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Third Edition 




All rights reserved no part of this book may be 
reproduced in any form without permission in 
writing from the publisher, except by a reviewer 
who wishes to quote brief passages in connection 
with a review written for inclusion in magazine or 

Printed in the United States of America 

Set up and electrotyped. Published April, 1935, 

Reprinted January, 1936. 

Copyrighted, 1923, 

By Gilbert M. Smith 

First edition copyrighted, 1924; second edition, 1928, 

By Tt}e Mftcmillan Company. 
First edition published Ju|y, 1024; second edition, September, 1928. 


This book is an outgrowth of the experience of the authors in 
the teaching of elementary botany at the University of Wisconsin. 
For the past three years the text, in successively revised form, 
has been used in our first-year courses. 

In its preparation, we have been guided by the view that the 
subject of botany should be presented as a unit. The beginning 
student is not interested in, and should not be burdened with, 
distinctions between the artificially abstracted phases of the 
subject morphology, physiology, ecology, and the like distinc- 
tions which have their place in defining and limiting the scope of 
more advanced and special courses. Especially should the study 
of structure and that of function be intimately correlated in an 
elementary course. 

It is hardly necessary, in the present state of development of 
the teaching of science, to point out that forms selected for study 
should, whenever possible, be such as are already known to the 
student, either because of their widespread occurrence in nature or 
in cultivation, or because of their economic importance; or that 
general conceptions should be illustrated by familiar facts. Par- 
ticularly in botany should the beginning of the study be an 
observation of everyday plants. Considerations such as these 
have guided us in the choice of material to be used in an elementary 

In a subject the teaching of which involves the introduction 
of the student to many new concepts, the use of a new terminology 
is unavoidable. However, the authors realize that each new term 
imposes an additional burden upon the student and correspond- 
ingly handicaps him in the mastery of the subject matter. We 
have attempted, therefore, to avoid technical terms except those 
which were found indispensable to a clear presentation. 

Only such facts and conceptions have been introduced as our 
experience has shown can be successfully treated in the Bourse of 
the beginning year. Necessarily the subject matter has been 
arranged in what seems to the authors a logical order* 
assumption of a continuous year's course. Howev&n 



institutions, conditions necessitate the offering of a briefer elemen- 
tary course in botany. We have tried to provide for the possibility 
of such a course by so treating various topics that, within reason- 
able limits, certain chapters and portions of chapters may be 
omitted without destroying the continuity of the course or im- 
pairing the utility of the later parts of the book. 



I. THE MAKE-UP OF4BBB .W^IWWV 11 *! * ^* . 1 


IV. ROOTS . fflBP^ 25 

V STEMS . ^rs?^ 50 

Vx. .bUDS A/^u^/^p^-r r 83 


Vllly RELATIONS OF PLANTS TO WATER ** r r . . 114 




-XII. NUCLEAR AND CELL DIVISION .--*r**r r : r 183 


2vIV.' v "CHLOROPHYCEAE (GREEN ALGAE) . ^^^^ . . 199 





\S , 


XX. ASCOMYCETES \^"'. 280 




XXIV. Musci (MOSSES) . 340 







XXVIII. GYMNOBPERMS .... ^~-r~r~~. . . 397 


XXX. SEEDS AND FRUITS . ..,- r *~ T""' 443 






INDEX 561 





1. A Plant and Its Parts. It is helpful to begin the study of 
plants by a consideration of one which is familiar, and at the 
same time large and easily handled. Such a plant is the sun- 
flower (Fig. 1). The sunflower is not only a member of the group 
of most complex plants (the seed plants) ; it is also one of the most 
highly developed members of that group. One noticeable char- 
acteristic of the sunflower plant is that it is made up of distinct 
parts. These parts of which the plant is composed are called 
organs. The organs that are present at almost any stage in the 
development of the plant are leaves, stem, and roots. Certain other 
organs may or may not be present; occasionally, for example 
(especially in certain varieties of the sunflower), branches, which, 
as will appear, can conveniently be classed with the stem; and 
flowers and fruits organs whose study may better be left until 

In order to remain alive, to grow, and to reproduce that is, 
to give rise to new plants the sunflower plant must do certain 
work. The different kinds of work that a plant carries on are 
spoken of as its functions. In order to understand a plant, it is 
necessary to learn as much as possible about its structure that 
is, about the parts of which it is composed, their form and arrange- 
ment; and also about its functions the work that it does. It 
is always true that, in a general way, a plant is so constituted as 
to be able to perform its functions successfully; and so the struc- 
ture of a plant can not be understood without a familiarity 
with its functions, nor can its functions be understood without a 
knowledge of its structure. 

What has just been said of a plant as a whole applies also to 
its separate organs. The work of a plant is in large measure 





divided between the organs of which it is composed. Each organ 
is so constituted that it is fitted to carry on a certain function or 
certain functions better than other organs of the plant can perform 

them. There is a consider- 
able degree of division of 
labor between the organs, 
each doing especially the 
work for which its struc- 
ture best fits it. It be- 
comes necessary, therefore, 
to study each separate 
organ likewise from the 
standpoints of structure 
and of function. 

2. Leaf of the Sunflower. 
A leaf of the sunflower 
' Nod * (Fig. 2) is made up of two 
-Intemode parts: a slender stalk, or 
petiole, and a broad blade. 
A leaf blade held between 
the eye and the light is 
seen to be marked by 
many light green lines 
which are called veins. 
There is one large central 
vein (midrib), from both 
sides of which run smaller 
branch veins; these branch 
veins send off finer 
branches, these in turn still 
finer ones, and so on; the 
smallest branches run to- 
gether here and there, so 
that the whole blacje is 
penetrated by a close net- 
work of veins large and 
small. The positions of the larger veins are marked by ridges 
on the under surface of the blade. The parts of the leaf petiole, 
blade, and veins have, to some extent, different functions; that 
is, just as there is a division of labor between the organs that 

FIG. 1. A sunflower plant. 


make up the plant, so there is a division of labor between the 
parts of a single organ such as a leaf. Similarly, a stem or a 
root is made up of different parts, 
each doing its share of the work 
of the organ as a whole. 

3. Other Types of Leaves. 
Although the more familiar and 
larger plants are in general com- 
posed of the same organs as is 
the sunflower, these organs differ 
greatly in form in different 
plants. A leaf of the Indian corn 
(Fig. 3) is long and slender, and 
is divided, not into blade and 
petiole as is the sunflower leaf, 
but into blade and sheath. The 
sheath, or basal part of the leaf, 
is a clasping structure which 
surrounds the stem for some 
distance above the level at, which 
the leaf is really attached. There 
is a difference also in the ar- 
rangement of the veins. Whereas 
the conspicuous veins of the sun- 
flower leaf are much branched 

and form a network, those of the corn leaf run approximately 
parallel from the base to the apex of the blade. The appearance 
of the vein systems in the two cases is very different; in reality 
both leaves have branch veins, but the branch veins in the corn 
leaf are very fine and not easily seen.* 

4. Stems and Branches. Both sunflower and corn (Fig. 4) have 
upright stems each consisting of nodes, or joints, at which the 
leaves are borne, and internodes (the portions of each stem be- 
tween successive nodes). The stem of the corn is commonly 
thought of as unbranched; there are, however, occasional branches 
(suckers) which grow from near its base. The tassels and ears, 
which bear flowers, are also branches, or systems of branches. 
An important characteristic of the sunflower plant is in the fact 
that older parts of the stem or branches gradually increase in 
thickness as long as the plant is growing. The method of growth 

FIG. 2. Leaf of a sunflower, showing 
the arrangement of veins. 


by which this secondary thickening takes place will be described 

in Chapter V. The corn stem, on the other hand, has no such 

means of secondary thickening. 

5. Roots. The original (primary) root of a sunflower plant is a 

direct continuation of the stem. Sooner or later this primary 

root produces lateral 
branches (secondary roots) 
which may themselves 
branch. Production of sec- 
ondary roots results in a 
rather widespread root sys- 
tem in which the branches 
are, for the most part, pro- 
gressively smaller, the ul- 
timate branches being very 
slender. The roots of the 
sunflower have a method 
of secondary thickening 
similar to that of stems 
and branches. 

The primary root of a 
corn plant, like the pri- 
mary root of a sunflower, 
is a direct continuation of 
the stem. It does not, at 
least under ordinary con- 
ditions, give rise to sec- 
ondary roots. Often this 

FIG. 3. Leaf of the corn, showing the sheath primary root dies early. In 

8Uch a Case the root ^ 
tern of the corn then con- 
sists chiefly of roots which have grown, not from the primary 
root as in the sunflower, but from the lower nodes of the stem. 
Many of these adventitious roots ( 31) arise at the underground 
nodes; but others commonly grow from one or more of the above- 
ground nodes of the corn stem. Some of the adventitious roots 
that arise above ground extend downward into the soil, so serv- 
ing as props to the stem; others remain too short to reach the 
surface of the soil. The roots of the corn, like its stem, do not 
undergo secondary thickening. 


6. Functions of Organs. Two important functions of a root 
system are, in most cases, anchorage of the plant in the soil and 
absorption from the soil of substances that are needed by the 
plant. The substances so taken in 

must be transported to the parts 
of the plant above ground, so that 
the conduction of absorbed sub- 
stances is also a part of the work of 
roots. The storage of reserve food, 
too, is a function of many roots, 
and this function is especially im- 
portant in such thick roots as those 
of the carrot, radish, and beet. 
The chief functions of the stem, 
and of the branches (if any), are 
usually the conduction of materials 
from roots to leaves and from 
leaves to roots, and the support 
of the leaves, as well as of the 
flowers, in a position favorable to 
the performance of their work. 
The leaves are, in the majority of 
seed plants, the chief food-manu- 
facturing organs; but some food is 
made also in the green parts of 
stem, branches, and flowers. 

7. Adaptation. It was suggested 
in 1 that in general a plant and 
its organs are so constituted that 
they are able to perform their func- 
tions successfully. It may be added 
that these functions can be per- 
formed most satisfactorily under 
the conditions, such as those of 
temperature, light, and moisture, 
to which the plant is ordinarily 

FIG. 4. A corn plant. 

exposed. These facts are summed up by saying that the plant 
is adapted to its environment. Adaptation in this sense is widely 
characteristic of living organisms, whether plants or animals- 
Adaptation is never perfect; but if organisms were not fairlj 


well adapted to their environment, obviously they would not 
long survive; nor will they usually survive if the environment 
is greatly altered. How the adaptation of organisms to their 
environment has come about is one of the major biological prob- 
lems. Something of what is known regarding this problem will 
be discussed in Chapters XXXII and XXXIII. 




8. Units of Structure. Every plant and every organ of a plant 
is made up of small parts, each of which is a cell. Cells are the 
unite of structure of plants and animals, much as bricks or stones 
may be the units of structure of the wall of a house. As we must 
know the nature of bricks or of stones in order to understand the 
strength and durability of the house which is built of them, so to 
understand the nature of a 
plant or of an animal we must 
know something of the cells of 
which it is composed. Some 
very simple organisms consist 
each of a single cell; but in 
general any one of the larger 
plants and animals is com- 
posed, like the sunflower, of 
many cells too small to be seen 
with the naked eye. 

The word cell commonly 
means a cavity or chamber 
which may be quite empty. 
But in speaking of the cells 
that compose a living organ- 
ism, the word is used in a 
different sense. These units of 
structure were first called cells 
by Robert Hooke (1635-1703). 
Hooke was interested in ex- 
amining various objects with 
the aid of the compound microscope, then a new toy, very crude as 
compared with present-day instruments, which had recently been 
introduced into England. Among other objects, as reported in 
his "Micrographia" (1665), he examined a thin slice of cork and, 
much to his surprise, found that it contained many little "pores 

FIG. 5. The cellular structure of cork. 
This, the first published illustration 
showing a tissue composed of ceils, 
appeared in Hooke's "Micrographia" 
in 1665. 



or cells" (Fig. 5). Years later it was recognized that cork, such 
as Hooke had studied, is made up of the walls of dead cells, and 
that the really living part of any cell is the liquid or jelly-like 
substance within its walls. Indeed, as is now known, many cells 
consist entirely of this liquid or semi-liquid substance and have 
no walls at all. However, the name cell has persisted and is ap- 
plied to all these units of living matter, whether or not they are 
surrounded by walls. 

Although the mature cells present in a plant are alike in fun- 
damental characteristics, they may differ greatly in size, form, 
and function. The cells of any organ (such as a root, stem, or 
leaf) are organized into tissues. A tissue may be composed of 
cells all of which are much alike. However, the term tissue is 
also often applied to groups of cells which differ in structure but 
which cooperate in the performance of a common function. Thus, 

a root or stem contains conduct- 
ing tissues each composed of cells 
of several different sorts as to size 
and form. Just as every plant, 
except some of the simpler ones, 
is composed of organs, so these 
organs are made up of tissues, and 
the tissues in turn are composed 
of cells. 

9. Organization of a Cell. In 
the study of the living cells of one 
of the more complex plants, seri- 
ous difficulty results from the fact 
that an organ of such a plant is 
usually of considerable thickness 
and composed of numerous cells. 
The microscope gives at best only 
a confused idea of any of the in- 
iividiml cells in such an organ. For this reason the cells can often 
DC seth more clearly in a leaf, because of its thinness, than in a 
stem or root, and the thinner the leaf the more distinctly can the 
structure of an individual cell be made out. A favorable leaf for 
;uch a study of a mature cell is that of Elodea (Fig. 6), an aquatic 
)lant sometimes called the "water pest/' a pative of North Amer- 
ca which grows in sluggish streams and in ponds throughout the 

FIG. 6. Elodea plants. 



continent, except in the extreme northern portions. The plant has 
a slender, branching, submerged stem varying in length, accord- 
ing to the depth of the water in which it grows, from a few inches 
to several feet. Slender roots growing from the stem anchor the 
plant more or less firmly in the soil at the bottom of the water. 
The many leaves are 
small, narrow, and 
pointed, and are usu- 
ally borne in circles of 
three, four, or more. 

A leaf of Elodea is 
only one cell in thick- 
ness at its margin, and 
for the greater part 
two cells in thickness 
elsewhere. Viewed 
from above (Fig. 7), 
the cells of several 
rows near the margin 
of the leaf appear nar- 
row and rectangular. 
At intervals, pointed 
cells project from the 
edge. Occupying the 
greater part of the sur- 
face of the leaf are a 
larger number of rows 
of wider and shorter 
cells which, as will be seen later, are concerned largely in food- 
manufacture. The cells of the lower layer are about hatfas wide as 
those forming the upper layer. In the central 
are several layers of narrow, elongated cells which 
midrib. Although a cell appears rectangular or squarei seen 
under the microscope (which shows only one plane), it S|gt be 
remembered that the cell has thickness as well, and is therefore 

The cells of the leaf (Fig. 8) are separated from one another, 
as well as botinded above and below, by transparent, ceM watts. 
The wall between any two adjacent cells is composed of sev- 
eral layers, of which the middle one is the oldest; during the 

FIG. 7. Portion of an Elodea leaf . 



development of the cells the other layers were deposited succes- 
sively on either side of this original layer through the activity of 
the living matter adjoining the wall on either side. 

All the material within a cell wall is referred to as protoplasm. 
Protoplasm is never homogeneous; it consists of numerous sub- 
stances differing in nature, which have a definite arrangement 
within the cell. The whole structure made up by the protoplasm 

Cell Wall Plasma Membrane Intercellular Space 

Dense Cytoplasm Nudeolus Nucleus Chloroplast 

FIG. 8. A living cell of a leaf of Elodea, as seen in optical section. 

that is, the body of the cell exclusive of the wall is sometimes 
called ihe protoplast. The protoplasm is divided into cytoplasm 
and nucleus. Each of these two main divisions is in turn composed 
of various different substances which are definitely arranged. The 
arrangement of the substances that compose cytoplasm and nucleus 
is the organization of the cell. Because dells, as well as plants and 
animals which consist of many cells, have a definite organization, 
a singlap&ll living alone, or a many-celled plant or animal, is an 

10. Structure of Cytoplasm. Just within the wall on all sides of 
a mature cell, including top and bottom, is a thin layer of the 
cytoplasm which appears relatively dense and often finely gran- 
ular; this thin layer will be referred to as the dense cytoplasm. 
Included in it are many ovoid, or at times somewhat flattened, 
green bodies, the chloroplasts. These, which are also parts of the 



dense cytoplasm, are the most conspicuous structures in most of 
the cells of a leaf. In the central part of the cell, enclosed by the 
layer of dense cytoplasm, is a large, transparent central vacuole. 
The cell sap which fills the central vacuole is a rather dilute solu- 
tion of food substances, salts, and numerous other materials. 
The very outermost film of the dense cytoplasm, next the cell 
wall, is the plasma membrane; a similar film next the central 
vacuole is the vacuolar membrane. 

Under certain conditions the layer of dense cytoplasm with the 
chloroplasts (but not including the plasma membrane) is in motion. 
The movement is mainly one of rotation (Fig. 9), usually about 

FIG. 9. Diagram showing the direction of rotation of the layer of dense 
cytoplasm in a cell of an Elodea leaf. 

the vertical axis of the cell. Commonly the movement is in the 
same direction in all the cells of a leaf; but frequent exceptions to 
this rule occur. Occasionally a cross strand of dense cytoplasm 
cuts from one side to another through the central vacuole. The 
dense cytoplasm is the active substance in this movement; the 
chloroplasts are carried along by the current, much as pieces of 
ice may be carried in a river. The cell wall is perforated by pores, 
usually too minute to be seen with the highest powers of the 
microscope. These pores offer means by which the protoplasts 
of adjacent cells are either continuous or in contact with one 

The similarity in color and in transparency between most 
parts of the cytoplasm, such as the dense cytoplasm and the 


vacuoles, makes it impossible to distinguish accurately the bound- 
aries of these parts when a cell is alive. On account of these dif- 
ficulties it is necessary, in order to study the finer details of struc- 
ture, to subject a leaf to a rather lengthy series of processes. These 
processes are, in brief: (a) killing and fixing the leaf in a poison 
or combination of poisons so selected as to kill the cells at once 
but to leave all parts of each cell in as nearly their original posi- 
tions as possible; (6) hardening by means of alcohol; (c) cutting 
into thin sections; and (d) staining of the sections. The stains 
used in the last-named process are, with a few exceptions, aniline 
dyes. Advantage is taken of the fact that protoplasmic substances 
in general show an affinity for aniline dyes, and that different 
parts of the protoplast have varying affinities for different dyes. 
If, therefore, a section of a leaf is subjected to the successive action 
of two or three properly selected dyes of different colors, various 
parts of the cell may take on contrasting colors and thus stand 
out distinctly one from another. 

The appearance of a cell in an Elodea leaf treated as just de- 
scribed (Fig. 10) is very different from that of a cell in a living 

Vacuolar Membrane Central Vacuok Plasma Membrane 

Chloroplast I I Dense Cytoplasm / 


Nuclear Sap . 
-,, . >r . / Nucleus 
Chromatic Network 

Nuclear Membrane 

FIG. 10. Portion of a cell from a leaf of Elodea, after it has been 
killed and stained. Highly magnified. 

leaf. The dense cytoplasm is seen not to be so nearly homogeneous 
as it appears to be in a living "cell. Included in it are many clear 
vacuoles, varying greatly in size and shape but all very small; 
between these is a somewhat more deeply stained, often finely 
granular substance. In addition to vacuoles, chloroplasts, and 
minute granules, the dense cytoplasm includes bodies of varied 
form, smaller than the chloroplasts, which may be deeply stained. 


These small cytoplasmic bodies (chondriosomes) are more readily 
observable in a young than in a mature cell (see Fig. 15). 

When the cell is living, each vacuole contains a transparent 
liquid and is surrounded by a thin membrane; the granular sub- 
stance between the vacuoles is also a liquid, but one of different 
nature. The visible structure of the dense cytoplasm is therefore 
that of an emulsion. 

11. Structure of a Nucleus. A nucleus is an approximately 
hemispherical, semi-transparent body, imbedded like the chlo- 
roplasts in the layer of dense cytoplasm, and therefore close to 
the cell wall. In a living cell the nucleus appears to be homo- 
geneous, except that it includes one or more fairly large, rounded 
refractive bodies, the nudeoli. The nucleus may be carried 
along by the cytoplasmic current, as the chloroplasts are, but 
more slowly. Consequently, at any moment it may lie at one 
side, at the top, or at the bottom of the cell. 

In a living cell the nucleus is barely distinguishable from the 
dense cytoplasm in which it is imbedded. In a killed and stained 
cell the nucleus, because of the strong affinity of some of its parts 
for dyes, is the most conspicuous portion. Staining demonstrates 
also that the nucleus is not homogeneous in structure but is it- 
self composed of parts differing in nature. It is bounded by a 
nuclear membrane/^ film similar to the plasma and vacuolar 
membranes. * Within the nucleus is usually at least one large, 
rounded, deeply stained nucleolus; some nuclei contain two or 
several nucleoli. There is also a network composed of material 
that is deeply stained but distinct from the nucleoli. Because 
it stains readily, this will be spoken of as a chromatic network. 
Its component strands vary in thickness; at the angles where 
they meet are granules or knots of varying sizes. As will be seen 
later (Chap. XII), the substances composing this network per- 
form functions of very great significance. The chromatic network, 
and the nucleolus or nucleoli, lie in a nuclear sap which is usually 

12. Nature of a Living Cell. All the protoplasmic structures 
that have been described, including those of both cytoplasm and 
nucleus, together constitute a living cell. Living cells are dis- 
tinguished from non-living forms of matter by certain activities 
or processes that go on within them. It is not possible to define 
living matter so sharply that we can say that certain parts of the 


protoplast are living and that other parts are non-living. So far 
as is known at present, the processes that distinguish living mat- 
ter from non-living matter are carried on within cells which are 
organized in general much like those of an Elodea leaf. It is pos- 
sible to say, also, that these processes which characterize living 
things have their seat primarily in certain portions of each cell 
particularly in the dense cytoplasm including the plasma and 
vacuolar membranes, in the chloroplasts, and in the chromatic 
network of the nucleus. To this extent, the parts of the proto- 
plast just mentioned are more living (or more actively living) 
than such other parts of the cell as the wall, the cell sap, and the 
nuclear sap. 


A mature cell, such as one in a leaf of Elodea, consists of the 
following parts: 

1. Wall: 

Middle layer. 
Later-formed layers. 
Minute pores. 

2. Protoplasm: 
(a) Cytoplasm: 

Dense cytoplasm: 

Plasma membrane. 

Small vacuoles, each with its membrane. 

Intervacuolar substance. 

Vacuolar membrane (next the central vacuole). 



Central vacuole, containing cell sap. 
(6) Nucleus: 

Nuclear membrane. 
Nucleolus (or nucleoli). 
Chromatic network. 
Nuclear sap. 



13. Elements and Chemical Compounds. The substances com- 
posing a plant, while of very many different kinds and degrees 
of complexity, are yet, like all substances, made up of molecules, 
and their molecules are composed of relatively few chemical 
elements. When a molecule of water is broken up into its con- 
stituent parts, each component is an atom of an element. A mole- 
cule of water is composed of two atoms of hydrogen and one atom 
of oxygen. In this case two gases have united to form a chemical 
compound that at ordinary temperatures is a liquid. The chemical 
formula for water, H20, indicates the elements of which it is 
composed and the proportion in which the atoms of those elements 
are present. Water is one of the simpler chemical compounds, 
and is of universal occurrence in living matter. 

Among the compounds found in plants, while some such as 
water are relatively simple, a great number are composed of 
three or more elements each, and their molecular structure is 
vastly more complex than that of water. Of the compounds that 
are found in and manufactured by plants, many, which are char- 
acterized by containing the element carbon, are spoken of as 
organic in contrast to inorganic compounds. Inorganic compounds 
occur in nature independently of the activities of living matter, 
but many of them are absorbed and used by plants. 

14. Diffusion. Before discussing the passage of substances into 
and out of a living cell, it is necessary to consider diffusion, a 
process that is very largely involved in such movements. 

If ether is poured into an open vessel, the odor soon indicates 
that the ether is becoming distributed through the air of the 
room. Liquid ether is composed of a vast number of molecules 
which are relatively close together and all in vigorous motion. 
Because of their comparatively crowded condition the molecules 
are constantly in collision, striking one against another and re- 
bounding. At the free surface of the liquid opportunity is greatest 
for the movement of the molecules. Hence many of them, when 



they reach this surface, fly off into the air. Having passed into 
the air, they are still in motion and tend to distribute themselves 
uniformly through the available space that is, through the room. 
TJie liquid ether thus rapidly becomes a gas. The change of ether 
from a liquid to a gaseous state is a consequence of the tendency 
of its molecules to become uniformly distributed throughout the 
space available. The process of distribution which results from 
this tendency is diffusion. A tendency to diffuse characterizes 
all matter. 

The space within the room into which the ether molecules pass 
is filled with air, a mixture of rapidly moving molecules of gases, 
those present in largest amounts being oxygen and nitrogen. 
When molecules of ether pass into the air, they tend to diffuse 
uniformly among the molecules of oxygen, nitrogen, and other 
gases that may be present. The diffusion of each gas throughout 
the space available is independent of the diffusion of each of the 
other gases. The consequence is that all the gases within a limited 
space, such as a room or a small container, will in time become 
so distributed that a uniform mixture will result. 

The same principle may hold when two or more liquids are 
in contact. For example, if liquid ether and alcohol are brought 
together in a dish, the alcohol diffuses throughout the ether, the 
final result being a homogeneous mixture. Just as diffusion may 
occur between gases or between liquids, so there may be diffusion 
between gases and liquids. For example, if oxygen and water are 
in contact, a certain proportion of oxygen diffuses into the water. 
Howeyer, the water is just as truly diffused through the oxygen as 
is the oxygen through the water. 

Similarly, a solid substance may diffuse through a liquid. A 
small amount of sugar or common salt in contact with a suffi- 
cient amount of water disappears, so that neither by the naked 
eye nor under the microscope can any evidence of the presence 
of the solid be detected. The mixture is homogeneous in character. 
If the solid is added bit by bit the result is similar, the mixture 
remaining homogeneous throughout the operation. Finally, how- 
ever, when enough of the solid is added, the process of mixing 
appears to stop; some of the .solid settles to the bottom, and the 
mixture is no longer homogeneous. 

Not all substances, when brought together, diffuse readily into 
one another. Olive oil and water, for instance, when in contact 


diffuse into each other but very slightly; the two liquids remain 
substantially separate. Whether or not substances will diffuse 
into one another depends upon their respective natures. 

When a solid or a gas diffuses into a liquid, the resultant homo- 
geneous mixture is spoken of as a solution. The liquid is the 
solvent, the substance diffusing into it the solute. When two liquids 
are concerned in the process, one of which is water, the water is 
commonly considered the solvent. It would be equally possible, 
however, to consider the other liquid, for example alcohol, as the 
solvent and the water as the solute. 

Water possesses the property to a remarkable extent of dissolv- 
ing other substances; in fact, a greater variety of substances are 
dissolved by water than by any other* known liquid. In living 
cells the principal solvent is water; the solutions that occur in 
plants are chiefly solutions of solids, liquids, and gases in water. 

It is important to note that the diffusion of any solute through 
water is independent of the diffusion of any other solutes present; 
in a solution of sugar and common salt the diffusion of the sugar 
is independent of that of .the salt. The capacity of different sub- 
stances to dissolve in water is extremely variable. For example, 
very large proportional amounts of sugar and salt will dissolve 
in water; only slight amounts of iron and some other metals will 
so dissolve. Some substances, like camphor, which are practically 
insoluble in water, are readily soluble in some other liquids such 
as alcohol or ether. The solubility of any substance in a liquid de- 
pends, then, upon the nature both of the solvent and of the solute. 
Surrounding conditions also influence solubility. For instance, 
changes of temperature affect the solubility of a solute in water. 
As a rule, solids become more soluble in water, and gases less 
soluble, as the temperature is raised. The solubility of some sub- 
stances in water increases rapidly with a rise in temperature, 
while for other substances the increase is slow. 

16. Osmosis. If two substances which tend to diffuse are 
separated by a membrane through which both separated sub- 
stances can readily pass, they will still diffuse despite the presence 
of the membrane. Thus if water and a solution of sugar in water 
are placed in the respective arms of a U-tube and are separated 
by a partition of rather coarse cloth, the sugar will pass through 
the cloth and diffuse in the water on the other side of the par- 
tition; the water will pass through the partition in the opposite 



direction. In time there will be a homogeneous mixture of sugar 
and water of equal concentration on both sides of the partition. 
In this case the membrane is permeable to both sugar and water. 
If the water and the solution of sugar are separated by a rubber 
membrane, neither water nor sugar will diffuse through the mem- 
brane. In this case 
the membrane is im- 
permeable to both 
sugar and water. 

If the water and 
sugar solution are 
separated by a mem- 
brane of parchment 
paper, the water will 
pass more rapidly 
through the mem- 
brane into the sugar 
solution than sugar 
wi|| pass into the 

A Membrane B water, and the liquid 

FIG. 11. An apparatus illustrating osmosis. A, at will rise in the arm 
the beginning of the experiment; B, after standing o f the tube con- 
for a time. One arm of the U-tube containing + Q1 . - +1^ , irTQ ~ 
pure water is separated by a differentially per- tamm g tne su S ar 
meable membrane from the other arm, containing solution (Fig. 11). 
a sugar solution. Note in B the rise of the liquid Although the mem- 
















in the arm containing the sugar solution. 

ig permeable 

to both water and sugar molecules, it is permeable to them in 
different degrees, sugar diffusing through it slowly and water more 
rapidly. The membrane differentiates between the two substances. 
In this case the membrane is differentially permeabk to sugar and 

The same principle may be illustrated by the use of a thistle 
tube (Fig. 12) over whose lower end parchment paper is fastened 
tightly. A fairly concentrated solution of sugar is placed in the 
lower end of the tube, which is immersed in water. Water will 
diffuse through the differentially permeable membrane into the 
sugar solution. Some of the sugar will diffuse very slowly through 
the membrane into the water. Since the water diffuses more 
rapidly than the sugar, the result will be a rise of the solution in 
the tube 


If a differentially permeable membrane separates aqueous so- 
lutions of two different substances (for example, a solution of 
sugar and one of salt), ordinarily water 
will at first diffuse chiefly from the 
less concentrated to the more con- 
centrated solution. Precisely what will 
happen, however, depends upon the 
nature of the membrane, the nature 
of the rolvent, and the natures and 
relative concentrations of the solutes. 

The diffusion of a gas, a liquid, or a 
dissolved substance through a mem- 
brane is commonly called osmosis. The 
direction of the major osmotic move- 
ment of any substance is determined 
by the relative number of particles of 
that substance present. In general, 
the direction of movement is from the 
side of the membrane where the pro- 
portioial number of particles of the 
diffusing substance is higher to that 
side on which the proportional number 
is lower. A diffusing substance may 
be moving in both directions through 
a membrane, but more particles of that 

substance are moving away from the 

side where .they are proportionally Fm 12 An apparatu8 to fl- 
more numerous than from the side lustrate osmosis. A, a thistle 
where they are less numerous. Unless 
prevented, the movement will con- 
tinue until equilibrium is established. 

16. Significance of Osmosis. A cell 
such as one of the Elodea leaf is an 
osmotic apparatus shut off from the 
outside world, and largely from adjoining cells, by a cell wall and a 
plasma membrane. Both are differentially permeable, but the wall 
permits the ready passage of many more substances than does the 
plasma membrane. In consequence, it is the plasma membrane 
which chiefly determines what substances shall enter or leave the 
cell. When substances entering the cell have once passed the wall 

tube whose lower end is closed 
by the differentially perme- 
able membrane B. The lower 
end (C) of the tube contains 
a concentrated sugar solu- 
tion. The vessel D contains 



and the plasma membrane, their further movement within the cell 
is regulated by a variety of other membranes. If substances are to 
reach the central vacuole they must pass the vacuolar membrane, 
which is itself differentially permeable. In a general way, whether 
or not a particular substance is to enter or leave a cell depends 
upon the relative concentration of that substance in the central 
vacuole and in the liquid outside the cell. The transfer of sub- 
stances between cytoplasm and nucleus is regulated by the nu- 
clear membrane, which is also differentially permeable. 

Under ordinary conditions the solutes in the cell sap are more 
concentrated than are those in the liquid surrounding the proto- 
plast. In consequence, water passes by osmosis into the cell. 

FIG. 13. A cell showing plasmolysis resulting from immersion in 
a strong solution of common salt. 

Continued movement of water into the protoplast results in a 
pressure within the vacuoles. This pressure is sufficient to distend 
the protoplast as far as the elasticity of the wall will permit. The 
pressure thus developed within the cell is called turgor and the 
cell is said to be turgid. A cell may be more or less turgid accord- 
ing to the amount of pressure within. 

