movements of the insect,
to keep the blood in motion through the blood sinuses. Snod-
grass ^ shows that there is a rapid and complete circulation of
blood through the main sinuses, the blood being forced backward
into the abdomen on the ventral side of the body by the pulsat-

smdl intestine
ventriculus.

rectal gkncC

The food tube of worker bee and glands con-
nected with it. The pharyngeal glands form the
royal jelly or brood food given to the larvae by the
workers. The postcerebral glands secrete a fatty
substance, which is thought to be mixed with wax
in making honeycomb. The salivary glands are
true digestive glands. (After Snodgrass.)

1 Snodgrass, Anatomy and Physiology of the Honey Bee, McGraw-Hill, 1925, pp. 189-190.

210 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

-airsccC/

.tracheal tube

spiracle^ 1—

ing vibrations of the so-called ventral diaphragm, a sheet of thin
tissue which is stretched across the ventral part of the abdominal
cavity, while on the dorsal side it is pumped by the heart toward the
anterior end of the body. The blood, which bathes all the tissues,
consists of a plasma, and colorless blood corpuscles or leucocytes.
The plasma is rich in food substances, but there are no oxygen-
carrying substances in it, so that insect blood only carries foods and
wastes.

Oxygen is brought directly to the tissues by a very efficient type
of respiratory organ, the so-called tracheae and their branches. Along
the sides of the thorax and abdomen of insects are found paired
openings called spiracles. In the worker bee there are ten pairs of

these openings, three pairs
in the thorax and seven
in the abdomen. The
spiracle is an oval open-
ing which can be opened
and closed by means of
a flat plate attached to
its rim. Each spiracle
leads into a tracheal tube,
the wall of which is
strengthened by a spiral
thread of chitin, thus
keeping the tube filled
with air. These tubes
branch again and again
until they finally end in
tiny tubules between the
body cells. Expansion
and contraction of the
muscles of the body wall
force air in and out
through the tracheae, thus securing circulation of oxygen to all body
cells. In addition to the tracheae, large air sacs are developed in
the thorax and abdomen, as are seen in the above diagram. Since
insects that fly rapidly usually have better developed air sacs than
those that are sedentary, it is evident that the air sacs must serve
to "lighten the load" of the body in its flight as a heavier-than-air
machine.

Spiracle 3..

spiracle 4....^
Spiracle 5...
Spiracle 6-
spiracle 7-

-i|K — anrsac

-,-tube5 join
■ dorsal sclcs

If. .commissure

A portion of the tracheal system of the worker
bee. The dorsal trachea and air sacs have been
removed. Three spiracles are not shown. What
advantages are there in having this type of re-
spiratory system? (After Snodgrass.)

THE POPULAR INSECT PLAN

211

The Nervous System

The nervous system of the bee is well developed, consisting of a

series of ganglia, forming a double ventral nerve cord with a dorsal

cerebral ganglion (brain),

antennal .

rain.

optic loloe

to -vin^?

to leg

connected by a circum-
esophageal nerve ring with
a subesophageal ganglio7i
directly underneath it.
Although typically in a
segmented animal there
should be one ganglion
for each segment, we find
fewer ganglia than seg-
ments in the adult bee.
This is because certain of
the ganglia have fused,
there being seven in the
ventral ner^'e cord of the
bee. From each of these
nerves efferent fibers ex-
tend to the muscles while
afferent fibers from sense
cells end in the ganglia to
make up the reflex arc
previously described (page
195). Not all co-ordi-
nation of muscles is controlled by the brain, for a headless bee will
still walk and experiments have shown that the body ganglia are
independent centers of control over the appendages. Insects of the
order Hymenoptera, to which the bee belongs, have the best brain
development of any of the insects, a fact that seems to be correlated
with their complex social habits and their keen senses.

Nervous system of worker bee. Why are the
ganglia in the thorax so much larger than those
of the abdomen? Note that the brain is on the
dorsal side, the esophagus (not shown) passing
between the two nerves that connect it with the
first thoracic ganglion. (After Snodgrass.)

Reproduction and Life History

Although the workers possess undeveloped ovaries, all the eggs are
laid by the fertile female or queen. While a worker may live about
six weeks in summer and never more than a few months, the queen
lives three or fotir years, or even longer. The ovaries of the queen are
made up of a number of tubules, in which are eggs in all stages of

212 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

development. The fully developed eggs pass out through the oviduct
into the vagina, where they are fertilized by sperm cells that were
placed in a sac called the seminal receptacle by a drone during the nup-
tial flight of the queen. The drones form the sperm cells in two testes,
but the sperms are stored in seminal vesicles from which, during mat-
ing, they are transferred to the seminal receptacles of the queen.

The queen lays fertilized eggs in honeycomb cells of the worker
and unfertilized eggs in the larger drone cells. Just how she controls

the actual fertilization
cocoon. .W-^

of the egg is not known.
According to Nolan,^ the
queen produces an
average of about 900
eggs a day during the
season, but may lay as
many as 2000 a day
during the period of
greatest honey making.
The queen places the
eggs in the cells by
means of an ovipositor,
which in the workers is
modified into a sting.
The latter structure is
made up of two darts,
closely applied to each other so as to form a tube through which
poison from a poison sac flows when the darts are forced out of their
sheath as the bee stings. Two different poisons are produced, one of
which is formic acid, the other an alkahne substance. Worker bees
usually die after stinging, as the sting with its attached parts, along
with some of the intestine, is left in the wound. The queen, which
also has a sting, uses it only in combat with other queens and does
not lose her life in its use.

The life history of the bee is rather brief. Three days after fertiliza-
tion the egg hatches into a larva which lies in the cell surrounded by
a plentiful supply of "bee milk," a mixture of digested honey, pollen,
and saliva. After three days of feeding by the young "nurse" bees,
the larvae are given more and more undigested food. Drones are

<^©er2 cell

Cells of hive of honey bee. Note the stages in
development of worker. How many kinds of cells
are shown? (Read page 213.)

1 Nolan, " Egg-laying Rate of the Queen Bee." Gleanings in Bee Culture, Vol. 52, 1924, pp. 428-

431.

THE POPULAR INSECT PLAN 2i;{

fed undigested honey and pollen after the fourth day, while young
queens are fed upon an especially nutritious albuminous "royal
jelly" until they pupate. During the larval period, the young
insects grow rapidly, changing their skins or molting several times
during the process. About the end of the fifth day the larvae are
given their last food by the attendant bees and the cell is capped with
wax. Then the larva spins a cocoon, molts for the last time, and
becomes a pujM. In this stage it begins to assume adult characters
and, after the next molt, emerges from the cell as an adult. This
process, in which the insect undergoes certain changes not in line
with its direct development, is called a metamorphosis. The last
molt in which the young adult is ready to emerge from the cell takes
place about 20 days from the time the egg was laid. The young
adult bee, or imago, chews its way out of the cell, usually emerging
on the 21st day. The metamorphosis of the drone takes 24 days and
the queen 16, the greater rapidity of the latter probably being due to
the more nutritious food received.

The Life in the Hive

The activities in a bee hive are numerous and interesting. Besides
collecting nectar and pollen and making honey, the most important
work is that of building the wax cells of the comb. Wax is secreted
by the wax glands on the abdomen and transferred to the mouths of
the workers, where it is mixed with saliva, kneaded by the mandibles,
and shaped into the familiar hexagonal cells of the honeycomb.
Six kinds of cells are made : (a) drone cells, (6) worker cells, (c) queen
cells, {d) transition cells between worker and drone cells, (<?) attach-
ment cells which fasten the comb in place, and (/) honey cells.
Worker bees also bring back propolis or "bee glue," resinous materials
collected largely from the buds of trees. The propolis is used to fill
up cracks in the hive and to strengthen the comb. Water is also
carried to the hive in dry, hot weather. Besides the above activities,
others must be performed if life in the hive is to go on. The workers
must have plenty of fresh air, for they do hard work. To this end
certain of the bees are delegated to the task of vibrating their wings
rapidly, thus creating currents of air through the hive. Some workers
rid the hive of excreta, dead bees, or any other substances that
interfere with its cleanliness. Still other bees guard the entrance of
the hive against such enemies as bee raoths or yellow-jacket hornets,
which come to steal honey.

214 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

One other activity is that of swarming. In early summer, the hive
frequently becomes overcrowded, and when such conditions arise
several queen cells are built and young queens are raised. When a
young queen hatches, the old queen gathers together several thousand
of the workers, who fill their honey stomachs with honey and then set
out to form a new colony. Sometimes scouts are sent out in advance
to seek a place for the new hive, which may be in a hollow tree.
Often the swarm, forming a large ball about the queen, will come to
rest on the branch of a tree and the beekeeper may then hive it in an
artificial hive. It is interesting to note that our honey bee {Apis
meUifica) is an emigrant from Europe and that there are no native
honey bees in this country.

This social life with its accompanying division of labor is seen in
varying degrees all through the order Hymenoptera. Beginning with
the solitary bees, we find increasing social complexity of life until, in
the ants, a highly organized group is developed having several different
kinds of workers, soldiers, and males. If you want fascinating reading
along this line, look into William Beebe's Jungle Life, or better, into
Wheeler's masterly volume on Ants.

SUGGESTED READINGS

Carpenter, G. H., The Biology of Insects, The Macmillan Co., 1928.

Chapters II, III, IV, V, VII, and IX make interesting reading.
Fernald, H. T., Applied Entomologij, McGraw-Hill Book Co., 1935. Chs. IV,

V, and XXXIII.

A useful book of reference.
Kellogg, V. L., American Insects, Henry Holt & Co., 1908.

Still an authentic book of reference.
Metcalf, C. L., and Flint, W. P., Fundamentals of Insect Life, McGraw-Hill

Book Co., 1932. Chs. II, IV, and V.
Plath, 0. E., Bumblebees and Their Ways, The Macmillan Co., 1934.

A fascinating study of one type of social insect.
Snodgrass, R. E., Anatomy and Physiology of the Honeybee, McGraw-Hill

Book Co., 1925.

Parts of Chapters II, III, and IV are particularly useful, but a student

can cull much from the entire book.
WeUs, H. G., Huxley, J. S., Wells, G. P., The Science of Life, Doubleday,

Doran & Co., 1931.

Pp. 1147-1182 give one phase of insect life worth reading about.

XII

THE ART OF PARASITISM

Preview. Who qualifies? • Some host-parasite relationships : The host-
parasite conflict ; effects of a parasitic life ; keeping the cycle going ■ The
complexity of parasitic relationships : External parasites ; temporary para-
sites, periodic parasites, pennanent parasites ; internal parasites ; parasites
requiring one host, parasites requiring two hosts, malaria, parasites requiring
more than two hosts • Suggested readings.

PREVIEW

According to the definition, a parasite is one that "lives on or
within, and at the expense of some other organism," and thus might
include forms from the smallest, such as filtrable viruses and bacteria,
to some of the largest species. As a matter of fact, parasitism is
well-nigh universal, for examples are found among nearly all groups
of plants and animals.

In many instances there appears to be a remarkable balance
between the parasite and its host. A dead host is of little use to a
parasite since it implies a loss of free transportation as well as board
and lodging. Consequently the existence of a parasite must be a
compromise, for it must be able to secure enough nourishment to
maintain and reproduce itself and yet do this either without injuring
too much the vitality of its host, or actually reducing its own numbers.
As a result of this rather elaborate compromise parasites have become
so adapted that they usually destroy only small portions of the host
tissue which usually can be replaced by regeneration.

Whenever a parasite reaches a final host, the problem of propaga-
tion arises. Most parasites produce large numbers of eggs, cysts, or
spores that are discharged with the waste products of the host.
Through the medium of food or drink, these reach the next host, which
is sometimes intermediate or secondary, the parasites thus becoming
dependent upon the food habits of more than one organism to main-
tain their cycles.

Most animal parasites are essentially carnivorous in their feeding

habits. True carnivores, however, destroy their prey, whereas

parasites as a rule do not, and while carnivores are much larger than

their prey, parasites are smaller. Elton says, to summarize, "The

H. w. H.— 15 21. "S

216 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

difference between the methods of a carnivore and a parasite is simply
the difference between Hving upon capital and upon income ; between
the habits of the beaver, which cuts down a whole tree a hundred
years old, and the bark beetle, which levies a daily toll from the tissues
of the tree ; between the burglar and the blackmailer. The general
result is the same, although the methods employed are different." ^

Who Qualifies?