If the concentration of the solutes in the liquid outside the 
protoplast is greater than the concentration of those in the cell 
sap, water will be withdrawn from the protoplast by osmosis. A 
sufficiently extensive withdrawal of water from the protoplast 
reduces the pressure to such a degree that the protoplast is partially 
or entirely withdrawn from the wall and becomes more or less 
rounded. In this state of contraction the cell is said to be plas- 


molyzed. The process of becoming plasmolyzed is plasmolysis. 
Plasmolysis may be illustrated by placing a living leaf of Elodea 
in a rather strong solution of cane sugar or of common salt (Fig. 13). 
Prolonged plasmolysis is usually fatal to a cell. If, however, a 
leaf in whose cells plasmolysis has not proceeded too far is removed 
from the sugar or salt solution and placed in approximately pure 
water, water will diffuse inward and the cells will return to their 
former turgid condition. 

The absorption of substances concerned in the nutrition of a 
plant depends largely upon osmosis, since all substances that enter 
or leave any living cell must be either liquid or dissolved and must 
be capable of diffusing through the plasma membrane. Water and 
a vast number of dissolved substances with which the activities 
of a cell are concerned diffuse into or out of the cell independ- 
ently of one another. Water and dissolved substances may enter 
or leave a cell at the same time at very different rates, some of 
them passing inward and others outward. The structure and 
composition of living membranes are important in determining 
what substances shall enter or leave a cell. While a plant is alive 
the permeability of any living membrane, such as a plasma mem- 
brane, may vary greatly from time to time; the variations de- 
pending upon numerous conditions internal and external to the 
cell, including age, temperature, arid light. At one time a mem- 
brane may be readily permeable to a particular substance; at 
another, relatively impermeable to the same substance. It is true 
also that there are great differences in permeability between the 
plasma membranes of different cells, the differences often being 
extremely great in this respect between cells of the same plant. 

Osmosis plays an important part in the transport of water and 
dissolved substances from cell to cell and from organ to organ, in 
the absorption of gases from the atmosphere by the cells of leaves 
and of other organs, and in the absorption of water and of other 
substances in solution from the soil by the cells of roots. The 
turgidity of cells is important in maintaining the rigidity of cer- 
tain parts of plants, such as leaves, young stems, and young 
roots. The form, and especially the position, of non-woody plants 
and parts of plants depend largely on the turgidity of their cells. A 
loss of turgidity by the cells of such plants results in their wilting. 

17. Imbibition. Osmosis explains in large part how water and 
dissolved substances enter the living protoplast. Another 


which plays a part in the passage of liquids and dissolved sub- 
stances into the cell walls and thence into the protoplast, is imbi- 
bition. A familiar illustration of imbibition is the soaking up of 
water by dry gelatine or wood. The absorbed water is drawn into 
the spaces between the particles that make up the gelatine, or into 
similar spaces in the walls of the cells that make up the wood. 
The water eventually becomes so arranged about the particles of 
such a substance that it forms films which crowd the particles of 
the imbibing substance apart and this substance swells. The 
extent to which the particles can be forced apart by the water 
depends upon the cohesion between the particles of the imbibing 
substance. In the case of gelatine, its particles may become so 
widely separated by the water that the mixture becomes a liquid. 
In the case of wood the limit of the separation of its particles is 
soon reached. Water is imbibed readily by the walls of living 
cells, except those of certain tissues which are impregnated with 
fat-like substances. In a cell of the Elodea leaf, the process of 
imbibition brings water and dissolved substances into contact 
with the plasma membrane. It is then possible for these substances 
to enter the protoplast either by osmosis or by imbibition, since 
the plasma membrane is capable of imbibing water and some dis- 
solved substances. The differential permeability of the plasma 
membrane is thus determined in part by the readiness with which 
it imbibes certain dissolved substances and by its inability to 
imbibe others. Imbibition and osmosis are closely related proc- 
esses; osmosis is made possible by the imbibing powers of the 
cell wall and of the plasma membrane. Some plant cells, especially 
those of certain desert plants, contain substances of a mucilaginous 
nature. These substances, located in the vacuoles, in the dense 
cytoplasm, or in both, by their power of imbibition increase the 
water-absorbing and water-holding capacity of the cells contain- 
ing them, to such an extent that the cells resist long periods of 

18. Suction Tension. The processes of osmosis and imbibition 
just discussed, together with other processes which may play a 
minor r61e, result' in a tendency on the part of every healthy proto- 
plast to draw in water, leading to the production of a suction 
tension. This suction tension is important not only in the activ- 
ities of a living cell but also, as will appear in later chapters, in 
*V transfer of water and dissolved substances throughout a plant 



Chemical compounds found in plants are composed of relatively 
few elements. The inorganic compounds are relatively simple; the 
organic compounds are more complex. 

Diffusion results from the tendency of molecules to become 
equally distributed, as illustrated by the mixing of two gases, of 
a gas and a liquid, of two liquids, or of a solid and a liquid, to 
form a homogeneous mixture. The mixture of a gas and a liquid, 
of a solid and a liquid, or of two liquids is a solution. The liquid 
involved in a solution is the solvent, the other substance the solute. 
If two liquids are concerned one of which is water, the water is 
commonly considered the solvent. Solutions occurring in plants 
are chiefly of solids, liquids, and gases in water. Solutes in water 
diffuse independently of one another, but the process is affected 
by the nature of the solute. Temperature affects solubility, solids 
as a rule becoming more soluble, gases less soluble, as the tem- 
perature rises. 

Osmosis is diffusion through a membrane. The rate of osmosis 
varies with the nature of the membrane, of the solvent, and of the 
solute, with the concentration of the solute, and with the tem- 
perature. Permeable membranes allow both solvent and solute 
to diffuse with little hindrance; impermeable membranes are 
permeable neither to solvent nor to solute. A differentially per- 
meable membrane is permeable to both solvent and solute but to 
different degrees. 

A living cell is enclosed by a cell wall and a plasma membrane, 
both of which are differentially permeable. Because of its lesser 
permeability to solutes, the plasma membrane chiefly determines 
what substances shall leave or enter the cell by osmosis. Since 
solutes are ordinarily more concentrated within a protoplast than 
in the liquid outside the cell, water passes into the protoplast by 
osmosis and develops a pressure (turgor), causing the cell to be- 
come turgid. If the conditions just mentioned are reversed, water 
is withdrawn and the protoplast is plasmolyzed. 

The structure and composition of living membranes are im- 
portant in determining what substances shall enter or leave a 
cell. The permeability of living membranes varies from time to 
time, depending upon conditions within and without the "* n 
such as age, temperature, and light. 


Osmosis is important in the transport of water and dissolved 
substances within the plant and in the absorption of substances 
in solution from the soil by roots. The form of a non- woody plant 
is maintained largely by the turgidity of its cells. 

Imbibition plays an important part in the absorption by cell 
walls of water and dissolved substances, bringing these substances 
into contact with the plasma membrane through which they may 
pass by imbibition and osmosis. The protoplasts of some plants 
contain substances which imbibe and hold large quantities of 
water, enabling them to endure periods of drought. 

Every healthy protoplast possesses a suction tension, resulting 
from its tendency to take up water by osmosis and imbibition. 
Suction tension is a factor not only in the absorption of water, but 
also in the transfer of water and dissolved substances. 



19. Regions of a Root. The roots of a sunflower are represent- 
ative of the type of roots normally growing in soil. Their structure 
is particularly suited for the performance of their principal func- 
tions: anchorage, the intake of water and of other materials, and 
the conduction of these to the stern. Since little or no food material 
is stored in the root system of the common sunflower, there are 
no prominently thickened portions except for the enlarged primary 
root. From this arise many smaller secondary roots, which in 
turn are branched and rebranched. These smaller roots, extending 
downward and outward, hold the plant firmly in place and provide 
a large absorbing area. 

The extreme tip of a root (Fig. 14) is covered by a root cap. A 
root as a whole, exclusive of the root cap, may be thought of as 
divided into four general regions which merge gradually one into 
the other. These regions are, beginning back of the cap: the form- 
ative or embryonic region, in which cell division takes place; the 
region of elongation, in which the cells grow, chiefly in length; the 
region of maturation, in which the various cells take on the char- 
acteristics distinctive of particular tissues; and the mature region, 
in which cells have become definitely differentiated, structurally 
and functionally. In consequence of the continued growth and 
maturation of cells formed in the embryonic region, as well as of 
cells formed by occasional divisions in the region of elongation, the 
length of the root is steadily increase^ and the root tip is pushed 
farther and farther into the soil. 

20. Root Cap. The cells of the root cap, like those in other parts 
of a root, are formed by divisions in the embryonic region. Be- 
ginning at this region and progressing forward through the cap 
there are, successively, a short region of elongation, one of matu- 
ration, and, at the extreme tip and outer sides of the cap, a mature 
region. Since the cells in these three regions go through the same 
course of development, though in an abbreviated form, as do the 
cells in the corresponding regions of the main body of the root, 





/legion of 

their history need not be followed in detail. The continued elonga- 
tion and maturation of cells does not, however, add to the volume 

of the mature portion 
of the cap, since the 
middle layers of the 
walls between the out- 
ermost cells in the ma- 
ture portion are con- 
stantly dissolving 
away and the cells are 
sloughing off as the 
elongating root forces 
the cap through the 
soil. The root cap 
serves to protect the 
embryonic region from 
possible injury as the 
root tip is pushed for- 

21. Embryonic Re- 
gion. The embryonic 
region, unlike other 
regions of the root, is a 
substantially uniform 
tissue composed of 
small, closely packed, 
angular cells (Fig. 14). 
The cells in this region 
are distinguished from 
those of most other 
parts of the root by 
their capacity for in- 
definitely repeated di- 
vision. Because of this 
capacity they are 
known as embryonic 

A section of a root 
FIG. 14. Lengthwise section of a root tip (dia- treated M Previously 

B . ^ ^ ' 

Region of 




Cell Wall 
Chondrio somes l 

that embryonic cells are different in appearance from mature 
cells. Since nuclear and cell division (Chap. XII) are likely to be 
going on in many of the cells in the embryonic region, it is neces- 
sary to select for the present comparison a resting cell that is, 
one whose nucleus and cyto- 
plasm are not in process of 

In the first place, the wall of 
an embryonic cell (Fig. 15) is 
very thin as compared with 
that of a mature cell. Second, 
the embryonic cell contains no 
central vacuole. The space 
within the wall is occupied by 
a nucleus and dense cytoplasm, 
the latter containing many 
minute vacuoles and being sim- 
ilar to the substance of the 
peripheral layer in a mature 
cell. Third, the embryonic 
cell contains no chloroplasts. 
Among the small darkly 
stained bodies (chondriosomes) 
in the dense cytoplasm, how- 
ever, are some which 

Nucleus Cytoplasm 

may FIG. 15. An embryonic cell from a root 

develop into larger structures 

tip of an. onion. 

(leucoplasts) that in certain important respects resemble chloro- 
plasts. Fourth, the nucleus is large in proportion to the size of 
the cell and lies in its central part. The nucleus of an embryonic 
cell, like that of a mature cell, is surrounded by a nuclear mem- 
brane and contains a nucleolus or nucleoli, a chromatic network, 
and nuclear sap. 

The continued division of cells in the embryonic region would 
result, if nothing occurred to prevent, in a steady increase in 
number of embryonic cells and in the size of the embryonic re- 
gion. No such increase occurs because the cells which come 
to lie in the anterior and posterior portions of the embryonic 
region cease to divide and begin to enlarge. These cells thus be- 
come constituents of the two regions of elongation those, re- 
spectively, in the root cap and in the body of the root. The 





volume of the embryonic region, therefore, remains approximately 

22. Region of Elongation. The cells which lie posterior to the 
embryonic region constitute the part of the body of the root that 
has been referred to as the region of elongation (Fig. 14). The 

cells in this region are enlarging, 
chiefly in length. As a cell enlarges, 
the amount of each of its component 
substances doubtless increases; but 
the largest factor in its enlargement 
is an increase in water content. Most 
of the additional water is taken into 
certain of the minute vacuoles that 
are scattered throughout the cyto- 
plasm. The result is a great increase 
in size of these particular vacuoles 
(Fig. 16); the majority of the vac- 
uoles, however, remain small. Now 
and then two or more of the enlarging 
vacuoles come into contact and coa- 
lesce. Gradually, therefore, the num- 
ber of conspicuous vacuoles in the 
cell becomes smaller, and finally, in 
the cells of many tissues, all the grow- 
ing vacuoles unite into a single large 
central vacuole. The coalescence of 
vacuoles results in pushing the rest 
of the cytoplasm to the outer part of 
the cell. This relatively thin layer 
just inside the wall is what was re- 
ferred to in the description of the mature cell ( 10) as the "dense 
cytoplasm." Since the numerous vacuoles that remain minute 
are included in it, the dense cytoplasm still has an emulsion-like 
structure. The nucleus remains imbedded in the dense cyto- 
plasm, and is therefore finally located near the cell wall. 

It might be expected, since the enlargement of a cell is due 
largely to the absorption of water by certain vacuoles, that it 
would expand equally in all dimensions. This is not generally the 
case; for most of the enlarging cells of a root undergo a greater 
increase in length than in either lateral dimension. 

FIG. 16. Stages in the devel- 
opment of an embryonic cell 
to the mature condition. 


All the cells of the embryonic region are substantially alike; 
but as they begin to enlarge differences appear. Certain groups of 
cells which are to constitute the respective tissues in the mature 
part of the root now begin to differ in size and shape. The majority 
of the cells in the axial portion of the region of elongation grow to 
many times their original length. The cells outside this central re- 
gion grow to but three or four times their original length. Thus, 
even in the region of elongation it is evident that the mature re- 
gion of the root will include a differentiated central portion (the 
stele) and a surrounding zone of different nature (the cortex). 

23. Region of Maturation. The differences in size and shape 
first evident in the region of elongation become more apparent 
as the fully enlarged cells take on their mature characteristics 
(Fig. 14). In general, the walls of maturing cells become thicker by 
the ^prmation of new layers of wall material. The original thin 
wall separating two adjoining embryonic cells, somewhat modi- 
fied in thickness and in chemical composition, remains as the 
middle layer of the mature wall. As will appear, the cells of 
various tissues differ materially in the amount of thickening that 
their walls undergo and in the manner of deposition of the addi- 
tional wall layers. 

24. Mature Region. The mature portion of a root (Fig. 14) 
consists of three parts, concentrically arranged: stele, cortex, and 
epidermis. The innermost of these parts, the stele, is a solid cylin- 
der. External to the stele and surrounding it is the cortex, several 
cells in thickness. The cortex, in turn, is ensheathed by the epider- 
mis which is but one cell in thickness. 

The cells of the young stele do not all mature into the same type 
of tissue. A cross section of a sunflower root cut at the posterior 
portion of the region of maturation (Fig. 17) shows that four 
(or more) groups of cells toward the periphery of the stele develop 
especially thick walls, constituting a tissue known as the primary 
xylem. Later the cells inward from these first-matured groups of 
primary xylem also mature into elements of primary xylem which 
are of larger diameter. When maturation is complete, the primary 
xylem often constitutes a mass that fills the entire central portion 
of the stele. Projecting outward from the central mass, as seen in 
cross section, are four or more radiating arms. These are sectional 
views of as many longitudinal ridges the first-matured groups 
of primary xylem. The outer portions of these ridges are composed 



g I Parenchyma 





FIG. 17. Cross section of a sunflower root (diagrammatic) at a level 
at which cambial activity has not yet begun. 


of elongated cells with pointed ends (tracheids, 
Fig. 18, A). During maturation the cell walls 
of young tracheids are thickened by the addi- 
tion of new layers. In some tracheids the new 
layers are deposited upon a limited portion of 
the inner surface of the wall in the form of a 
spiral band that runs the whole length of the 
cell; in other tracheids the internal thickening 
takes the form of a series of rings which en- 
circle the cell transversely. Cytoplasm and 
nuclei disappear from the tracheids after the 
formation of these additional wall layers. Ma- 
ture tracheids, therefore, consist of cell walls 
only, and each is the result of the maturation 
of a single embryonic cell. 

Many of the cells inward from those that 
become tracheids mature into vessels (Fig. 

18, B). A vessel is an element resulting from 
the maturation of a vertical row consisting of 
a variable number of embryonic cells joined 
end to end. After the increase in length which 
these cells undergo in the region of elongation, 
their maturation begins with a great increase 
in the diameter of each cell of the row (Fig. 

19, B). After this enlargement is completed, 
additional layers of wall material are deposited 
on the lateral walls of the cells. In the first 
vessels to mature, the additional wall material 
is laid down in transverse rings or in a con- 
tinuous spiral. The thickened walls of later- 
matured vessels often contain many thin areas 
(pits) in which the material of the later-formed 
layers was not deposited. After the thickening 
of the lateral walls the cross walls between 
adjoining cells of the vertical row disappear, 
except for the outer margin of each cross wall. FlG ' 
Finally the protoplasts disappear (Fig, 19, D). cheid with 
Hence a vessel is a tube formed from a row of thickened 
originally separate cells; it differs in this re- 

spect from a tracheid, which is at all stages a ened wall. 

a tra _ 

wall B, 



single cell. Tracheids and vessels are not functionless when mature, 
even though they lack protoplasts : as will appear in a later chapter, 
water passing through the root to the stem moves chiefly through 
these dead elements. 

In the angles between the radiating ridges of primary xylem are 
groups of cells that mature into primary phloem. The alternate 



A B C D 

FIG. 19. Stages in the development of a vessel from a row of 
originally separate cells. 

distribution of xylem and phloem groups in the peripheral portion 
of the stele is characteristic of young roots in general. The sieve 
tubes (Fig.* 20) are the most conspicuous elements of the phloem. 
Each sieve tube is formed by the maturation of a vertical row of 
embryonic cells. It is called a " sieve tube" because the walls 
between adjacent cells in the row are perforated by groups of 
pores which, though small, are much larger than the minute pores 
referred to in 10. In a lengthwise section a mature sieve tube 
appears as a series of elongated cells arranged end to end, with 
strands of cytoplasm extending through the pores. Manufactured 
foods, and possibly other materials, are moved through sieve tubes. 
Interspersed among the sieve tubes are other thin-walled cells 
whose walls do not develop pores; these cells, which may be long 
or short, contain cytoplasm and nuclei, whereas the nuclei of the 
sieve tubes sooner or later disappear. In the roots of some plants, 
though not of the sunflower, a narrow, elongated companion cell 



lies beside each sieve cell that is, 
beside each segment of a sieve 

Lying between the primary xy- 
lem and the primary phloem are 
many thin-walled cells. Thes^, 
like the slightly elongated, usually 
thin-walled cells found elsewhere 
in roc t and stem, are parenchyma- 
tons cells. A tissue composed of 
such cells is a parenchyma. Sur- 
rounding the phloem and xylem, 
and appearing as a ring in cross 
section, is a sheath of parenchym- 
atous tissue, the pericycle. It is 
in this outermost region of the 
stele that secondary roots origi- 

Immediately outside the peri- 
cycle is a single layer of cells, the 
endodermis, which is the inner- 
most region of the cortex. Early 
in the maturation of endodermal 
cells (in many plants), the middle 
part of each radial wall becomes 
thickened. Further deposition of 
wall material may result in a uni- 
form thickening of the radial walls. 
In some plants, including some 
kinds of sunflower, there follows, 
in turn, a thickening of the tan- 
gential walls at the inner faces of 
the cells. The remaining portion 
of the relatively thick cortex is 
made up for the most part of 
rounded parenchymatous cells. 

The surface layer of cells in 
the embryonic region of a root 
matures into a tissue, one cell in 
thickness, known as the epider- 

FIG. 20. A, lengthwise section of a 
sieve tube segment and its com- 
panion cell, from the secondary 
phloem of a i-oot of the squash. B t 
the end wall of a sieve tube seg- 
ment, showing a group of porea. 



mis. In the posterior part of the region of elongation, many of 
the epidermal cells develop long tubular projections from their 
outer sides. These projections, the root hairs, elongate to many 
times the diameter of the epidermal cells and thus increase very 






I Pericycle 





FIG. 21. Cross section of the stele and endodermis, and of a portion of the 
cortex of a corn root. 

greatly the absorptive surface of the root. It is largely through 
the root hairs that water is absorbed from the soil, together with 
some other materials, chiefly mineral salts, which are dissolved in 
the soil water. This water passes from the root hairs through the 
cortex to the tracheids and vessels, whence it rises into the stem 
and is distributed throughout the plant. The length of the portion 
of a root on which root hairs persist extends back from the region 
of elongation for a variable distance, depending upon the species 
and upon the environment of the root. The number of root hairs 



developed depends largely also upon the medium in which the root 
is growing. After they have ceased to function, either the root 
hairs remain in a shriveled condition or the epidermal layer with 
the root hairs is sloughed off. New hairs are formed only in the 

Primary Phloem Parenchyma Primary Xylem 


Cortical Parenchyma 



FIG. 22. Cross section of the central portion of a root (diagrammatic) at a level 
at which cambial activity is beginning. 

elongating region of the root. Therefore, as the root penetrates 
farther into the soil its absorbing portion (that bearing root hairs) 
comes to be more remote -from the stem. 

26. Secondary Thickening. All the tissues thus far considered 
were formed by the maturation of cells produced by cell divisions 
in the embryonic region or, less frequently, in the region of elon- 
gation. Tissues produced by the maturing of such early-formed 
cells Are termed primary tissues in distinction to secondary tissues 
which originate in a different manner. 



Seed plants are divided into gymnosperms, among which are the 
pine, larch, and spruce; dicotyledons, including many trees, shrubs, 
and the majority of other seed plants; and monocotyledons, of 
which corn, wheat, and the lilies are examples. One respect in 
which monocotyledons differ from gymnosperms and dicotyledons 

Primary Phloem 

Secondary Phloem 


Primary Xylem 

Secondary Xylem 



Cortical Parenchyma 

FIG. 23. Diagram of the central portion of a root after the formation of a con< 
tinuous zone of cambium outside the xylem. 

is that monocotyledons produce no secondary tissues in their 
roots, whereas the roots of gymnosperms and dicotyledons usually 
form secondary tissues. 

The sunflower is a dicotyledon whose roots increase in thickness 
by the formation of secondary tissues. The production of secondary 
tissues in a sunflower root results from the activity of an embryonic 
tissue known as the cambium. First to function as cambium are 
certain cells lying just within each phloem group and outside the 






y Cambium 


central mass of xylem (Fig. 22). In a root with four phloem groups, 
therefore, four cambium regions appear. The cambial cells, al- 
though they have enlarged and changed more or less in structure, 
have nevertheless retained their embryonic character. Each one 
divides tangentially (that is, in a plane at right angles to the 
radius of the root) into two daughter cells. Further tangential 
divisions increase the diameter oi each strip of cambial tissue. As 
tangential division continues, the cells that come to lie toward the 
inner face of each cambial strip begin to enlarge, chiefly in a radial 
direction; then they ma- 
ture gradually into sec- 
ondary xylem. Repeti- 
tion of this history 
results in the continued 
formation of additional 
secondary xylem ele- 
ments between the outer 
face of the xylem mass 
and the inner face of 
the cambium. Occa- 
sional cells on the outer 
face of the cambium 
mature into secondary 
phloem elements, which 
lie, therefore, between 
the cambium and the 
inner face of a primary 
phloem group. 

Shortly after cambial FIG. 24. Small portion of a cross section of a 
activity begins between root of Ficus ( the rubber plant), showing the 
the primary xylem and 
the primary phloem 
groups, some of the pericyclic cells just outside each ridge of 
primary xylem also begin to function as cambium. The cambium 
now forms a continuous sheath which completely surrounds the 
primary xylem (Fig. 23). In consequence of the method of their 
formation, the secondary xylem and the secondary phloem con- 
stitute two concentric cylinders that lie, respectively, within and 
without the cambium (Fig. 24). The cylinders of secondary xy- 
lem and secondary phloem are not continuous, because, at occa<- 


formation of secondary phloem and secondary 
xylem by the cambium. 


sional narrow regions in the cambium, cells are formed which 
remain thin-walled instead of developing into secondary xylem 
and secondary phloem. Such parenchymatous cells constitute 
strips (medullary rays) which run radially through secondary xy- 
lem and secondary phloem. 

26. Secondary Tissues. The secondary xylem is variously 
organized in different plants. The roots of some plants contain a 
large proportion of water-conducting elements. In such roots 
these elements may consist of tracheids only, as in the pine; of 
vessels only, as in the willow; or, as in the oak, of both tracheids 
and vessels. In the roots of other plants the greater portion of 
the secondary xylem is composed of elements other than tracheids 
or vessels. Such elements, not primarily concerned in conduction, 
may be thick-walled (mechanical) elements which contribute to 
the efficiency of the root as an organ that anchors and supports 
the stem; or they may be parenchymatous cells which store reserve 

While they are very young the elements of secondary xylem and 
secondary phloem are arranged in definite radial rows in conse- 
quence of the method of their formation by repeated tangential 
divisions of the cambial cells. In such plants as the pine, whose 
secondary elements do not enlarge greatly during maturation, 
this radial arrangement persists. On the other hand, in many 
roots, such as those of the sunflower, vessels enlarge greatly while 
other elements of the secondary xylem remain small; consequently, 
in the mature secondary xylem the various elements are irregularly 
disposed, although some indication still remains of the original 
radial ^arrangement. 

27. Annual Rings. In the roots of trees and shrubs, which live 
for several or many years, the formation of new secondary xylem 
and phloem continues from year to year. However, the xylem 
elements formed at the beginning of each season of growth com- 
monly differ greatly from those formed later in the season. In the 
willow, for example, the vessels matured early in the spring are 
large and relatively thin-walled. As the season advances the suc- 
cessively formdtt vessels do not develop to so large a size, but 
their walls become thicker. The vessels produced toward the close 
of the growing season are smallest and thickest-walled of all. In 
late summer or early fall the formation of vessels by the cambium 
ceases. When, in the following spring, the cambium again begins 



/ Cambiu 

to form new secondary xylem, the first /^Cambium x 1 X* 
vessels matured are large and thin- 
walled. Consequently, a cross section 
of the root shows a sharp line of 
demarcation between the small-celled, 
thick-walled xylem of the preceding 
summer and the large-celled, thin- 
walled xylem of the spring. This is 
the explanation of the occurrence of 
concentric annual rings (Fig. 25), 
which are so conspicuous in the cut 
ends of roots of many trees and shrubs. 

There are similar, though less 
sharply marked, differences between 
secondary phloem elements formed at 
the beginning, and those formed to- 
ward the end, of the growing season. 
Any annual ring of phloem does not, 
however, long remain intact, since the 
constantly enlarging cylinder of xylem 
and phloem within crushes it against 
the encircling tissues. 

28. Cork Cambium. In the roots 
of many plants which form secondary 
xylem and phloem there are pericyclic 
cells that have retained the power of 
division. The tangential division of 
these cells results in the establishment 
of another cylindrical sheet of cam- 
bium (cork cambium, Fig. 26) which, 
instead of phloem and xylem, produces 
cork cells on its outer face and paren- 
chymatous cells on its inner face. The 
walls of cork cells become impregnated 
with a fat-like substance; conse- 
quently the layer of cork so 

FIG. 25. Diagram of a root in lengthwise section, showing the relations of 
the annual rings of xylem and phloem. X\ xylem (primary and secondary) 
formed during the first year. X*, X*, xylem formed during the second and 
third years. P 1 , phloem formed during the first year. P\ P 3 , phloem formed 
during the second and third years. 

Root Hair 

Root Cap N 




Endodermis Cork Cambium 

deep within the root that is, just inside the endodermfe ^is im- 
permeable to water and to dissolved food substances. As a result, 
the cortical and epidermal cells outside the cork layer die and are 

eventually sloughed off. 
Thus, excepting for 
some plants whose roots 
form a cork cambium in 
their cortex, the mature 
portions of a root are 
derived entirely from 
the stele. 

The cork cambium 
just described ordina- 
rily forms new cork cells 
for a few years only 
and then becomes inac- 
tive. It does not follow, 
however, that no more 
cork is formed in the 
root ; for additional cork 
cambium layers are suc- 
cessively developed in 
parenchymatous cells of 
pericycle and phloem 
inward from the original 



from Cork Cambium 

FIG. 26. A, cork cambium being differentiated 
in the pericycle of a root of the grape; adapted 
from Bonnier and Sablon. /?, cork cambium, 
and secondary tissues formed by its activity, 
from a root of the rubber plant. 

cork cambium, and therefore, no matter how old the root becomes, 
new cork cells are continuously formed. 

29. Secondary Roots. It has already been said that branch or 
secondary roots may arise from a primary root. The development 
of such a secondary root begins with the division of certain peri- 
cyclic cells of the primary root adjacent to the group of tracheids 
in one ridge of primary xylem. As a result of these and of suc- 
cessive similar divisions, a small lens-shaped mass of embryonic 
tissue is formed. The outermost cells of this mass develop into 
a root cap; the innermost cells form the embryonic region of the 
secondary root. The secondary root grows through the cortex of 
the primary root to the surface (Fig. 27) and then pushes through 
the soil precisely as did the parent root. In penetrating the cortex 
of the primary root a young secondary root does not necessarily 
push aside the cortical cells; it may secrete substances that render 


the materials of these cells soluble, the secondary root then grow- 
ing into the cavity formed by their dissolution. Early in the 
development of a secondary root, the cells in the anterior and 
posterior portions of its embryonic region begin to undergo the 
processes of elongation and maturation already described. The 
various tissues matured in the secondary root are continuous with 
corresponding tissues of the parent root. Hence water and dis- 

FIG. 27. Cross section of part of a primary root of the sunflower, showing 
the origin of a secondary root from the pericycle and its growth through 
the cortex of the primary root. 

solved mineral salts can pass through the xylem of the secondary 
root to the xylem of the primary root and thence into the stem. 
Conversely, manufactured foods entering the phloem of the pri- 
mary root may pass into the phloem of secondary roots. In plants 
whose primary roots undergo secondary thickening, each second- 
ary root also may develop a cambium. Secondary phloem and 
secondary xylem, like primary phloem and primary xylem, are 
continuous from secondary root to primary root. 

Since the development of secondary roots begins adjacent to the 
groups of tracheids in the ridges of primary xylem, secondary roots 
are formed in more or less definite rows, the number of rows usu- 
ally corresponding to the number of ridges. For example, in the 
sunflower 3, 4, or 5 rows of secondary roots are formed at some 
distance back from the tip of the primary root, in its recently 
matured portion. Secondary roots usually begin to develop be- 
fore the cambium has become a complete cylinder; if they develop 
after it has become continuous, the secondary roots originate in 



the cambium rather than in the pericycle. Secondary roots so 

formed are not in regular vertical rows. 

A secondary root, in the same manner as a primary root, may 

produce branch roots. The repeated formation of new branch 
roots, together with the growth in length 
of roots already formed, results in a con- 
tinuous extension of the root system 
through the soil. The increase in num- 
ber of roots, each bearing root hairs, 
increases the absorptive surface of the 
root system; the heightened water-absorp- 
tion meets the growing need of the plant 
for water which results from the contin- 
uous enlargement of the aerial portion of 
the plant. 

30. Types of Root Systems. It is a 
familiar fact that the aerial portion of a 
plant of a given species usually assumes 
a characteristic form. The root system of 
a plant of any species likewise commonly 
takes on a typical form. Some plants, 
including the corn and the pine, have 
relatively shallow root systems in pro- 
portion to the height of the stem; other 
plants, as the dandelion and the alfalfa, 
have comparatively deep root systems. 
It is to be remembered, however, that 
environmental conditions, especially the 
nature of the soil and the amount of 
moisture in the soil, profoundly affect 
both the shape and the extent to which 
a root system develops. For example, 
two-months-old alfalfa plants grown in a 
prairie region of the Missouri Valley had 

FIG. 28. The tap root sys- roots extending but a foot and a half be- 

tem of alfalfa. After low the surface of the soil; whereas plants 

from the same lot of seed and of the same 

age grown in more arid grasslands to the west developed roots 

extending to a depth of five feet. 
Structurally, two main types of root systems are recognized: 



tap root (Fig. 28) and fibrous root systems (Fig. 29). In a system of 
the former type, the primary root grows much more rapidly than 
any of the branch roots; the primary root then constitutes a central 
axis from which small branch roots arise. Sometimes, as in the 

FIG. 29. The fibrous root system of corn. After Weaver. 

pine, the primary root (a tap root) may die early. In such a case 
a secondary root takes on the appearance of a tap root. The tap 
roots of many plants attain a diameter approximately equal to 
that of the stem. This diameter is relatively small in plants that 
live but a year or less, but some trees, hickories for example, form 
thick, woody tap roots of large diameter and extending many 
feet downward. In some plants the diameter of the tap root may 
exceed by several times that of the stem. The great diameter of 
such a root is due chiefly to the formation of parenchymatous cells 
by the cambium; the cambium adding many such cells to the sec- 
ondary phloem, as in the parsnip, or to the secondary xylem, as 
in the carrot and the radish. The economic importance of many 


plants with fleshy tap roots results from the storage of large 
amounts of reserve starch in such parenchymatous cells. 