Parasites vary greatly in their types of relationships. Some may
be classified as either internal or external, according to their location in
or upon the host. They may be otherwise classified as temporary, or
free-living during a part of their life cycles ; yermanent, or parasitic

throughout their life span ;
and 'periodic, only visit-
ing their hosts to obtain
nourishment. Actually
there are almost as many
gradations and variations
in the degree of parasitism
among animals and plants
as there are kinds of par-
asites. Mosquitoes and
some fleas visit their hosts
just long enough to sat-
isfy their appetites. The
cattle tick, Boophilus an-
nulatus, never leaves its
host except when ready to
lay eggs. Scab mites and
some lice are permanent,
living upon the same host
from one generation to the next, only leaving or being transferred
by direct contact. In between these extremes occur such well-known
forms as the hookworm, which has a free-living larval stage, and
the botflies that pass their larval existence as parasites.

Among plants, the large and heterogeneous group of bacteria exhibit
many varieties of parasitism, while higher in the plant scale such forms
as dodder and broomrape exemplify true parasitism. Other groups
are partially parasitic during their life cycle.

■ From Elton, C, Animal Ecology. By permission of The Macmillan Company, 1935.

Wright Pierce

Dodder, an example of a plant parasite which
starts life as a self-respecting plant growing in
soil.

THE ART OF PARASITISM

217

Some Host-Parasite Relationships

In the event of parasitism, the association is definitely in favor of
the parasite, since it usually "lives on" the second party concerned,
the host. Such a relation-
ship constitutes a fourth
type of habitat, namely
parasitic, that is available
to both plants and ani-
mals along with the well-
recognized terrestrial,
fresh-water, and marine
habitats. That many or-
ganisms take advantage
of this type of existence
may be clearly proved by
observing the plants and
animals of any locality.

The Host-Parasite
Conflict

Theoretically a conflict
exists between the para-
site and the host. The
latter has as its chief
weapon a lytic or dissolv-
ing power which is a nor-
mal physiological reaction. Likewise the weapons that probably
were first brought by the parasite from its hypothetical free living
ancestral state must also have been of a lytic, toxic, or otherwise
destructive nature. In many cases the host seems to have adapted
itself to bear the burden of parasitism with the least possible
outlay of energy on its own part, so that eventually there has devel-
oped a balance between the two organisms, which might be called
a Jwst-parasitc equilibrium. In order to reach this equilibrium the
parasite has likewise gradually e^'olved some sort of protectixe device,
often a capsule which becomes interpolated in the cycle, or an anti-
enzyme or anticoaguUn to counteract the destructive action of the
host's secretions, thus necessitating a counter attack upon the part
of the host. This apparently was made, first, through the o\or-

W right I'icTCe

The large masses in this tree represent a true
plant parasite, the mistletoe.

218 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

development of the host mechanism which elaborates specific pro-
tective substances called antibodies, and secondly, through "the
adaptation of certain normally phagocytic cell groups to the intern-
ment and gradual destruction of the parasite." ^

Sometimes the introduction of a parasite has a visible effect upon
the host. In the case of certain gall insects, such as the Cynipoidea,
the deposition of an egg by the female in a plant tissue, or the subse-
quent movement of the larva, furnishes the stimulus which causes

H^HI^^^^^^^HBf^#i ^

^^H

I^^HRP^'

^Hf^l

^^^^^^H^^.'

iPn ■'■

^^^^^^^^^.^TlK^^^^I

Wright Pierce

Unopened oak gall beside one which has been opened to show the enclosed

larvae.

abnormal proliferation of tissue, resulting in the enclosing of the insect
larva and the production of a so-called gall. The type of gall pro-
duced on a given plant appears to be specific, whether it occurs in
root, leaf, twig, or stem. Usually a gall ceases to grow about the
time when the enclosed larva finishes feeding. In such instances it
dries and forms a protective covering inside of which the insect
pupates, ultimately gnawing its way out.

Effects of a Parasitic Life

Parasitism as a biological phenomenon probably has a more far-
reaching effect upon the structure of the parasite than upon the host.
In the first place the former no longer has to worry about locomotion
or the securing of food because these two important functions are
taken care of by the host. Consequently a gradual simplification
of the organs of a parasite takes place, until in forms like the tape-

> Smith, T., Parasitism and Disease, Princeton Univ. Press, 1934, p. 111.

THE ART OF PAIUSITISM 219

worm, and the spiny-headed worm, for example, there is no trace
whatever of a digestive tract in the adult. Such worms, however,
have access to various digested foods which are ready for absorption
by the host and it appears certain that these gutless forms must be
able to absorb and utilize materials from the alimentary canal of
their benefactor. Other worms, such as the flukes or trematodes, and
roundworms, possess a well-developed alimentary canal, the secretions
of which, in some instances at least, cause a liquefaction of the tissue
in the immediate vicinity of the parasite, thus making it available as
food for the organism.

Another problem which parasites have had to solve is that of respira-
tion. In the case of cellular or blood-inhabiting forms the parasite
obviously has access to plenty of oxygen, whereas intestinal parasites
face a difficulty, since the alimentary canal is known to contain little
oxygen. Many investigators now believe that these worms secure
their energy from the breakdown of dextrose. This substance results
from the hydrolysis of more complex carbohydrates and is the form
in which it is absorbed from the intestine into the blood stream.
Presumably oxygen is secured during the process of anaerobic fermen-
tation that results in the splitting of dextrose or glycogen (if the
carbohydrate has been converted into glycogen during the metabolism
of the parasite) into fatty acids and carbon dioxide. This type of
metabolism is characteristic of some bacteria and yeasts.

One of the most striking effects of the parasitic habit lies in the
tremendous development of the reproductive capacity of the parasite,
a process undoubtedly correlated with the numerous hazards which
must be met if its life cycle is to be completed. The development
occurs in two ways, — first by the production of enormous numbers
of eggs, and secondly by the interpolation of asexual stages in the cycle.
Thus it has been estimated that a single free-swimming, ciliated stage
{miracidium) of a fluke may be the indirect parent of as many as
10,000 free-swimming, tailed larvae {cercariae).

External or ectoparasites also show marked evidence of adaptation to
their type of existence, as shown by the piercing and sucking mouth
parts of the parasitically inclined arthropods or the degeneration
of the mouth parts in the case of the adult botflies, as well as by
the laterally compressed body of the flea, and the loss of wings in
lice and bedbugs. Limitation as to the host and as to the location
on the host shows specialization among this group. These factors
tend to illustrate stages in the development of ectoparasitism.

220 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

Keeping the Cycle Going

The chief problem of any species centers about maintaining itself,
a statement to which there is no exception in the world of parasites.
Obviously those organisms which have become adapted as ectopara-
sites are not faced with complicated problems relating to the transfer
from host to host. By means of simple contact a new host may be
reached, or if a portion of the life cycle of the parasite is free living,
it may leave the host to deposit its eggs. Even in cases where the
eggs are laid among hairs of the host they usually fall to the ground
to develop.

A more difficult problem of maintaining the species must be faced
when internal parasites are involved. Bacteria which are capable of
producing protective capsules or spores of one sort or another are
tided over unfavorable periods and so aided in reaching new hosts.
They are adapted also for rapid reproduction. One worker has esti-
mated that if the multiplication of bacteria were unchecked one cell
would be the parent of 281,500,000,000 bacteria in two days. Such a
mass at the end of the third day would weigh about 148,356,000
pounds.

Many parasitic protozoa as well as metazoa are adapted to be
transferred from one host to the next by means of resistant cysts
secreted by the organism. Others, like the blood-inhabiting try-
panosomes or malarial organisms, secure transference by adapting
themselves to insects which act as wholesale distributors for the
parasites. Some produce harmful toxins which occasionally kill the
host. In such instances, however, one may be sure that the host is
abnormal and the parasites have not become adapted to it. In the
case of the trypanosomes of man in Africa, antelopes are their natural
hosts and are quite tolerant to these blood parasites. Since man and
domestic animals are unnatural hosts, they are consequently much
more severely affected by them.

The Complexity of Parasitic Relationships

The most satisfactory way to secure a general idea of the surpris-
ingly varied adaptations to a parasitic existence is by a study of
a few examples. Such a study emphasizes clearly the almost uncanny
adaptations which have been made by parasites to insure the com-
pletion of their life cycles. While various types of parasitism clearly
exist, nevertheless the line that demarks one kind of parasite from

THE ART OF PARASITISM

221

another may not always be sharply drawn. However, for the sake of
convenience an attempt will be made to outline briefly a few examples
of such relationships.

External Parasites

External parasites are found throughout the plant and animal king-
doms. Even among the minute protozoa, ectoparasitic organisms
occur, such as Cyclochaeta, a parasite on fishes, which may cause an
appreciable economic
loss under epidemic
conditions. The lam-
prey eel among the
chordates is a large
external parasite on
certain fishes.

For the sake of con-
venience, external par-
asites may be classified
as to whether they are
temporary, periodic, or
permanent. Some
forms, like the house
fly, do not really belong
in any of these cate-
gories. Yet the house
fly certainly deserves
mention, since it serves as a mechanical carrier from one host to
another for the transfer of numerous bacteria and their spores, as
well as the cysts and eggs of various other parasites.

Temporary Parasites. As an example of temporary parasitism
may be mentioned the parasitic Hymenoptera that lay their eggs
on the eggs, larvae, or even the adults of other insects. During the
developmental interlude they remain as true parasites within the
body of the host until they eventually destroy it, at which time they
cease their parasitic existence and become free living. The ichneu-
mon flies, that belong in this group of parasitic Hymenoptera, each
year attack and destroy great numbers of injurious as well as some
beneficial insects. Another example of a temporary parasite is the
ox botfly, the free living adult of which attaches its eggs to hairs on
the legs of cattle. Upon hatching, the larvae penetrate the hide and

1^^

"-^l^'^^^LjEPmS^^^^^Mnr

ll^Sj^Slmr'

1^' '^HP

^^^^^S.1

fW^'^^J

American Museum of Natural History

These brook lampreys are close relatives of a
larger form which frequently attacks fish and remains
as a temporary external parasite until the host is
destroyed. What type of mouth is characteristic
of this group ?

222 ORGANISMS ILLUSTI\ATING BIOLOGICAL PRINCIPLES

wander through the underlying tissues of the host until in the spring
of the year they come to lie beneath the skin, which is soon punctured
to serve as an air vent. Finally, when the larvae are full grown they
burrow out, fall to the ground, and there pupate, finally emerging
as adult free living flies, destined to ruin many million dollars' worth
of hides annually.

i>e;Comes

larva fall? to
drouncC , pupatss

rnatas,
Iccys

becomes j

lodgecC

xxndzr

biole.

tovaroC

spring

penetratss
hide of
cattle and ,^^

tissue during
the. vinter

Life cycle of the ox botfly.

Periodic Parasites. Other arthropods definitely fall into the
group of those that are periodically parasitic. Such forms are
predators, and most of them are blood suckers, in which manner they
may serve as a link in a chain of parasitism. Thus the female mos-
quito serves as the carrier for organisms that cause malaria, yellow
fever, and filariasis. Others like the tick or rat flea may not only
secure a meal of blood from one host but at the same time be the means
of transmitting Rocky Mountain spotted fever or bubonic plague to
some other host. Certain species fall into the realm of parasites by
their own right, the tick and botfly clearly belonging in this latter
group.

thp: art of parasitism

223

\\ ri(jhi I'iirct

Longitudinal section showing mistletoe invading
the tissues of its host.

Permanent Parasites. Comparatively few organisms belong in
this category. Some of the flukes with a continuous life cycle like
the marine Epidella melleni, or the gill fluke, Ancyrocephalus, pass
their entire cycle upon the
same host, adding their
progeny to the same ani-
mal and so on ad infinitum.
Similarly, the female head
louse that cements her
eggs, or "nits," to human
hair from which newly
hatched lice appear within
six to ten days is another
example of a permanent
parasite. The new addi-
tions to the community of
head lice must soon feed
upon the roots of the host's
hair or else they will die.

The parasitical mistletoe is practically permanent in habit, since
it not only taps the life-giving fluids of its host but also lives for
many years upon the same tree.

Internal Parasites

The food cycle plays a vital role in the dispersal of all internal
animal parasites. It frequently happens that animals which suck
the juices of plants or the blood of other animals play an important
part as an intermediate host. It should be borne in mind that when
carnivores are included in the chain of parasitism, the cycle tends
toward greater complexity. A few examples will serve to illustrate
this point.

Parasites Requiring One Host. The adult hookworm, Necator
americanus, lives in the small intestine of man, where the adult female
is attached to the walls of the intestine and produces great numbers
of eggs which are eliminated from the digestive tract in the early
developmental stages. Under proper conditions of soil, temperature,
and moisture, development of the larvae proceeds rapidly, so that
hatching may take place within 24 hours. The small larval form is
only about 0.25 mm. in length, but by the end of the third day it has
nearly doubled in length and soon molts twice, then being in the

224 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

infective stage. Hookworm larvae may enter the body in food or
drink, but the normal method of entry is by actively boring through
the skin of the human hand or foot. For this reason the disease is
called "ground itch" because of the inflamed areas caused by the

to stomach

egg passed,
in fexies

larva ready''
to infect -^
human iTost

Life cycle of the liookworm.

r-yo

entrance of the larvae. Once liaving effected entrance into the host,
the minute worms are passively carried through the blood stream to
the heart and thence to the lungs, where they migrate out from the
capillaries of the lungs and work their way through the delicate
walls of the air sacs into the lung cavities. They next migrate up

THE ART OF PARASITISM 22:,

the lung passages over the "saddle" to the esophagus, and there
are swallowed, reaching the stomach and eventually the intestine.
Within the next fortnight two more molts occur, after which the
parasites reach maturity, copulate, produce eggs, and continue the
cycle.