A fibrous root system has no central axis. Instead, many of the 
secondary roots, arid the primary root if it persists, grow to ap- 
proximately the same length and diameter. Such a root system 

may be composed of rel- 
atively slender roots, as 
in the sunflower; or, if 
secondary thickening is 
long continued, various 
members of the root sys- 
tem may attain to a large 
diameter, as in most fa- 
miliar trees. In the 
fibrous root systems of 
some plants, such as the 
dahlia (Fig. 30), some of 

the secondary roots be- 

"'IG. '60. fascicled roots or tne cianna. , , 

come much swollen and 

enlarged for a part of their length and filled with food or water. 
The enlarged diameter of such a root is not usually the result of 
extensive secondary thickening, but is due rather to the enlarge- 
ment of parenchymatous cells in the cortex or pith. Fibrous sys- 
tems with such enlarged secondary roots are often termed fascicled 
root systems. 

31. Adventitious Roots. Secondary roots may develop directly 
from stems and leaves much as they regularly develop from primary 
roots. Roots arising directly on stems and leaves are adventitious 
roots. Most plants with trailing or horizontal underground stems, 
such as the iris, most grasses, many ferns, and various vines, reg- 
ularly produce numerous adventitious roots on their stems. Some 
plants with erect stems, like the corn, also regularly form adventi- 
tious roots. 

Adventitious roots may supplement the work of primary and 
branch roots in absorbing water and food materials from the soil. 
In some plants, such as the corn and certain other cereal grains, 
the primary root and its branches may entirely disappear, all 
intake of water then being through the fibrous system of adventi- 
tious roots. Adventitious roots may function also as supports that 
help to hold erect the stem or its branches. Instances of this sort 


are seen in the corn (Fig. 4), whose "prop roots" grow downward 
from the lower nodes of the stem into the soil; or in the banyan 
tree of India (Fig. 31), whose adventitious roots grow directly 
downward from horizontal branches to the soil. Some climbing 
plants, including the English ivy, develop many short adventitious 

FIG. 31. A banyan tree, with many vertical adventitious roots. After Baillon. 

roots along their internodes that aid the plants in clinging to their 

Many plants that do not regularly form adventitious roots 
do so under special conditions. When the tip of a raspberry stem 
bends over and touches the ground, the terminal portion in con- 
tact with the soil forms adventitious roots. Stems and branches of 
other plants, including the rose, geranium, and Coleus (Fig. 32), 
ordinarily form adventitious roots only when a stem or branch has 
been severed from the root system and placed in damp soil. 

The common practice of propagating plants by cutting a stem 
or branch into segments is based upon the ability of such pieces 
(cuttings) to form adventitious roots when placed under appropri- 
ate conditions. This practice makes it possible to obtain a large 
number of new individuals of some species, such as the potato, 
that rarely form seeds. Propagation by cuttings also has the great 
advantage that it almost always results in the production of 
new plants precisely like the parent. In this way some especially 


desirable strawberry, raspberry, or rose plant which, because of 
its hybrid nature, will not breed true by means of seeds, may be 
indefinitely multiplied. 

32. Environment of Roots : The Soil. The soil is of importance 
to a plant in three main respects: it affords a dense medium in 

FIG. 32. Adventitious roots of Coleus, formed from the 
stem of a cutting placed in the soil. 

which the plant is firmly anchored and supported; it is the source 
of a supply of water; and from it the plant takes in certain sub- 
stances essential to its growth and development. The soil also 
provides a protective environment, during periods unfavorable for 
growth, for roots and other underground structures. 


Although soils are exceedingly variable, most of them con- 
tain rock particles, humus, water, substances dissolved in water, 
air, and microorganisms. The particles which constitute the 
bulk of the soil have been formed chiefly from rocks by various 
disintegrating agencies such as glacial movements, frost, rain and 
running water, gases of the air, find winds. Some simple plants, 
including blue-green algae and lichens, by their gradual penetra- 
tion of rocks aid in the process of disintegration. Such plants 
obtain needed mineral substances from the dissolved portions of 
the rocks upon which they grow. After the death of these plants 
their decaying bodies together with the mineral substances they 
have absorbed are returned to the soil. Further organic sub- 
stances are added by the decay of the bodies of other and larger 
plants which may follow, and by the decay of animal bodies and 
of animal excreta. Thus organic matter accumulates in the soil. 
Through the agency of minute organisms the organic matter is 
constantly undergoing change, in the course of which it is trans- 
formed into simpler compounds. When the organic matter reaches 
a certain stage of decomposition it is called humus. 

Humus, by separating the soil particles, increases the space 
which may be occupied by air; the physical texture of the soil is 
thus made more favorable to the growth of plants. Humus readily 
absorbs and holds water, so increasing the water-holding capacity 
of the soil. Although humus may remain in the soil for some time, 
it is finally completely decomposed into simpler compounds. 
While present it is the seat of an abundant and flourishing growth 
of minute organisms, many of which are of great importance to the 
nutrition of green plants. Certain animals, such as rodents, in- 
sects, and earthworms, also modify the physical condition of the 

Rock particles in the soil vary greatly in size. Soil materials 
may be classified on the basis of the size of their included rock 
particles as coarse gravel, fine gravel, coarse sand, fine sand, silt, 
fine silt, and clay. Clay, the commonest soil material, is found to 
some extent in soils of all classes. The smallest clay particles are 
too fine to be seen with a microscope. Although the small size and 
irregular shape of these particles enable many of them to fit 
together closely, nevertheless a considerable amount of space, 
occupied by air and water, is left between them. Sand, composed 
of larger particles, plays an important part in keeping the soil 



open, but soils with a too high content of sand are infertile. The 
water-holding capacity of sandy soils is low; clays and silts hold 
water tenaciously. Soils containing both sand and clay are 
loams. Silt forms a considerable portion of loams. Loams may 
also contain humus, which is intermingled with the rock par- 
ticles, and which further adds to the water-holding capacity 
of the soil. Soil particles may be held together by the films of 
water about them, or by substances such as clay which act as 

33. Functions of Roots. The functions of roots that have been 
Already noted are: absorption of water and dissolved substances 

from the soil, anchorage 
arid support of the stem, 
and storage of reserve 
foods. The roots of plants 
growing in unusual situa- 
tions may function in still 
other ways. For example, 
the roots of many sub- 
merged water plants, as 
well as those of many 
swamp plants, are provided 
with large intercellular 
spaces which store the gases 
(chiefly carbon dioxide and 
oxygen) that are taken in 
by the plant or are given 
off by its cells. The roots 
of certain tropical plants that grow on rocks in swiftly running 
streams are flattened and blade-like and their cortical cells con- 
tain numerous chloroplasts. These roots function like leaves in 
the manufacture of foods. 

Other plants, including some tropical orchids (Fig. 33), grow- 
ing high above the soil on stems and branches of other plants, 
bear roots many of which are exposed to the air. Each such 
aerial root usually has a spongy outer cell layer which holds 
and absorbs rain that falls or dew that condenses on its surface. 
Although the aerial roots may never penetrate the soil, they 
perform much the same absorptive functions as roots growing in 
the soil. 

FIG. 33. Aerial roots of an orchid. 




A root is divided into the following general regions: root cap, 
embryonic region, region of elongation, region of maturation, and 
mature region. The mature region consists of stele, cortex, and 
epidermis. The stele is composed of xylem (including tracheids 
and vessels), phloem (including sieve tubes), pericycle, and paren- 
chyma. The cortex includes endodermis, parenchyma, and some- 
times mechanical tissue. The epidermis is a single superficial layer 
of c lis, many of which develop tubular projections (root hairs). 

Ail the tissues thus far mentioned are primary since they result 
from the maturation of cells of the embryonic region. Some roots 
develop primary tissues only. Others develop secondary tissues as 
a result of carnbial activity. The functioning of the cambium re- 
sults in a formation of secondary phloem and secondary xylem, 
both interrupted by medullary rays. The secondary xylem pro- 
duced in a single year's growth constitutes an annual ring. Cork 
cells and parenchyma formed by cork cambium are also secondary 

Secondary roots developed from a primary root always have a 
deep-seated origin and arise in the pericycle. As they grow through 
the cortex they become differentiated into the parts characteristic 
of a primary root. Secondary roots developing directly from stems 
or leaves are adventitious roots. 

The root system of a plant may be derived wholly from the 
primary root, or in part from the primary root and in part adventi- 
tiously, or wholly adventitiously. According to its external ap- 
pearance it is a tap root system or a fibrous root system. Fibrous 
systems with enlarged secondary roots are fascicled root systems. 

The functions of roots ordinarily include absorption of water 
and dissolved substances, anchorage and support of the stem, and 
storage of manufactured foods. Various special types of roots 
perform other functions. 



34. Regions of a Stem. As in the case of a root, the growing end 
of a stem includes an embryonic region, a region of elongation, a 
region of maturation, and a mature region. There are, however, 
certain differences. First, a stem has no structure corresponding 
to a root cap. Second, the transition from embryonic to mature 
region is not continuous in a stem as it is in a root, but is interrupted 

FIG. 34. Lengthwise section of the apex of the stem of a honeysuckle. 

at intervals by the nodes, in which the processes of maturation lag. 
This lagging is especially noticeable in certain of the grasses, por- 
tions of whose nodes remain embryonic even after the internodal 
tissues are mature. 

In the embryonic region at the apex of a stem (Fig. 34) and of 
each branch many of the cells are dividing. In this region the cells 
are all essentially alike. Along the sides of the embryonic region are 
small dome-shaped or spoon-shaped superficial outgrowths, each 





Region of 




composed of embryonic cells. These outgrowths (leaf primordia) 
are the beginnings of young leaves. The level at which one or more 
, leaves are being formed is a node; the embryonic region of the stem 
is therefore beginning to be differentiated into nodes and inter- 
nodes, but the inter- 
nodes are as yet so Immature Leaf 
short that successive 
nodes appear to adjoin 
one an other (Fig. 35). 
As already noted, the 
enlargement and mat- 
uration of cells that 
follow are generally 
slower in the nodal 
than in the internodal 
portions of the stem. 

In an actively grow- 
ing stem elongation 
usually begins in the 
third or fourth inter- 
node from the stem's 
apex. The cells of such 
an internode are grow- 
ing chiefly, though not 
exclusively, in length. 
Here, certain groups of 
cells which are to con- 
stitute the respective 
tissues in the mature 
part of the stem are 
becoming differenti- 
ated in size and shape. 
Among the earliest 
cells to become differ- 
entiated in an elon- 
gating internode are 
those immediately beneath the epidermis. These cells enlarge 
somewhat and then deposit additional layers of wall material at 
their angles and on their outer and inner tangential walls. Such 
localized thickening of the walls results in the formation of a 

FlG ' 35 ' 

of the a P ical Portion of a 

honeysuckle stem. 


mechanical tissue of considerable rigidity, whose chief function is 
to help hold the stem erect until other tissues are mature. After 
the stem is mature this mechanical tissue, though persistent, is of 
faainor importance in the support of the stem. 

In an elongating internode, at some distance inward from the 
surface of the stem, strands of cells are differentiated that extend 
vertically, and parallel with one another, through the internode. 
These strands, composed chiefly of elongated cells, will eventually 
mature into vascular bundles. They may therefore be spoken of 
as provascular strands. In gymnosperms and in most dicotyledons 
the provascular strands, as seen in cross section, constitute an 
interrupted ring. In most monocotyledons the provascular strands 
are numerous and scattered, although they also run vertically 
through each internode. 

35. Primary Tissues of Dicotyledonous Internodes. Since the 
difference just noted is correlated with certain other differences 
between gymnosperms and dicotyledons on the one hand and 
monocotyledons on the other, it will be necessary to consider 
these two types of stems, separately. The sunflower may be 
taken as typifying the usual arrangement of tissues in a dicoty- 
ledonous stem. The cells of the central part of the sunflower 
stem mature into large parenchymatous cells, the vertical length 
of each of which is about double its thickness. These parenchym- 
atous cells constitute the pith (Fig. 36). 

Just outside the pith is the region in which lie provascular 
strands. Beginning at the inner face of each provascular strand 
and progressing outward, the cells mature into primary xylem. 
Simultaneously, the cells constituting the outer portion of each 
strand mature, progressively from the outer face inward, into 
primary phloem. The portions of the strand maturing into xylem 
and phloem eventually approximate but never meet, because a 
narrow strip in the center of the strand remains embryonic and 
may later function as a cambium. A mature vascular bundle, 
therefore, consists of three main parts: the xylem on the inner 
side toward the pith, the phloem on the outer side, and the cam- 
bium between xylem and phloem. This side-by-side arrangement 
of phloem and xylem in vascular bundles is characteristic of the 
structure of stems, in contrast to the alternate arrangement 
of separate xylem and phloem bundles which characterizes the 
primary tissues of roots. 



The most conspicuous elements in the primary xylem are ves- 
sels, comparable in every way with the vessels of roots. Primary 


Mechanical Tissue \ 
I > Cortex 

/Parenchyma * 

'Medullary Ray 

^Mechanical Tissue 


i / Phloem 

Vascular Bundle ? I Cambium 
' Xylem 

FIG. 36. Portion of a cross section of a sunflower stem. 

phloem in stems, like that in roots, contains sieve tubes which are 
usually accompanied by companion cells. 

The vascular bundles are separated from one another by radial 
strands of parenchymatous cells the medullary rays. Surrounding 
the cylinder of bundles is a rather poorly delimited pericycle. Just 



Mechanical Tissue n\ 
/Parenchyma \ / , 

outside the phloem portion of each vascular bundle the pericycle 
consists of long needle-shaped cells whose walls, during maturation, 
are toughened and stiffened by substances with which they be- 
come impregnated. Since this change in the pericyclic cells does 
not take place until the internode is well along toward maturity, 
the mechanical tissue thus formed helps to hold erect only the 
mature portions of the stem. The pericyclic cells adjacent to the 
medullary rays are parenchymatous and scarcely distinguishable 

from ray and cortical 
cells. The pericycle 
constitutes the outer- 
most portion of the stele. 
A relatively thin cor- 
tex surrounds the stele. 
In the sunflower the 
inner portion of the cor- 
tex consists of large, 
loosely arranged paren- 
chymatous cells. An 
innermost endodermal 
layer is not differenti- 
ated. Stems and 
branches of some dicot- 
yledons, especially cer- 
tain horizontal under- 




FIG. 37. Portion of a lengthwise section of a sun- 
- flower stem. 

ground branches like 
those of the potato, 
have an endodermis 
similar in structure to 
that of a root. As already mentioned, cells in the outer portion 
of the cortex with greatly thickened tangential walls constitute a 
mechanical tissue. These cells also contain many chloroplasts and 
manufacture a limited amount of food. All the cells of the cortex 
are elongated in the direction of the length of the stem (Fig. 37). 
The cortex is bounded on its outer face by an epidermis. The 
epidermis is a single layer of slightly flattened, vertically elongated 
cells. Their outer walls are thickened and impregnated with a 
waxy substance (cutin), which makes the walls almost imper- 
meable to water. Comparatively little water is lost, therefore, 
from the surface of a sunflower stem. Here and there an embryonic 


epidermal cell matures into a long, pointed hair. Certain other 
epidermal cells develop into hairs, each composed of a row of 
several cells, the terminal cell being pointed. Stomata (which 
can be more satisfactorily studied in the leaf) also occur sparingly 
on the stem. 

36. Structure of a Dicotyledonous Node. A very young node of 
a sunflower stem is a homogeneous mass of embryonic cells. 
One of the first changes as the node develops is the appearance of 
numerous orovascular strands whose ends are continuous with 
the provascular strands of adjacent internodes. Although these 
strands are parallel, as has been seen, throughout each internode, 
they are laterally united with one another at various points in 
each node. In the mature stem, therefore, most of the vascular 
bundles of any internode are continuous with those of the inter- 
nodes above and below; but in the nodes there are cross connections 
between the bundles. 

In the cortical portion of each node of the sunflower stem 
three provascular strands are differentiated which run diagonally 
or almost horizontally through the cortex. Each such strand 
connects at its inner end with a provascular strand of the inter- 
node below and at its outer end with the base of an immature 
leaf. It follows that in the mature stem three of the bundles in 
each internode, instead of running vertically through the node 
above, bend outward in that node and run outward through the 
cortex to the base of a leaf. The portion of each such bundle thai 
is developed in the cortex is a leaf trace. The leaf traces provide 
paths of transfer to the leaves for water and food materials thai 
are moving up the stem and, conversely, paths of transfer to the 
food-conducting tissues of the stem for foods manufactured in 
the leaves. 

Although seed plants in general have cortical leaf traces con- 
necting their leaves with the vascular cylinder, the number of 
traces passing to each leaf varies in different species. For example, 
in tomatoes and carnations there is but one, in peppers there 
are five or seven, and in the buckwheat still more numerous 
traces connecting with each leaf. 

37. Secondary Thickening in Dicotyledons and Gymnosperms. 
After primary xylem and primary phloem have been formed in 
the stem of a dicotyledon or of a gymnosperm, the cambial cells 
in the central part of each bundle begin to divide tangentf-Nv 


As in a root, the new cells formed on the inner face of the cambium 
mature into secondary xylem elements and those formed toward 
the outer face mature into secondary phloem elements. The 
process of division extends to those layers of medullary ray cells 
which connect the cambial regions of adjacent bundles; the cam- 
bium thus in time becomes a continuous zone or cylinder (Fig. 36). 
The secondary xylem matured from the cambium now also con- 
stitutes a cylinder, continuous except where it is interrupted by 
medullary rays. Similarly, a cylinder of secondary phloem, con- 
tinuous but for medullary rays, is formed outside the cambium. 
A few cells formed from the cambium remain thin-walled and are 
added to the medullary rays, so that these rays remain continuous 
parenchymatous layers connecting cortex and pith. 

In the stem of a sunflower the cambium ordinarily produces 
secondary xylem and phloem for one season only, after which the 
stem dies. Many trees and shrubs have stems whose cambium, 
like that of their roots, continues the formation of xylem and 
phloem elements from year to year. 

38. Structure of Secondary Phloem (Fig. 38). Secondary phloem 
may be simple or relatively complex in structure. In most gymno- 
sperms the secondary phloem consists chiefly of sieve tubes on 
whose side and end walls are groups of pores; intermingled with 
the sieve tubes are a few parenchymatous cells. In the sieve tubes 
of a dicotyledon the groups of pores may occur in both end and 
side walls or in the end walls only. In the secondary, as in the 
primary, phloem of dicotyledons, a companion cell lies beside 
each segment of a sieve tube. The secondary phloem of dicoty- 
ledonous stems also includes parenchymatous cells, which are 
often densely filled with reserve foods. Vertically elongated 
cells (bast fibers), resembling, with their pointed ends and thick 
walls, the wood fibers of secondary xylem, occur in the secondary 
phloem of most woody dicotyledons and of some gymnosperms. 
In some woody stems, such as those of basswood and hickory, 
masses of bast fibers and of thin-walled elements (sieve tubes, 
companion cells, and phloem parenchyma) are formed alter- 
nately by the cambium. 

Toward the end of the growing season, further functioning of 
the sieve tubes is usually prevented by the development of pads 
of additional wall material over each group of pores. During the 
next growing season the non-functioning sieve tubes with their 



companion cells become crushed between the rigid "hard bark" 
outside and the expanding cylinder of newly formed secondary 
xylem and phloem within. Such parenchymatous cells and bast 
fibers as are included in the secondary phloem are more resistant 




Medullary Ray 

Sieve Tube 



Medullary Ray 

Companion Cell 

FIG. 38. Portion of the secondary phloem of a basswood stem. 

to this crushing and usually persist in a more or less unmutilated 
state for several years. 

39. Structure of Secondary Xylem (Fig. 39). The secondary 
xylem in the stems of gymnosperms, like that in their roots, con- 
sists chiefly of tracheids. Secondary xylem in stems of woody di- 




Medullary Ray 


Medullary Ray 



FIG. 39. Portion of the secondary xylem of a willow. The only parenchyma- 
tous cells here present constitute the last-formed layer of the summer 
wood. This layer of parenchyma, therefore, marks the boundary between 
the summer wood of one year and the spring wood formed the next year. 


cotyledons, with a very few exceptions, contains vessels and may 
contain tracheids in addition. However, in stems these water- 
conducting elements constitute a smaller proportion of the second- 
ary xylem than in roots. The remaining portion of the secondary 
xylem in stems consists largely of elongated, empty, pointed cells 
whose walls are strengthened and stiffened by substances with 
which they are infiltrated. Such wuod fibers contribute to the 
rigidity of the stem. The secondary xylem of woody dicotyledons 
also contains parenchymatous cells. In some stems, such as those 
of the walnut and the hickory, parenchymatous cells are numerous 
in the secondary xylem; in other sterns, as those of the willow and 
the cottonwood, only a few parenchymatous cells occur in the 
secondary xylem. 

40. Medullary Rays. Each medullary ray in a sunflower stem 
is thick, that is, it consists of many layers of cells; it extends 
vertically from one internode to the next internode above. In 
most woody stems that increase in diameter for many years, such 
as those of the pine, willow, and apple, the rays are from one to 
five cells in thickness and from three to thirty cells in vertical 
height. In some woody dicotyledons, such as the oak and syca- 
more, certain rays arc many cells in thickness and very many 
cells in height, whereas others are shorter vertically and but one 
to three cell layers in thickness. In all these woody stems, how- 
ever, differently from the condition in the sunflower, the height 
of any ray is but a fraction of the length of the internode. 

The secondary thickening of a woody stem is accompanied by 
the addition, through carnbial activity, of new cells to the por- 
tions of the original rays imbedded in xylem and phloem. As the 
formation of secondary tissues continues, new rays originate from 
year to year at various points in the cambium. Such rays run 
radially, not through all the annual rings but only through some 
of the younger rings. Thus the number of rays increases from the 
inner rings outward, approximately in proportion to the increas- 
ing circumference of successively formed rings. The rays of a 
woody plant serve as a pathway for the lateral transfer of water, 
foods, and food materials between the inner and outer parts of 
the stem. 

41. Annual Rings. The size differences between the secondary 
xylem elements produced at the beginning of each growing season 
(spring wood) and those produced later in the season (summer 



wood) are even more noticeable 
in stems than in roots (Fig. 40). 
Such differences are especially 
characteristic of vessels, both in 
those species in which there is 
a gradual diminution in size of 
vessels from spring to summer 
wood (Fig. 41, A) and in those in 
which the transition is rather 
abrupt (Fig. 41, B). As a result 
of these differences in size of 
vessels, the annual rings in steins, 
especially of woody dicotyledons, 
are more sharply defined than 
are the annual rings in roots. 
Annual rings produced in suc- 
cessive years often vary greatly 
in thickness in consequence of 
variations in environmental con- 
ditions, especially in the supply 
of water to the plant. A per- 
manent change in the environ- 
ment, such as the felling of 
surrounding trees or the draining 
of a swamp in which a tree is 
growing, may result in marked 
differences in thickness between 
the annual rings produced before 
and those produced after the 
change (Fig. 42). 

The number of rings in the 
xylem is not a perfectly accurate 
measure of the age of a tree, 
because sometimes, as a result of 
exceptional weather or other con- 
ditions, two rings are formed in 

FIG. 40. Portion of a cross section through the secondary xylem of a trunk 
of the shortleaf pine. Two annual rings and parts of 2 others are shown. 
In this, as in other gymnosperm stems, no vessels are present. Photograph 
by the Forest Products Laboratory, Madison, Wis. 

A B 

FIG. 41. A, secondary xylem of the quaking aspen. B, secondary xylem of 
the black ash. Photographs by the Forest Products Laboratory, Madison, 





a single year. In such a case the outer face of the inner or "false** 
ring is not sharply defined, but shows a gradual transition to the 

xylem formed later 
in the same season. 
Annual rings are 
sometimes much 
thicker in certain 
portions than in oth- 
ers. Differences of 
this nature may be 
brought about by a 
variety of condi- 
tions; they may re- 
sult, for example, 
from a greater de- 
velopment either of 
roots or of branches 
on one side, as in a 
tree standing next 
to a clearing in a 

42. Sapwood and 
Heartwood. As the 
number of annual 
rings in the wood in- 
creases, there comes 
a time when, in 
many stems, the 
vessels and tracheids 

FIG. 42. A, cross section of a trunk of lodge- of the inner rin S 
pole pine, showing the greater width of an- become filled with 
nual rings produced after a thinning of the 
stand. B, cross section of a portion of a trunk 
of black spruce, showing the greater width of 
annual rings produced after draining the 
ewamp in which the tree grew. Photographs 
by the Forest Products Laboratory, Madison, 

gums and resins or 
blocked by bladder- 
like growths (Fig. 43) 
from neighboring 

cells of the xylem or 
of medullary rays. Such blocking of the water-conducting elements 
is usually followed by the death of all living cells in the annual ring, 
and frequently by an impregnation of the cell walls with dark- 


colored substances. The xylem of the rings so modified becomes dry 
and is then known as heartwood. The sap (water and dissolved 
substances from the roots) must now travel upward in the outer, 
younger rings which constitute the sapwood. From year to year 
more rings in the inner part of the sapwood are changed to heart- 
wood. Hence the sapwood of any particular tree remains of about 
the same thickness from year to year, whereas the heartwood is 

FIG. 43. Portion of a cross section through the heartwood of an oak. The 
vessels are filled by growths from neighboring cells. Photograph by the 
Forest Products Laboratory, Madison, Wis. 

continually increasing in diameter. The sapwood may include 
only a few annual rings, as in the black locust or the black cherry, 
or, as in the hickory and the maple, it may be many rings in thick- 
ness. Xylem that has been changed to heartwood is often of 
greater mechanical strength than before the change. The develop- 
ment of heartwood in a tree may, therefore, increase the rigidity 
of the stem independently of any increase in the number of cells. 
It often happens that in very old trees the heartwood has decayed 
and disappeared, leaving the now hollow trunk composed almost 
wholly of sapwood. 

43. Bark. The term bark is often used to designate all the tissues 
of woody stems from the phloem outward. The plane of separa- 
tion, therefore, between wood and bark is the cambium. In a very 



young branch or stem the bark is composed of phloem, pericycle, 
cortex, and epidermis. The stems of most trees and shrubs and of 
some herbaceous plants develop a cork cambium during their first 
growing season (Fig. 44). This cork cambium usually develops in 
the outermost portion of the cortex; but it may develop from the 
epidermis, as in the apple, or within the pericycle, as in the cur- 

Mature Cork Cell Cork Cambium 

\ Immature Cork Cell \ Epidermis 

\^ - 1 - L _ 

Parenchyma from 
Cor* Cambium 


FIG. 44. Portions of cross sections of the outer region of a geranium (Pelar- 
gonium) stem. A, a young stem. B y the cork cambium beginning to 
function. C, the cork cambium has produced several layers of cork cells. 

rant. The cork cambium of a stem, like that of a root, produces 
parenchymatous ceils on its inner face and cork cells on its outer 
face. As in a root, the walls of the cork cells become impregnated 
with a fat-like substance which renders them impermeable to 
water; thus-* water is prevented from passing outward to the epi- 
dermal and cortical cells lying outside the cork layer; these outer 
cells die and their walls become dry and hard. Such outer dry 
tissues, together with the cork, are sometimes called "hard bark" 
in contrast to portions of the bark within the cork cambium, which 
are the "soft bark." 

The cork at the inner face of the hard bark is usually but a few 
cells in thickness. In some plants, including the cork oak, which 
supplies the cork of commerce, many more layers of cork cells are 
formed, and the cork may attain a thickness of a half inch or 
more. In most perennial stems the original cork cambium becomes 
inactive after a few years; although this layer in some trees, such 
as the beech, cork oak. and some cherries, continues to form new 



cork cells for many years. In woody plants whose first-formed 
cork cambium becomes inactive after a few years, additional cork 
cambiums are developed in parenchymatous tissues inward from 
the original cork cambium (Fig. 45). These newer cork cambiums 
appear successively in the inner portion of the cortex, in the peri- 
cycle, and in the outer layers of the phloem. Each successively 

Pith Xyletn ^Cork Layers 

-Hard Bark 
Soft Bark 

FIG. 45. Diagrams showing the development of cork layers and the re- 
sultant production of hard bark. A, before the differentiation of a cork 
cambium. B, the first cork cambium has produced a zone of cork (shown 
in black). C, later cork cambiums have developed and have produced 
strips of cork inside the original cork layer. I), an old stem with many 
cork strips, showing the cracking of the hard bark. 

formed cork layer shuts off the supply of water and foods to such 
cells as lie outside it, the starved elements being added to the 
hard bark. 

The cell layers constituting the hard bark often become cracked 
in consequence of the pressure of the expanding cylinder of xylem 
and phloem from within, and portions of the hard bark may be- 
come separated from the tissues within and fall away. The size 
and form of these separating portions of hard bark are influenced 
by the relative positions of successive cork cambiums, which are 
usually not continuous concentric zones but relatively short inter- 
secting arcs. The manner and pattern of cracking of the hard bark 
are often distinctive of the species. Examples of characteristic 



cracking are seen in the shaggy bark of hickories, the rough, deeply 
furrowed bark of oaks, and the shredded bark of grapes. Smooth 


Cork Cells 

FIG. 46. Cross section of a lenticel of the elder. 

bark, like that of a birch, is usually produced by the continued 
activity of the original cork cambium. 

44. Lenticels. In the young portions of many stems certain 
limited regions of the cork cambium, usually beneath stomata, 
become particularly active. Repeated tangential divisions in each 
such region produce a mass of cells, 
the pressure caused by whose en- 
largement ruptures the epidermis; 
the cellular mass then protrudes as 
a small rounded or elongated swell- 
ing called a lenticel (Fig. 46). The 
characteristic horizontal markings 
of birch (Fig: 47) and cherry sterns, 
as well as the rounded markings on 
young twigs of the horse-chestnut, are lenticels. Between the cells 
of a lenticel are intercellular spaces continuous with those of the 
cortex, which make possible a free interchange of gases between the 
outer atmosphere and the interior of the stem. 

46. Tissues of a Monocotyledonous Stem (Fig. 48). The stem 
of the corn may be taken as a typical monocotyledonous stem. 
While the young internodes are elongating, many scattered, paral- 
lel provascular strands appear in each. Later these strands mature 
into vascular bundles. The peripheral cells of each strand mature 
into long, thick-walled cells which constitute a mechanical tissue 
completely enclosing the bundle (Fig. 49). This sheath of mechani- 

FIG. 47. Portion of a birch stem, 
with many horizontal lenticels. 


cal tissue is thickest on the inner and outer sides of the bundle. 
The remaining portion of the strand matures into primary xylem 
and primary phloem. As in the sunflower, there is a progressive 
maturation of phloem beginning at the outer face, and a progres- 
sive maturation of xylem beginning at the inner face, of each 
strand. Differently from the condition in the sunflower, matura- 





FIG. 48. Cross section of a portion of a corn stem. 

tion continues until the whole strand (except the sheath of mechan- 
ical tissue) L converted into xylem and phloem. There is, there- 
fore, no embryonic region (cambium) separating xylem and 
phloem, and no possibility of the formation of secondary elements 
between primary xylem and primary phloem. The phloem con- 
sists of regularly arranged sieve tubes and companion cells. The 
xylem of each mature bundle includes two large vessels with pitted 
walls adjacent to the phloem, and between these vessels a few 



tracheids. The innermost portion of the xylem contains one or 
two vessels whose walls have spiral or ring-shaped thickenings. 
Just outside the latter vessel or vessels is a large intercellular space 




FIG. 49. A vascular bundle from a corn ste: 

that separates this region of the xylem from the she; 
ical tissue. 

The greater part of the stem consists of large 
cells. Scattered throughout this tissue are vasculi 
parenchymatous cells in the central part of the stei 
pared with the pith, and those in the peripheral 
cortex, of the sunflower. It is impossible, however, 
sharply in the corn stem between pith and corte: 
parenchymatous tissue is a narrow cylinder of m< 



[es. The 
be com- 

ith the 
ide the 




most of whose cells are small and thick- walled. Outside this is an. 
epidermis of relatively small, thick-walled cells. 