The large roundworm, Ascaris lumhricoides, lays eggs which de-
velop into infective embryos within three weeks under proper con-
ditions of temperature and moisture. After reaching the digestive
tract of the host together with food or drink, the newly hatched
larva burrows through the mucous layer and starts on a "10-day
tour" following essentially the same itinerary as that of the hook-
worm.

Among the protozoa the Ameba, Endamcha histohjtica, the cause
of amebic dysentery, is transmitted from one human host to another
and thence to the outside world, and back again to the human large
intestine by means of resistant cysts carried in contaminated food
and drink.

Parasites Requiring Two Hosts. The dread pork roundworm,
Trichinclla spiralis, while a permanent parasite having a relatively
simple life cycle, nevertheless requires two hosts to complete its cycle.
The encysted larvae occur in a variety of hosts, but are normally
secured by man through eating insufficiently cooked pork. The
parasites mature rapidly in the small intestine and reproduce within
twenty-four to forty-eight hours of their arrival. Each viviparous
female produces between 10,000 and 15,000 larvae, which are depos-
ited directly in the lymph or capillaries lining the intestine, and are
thus circulated by the blood until they reach the voluntary muscles of
the body. There, these minute roundworms leave the blood stream,
enter the muscle fibers, where within a month a lemon-shaped cyst is
deposited about them. Since man is not cannibalistic, the introduc-
tion of these parasites into his body becomes a blind alley so far as
completing the life cycle is concerned. Unfortunately, when these
parasites are once established in the body, there is no way of getting
rid of them. In due course of time, calcium carbonate is deposited
about the cyst and eventually the parasite dies, but the obnoxious
cyst remains to remind the infected person of his injudicious meal
by frequent muscular pains which may accompany this infection
for years. The normal hosts of TrichineUa seem to be the rat,
mouse, and pig. The former are commonly found in numbers about
slaughter houses and the percentage of their infection is usually

226 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

liigh. A great number of other animals have been experimentally
infected.

Nearly all of the taenioid tapeworms have a rather simple life
cycle. In the case of the beef and pork tapeworms, for example, the
infective stage occurs in the flesh of the host in a milky white cyst.
When this larval tapeworm, or cysticerus, consisting of an inverted
head or scolex and its outer cyst wall, is ingested by man, the head

if imppcfparly
cooked. , pork with
"^ cysLs In musda.
f ibens may develop
vhan eaten 'by

encysts in pork
if hog' is "host-

vulva

oviduct

tacome adults

in smoll intestine,

vithin afev dajs-

■females burro*^ into

U5C mucosa , depos-it

over 10,000 larvae^

intestine
of hog-

larvae enter blooeC
stream , are carried
to vol untoj'y muscles

•muscles ^ , . , ,

of xnan which larvae penetrate and then

The life cycle of Trichinella.

becomes everted, and then attached to the intestinal wall, where the
worm starts budding segments or proglottids and soon reaches sexual
maturity. Proglottids of Taenia saginata, or proglottids together
with free eggs in the case of T. solium, are passed with the feces and,
when eaten by the proper intermediate host, develop into cysticerci.
Cattle, buffalo, giraffes, and llamas may harbor the larval form of the
beef tapeworm, while the hog, camel, monkey, dog, and man are the
only known hosts for the pork tapeworm. The chief difference
between the cycle of these two parasites centers around the possibility
of auto-infection in the case of the latter. This occurs by ingesting
the eggs destined for the outside, which hatch in the intestines,

THE ART OF PARASITISM

227

lorozoitsi

ifeetecL
sctUvory
glcmcd

gametoejtc

migrate to the blood stream and so reach various parts of the body,
there producing cysticerci. As in the case of Trichinella, human
infection really becomes a blind alley for the parasite.

Malaria. One of the most economically important parasites is
the causative organism of malaria, a minute spore-forming protozoan
of the genus Plasmodium.
The infective stage, or spo-
rozoite, reaches the blood
stream of man in the saliva
of the mosquito, which is
poured into the wound im-
mediately after the victim
is punctured. This minute
parasite promptly pen-
etrates a red corpuscle and
starts to de\'elop asexually,
growing until it fills about
one half of the corpuscle.
It is now ready to undergo
the asexual reproductive
cycle. The chromatin mate-
rial is gradually separated
into a number of tiny
masses, each one of which
finally becomes surrounded
by a bit of cytoplasm.
Growth continues until the
red corpuscle is filled with e eye e o

a number of new indi\-iduals called merozoites. Soon the corpuscle
bursts, liberating these merozoites, each one of which seeks out a
new corpuscle and begins the asexual cycle all over again.

This asexual cycle recurs regularly, the intervals depending upon
the species of parasite infecting the blood stream. Thus in the case
of tertian malaria, schizogony is completed every twenty-four hours,
while in the quartan type it takes forty-eight hours to complete it.
The periodic chills and fever so characteristic of malaria occur at
the time of the bursting of the red corpuscles with the subsequent
release of the asexually formed merozoites and the accompanying
waste matter. Quinine is the most widely used drug to combat the
infection as it destroys the newly "hatched" merozoites.

jneTO*oiteS^&S®

228 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

After a number of asexual generations have been produced, special
larger, sausage-shaped crescents appear within the red corpuscles.
These are the gametocytes, or sexual forms. If a female mosquito
sucks blood from a person having mature male and female stages of

the parasite in the blood, such
i^a, parasites are taken mto the diges-

tive tract of the mosquito, where
union of the male and female
gametocytes takes place. After
conjugation the resulting zygote
forms an ookinete or cyst that
enters the lining of the stomach
of the mosquito, in the outer
walls of which a complicated de-
velopment then ensues for about
twelve days, ending with the
formation of a large number of
spindle-shaped structures called
sporozoites. The cyst then bursts
and the sporozoites migrate to the
salivary gland of the mosquito.
After that time, if the female mos-
quito bites an uninfected human
host she infects him with the sporo-
zoites, which enter red blood cells.
Animals are not the only group
having complicated parasitic
cycles. The various smuts, mildews, and rusts are plant parasites
that annually take their toll throughout the country. Wheat rust is
probably one of the most destructive of the parasitic fungi. This
rust has been the most dreaded of plant diseases because it destroys
the harvest upon which the civilized world is most dependent. Wheat
rust has long been associated with barberry bushes. As early as
1760, laws were enacted in New England providing for the destruction
of barberry bushes near wheat fields, although nothing was actually
known of the relationship between the barberry and rust until com-
paratively recent years. It is now known that wheat rust may pass
part of its life as a parasite on the barberry, whence it migrates to the
wheat plant and there undergoes a complicated life history. Since
the nourishment and living matter of the wheat are used as food by

Diagram of eggs, larva, pupa, and
adult of Culex (left) and the malarial
carrying Anopheles (right). How could
you tell the eggs, larvae, and adults of
these two genera apart ?

THb: AllT OF PAllASITISM

229

the parasite, the plant is weakened and Httle or no grain is produced.
A few of the wheat rusts do not require two hosts but complete their
life cycle on wheat alone. Such rusts pass the winter by means of
thick-walled spores which may remain in the stubble or in the ground
until the young wheat
plant appears the follow-
ing year, or the spores are
carried by the wind from
other regions.

Parasites Requiring More
Than Two Hosts

Tapeworms show a va-
riety of adaptations and
exhibit a unique and deli-
cate balance that permits
the completion of their
various cycles. Roughly
they may be divided into
two groups, one in which
the eggs reach water, sub-
sequently passing through
some aquatic organism,
and a second in which ova
are scattered in the soil
and reach the intermedi-
ate host by means of food
or drink. In the first
group are the broad tape-
worm of man, the bass
tapeworm, and many
others, while the second
includes the various tae-
nioid worms and their
relatives. All of these par-
asites show a remarkable
degree of specialization.

N. Y. State Conservaiion Depi.
The life cycle of the bass tapeworm {P. arnblo-
plilis). (1) The mature tapeworm occurs in the
intestines of the large- and small-mouthed hass.
(2) Contact with water causes the proglottids
to liberate the eggs which are eaten, (.3) by
various copepods. \\ hen infected copepods are
eaten by many species of plankton-feeding fish
(1) a larval tapeworm (plerocercoid) develops in
the mesenteries, liver, spleen, or gonads of these
fish. Heavy infections in the small-mouthed
bass affect reproduction. The tapeworm reaches
maturity when fish infected with the larval stage
are eaten by larger ones. How could this cycle
be controlled in fish hatcheries!'

The broad tapeworm of man, Diphyllobothrium latum, was brought
to this country sometime during the last century by immigrants from
the shores of the Baltic Sea. The worm matures in

the digesti\e

230 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

tract of the host, producing a string of as many as 3000 to 4000 seg-
ments or proglottids, often reaching a length of ten meters. Mature
proglottids, passed from the host with the feces, must reach water,
where the eggs are shed. After a developmental period in the water,
the eggs hatch into free-swimming larvae (coracidia), which to continue
development must be eaten by a copepod. The parasites penetrate
the intestinal wall and so reach the body cavity of this host, where
they develop until the copepod is in turn eaten by a fish, when they
usually penetrate to the flesh of the host and grow to approximately
six millimeters in length. Various fishes, such as the northern pike,
Esox lucius, wall-eyed pike, Stizostedion vitreum, sand pike, S. cana-
dense griseum, as well as the burbot, Lota maculosa, may all serve
as second intermediate hosts for this important parasite. Man and
other carnivores acquire the infection by eating improperly cooked
fish.

The bass tapeworm which matures in large- and small-mouthed
black bass also requires three hosts — copepods, small fishes which
carry the larval stage encysted in the viscera, and the final host, or
adult bass. The life cycle of this parasite illustrates very clearly
the interdependence of organisms necessary for the completion of the

Adult yellow grub, enlarged
from mouth cavity of hejxn«

•i>;^@ e-Maturtegg ^o^' — "— "^^--tS

N. Y. State Conservation Dept.

Diagram of the life cycle of the yellow grub of bass (C. marginatum). (1) The
adult fluke in buccal cavity. (2-4) Embryo within egg hatches as free living
miracidium which, upon entering snail, produces a mother sporocyst and two
generations of rediae (5-8), cercariae (8-9), liberated by the daughter redia,
penetrate many species of fish (10-11) and mature when eaten by various
herons (12).

THE ART OF PARASITISM

2:{i

cycle. The adult tapeworm matures sexually in the spring of the
year, the mature eggs being shed into shallow water where the fishes
come inshore to spawn. The eggs of the parasites are soon eaten by
copepods and the developmental period necessary for the larval
parasite to reach its second infective stage is closely correlated with
the time interval between the laying of the bass eggs and the absorp-
tion of the yolk sac of the bass fry. At the time the young fishes
begin feeding upon plankton, the copepods in the vicinity of bass
nests are found to be much more heavily parasitized than at other
seasons of the year. It is adaptations such as these which enable
parasites to complete complex life cycles.

Flukes, or trematodes, probably undergo more complicated cycles
than any other group of parasites. In considering the complex
cycle of a trematode one should keep in mind that there are usually

Diagram explaining the life cycle of endoparasitic trematodes.

two free-living stages, — the miracidium and the cercaria. The
variations that may be expected in such a cycle are apparent upon
inspecting the above diagram.

The frequent presence of a second intermediate host suggests a
characteristic of most trematodes. For example, the great blue

H. W. H. — 16

232 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

heron harbors an adult fluke, Clinostomum marginatum, in its mouth
cavity. Eggs discharged by the parasite reach the water and soon
hatch, the miracidia penetrating snails. After several generations
in the snail, fork-tailed cercariae emerge to penetrate under the
scales into the flesh and sometimes on the fins of many species
of fresh-water fish. Here they grow into the typical yellow grubs
commonly found surrounded by a cyst formed by the connective
tissue of the host.

As a result of the above discussion of parasitism it is hoped that
some concept of the elaborate food chains and interrelationships and
interdependence characteristic of the various groups of parasites and
their hosts may be gained. Because these relationships are so com-
plicated and form so intricately woven a pattern, it becomes prac-
tically impossible "to predict the precise effects of twitching one
thread in the fabric."

SUGGESTED READINGS

Cowdry, E. V., et al., Human Biology and Racial Welfare, P. B. Hoeber, 1930.