Since the corn lacks a cambium, its stem can not increase in 
diameter after all the primary tissues are mature. The absence of 
a cambium is the chief feature distinguishing most monocotyle- 
donous stems from stems of gymnosperms and dicotyledons. Other 
respects in which monocotyledons differ from the majority of di- 

i^*^ffiif'fe^i ftf ' r r T '" F '! ^i ^ 

FIG. 50. The Joshua tree (Yucca arborescens), a mono- 
cotyledonous tree of the Mojave Desert. Photograph 
by Forrest Shreve. 

cotyledons and from gymnosperms are the scattered arrangement 
of the vascular bundles, the presence of a sheath of mechanical 
tissue partially or completely surrounding each vascular bundle, 
the lack of a well-defined cortex, and the lack of medullary rays. 
46. Growth in Thickness of Monocotyledonous Stems. The 
sterns of most monocotyledons, since they contain no cambium, are 
incapable of growth in thickness after the cells in any particular 



region have attained their full size. Hence certain monocotyledons, 
such as some bamboos, although they grow to a considerable 
height, remain slender. The trunks of some other monocotyledons, 
like the date and the coconut palm, taper gradually from base to 
apex. In such a case there is a progressive enlargement and matura- 
tion of cells from the apex of the stem to the base. The greater 
diameter of the basal portion of the trunk is due, therefore, not 
to the formation of new cells but to the delayed enlargement of 

the cells which were formed when 
that region was embryonic. 

The stems of a few monocotyle- 
dons, such as Yucca (among 
whose species are the Joshua tree 
of the Southwest, Fig. 50), Aloe, 
and Dracaena (the dragon tree), 
undergo a true secondary thicken- 
ing, although their vascular bun- 
dles are without cambium. In 
Dracaena, a cylinder of embryonic 
cells in the pericycle functions as 
a cambium, by whose means the 
stem grows slowly in thickness 
from year to year. Groups of new 
cells formed on the inner side of 
this cambium develop into vascu- 
lar bundles; the new cells formed 
on the outer side of the cambium 
remain parenchymatous. A fa- 
mous dragon tree of the Island of 
TenerifTe attained a height of ap- 
proximately 70 feet and a circum- 
ference of 45 feet. 
FIG. 51. Growth of wound tissue 47. Wound Tissue. An im- 
about the base of an amputated rt t distinction between em- 
apple branch, f . 

bryomc and mature cells consists 

in the power of division of embryonic cells. This distinction 
is, however, a relative one, since under unusual stimuli many 
apparently mature cells divide. One stimulus which so affects 
mature cells is that supplied by a wound. When a branch is 
cut from a stem, cell divisions begin in such parenchymatous 



tissues as lie immediately adjacent to the cut surface, espe- 
cially in those of the cambium and the soft bark. The result is 
the formation of a wound tissue- or callus (Fig. 51). The walls 
of the outer cells of the callus become impregnated with waxy 
compounds and function in the same manner as walls of cork 
cells. Certain deeper-lying callus cells become a cambium which 
is continuous with the cambium in the stem. The cambium de- 
veloped within the callus forms secondary xylem and secondary 
phloem in the usual manner, and the resultant tissues, together 
with such callus cells as lie external to them, gradually grow over 

A B C D 

FIG. 52. Methods of grafting. A, /?, between stock and scion of similar size. 
C, D, E, between a large stock and small scions. F, G, H t bud-grafting. 

the xylem exposed by the wound. In time the callus may extend 
completely across the wounded surface, which, as additional ele- 
ments are added, is moire and more deeply buried. The healing or 
covering of exposed surfaces by a callus largely prevents the en- 
trance into the wood of decay- and disease-producing organisms. 
The formation of wound tissue on cut surfaces makes graft- 
ing possible. Grafting is commonly used in the propagation of such 
woody plants as do not readily form adventitious roots on cuttings. 
By this means it is possible to obtain new individuals of species 
which produce no seeds, such as seedless grapes or navel oranges, 
or to obtain duplicates of some desirable plant which, because of 
its hybrid nature, will not breed true by means of seeds. The two 
members of a graft are the stock, a root or the base of a stem with 
the attached root system, and the scion, a branch or bud which is. 



to grow into the fruit- or flower-bearing portion of the grafted 

Grafting (Fig. 52) may be between a stock and a scion of the 
same size, between a large stock arid a small branch, or between a 
large stock and a bud. In all cases the success of the graft depends 
upon the close proximity of the cambiums of the two members, 
and upon the prevention of a drying of the united faces during the 
establishment of the graft. Grafts are usually established only 
when the two members are from plants of the same or of closely 
related species. 

48. Structure of Wood. The great diversity among trees of 
different species in type, number, and distribution of component 

FIG. 53. Methods of sawing a log. A, plain sawing. B, quarter sawing. 

elements, the "variations in width of annual rings and in relative 
amounts of spring and summer wood, and differences in size of 
medullary rays, result in marked differences in strength, workabil- 
ity, texture, and surface pattern of their woods when used for 
building or other purposes. The presence of numerous thick-walled 
fibers in the xylem of the oak or of broad layers of thick-walled 
summer tracheids in the annual rings of southern yellow pine re- 
sults in a timber that is most suitable for use where strength is 
especially required. Sharp differences between spring and summer 
wood, or the presence of masses of fibers, may, however, result in a 
timber that is hard to work and therefore less desirable than one 
which, like that of the white pine, has a uniform texture. Even 
when the secondary xylern of two species is composed of similar 


elements and includes similar proportions of spring and summer 
wood, differences in the length of elements and in the nature of the 
cell walls may make the two sorts of timber markedly different in 
character. It is for such reasons that spruce is much tougher and 
lighter than most pines. 

Woods differ markedly in color, in figure or grain, and in surface 
texture. Color is affected chiefly by changes incident to the trans- 
formation of sap wood into heartwood; the heartwood may be 
black as in ebony, brown as in walnut, or red as in mahogany. The 


FIG. 54. A, plain-sawn bird's-eye maple. B, ribbon effect of quarter sawing 
"Philippine mahogany." C, quarter-sawn oak. Photographs by the 
Forest Products Laboratory, Madison, Wis. 

patterns produced on an exposed surface by medullary rays, fibers, 
and other elements, as well as by the alternations of summer and 
spring wood, make lumber cut from certain species especially 
valuable for furniture and for the interior finish of buildings. The 
pattern depends largely upon the manner in which the lumber is 
sawn from the log. There are two general methods of sawing logs 
(Fig. 53): " plain sawing," the lumber being cut at right angles to 
the medullary rays; and "quarter sawing," the cutting being 
parallel to the rays. Plain sawing is especially desirable in logs 
with contrasting cylinders of spring and summer wood, such as 


those of southern yellow pine, cypress, and redwood, or with eleva- 
tions or depressions in the cylinders of summer wood. Plain sawing 
of this latter type of log, as in curly and bird's-eye maple (Fig. 54, 
A), produces a pattern of circular and wavy summer-wood lines 
against a background of spring wood. Quarter sawing, on the other 
hand, may give a surface with a more striking pattern than does 
plain sawing (Fig. 54, B, C). In oaks and sycamores this pattern 
results largely from contrasts in color between medullary rays and 
other elements. Quarter sawing of maples, birches, and mahoganies 
gives a wavy surface view of the fiber mass, or, as in some mahog- 
any logs, a ribbon-like surface view of alternately inclined fiber 
masses in adjacent rings. 

49. Branches. A branch begins as a small rounded hump of 
embryonic tissue in the axil (the angle between leaf and stem) 
of a young leaf just back of the embryonic region of the stem 
(Fig. 35). Although there is usually such a branch primordium in 
the axil of each leaf, only a few of the primordia ultimately de- 
velop into branches. 

If the plant is one whose normal span of life is a single year, the 
primordia destined to become branches begin to elongate soon 
after they appear. Elongation is soon followed by an organization 
in each elongating primordium of regions corresponding to those 
present in the stem namely, an embryonic region, a region of 
elongation, a region of maturation, and a mature region. The 
tissues matured in the branch are similar to those of the stem, and 
each tissue of a branch is continuous with the corresponding tissue 
of the stem. Water and food materials may therefore pass from the 
xylem of the stem to that of the branch, and foods manufactured 
in the branch may pass through its phloem to that of the stem. 
The secondary tissues of the branch are similarly continuous with 
those of the stem. 

Branch primordia of most trees and shrubs do not develop into 
branches during the year that they appear, but, after developing 
to a certain stage, they remain dormant or continue to grow very 
slowly. Such buds may begin to elongate rapidly the year after 
they are formed, or they may remain dormant for several or many 

The growth in length of a stem and of its branches is accom- 
panied by the formation of an ever-increasing number of new 
branches. If all such branches were to persist and to increase in 


diameter by secondary thickening, the system of branches might be- 
come so closely crowded that they with their leaves would densely 
shade one another. Such a condition is rarely found in trees, 
however, because many of the young branches die within a few 
years of their formation. In some trees, such as the cottonwood, 

FIG. 55. A pine growing at a relatively high altitude (Sentinel Dome, Yo- 
scrnite), showing the dwarfing and distorting effects of snow and wind. 
Photograph by Phillips D. Schneider. 

the death of a branch is due to the development of a basal trans- 
verse tissue, an abscission layer, which causes the branch to be- 
come separated from the stem or older branch bearing it. In most 
trees no abscission layer is formed at the base of a branch; a dead 
branch remains attached to the tree until it is broken off by storms 
or by other means. 

60. Kinds of Stems. Plants which develop tall, woody sterns 
capable of standing erect without support are called trees. Each 
species of tree has, in general, a characteristic form. The trunk of 
the black spruce tapers gradually without forking from base to 
apex and produces whorls of branches, the branches of the older 
whorls being largest and those above progressively shorter. The 
outline of such a tree is that of a cone. Some other gymnosperms, 



FIG, 56. One of the "big trees" of California. Photograph by 
Lenwood Abbott, from American Forestry. 



such as the larch (Fig. 70), have a similar habit of growth. The 
form of such trees as the elm (Fig. 72) and the oak is usually very 
different from that of the spruce. At some distance above the 
ground there ceases to be a single trunk; instead, a varying number 
of large branches appear, which are themselves often much 
branched. Such a tree fre- 
quently has a rounded form 
(see also 58). 

The height of a tree and the 
thickness ot its trunk vary with 
the species, with the age of 
the tree, and with the environ- 
ment. At high elevations, 
plants of species which at lower 
levels develop into trees often 
have small, twisted, gnarled, 
and more or less prostrate 
sterns (Fig. 55). Under con- 
ditions that favor growth, trees 
of certain species attain a great 
height and develop huge 
trunks. The "big trees" (Se- 
quoia gigantea) of California 
are notable illustrations 
(Fig. 56). One of the largest of 

these trees is 325 feet in height, -, c( . r , . . , r . 

. _ _ 11 FIG. 57. The twining stem of a bean, 

and the trunk, at a short dis- 
tance above the ground, has a diameter of nearly 30 feet. The 
numbers of annual rings counted in the stumps of a few Sequoias 
indicate that the trees were over 3,000 years old when felled, 
the oldest one thus far counted having been somewhat more than 
3,200 years of age. 

Plants which develop relatively short and usually freely branched 
woody stems are called shrubs. Since trees and shrubs intergrade, 
the words " shrub " and "tree " are convenient, but not exact, terms. 

Those plants whose stems develop a small proportion of xylem, 
their stems therefore frequently remaining relatively soft, are 
called herbs. The distinction between herbaceous and woody plants 
is likewise not a sharp one, for almost all gradations in the amount 
of xylem developed may be found in different plants. 



The stems of many plants can not hold themselves upright, 
either because of their slenderness or because of the small propor- 
tion of mechanical tissue. Some such weak-stemmed plants merely 
creep or clamber along the ground or over rocks. Others (often 
called vines) attach themselves to supporting objects, such as 
other plants, and so attain an approximately upright position. 

Clambering and climbing 
plants may be either woody 
or herbaceous. Some vines, 
such as the morning glory 
and the lima bean (Fig. 57), 
climb by means of a twining 
of their steins, the terminal 
portion of such a stem mov- 
ing through a rather large 
spiral as a result of irregu- 
larities of growth on differ- 
ent sides. If a stern of this 
type comes in contact with a 
suitable support, the spiral 
movement of the terminal 
portion causes the stem to 
twine about the supporting 
object. The coils are at first 
often very loose, but later, 
through a straightening of 
the stem, the spirals become 
steep and firmly bound about 
the support. Other vines 
climb by means of tendrils. 
The tendrils of the garden 
pea (Fig. 76, A) correspond to leaflets; those of the grape 
(Fig. 58) to branches. Tendrils are usually sensitive to con- 
tact, especially toward their tips. Contact with an object serves 
as a stimulus, and, in the cases of most tendrils, the end within 
a short time becomes tightly wound about the object touched. 
In the Japanese ivy and the Virginia creeper, the small branches 
of the tendrils end in knobs. Upon contact these knobs broaden 
into disk-shaped structures that adhere with extreme tenacity 
to the surface with which they are in contact. Certain plants, 

FIG. 58. Tendrils of the grape. 



such as the English ivy, climb by means of adventitious aerial 

The stems of many plants grow underground. A whole stem 
with its branches may be underground, as is the case in many 

FIG. 59. Underground stem of Solomon's seal. 

ferns; or, more frequently, the underground stem produces aerial 
branches which bear foliage leaves and flowers. The latter case is 
illustrated by cat-tails, sedges, grasses, and golden-rods. Under- 
ground stems (Figs. 59, 60) often contain a considerab lck amount of 


FIG. 60. Underground stem of quack grass. 

food. If such a stem is broken into numerous parts, roots and aerial 
branches may be developed at each node, new plants thus being 
produced. It is for this reason that certain grasses, like the quack 


grass, are often pests in fields and gardens. Tubers, such as those 
of the potato, are enlarged portions of underground branches in 
which a great amount of food is stored. The "eyes" of a potato 
contain buds each of which is capable of forming a shoot. 

Many plants have very short stems that are partly or wholly 
buried in the ground. Short stems of this type are often associated 
with roots containing large reserves of food, as in the parsnip, 
carrot, beet, and dandelion. In other cases, such as the jack-in- 
the-pulpit and the crocus (Fig. 61), the short 
stem is itself the storage organ and is conse- 
quently enlarged and fleshy. 

According to their longevity, plants are 
classed as annuals, biennials, and perennials. 
Annuals live for but one season. They pro- 
duce leaves, flowers, fruits, and seeds, and then 
die. In biennials, like the cabbage, turnip, and 
beet, the leaves formed the first year produce 
FIG. 61. The short, a quantity of food that is stored in the stem, 
thick underground leaves, or root, the storage organ or organs 

g enerall y bcin * thick and flesh y- The next 

year this stored food is used in the production 
of new organs, including flowers, fruits, and seeds, after which the 
plant dies. Perennials (plants that live for many years) may be 
either herbaceous or woody, woody perennials being trees, shrubs, 
or vines. Herbaceous perennials usually store reserve foods in 
underground stems, branches, or roots. Buds developed on these 
organs may grow into new shoots. The part of a herbaceous 
perennial which lives from year to year is usually, therefore, es- 
pecially in temperate regions, an underground part, which may 
be a stem, part of a stem, a branch, or a root. 

61. Functions of Stems. The chief functions of a stem or branch 
are the support of leaves and flowers, and conduction. Water with 
mineral nutrients in solution is conducted through the xylem and 
in the direction of the leaves. The foods made in the leaves pass 
through the conducting cells of the phloem to the parts of the plant 
where active growth is going on or where foods are being stored. 

The green parts of a stem play a part also in the manufacture 
of foods, although in the sunflower, as in most familiar plants, it 
is chiefly in the leaves that this work is performed. In some 
plants, however, including cacti, practically all the foods are made 


in the stem; in others, such as the asparagus, some of the branches 
do the work of food-making. Sometimes, as in the greenhouse 
"smilax," branches assume the form, as well as the functions, of 
leaves. In woody plants (trees and shrubs) it is often only the 
youngest branches that are green and therefore capable of manu- 
facturing foods. 

Usually some foods are stored for a longer or shorter time in 
certain parts of the stem, especially in the parenchyma of pith, 
cortex, and medullary rays. In many plants, especially those 
which live for more than a year, food-storage is an important func- 
tion of the stem. For example, during the winter the parenchyma- 
tous cells in the stems of trees and shrubs often contain large 
amounts of starch and fats. The stem of the kohlrabi, potato 
tubers, and many other underground branches and stems are 
especially adapted for the storage of foods. 


The growing apex of a stem includes an embryonic region, a 
region of elongation, a region of maturation, and a mature region. 
The three latter regions are differentiated into nodes and inter- 
nodes. In the stems of dicotyledons and of gymnosperms the vas- 
cular bundles are arranged in the form of a hollow cylinder. In 
stems of monocotyledons the vascular bundles are as a general 
rule numerous and scattered. 

Internodes of a dicotyledonous stem consist of stele, cortex, and 
epidermis. Tissues of the stele include pith, xylem, cambium, 
phloem, medullary rays, and pericycle. The cortex includes 
mechanical tissue, parenchyma, and endodermis. A node differs 
from an internode in that leaf traces are present in its cortex. 

A stem of a dicotyledon or of a gymnosperm develops a cambium, 
the division of whose cells gives rise to secondary xylem and second- 
ary phloem. The secondary xylem (with included medullary rays) 
formed in each growing season constitutes an annual ring. Annual 
rings are distinguishable because of differences in size between 
elements formed at the beginning and those formed at the end of 
the growing season. Older annual rings toward the center of a tree 
may be modified into heart wood. In such case, movement of sap 
is restricted to the outer annual rings, the sapwood. 

Bark comprises all tissues outside the cambium. In a young 
stem the bark is composed of phloem, medullary rays, pericycle, 


cortex, and epidermis. The bark of an older stem includes also a 
cork layer (or layers) produced by a cork cambium, which usually 
arises in the outermost portion of the cortex. Several successive 
cork cambiums may be formed by a stem. Lenticels may be 
present in the outer surface of the bark. 

Internodes of monocotyledons are ordinarily not sharply dif- 
ferentiated into cortex and stele. In the numerous scattered vas- 
cular bundles there is no cambium between xylem and phloem. 
Most monocotyledonous stems have no secondary thickening. 
Those which thicken secondarily have a cambium which produces 
new vascular bundles. A wounded portion of a stem may form a 
wound tissue (callus). 

Branch primordia are formed in the embryonic region in the 
axils of leaves. A branch primordium may develop immediately 
into a branch or may remain dormant for one or more years. 

According to proportion of wood and length of stem, plants are 
classed as trees, shrubs, and herbs. A plant whose stem can not 
hold itself erect may creep or clamber; if the stem can attach itself 
to a support and so grow upward, the plant is a vine. Vines may 
be woody or herbaceous. Stems that grow underground are either 
erect or horizontal. They may be entirely subterranean, or may 
produce aerial branches. According to their longevity, plants are 
classed as annuals, biennials, and perennials. 

The functions of a stem include support of leaves and flowers, 
conduction of materials, manufacture of foods, and storage of 



52. Nature and Positions of Buds. As noted in 34, the em- 
bryonic region of a stem or branch gives rise to numerous leaf 
primordia (Fig. 62). Immediately below the apex of the embryonic 
region such primordia are but small humps of embryonic tissue. 
Farther back from the apex the young leaves, developed from 
these primordia, have grown more rapidly in length than the em- 
bryonic region of the stem, and, since their growth has been more 
rapid on their outer than on their inner surfaces, they have curved 
so as to enclose the embryonic region. Such a terminal portion of 
a stem or of a branch with its urielongated internodes bearing im- 
mature leaves, or immature and mature leaves, which curve and 
enclose the growing point, is termed a bud. 

New growing points which develop into buds arise also in the 
axils of many leaves, usually a single growing point in an axil. 
Buds that develop from such growing points are called axillary 
buds to distinguish them from the terminal bud of a stem or branch. 
An axillary bud is often inconspicuous until the leaf in whose axil 
it occurs has fallen (Fig. 63). 

63. Naked Buds. If the leaves borne by a bud are all foliage 
leaves, the bud is naked. Naked buds are characteristic of all 
annuals, of most biennials, and of a few perennials such as Elodea 
(Fig. 62), juniper, and the staghorn sumac. During the growing 
period, new leaf primordia, nodes, and internodes are being formed 
in the anterior portion of a bud. Farther back in the bud the inter- 
nodes undergo elongation and the leaf primordia grow into young 
leaves. As a result of the continuation of these activities during 
the growing period, such a naked bud is constantly changing in 
its composition of leaves, nodes, and internodes. In the axils of 
leaves within the bud may be the primordia of branches. In some 
few plants the older internodes elongate but slightly as new leaf 
primordia and internodes develop in the embryonic region. The 
result is a continual increase in the size of the bud which may, as 
in the head lettuce, become relatively enormous. 




Terminal Bud* 


Leaf Scar * 





FIG. 62. Lengthwise section through 
a terminal naked bud of Elodea 

FIG. 63. A branch of the 
horse-chestnut bearing ter- 
minal and axillary protected 



Naked buds are of various types as regards the immature parts 
which they contain. Some are leaf buds; such a bud (Fig. 62) in- 
cludes an embryonic branch or stem tip with its nodes and unelon- 
gated internodes and the primordia of leaves. At some of the nodes 
may appear the primordia of branches. Other naked buds are 


Young Foliage Leaf 

Foliage Leaf 

FIG. 64. Lengthwise section of an 
axillary naked floral bud of the but- 
tercup, containing the parts of a 
single flower enclosed by foliage 
leaves (diagrammatic). 

floral buds. A floral bud con- 
tains the primordia of a flower FlG - 6 \ Lengthwise section through 
- , . f a , an axillary protected leaf bud of the 
or of a cluster of flowers borne elm (diagrammatic). 

on an embryonic branch. A 

floral bud of a buttercup (Fig. 64) contains the embryonic parts 
of a single flower; one of a sweet pea possesses the embryonic 
parts of a cluster of flowers. If, as happens in some instances, a 
floral bud contains primordia of leaves and possibly of branches 
in addition to those of a flower or flowers, it may be called a mixed 
bud. These three types of buds to some extent intergrade; hence 
the names applied to them should be regarded not as exact but 
rather as convenient and general expressions of differences. 

In contrast with the active naked buds just described are those 
which enter upon a period of rest or of very slow development. 
It is in such a resting condition that the naked buds of most bi- 



ennials, as for example the cabbage, pass the winter. Certain 
perennials also, including Elodea, have resting naked buds at the 
tips of stems and branches during the winter. 

54. Protected Buds. If the outermost leaves of a bud, instead 
of being foliage leaves, are scale-like, it is a protected bud. Most 
woody perennials of temperate regions, such as the elm, hickory, 
and horse-chestnut (Fig. 63), form buds of this type. In such a 
Floral S ***^ kud the outermost scale 

Primordium ^r ^/^^. ca // ^^ leaves commonly overlap 

and are often heavily cu- 
tinized or covered with 
waxy or resinous sub- 
stances. In addition, nu- 
merous hairs may be borne 
on the surfaces or margins 
of the scales. Such secre- 
tions and outgrowths tend 
to prevent the loss of water 
and the consequent drying 
out of the enclosed im- 
mature parts of the bud. 
They also afford protection 
against mechanical injury. 
As regards the immature 
parts that they contain, 
protected buds like naked 
buds may be either leaf 
buds, floral buds, or mixed 
buds. For example, the elm has both leaf buds (Fig. 65) and 
floral buds, a floral bud containing the embryonic parts of a 
cluster of flowers (see also Fig. 68). The apple (Fig. 66), lilac, and 
horse-chestnut form mixed buds. 

65. Unfolding of Protected Buds (Fig. 67). Protected buds, 
at least in temperate regions, ordinarily undergo a period of rest 
-during the winter and resume growth with the coming of spring. 
When a protected leaf bud resumes growth the scale leaves expand. 
Sooner or later the scale leaves are shed. The opening of a bud is 
accompanied by a very rapid elongation of the internodes within 
it and by a rapid development of the embryonic foliage leaves to 
maturity. This rapid maturation of embryonic leaves accounts for 

Branch Leaf 

Primordium Primordium 

FIG. 66. Lengthwise section of an axillary 
protected mixed bud* of the apple (dia- 



the suddenness with which a tree puts forth a crop of new leaves 
in the spring. All the leaves borne by some trees, such as the 
horse-chestnut, were present in an immature condition in pro- 
tected buds at the beginning of the growing season; in other trees, 
such as the elm, additional foliage leaves are formed from the em- 
bryonic regions of stem and branches after the maturation of such 
immature leaves as were present in the dormant protected buds. 
After the shedding of the scale leaves a scar remains on the stem 
or branch marking the former place of attachment of each scale. 

FIG. 67. Protected leaf buds of the hickory in different stages of develop- 
ment. A, winter condition. B, beginning of elongation in the spring; 
scale leaves greatly expanded. C, after the foliage leaves have emerged; 
scale leaves still present. 

Since the internodes between the scale leaves elongate but slightly, 
the scars left by the scales form a ring about the stem (Fig. 63). 
In geographical regions where scale leaves are shed but once a 
year, it is therefore possible to gauge the age of a branch by count- 
ing the groups of scale-leaf scars. 

Protected leaf buds may be either terminal or axillary. Those 
which elongate are usually at or near the apex of the stem or branch; 
the remaining leaf buds latent buds ordinarily do not open 
and elongate. Under changed circumstances latent buds may 
develop into branches, as, for example, when the active buds 
above them are destroyed. Buds may thus be stimulated to de- 
velopment after years of latency. 



Protected floral buds are usually axillary. When such a bud 
opens, the scale leaves, as in the case of leaf buds, expand and fall 

FIG. 68. The seasonal history of an elm twig and of its protected buds. 
A, winter condition. B, early spring; the floral buds have developed into 
floral branches. C, midsummer; 2 axillary leaf buds have developed into 
branches and produced leaves; 2 have remained latent; the floral branches 
have developed mature fruits and have fallen, leaving scars. Z>, late 
autumn; the leaves have fallen; in their axils are protected leaf and floral 
buds. TB, minute undeveloped terminal bud; LBi-LB*, axillary leaf buds; 
FBj axillary floral buds; SLS, scale-leaf scars; FBS, floral branch scars. 

off, leaving a group of scale-leaf scars. The embryonic branch 
within the bud elongates somewhat and the floral primordia which 
it bears develop into one or more flowers. The unfolding of floral 



buds frequently occurs before the unfolding of the leaf buds. In- 
deed, as in the elm (Fig. 68), the flowers may produce mature 
fruits and the fruits may be shed by the time the foliage leaves 
have come to maturity. In other cases, as in the cherry, the fruits 
do not mature until some weeks after the foliage leaves have 

In the unfolding of a protected mixed bud, floral and leaf pri- 
mordia may mature simultaneously, or the floral primordia may 
mature more rapidly than those of the leaves. The horse-chestnut 
is an example of the former type; the apple, of the latter. 

56. Adventitious Buds. It is possible for new growing points 
to become organized in various parts of a plant other than the 
axils of leaves. Buds that develop from 

these new growing points are similar in 
structure to the terminal and axillary 
buds already described and differ only 
in the position in which they are borne 
on the plant. Such adventitious buds may 
arise from various tissues of a leaf (as in 
begonia); from the cortex of a stem; 
from the peri cycle of a stern or root; or 
from wound tissue of a stern or root. 
The sprouts which appear on the cut 
surfaces of stumps of trees of many 
kinds result from the elongation of ad- 
ventitious buds. 

57. Bulbs. Some plants produce a 
special type of underground bud com- 
monly known as a bulb. Among such 
plants are the hyacinth, narcissus, onion, 
and tulip. A median lengthwise section 

(Fig. 69) of one of these bulbs shows FIG. 69. 
that it has a short conical stem. The 
upper surface of the stem bears numerous 
broad, fleshy scale leaves enclosing the terminal growing point and, 
in some plants at least, the primordia of foliage leaves and flowers. 
Many of the scale leaves are storage organs containing abundant 
reserve food, often in the form of sugar. From the outer lower 
edge of the stem grow many adventitious roots. Among the new 
growing points which arise in the axils of the scale leaves, an 

The bulb of a 
hyacinth in lengthwise 



occasional one may itself develop conspicuous scale leaves and so 

become a new bulb. 

Unlike those of other protected buds, the scale leaves of a 

bulb never unfold when growth is resumed. At this time the 

foliage-leaf primordia, 
if present, as in the 
narcissus, develop 
rapidly into foliage 
leaves. From the cen- 
tral region of the 
growing point arises 
a floral axis which 
bears a single flower 
as in the tulip, or a 
cluster of flowers as 
in the hyacinth. 

58. Buds and Plant 
Form. The form char- 
acteristic of a plant is 
an expression of the 
type and degree of the 
development which 
has taken place from 
its buds. Many coni- 
fers, such as the pine, 
spruce, and larch 
(Fig. 70), produce 
strong terminal pro- 
tected buds and, in 
close proximity to each 
terminal bud, several 
smaller axillary pro- 
tected buds. During 
successive growing 
seasons development 
from the terminal bud 
of the stem is more 

rapid than that from the terminal buds of the branches, with the 

result that a single strong central shaft or trunk is produced. 

Trees like the maple (Fig. 71) and the elm (Fig:. 72), although 

FIG. 70. The larch/ a tree with a conspicuous 
central shaft; growth from the terminal bud of 
the stem IB more rapid than from the terminal 
buds of branches. 



they have a different arrangement of buds, show much the same 
growth tendencies but to a lesser degree; consequently, growth 
of one or more of the lateral branches may be nearly or quite as 
rapid as that of the original trunk, and thus several large branches 
are formed. The tend- 
ency for many axillary 
buds on these branches 
to elongate results 
finally in the produc- 
tion 01 a much- 
branched, spreading 
top, very different 
from that of a spruce 
or larch. 

Sometimes, as in the 
lilac, a mixed bud of- 
ten occupies the ter- 
minal position on each 
of the larger branches. 
Since this bud termi- 
nates in a cluster of 
floral primordia, fur- 
ther extension of the 
vegetative parts of the 
plant must be brought about by growth from axillary buds. If 
many of these axillary buds elongate, a densely branching plant 
is produced. Some plants, like the elm (Fig. 68) and the poplar, 
have many axillary floral buds and relatively few axillary leaf 
buds back of each terminal bud. Later in the same spring the 
floral branches, now bearing fruits, fall from the tree, leaving 
conspicuous bare spaces on the twigs which bore them. 

Although each kind of tree tends to develop a characteristic 
form, it must not be overlooked that environmental factors 
also play an important rdle in determining the shape of the tree. 
Among these environmental factors are wind, shade, and tempera- 
ture (see Fig. 55). The form of a tree may be profoundly in- 
fluenced by the presence of surrounding vegetation. A pine 
tree growing in the open takes a very different form from that 
which it would assume in a dense forest. The effects of the 
human element in the environment are illustrated by the re- 

FIG. 71. The rnaple, a tree; in which the terminal 
growth of branches is nearly as active as that of 
the central axis. Photograph by George Kem- 


suits of pruning, sometimes resulting in the production of bizarre 


Buds may be either terminal or axillary. Either terminal or 
axillary buds may be naked or protected. Whether terminal 

FIG. 72. The elm, whose habit of growth is similar to tftat ot 
the maple. Photograph by George Kemmerer. 

or axillary, naked or protected, a bud may be a leaf bud, a floral 
bud, or a mixed bud. 

Naked buds, while for the most part actively growing, may in 
some plants remain for a time dormant. Protected buds in general, 
at least in temperate regions, undergo a dormant period. Some 
protected buds remain indefinitely in a latent condition unless 
induced to develop by some special stimulus. 

When a protected bud opens, the scale leaves fall, leaving 

BUDS 93 

scars at their former places of attachment. The primordia (of 
leaves, branches, or flowers) previously enclosed within the scales 
then develop. 

Buds which appear in other places than at the ends of stems 
or branches or in the axils of leaves are adventitious. 

A bulb is a special type of underground bud. 

The form of a plant is determined by the type and degree 
of the development that takes place from its buds. 



69. Development of Leaves. Leaves are first recognizable as 
small dome-shaped primordia along the sides of the embryonic 
region of a stem or branch. In most cases the continued divi- 
sion of cells in a leaf primordium results in the development 
of a young leaf which, though composed of embryonic cells, has 
much the form that will characterize it at maturity. Approxi- 
mately the number of cells that will be present in the mature 
leaf is reached while the cells are still embryonic and the leaf 
is small. The rapid increase in size of an unfolding leaf is due, 
therefore, not chiefly to the formation of new cells, but to the 
enlargement and maturation of embryonic cells already formed; 
in consequence, all the cells enlarge and mature simultaneously, 
and there are no localized regions of elongation and maturation. 
The lack of such distinct regions is one important respect in which 
the development of most leaves differs from that of a root, stem, or 
branch. Another important difference lies in the fact that in gen- 
eral the cells of a leaf are formed once for all, cell division then 
ceasing; whereas division, and therefore growth, may continue 
indefinitely in the apical region of a root or stem. 