Ch. XVII.

Popular discussion, resistance, etc., from the bacteriological point of

view.
Elton, C., Animal Ecology, The Macmillan Co., 1935. Chs. V, VI.

Excellent readable discussion of parasitism from an ecological view-
point.
Massee, George, Diseases of Cultivated Plants a>}d Trees, The Macmillan Co.,

1910, pp. 1-23, 59-77.

A good discussion of parasitic plants.
Needham, J. G., Frost, S. W., Tothill, B., Leaf-Mining Insects, The Williams

& WUkins Co., 1928. Ch. I.

Deals with natural history of group.
Smith, T., Parasitism and Disease, Princeton University Press, 1934.

Excellent general, but somewhat technical, discussion of the parasitic

habit.

XIII

ADVANTAGES OF BEING A VERTEBRATE

Preview. Vertebrate cliaracteristics • Skeletons • Invertebrate attempts •
The vertebrate endoskeleton • Suggested readings.

PREVIEW

How fortunate it is that we are vertebrates, not only vertebrates
in general but mammalian vertebrates of the royal primate line which
has blossomed finally into human beings !

When one thinks over the myriads of lowly, less endowed animals
scattered along the devious highways of evolution, who might have
been our near relatives, it is a real privilege to claim relationship
with such highly endowed primates as monkeys and apes. With
the inclusive vertebrate type, to say nothing of the specialized Pri-
mates, there are certain outstanding structures and qualities which
we as mankind are thankful to possess. They are so famihar to us,
however, that we are apt to forget how far our fortunate biological
heritage is dependent upon them.

Only a few of these distinctive vertebrate characteristics that give
us occasion for self-congratulation may be pointed out here. A
consideration of the Vertebrates as such forms a biological science
in itself, set forth in a voluminous Hbrary of descriptive and inter-
pretative books.

Vertebrate Characteristics

Even a partial list of the distinctive vertebrate endowments
would include the following: 1, a highly developed nervous system,
based upon a hollow dorsally-located nerve cord ; 2, a unique embry-
onic skeletal axis, called the notochord, which is the foundation for a
living internal skeleton, adaptable to the changing demands of growth ;
3, a peculiar kind of blood, that in the higher forms makes the mainte-
nance of a constant body temperature possible regardless of the sur-
rounding temperature ; 4, various devices for effectually transporting
the blood to every part of the body, devices that are as superior to the
methods employed by non-vertebrates as modern highways and
means of transportation are better than the conditions encountered
in the days of the trackless wilderness ; and 5, locomotor organs for

233

234 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

getting about on land, in water, and in air, far surpassing those
employed by lower animals.

Skeletons

Let us consider briefly just one vertebrate feature, namely, the
living inside skeleton, which gives the name "vertebrate" to the group.
It is the culmination of an endless array of experiments and adapta-
tions that have been going on since the beginnings of life on this
planet, and there is every indication that the end is not yet. The
skeleton of man, for example, is by no means the final mechanism of
its kind. There are to be expected in the future other models nearer
to perfection, though based upon all that has gone before.

Invertebrate Attempts

The microscopic protozoa made brave experiments with the idea
of a skeleton, in their case an armor mostly for protective pur-
poses and consequently found located on the outside of the animal
itself. In fact, protection seems to be the primary service of skeletal
structures in general, although secondarily supplanted largely by the
function of support and muscle attachment. It still plays an important
role even in the vertebrates, since the brain and cord, being ex-
tremely liable to injury, are enclosed within a protective skull and
enveloping vertebral arches, while the viscera are in part stowed away
within a bony thoracic basket.

In the great group of arthropods, that includes both crustaceans
and insects, the skeleton is plainly a protective external covering
which, being a lifeless excretion of the skin, does not change in size
after it has been laid down. As the anima'l grows, the dead inelastic
skeletal armor thus formed fits more and more tightly over the
enlarging body until finally it has to crack open in order that the
animal may emerge and become refitted, after an interval of rapid
bodily expansion, with a new and larger skeletal garment. This
complicated process is called molting. To elaborate and then periodi-
cally to reject all this material is not only a physiologically expensive
performance but it is also a hazardous one, since while shifting into
a more commodious suit of armor, the animal may lose a leg or two,
and is always exposed for some time to enemies while in its defenseless
shell-less condition.

Insects, caught in the same evolutionary blind alley with their
crustacean cousins, have taken an upward step by secreting a much

ADVANTAGES OF BEING A VERTEBRATE 235

thinner chitinous envelope than the more cumbersome "crust" of the
crustaceans. Instead of molting at repeated intervals throughout
life, they have hit upon the idea of metamorphosis, whereby they do
all their molting early during the growing larval stages. Then, as
adults of established and unchanging size, they live happily ever
after without being troubled by the inconveniences and perils of
growth within an unada])tive external encasement.

Another and paramount objection to a protective exoskcleton is
the increasing burden of a heavy armor which soon becomes insup-
portable, necessitating a limit to the size of the body encased within
it. The largest known representative of the enormous group of the
insects is probably smaller even than the smallest adult vertebrate.

The mxoUuscs have gone at the problem of evolving a skeleton in
another way. Although the skeleton is still on the outside, excreted
and consequently lifeless, it is never wastefully molted after the
crustacean fashion. The parsimonious molluscs keep every particle
of their old dead shells and simply add new layers on the inside, as
growth demands. The layers, being a little more extensive with each
addition, form by their edges the familiar "lines of growth" showing
as parallel ridges on the outside of the shell. This particular experi-
ment in skeletons, however, has cost the group of molluscs dear, for
the heavy shell, together with the accompanying policy of passive
defense, has either impeded the power of locomotion with all attendant
advantages that would accrue for the evolution of the nervous system,
or has brought about its complete abandonment. The clams and
their allies, therefore, have stuck conservatively in the mud and
lagged behind in the race for life, while other animals without the
incubus of a molluscan shell have toiled successfully on to higher
levels of attainment in working out their organic salvation.

The Vertebrate Endoskeleton

The vertebrates alone exploit a fundamentally different model in
skeletal structure.

An increase in size being necessary for dominance in the struggle for
existence, an adequate supporting scaffolding for the body is de-
manded, and as a result the skeletal function of protection now be-
comes secondary. Levers and muscles to work them to attain
locomotion, with ample skeletal surface for their attachment, are also in-
dispensable for animals that are to develop a successful nervous sys-
tem. The vertebrate skeleton provides for these adaptive advances.

236 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES

The fact that the vertebrate skeleton is inside the body makes it
a changeable living structure which, by reason of its capacity for
continuous growth, keeps pace with the increasing demands of the
enlarging organism. With the introduction of such a scheme of
mechanical support, the ban upon size imposed by a lifeless exoskele-
ton is lifted, so that during the Age of Mesozoic monsters there were
dinosaurs and similar beasts, for example, that were able to lift tons
of flesh into the air upon majestic bony scaffoldings. These prehis-
toric giants proved impracticably large, however, and vanished
forever from the face of the earth after recording by means of their
fossil remains the results of these colossal experiments in the mech-
anism of living inside skeletons. There still remain today, elephants
on land and whales in water as living illustrations of how far it is
possible to go in the matter of size when an adequate living internal
support is provided.

The remarkable superiority of the vertebrate endoskeleton over
all other skeletal devices is evident. It would be possible to go much
further and to unfold some of the marvelous details of adaptation
which every separate part of the vertebrate skeleton presents. That
would call for many pages. It is the task of the comparative anatomist
to assemble and elucidate the innumerable facts about the vertebrate
plan of structure, of which those involved in the skeleton are a
sample, and to point out wherein we are fortunate to be constructed
as we are. This is a fertile field, inviting intellectual adventure for
those who have the curiosity to explore it.

SUGGESTED READINGS

Adams, L. A., An Introduction to the Vertebrates, John Wiley & Sons, Inc.,

1933.

A fine text.
Keith, Sir A., The Engines of the Human Body, J. B. Lippincott Co., 1919.

Parallels intriguingly worked out for the mechanically minded.
Neal, H. V., and Rand, H. W. Comparative Anatomy, P. Blakiston's Son

and Co., 1936.

Written by two masters of the subject.
Newman, H. H., Vertebrate Zoology, The Macmillan Co., 1920.

Just what the title indicates.
Walter, H. E., Biology of the Vertebrates, The Macmillan Co., 1928.

Many illustrations. Bibliography.
Wilder, H. H., History of the Human Body, Henry Holt & Co., 1923.

Told with literary grace without sacrifice to accuracy.

THE MAINTENANCE OF THE INDIVIDUAL

XIV

THE ROLE OF GREEN PLANTS

Preview. Structure of green plants • The raw food materials used by
plants • The root and its work • The stem, structure and function • The
structure of the leaf • How green plants make food ; carbon dioxide as raw
material ; the role of water ; chlorophyll and light ; relation of artificial
light to food making ; what goes on in the green leaf in sunlight ; chemistry
of food making • Enzymes and their work • How food is used by the plant
body • Respiration • Transpiration • The rise of water in plants • Produc-
tion of oxygen by plants • Suggested readings.

PREVIEW

It is a trite statement to say that the destiny of man on the earth
depends upon green plants. All living stuff is made up of the ele-
ments found in air, in water, and under the earth's surface, yet
no laboratory technician has ever been able to put this material
together and make protoplasm. That energy is displayed by plants
and animals is obvious, but no man has ever been able to energize
matter and create a living organism. We know that the units of
structure, the cells, do release energy and that this energy comes, as
does all other energy, from the oxidation of fuel substances. Such
fuels used by living things we call foods. Moreover, these foods,
be they from plant or animal, in the long run depend upon energy
derived from the sun. The Biblical declaration that ''all flesh is
grass" is literally true, for without green plants animals would have
no food.

We do not think of plants as very dynamic objects compared with
animals. Nevertheless, if we look at the soil pushed up by growing
seeds, the pavement broken by the growth of trees, and even the
hardest rocks split apart by the wedge action of growing stems and
roots, we realize that plants are very much alive. They respond to
the various stimuli in the environment, reacting like animals to
temperature changes, to gravity, to various chemical substances, or
to the directive force of currents of water.

Unlike animals, whose metabolism is catabolic, the green plant's
metabolism is more completely anabolic. It builds up materials to

237

238 THE MAINTENANCE OF THE INDIVIDUAL

a greater extent than it tears them down in the metabolic process.
Compare the growth of an animal with that of a plant such as a big
tree. While the animal is more fixed in size and limited in age, the
tree grows for a longer period of time and grows to a greater size.
These differences are due to a continuous growth of the meristemic
tissue already mentioned, and also to the fact that new tissues and
organs grow continuously from this area of meristem that is found in
growing buds, stems, and roots. The most important difference
between green plants and animals lies in the fact that the green plant
can make use of the sun's energy to manufacture foodstuffs on which
not only it, but also the animals which eat it, depend.

In an investigation of a living green plant, two methods of study
present themselves. We can rather carefully dissect and study each
system of structures which makes up the living organism and direct
our attention to the microscopic make-up of each part. In this way
a fairly complete picture will be had of the organism in its entirety.
But such a picture will lack vitality. If the plant is a living thing,
then why not study it from the point of view of function, of what it
does and how it lives, using only so much reference to the structures
as will make intelligible the work of the parts of the plant? This is
the viewpoint adopted for this unit. The plant is to be thought of as
a living, working organism, performing the same metabolic functions
as any other organism, but in addition doing a different kind of work
from that of animals, that of synthesizing organic foodstuffs out of
chemical raw materials from the air, the soil, and the absorbed water.
This unit, then, will bring up a number of important points. Among
them will be such questions as these : What are the adaptations
which enable the green plant to do its work? Where does the raw
material from which food is manufactured come from and how does
the plant get it ? Under what conditions is the work of food manu-
facture performed? Where is food made and how does it get to the
cells where work is done? Why is light necessary for green plants
and why do they bleach in the darkness? Are green plants really
as important as is here indicated ? These and similar questions will
be answered in the pages that follow.

Structure of Green Plants

It is not easy to give a general description of a green plant. In the
higher plants it is obvious that there are several well-defined regions
which are called root, stem, leaves, flowers, and fruits. In these

Wright Pierce

(1) Eucalyptus trees, natives of Australia , which have found California so well
suited to their needs that they are a dominant form there. (2) Rose, shrub showing
bushy habit. (3) Snapdragon, a common annual. (4) Carrot, a biennial; note
the food storage in the root. Of what value is this to the plant ?

239

240

THE MAINTENANCE OF THE INDIVIDUAL

e e i

regions several different methods of growth occur which will be
described later. Some plants that grow more or less continuously,
forming a woody body which resists cold and storm, are called trees
or shrubs. Others die down at the end of the year, although they
have some wood fiber in the body. These are the herbaceous plants,
examples of which are peas, beans, and a variety of garden plants and

roadside weeds. Herbaceous plants
DiCotyledoiv MoaocotyledoR that produce seeds and die before

the following winter are called
annuals.

A second group of herbaceous
plants, called biennials, store food
in the roots or underground portion
of the stem. After the upper part
of such a plant is cut down by un-
favorable weather conditions, the
following spring the underground
portions send up a new shoot from
the subterranean food supply. This
gives rise to flowers and seeds at
the close of the second year. Ex-
amples of biennials are carrots,
parsnips, and beets.

A third type of herbaceous plants
is the perennial, which grows each
spring from the underground parts
that remain alive during the winter.
Many of our common weeds have
this habit, which makes them
difficult to eradicate.

Woody plants, such as trees and
shrubs, as we have seen in the
unit on classification, are grouped either as conifers (the softwoods,
pines, firs, hemlocks, and their relatives) or as deciduous hardwoods.
The latter are placed with the flowering plants, and may be either
monocotyledons or dicotyledons. These two groups have differences
in the structure of leaf, stem, and seed. The monocotyledons usu-
ally have parallel-veined leaves, like those of grass or lily. Their
stems have scattered "closed" woody vascular bundles and a single
cotyledon in the seed. The dicotyledons have netted-veined leaves,

iS tern

Differences between monocoty-
ledons and dicotyledons; c, cotyle-
don ; e, endosperm ; fb, fibrovascular
bundles; h, hypocotyl; p, plumule.

THE ROLE OF GREEN PLANTS 241

sudh as are seen in the elm, oak, or sassafras; stems with "open"
vascular bundles which usually appear as a ring of growing tissue;
and seeds with two cotyledons or seed leaves. These structures will
be referred to in more detail later.

The Raw Food Materials Used by Plants

For a good many centuries after the time of the Greek philosophers
who first hold this theory, it was thought that green plants absorbed
food from the soil, but it was not until the time of the Belgian philoso-
pher van Helmont, who lived in the sixteenth century (1577-1644),
that it was clear that water played a very important part in the
growth of a plant. One of van Helmont's experiments consisted of
placing a willow slip weighing five pounds in a vessel containing two
hundred pounds of dried soil. For five years he watered the tree with
distilled water, making careful observations on it until it had grown
to be a sapling weighing one hundred and sixty-nine pounds and three
ounces. But when he weighed the soil in which the tree had grown,
he found it had lost only two ounces. Clearly then, the gain came
largely from sources other than soil, and he rightly concluded that
water was largely responsible for the increase in weight. In the first
half of the eighteenth century, an English clergyman, Stephen Hales,
worked out the daily water consumption of a plant by ascertaining
the relation between leaf and stem surface and the quantity of water
absorbed. He went a step further than van Helmont in suggesting
that plants take something from the air as well as the soil with which
to build up their body material. In 1779, Ingen-Housz, a Dutch phy-
sician, who was a co-worker with the famous surgeon, John Hunter,
showed that the green part of a plant, when exposed to light, uses the
free carbon dioxide of the atmosphere, but that it does not have this
power when kept in darkness. A little later, in 1804, de Saussure,
by a series of experiments, proved that carbon dioxide and water
were both used by plants in the sunlight and that as carbon dioxide
was taken from the atmosphere, about the same amount of oxygen
was returned to it. He, however, missed the use of the green coloring
matter of the leaf in its connection with the sun's energy in building
living matter and food. The real explanation of the function of this
green substance (chlorophyll) was left for Julius von Sachs, a famous
botanist of the nineteenth century. He was the first investigator to
demonstrate the fact that green plants make food for the world. Just
how they do this is still not fully known, although plant physiologists

242 THE MAINTENANCE OF THE INDIVIDUAL

have been experimenting and are still experimenting in the attempt
to solve the problem.

With this background, our point of view is to consider the living
green plant as an organism, faced by the same kinds of problems as a
living animal, taking a living from its environment, storing up food
for the inevitable time of food shortage, and eventually forming
fruits to hold the seeds which are necessary to pass the stream of life
on to the next generation. Unlike an animal, the green plant takes
raw food materials from its environment and, under certain favorable
conditions, synthesizes them into organic foods, a process effected
by means of a number of adaptive structures, in certain, favorable
environmental conditions, the chief of which is sunlight.

By burning the body of a hving plant until nothing but ash remains,
and then making a careful analysis of this residue, frequently as many
as thirty chemical elements are found. Twelve are nearly always
present, eight of which are essential to plant growth. The latter are
boron, calcium, iron, magnesium, manganese, phosphorus, potassium,
and sulphur. It will be noticed that this list does not agree exactly
with the previous list of elements usually found in the protoplasm of
Hving things (page 131), but the implication is clear. The chemical
elements found in living matter, as previously noted, are also found
in rocks or soil, air, and water. The stage is set and it remains for
the scientist to discover just how these elements, found in the environ-
ment, can be made into food and living stuff by the green plant.

A good many experiments have been made with plants to determine
more exactly the function of these elements. It has been shown
that if green plants are placed in a nutrient solution containing the
necessary elements,^ growth will take place. If, however, certain
elements are subtracted from the solution, the plants will not develop,
or their growth will be considerably slowed down. Such experiments
give us our first clue to one important use of the root. It is evidently
an absorbing organ through which the plant takes in not only water,
but some of the essential mineral materials necessary for its growth.

1 A list of the most commonly used nutrient solutions for plant growth are given below.

Crone's solution : Water, 2.0 1. ; KNO3, 1.0 g. ; FeP04, 0.5 g. ; CaS04, 0.25 g. ; MgSOj, 0.25 g.

Detmer's solution: Water, 1000 g. ; Ca(N03)2, 1.0 g.; KCl, 0.25 g. ; MgS04, 0.25 g. ; KH2PO4,
0.25 g. ; FeClj, trace.

Knop's solution: Water, 1000 g. ; Ca(N03)2, 1.0 g. ; KNO3, 0.25 g. ; KH2PO4, 0.25 g. ; MgS04,
0.25 g. ; FeP04, trace.

Pfeffer's solution : Water, 3-7 1. ; Ca(N03)2, 4 g. ; KNO3, 1 g. ; MgS04, 1 g. ; KH2PO4, 1 g. ; KCl,
0.5 g. ; FeCls, trace.

Sach's solution : Water, 1000 g. ; KNO3, 1.00 g. ; NaCl, 0.50 g. ; CaS04, 0.50 g. ; MgS04, 0.50 g. ;
Ca3(P04)2, 0.50 g. ; FeCls, 0.005 g.

THE ROLE OF GREEN PLANTS

243

The Root and Its Work

Recent experiments made by Weaver ^ and others show that plants
have extremely comphcated root systems. The roots of an old oat
plant, for example, although extending through only about two cubic
yards of soil, were found to have a total length of over 450 feet.
Weaver found that hardy wheat plants sent their rootlets into the
soil six feet below the surface

^,CeJ7tml Cylinder-
-_>v&ocf^ bundle

-root "hciiT~

ictermis

of the ground. In the bush
morning-glory, a common
plant of the mid-western
plains, the roots may extend
ten feet into the ground and a
distance of twenty-five feet
away from the parent plant.
The roots of corn extend
laterally three to four feet
from the stem and sometimes
over seven feet into the soil.
All this is evidence for the
great importance of the root
as an absorbing organ.

Examination of longitudinal
sections cut from growing
roots shows that the body of
a root is made up of a central
woody cylinder surrounded
by layers of softer cells, collec-
tively called the cortex. Over
the lower end of the root is
found a collection of cells,
most of which are dead, ar-
ranged in the form of a cap

covering the growing tip. As the root pushes through the soil, the
outer cells of this root cap are sloughed off, and are rapidly replaced
by growing cells of meristem that are just under the root cap. The
root cap proper is evidently a protective adaptation. In the woody
region of the root are vascular tissues consisting of xijlc77i and phloem.
These tissues form a series of tubelike structures which together with

. >■-' J~OOt/

cctp

Root of a dicotyledon, greatly magnified.
Find the functions of each part labeled.
How might soil water get from the outside
of the plant into the woody bundles.'*

' Weaver, Root Development of Field Crops, McGraw-Hill Book Co.

244

THE MAINTENANCE OF THE INDIVIDUAL

strong supporting woody cells constitute the vascular bundles that

put the root in connection with the stem and leaves above it.

If mustard seeds, for example, are germinated in a moist chamber,

a few days after germination the lower part of the root will be found
to be covered with a delicate, fuzzy growth. Ex-
amination of the root at this stage shows an actively
growing area of meristem, an elongating zone of tissue
directly back of it, with a zone of maturing tissue
above, which together make a zone of growth coincid-
ing more or less directly with an area covered with
fuzzy structures known as the root hairs.

Root hairs vary in length according to their posi-
tion on the root, the longer ones being found at some
distance from the tip. They are outgrowths of the
outer layer of epidermis. A single root hair examined
under the compound microscope is found to be a
threadlike, almost colorless structure. The delicate
cellulose wall is lined by the
protoplasm of the cell, the
outer layer of which forms a
selectively permeable mem-
brane. Inside the root hair

are found numerous vacuoles filled with cell

sap. A nucleus is always present and may be

found in the basal part of the cell, or in the

hairlike portion itself. The root hairs are

evidently living epidermal cells.

An examination of a young plant growing

in moist soil shows that the root hairs reach

out between the particles of soil, apparently

being closely cemented or attached in places

to them. Each particle of soil is surrounded

by a delicate film of water, which, with the

dissolved minerals found in it, is absorbed

into the root hair by the process of osmosis.

The wall of the root hair is covered with a

delicate layer of mucilagelike pecten formed

by the outer layer of the cell wall and is also

in contact with the moist protoplasm within

the cell, which forms a delicate membrane

Root hairs of
corn, showing
their relation to
the root tip.

Root hair, showing its
relation to an epidermal
cell. How do you account
for the attachment of the
soil particles to the surface
of the root hair ?

THE ROLE OF GREEN PLANTS 245

just under the wall. Diffusion takes place following the laws of
osmosis, according to which water passes through a selectively per-
meable membrane from a point of its greater to a point of its lesser
concentration. This means that water passes from the soil into the
cell sap, which has a higher concentration of solutes than does the
water. Since the cell sap within the root hair has received a greater
quantity of water, it in turn becomes a point of higher concentration
of water than the cells lying next to it interiorly, and consequently,
the flow continues from these outer cells to the adjoining cells which
have a higher concentration of solutes. In this manner water is
passed through the cells of the root i)ito the woody cylinder inside
the cortex. Once having reached this region it passes up the tubes
into the stem and on into the leaves as will be shown later.

The Stem, Structure and Functions

In thinking of the tree as a li^'ing organism, we are not so much
concerned with the internal structure of the stem as with the way it
functions. For many centuries it has been known that water passes
up through the wood. If a tree is girdled — that is, if a narrow strip
of bark extending inward as far as the wood is removed — the tree will
keep its leaves for some time, indicating the upward passage of water
which keeps them from wilting. If, however, a strip of wood directly
under the bark is removed, enough of the bark being left intact to
allow for passage of fluids, the leaves will wilt within a very few mo-
ments. A cut branch of apple or willow placed in red ink after a few
hours shows by a red circle, visible in sections cut across the stem,
that the colored water has passed up through the outer layers of the
new wood.

In order to understand better the pathways for the rise of sap in
the dicotyledon stem, one must study its growth. When seen in
cross section, the vascular tissues of such stems are arranged in a circle.
In some herbaceous stems, the woody bundles are separated by a
parenchyma, but in trees, shrubs, and a good many herbs, the bundles
are united to form a complete ring around the stem. These vascular
bundles are open at each side and grow more or less continuously
from a single row of meristem or embryonic cells which form a layer
around the stem. This layer is called the cambium, and the growth
of the wood and bark of our large trees is due to the activity of this
always youthful layer of cells which, like the cells of embryonic tissue,
continually divide and multiply to form internally the xylem or wood

246

THE MAINTENANCE OF THE INDIVIDUAL

and externally the phloem tissue. In the spring when this tissue is
very active, it forms a soft layer of cells that allows of the easy sepa-
ration of bark from wood, a fact well known to any small boy who
has made a willow whistle.

It is not necessary to go into the details of stem structure, except
to note that the cambium layer gives rise each year to new layers of

Cross section of stem of Ricinus communis, a dicotyledon, showing cambium
ring. In what area of the diagram does growth take place ?

tissues, both internally and externally. The inner layers made up
of secondary xylem are from the annual rings of a tree. In spring
the growth of the tissue is rapid, while in winter it is very slow indeed
or stops entirely, thus making the differences in the cross section
shown in the figure. As the tree ages, changes may be noticed in the
appearance of the older woody area forming the interior of the trunk.
This wood becomes darker in color, its chemical composition changes,
and it loses its ability to conduct water. It is known as the heart-
wood as distinguished from the outer rings of wood called sap-wood.