In certain monocotyledons, however, especially in some grasses, 
the basal portion of each leaf remains embryonic even after the 
apical portion is mature. The leaf continues to grow in length as 
cells formed in the basal embryonic region successively enlarge 
and mature. 

60. Arrangement. The arrangement of leaf primordia on a stem 
or branch (as well as of the leaves developed from these primordia) 
varies greatly as between different species, and may vary con- 
siderably on different parts of a single plant. In some plants, 
leaves are borne in pairs at each node and opposite each other 
(Fig. 73, Z>), each pair usually standing at right angles to the pair 
below. Coleus, maples, and lilacs have leaves arranged in this way. 
If three or more leaves are borne at a single node, as in Elodea, 
they are whorled. 




Leaves are alternate when but a single leaf occurs at each node 
and the leaves follow each other on the stem in a spiral manner; 
that is, a line drawn from any leaf-insertion to the next above, 
and so on, forms a spiral around the stem. An arrangement of 

FIG. 73. Types of leaf -arrangement. A, B, C, various alternate arrange- 
ments. D, opposite arrangement. The upper figure in each case represents 
the leaf-arrangement as seen from above. 

this nature is most common. In a species whose leaves are alter- 
nately arranged, the distance around the stem from any one leaf- 
insertion to the next is substantially equal, although the vertical 
distance from leaf to leaf may vary greatly. In some plants, as 
the grasses and elms, the second leaf is on the side opposite the 
first and the third on the side opposite the second that is, directly 
over the first (Fig. 73, A). In other plants, as the hazel, each leaf 
is one third the distance around the stem from the leaf next below 
(Fig. 73, B). In still other plants, as the cherry, apple, and willow, 
any two successive leaves are separated by two fifths of the circum- 
ference of the stem (Fig. 73, C). In the latter case a spiral con- 



necting the leaves passes twice around the stem before a leaf is 
reached which is directly over that with which the spiral started. 
In still other plants, more than two turns of a spiral are necessary 
to reach a leaf directly above that with which the spiral began. 
In many common plants each leaf is so located as to shade 
its neighbors as little as possible. If leaves are numerous, nearly 

FIG. 74. The rosette arrangement (leaf mosaic) of the first-formed 
leaves of a mullein. 

all the space exposed to light is occupied. A leaf-arrangement of 
this nature is spoken of as a leaf mosaic. When Japanese ivy grows 
upon a wall, it furnishes a striking example of a mosaic. Leaves 
that lie prostrate on the ground, such as those of the dandelion 
or the basal leaves of the mullein (Fig. 74), form a rosette whose 
lower leaves are longer than those above, the greater length of 
the lower leaves being chiefly in their petioles. 

61. External Structure. Most leaves are made up of certain 
definite parts which are externally recognizable. Often a foliage 
leaf is composed of blade, petiole, and stipules; many foliage leaves, 



however, lack petioles, stipules, or both. In the grasses and in 
some other monocotyledons, both petiole and stipules are lacking 
and, as illustrated by the corn, the basal part of each leaf is a 

The petiole may be short or long, stout or slender, cylindrical 
or flattened, and in some cases grooved or winged. At its base, 
where it is attached to the stem, the petiole may be swollen or 
may clasp or ensheathe the stem. When a petiole is lacking and 
the blad is attached directly to 
the stem, the leaf is sessile. The 
blades of some sessile leaves partly 
surround the stem, as in the wild 
lettuce; or, as in the bellwort 
(Fig. 75, A), they completely en- 
circle the stem. When sessile leaves 
are opposite, the blades of a pair 
may be united around the stem, 
as in some of the wild honey- 
suckles (Fig. 75, B). 

Stipules may be either blade-like 
or spine-like. When present they 
are borne one on either side of the 
base of the petiole. Certain blade- 
like stipules are green and, like the 
leaf blade, manufacture foods. In 
this respect such stipules are usu- 
ally of minor importance as com- 
pared with the blade, but in the 
pea (Fig. 76, A) and in some other 
plants their importance in food- 
manufacture approaches that of the rest of the leaf. Other blade- 
like stipules furnish a protective covering for immature leaves 
and, as in certain oaks and willows, are shed after the leaves are 
.mature. The common black locust and certain Euphorbias 
(Fig. 76, C) are examples of plants whose stipules are spine-like 
and persist after the other parts of the leaves are shed. 

In spite of great variation between plants of different kinds 
in shape of leaf blades, the arrangement of the veins is according 
to one of two main plans. In the majority of monocotyledons, 
as in the corn and the lily of the valley (Fig. 77, A), several main 


FIG. 75. A, stem of the bellwort 
with leaves which encircle the 
stem. B, stem of a climb- 
ing honeysuckle with opposite 
leaves, the two of each pair 
united about the stem. 


veins run approximately parallel to the leaf axis, extending from 
the base to the apex of the blade. Such leaves are paralkl-veined. 
The leaves of the banana (Fig. 77, B) and of some other mono- 

FIG. 76. Leaves with stipules. A, leaf of the pea; some leaflets replaced by 
tendrils. B, rose leaf. C, twig of a Euphorbia, with spine-like stipules. 

cotyledons show a deviation -from the ordinary parallel-veined 

arrangement. The leaf blade of such a plant has a midrib and 

many branch veins; the latter extend, parallel with one another, 

from the midrib nearly or quite 

to the margin. In both types 

of parallel-veined leaves the 

conspicuous veins really give 

off many fine branches which 

constitute a network; but these 

slender branches are not readily 


In the second main plan of 
vein-arrangement, which is 
characteristic of the leaf blades 
of the majority of dicotyledons, 
the main vein (midrib) or main 
veins branch repeatedly, many 
of the ultimate branches meet- 
ing one another so as to form a 
rather conspicuous network parallel-veined leaf of banana. 


(Fig. 78). Such leaves are netted-veined. A netted-veined leaf 
with a conspicuous midrib, from different points on which large 
branch veins diverge, as in a leaf of the elm (Fig. 79, A), IB 
pinnately netted-veined. If several main veins diverge from a 
single point at the base of the leaf, as in the maple (Fig. 79, J5), 
the arrangement of veins is 

In leaves the margins of 
whose blades are lobed, the 
method of lobing usually cor- 
responds to the arrangement 
of the veins; hence, if a leaf is 
pinnately veined, it may be 
pinnately lobed. If the veins 
are palmately arranged, the 
leaf may be palmately lobed. 
In case the lobing extends to 
the midrib or to the base of the 
blade, the leaf is compound; 
the parts into which the blade 
is thus divided are leaflets. If 
the various leaflets are sessile 
upon the midrib or upon the 
petiole, it may be difficult to 
distinguish between a com- 
pound and a deeply lobed leaf. 
No such difficulty obtains, 
however, in compound leaves each of whose leaflets has a separate 
stalk. Compound leaves may be pinnately compound, like those 
of the ash (Fig. 80, A), the rose (Fig. 76, JB), and the pea (Fig. 76, 
A)] or palmately compound, like those of the horse-chestnut 
(Fig. 80, B). The leaflets may themselves be divided, as in many 
ferns. Some compound leaves are three times divided, and the 
common meadow rue (Fig. 80, C) has a four-times-divided leaf. 

The surface of the blade of a foliage leaf may be smooth, rough, 
or hairy. The surfaces of most leaves are cutinized, and some are 
coated with wax; when the waxy coating is broken up into minute 
rods or plates it appears as a "bloom/ 7 like that on the leaves of 
the cabbage and the tulip. The blades of many young leaves are 
densely covered with hairs which check, more or less, the evapo- 

FIG. 78. The system of veins of a 
netted-veined leaf (black oak). 



ration of water from the immature blades. As a blade matures, the 
hairs may shrivel and disappear, or they may persist. Some leaf 

FIG. 79. A, elm leaf, with pinnate venation. B, leaf of maple, palmately 
lobed and with palmate venation. Only the larger veins are shown in 
each case. 

blades, like those of the mullein, are covered by a dense matting 
of hairs (Fig. 81, (?). The leaves of some plants bear glandular 

FIG. 80. Compound leaves. A, pinnately compound leaf of an ash. B, pal- 
mately compound leaf of the horse-chestnut. (7, four-times-divided leaf 
of meadow rue. 

hairs (Fig. 81, E, F) which secrete special substances. The strong 
odors given off by mints are due to volatile oils secreted by the 
terminal cells of glandular hairs on the leaves and stem. 



62. Tissues of a Leaf Blade. The blade of a dicotyledonous leaf 
(Fig. 82) typically includes several distinct tissues. On its upper 
side is an epidermis, whose cells are often irregular as seen in sur- 
face view but appear nearly rectangular in cross section. The 
outer walls of these epidermal cells are cutinized. Next beneath 
the upper epidermis is a layer (or under certain conditions two or 
more layers) of palisade cells, the long axis of each cell being perpen- 
dicular to the surface of the leaf; these cells are frequently separate 
from one another at their lower ends. The palisade cells contain 
many small chloroplasts. Next beneath them, commonly making 


FIG. 81. Epidermal hairs from various leaves. A-D, from the sunflower. 
E y F, young and old glandular hairs of the geranium. G, branching hair 
of the mullein. 

up the greater part of the thickness of the leaf, are several layers 
of rounded or irregularly shaped cells, constituting the spongy 
tissue; between these cells are numerous, often large, intercellular 
spaces. The cells of the spongy tissue also contain chloroplasts, 
though usually not so many in proportion to their size as do the 
palisade cells. Adjoining the spongy tissue is the lower epidermis, 
whose cells are similar to those of the upper epidermis. Here and 
there in the lower epidermis is an opening called a stoma (plural, 
stomata). Each stoma lies between two almost crescent-shaped 
guard cells which are smaller than the other cells of the epidermis 
and which contain chloroplasts. The stoma opens into an inter- 
cellular space, which in turn is continuous with spaces between 
the cells of the spongy tissue. All the intercellular spaces in the 
spongy tissue are likewise connected with one another and with 



such spaces as occur between the palisade cells. Thus the stomata 
and the intercellular spaces constitute an aerating system, by 
means of which gases may be freely interchanged between the 
atmosphere and the interior cells of the leaf. In many leaves 
stomata are present also in the upper epidermis. 

Both upper and lower epidermis may bear hairs. Some hairs 
are merely single elongated epidermal cells. In other cases a hair 

Cuticle Upper Epidermis Palisade Tissue 

Sheath of Vein 


Lower Epidermis 

Guard Cells 

FIG. 82. Portion of the blade of a sunflower leaf, showing the relations 
of the various tissues. 

is a row of two^or more cells resulting from the growth and division 
of an epidermal cell. In still other instances a hair is composed of 
more than one row of cells. 

The veins of a leaf are vascular bundles which are continuous 
through the petiole with the vascular bundles of the stem. Like 
a stem bundle, a vein contains xylem and phloem, the xylem being 
toward the upoer side of the blade and the phloem toward the 
lower surface. Xylem and phloem consist, respectively, of the 
same types of elements as are found in the bundles of the stem. 
As the branches from the midrib and the main veins become pro- 
gressively smaller, the phloem and xylem contain fewer and smaller 
elements. Each of the ultimate veinlets in which the system termi- 
uates may possess no phloem and but a single tracheid. Surround- 





ing the xylem and phloem of a vein is a sheath whose thickness 
varies with the size of the vein. In the case of the midrib, and often 
of some of the larger veins, the sheath is usually a compact mass 
of cells extending from upper to lower epidermis, consisting of 
parenchyma and mechanical tissue, and occupying in this par- 
ticular region the place of palisade and spongy tissue. The sheath 
about one of the smaller veins or veinlets consists usually of but 
a single layer of parenchymatous cells and lies wholly within the 
spongy tissue. 

63. Tissues of a Petiole. The vascular bundles of a petiole run 
lengthwise from the junction of the petiole with the stem to its 
junction with the leaf blade. A petiole may contain one, three, 
five, or more bundles. If more than one is present, the various 
bundles may be parallel throughout the length of the petiole, or 
they may lie so close to one another that at the outer end of the 
petiole they appear to be a single bundle. As seen in cross section 
(Fig. 83), the separate bundles in the petiole usually lie in an arc 
which is concave to- 
ward the upper side 

of the petiole. 

Petiolar bundles, 
like leaf veins, con- 
tain xylem and 
phloem. The phloem 
provides a pathway 
for the transfer to 
the stem of food 
manufactured in the 
blade ; the xylem pro- 
vides a pathway for 
the transfer of water 
and mineral nutrients 
from stem to blade. Surrounding each bundle is often a sheath of 
thick-walled mechanical cells. There may be mechanical tissue 
also just within the epidermis of the petiole. Both systems of 
mechanical tissue aid materially in the functioning of the petiole 
as a support for the blade. The remaining cells of the petiole, 
except the epidermis, are parenchymatous. 

64. Variation in Foliage Leaves. It has been suggested in pre- 
vious paragraphs that leaves borne by plants of different kinds 

FIG. 83. 


Diagram of a cross section through the 
petiole of a leaf of the beet. 



often differ greatly in form, structure, and even in function. Such 
variations are due chiefly to differences in the arrangement and 


\ Lower 
f Palisade 

\ Lower 
i Epidermis 


Pit Containing Stomata 
and Hairs 

FIG. 84. Cross section of a portion of an oleander leaf. 

proportions of the various tissues rather than to the presence of 
tissues of different sorts. A special type of arrangement is found 
in such leaf blades as those of the oleander (Fig. 84) and the rubber 

FIG. 85. Portion of a cross section of a corn leaf. 

plant, which have a palisade layer next the lower, as well as one 
next the upper epidermis, the spongy tissue thus lying between 
two palisade layers. 
Some leaf blades lack certain of the tissues that have been de- 



scribed. Many plants, like the compass plant and Eucalyptus, 
whose leaf blades are vertical rather than horizontal, have no 
spongy tissue, all the chloroplast-containing cells within each 
blade being palisade-like and perpendicular to the epidermis. In 
other leaves, like those of the grasses, the interior chloroplast- 
bearing region is not clearly differentiated into palisade and spongy 


Palisade Tissue 



FIG. 86. Cross section of a leaf of the Russian thistle, with large 
parenchyrnatous water-storage cells. 

tissues. In the corn leaf (Fig. 85), for example, intercellular spaces 
occur only immediately within the stomata, which are very numer- 
ous on both sides of the leaf; the remaining tissue between upper 
and lower epidermis, apart from the veins, is composed of com- 
pactly arranged cells with numerous chloroplasts. 

Ordinarily the foods manufactured in a foliage leaf do not re- 
main long in the leaf but are transferred to, and stored in, some 
other organ such as root or stem. Sometimes, however, as in the 
cabbage and the century plant, considerable amounts of foods are 
stored in foliage leaves, which in such plants are usually relatively 

The foliage leaves of many plants characteristic of dry regions 



have thick, fleshy leaves composed in the main of a water-storage 
tissue. This tissue may be an epidermis several cells in thickness, 
as in the leaf of the begonia; but more commonly the water-stor- 
age tissue is internal, as in leaves of the Russian thistle (Fig. 86), 

the century plant, and the 
aloe (Fig. 87). In either 
case the storage tissue is 
composed of large paren- 
chymatous cells with few 
or no chloroplasts. The 
imbibing power of muci- 
laginous substances in the 
dense cytoplasm and in 
the central vacuoles of 
these cells greatly increases 
their water-absorbing and 
water-retaining capacity. 
Water so held within the 
storage tissue may, how- 
ever, move to other parts 
of the plant after the water 
supply of the soil is ex- 
hausted, thus keeping the 
plant alive for some time. 
The leaves of certain 
plants obtain a part of 
their food from the bodies 
of insects. An example is 
the common pitcher plant 
(Fig. 88) growing in 
marshes, whose pitcher- 
like leaves are usually 
partly filled with water. 

On the inside of the tip of each leaf are stiff hairs pointing in- 
ward and downward, and glands which secrete a fluid attrac- 
tive to insects. After insects enter the pitcher, some of them 
are prevented by the hairs from escaping, and many insects 
ultimately drown in the water at the base of the pitcher. Sub- 
stances from their decomposing bodies are used as foods by the 
plant. Another plant growing in similar situations is the sun- 

FIG. 87. An aloe, in whose thick leaves con- 
siderable amounts of water are stored. 



dew (Fig. 89). Its leaves are provided with slender, sticky hairs 
which are sensitive to contact with protein-containing bodies. If 
a small insect touches one of the hairs it sticks to the hair, and in 
its struggles comes into contact with neighboring hairs which then 
bend over and hold the insect fast. After the death of the insect, 

FIG. 88. A pitcher plant (Sarracenia). 

the soft parts of its body are dissolved by secreted digestive juices. 
In time the hairs resume their ordinary position. 

65. Variation in Leaves on the Same Plant. Foliage leaves borne 
on different parts even of the same plant may differ considerably 
in form and structure. The leaves of a tree that are freely exposed 
to sunlight frequently have a thick, heavily cutinized epidermis, 
a two- or three-layered palisade tissue, and a spongy tissue with 
small intercellular spaces; whereas leaves in the interior of the 
crown of the same tree may have a thinner, less heavily cutinized 
epidermis, a single palisade layer, and a spongy tissue with large 
intercellular spaces. 



Even more striking differences appear between the aerial and 
the submerged leaves of some water plants. The aerial leaves of 

FIG. 89. A sundew (Drosera). 

the water crowfoot, for example (Fig. 90) , have well-defined pali- 
sade and spongy tissues; the submerged leaves, on the other hand, 
have no palisade tissue, and the intercellular spaces in their spongy 
tissue are small. The aerial leaves 
are few-lobed; the submerged leaves 
are divided into many narrow, thin 

Differences may appear also be- 
tween the leaves first produced by a 
plant and those formed later. The 
leaves formed by a young seedling of 
arbor vitae are needle-shaped (Fig. 91) ; 
but after one or more seasons' growth, 
scale-like appressed leaves develop and 
ordinarily continue to be formed dur- 
ing the life of the tree. Another ex- 
ample of "juvenile" leaves is seen in the bean, whose first- 
formed foliage leaves are undivided whereas those formed later 
are compound. 

A B 

FIG. 90. Aerial (A) and sub- 
merged (B) leaves of a water 
crowfoot. Modified from 



Leaves of juvenile form are not confined to seedlings; not un- 
commonly, leaves borne on branches developed from adventitious 
buds formed in a callus are of the juvenile type. A change in the 
environment of an older plant, too, may result in the formation 
of juvenile leaves. 
The basal (juvenile) 
leaves of the harebell 
are rounded, whereas 
the leaves borne on 
the upper portion of 
the stem are long 
and slender (Fig. 92). 
An environmental 
change, such as a 
marked change in il- 
lumination, causes a 
stoppage of terminal 
growth, followed by 
the development of 
lateral shoots whose 
basal leaves are 

66. Fall of Foliage 
Leaves. Intemperate 
climates the autumnal 
shedding of leaves by 
dicotyledonous trees 
and shrubs is a well- 
known phenomenon. 
It is brought about by 
the development of 
a special layer of cells (an abscission layer. Fig. 93) across the base 
of each petiole, and sometimes, in a compound leaf, across the base 
of the stalk of each separate leaflet as well The cell walls of the 
abscission layer are thin; the middle layer of each wall becomes 
dissolved, and finally nearly the whole thickness of the wall is 
softened and dissolved. The abscission layer does not extend 
across the vessels and tracheids of the bundle or bundles, whose 
walls, however, are easily broken by the wind or by the weight of 
the leaf after the disintegration of the walls of other cells. In some 

FIG. 91. Seedlings of the arbor vitae bearing 
leaves of juvenile and adult forms. 



oaks and other trees, the abscission layer is not well developed in 
the autumn; dead leaves, therefore, may remain on such trees 

well into the winter or even into the 
spring. The cells of the basal part of a 
petiole immediately below the abscission 
layer usually develop into a corky tissue 
which is externally visible on the stem 
as a leaf scar. The fall of a scale leaf, 
like that of a foliage leaf, is brought 
about through the formation of an ab- 
scission layer. 

Many trees and shrubs indigenous to 
regions without pronounced seasonal 
changes do not shed all their leaves sirnul- 
Vascular Bundle 


FIG. 92. Harebell (Cam- 
panula rotundifolia), with 
leaves of juvenile form at 
the base, and of adult form 
on the upright stalk. 




FIG. 93. Diagram showing the attach- 
ment of a leaf petiole to the stem, 
and the position of the abscission 

taneously. These plants, exemplified by eucalyptus, oranges, and 
live oaks, form new leaves and shed old ones continuously through- 
out the entire year. Such " evergreen " plants are always in foliage. 



67. Scale Leaves. In many cases a leaf primordium matures 
into a flattened leaf which is attached to the stem by a broad base, 
and which carries on little or no food-manufacture. Such a scale 
leaf is usually relatively small, without chloroplasts, and brownish 
or yellowish in color. Scale leaves about a protected bud prevent 
mechanical injury of the embryonic parts within. They aid also 
in checking evaporation from structures within the bud, and so 
minimize the harmful effects of sudden changes in temperature. 
The scale leaves of some buds are coated with resin, as in the 
poplars, and they may be provided with a dense coating of hairs, 
as are the inner scale leaves of a horse- 
chestnut bud. Frequently there is no 

sharp distinction between scale and foli- 
age leaves, and often, as in the lilac, 
there are all gradations from scale leaves 
at the outside of a bud to foliage leaves 
within. After foliage leaves have 
emerged from the bud, each scale leaf 
usually falls away. In some buds, like 
those of the hickory, the inner scale 
leaves become large and brightly colored 
before they fall. 

Scale leaves develop on subterranean, 
as well as on aerial, stems and branches. FI G . 94. 
The scale leaves surrounding the em- 
bryonic region of a subterranean stem 
or branch constitute a protective sheath 
which prevents the abrasion of the embryonic region as the stem 
pushes through the soil. 

68. Tendrils and Spines. An entire leaf primordium or a portion 
only of such a primordium may mature into a tendril. A tendril, 
therefore, may represent a whole leaf or only a part of a leaf. In 
peas (Fig. 76, A) and vetches one or more leaflets toward the 
terminal end of the leaf are tendrils. In some smilaxes (not in- 
cluding the greenhouse "smilax," which is an asparagus), the 
stipules are tendrils. In clematis and the nasturtium (Fig. 94) 
the petioles may function as tendrils, winding about a support 
and enabling the plant to climb. The term " tendril" is, in fact, 
applied to any twining portion of a plant which helps to attach 
the plant to a supporting object. The twining organs of the 

Leaves of the 
nasturtium (Tropaeolum), 
whose petioles function as 



grape (Fig. 58) are tendrils, although they are branches rather 

than leaves. 

Spines and thorns, likewise, may be branches, leaves, parts of 

leaves, or in some cases roots. The common barberry has one to 
five (typically three) spines at each node 
(Fig. 95), the spine or group of spines in each 
case representing a leaf. Some of the spines of 
cacti (Fig. 96) are leaves and some are branches. 


Leaves differ from other vegetative organs 
in that in general all their cells enlarge and 
mature simulta- 
neously. Mature 
leaves may be op- 
posite, whorled, or 
alternate in ar- 
rangement on the 
stem. A foliage 
leaf is commonly 



condition of a 

stem of the com- composed 


mon barberry; the blade, petiole, and 
spines are leaves stipuleg but gome 
of special form. , ' 

lack petiole, stip- 

ules, or both. Leaf blades are either 
parallel- veined or netted- veined. In 
the latter case the arrangement of 
veins may be palmate or pinnate. 
When marginal lobes of a blade ex- 
tend to the midrib or to the base of 
the blade, the leaf is. compound. 
Leaflets of a compound leaf may be 
pinnately or palmately arranged. 
Leaflets may themselves be divided. 

Apart from veins, the blade of a 
dicotyledonous leaf usually consists 
of the following tissues: upper epidermis, palisade tissue, spongy 
tissue, lower epidermis. Both lower and upper epidermis may 
contain stomata. A stoma is a space between two guard cells, 

Veins are vascular bundles, continuous with those of the steiE ; 

FIG. 96. Portion of the stem of 
a cactus (Carnegiea gigantea), 
bearing spines and flowers. 
Photograph by D. T. Mac- 


and consisting of the same primary tissues. Surrounding a vein 
is a sheath composed of parenchyma and including, in the cases 
of many large veins, mechanical tissue as well. A petiole contains 
one or more vascular bundles which connect the vascular bundles 
of the stem with the veins of the leaf blade. 

Structural differences between leaves of various plants are due 
largely to the proportional amounts present of the various tissues 
already mentioned. Leaves other than foliage leaves may be scale 
leaves, tendrils, and spines. The separation of leaf from stem is 
due to an abscission layer formed at the base of the petiole. 



69. Importance of Water to Plants. In previous chapters it has 
been noted that water, together with nutrient substances in solu- 
tion, is absorbed by roots and is conducted through the vascular 
tissues of roots, stems, branches, and leaves to all parts of a plant. 
An ample supply of water is necessary for all the activities of the 
plant. The importance of water results in part from the fact that 
it is the liquid in which all or almost all other substances that are 
to be utilized or are to be moved from cell to cell must be dissolved; 
and in part from the fact that water is the largest single constituent 
of protoplasm, which in active cells is ordinarily in a semi-fluid 
condition. The different organs of a protoplast such as dense 
cytoplasm, plastids, and nucleus differ in the amount of water 
they contain, but all parts of a living cell must be nearly saturated 
with water in order to carry on their ordinary functions. The need 
of a water supply is greatly increased by the fact that water is 
constantly being lost by evaporation from cells of all aerial parts 
of a plant, especially from the leaves; and this loss can be com- 
pensated only by an intake of water from the soil or from other 
available source. 

As between various organs and tissues, the amount of water 
present may fluctuate within rather wide limits. In root tips, 
fruits, and young leaves, the proportion of water may be as high as 
90 to 95 per cent. In woody stems the proportion of water is often 
about 50 per cent; in dormant winter buds 40 to 50 per cent; 
and in dormant seeds the proportion may be as low as 10 to 15 
per cent. The greatest proportion of water is required for activities 
connected with constructive processes such as those of growth, 
and the smallest proportion is required for destructive processes 
such as respiration. Water, however, is necessary to all the physical 
and chemical changes that occur within the plant. 

70. Transpiration. Water evaporates from a free water surface 
or from a surface containing water; that is, it changes from a 
liquid state to a vapor and passes into the atmosphere. The evap- 



oration of water from the exposed surfaces of a plant is transpi- 
ration. Since in most familiar plants transpiration is chiefly from 
the cells of leaves, it is in leaves that this process is most readily 
studied. On the other hand, the structure of a leaf is best under- 
stood if the leaf is considered in its relation to transpiration. The 
spongy tissue of a leaf is constantly evaporating water into the 
intercellular spaces, from which the water vapor passes into the 
outer atmosphere, mainly through the stomata. The epidermis of a 
leaf allows some water to pass through it, but the amount of water 
lost to the atmosphere from the epidermis is relatively small in 
land plants because of the presence in and on their walls of cutin. 

That water is lost from the surface of a leaf, and that this loss 
is mainly through the stomata, may be shown by the following 
experiment. A geranium leaf is removed from the plant, its lower 
surface (in which most of the stomata are located) is coated with a 
layer of wax or vaseline, and the leaf is then laid on a table. An- 
other similar leaf cut at the same time, whose upper surface only 
is coated, is placed beside the first. The second leaf will wilt much 
more quickly than the first. The wilting is caused by the loss of 
turgidity of the cells of the leaf; the loss of turgidity results from a 
loss of water. The slower wilting of the leaf whose lower surface 
was coated is due to the fact that most of the water loss was through 
the stomata. In various cases, from 80 to 97 per cent of the water 
lost by transpiration passes through stomata, the remainder being 
lost through the cutinized epidermis. 

71. Amount and Rate of Transpkation. Some conception of the 
extent to which transpiration goes on may be gained by comparing 
the loss of weight from a pot containing a geranium plant in soil 
with the loss from a pot of the same size -containing soil but no 
plant. The soil in both pots is well watered at the beginning of the 
experiment, and both pots are w r eighed. They are weighed again 
at the end of 24 hours. The pot containing no plant will be found 
to have lost some weight because of the evaporation of water from 
the soil. The other pot will have lost about the same amount in 
the same way; but the total loss of weight from the pot containing 
the plant will be much greater. The difference between the losses 
of weight in the two cases is an approximate measure of the loss 1 - 
by transpiration from the leaves of the plant. 

The approximate rate of transpiration may be determined by 
the use of a potometer (Fig. 97). A cut shoot is fitted into an up- 



right tube (A); to this tube an empty horizontal tube (B) is at- 
tached whose free end is bent and immersed in water (C). By 
opening the stop-cock of the vessel D, water is driven into tubes A 
and B. By removing the end of tube B from the water in C and 
allowing the plant to transpire for a short time a bubble of air (E) 
is introduced; the lower end of the tube is then replaced in C. Now, 
as water evaporates from the plant it is replaced by water drawn 

FIG. 97. Potometer, an apparatus used to determine the amount and rate of 
water-absorption and water-loss by a plant during transpiration. 

from tube B; the result is a shifting of the bubble E toward the 
plant. By noting the time required for the bubble to move a cer- 
tain distance, the rate of transpiration may be estimated. If the 
apparatus is placed under different external conditions, the effects 
of the environment on transpiration may be studied. 

The method just described is not a strictly accurate one for 
determining the rate of transpiration because it measures the rate 
at which water is being taken into the plant, and the amount of 
water transpired may be different from the amount absorbed. The 


amount of transpiration and its relation to the amount of water 
absorbed can be determined by standing the apparatus just de- 
scribed on one pan of a balance, the other pan being weighted to 
bring the two to the same level. After a half hour or more the pans 
will no longer be in balance because of the water lost by transpira- 
tion. Such a procedure gives some idea both of the amount of 
water absorbed by the plant and of the amount lost by tran- 

By far the greater part of the water absorbed by plants is lost 
by transpiration, and the amount lost is surprisingly large. Under 
ordinary growing conditions, a square foot of the leaf surface of 
a sunflower transpires about four ounces of water in the course of 
24 hours. In a growing season of 100 days, this would imply a 
loss of 25 pounds of water per square foot of leaf surface. A single 
corn plant growing in Kansas has been shown to remove 54 gal- 
lons or 1 y barrels of water from the soil in a single season, which 
is 90 times as much water as is needed by the plant for all purposes 
except to replace the loss by transpiration. An apple tree 30 years 
old may lose 250 pounds of water in a day, or 36,000 pounds 
during the growing season. At this rate an acre of 40 apple trees 
would transpire 600 tons of water per year. 

Experiments show that the amount of water transpired by 
a plant fluctuates from hour to hour, from day to day, and from 
season to season. Such fluctuations are due largely to variations 
in the external conditions, although conditions within the plant 
and within its individual cells also affect the rate of transpiration. 
Important among external factors that influence transpiration 
are the temperature and the humidity of the surrounding air. 
Winds and air currents affect the process as they affect humidity 
and temperature. Other things being equal, the drier and warmer 
the air the more rapid is transpiration. In very moist and cool 
air transpiration is comparatively slow. Quite apart from the direct 
effects of the sun's rays upon the temperature of the air, the inten- 
sity and quality of light are also important in affecting transpira- 
tion. Green leaves and other green parts of a plant absorb a con- 
siderable portion of the light rays falling upon them. A small 
part of the solar energy thus absorbed is used, as will be seen 
(Chap. IX), in food-manufacture; but in bright light the greater 
portion is changed to heat and increases the rate of transpiration. 

Some of this excess heat vaporizes the water which is then lost 


in transpiration. The leaf or other transpiring organ is thus cooled, 
so that its temperature is kept at or near that of the surrounding 
air. Transpiration, therefore, tends to regulate the temperature 
and to prevent excessive heating of the organs of the plant, al- 
though this is not the principal significance of transpiration. 
As will appear later, transpiration is important also as a factor 
in the transfer to the aerial portions of the plant of water and 
mineral salts absorbed by the roots. 

72. Functions of Stomata and of Intercellular Spaces. The im- 
portance of a large leaf surface lies in the fact that it is necessary 
in the case of a green plant both that a considerable surface be 
exposed to sunlight, and that a large proportion of its cells have 
access to certain gases of the air (carbon dioxide and oxygen) 
which are used by the plant. The large leaves borne by many 
common plants are adapted to meet these needs ; but the presence 
of such leaves increases the danger of too rapid transpiration. 