THE ROLE OF GREEN PLANTS

247

The latter conducts water, while the heart-wood functions merely as
a supporting tissue. As the tree increases in diameter, the area of

,_bark

.-Cambium
layer-
annual

pith
rays

Section through a dicotyledonous stem. Explain its method of growth.

heart-wood increases while the sap-wood, although greater in cir-
cumference, gets proportionately smaller in extent.

The bark, or area outside the cambium, is made up of several
different tissues, which have a somewhat different
arrangement in conifers than in deciduous trees. The
area known as phloem is formed immediately outside
the cambium. This area contains many living sieve
tubes through which elaborated food is carried down
from the upper part of the plant. The sieve tubes
in the conifers are more or less regular in arrangement
while in deciduous trees they are scattered. In both
stems they are all surrounded by parenchyma.
Scattered through the bark of deciduous trees are
masses of tough, stringy schlercnchyma cells of two
types, phloem fibers — fibrous, elongated cells that
give strength and elasticity to the trunk — and thick-
walled, hard stone cells. Outside the latter area is
formed the corky layer, produced by a layer of growing
cells known as the cork cambium. Cork cells, which
have their walls impregnated with an insulating sub-
stance called suherin, are of great value to the tree
because they prevent a rapid loss of water from the
tissues. It is this layer in the Spanish cork oak which
is of commercial value. In some trees, such as the
redwoods, the bark forms a coating highly resistant
to fire.

H. w. H. — 17

Above, sieve
\ (' s s e 1 (of
phloem) with
c()mf>anion cell;
below, sieve
plate, with
section of com-
panion cell.
(After Stras-
burger.)

248

THE MAINTENANCE OF THE INDIVIDUAL

Wright Pierce

The characteristic lenticels of the white birch
{Betiila populi folia). Note the placement of the
lenticles.

Scattered over the surface of twigs and young tree trunks are

found many lenticels, openings in the corky layer which become filled

with loose masses of cells.
They are found both on
roots and stems and act
as pores which allow for
the exchange of gases be-
tween the living cells of
the cortex and the me-
dium outside. Lenticels
are often spoken of as
"breathing jDores" and
experimental evidence
seems to make this title
valid.

As the stem or trunk
of a tree grows larger in
diameter, there is an in-
creasing area that uses

water and foods. Cells cannot grow without food, and food in a

growing plant cannot be made without water. The structures which

put the water-conducting tissue of

the inside of the stem in connection

with the phloem of its outer part

are known as vascular rays. They

may be seen in almost any cross

section of a tree which has

produced secondary xylem and

phloem. Here the cambium has

rows of irregularly placed cells

that instead of forming xylem and

phloem produce ingrowing masses

of more uniform parenchymatous

cells making vertically placed

strings of tissue. These bands

act as conducting pathways

for water from the xylem to

the phloem and also as chan-
nels for elaborated food from the phloem to the xylem, thus dis-
tributing these materials to the growing trunk. Experiments by

phi

.oem

(i:am\:)i.imri

\ — •^yiein

pith

Note the bands of living parenchym-
atous tissue that grow inward toward
the pith.

THE ROLE OF GREEN PLANTS

219

Aiichtor ^ have shown that food and water are not transferred from
one side of a tree to the other, but instead that ahnost all of the
water taken in is used directly above where it is absorbed, while
food passes down from the leaves on the same side of the tree. There
is seemingly little cross transfer of food or water in a plant stem.

Vascular rays must not be confused with the so-called pith rays
which are formed in herbaceous stems such as Ranunculus or in the
stem of Clematis where, as the primary wood bundles grow in the pith,
the pith forms narrow plates between the bundles. These appear as
the pith rays in a cross section.

Conditions of growth upon which the passage of food and water
depend differ in monocotyledons from those in dicotyledons. If a
stalk of celery or asparagus is placed in red ink over night, the color
is seen to be located in little fibrous bundles of tissue which are scat-
tered throughout the stem. If such a stained stem is examined in
cross section under the microscope, it is found to be made up of pa-
renchyma or pith which is dotted with little groups of woody cells
of irregular size and shape. These are the vascular bundles which,

Transverse section of stem of corn, a monocotyledon, showhiK the " scattered "
vascular bundles which are cut in cross section.

■ Auchter, E. C.
in Woody Plants?'

" Is There Normally a Cross Transfer of Foods, Water, and Mineral Nutrients
Univ. Maryland .\gric. Exp. Station, Bull. 251, Sept. 1923.

250

THE MAINTENANCE OF THE INDIVIDUAL

instead of being located in a ring as in the dicotyledons, are scattered
through the pith although more concentrated toward the outer edge of
the stem. Examination of this outer edge or rind shows that there
is no true bark, but that this outer area is made up of these same
woody bundles closely massed together. Under high power, the
bundles are seen to have outer strengthened walls of wood cells

enclosing tubelike cells of
different diameters of
which the larger have
pitted surfaces. The area
containing these tubes is
the xylem. Other elon-
gated tubular cells having
their ends perforated with
small holes like a sieve,
form the sieve tubes,
w^hich are the conducting
tissues of the phloem. In
the phloem, the tubes pass
foods down from the
leaves, while the xylem

A cross section through a closed monocotyle- carries water up from the
donous bundle. Note that the thick-walled roots to the leaves. The
xylem cells completely enclose the cells of the entire WOody bundle is en-

^ °^™' closed w^th a tough wall of

sclerenchyma which gives strength and resiliency to the stem. Since
this hard tissue binds the entire bundle, it is called a closed bundle.
Monocotyledonous stems grow, then, through an increase and
lengthening of closed bundles in the parenchyma of the stem.

The end result in both monocotyledon and dicotyledon stems is the
same. The vascular bundles put the root, stem, and leaves in direct
communication. The root hairs at one end and the cells of the leaf
at the other end are the opposite terminals of long communicating
woody tubes. These tubes carry water and solutes up from the soil
to the cells of the leaf, and, as will be shown presently, carry elaborated
food materials down from the leaves to various parts of the plant,
where they may be stored for future consumption or used immediately
to liberate the energy needed in growth and in destructive metabolic
changes. The vascular bundles which leave the stem to enter the
leaves do so by way of the petiole or leaf stalk. As they enter the blade

THE ROLE OF CxREEN PLANTS

251

of the leaf, they branch into bundles of ever smaller and smaller
diameter to form the veins of the leaf. In the monocotyledonous
leaves, these veins run in a more or less parallel direction as seen in
grass blades or palm leaves. In the case of the dicotyledonous plants
characteristic irregular and netted veins
are found, reminding one of the branch-
ing of the capillaries in the human body.
These veins are made up structurally
of tracheids and tracheal vessels, ser\'ing
as water-conducting tissues ; sieve tubes,
which carry out food materials from the
leaf; and supporting tissue, which
makes up the mechanical framework of
the veins. Thus the veins act as a sup-
porting skeleton for the leaf as well as
conduits.

The Structure of the Leaf

The outer covering of the leaf (epi-
dermis) is composed of a layer of
irregularly shaped cells, usually rather
flattened. In some plants, like the
mullein, these cells are prolonged into
hairs, or again the layer, as a whole, is
frequently covered with a waxy cuticle
which is impermeable to gases and
water. The under surface of the leaf,
as seen through the compound micro-
scope, shows many tiny oval openings,
which are called stomata. The position
of the stomata varies in different leaves.
Some plants, as, for example, water
lilies, whose leaves float on the surface
of the water, have them in the upper
epidermis. Others have them on the
under .side, and .still others have them
on both surfaces. The estimated num-
ber of these openings varies. Mac-
Dougal estimates that as many as two
million are on the under surface of an

Stomata from the loaf of an
Easter lily (Lilium lonyiflorum) :
Above, a stoma, as seen in sur-
face view, showing the two
kidney-shaped guard cells {g),
which enclose the stomatal aper-
ture (s), the more deeply shaded
portion representing the central
slit ; note the chloroplasts in the
guard cells; (b) subsidiary cells.
Below, a stoma, as seen in cross
section ; note the guard cell (g)
next to the subsidiary cell (6) ; the
outer slit (o) is enclosed between
the cutinized outer guard-cell
ridges (r), the enlarged area just
below being the outer vestibule
(o') ; below the central slit (s) is
the inner vestibule (('), which
here opens directly into the
cavity (c) underneath the stoma.

252

THE MAINTENANCE OF THE INDIVIDUAL

oak leaf of ordinary size, while four or five hundred thousand to a
leaf is a common estimate. Surrounding the opening of each stoma
are found two kidney-shaped cells, the guard cells, which can easily
change their shape under certain conditions. They are of great
importance in the life of the plant, since they control to a great
extent the amount of moisture that may be lost from the leaf's sur-
face. The guard cells are noticeably greener than the epidermal cells,
the color being due to many tiny green chloroplasts.

If the leaf is cut in cross section and examined under the microscope,
it will be found to be made up largely of a tissue known as mesophyll.

Lying close to the epi-
dermis are one or two
layers of elongated cells
with the long axis placed
at right angles to the sur-
face of the leaf. These
layers of cells are collec-
tively called the palisade
layer. Each cell of this
layer contains numerous
chloroplasts which are
found in the protoplasm
close to the cell wall. It
has been estimated that a
square inch of a sunflower
leaf contains as many as
thirty million of these
chloroplasts, which are
most important structures
in the plant so far as food
making is concerned.
Below the palisade layer
is a layer of numerous irregular cells containing fewer chloroplasts.
These cells are known collectively as the spongy parenchyma. Be-
tween them are found air spaces connected with the exterior of the
leaf through the stomata. We have already noted that the veins
form the framework of the leaf and in a cross section are often found
occupying part of the area of spongy parenchyma. These veins
connect the vascular tissue of the root and stem with the leaf. The
petiole, or leaf stalk, is made up largely of vascular and supporting

Cross section through a leaf; e, upper epider-
mis, e', lower epidermis, showing stomata (s) ;
I, intercellular spaces in the spongy parenchyma.
Note the cross section of the vein (v). Why is
the palisade layer (p) so placed ?

THE ROLE OF GREEN PLANTS

253

woody tissue. At one point on the petiole, usually close to the main
stalk, a little time before the leaves drop from deciduous trees in the
fall, a layer of delicate, thin-walled cells is formed which extends
completely across the petiole. This is called the separation or ab-
scission layer, and it is at this point that the leaf is cast off.

How Green Plants Make Food

The general biologist is concerned not so much with the structure
of the organism or with detailed minutiae as with the general
metabolism of an organism as a whole. He wants to know how plants
and animals act as living things, both alone and in relation to each
other. We have examined the green plant from the standpoint of
structure and are ready to consider it as an organic whole, as a living
organism that releases en-

leof on live plant
+ light-

boilecL

■— r^ in -^oodi
■\>^ alcoViol

positive reaction
"where sta.r-cVi was
locctte<:C

ergy, respires, feeds, repro-
duces, and in time dies.
But we must remember that
in addition, the green plant
makes food, and it is this
process upon which we will
now focus our attention.

It is a relatively simple
matter to prove that sun-
light is necessary for starch
making in a leaf. Place a
healthy green plant in dark-
ness for a couple of clays.
Then pin strips of black
cloth over parts of some of the leaves and expose the plant to bright
sunlight for a few hours. Later, remove the leaves and boil them to
soften the tissues, adding alcohol to extract the chlorophyll, and
finally, place them in a solution of iodine. A blue color will appear in
those parts of the leaves exposed to sunlight, while the covered areas
will be colorless. The appearance of the blue color in the presence of
iodine is the regular test for starch, thus showing clearly that sunlight
is necessary for starch making.

Another simple experiment may be performed to show that air is
also a necessary factor. Place a healthy green plant in darkness for
two or three days, then carefully smear vaselin(> on th(> ui)i)(>r and
lower surface of two or three l(?aves, leaving the others uiitoiiclicd.

Proof that light is necessary for starch forma-
tion in green leaves.

254 THE MAINTENANCE OF THE INDIVIDUAL

Place the plant in full sunlight for a few hours, then remove the
vaselined and untouched leaves, and treat both in the manner de-
scribed in the last experiment. The leaves to which no air penetrated
will be shown to have no starch.

The need of carbon dioxide in the process of starch making may also
be demonstrated by a relatively simple experiment. If plants are
grown under similar conditions in two bell jars, but in one case the
carbon dioxide in the atmosphere is removed by means of soda lime,
while the other plant is left in the bell jar containing normal air, the
latter continues to grow while the one lacking carbon dioxide does
not increase in size.

By burning a plant in a hot flame, it can be ultimately reduced to
mineral ash equaling about 4 to 5 per cent of the entire weight. Ac-
cording to Raber, from 1 to 55 per cent of the plant is consumed,
while from 40 to 95 per cent, roughly speaking, consists of water.
Since a green plant is immobile and since it has no way of obtaining
material except from the air, water, and the soil that surrounds it,
it may be safely assumed that if food is found in the plant body,
it must be made there. That foods are found in plants is common
knowledge. We eat roots, stems, fruits, and leaves of plants. Grains
form our staples of food. Roots and various types of fruits form
part of our dietary, while herbivorous animals live upon grasses and
fodder crops. This brings us then to the sources of the raw
materials out of which these elaborated foods must be formed.

Carbon Dioxide as Raw Material

Carbon dioxide is not only a product of respiration of animals but of
plants as well. A man gives off about nine hundred grams of carbon
dioxide daily into the air. Carbon dioxide also gets into the air from
the combustion of inflammable materials. Volcanic eruptions and
other sources of combustion increase the amount, while decaying
organisms and the oxidation of rocks and soils add a very appreciable
amount daily to the store. While it is estimated that there are only
two grams of carbon in each ten liters of air, nevertheless the fact
that carbon dioxide is universally available in the air and oceans close
to the surface of the earth shows that it may readily be made use of
by growing plants. Its need in food manufacture is well illustrated
by the statement that the world crop of wheat requires annually one
hundred and fourteen million tons of carbon dioxide in order to pro-
duce the seventy million tons of carbohydrates which form this crop.

Wrijjla Pierce

The role of water. Upper photograph : The Mohave River near Victorville.
This river rises in the San Bernardino Mountains and loses itself in a desert sink.
What effect does it have upon the desert .►>

Lower photograph: An irrigated orange ranch in the desert near Clareniont,
California. Thousands of acres of trees now grow where desert conditions
existed before irrigation.

255

256 THE MAINTENANCE OF THE INDIVIDUAL

The Role of Water

Water as a raw material needs little mention. The soil always con-
tains more or less water, and the original source of water in its cycle
through the oceans, the air, the clouds, and rain gives the earth a
never ending water supply. When mm aids Nature in carrying
water to dry areas by irrigation the desert literally is made "to blos-
som as the rose." Certain chemical elements find their way into the
plant body with this water. If the green plant is to manufacture
organic food substances, it is evident that the elements carbon, oxy-
gen, and hydrogen must come from the water and air. Various
mineral salts, taken in by the root, furnish the necessary amounts of
calcium, iron, potassium, sodium, and other elements, which leaves
only nitrogen to be accounted for. Although nitrogen makes up
approximately four fifths of the atmosphere, it is nevertheless unusable
in that free form. It is an extremely inert gas and does not unite
readily in combination with other substances. By means of the proc-
ess of decay, however, and particularly through the nitrogen-fixing
bacteria found on the roots of certain types of plants, this highly im-
portant element is made available to plants. So much for the raw
materials. Now let us turn to the machinery of food manufacture.

Chlorophyll and Light

Common observation shows that there is a relation between light
and the green color of plants. We are familiar with the bleaching of
celery stalks, with the curious blanched elongated shoots of a potato
which sprouts in darkness, and with the fact that young seedlings are
devoid of chlorophyll until after they have sprouted. Seedlings
grown with light coming from one side turn to the source of light, while
plants grown in a dark box having a hole on one side work their way
toward the light. Obviously light has a very potent effect on the
plant.

Sunlight passed through a prism is broken up into seven primary
colors ranging from violet to red, but passed through a spectroscope
shows numerous dark lines traversing different areas in the spectro-
prism. The most conspicuous are used as landmarks by physicists
and for convenience have been designated by the letters A to H by
Fraunhofer, their discoverer. These several wave lengths of light can
be measured and it has been fovmd that they vary from 0.00076 mm.
at the red end of the spectrum to 0.00039 mm. at the violet end.

THE HOIJ-: OF r.REF^.N PLVNTS 257

Rays of greater and shorter length are also found at eaeh end of the
spectrum forming the ultraviolet and infrared portions. The heat
of light rays varies, Ijcing greater at the r(>d end of the spectrum.
Since all life depends upon this I'adiant energy whose source is the

1 z ^ 4 I n m EA

When a green leaf is placed in the path of light passing through a {)risni. dark
strips appear, due to the partial or conipleh^ blocking of the light energy. These
are shown in the absorption spectra above. .4, chlorophyll of Alliumiirsi-
mim in alcohol; B, chlorophyll of English ivy {Iledera helix) in alcohol;
C, chlorophyll of OscUlatoria in alcohol; D, carotin. 1, 2, 3, 4. absorption bands
of chlorophyll; /, //, III. absorption bands of carotin; EA, end absorption.
The lettered broken lines mark the position of the principal absorption hnes of the
solar spectrum (Fraunhofer lines); the numbered solid lines form a scale from
which wave lengths (X) in nullionths of a millimeter may be found by adding
a cipher; note the increasing dispersion from left (red) 1o right (violet).
(After Kohl.)

sun, the green plant is no exception to this rule. Certain parts of
the plant, however, are more susceptii)le than other portions to ra-
diant energy. While the green leaf as a whole needs sunlight, it is
only chlorophyll in the chloroplasts that is al:)le to utilize it for food
making.

If a chloroplast is examined under a very high magnification of the
microscope, it is found to be a mass of living matter somewhat
denser than the protoplasm surrounding it. In its disk-shaped struc-
ture the green coloring matter is arranged around the outer part of
the chloroplast, while the central portion usually contains a clear area

258

THE MAINTENANCE OF THE INDIVIDUAL

filled with fluid. Chlorophyll is a very complex protein, apparently
made up of two substances known as Chlorophyll A, having the chemi-
cal formula C55H7205N4Mg, and Chlorophyll B, C55H7o06N4Mg.
It is found to be somewhat like the hemoglobin of the human blood
except that it has an atom of magnesium instead of iron and the
property of fluorescence, its color being different in transmitted or
reflected light. Chlorophyll in solution, when extracted from the
leaf by means of alcohol, appears green as light passes through it, but
red when light is reflected from it against a black background. Other
pigments are closely associated with chlorophyll, a group of yellow

pigments called carotins,
which give the yellow
color to carrots and other
fruits or vegetables, and
xmithophylls, pigments
that help give color to
leaves in the fall.

Numerous experiments
have been made to dis-
cover how chlorophyll
does its work. It has
been found that if light is
passed through this sub-
stance and then broken
up by a prism, that part
of the light which is
absorbed by the chlorophyll may be detected by the presence of
absorption bands in the spectrum where the chlorophyll has taken
out the light. By this means we learn that the red band of the spec-
trum is most active while parts of the blue, violet, and indigo regions
of the spectrum are also absorbed. A classic experiment by Engel-
mann illustrates this in another w^ay. A filament of an alga was
placed in a culture of bacteria which were active only in the pres-
ence of oxygen. The filament was then put in darkness until the
bacteria had used up all the oxygen present. Then the slide con-
taining filament and bacteria was placed on a microscope under a
solar spectrum. In a short time the bacteria were found to mass
themselves in abundance at the red end of the spectrum and to a lesser
extent at the blue end, because at these points more oxygen was
given off by the alga, thus indicating activity in starch formation.

3 C

D F

'.■.'/•

ii;|

y.*''"^'--"'.v-/

■'■*.

«%.,.

'-■■■.'

■'«■- •-•'.■'..•?-'-

^/A_ _ _

: •.•.-'•* .■-*■.■.'.■?... .

1 ^

:::;•:.;;

■■^.■'■'•'.•.■■-- X-'

;p^^'-"

."*■'■■

-,•':

;■;v■■iv^,v:;::

^^^'^^^S:0'

•' ''Z^'J^'

I"."

Engelmann's experiment to show the areas in
the spectrum most favorable for oxygen release
in a green alga. The dots represent bacteria.

THE ROLE OF GREEN PLANTS

259

Relation of Artificial Light to Food Making

We have already noted that there are great differences in the
amount of sunhght required by plants. As a matter of fact, very
strong sunlight may cause harm since it overheats the protoplasm,
thus endangering the life of the plant. Moreover, it increases the
rate of transpiration so that water is evaporated too rapidly. Experi-
mental evidence with growing plants shows also that too much sun-
light may retard growth. Some plants are shade loving, as may be

Shade loving plants on a forest floor. Note the leaf arrangement with reference

to light.

seen in any field trip to a forest. The differences in illumination are
correlated with differences in the structure of the leaf, the ])lants
which are exposed to bright sunlight having a well developed palisade
layer, while the spongy parenchyma is not so well developed. The
reverse is true in shade-loving plants. In addition, plants that live
in the shade are apt to have a very thin epidermis and usually ha\-e
dull leaf surfaces which do not reflect the light as reatlily.

Contrary to common belief, it is possible to grow i:)lants without
sunlight as pro^'ed by recent experiments (Harvey) with a large
number of different crop plants such as grains, tomatoes, squash,
peas, potatoes, and others. Plants exposed continuously to the light

260

THE MAINTENANCE OF THE INDIVIDUAL

of nitrogen-filled tungsten lamps of from 200 to 1000 watts produced
both viable fruits and seeds. The bearing of this experiment upon
growing crops in areas where the days are short and the intensity of
sunlight not great is readily seen. Lamps have been put on the
market for use in the home which provide space directly underneath
the bulb for stimulating plant growth during the winter season.

What Goes On in the Green Leaf in Sunlight

When we examine the green leaf to see how it is adapted to use the
energy of sunlight, several interesting facts are discovered. One is
that a plant places its leaves so that they get the largest possible
amount of sunlight, in a given period. Petioles and even stems of
plants turn with the sun so that a maximum amount of green surface
is exposed to its rays. Looking at a tree from above as the bird sees

Diagram to show the cells of the palisade layer of a leaf at two different times
during the day. Which of the two receives full sunlight ?

it, leaves are found to be so arranged that there is a minimum amount
of overshading, the leaves forming a sort of mosaic or pavement on
which the sunlight falls. Examination of the internal structure of the
leaf also shows that the palisade layers which contain the greatest
number of chloroplasts per cell are massed close under the upper part
of the epidermis. It is this layer of palisade cells wdiere most of the
work of starch or sugar making takes place. In the cells themselves,
the green chloroplasts are so placed that a maximum amount of light
falls upon them. When the sun's rays are slanting during the morning
and afternoon, light can reach all of them readily, while at the period
of greatest illumination, when the sun's rays are direct, less light
reaches them as they lie one above the other. Their position may be
changed in the protoplasm, their movement being controlled by the

THE ROLE OF GREEN PLANTS o^.i

liviii<i; substance in which they rest. The ciiloroi)hists are the
structures in the cells which utilize the sun's rays, and it is within
them that the raw materials, carbon cUoxide and water, are manu-
factured into sugar.

Chemistry of Food Making

The actual processes of sugar and starch formation in (he ](-if are
not fully known. The end process can easily l)e shown by the
equation :

6 COo + 6 HoO = CeHioOe + 6 O2

(carbon dioxide plus water = gluco.se plus oxygen)

but how this glucose actually comes into existence is still problem-
atical. Many theories have been advanced to account for the con-
version of raw materials into foods. The one proposed by von
Baeyer in 1870 is still accepted with modifications. He assumed that
formaldehyde is formed by the breaking down of carbon dioxide into
carbon monoxide and oxygen at the same time the water in the leaf
is broken up into hydrogen and oxygen. The carbon monoxide and
hydrogen unite to form formaldehyde, which is then built into
glucose as shown by the following formula :

CO2 — ^ CO + O
H2O — >■ \h + O
CO + H2 — ^ CH2O (formaldehyde)
6 CH2O — ^ C6H12O5 (sugar)

One objection to this theory is that carbon monoxide is extremely
poisonous and is almost never found free in plants, while the product
formaldehyde is also a poison. Later theories postulate that by
first reducing carbon dioxide and water to carbonic acid, then to
formic acid and hydrogen peroxide by the addition of a molecule of
water, formaldehyde and hydrogen peroxide result, the peroxide being
finally reduced to water and oxygen :

C02+H20 = H2C03 (carbonic acid)
H2C03+H20 = HCOOH (formic acid)+H202 (hydrogen peroxide)
HCOOH+H2O = CH2O (formaldehyde) +H2O2
2Ho02 = 2H20+02

The last step in this process is brought about by an enzyme, known
as catalase. Plant physiologists believe that although formaldehyde

262 THE MAINTENANCE OF THE INDIVIDUAL

is a poison, it is probably changed into sugar so rapidly that at no
time is there much present in the cells of the leaf. The last part of
this process, that of changing the formaldehyde to sugar, seems to be
brought about by the action of the two chlorophylls, A and B. One
recent writer, Gordon, > has given the following suggestive formula:

6 C55H70O6N4Mg + 6 H2O = 6 C55H7205N4Mg + 602
(Chlorophyll B) (Chlorophyll A)

6 C55H7205N4Mg + 6 CO2 = 6 C,r,H7o06N,Mg + CeHisOs

(Chlorophyll A) (Chlorophyll B) (sugar)

To the amateur chemist this means very little, but it suggests the
double action of the two chlorophylls in the formation of sugar.
All we really know is that sugar is first formed in the green leaf and
that later this is changed to starch and stored in that form in various
parts of the plant.