The aerating system consisting of stomata and intercellular 
spaces permits, as shown in 62, the exchange of gases by dif- 
fusion between the cells inside and the air outside the leaf. In order 
that gases may be absorbed by the plant they must be in solution 
in water. The gases in the intercellular spaces of a leaf pass into 
solution in the water in the cell walls; the dissolved gases may 
then diffuse by osmosis into the protoplasts. At the same time, 
water evaporates from the surfaces of the walls abutting upon the 
intercellular spaces. The air in these spaces thus tends to become 
saturated with water vapor. This water vapor can pass to the 
outside of the leaf only by diffusion through stomata (Fig. 98). 
The further movement of the water vapor from the outside sur- 
face of the leaf depends, among other things, upon the carrying 
away of the water-laden air by winds and other air movements. 
Except for the stomata, the leaf is covered by a continuous layer 
of epidermal cells whose outer walls are in general cutinized and 
so are relatively impervious to water. Because of the saturation 
of the air in the intercellular spaces, loss of water from the cells 
lining these spaces is less rapid than it would be from cells exposing 
an equal area on the surface of the leaf. The whole arrangement 
of stomata and intercellular spaces results, therefore, in greatly 
increasing the area of the leaf that can take in gases from, and 
give off gases to, the air. The location of the stomata, or of most 
of them, in the lower surfaces of most leaves results in the loss of 



less water than would be lost from the same number of stomata in 
the upper surfaces, which are more or less exposed to the direct 
rays of the sun. 

Stomatal transpiration consists, therefore, first, in the evapo- 
ration of water from the saturated walls of cells lining the inter- 
cellular spaces, and second, in the diffusion of the water vapor 






FIG. 98. Diagram showing the paths of movement of water through and out of 
a leaf. The movement of water in liquid form is indicated by black arrows; 
that of water in the form of vapor by light arrows. 

through stomata. The evaporation of water from the walls of 
cells adjoining an intercellular space tends to dry out the walls. 
When this drying begins, the walls of each cell imbibe more water 
from the included protoplast. Withdrawal of water from the 
protoplast increases the concentration of substances dissolved in 
the cell sap, and the cell then tends to draw water by osmosis 
from neighboring cells that contain proportionally more water. 
The latter cells in like manner draw from their neighbors, and 
eventually water is withdrawn from the tracheids and vessels of 
the veins. These elements of the veins are connected by the tra- 
cheids and vessels in the xylem of petiole, stem, and root with 
the cortex of the root. Thus a continuous stream of water is 
made possible through root, stem, and branches to the leaf, com- 


pensating for the loss of water in the form of vapor from the 

The great number of stomata in the epidermis strongly favors 
diffusion of water vapor outward and of gases inward. Although 
stomata occupy but one to two per cent of the surface area of a 
leaf, diffusion through them can go on almost as readily as though 
the interior cells were exposed directly to the outside air. The 
rate of diffusion through an epidermis containing numerous sto- 
mata rarely rises to its possible maximum. 

73. Guard Cells. One striking characteristic connected with the 
functioning of stomata in many plants is the ability of the guard 
cells to undergo changes in turgidity and so to change the sizes 

of the stomatal openings. The 
mechanism controlling the 
movements of guard cells is 
rather complex, and the re- 
sponses of guard cells to en- 
FIG. 99. A~ stoma in cr^s section, vironmental conditions are also 
showing its opening and closing in complex. In general, when 
consequence of changes in the turgid- guard ce j ls are turgid they are 
ity of the guard cells. The thick walls & . . . A , . , . , ~ 
of the guard cells in the open (turgid) arched and the included stoma 
condition are indicated by diagonal is wide open, but when they 
shading; in the closed (non-turgid) are not turgid they stra ighten 
condition, by stippling. Adapted from , , , . , . . 

Schwendener. anc * c * ose or re duce the size of 

the stoma (Fig. 99). The tur- 
gidity or non-turgidity of guard cells is, however, affected by a 
number of factors, chief among which are the intensity of illumi- 
nation and the water content of the leaf, especially of its guard 
cells. In most cases, stomata are open in light and closed in 
darkness. When water is abundant in the leaves the stomata 
are usually open; they are usually closed when water is deficient. 
The behavior of guard cells varies in different kinds of plants. 
In some common plants, such as the potato, cabbage, and beet, 
stomata are usually open both day and night if the water supply 
is abundant. On the other hand, in cereals, such as wheat and 
oats, stomata are always closed at night, and may even close in 
the daytime if there is a slight deficiency in the water content of 
the plant. In very many plants the behavior of the stomatal ap- 
paratus is intermediate between the extremes just mentioned. 
In leaves with stomata on both sides, those in the upper surface 



open later and for a shorter time than those in the lower surface. 
Stomata near the tip open later and close earlier than those near 
the base of a leaf. So many factors affect transpiration, however, 
that this process is not always most rapid when the stomata are 
most widely open; and therefore changes in shape of guard cells 
are not so efficient as might be 
imagined in regulating transpi- 

74. Means by Which Tran- 
spiration Is Checked. When 
the stomata are completely 
closed transpiration is stopped. 
A decrease of 50 to 75 per cent 
in the diameter of stomata ap- 
parently affects transpiration 
but slightly; a further decrease 
in diameter, however, results in 
a perceptible reduction of the 
rate of transpiration. 

The shape and size of a leaf 
markedly affect the amount of 
water transpired. Some plants 
of dry regions, like the century 
plant and the aloes (Fig. 87), 
have large, thick leaves. Tran- 
spiration from such leaves tends 
to be slow, because even though 
the area of the leaves may be 
considerable it is small in pro- 
portion to their volume. In 
some plants, such as certain 
cacti (Fig. 100), the loss of water is relatively low because the 
leaves are small or absent, or in some cases are spine-like. Some 
desert shrubs and trees have leaves during the rainy season but 
shed them in dry periods. Another extreme is illustrated by plants 
that, like Elodea, live submerged in water, in which situation tran- 
spiration is practically impossible. Such plants ordinarily have 
very thin leaves. The sunflower, like many common plants, has 
large leaves which, however, are fairly thick, and are adapted by 
their structure to limit materially the loss of water by transpiration. 

FIG. 100. A cactus (Camegeia gi- 
gantea), adapted by its structure 
to life in a desert. Photograph by 
D. T. Macdougal. 



Impregnation of the outer walls of the epidermis with cutin 
tends to check transpiration. Cutinized walls are characteristic of 
epidermal cells of leaves and stems that are exposed to the air. In 
some leaves so much cutin is present that it forms a thin layer 
(cuticle) on the surface. In consequence of the presence of cutin 

little transpiration 
takes place from epi- 
dermal cells, which 
otherwise, since they 
are directly exposed to 
the air, would lose rel- 
atively large amounts 
of water. A coating of 
wax, found on such 
fruits as those of ap- 

Guard Cells Cuticle pies, plums, and mel- 

FIG. 101. Cross section of a portion of the lower ons, and even on such 
surface of a leaf of the rubber plant. The i pflvps am a sfpms as 
heavily cutinized epidermis and the sunken leaVCfe anQ &lems aS 
stornata tend to limit transpiration. those of the cabbage, 

also checks epidermal 

evaporation. Cork acts in the same manner as cutin and wax. 
The presence of hairs on the surface of a leaf or stem, so conspicuous 
in the mullein, also may limit transpiration. The location of 
stomata at the bases of pits, as in the leaves of the pine and of the 
rubber plant (Fig. 101), has a similar effect. 

Any condition within a protoplast which tends to retain water 
or to prevent its being imbibed by the walls hinders transpiration. 
Living matter, because of its imbibing power, always offers a con- 
siderable resistance to the removal of water which it contains. 
The proportion of water in the walls depends, therefore, upon 
whether or not their imbibing capacity is greater than that of the 
included protoplast. The degree of saturation of cell walls is in- 
fluenced also by the osmotic concentration of substances in solu- 
tion in the cell sap, which may tend to retain water against the 
imbibitional force of the walls. On the other hand, when a cell is 
fully turgid, the turgor pressure within its protoplast may be so 
great as to press out water into and through the wall, so that the 
outer surface of the wall may become covered with a film of water 
which then evaporates into an intercellular space. Hence the rate 
of transpiration may be increased or decreased, regardless of the 


size of stomata, by any condition or change in condition of the 
living matter which affects turgor or the capacity of a cell to take 
up water (suction tension); by a deficiency of water in the cell 
owing to excessive transpiration, or by an insufficient supply of 
water from the soil. 

In the cells of thick, fleshy stems or leaves, substances of a 
mucilaginous nature are often present, which tend to imbibe and 
retain water. As already mentioned, the leaves of Russian thistle 
(Fig. 86), the century plant, and the aloe have internal water- 
storage tissues which can hold large quantities of water. Such 
plants usually have root systems near the surface of the soil which 
absorb water quickly after a rain, the water being collected in the 
stem and leaves. This water is lost very slowly, arid some plants 
with adaptations like those just mentioned can live for months 
without an external water supply. 

Generally speaking, plants native to regions or conditions in 
which the supply of available water is very limited, such as the 
semi-deserts of the southwestern United States, possess the most 
highly developed means of checking transpiration; whereas plants 
living in habitats permitting access to an abundance of water 
rarely have special means of hindering transpiration. Plants whose 
structure fits them to live in deserts and other very dry localities, 
or elsewhere where water available to the plant is very limited, 
are xerophytes; those fitted for life in water or under extremely 
moist conditions are hydrophytes; and those which stand midway 
between these two classes, being suited to approximately average 
conditions with reference to a supply of water, are mesophytes. 
The differences between xerophytes, mesophytes, and hydrophytes 
illustrate the general rule that living organisms are adapted by 
their structure and functions to existence in particular types of 

75. Water in the Soil. The chief source of soil water is rain. 
Much of the water falling as rain runs off and some of it evaporates, 
but some of it enters the soil by gravity between the soil particles. 
The amount of gravitational water which thus enters the soil and 
its rate of entry depend upon the nature of the soil, the kind and 
amount of vegetation on the surface, the amount of precipitation, 
the slope of the land, and the amount of water already in the soil. 
After a rain some of the water gradually sinks under the influence 
of gravity until it arrives at the water tabk, the level at and below 


which all the spaces in the soil are filled with standing water. As 
water enters the soil and passes downward, much of it is retained 
at each level for a time in the form of thin films about the soil 
particles as well as in the more minute spaces between them. This 
capillary water adheres so closely to the soil particles that it is not 
influenced by gravity. The films about adjacent soil particles are 
united, forming a continuous water system which is the source of 
most of the water absorbed by plants. As water evaporates from 
the surface of the soil or is absorbed by plants, capillary water 
moves from adjacent regions where the films are thicker to regions 
where they are thinner, this movement tending to bring the film- 
system into equilibrium. Loss of water from the surface of the 
soil causes an upward movement of capillary water; the effect of 
this upward movement may extend as far down as the water 
table, which is then lowered. Lateral movements of capillary 
water in the soil occur also from regions where water is more abun- 
dant to regions where it is less abundant. The mobility of capillary 
water varies, however, with the character of the soil. 

In soils that have lost their capillary water by evaporation, 
there still remains about each soil particle a thin film of water 
which is held so firmly that it can be removed only by heating the 
soil to a relatively high temperature. This hygroscopic water ad- 
heres so tenaciously to the soil particles that plants are unable to 
absorb it. 

The water in the soil is not pure but contains many dissolved 
substances, some derived from humus and others from inorganic 
rock particles. Among these dissolved substances are compounds 
of nitrogen, sodium, potassium, calcium, magnesium, iron, phos- 
phorus, arid sulphur, all of which are significant in the nutrition 
of plants. The soil water also contains many other solutes that 
are less important to plants. The actual proportions of the sub- 
stances in solution in the soil water are usually small, although in 
some soils relatively large amounts of solutes are present. In some 
cases much lime is present; "alkali" soils are rich in soluble salts. 
76. Air in the Soil. If the spaces in the soil are filled with water, 
most of the air present is driven out. Ordinarily when water enters 
the soil it passes downward or evaporates, and the spaces between 
the soil particles not occupied by water become filled with air. 
The proportion of carbon dioxide increases and that of oxygen 
correspondingly decreases with the depth of the soil. The texture 



of the soil influences the rate at which oxygen enters and carbon 
dioxide escapes. Since oxygen is necessary for the respiration and 
growth of the roots of most plants, an adequate supply of air in 
the soil is of great importance. In poorly aerated soils not only 
does the absence of oxygen retard growth, but the concentration 
of carbon dioxide exerts a toxic action which hinders the growth 
of roots. The reactions of different plants to the aeration of the 
soil vary greatly. Most cereal grains and some other plants turn 
yellow and die when the soil in which they grow is saturated with 
water for a lon^ period because the presence of water in the spaces 
between the soil particles excludes the air. 

77. Absorption of Water and Solutes. If young roots are re- 
moved from the earth, some of the soil adheres to them. This is 
because the root hairs have grown against and about the soil 

FIG. 102. Diagram showing the relation of root hairs to the soil. The con- 
centric lines about the soil particles show the distribution of water in the 
soil; the clear areas represent spaces filled with air. 

particles, becoming firmly attached to them and so being brought 
into intimate contact with the water about the particles (Fig. 102) 
and with organic and inorganic substances, including gases. Roots 
can absorb much of the capillary water, but can not absorb the 
thin films of hygroscopic water held about each soil particle. Root 
hairs greatly increase the absorbing area of roots. 

Generally speaking, root hairs are borne on a limited zone 1 to 
4 mm. in length near the tip of a root. As a rule, only a limited 
number of the epidermal cells in this zone bear root hairs; but the 


other epidermal cells in the root-hair region, and those in the 
region immediately back of the root cap, also play a part in ab- 
sorption. The absorption of water and dissolved substances by 
roots is determined in part by the laws of diffusion. Water is im- 
bibed by the walls of the root hairs and of the adjacent absorbing 
cells. In consequence, the water on the outside is continuous 
through the walls with the water on the inside of the cells, even 
through the plasma membranes. The liquid in the soil outside 
each absorbing ceil is a dilute solution of very many substances. 
The cell sap is also a solution, usually on the whole of greater 
concentration than the soil solution. In consequence of the dif- 
ference in concentration and of the presence of a differentially 
permeable plasma membrane, water and its solutes in the soil will 
tend to diffuse through the cell wall and the plasma membrane 
into the protoplast. The solutions inside and outside an absorbing 
cell contain largely different substances; or, if a given substance 
is present in both solutions, it is likely to be in different concen- 
trations within and without the cell. As a result of such differences 
an osmotic interchange of great complexity may go on between 
the cell sap and the soil solution. 

The entrance of water into root hairs and adjacent cells depends 
also upon the suction tension ( 18) of these cells. Water will 
enter the protoplast of an absorbing cell of a root and distend the 
cell wall until the pressures within the cell balance the backward 
pressure of the elastic wall, when no more water will enter the 
cell. Under these conditions the cell has lost its suction tension 
and can absorb no more water whatever the osmotic concentration 
of the cell sap. If, however, the turgor pressure of the absorbing 
cell is diminished, its suction tension correspondingly rises. 

Water lost by evaporation from the aboveground portions of 
a plant is replaced by that absorbed by the roots. The loss of 
water by evaporation tends to increase the concentration of dis- 
solved substances in the cells of the roots including the absorbing 
cells, and thus to increase the tendency of water to pass into root 
hairs and adjacent cells, provided these cells are not fully turgid 
and possess a degree of suction tension. The results of these rela- 
tions are an entry of water into roots and an absorption from the 
soil of water and various solutes, many of which are later utilized 
by the plant; at the same time, other substances dissolved in the 
cell sap of root hairs and adjacent cells are given off to the soil. 


The rate of water-absorption by roots varies with the rate of 
loss by evaporation of water from the aerial parts of the plant, 
with the extent of the root system, with the character of the soil 
and its solutions (some of which may be injurious), with the amount 
of water in the soil, and with the condition of the absorbing cells. 
Low temperatures retard absorption. Absorption is influenced 
also by respiration, which depends upon the presence of oxygen 
in the soil. 

78. Root Pressure. This term designates the pressure exerted 
by roots under 3ertain conditions in forcing water into and up the 

FIG. 103. Diagram showing the path of water-movement from the soil 
through a root hair and the internal cells into the conducting elements 
of a root. 

stem, as is evidenced by the exudation of water from water pores 
and from wounds. As has been seen, water can enter root hairs 
and adjacent epidermal cells when the osmotic concentration of 
the cell sap is higher than that of the soil solution. The entrance 
of water into, and its passage through, the cells in the cortex and 
stele of a root are influenced by the suction tension of these^ cells 
as well as by osmotic relations. Water passing through these cells, 
finally reaches the conducting tracts. Only when the suction 
tension of the cells in the interior of a root is greater than that 
of the cells farther outward will the interior cells draw water from 
the outer ones. In such a series of cells as those labeled A to M 
in Figure 103, the suction tension is considered as increasing from 
A toward M. The cell M , having a higher suction tension than 
Z/ f may take water from L until M becomes so turgid that its 



suction pressure is reduced to equal 
that of L. In like manner water 
may be drawn from cell to cell pro- 
gressively from A toward M, provid- 
ing that the suction tensions of the 
intermediate cells are progressively 
higher from A toward M, as, it is 
actually observed, may be the case. 
When the water reaches any cell 
from // to M, adjoining a vessel N, 
it may enter the adjacent vessel. 

By the process just described, a 
considerable quantity of water and 
dissolved substances may enter the 
cortex of a root and thence pass into 
the stele, whose tissues are rather 
rigidly confined by the surrounding 
endodermis. The cells of the endo- 
dermis function to a certain extent 
in preventing a backward movement 
of water and of dissolved substances 
into the cortex. A considerable pres- 
sure (root pressure) therefore develops 
within the stele, which may be suffi- 
cient to force water and dissolved 
substances into the conducting ele- 
ments of the xylem, where they are 
free to move upward. 

The existence of a root pressure is 
easily demonstrated by removing the 
top of a vigorous single-stemmed 
plant a short distance above the soil 
and slipping over the cut stump a 
piece of rubber tubing into which has 
been inserted a glass tube, arranged 
with a device to measure the force 
exerted by root pressure (Fig. 104). 

Water will exude from the cut surface of the stem and be forced 
upward in the glass tube. The exudation of water and the 
rise of the water in the tube result from a pressure in the steles o^ 

FIG. 104. Experiment dem- 
onstrating root pressure. 
The stem of a potted cut- 
ting of a geranium was cut, 
and its upper part was re- 
placed by a bent closed 
tube containing mercury. 
Water exudes from the cut 
end of the stem, forcing the 
mercury upward in the 



the roots. The magnitude of 
root pressure and the quantity 
of water exuded vary in differ- 
ent plants and under different 
conditions. The exudation 
from woody stems when cut or 
wounded is greater than that 
from herbaceous stems. 

79. Loss of Water in Liquid 
Form. Under certain condi- 
tions some plants give off water 

as a liquid. At the ends of Fm m Water pore at ^ tip of a 
the veins of the leaves of such leaf of Fuchsia. Water in liquid form 
plants as the cabbage, nastur- exudes from such a pore, 
tium, and Fuchsia are large pores (Fig. 105) which differ from 
ordinary stomata in that they always remain wide open. Im- 

FIG. 106. The exudation of droplets of water from water 
pores at the tips of the leaves of wheat. 

mediately beneath each water pore is a loose tissue devoid of 
chlorophyll, which is in contact with the end of a vein. When 
there is an abundant supply of water, and transpiration from 
such a leaf is limited, water in liquid form escapes through these 



pores. In the strawberry and primrose, water pores occur at the 
tips of the teeth of the leaves. Similar pores are present also at 
the tips of the leaves of most grasses, such as wheat (Fig. 106) and 

The process of exudation of water from water pores may 
be easily observed if a pot of young, well-watered wheat or 
barley plants is covered with a bell-jar. In a short time, 
drops of water appear on the tips of the leaves at the ends of 
the veins. 

Many plants exude liquid water from nectaries (Fig. 107) and 
other glands. The water thus lost contains substances in solution. 

The process by 
which water is given 
off from glands is 
not well understood, 
but it is probably 
connected with the 
osmotic activities of 
particular cells in, 
or in the neighbor- 
hood of, the glands. 
Plants lose water 
and solutes from 
wounds also, and 
from the cut ends 

Nectarie8 branches. Large 

FIG. 107. Flower ofjhe^buckwheat, with nectaries quantitieg of water 

may escape during 

late winter and early spring from such plants as the grape and 
maple. The sap exuded by wounded stems is never pure water 
but always, as in the case of the sugar maple, contains solutes. 
The exudation of water and solutes from wounds is often called 
bleeding. Under conditions favoring absorption of water from 
the soil, if transpiration is limited, water is forced into and up 
the xylem of root and stem under considerable pressure and 
exudes from water pores or wounds. Bleeding is not, however, 
always due to such root pressure. In the maples, for example, 
root pressure is very infrequent and bleeding is attributed to the 
activity of living cells in the neighborhood of the wound. 


80. Movement of Water in Stems. When water is absorbed by 
one part and given off by a distant part of the same plant, a 
movement of water must take place in the intervening regions. 
In a very small plant the water transpired may be replaced merely 
by osmotic movements from cell to cell the force concerned in 
these movements being equivalent, not to the osmotic pressure 
of the cell sap, but to the difference in suction tension between the 
cells concerned. The conditions in such a plant are very different, 
however, from those in a sunflower or in a tree. Osmotic move- 
ment of water from cell to cell can not possibly suffice to replace 
the amounts lost by transpiration in these larger plants. The 
great resistance offered to the passage of water from cell to cell, 
and the great distances to which water is conducted in large plants, 
imply the existence of some means for rapid conduction. The 
long conducting elements of the xylem permit the movement of 
water and of substances in solution as a mass and at a more rapid 
rate than would be possible if there were many interposing mem- 
branes to be traversed. The movement of water is mainly in the 
cavities of tracheids and vessels. Water moves in those tracheids 
and vessels only which are unobstructed and entirely filled with 
water. The mass of water and solutes moving in the conducting 
elements of the xylem is the transpiration stream, and the move- 
ment of this mass is the sap flow. 

That the path of the transpiration stream is in the xylem may 
be shown by the following experiment. Take four potted her- 
baceous plants of a type in wiiose stem the vascular bundles are 
separate. From the stem of one plant the pith is removed for a 
distance through an opening cut in the cortex; from the stem of 
one the xylem is removed, from another the phloem, and from 
still another the cortex. If the wounds are protected against dry- 
ing, as with w r axed or oiled paper, the leaves will wilt only in the 
specimen whose xylem strands have been interrupted. 

The path of the transpiration stream may be shown also by 
cutting off the shoot of a plant which, like Impatiens, has a trans- 
lucent stem, and placing the shoot in a solution of a dye such as 
eosin or fuchsin. The general course of the dye can be traced 
through the stem, the petioles, and the veins of the leaves. Micro- 
scopic examination of cross sections of the stem will show that 
the only elements containing the dye are those of the xylem. 

But not all the vessels and tracheids of the xylem necessarily 


conduct water. As has been seen ( 42), the water-conducting 
elements in trunks of trees become blocked when sapwood changes 
to heartwood. Hence there can be no movement of water 
through heartwood. Conducting elements even of the sapwood 
may become partly or completely filled with gas at certain 
seasons of the year, in which case there is no conduction through 
these vessels and tracheids. Such blocking of conducting ele- 
ments with gases is often so extensive that half of the vessels 
and tracheids in an annual ring of sapwood are temporarily non- 

There is a general relation between the volume of transpiration 
and the amount of conducting tissue developed by a stem. Sub- 
merged water plants have relatively few vessels and tracheids. 
When plants of the same species are grown on land, or when water 
plants bear branches and leaves above the surface of the water, 
a proportionally greater amount of xylem is produced. 

81. Forces Concerned in Sap Flow. In some trees water is 
raised to great heights. Such common trees as oaks, maples, 
and elms often reach heights of 50 to 100 feet. The redwood and 
some other timber trees of the Pacific Coast states grow to 200 
feet or more. The amount of work necessary in lifting enough 
water to these heights to supply transpiration needs is very great 
sufficient not only to raise the water, but also to overcome the 
resistance encountered in its passage. 

The conducting elements, being tubes without protoplasm, 
can in themselves exert neither suction tension nor any other 
force that may cause the movement of water. Capillarity is the 
force which causes water to rise in a slender glass tube, and might 
therefore be considered responsible for the rise of water entering 
the very fine tracheids and vessels. However, capillarity operates 
only where there are free surfaces in open tubes. Within tracheids 
and vessels which are entirely filled with water no free surfaces 
are present. Even if they existed, water could not rise in such 
elements higher than about 30 cm. Therefore, capillarity is en- 
tirely inadequate to explain the height to which water is lifted in 
many trees. 

Root pressure plays a minor role in bringing about the rise of 
sap in stems. It has been shown that the parenchymatous cells 
of the stele force water into tracheids and vessels with which they 
are in close contact (Fig. 103). Root pressure, however, is not of 


prime importance in forcing water upward in a stem, since it is 
least when transpiration and water movement in the stem are 
most rapid. 

The force which chiefly accounts for the ascent of sap is that 
exerted by the transpiring cells of the leaves. As has been seen, 
a living cell of a leaf, because of the presence of dissolved substances 
in its cell sap, exerts a suction tension by virtue of which it tends 
to draw in water. As water evaporates from the leaf the suction 
tension of the transpiring cells rises; as a result, they take water 
from the adjoining cells within the leaf, and these in turn draw it 
from still more deeply lying cells. Ultimately the suction force is 
applied to the columns of water in the tracheids and vessels of the 
veins of the leaf, which are connected through the petiole with 
xylem elements in the branch; the latter are connected with xylem 
elements in the stem or trunk, and these in their turn with elements 
of the xylem in the root. In effect, there are continuous columns of 
water in the xylem elements of root, stem, branch, and leaf; and 
upon the upper ends of these columns the force generated in the 
leaf cells exerts an upward pull. 

The suction force so exerted by the cells of leaves is found to 
run from 5 to 10 atmospheres, and in some cases very much higher. 
Since a suction force of one atmosphere can raise a column of water 
about 30 feet, it is evident that the force exerted by leaf cells is 
sufficient to raise water to the height of the tallest trees if that 
force is efficiently applied, and if the columns of water in the con- 
ducting tracts are not broken. 

82. Tensile Strength of Water. Contrary to what would be 
imagined from ordinary experience with water, experiment shows 
that a column of water has great tensile strength. When the tensile 
strength of a bar of metal is tested, the force required to break it is 
considered equal to the force of cohesion of its molecules. In 
liquids the molecules are more mobile and their cohesive force is 
more difficult to measure. However, by means of proper apparatus 
it is shown that the molecules of water enclosed in a tube tend very 
strongly, like those of a metal bar, to cohere. Because of the cohe- 
sion of the water columns in the xylem, the pull exerted by the leaf 
cells lifts the columns. In a tree 100 feet tall, the effect of the pull 
is felt for 100 feet plus the distance to the most remote root. The 
pull upon the water columns is continuous so long as transpiration 
is active. If intake of water by the roots is ample, a stream of 


water is kept flowing continuously upward through the xylem to 
the leaves. 

83. The Water Balance in Plants. By far the greater part of the 
water absorbed from the soil by a plant is lost as water vapor. 
From what has been said it is evident that the movement of water 
in the plant is affected by the rate and amount of absorption of 
water from the soil and by the rate and amount of transpiration. 
Any condition in the soil, such as its proportional water content or 
the concentration of solutes in the soil water, usually affects the 
rate of transpiration. However, transpiration and absorption are 
not always directly proportional, and water may be transpired by a 
plant more rapidly than it is absorbed by the roots. Conversely, 
water may be absorbed more rapidly than it is lost by transpiration. 
When water is lost more rapidly than it is absorbed, the water 
balance is changed and a deficit exists. A considerable water 
deficit is likely, therefore, to exist in a plant during periods of ac- 
tive transpiration. About midday on bright, sunny days many 
plants lose considerably more water than they can absorb; the 
resultant water deficit in a sunflower plant may reach 28 per cent 
of its maximum water content. Fluctuations in the water content 
of leaves of common plants (such as wheat, sugar beet, or pumpkin) 
may under some climatic conditions be as great as 20 per cent. 
Even in plants with an extensive water supply arid whose tran- 
spiration is limited, a daily deficit may occur. When transpiration 
is excessive, as on very warm, dry days, a plant can not make up 
the water deficit during the night, so that it may still have a deficit 
the next morning. 

With continued^ drought and consequent lack of water in the 
soil, the daily water deficit in the plant increases. The loss of 
turgidity in the cells leads to a loss of rigidity, and the leaves and 
young stem and branch tips droop or wilt, although elsewhere in 
the plant the water content may be relatively high. Such wilting 
often occurs at midday when transpiration is active ; but ordinarily 
toward evening, when transpiration decreases, the water deficit 
is restored and the plant recovers its rigidity. This occurs even 
without any appreciable absorption of water from the soil, the 
water already in the plant being redistributed. Under certain con- 
ditions the water content may become so depleted that the plant 
recovers from a wilted condition only with difficulty if at all. 
Permanent wilting occurs when soil water is no longer available 


to the plant, and the living cells of all organs, including root hairs, 
gradually lose their turgor. After permanent wilting the plant 
absorbs water, if at all, very slowly from moist soil, and not until 
new root hairs are formed. In most plants, when the leaves have 
wilted, the guard cells lose their turgidity and close the stomata, 
thus interfering with an exchange of gases between the inside and 
the outside of the leaf. During permanent wilting, structures such 
as chloroplasts are injured, the result being usually permanently 
harmful to the plant. 

In some plants special water-storage tissues or regions, occurring 
in certain leaves and in stems adapted to hold water like those of 
cacti, may contain sufficient water for considerable growth and 
transpiration; but unless water is supplied from other sources, even 
plants with such structures eventually perish. Some plants growing 
in dry regions, because of their power of storing and retaining 
water, can live for months without an external supply. 



84. Plastids: Chloroplasts. An important part of the work of 
food-manufacture in living cells is carried on by bodies of a class 
known as plastids. The plastids of mature cells, at least in the 
higher plants, are developed from the chondriosomes ( 21) of 
embryonic cells. As an embryonic cell becomes mature, some (by 
no means all) of its chondriosomes enlarge and take on the 

characteristics of plastids. Those 
plastids which are green in color 
are chloroplasts (Fig. 108). Cer- 
tain types of plastids other than 
chloroplasts will be discussed later. 
The chloroplasts of most of the 
larger land plants are usually 
small and spheroidal, ellipsoidal, 
or (if crowded) polyhedral; the 
number in a cell varies from one 
to many. In green food-manu- 
facturing tissues, such as the 

^, , A palisade tissue of a leaf, many 

FIG. 108. Chloroplasts. A, from the , , , , , . T. 

leaf of Elodea, each chloroplast chloroplasts are present in each 
containing l starch grain. B, cell. A chloroplast appears to be 
from the leaf of a moss, each o f a S p On gy nature; its frame- 
work is not fundamentally differ- 
ent, except for its firmer consist- 
ency, from the emulsion-like structure of the surrounding dense 
cytoplasm. A continuous sheath of dense cytoplasm immediately 
surrounds the chloroplast. The center of the body is less dense, 
or vacuolate, and may contain a single large vacuole (Fig. 109). 

85. Chlorophyll. The green color of chloroplasts is due to the 
presence of two green pigments, which together are called chloro- 
phyll. Little is known as to just how these pigments are held in a 
chloroplast. Apparently they exist in a finely divided state in the 
peripheral framework. That the pigments are distinct from the 


chloroplast containing 1 or more 
starch grains. 


chloroplast is shown by killing a leaf in boiling water and then 
immersing it for some time in alcohol. The alcohol becomes green r 
and when the leaf is removed it is found to be colorless, the chloro- 
phyll having been dissolved in the alcohol. Microscopic examina- 
tion discloses the now colorless chloroplasts still present in the 
cells of the leaf. Chlorophyll is a mixture of chlorophyll a, which 
is blue-green, and chlorophyll b, which is yellow-green. These pig- 
ments ordinarily occur in the 
proportion of about 72 per 

cent of chlorophyll a to 28 ~"~\ Portion" Pore 

per cent of chlorophyll b. 

Chlorophyll and the other 
pigments contained in chloro- 
plasts are insoluble in water 
and in the cell sap, but are 
soluble in alcohol, ether, ace- 
tone, and various other liquids. ^L^Z^J C 
The extract obtained from a / \ 
leaf includes, besides chloro- Starch Grain Vacuole 
phyll a and b, certain yellow FIG. 109. Diagram of a chloroplast (in 
pigments. If an alcoholic leaf section), showing distribution of chlo- 

, ,. i-i^j -,,1 4. rophyll. Modified from Zirkle. 

extract is diluted with water 

and then benzol is added, the benzol soon rises to the top of the 
mixture as a sharply defined, deep green layer, while the water 
and alcohol below show a pale yellow or straw color. The green 
color at the top is due to the two chlorophylls; the yellow color at 
the bottom to the yellow pigments. Of the yellow pigments 
occurring in plastids, the best known are carotin, usually deep 
yellow or orange in color, and xanthophyll, which is light yellow or 
lemon-colored. The presence of carotin and xanthophyll affects 
the color of leaves, which is yellow-green rather than pure green. 
When plants are grown in darkness, the plastids lack chlorophyll 
but contain yellow pigments. In some plants growing in the light, 
chlorophyll fails to develop in the plastids of certain cells. In the 
silver-leaf geranium chlorophyll-development is limited to certain 
areas of the leaf blade, other areas being white or yellowish white 
(Fig. 113, A). Sometimes a whole leaf or branch is entirely devoid 
of chlorophyll. The plastids in the lighter portions of such a plant 
contain carotin and xanthophyll, whereas in the plastids of the 
green areas chlorophyll also is present. 