Of the manufacture of foods other than sugar very little is known.
There are tiny droplets of fat in the vacuoles inside the chloroplasts.
We know that fats can be synthesized out of carbohydrates by
animals. Therefore, a similar process may take place in plants.
Fatty tissue is undoubtedly manufactured out of the carbon, oxygen,
and hydrogen contained in the sugar molecule. Probably a like
situation exists in the chloroplasts of the leaves, although we do not
know just how this process takes place.

Proteins are even more complex than carbohydrates and fats.
Their molecule contains nitrogen and a number of mineral salts,
in addition to carbon, oxygen, and hydrogen. Protein foods are
found not only in leaves, but in most of the storage organs of the plant.
Apparently proteins can be synthesized out of the sugar plus the
elements nitrogen, sulphur, and phosphorus, wiiich combine with
the carbon, oxygen, and hydrogen of the glucose. Proteins are
probably manufactured in other cells than those containing chloro-
phyll, wherever .starches, sugar, and the essential salts are found,
although light does not seem to be a necessary factor in the process.
Proteins are undoubtedly used in any of the cells of the plant, just as
they are in animal cells, for the making of protoplasm, since the plant
is a living organism composed of cell units each of which is doing a
common work for the plant as a whole.

1 Gordon, R. B. : " Suggested Equation for the Photo-synthesis, Action." Ohio Journal of
Science, 29: 131, 1929.

THE ROLE OF GREEN PLANTS 26:5

Enzymes and Their Work

The changes just described which take place in food making as well
as in food storage, all belong to a series of oxidative and reducing
changes that are presided over and brought about by enzyme action,
another indication of the importance of these omnipresent substances.

We have already spoken of enzymes and their work, but reference
to them again may not be amiss at this point. They are found
practically everyw^here in the living cells of plants and animals, being
much more numerous than was at first believed. Although their
nature is not fully known, we do know that they are colloidal sub-
stances, because they will pass through porcelain filter, but not
through membranes. We also know that some of them are doubtless
proteins, and that they are sensitive to light and ultraviolet rays as
well as to heat, acid, alkali, and substances which are toxic to proto-
plasm. They are powerful catalyzers, as is shown by the fact that a
single gram of the enzyme invertase, for example, will quickly hy-
drolyze one million times its weight of sugar. Enzymes are found in
all living cells and are specific in action, that is, one enzyme will only
do a certain type of w^ork. In general, they may be divided into a
number of groups, depending upon their function, such as the hy-
drolases, that act in the digestive processes of plants and animals by
hydrolyzing materials ; the oxidases, which enable cell respiration to
take place ; the fermentases, as, for example, remiin, that is used in
cheese making, and the coagulascs, to which pedasc belongs that is
used commercially in substances sold for use in jelly making; and
finally, the carboxylases, which cause organic acids to split into carbon
dioxide and other simpler substances.

Specific examples of these various plant enzymes include the en-
zyme, diastase, that causes the digestion of starch. Another enzyme,
maltase, aids in the digestion of maltose to glucose, a still simpler
sugar. A similar action takes place by means of the enzyme, ptyalin,
in our own salivary digestion. Bacteria carry on a slightly different
type of digestion in which cellulose or wood fiber is broken down and
used as food. Here another enzyme, cellulase, causes this digestive
change. Still another enzyme, called lipase, is instrumental in the
digestion of fats. In fruits and seeds rich in fat, such as the avocado,
Brazil nut, walnut, almond, or pecan, the fats are broken down into
fatty acids and glycerine just as in animals where lipase is formed by
the pancreas.

H. w. H. — 18

264 THE MAINTENANCE OF THE INDIVIDUAL

Protein digestion is l^rought about by a different group of enzymes,
called proteases. These enzymes are found in abundance in leaves and
germinating seeds of plants and to a lesser extent in practically all
plant tissues. In the living plant, the digestive enzymes carry on a
necessary and important work. If plants make foods in the green
leaf, and they do, and if they store foods in the root, stem, fruit, and
seed, then there must be some way to transfer the foods made in the
leaf in a soluble form to those parts of the organisms where the food
is finally used. This work of changing insoluble foods to soluble foods
is obviously performed by enzymes. A still more interesting phenom-
enon sometimes takes place. Many of these enzymes under certain
conditions are capable of reversing their actions, that is, of converting
a soluble substance like sugar into an insoluble one such as starch,
or of changing proteins to soluble forms so that they can be transported
through the vascular system of the plant and stored in insoluble form
in seeds, nuts, and roots.

The changes from sugar to starch may take place in leaves wherever
certain plastids known as amyloplasts exist. These bodies have
the power to form starch in the presence of a series of enzymes which
first bring about the transformation of simple sugars to more complex
sugars, and then to an intermediate substance between sugars and
starches, called dextrin. Dextrin is changed into soluble starch by the
enzyme, amijlase, and finally the soluble starch is converted into
insoluble starch by the enzyme, coagulase. Thus we see that the
work of enzymes is absolutely essential to the life of the plant. Al-
though plants and animals obtain their foods in different ways, they
probably assimilate it in much the same manner, for foods serve
exactly the same purposes in plants and in animals, namely, they are
oxidized to release energy and they build up living matter.

How Food Is Used in the Plant Body

Although, basically, the uses of food are production of energy and
making of protoplasm, certain substances are produced by plants
which are not found in animals. For example, the plant cell is charac-
terized by its cellulose wall which in old cells is strengthened by the
addition of a complex substance, known as lignin. This forms the
useful substance we call wood. In addition, other products charac-
teristic of plant activity should be mentioned : the fatty substances,
known as cutin and suherin, as well as waxes which give the "bloom"
to certain fruits ; the essential oils in resins, such as lemon, pepper-

THE ROLE OF GREEN PLANTS

265

mint, wintergreen, menthol, eucalyptus, camphor, and the like ; va-
rious alkaloids ; poisonous substances such as nicotine and strych-
nine ; acids such as mahc, citric, and tartaric. Plant protoplasm, in
addition, as we have seen, manufactures many characteristic enzymes
and produces pigments like the
chlorophylls and carotins al-
ready mentioned. The carotin
present in green grass fed to
dairy cows gives the deeper
color so much desired in cream
and butter. Another interest-
ing substance found in carotin
is a precursor of Vitamin A
which exists in plant bodies as
a form of carotin and is prob-
ably transformed by the liver of
animals into Vitamin A. This
is another example showing how
closely the lives of plants and
animals are interwoven. (See
pages 277-279.)

A diagram of the outer portion of a
cross section of a wheat grain showing the
various layers of tissues : h, the different
integuments of the ovary and seed which
make up the husk ; o, the cells of " tileu-
rone layer" of the endosperm, which are
loaded with protein grains : and b, the layer
of starch-bearing cells. (After Cobb.)

Respiration

Respiration is essentially the
same process in plants as in
animals. In its simplest terms it is the release of potential energy
from foods by means of the process of oxidation, whereby oxygen
is used and carbon dioxide is given ofT. Glucose is perhaps the
chief fuel of the plant body, although fats also serve this purpose.
The latter are probal^ly changed to sugar before actually being
utilized in the respiratory process.

In order to have respiration take place, there must be an exchange
of gases through a selectively permeable membrane. This means that
there will be an exchange of oxygen and carbon dioxide in the cells
where the oxidative process is taking place. Sin.ce respiration occurs in
all living cells and since there is a greater volume of carbon dioxide and
oxygen in parts of plants that are growing rapidly, it is obvious that
growing roots must have a supi)ly of oxygen. This is a reason for the
loosening of soil particles around plants in cultivation to allow air to
have access to the root hairs. The actual oxidative |)rocess is con-

266

THE MAINTENANCE OF THE INDIVIDUAL

siderably influenced by external conditions. Low temperatures slow
up the process as do very high temperatures, there being an optimum
temperature for each organism at which the rate of respiration goes on
best. Seeds have survived a temperature of —250° C. Experiments
with leaves show that the respiratory rate increases rapidly from
0°-40° C, from which point it falls slowly until the death of the
organism. The amount of food present in the plant is a second
factor influencing the rate of respiration, while the rate also varies
with the amount of protoplasm in the cells. Light usually increases
the respiratory rate, probably because of a parallel increase in food
and temperature. It is also found that wave lengths which increase
photosynthesis also increase the respiratory rate. Finally, the rate
of respiration is greatly affected by poisons or anesthetics, at first
being increased, but later slowing down rapidly. In brief, respiration
in plants, as in animals, is induced by the action of enzymes, and
results in the release of energy.

Transpiration

If a healthy potted plant is placed in a dry bell jar and left in the
sun for a few minutes, drops of water are seen to gather on the inside
of the jar. By covering the pot with a rubber tissue to exclude the

large.
battery
jar-.

(Jovcrect
vith 5heet
rubbe'T....

moifture

star-t-

24 Viours later

Experiment to show transpiration. Read your text and explain what has

happened.

possibility of the evaporation of water through its surface and return- ,
ing it to the jar under similar conditions, drops of moisture are again
found after a time on the inner surface of the jar. Obviously, water
must come out through the leaves or stem of the plant, a fact which
can be demonstrated by weighing it before placing it in the jar, and

THE ROLE OF GREEN PLANTS

267

again after a brief period of exposure to sunliglit, when it will be found
to have lost weight. This loss ol' water takes place through the sto-
mata and to some extent through the lenticels of the stem, a loluniom-
enon closely associated with the process of i)hotosynthesis, for which
a relatively enormous amount of water is required. The reasons for
this are that living matter is largely composed of water ; that the pro-
cess of food making cannot take place in plants unless the interior of
their leaves are moist ; and, in the third place, because water is one of
the raw materials used in making sugar. The amount of water given
off by plants through transpiration is very great. Early in the eight-
eenth century Stephen Hales (see p. 241) estimated that an average
crop of cabbages loses from three to four tons of water per day per acre
in warm weather. An acre of pasture grass is said to give off over
100 tons of water on a hot, dry day. A medium-sized tree will evapo-
rate about five to six tons of water on a hot day. One writer, von
Hohnel, estimated that an acre of large beech trees would transpire
30,000 barrels of w^ater in one summer. Such figures show that a
green plant loses water very rapidly during hot, dry days.

The amount of water lost differs greatly under different conditions.
If the air is humid, or if the temperature is lowered, or if the tempera-
ture of the plant becomes low, the rate of transpiration is greatly

Diagrammatic cross section through a stoma to show movement of guard cells.
The dotted lines show the closed position. Closure is brought about by the
guard cells becoming more elongated and flattened, while the outer wall (w)
remains in place, the ventral wall (/) and dorsal wall (V/) assume the positions (/')
and id') moving toward the central slit (s) of the opening of the stoma. This
movement is largely brought about through the change in position of the base
or hinge {h) {h') of the guard cell. (After Schwendener.)

reduced. The stomata tend to close under certain conditions, thus
helping to prevent evaporation. The opening and closing of the
stomata depend on changes in turgor of the guard cells. The stomata

268

THE MAINTENANCE OF THE INDIVIDUAL

open when the guard cells become more than normally turgid, but if
the turgor of all of the living cells of the leaf is reduced by water loss,
then the stomata seem to close automatically.

Light increases the amount of sugar formed in the guard cells
because of the chloroplasts present, which results in a concentration
of sugar, thereby causing a change in turgor. When the leaf is not
illuminated by direct sunlight, or at night, the amount of sugar con-
centration in the guard cells becomes less, and consequently the
stomata close. They usually are closed at night but remain open
from shortly after sunrise until late in the afternoon. Toward the
middle of the afternoon they begin to close, thus decreasing the
amount of water lost in the latter part of the day. Plants wilt on
hot, dry days because they cannot obtain water rapidly enough from
the soil to make up for the loss through the leaves. Many adapta-
tions are found in the leaves which help prevent this water loss, such
as waterproofing of the outer cells, hairs on the leaf surface, the
absence of leaves, as in the cactus where the minute leaves are early
replaced by spines, or the actual turning of the leaves in order to
place a small surface to the sun, as in the compass plant, thus causing
the rate of evaporation to decrease.

; ]

■■

i
> 1

■

-LI

mmd

■WSfflTslSlpiJI

W riylu J'icrcc

Capillary tubes of various sizes. Is there any relation between the size of
the bore of the tube and the water level in the tube ? Explain.

THE ROLE OF GREEN PLANTS

269

The Rise of Water in Plants

We have spoken of the passage of water from the root up the stem
into the leaf. Osmotic pressure has been shown to be sufficient to
start this column of water on its way up the stem, but it is not enough
to account for the rise of water sometimes hundreds of feet into the
air in the stems of trees. Several theories have been advanced to
account f