86. Formation of Chlorophyll. Except in some very simple 
plants, chlorophyll occurs only in definitely organized chloroplasts. 
It is manufactured by the living matter of the cell, apparently by 
that portion of the cytoplasm which constitutes the chloroplasts. 
The production of chlorophyll is dependent upon various factors 

both within and without the 

Light is one of the factors 
necessary to chlorophyll-for- 
mation, which occurs in the 
presence of light of most wave 
lengths (that is, of most col- 
ors). It is much more affected 
by the intensity than by the 
wave length of light, although 
for light of equal energy values 
the red rays are more effective 
than the green, and the green 
are more effective than the 
blue. A medium intensity of 
light is most favorable for the 
production of chlorophyll. 

The chlorophyll formed 
within a chloroplast is not 
stable but breaks down within 
a relatively short period. This 
fact is not evident in plants 
growing in the light, since the 
disintegrating chlorophyll is 
being continually replaced by 
that which is newly formed. 
It becomes evident, however, 
when green plants are placed in darkness, since the plant is then 
unable to form new chlorophyll. In sufficiently intense light, 
although its formation and decomposition occur simultaneously, 
chlorophyll is destroyed faster than it is formed, whereas in diffuse 
light the reverse is true. 

Plant parts which would be green if grown in light, when grown 
in darkness do not form chlorophyll and hence are whitish or pale 
yellow. Such parts contain some of the yellow pigments, especially 

FIG. 110. Potato shoots grown in dark- 
ness (left) and in the light (right). 



carotin and xanthophyll. Plants grown in darkness, as compared 
with those grown in light, also frequently show marked differences 
in the forms of their aerial organs; these differences, together with 
the absence of green color, are summed up under the term " etiola- 
tion. " Etiolated shoots of the potato (Fig. 110) or of the bean have 
long, slender internodes, elongated petioles, and small leaf blades. 
Etiolated shoots of wheat / barley, or corn have greatly elongated 
leaves, and sometimes the internodes are 
unusually long. 

Chlorophyll is formed only within a rel- 
atively narrow range of temperature. In 
etiolated plants brought into light, chloro- 
phyll is produced most rapidly between 18 
and 30 C. A chlorophyll molecule, whether 
of chlorophyll a or 6, contains the elements 
carbon, hydrogen, oxygen, nitrogen, and 
magnesium. The presence of each of these 
elements, therefore, in appropriate com- 
pounds is necessary to the production of 
chlorophyll. Carbohydrates also must be 
present if chlorophyll is to be formed. Al- FIG. 111. 
though iron is not a constituent of the formed 
chlorophyll molecule, its presence is neces- 
sary for chlorophyll-formation, and plants 
grown in the absence of iron are yellowish. Variations in the 
amounts of calcium, sulphur, potassium, manganese, and phos- 
phorus present, as well as the absence of iron, also may cause a 
yellowing, although none of these elements enters into the com- 
position of chlorophyll. The presence of a large number of ele- 
ments and compounds, therefore, favors chlorophyll-formation; 
on the other hand, the presence in too great quantities of some 
substances, one of which is common salt, impedes the process. 

87. Plastids Other than Chloroplasts. Leucoplasts are similar 
in appearance to chloroplasts, except that they contain no pig- 
ments. Lacking chlorophyll, they can not carry on photosynthesis; 
they can, however, like chloroplasts, manufacture starch from 
sugars (Fig. 111). They occur chiefly in parts of plants not ex- 
posed to light, in which starch is stored; such as the cortices of 
aerial stems and various tissues of underground stems and roots. 
They are particularly abundant in many seeds and fruits, as ia 


Starch grain 
in a leuco- 

plast; from a potato 



those of the cereal grains. Not infrequently, if such an underground 
organ as a young potato tuber is exposed to light, chlorophyll 
appears in the leucoplasts which thereby become chloroplasts. In 
this case, whether a particular plastid is to be a leucoplast or a 
chloroplast is determined .by external conditions, particularly by 
the presence or absence of light. Some colorless plastids, how- 
ever, like those in the marginal cells of the silver-leaf geranium, 
can not under any conditions become chloroplasts. 

Chromoplasts lack chlorophyll but contain some of the other 
pigments, especially carotin and xanthophyll. While carotin and 

xanthophyll are typically 
yellow or orange, they 
may vary in depth of color 
to orange-red, brick red, 
or reddish brown. These 
colors occurring in flow T ers, 
fruits, and various other 

organs are due usually to 
FIG. 112. Chromoplasts in a cell of a fh nrospnofi o f -hromo- 

n f , i A fj cij. i Lilt/ LJ1 v/ov>llUt/ vjl L/11J wlllvl 

flower of nasturtium. After Strasburger. * 

plasts. Chromoplasts are 

more variable in form than are either chloroplasts or leucoplasts. 
In the cells of some of the floral parts of the nasturtium (Fig. 112), 
the chromoplasts are angular and orange-red. Those in the fruit 
of the climbing bitter-sweet are crescent-shaped and reddish 
brown. Other fruits whose colors are due to the presence of chro- 
moplasts are those of the tomato, rose, and red pepper. ., 

Elaioplasts, whose particular function seems to be the storage 
of fats, occur in some plants. 

88. Photosynthesis* As has been seen, elements unite to form 
simple compounds; these may be combined into more complex 
substances. One particular process which is fundamental to the 
activities and continued existence of plants is the combination of 
carbon dioxide and water into a sugar. This combination is brought 
about only in the chlorophyll-containing cells of living plants, and 
only in the presence of light. Since it involves a putting together 
(synthesis) of simple substances into one that is more complex, and 
since light is essential, the process of thus combining carbon di- 
oxide and water is called photosynthesis. 

Photosynthesis is a complex process whose early stages are not 
certainly known. Carbon dioxide and water are decomposed, and 


the products of their decomposition are recombined. In the course 
of these changes oxygen is given off. The first product of photo- 
synthesis that can be detected is usually a sugar, most commonly 
glucose (C 6 Hi 2 6 ). It is probable that a relatively simple com- 
pound, such as formaldehyde (CH 2 O), is first produced from 
carbon dioxide (CO 2 ) and water (H 2 0), and that molecules of 
this substance are then combined to form glucose. Whatever the 
intermediate stages, the end result of photosynthesis may be ex- 
pressed by a formula like the following: 

6 C0 2 + 6 H 2 = C 6 H 12 6 + 6 2 . 

In any event, the substances formed in photosynthesis are 
carbohydrates, the basic substances from which all other organic 
compounds in animal and plant bodies are produced. Glucose, 
like other sugars, is a food that may itself be used by the cell 
which formed it, in the building up of other compounds; or it may 
have the effect of maintaining or increasing the osmotic concen- 
tration of the cell contents. Being readily soluble in the liquids 
of a plant, glucose may diffuse from cell to cell in solution. Its 
diffusion is slow, however, and when photosynthetic activity is 
considerable, glucose (or another sugar) is formed more rapidly 
than it diffuses away from the cell in which it is made. 

An accumulation of large quantities of a sugar greatly increases 
the osmotic concentration within a cell and thus interferes with 
various activities of the living matter. This difficulty is obviated 
in many plant cells by a change of glucose into another carbohy- 
drate which is not readily soluble in water. The insoluble carbo- 
hydrate into which sugars are most commonly converted in plant 
cells is starch. 

Not only is the sugar produced by photosynthesis necessary 
to the plant that manufactures it; sugars, or substances derived 
from sugars, are indispensable to those organisms, plant or animal, 
which lack chlorophyll, with the exception of a few groups of 
bacteria which can live independently of the compounds formed 
by green plants. Apart from these bacteria, the existence of all 
living organisms is dependent upon the occurrence of photosynthe- 
sis in plant cells containing chlorophyll. 

89. Conditions Essential to Photosynthesis. Photosynthesis 
is possible only at a suitable temperature and in the presence of 
living matter, chlorophyll, water, carbon dioxide, and light. That 



the process is dependent upon the presence of chlorophyll may 
be shown in the following way. If a leaf of the silver-leaf gera- 
nium, already referred to, after being exposed to the light for 

FIG. 113. Experiment illustrating the necessity of the presence of chlorophyll 
for photosynthesis. A, fresli leaf of the silver-leaf geranium. B, the same 
leaf with the chlorophyll extracted. C, the starch-containing portion of 
the leaf has turned dark blue after treatment with iodine. Note that the 
starch pattern in C corresponds with the chlorophyll pattern in A. 

an hour or more, is killed with boiling water, the chlorophyll 
extracted with alcohol, and the leaf placed in a solution of 
iodine, the portions which formerly contained chlorophyll turn 
dark blue, but those which 
lacked chlorophyll remain 
white or become yellowish 
(Fig. 113, C). Since starch in 
contact with iodine turns blue, 
the experiment shows that 
starch was formed only where 
chlorophyll was present. An- 
other method of demonstrating 
the same fact consists in plant- 
ing specially selected kernels of 
corn, some of which will pro- 
duce green and others white 
plants (Fig. 114). For the first 
few days plants of both types 
grow with equal rapidity, but 
after the food reserves of the 
kernels have been used by the 
seedlings the green plants will 
continue to grow while the FIG. 114. Green and white corn plants 
i .j i , -11 i- T of the same acre. Note the greater 

white plants will die. Because ^^ of th( f p]ant at the g right , 
of the presence of chlorophyll, which contains chlorophyll. 



Soda Lime 


the green plants can manufacture their own food; the white plants, 
having no chlorophyll, can not. 

That carbon dioxide is necessary for photosynthesis, and that 
this atmospheric gas is used in sugar-manufacture, can be dem- 
onstrated by selecting two vigorous Coleus or nasturtium plants 
and placing them in the dark until the iodine test no longer dis- 
closes the presence of starch in their leaves. One of these plants 
is then placed under a transparent glass bell-jar (Fig. 115); the 
air that enters the bell-jar must pass over some substance, such 
as soda-lime, which absorbs the carbon dioxide but not the other 
atmospheric gases. Within the bell-jar is placed a dish containing 
a solution of soda-lime 
to absorb any carbon 
dioxide there present. 
The other plant is 
placed in a bell-jar 
similarly equipped, ex- 
cept that particles of 
brick of the same size 
are substituted for 
those of the soda-lime, 
and that water replaces 
the soda-lime solution 
within the bell-jar. The 
brick does not absorb FIG. 115. 
carbon dioxide, so that 
the plant can obtain 
this gas as well as the other atmospheric gases. If, after several 
hours' exposure to sunlight, leaves from each plant are tested with 
iodine, those from the plant in the first bell-jar remain colorless 
but those from the second turn blue, showing that sugar, and 
later starch, were formed only when carbon dioxide was available 
to the plant. 

The amount of carbon dioxide in the air is only about 3 parts 
in 10,000 (0.03%). In some localities, as in the neighborhood of 
cities and factories, the proportion may be slightly higher. Al- 
though a large amount of leaf surface which can absorb carbon 
dioxide is exposed to the atmosphere, the ordinary supply of this 
gas is often insufficient for the maximum possible amount of photo- 
synthesis. When the supply of carbon dioxide is artificially in- 



Solution < 


Experiment showing the necessity of 
carbon dioxide for photosynthesis. For ex- 
planation see 89. 



creased to 1 per cent, photosynthesis becomes correspondingly 
more rapid. Since carbon dioxide enters a leaf through its stomata, 
the number and distribution of stomata and the extent to which 
they are open affects the entrance of the gas into the leaf and there- 
fore affects the rate of photosynthesis. That this is true can be 
shown by coating with vaseline a portion of the surface of a leaf 
having stomata only on its lower side. This blocks the stomata 
and so prevents the entrance of carbon dioxide. If the leaf is 
tested with iodine after sufficient exposure to light, it will be found 

that no starch is present in 
that portion of the leaf whose 
stomata were blocked. 

The necessity of light for 
photosynthesis can be shown 
by covering the whole or a part 
of the surface of a green leaf 
with a screen made of some 
opaque substance, such as tin- 
foil or black paper (Fig. 116, A), 
so arranged that it will exclude 
the light but will not interfere 
with transpiration or with the 
exchange of gases between 

the inside and the outside of 
FIG. 116. A light screen attached to a th leaf It is best to gelect a 
leaf. B y leaf after exposure to light . 

under a light screen. The dark por- leaf On a Vigorously growing 
tions of the leaf show the distribution plant which has been kept in 
of starch as demonstrated by treat- ^ dark f Qne Qr more d 
ment witn iodine. ^ 7 

because leaves on such a plant 

do not ordinarily contain starch. If, after adjusting the light screen, 
the leaf is exposed to the sun for an hour or more, then removed 
from the plant and tested, it is found that the portions exposed to 
the light contain starch (Fig. 116, B), but that no starch is present 
in the portion from which light was excluded. This experiment 
shows, not that light is concerned directly in the formation of 
starch, but that light is necessary to the manufacture of a sugar 
which, after its formation, is converted into starch. 

Photosynthesis proceeds best in sunlight of appropriate intensity, 
but it goes on in greenhouses in the presence of a suitable kind of 
electric light. For many plants ordinary direct sunlight is too 


intense, and photosynthesis is most rapid in such plants in strong 
diffuse light. 

90. Energy and the Function of Chlorophyll. In the study of 
photosynthesis it is relatively easy to determine, at least in out- 
line, the material changes that are involved in the combination of 
carbon dioxide and water into such a carbohydrate as glucose. 
Intimately connected with these material changes are changes 
that involve energy. Every substance contains, or possesses, a 
certain quantity of energy. Some of the energy possessed by a 
substance may more or less readily be changed into another form. 
For example, chemical energy is changed in the process of com- 
bustion (burning) to heat energy. Energy which may thus fairly 
readily be changed is available energy. Great amounts of energy 
are locked up, as it were, in the atoms of any substance and are 
not available by any ordinary means. The available energy content 
of carbon dioxide and that of water are relatively low; on the 
other hand, the available energy content of glucose or of any 
other carbohydrate is relatively high. When, therefore, through 
the agency of chlorophyll, carbon dioxide and water have been 
combined to form a sugar, the newly formed sugar possesses a 
stock of energy which was not present in the water and carbon 
dioxide. The energy that has thus been stored in the sugar was 
obtained from the sunlight by the plant cell through the agency 
of chlorophyll. In the process, light energy was changed into 
chemical energy. The function of chlorophyll in this change is 
comparable to the function of an automobile motor which changes 
the chemical energy of gasoline into the mechanical energy that 
moves the car. 

The energy thus utilized and transformed by the agency of 
chlorophyll must first be taken by the chlorophyll from the sun- 
light. Part of the energy of the light which penetrates any sub- 
stance is absorbed by that substance; no effect can be produced by 
light unless it is absorbed. An important characteristic of chloro- 
phyll is its capacity to absorb a considerable fraction of the energy 
of the light falling upon it and to transform a portion of this 
energy into chemical energy. 

Sunlight is composed of different kinds of light. When a beam 
of light passes through a prism, it is split into its component parts, 
the visible ones of which produce on the human eye the sensa- 
tions of the colors red, orange, yellow, green, blue, and violet. 



The splitting of the beam is due to the different degrees to which 
its component rays are deflected by the prism. The band of varied 
colors thus produced is the visible spectrum. If an alcoholic ex- 
tract of chlorophyll from a leaf is placed in the path of a beam of 
sunlight, and the beam after passing through the extract is dis- 
persed by a prism, the spectrum appears interrupted by several 











FIG. 117. A, approximate distribution of colors in the spectrum of sunlight. 
Numbers refer to wave lengths measured in Angstrom units. B, ab- 
sorption spectrum after sunlight has passed through a leaf extract. C, ab- 
sorption spectrum after sunlight has passed through a single leaf. The 
dark hands in B and C correspond to those portions of the sun's rays 
which have been absorbed. Adapted from Willstiitter. 

dark bands (Fig. 117, B). The dark bands correspond to the por- 
tions of the beam of light which have been absorbed by the extract. 
Some of the bands located toward the red end of the spectrum, 
and some toward the violet end, are caused by the absorption of 
the corresponding rays of light by chlorophyll a and chlorophyll 
b. Two or three bands toward the violet end of the spectrum are 
caused by the absorption of light by carotin and xanthophyll. 
Experiments show that the light of particular wave lengths which 
is absorbed by chlorophyll is the energy used in photosynthesis. 
Plants which lack chlorophyll, with the possible exception of 
certain bacteria already mentioned, can not transform the energy 
of sunlight into the chemical energy used in photosynthesis. In 
strong direct sunlight, the energy absorbed by chlorophyll from 
the red portion of the spectrum is most efficient in photosynthesis; 
in diffuse light those rays toward the blue end of the spectrum are 
more important in the process. Only about 0.5 to 3 per cent of 


the light energy falling upon a leaf in direcft sunlight is used in 
photosynthesis; some of the light is reflected, some is transmitted, 
and a considerable proportion of that which is absorbed is trans- 
formed into heat ; the heat in turn is used to evaporate the water 
lost in transpiration. 

It is evident from the preceding discussion that the process of 
photosynthesis is affected by both external and internal conditions. 
The factors chiefly affecting it are the carbon dioxide content of 
the air, temperature, light, and the amount of chlorophyll present. 

Any substance that possesses energy which is available to a 
plant or animal is a food. Among the substances that may serve 
as foods are glucose, other sugars, and other carbohydrates into 
which sugars may be converted. Since light energy has been trans- 
formed into the chemical energy of carbohydrates, these are reser- 
voirs of the energy of sunlight. It is only in cells containing chloro- 
phyll that this transformation of the energy of sunlight can occur. 

91. Carbohydrates Other than Glucose. Glucose may be trans- 
formed, in the cell in which it is produced or in another cell to 
which it has passed, into any one of a considerable number of 
sugars. Among these are sugars having the same formula as 
glucose 1 (CeH^Oe), but in whose molecules the atoms are differ- 
ently arranged. Some sugars are simpler than glucose, having 5 
carbon atoms in each molecule instead of 6. Among those more 
complex than glucose are cane sugar (Ci2H 22 On), found in the 
sugar beet, sugar cane, and sugar maple; and malt sugar, having 
the same formula as cane sugar, which is present in sprouted 

It has been seen that another carbohydrate into which glucose 
may be changed is starch. In a cell in which photosynthesis is 
going on, starch is often deposited in the form of a small grain or 
of several small grains within each chloroplast (Fig. 108). The 
amount of starch formed in such a cell is approximately propor- 
tional to the rate at which photosynthesis is proceeding. In the 
produetion of starch it is thought that the equivalent of a mole- 
cule of water (H^O) is extracted from each molecule of glucose, 
so that a new unit of the composition C 6 Hi O5, instead of CeH^Oe, 
is formed, and that many such units are combined to form a 
molecule of starch. The formula of starch, therefore, is written 
(CeHioOe) , the n indicating an indefinite but large number of units. 

Another carbohydrate produced by the transformation of 


sugars is cellulose, Which frequently forms a large proportion of 
the substance of cell walls and which exists in an almost pure 
state in cotton fibers. Cellulose is deposited in the cells of some 
plants as a reserve food. Other carbohydrate foods produced by 
certain plants are glycogen and inulin. 

92. Fats. Some of the carbohydrates seem to serve as the chief 
building materials for fats, examples of which are olive oil, cotton- 
seed oil, and linseed oil. Many seeds and fruits, as for instance 
the castor bean, soybean, peanut, and olive, are particularly rich 
in fats. Although composed of the same elements as carbohydrates 
(carbon, hydrogen, and oxygen), a fat molecule differs markedly 
in its organization from a carbohydrate molecule, one important 
difference being its lesser proportion of oxygen. As an illustration, 
olein, the fat most largely present in olive oil, has the formula 
CBTHunOe. The energy content of fats is higher in proportion to 
their volume than is that of carbohydrates, and consequently 
fats are foods by means of which a large amount of available 
energy may be stored in a very limited space. Fats, like starch, are 
practically insoluble in the cell sap. 

93. Proteins. Carbohydrates and some nitrogen-containing 
compounds are the chief sources for the synthesis of proteins. 
Proteins constitute an essential part of the living matter of all 
plant and animal cells. They are also often present as reserve 
foods, being especially abundant in peas, beans, and similar seeds, 
and in the outer portions of the kernels of wheat, oats, and corn. 

All proteins contain carbon, hydrogen, oxygen, and nitrogen; 
many contain also small proportions of sulphur and phosphorus. 
The nitrogen, sulphur, and phosphorus are derived from relatively 
simple compounds, such as nitrates, sulphates, and phosphates, 
which most of the familiar green plants obtain in solution from the 
soil. Phosphorus-containing proteins are particularly characteris- 
tic of nuclei, although some cytoplasmic proteins also contain this 

Proteins are extremely complex, their molecules being composed 
of very large numbers of atoms. For example, the formula of 
gliadin, found in the wheat kernel, is CegsHioesC^iiNigeSe. Al- 
though formulas of this nature have been determined for a few 
proteins, the exact chemical constitution, that is, the arrangement 
of the atoms in the molecule, is not 'mown for any protein that 
occurs in living cells. However, much has been learned regarding 


the simpler compounds (amino-acids) of whi(l a protein molecule 
is composed ; and a considerable number of these simpler compounds 
have been artificially combined into substances that may be 
considered comparatively simple proteins. 

Proteins, as well as many other components of living matter, 
occur in the colloidal state a state marked by the distribution 
of the substances in question in a more or less finely divided con- 
dition through a continuous medium. This medium, in proto- 
plasm, is always water containing numerous substances in solution. 
Egg albumen is a colloidal mixture of proteins in water, containing 
also, however, various other substances suspended or dissolved in 
the water. A colloidal suspension of proteins varies in its con- 
sistency from that of a viscous liquid like egg albumen to that of 
so solid a substance as a firm gelatine. The differences in consist- 
ency depend in part upon the proportion of water present, and in 
part upon the size and arrangement of the protein particles. 

An important characteristic of many proteins in the colloidal 
state is their tendency to undergo, under high temperatures and 
other conditions, the change known as coagulation. This change 
involves modifications in the physical state and probably in the 
chemical constitution of a protein. The albumen of a cooked egg 
is an excellent illustration of coagulated proteins. In many cases 
coagulation is irreversible; the coagulated protein can not again 
be brought into suspension in the medium from which it was 

Another significant characteristic of proteins and of other sub- 
stances in the colloidal state is their general inability to pass through 
ordinary membranes, even through so permeable a membrane as 
a cell walL Protein foods can not, therefore, pass from one cell to 
another unless the two cells are connected by openings of some 
size through the dividing walls. Such openings, it has been seen, 
are present in the walls between the cells of sieve tubes, and it is 
probable that sieve tubes serve for the translocation of protein 

94. Chemical Elements Utilized by Plants. In addition to the 
elements which enter into the composition of carbohydrates, fats, 
and proteins, namely, carbon, oxygen, hydrogen, nitrogen, sulphur, 
and phosphorus, at least four others are essential to the com- 
plete development of the more complex plants. These are 
calcium, magnesium, iron, and potassium. With the exception of 



hydrogen, oxygen, and nitrogen, which are gases, all the elements 
named are solids. Nitrogen, iron, and sulphur may exist in the 
soil as elements; the others only in compounds. None of the ele- 
ments of this list can be used by complex plants except in certain 
definite compounds. These elements may unite with one another 
and with other elements into such compounds as potassium sul- 
phate, calcium phosphate, magnesium carbonate, and many others 

FIG. 118. A series of cultures of plants; in culture 1 all the essential ele- 
ments were present; in each of the other cultures at least 1 of these 
elements was absent. The cultures show the effects on growth of the 
omission of the respective elements; culture 2, lacking iron; 3, lacking 
magnesium; 4, lacking sulphur; 5, lacking calcium; 6, lacking nitro- 
gen; 7, in distilled water only; 8, lacking phosphorus; 9, lacking 

far more complex. Such compounds can be taken into a plant 
only in solution arid only from soil or from water. 

Although nitrogen constitutes nearly 80 per cent of the earth's 
atmosphere, it is riot available in the elemental form as a nutrient 
material except to some simple plants and, through the agency*of 
bacteria, to certain of the more complex ones. In general, it must 
be combined with oxygen and some other element in the form of a 
nitrate, such as potassium nitrate (KNO 3 ). Nitrates are absorbed 
by plants from the soil solution. A few familiar plants, such as 
rice, com, and potato, can use nitrogen when combined with hy- 
drogen in the form of ammonia (NH 3 ). Nitrogen forms a part 
of a very large number of substances occurring in plants. The type 


of growth that characterizes any particular plant is often directly 
related to the amount of nitrogen available. 

Phosphorus is a constituent of many highly complex organic 
substances in plant cells. It is essential to a rapid multiplication 
of cells, which is usually associated with active growth. Phos- 
phorus is stored in large quantities in most seeds, and is taken 
from the soil in relatively large amounts dunng the early stages 
of growth of young plants. 

Sulphur also is a constituent of some of the complex organic 
substances, in addition to proteins, needed for the maintenance 
of living matter. 

Without calcium most green plants are unable to grow T , and if 
calcium is not supplied to tissues already formed, the cells compos- 
ing such tissues die and tend to disintegrate. This element also 
neutralizes acids, both within and without a plant, which otherwise 
might be injurious and might restrict or completely prevent the 
plant's growth. 

Magnesium forms an essential part of the chlorophyll molecule. 
Iron is not a constituent part of this molecule, but when iron is 
lacking chlorophyll can not be formed. An insufficient supply of 
chlorophyll results in the diseased condition of plants known as 
" chlorosis." Potassium probably aids in the manufacture of car- 
bohydrates and in their movement from cell to cell. Consequently, 
a diminution in the amount of either magnesium, iron, or potas- 
sium available to the plant would greatly decrease growth by 
limiting the formation of carbohydrates. 

Other elements than those already mentioned, although ab- 
sorbed in relatively small amounts, are as essential to the growth 
and development of green plants. Boron, manganese, and copper 
occur in very many plants, and these elements have been shown 
to exert a beneficial effect upon development. Others, including 
aluminium, silicon, sodium, and zinc, seem not to be essential; 
but their presence may be beneficial, and some of them have very 
specific effects upon the plants in which they occur. The presence 
of aluminium or zinc may cause definite variations in color and 
form in such plants as the hydrangea and pansy. The r61e of 
some of the elements mentioned may be stimulatory rather than 



95. Translocation. The chlorophyll-containing cells of stems 
or other organs are capable, like the green cells of leaves, of making 
certain sugars by photosynthesis. A similar statement applies to 
the cells of thousands of simpler green plants which possess no true 
leaves or stems, and many of which consist of but a single cell each. 
These carbohydrate foods may be used by the cells in which they 
are made, or they may be stored in the same cells, or in many- 
celled plants they may be moved to other cells. In all the larger 
plants, many cells more or less remote from those in which foods 
are made do not contain chloroplasts. Foods, in order to reach 
these distant cells, must pass in a dissolved condition, or possibly 
in some cases in an exceedingly finely divided state, out of the cells 
that made them and must be transported to their destination. Thus 
some glucose is used by the chlorophyll-containing cells that make 
it ; but, being easily soluble, some of it is conducted to cells in other 
parts of the same plant. 

If during photosynthesis the manufacture of sugars is more rapid 
than their use, sugars will accumulate in the cells in which they 
are formed. Commonly, however, under such conditions, much 
of the accumulated sugar is converted into starch, which appears 
in the form of grains jn the chloroplasts ( 91). Such an accumula- 
tion of starch occurs ordinarily during the daytime, but during the 
night the starch is usually changed back to a sugar which moves to 
and through the phloem to various regions of the plant; during 
this movement a portion of the sugar may pass out from the 
phloem at any level into other cells., The sieve tubes, by virtue of 
their great length and of the presence of numerous pores in their 
walls, appear to be especially suited for the rapid movement of 
foods. At certain times, and under certain conditions, foods may 
be transported in the elements of the xylem. All these movements 
of foods from place to place within the plant are included under 
translocation. Foods other than carbohydrates, such as proteins 
and fats, also are translocated from places where they are made t,o 




places where they are to be stored, and from places of storage to 
regions where they are to be used. 

96. Food-storage. Most of the foods whose manufacture has 
been discussed are often produced by a plant in far greater amounts 
than are immediately used. The surplus is stored. If photosyn- 
thesis is more rapid than the removal of its products, these prod- 
ucts accumulate in the cells in which they originate and are 
ordinarily stored as starch in the chloroplasts; hence there may be 
a temporary storage in the chlorophyll-containing cells of leaves 
or of other organs. When starch is once formed, therefore, being a 
solid not readily soluble in water, it can not be moved. It must 
first be changed into a substance that is soluble, such as glucose 
or another sugar, if it is to be translocated. When sugar reaches a 
storage cell, some of it may 
be changed to starch, this 
change being effected by 
plastids, usually leuco- 
plasts (87). 

The starch grains formed 
in leucoplasts are commonly 
larger than those formed in 
chloroplasts. Starch grains 
deposited in leucoplasts are 
stratified, being composed 
of successive layers depos- 
ited about an original small 
core. Their shapes vary 
greatly, those starch grains 
produced by each kind of 
plant having a more or less 
characteristic form (Figs. Ill, 119). The layers that make up 
a grain are commonly of two different sorts. Some layers allow 
light to pass through rather freely and therefore appear clear in 
transmitted light. Layers of a second sort, alternating with those 
of the former type, do not allow light to pass through so readily; 
they appear, therefore, as comparatively dark, usually narrow 
zones. In many plants, including oats (Fig. 119, A) and rice, several 
starch grains are formed within each leucoplast; as these grains 
grow, they come into contact and commonly remain together as 
compound grains. 


FIG. 119. Starch grains. A, compound 
and individual grains from the oat. 
B, grain from a seed of the bean. 
C } from the stem of canna. 



In occasional instances, comparatively large storage-starch 
grains are formed in chloroplasts instead of in leucoplasts. An 
example of this type of starch-formation is furnished by the rel- 
atively thick stems of Pellionia. The stems are green, since many 
of their cells contain chloroplasts. Photosynthesis may go on in 
these cells; and the accumulating sugars are transformed into 
starch. But the stems serve also as storage organs. Hence many 

of the starch grains, instead of 
being small temporary struc- 
tures, as are most starch grains 
formed in chloroplasts, become 
large and similar in structure to 
the grains commonly deposited 
in leucoplasts (Fig. 120). 

Stored fats appear in various 
parts of cells as droplets of 
varying size. They occur in 
vacuoles and in the dense cyto- 
plasm, as well as in, or attached 
to, chloroplasts, leucoplasts, arid 

FIG. 120. Stages in the development Protein foods also may be 
of starch grains in chloroplasts of stored in the form of small 
cortical cells of a stem of Pellionia. i i_ i / ? 
The chloroplast is shaded in each globules (aleurone grains, 
case. A, B, grains composed of but Fig. 121). Aleurone grains are 
few layers. C, older grain with many especially characteristic of the 


storage cells of the seeds of 

beans, peas, and other members of the same family, as well as of 
a particular layer of the kernels of the cereal grains. In the castor 
bean, the aleurone grains are comparatively large and of complex 

The portions of different plants in which foods are stored vary 
greatly, depending largely upon the length of life of the plant. In 
annual plants foods are stored chiefly in seeds and fruits. The corn 
kernel is a fruit in which are stored quantities of starch and fats 
and some proteins; the bean seed is rich in carbohydrates and pro- 
teins. In trees and shrubs, as well as in other perennials, storage 
may occur in any of the living tissues of the vegetative organs, 
most frequently in those of stems and roots. A potato tuber is an 
enlarged underground branch, or part of a branch, many of whose 



cells are packed with starch; a head of cabbage consists dbiefly of 
leaves containing much water as well as some fats, carbohydrates, 
and proteins. The sugar beet has a much-enlarged root in the sap 
of whose cells large quantities of cane sugar are dissolved. 

97. Digestion; Enzymes. The term digestion is applied to those 
chemical changes in a substance which render that substance solu- 
ble and capable of diffusing readily through cell membranes. 
Many stored foods, such as starch, are insoluble or practically so 
in cell sap, and hence must be digested before they can be utilized 

Aleurone Grain 

Aleurone Grains 

Fia. 121. Aleurone grains. A, cell of the castor bean containing large, com- 
plex grains. B, cross section of the outer portion of a kernel of corn, 
showing the layer of cells which contain many small aleurone grains. 
After Strasburger. 

or translocated. Some other stored foods, such as cane sugar, while 
soluble in cell sap, do not diffuse readily through cell membranes; 
they also, therefore, must be digested before being translocated. 
The protoplasm of plant and animal cells produces special sub- 
stances known as enzymes whose function it is to bring about or to 
accelerate chemical changes. Each class of compounds, such as 
fats, proteins, or starch, is in general acted upon by a particular 
type ef digestive enzyme. Starch is digested by diastases, fats are 
digested by Upases, proteins by proteases, and cane sugar is changed 
into two simpler sugars (dextrose and levulose) by invertase. In the 
course of any such process the enzyme concerned is not used up, 
and exceedingly small quantities of a digestive enzyme are capable, 
consequently, of digesting large amounts of the particular food 
upon which it acts. Some foods are digested quite as well outside 


a living cell as in it, when they are brought into contact under 
appropriate conditions with the proper enzyme. An extract of 
diastase, for example, derived from barley kernels, will digest 
starch if placed with the starch in a little water in a test tube. 
Experiments such as this demonstrate that enzymes are distinct 
from living matter, since they can be extracted from cells without 
loss of their characteristic properties by processes that remove or 
kill the living matter. 

Although enzymes play a conspicuous and important part in 
digestive changes, it must not be thought that these are the only 
processes with which enzymes are concerned. As a matter of fact, 
they take part in most if not all of the chemical changes, including 
those involved in the manufacture of foods and other compounds, 
that go on in living cells. The several steps in photosynthesis itself 
may possibly be due to specific enzymes. When living matter 
brings about chemical changes, therefore, it does so largely through 
the agency of enzymes that it has produced. It follows that a great 
variety of enzymes are present in every living cell. 

98. Living Matter. In all the living parts of a plant, substances 
are continually being changed from one form to another. In some 
cases, simpler compounds are changed to more complex ones; in 
others, the change is from a more complex to a simpler form. The 
actual incorporation of non-living substances into the living matter 
itself is assimilation. Assimilation usually involves digestive action ; 
and it always involves numerous building-up processes in which 
enzymes are often concerned. One of the characteristics of living 
matter is its power of growth and repair that is, its ability to 
manufacture new matter like itself out of foods. Another character- 
istic is its instability the readiness with which it undergoes 
changes as a result of changes in the conditions which surround it. 
At all times, living matter is being continuously built up and con- 
tinuously broken down. It possesses also the capacity of liberating 
and of utilizing for various purposes energy stored in foods. 

Living matter is probably not a single substance, but rather a 
combination of substances more or less closely united physically 
and chemically, and definitely arranged and organized in each 
particular kind of cell. As yet, all attempts to determine the exact 
chemical composition of living matter have failed, because in any 
such attempt its condition is so markedly changed that it ceases 
to be living. At best, therefore, an analysis can furnish only 


an indication of the substances which entered into the living 

By an examination of living cells it is possible to determine some- 
thing of the physical relations of the substances present. A large 
part of living matter is always water in which various substances 
are dissolved. The less liquid portions of living matter include 
carbohydrates, proteins, and fats in a colloidal state. The particles 
are often aggregated to form granules, globules, and strands of 
various forms and sizes which are large enough to be visible under 
a microscope. These larger aggregations are themselves suspended 
in the more transparent, and often more liquid, portion of the living 
matter. Very often this liquid portion contains ultramicroscopic 
droplets which may be aggregated into visible drops or vacuoles 
that are more transparent than the remaining liquid. It is evident 
that there is a still finer structure inherent in living matter which 
microscopes of the highest powers can not reveal, and upon which 
as yet only indirect and conflicting evidence is available. 

99. Growth. Growth is commonly thought of as an increase in 
size. Frequently increase in size that is, in volume involves an 
increase in the amount of water present in the plant, as well as an 
increase in the amount of those substances other than water which 
remain if the plant is completely dried. In many instances, how- 
ever, plants or some of their organs increase greatly in their con- 
tent of substances other than water, such as starch, sugars, and the 
like, although such plants or organs do not increase appreciably in 
volume. Such an increase in materials other than water is often 
referred to as growth, whether or not it is accompanied by an in- 
crease in size. On the other hand, many plants, especially those 
containing large amounts of stored foods, when placed under con- 
ditions favoring development, may increase greatly in size, largely 
through the absorption of water, but may actually decrease in the 
amount of substances which remain when the plants are dried. 
Such an increase in size is also often considered as growth even 
though there is no increase in the amount of any substance present 
other than water. Thus grow r th, depending upon the sense in which 
the word is used, may be measured either by increase in volume, 
by increase in dry weight, or in both ways. 

Growth of a plant and of its various organs, in any sense of the 
term, depends upon the division and growth of the individual 
cells of which it is composed. Because the greater portion of the 


plant consists of mature cells, not all its cells at any one time are 
concerned in growth. As has been seen, the cells in the embryonic 
regions of roots and stems are both growing and dividing; growth 
here consisting mainly in an increase in the amount of living 
matter. In regions of elongation the enlargement of cells is brought 
about almost entirely by an increase in their water content. In 
regions of maturation growth involves an increase in the amount 
of cell wall material. Cells produced by cambial activity un- 
dergo a similar sequence of phases of growth: increase in living 
matter, increase in water content, and the deposition of cell wall 

The amount and kind of growth that a plant undergoes depend 
not only upon the total quantities of foods present or being manu- 
factured, but also upon the proportional amounts of water as well 
as of mineral nutrients available. A plant may possess an abun- 
dance of carbohydrates, but in the absence of sufficient water 
they are not utilized; or, if it has both carbohydrates and water, 
and lacks certain elements which these do not contain, the car- 
bohydrates and water are not combined into other substances. 

WTien a biennial or perennial plant resumes growth in the spring, 
its growth at first depends upon the utilization of reserve foods, 
together with the absorption of water and of mineral nutrients. 
Plants which lost their leaves in the autumn must produce new 
leaves in the spring, at the expense of stored foods, before photo- 
synthesis can be resumed. If the surviving part of the plant is 
an underground portion only, such as the tuber of a potato, growth 
goes on at the expense of the stored foods until a leaf-bearing 
shoot has been formed. In the leaves of evergreen plants, such as 
a pine, the green substance present in the leaves in the spring is 
not true chlorophyll, at least in regions with cold winters. With 
the advent of higher temperatures in the spring, however, the re- 
formation of chlorophyll in such leaves quickly makes possible 
the resumption of carbohydrate-manufacture. 

The development of the embryonic plant in a germinating seed 
to the point at which it can carry on photosynthesis involves the 
use of foods stored either in the seed leaves of the embryo or in 
tissues of the seed outside the embryo. The extent to which the 
reserve foods in a seed are used in the early development of a young 
plant may be determined experimentally by removing all or most 
of these foods. For examole. in a bean embrvo the two seed leaves 


contain nearly all the reserve foods. Their utilization may be shown 
by comparing the amount of growth of two young bean seedlings 
of similar age; both seed leaves are removed from one seedling 
and both are left attached to the second. During several days 
following, the plant with both seed leaves will develop the more 
rapidly and the one from which both seed leaves have been re- 
moved will develop more slowly, if it continues to develop at all. 
There is thus a direct proportional relation between the growth of 
the young seedling and the amount of foods available for its 
development. The later the seed leaves are removed during the 
period of germination and early growth of the seedling, the less 
pronounced is the effect of such removal upon the young plant. 
In fact, the seed leaves are shed by the bean plant itself after most 
of the foods which they contained have been utilized. 

Plants without chlorophyll must acquire foods directly from 
living green plants, or must utilize foods which were originally 
manufactured by such plants. Even in green plants, especially in 
the more complex ones, there are many organs and living cells 
that lack chlorophyll. The foods required by such cells for main- 
tenance of their living matter, growth, and reproduction must be 
conducted to them from other cells which have made more foods 
than were needed for their own use. 

100. Secretion. The production by plant cells of compounds 
which are stored for short or long periods is secretion. Such foods 
as sugars, starch, fats, and proteins are secreted. The formation 
of chlorophyll and other pigments and the deposition of cellulose 
and other substances in the form of cell walls are processes of 
secretion. In addition to those already mentioned, a vast number 
of substances are secreted by the cells of various plants. These 
substances include glucosides and alkaloids, many of which are 
of medicinal value; volatile oils, which cause the characteristic 
odors of plants; organic acids, such as malic acid found in apples, 
and citric acid in lemons, oranges, and grape fruits; mucilages, 
oleoresins, latex (milky juices), and tannin. Many secreted sub- 
stances are reserve foods; others are useful in a variety of ways 
to the plant which makes them; but some are by-products or 
waste substances. The deposition or extrusion of useless or pos- 
sibly harmful substances is often referred to as excretion; but it 
is not easy to distinguish sharply between excretion and secre- 


101. Pigments. Certain secreted pigments, especially the chlo- 
rophylls, carotin, and xanthophyll, which are present in plastids, 
have already been described. Certain other pigments that are 
secreted by plant cells occur, not in plastids, but in solution 
in the cell sap. Most abundant of these water-soluble pigments 
are the anthocyans y which are usually red or blue. In some 
plants anthocyans are present in varying amounts at all times. 
In others they appear only in the spring or in the autumn. They 
are abundant in the roots of the red beet and the radish, in red 
onion bulbs, in the leaves of some plants, such as Coleus, and 
in many red and blue flowers and fruits. In some plant parts, 
these pigments appear only in cells of the epidermis; in others, 
they occur in cells of various tissues. 

The formation of a.n anthocyan is dependent especially upon 
the presence of large amounts of sugars. When conditions are 
such that sugars are being used rapidly by a plant so that they do 
not accumulate, anthocyans are not formed. At low temperatures 
sugars accumulate and anthocyan-formation results. Many plant 
parts are brilliantly red in early spring and in autumn, because 
of the abundance of sugars and the consequent formation of 

Although the functions of anthocyans have not been definitely 
determined, it is possible that they aid in the absorption of cer- 
tain rays of light which would be injurious to chlorophyll. Antho- 
cyans absorb some light energy, which is converted into heat, so 
that under like conditions red leaves have a higher temperature 
than green ones. Such rise in temperature may accelerate the 
activities of living matter, and perhaps aids in protecting a plant 
from the effects of low temperatures in the surrounding air. 

The brilliant colors characteristic of autumn leaves in temperate 
regions result from the presence of pigments. The yellow colors 
are due to the yellow pigments present in chloroplasts. During 
the summer chlorophyll is usually formed about as rapidly as it 
is destroyed, but as autumn progresses the production of chloro- 
phyll is slower than its destruction, so that the green color fades, 
leaving only the color of the yellow pigments. Both low tempera- 
tures and drought tend to check the formation of chlorophyll, 
but frost is not necessary to its disappearance. It is a matter of 
common observation that leaves often become yellow during dry 
periods even in summer. Bright red colors are due to anthocyans 


which are produced in many plants in the autumn. Autumnal 
conditions favor the accumulation of sugars in leaves and hence the 
formation of anthocyans. When a leaf is alive, the cell walls are 
light-colored and translucent. They become brownish upon the 
death of the cells. The protoplasts often blacken after death. 
In these ways the brown and black colors of autumn leaves are 
caused. Various combinations of yellow, red, brown, and black 
colors produce intermediate shades or cause a mottling of the 

102. Respiration. It has been seen that in photosynthesis a 
part of the energy which comes to the earth's surface as light is 
used in the production of a sugar out of carbon dioxide and water. 
In other words, some of this energy has entered into, and is bound 
up in, the sugar itself. If by any means the sugar is decomposed 
into simpler substances, energy is set free. It would be possible 
completely to release the bound-up energy by changing the sugar 
back to carbon dioxide and water. 

Since all foods utilized by animals come directly or indirectly 
from plant cells, the continued existence of animals as well as of 
plants depends upon the available energy accumulated in the 
process of photosynthesis. Man makes use of such stored energy 
not only for his bodily needs but also in various other ways. The 
use of fuels as a source of heat or of mechanical energy is, in the 
last analysis, merely the transformation of energy previously 
absorbed from the sunlight by chlorophyll. The use of w r ood as 
a fuel involves the utilization of energy that has thus been accumu- 
lated comparatively recently, whereas coal and petroleum repre- 
sent stores of energy accumulated millions of years ago. 

A living plant requires a source of energy for carrying on its 
various functions, such as the formation of the more complex 
foods, assimilation, growth, and movement. Since for most of 
these functions the energy of light can not be used, as it is in 
photosynthesis, some of the foods already built must be destroyed 
in order that their stored or potential energy may be liberated. 
In plant cells carbohydrates and other organic substances, when 
they are utilized as foods, are to a large extent decomposed by 
processes analogous to, but essentially different from, combustion, 
and the energy so released is utilized by the cells in which the 
decomposition occurs. The process by which, in living cells, 
energy is obtained or released through the destruction of foods is 


respiration. In respiration the foods destroyed are not merely the 
sugar or sugars first formed in photosynthesis; they may include 
a considerable variety of substances, among them fats and pro- 
teins, which like sugars possess stored-up energy. Commonly, 
though not in all cases, respiration entails the absorption of free 
oxygen and its combination with the substances undergoing 
destruction, which thus become oxidized. Land plants obtain 
oxygen needed for respiration from the air, including that present 
in the interstices of the soil. Plants living submerged in water are 
dependent upon oxygen that is dissolved in the water. It has been 
seen that cells carrying on photosynthesis liberate oxygen, which 
is available for their respiration as well as for that of other cells. 
If the substance being respired is a carbohydrate, and if it is 
completely oxidized, carbon dioxide and water are produced. 
Hence the respiration of carbohydrates is in some of its effects the 
reverse of photosynthesis. It will be remembered that in photo- 
synthesis carbon dioxide and water are combined, oxygen is liber- 
ated, sugar is formed, and energy is stored. During respiration 
of a sugar, oxygen is combined with the sugar, carbon dioxide and 
water are re-formed, and the stored energy is released. Oxygen is 
set free as a result of photosynthesis; carbon dioxide is set free 
as a result of respiration. Unlike photosynthesis, however, res- 
piration is going on all the time, night and day, in every living 
cell, whereas photosynthesis takes place only in chlorophyll- 
containing cells and only in the presence of light. It is, therefore, 
only when the rate of photosynthesis exceeds the rate of respira- 
tion that oxygen is given off by a leaf, and, conversely, only when 
respiration is more r^pid than photosynthesis is carbon dioxide 
given off. If the two processes were to go on at exactly equal 
rates, neither carbon dioxide nor oxygen would be evolved from 
the cells concerned. Likewise, since the initial sugar is made only 
through photosynthesis, and since foods are constantly being de- 
stroyed through respiration, it is only when the quantity of mate- 
rials built up exceeds the quantity destroyed that a green plant can 
actually gain in dry weight. A green plant grown in the dark 
loses in dry weight because its stored foods are respired; a similar 
plant grown in the light gains in dry weight because photosynthesis 
is possible and because, although respiration is going on, more 
foods are manufactured by the plant in the light than are de- 



A part of the energy released in respiration appears in the form 
of heat; but because of the slowness of the process, and because of 
the large radiating surface of the plant compared with the amount 
of heat being radiated, it is difficult to measure the heat generated 
at any specific point. When respiration is very active, however, 
as in some germinating seeds, the liberated heat can be measured. 
Provided special precautions are taken, a rise in temperature may 
often be demonstrated. During the active growth of bacteria in 
certain media, temperatures of 35 to 40 C. are not uncommon, 
as compared with ordinary room temperatures of 18 to 20 C. 

That large quantities of carbon dioxide are given off in res- 
piration can be determined by passing air freed from carbon di- 
oxide through a vessel containing germinating seeds (Fig. 122), 


FIG. 122, An experiment demonstrating the evolution of carbon dioxide 
during respiration. The baryta water in the first bottle removes all carbon 
dioxide from the air, as shown by the lack of a precipitate in the second 
bottle; the air then passes into the third bottle, containing germinating 
peas; the air passing from this into the last bottle again contains carbon 
dioxide, and a precipitate is formed. 

and then testing the air with baryta water for the presence of 
carbon dioxide a white precipitate being formed when carbon 
dioxide is passed into baryta water. 

When green leaves are used to illustrate the liberation of carbon 
dioxide in respiration, it is necessary to place them in darkness, 
since in light the carbon dioxide produced by respiration might be 
utilized in photosynthesis and therefore not liberated from the 
leaves. If, then, green leaves are placed in a flask containing air, 
and the flask is closed and kept for a time in darkness, it is possible 


to determine that carbon dioxide has been liberated by testing the 
gases in the flask with baryta water and noting the precipitate 
formed. That most of the oxygen originally present in the air 
contained in the flask has been consumed by these leaves can be 
shown by the effects produced upon a lighted splinter lowered into 
the flask containing the leaves, as compared with the effects pro- 
duced upon a splinter lowered into a flask containing air alone. 
The splinter in the flask containing air will continue to burn until 
most of the oxygen has been consumed, whereas that in the flask 
containing the leaves is immediately extinguished. 

103. Anaerobic Respiration. Another type of respiration differs 
from that just discussed in the fact that, while foods are destroyed 
and energy is released, no atmospheric oxygen is absorbed during 
the process. Instead, the elements in the molecules of which the 
foods are composed are rearranged into other substances, and the 
oxygen necessary for respiration is released. Since these changes 
take place in the absence of atmospheric oxygen, the process is 
called anaerobic respiration in contrast to aerobic respiration. 
Anaerobic like aerobic respiration is a means by which the respir- 
ing organism releases energy. Anaerobic respiration may be illus- 
trated by inserting several pea seeds previously soaked in water 
into the mouth of a test tube filled with mercury and inverted 
over a dish of mercury. The seeds will rise to the top of the mercury 
in the test tube. After a time, as the seeds respire, the mercury 
will be forced down and out of the test tube by the pressure of 
the carbon dioxide produced. Since in this type of respiration 
the foods may not all be completely broken down into carbon 
dioxide and water, more foods may be consumed in proportion 
to the amount of energy released than in aerobic respiration, 
and substances are often formed which are poisonous to the plant. 
At times very high temperatures are generated in the course of 
Anaerobic respiration. 

In higher plants respiration of this type is but a temporary 
substitute for aerobic respiration, and such plants will eventually 
perish if permanently deprived of atmospheric oxygen. Many 
bacteria, yeasts, and molds, however, may use only small amounts 
of atmospheric oxygen. For such organisms anaerobic respiration 
is therefore the fundamental process concerned in the release of 
energy. Anaerobic respiration on the part of these simple organ- 
isms is fermentation. 


104. Metabolism. The whole complex of material and energy 
changes which go on within the plant is metabolism. Metabolism 
includes constructive processes, such as photosynthesis, other 
forms of food-manufacture, and assimilation, as well as destruc- 
tive processes, most conspicuous among which is respiration. An 
excess of constructive over destructive metabolism results in an 
increase in the amount of those substances which would remain 
if the plant were dried. An excess of destructive over constructive 
metabolism results in a decrease in the amount of such substances 
present. It is only an excess of constructive over destructive proc- 
esses that permits of the continued existence and growth of a 
plant body as a living unit. Every living animal, and every living 
plant except some of the bacteria, is ultimately dependent upon 
photosynthesis for its existence, because at present this is the only 
efficient means of combining inorganic substances into a food 
that is, into a store of reserve energy. 



106. Irritability. As has been seen ( 10), the dense cytoplasm 
in a cell of a leaf of Elodea may be in motion at ordinary room 
temperature. If the temperature is gradually raised, movement 
becomes more and more rapid, until at a particular point (about 
37 C.) the greatest rapidity is reached. As the temperature is 
raised further, movement becomes slower, and at about 38.7 C. 
it ceases entirely. If, on the other hand, a cell showing streaming 
at room temperature is cooled, the motion becomes gradually 
slower until at about C. it stops. It is evident that this form of 
activity of living matter is influenced by an external condition, 
namely, temperature. All other forms of activity of living matter 
are likewise affected by this and by other external conditions. Any 
external condition that affects a process going on within a cell is 
a stimulus. The external condition, in order to serve as a stimulus, 
must exert some force upon the living matter; a stimulus, there- 
fore, implies an expenditure of energy. The change within the 
cell which results from the application of a stimulus is a response. 
In the case of streaming cytoplasm, the response is a quickening 
or slowing of the rate of movement. In very many instances the 
response to a stimulus is similarly a slowing or an acceleration in 
the rate of some intracellular process. Responses, however, include 
changes of any nature in the activities of a cell, including the 
initiation of a new process, or changes in activities common to a 
group of cells such as a tissue or an organ. A response involves 
an expenditure of energy on the part of the living matter. But 
the energy which is expended in the response has no necessary 
relation to the energy that was manifested in the stimulus. The 
energy of the response is energy that was already present, but in 
latent form, in the living matter; not that which was supplied by 
the stimulus. A stimulus is recognizable as a stimulus only if it 
produces a perceptible response. Practically, therefore, only those 
stimuli are treated as such which result in a response that can in 
some way be observed. 



The capacity to respond to a stimulus is irritability. Irritability 
may be manifested by non-living as well as by living matter, but 
living matter is marked by the ability to respond readily in a vari- 
ety of ways to a great variety of stimuli. Irritability is considered, 
therefore, one of the fundamental characteristics of living matter. 

As just noted, streaming movements cease if the cells of an 
Elodea leaf are cooled to about C., or if they are heated to 
about 38.7 C. These temperatures are respectively the minimum 
and the maximum temperatures for streaming. The movement is 
most rapid at about 37 C., which is the optimum temperature 
for streaming. In general, an external condition, such as tempera- 
ture, in its effect upon a particular activity has a minimum, an 
optimum, and a maximum point. Among the stimuli that affect 
cytoplasmic streaming in the cells of an Elodea leaf are, besides 
temperature: mechanical shock, electric currents, illumination, and 
the proportions of salts, acids, alkalis, sugars, oxygen, or carbon 
dioxide in the liquid surrounding the leaf. 

106. Other Relations of Temperature to Living Matter. If the 
rise in temperature is stopped as soon as streaming ceases, and the 
leaf is slowly cooled, streaming may begin again. A stoppage of 
the streaming movement, therefore, does not necessarily mean 
that the protoplasm is dead. Death involves the cessation of a 
complex of activities which characterize matter in a living con- 
dition. Each activity of living matter has its minimum, optimum, 
and maximum temperatures, and these temperatures may be 
different for each of the various activities of a single cell; or they 
may be different for the same activity in cells of different kinds. 
In general, the temperature range for activities characteristic of 
living matter is between and 50 C. Under some conditions, 
however, especially when the protoplasm contains relatively little 
water, such temperature limits may be greatly exceeded without 
a resultant death of the cells concerned. Seeds of low water con- 
tent may be kept alive for long periods at temperatures of from 
90 to 100 C. without causing the death of their cells. Many 
bacteria are killed at 70 C., but some will endure temperatures 
above 90 C. Certain bacteria in a resting condition can be killed 
only by prolonged heating at 100 C. At the other temperature 
extreme, some seeds and some dormant bacteria can endure the 
temperature of liquid hydrogen (252 centigrade degrees below the 
freezing point). 


The temperature of the air, as well as that of the soil or water 
with which a plant is in contact, affects the activities of the plant's 
component cells, and therefore the activities, including growth, 
of the plant as a whole. Any kind of plant thrives best within a 
certain range of temperature which is much narrower than that 
within which the plant can remain alive. The more the tempera- 
ture varies from this favorable range the less vigorously does the 
plant grow. Hence, the distribution of plants on the earth's sur- 
face is in part determined by temperature. 

Plants do not ordinarily possess the relatively high internal 
temperatures which characterize the bodies of many of the larger 
animals. Instead, the internal temperatures of most plants are not 
greatly different from the temperature of their environment. In 
this connection it is of interest to note the effects of transpiration 
on the temperature of leaves. Rapid transpiration has a marked 
cooling effect, and on a hot summer's day may result in lowering 
the temperature of a leaf several degrees below that of the sur- 
rounding air. On the other hand, vigorously growing regions of 
plants may develop an internal temperature several degrees higher 
than that of the environment. In such cases respiration goes on 
very rapidly and much of the released energy takes the form of 
heat. These facts explain how such plants as the skunk cabbage 
may be seen growing through snow and frozen soil in early spring. 

107. Responses to the Stimulus of Gravity. All processes going 
on in living organisms are affected by many environmental fac- 
tors. Of these factors gravity is the only one which acts constantly 
in the same direction and with the same intensity at any point 
on the earth's surface. Gravity affects the direction of growth of 
plants. In general, stems grow vertically upward, primary roots 
vertically downward, and branches, leaves, and branch roots grow 
transversely or at widely varying angles to the direction of the 
force of gravity. The ability of plants or of their parts to respond 
in these various ways to gravity as a stimulus is geotropism. Or- 
gans which respond to the stimulus of gravity by growing toward 
the center of the earth are positively geotropic; those which respond 
by growing away from the center of the earth are negatively geo- 
tropic; those which grow at right angles or obliquely to the direc- 
tion of the force of gravity are transversely geotropic. To illustrate 
responses to gravity, seedlings of bean or corn whose roots and 
stems are a few centimeters long may be placed in a moist chamber 



with the stems and roots at various angles to the direction of 
gravity. In a few hours the stems will have turned upward and 
the primary roots downward. 

In most roots growing in the soil, the part that is capable of 
receiving the stimulus of gravity, and which is therefore said to 
perceive the stimulus, appears to be the terminal portion, including 

FIG. 123. Roots with tips in glass slippers. A shows tip bent at right angles. 
B, same root placed with its tip vertical. C, same root after 18 to 20 hours. 
E, curvature 18 to 20 hours after the root is placed in position D. 

the root cap; the responding portion is farther back, in the region 
of elongation. The location of the perceptive portion may be shown 
by making root tips grow into small glass slippers (Fig. 123, A) 
so that the terminal few millimeters of each root are bent at 
right angles. One of these roots is then placed horizontally with 
its bent tip turned downward (Fig. 123, 5). Since the perceptive 
portion of the root is now in a vertical position, no effect is trans- 
mitted to the horizontally placed region of elongation; conse- 
quently no further change in form occurs, and the region of elon- 
gation continues to extend in the direction in which it is placed 
(Fig. 123, C). If, however, the root is so placed that the region 
of elongation is vertical and the bent tip horizontal (Fig. 123, D), 
the root will curve until the perceptive portion is in a vertical 
position (Fig. 123, E). 

The location of the responding portion can be determined by 
marking with India ink the primary root of a bean seedling at 
intervals of one millimeter and placing the seedling in a moist 



chamber with its root in a horizontal position (Fig. 124). After 
24 hours it can be determined by measuring the distances between 

A B 

FIG. 124. A, bean seedling whose root has been marked at equal intervals 
and placed in a horizontal position. B, the same seedling 24 hours later. 

markings that the region of downward curvature corresponds 
with that of greatest elongation. This curvature observable in 
the region of elongation is due to an elongation at different rates 
on opposite sides of 
the root. The experi- 
ments just described, 
taken together, show 
that in this root the 
region of perception 
and the region of re- 
sponse are distinct. 

In the majority of 
stems both the per- 
ception of the stimulus 
of gravity and the re- 
sponse to this stimulus ~ 
are localized in the re- 
gion of elongation. 
The location of the * ?IG< *^5. At the left, a plant of Iresine in the 
,. , . ordinary position: at the right, a plant which 

responding portion of has been turned on its side The negative re _ 

a stem may be shown sponse to the stimulus of gravity consists in 

by marking with India bending at the nodes as well as in the elongat- 

i n , ing region near the tip. 

ink sunflower or to- 

mato stems at equal intervals and placing the plants parallel to the 
earth's surface. After a few hours it will be found that an upward 
curvature has occurred in the elongating internodes; the mature 
internodes show no response. However, in some stems geotropic 
response is not localized exclusively in the region of elongation. 



If the relatively mature stem of Iresine (Fig. 125) is placed in a 

horizontal position, upward bending will occur at each of the 

younger nodes, as well as in the region near the tip where the 

internodes are elongating. 

If a similar experiment is 

performed with the stem 

of some grass, such as 

wheat, bending will occur 

only at the nodes. This is 

because of the delayed 

maturation of the nodal 

tissues ( 34). 

To determine whether 
such changes in position of 
stem and root are geo- FlG 126 An apparatus (c iinostat) for ro- 
tropic responses, it is nee- tating plants slowly about a horizontal 
essary to counteract the axis > constantly changing their relation to 

,. ., ,. the direction of the stimulus of gravity. 

action of gravity as a stim- & 

ulus and to observe the direction of growth under the new con- 
ditions. This can be done by rotating the plant slowly about a 
horizontal axis, thus constantly altering the plant's relation to 

FIG. 127. Knight's experiment, in which the stimuli of both centrifugal force 
and gravity are acting upon stems and roots. 

gravity (Fig. 126). Gravity is now no longer operating as a one- 
sided stimulus. Under such conditions the stem does not grow 
vertically upward nor the primary root vertically downward, but 


these organs tend to grow horizontally that is, in the directions 
in which they were placed when the experiment began. 

In another experiment, first performed by Knight in 1805, the 
seedlings are placed on the rim of a horizontally revolving wheel 
(Fig. 127) and the wheel is rapidly rotated. The primary roots of 
the seedlings now grow diagonally downward and outward, while 
the stems grow upward and inward. The stems and roots are 
responding both to the stimulus of gravity and to that of cen- 
trifugal force. 

It is impossible as yet to explain fully how the stimulus of gravity 
is perceived and how it is transmitted from the perceptive to the 
responsive portions of a root or stem. Experimental evidence 
seems to indicate that geotropic curvature is influenced mainly by 
the production of growth-regulating substances and by their un- 
equal distribution under the influence of gravity. The difference 
between the reactions of roots and those of stems to the stimulus 
of gravity is explained by a difference in the action of the growth- 
regulating substances in these respective organs. It is thought 
that, in a root which responds positively, a growth-regulating sub- 
stance is produced in the tip. If the root is placed horizontally, 
the accumulation of this substance on the lower side of the root 
results in a slowing of growth on that side. The upper side of the 
root continues to grow, and the consequence is a downward bend- 
ing of the root in the region of elongation. If a negatively geo- 
tropic stem is horizontally placed, an accumulation of a growth- 
regulating substance in the lower side accelerates growth on that 
side, and the stem bends upward. In the two cases just cited the 
growth-regulating substance is the same; but the responses that 
it causes in root and stem are opposite in kind. 

Not all investigators accept the explanation of geotropic re- 
sponse just outlined, and other explanations of such response have 
been proposed. 

108. Responses to the Stimulus of Light. Unlike gravity, light 
is neither constant nor uniform in amount, direction, or intensity. 
Like gravity, light affects the direction of growth of plants. Stems 
are usually positively phototropic, bending toward a source of light. 
Branches and leaves are usually transversely phototropic. Some 
roots, such as those of certain members of the mustard family, 
are negatively phototropic. 
Not all organs of plants are sensitive to light as a stimulus. 



Phototropic responses are in general characteristic only of the 
aerial parts of plants, although the sensitivity to light is in no way 
connected with the presence of chlorophyll. Such underground 
organs as roots and rhizomes, which grow in darkness, usually 
show no reaction to light. 

To determine the regions at which phototropic responses occur, 
the stems of young plants, as of sunflower or geranium, are marked 
with India ink at intervals of 5 mm. (Fig. 128) and then grown 
in darkness for 24 hours. The intervals are then remeasured. 

FIG. 128. A, part of a sunflower plant with its stem marked at equal inter- 
vals. B y the same plant after exposure to a one-sided illumination. The 
bending occurs in the region of elongation. 

The plants are next so placed that they are illuminated from one 
side. In a few hours the stems will show phototropic curvature, 
and it can be determined by measuring the intervals between the 
marks that the regions of curvature correspond with the regions 
where the greatest elongation has occurred. Not all parts of any 
particular organ are equally sensitive to the stimulus of light. In 
general, the apical part of a stem is most sensitive; the older por- 
tions also are sensitive but usually to a lesser degree, the sensitive- 
ness decreasing as the distance from the tip increases. 

The positions of some leaves are changed in response to light 
stimuli by alterations in the direction of growth of their petioles. 
The new position may be either temporary or permanent. The 
leaves of house plants such as the geranium which are growing 
in a window change their positions so that the upper surface of 
each blade faces the source of light. That it is the blade rather 
than the petiole which perceives the light stimulus can be shown 


by wrapping the petiole of a young leaf of nasturtium or of begonia 
with tin-foil so as to exclude all light, and by covering in the same 
way the blade of a similar leaf whose petiole is left exposed. If 
the leaves so treated are now illuminated from one side, only the 
leaf with the exposed blade will respond by a curvature of its 

The response of most broad foliage leaves to light as a stimulus 
may occur, not only in the petioles, but also in the growing parts 
of the blades or in parts still capable of growth. In general, each 
such leaf finally assumes a relatively fixed position, which tends 
to be such as to expose the surface of the blade most directly to 
the incident rays of light. The movements of leaves in attaining 
their fixed positions may be due to curvature, increase in length, 
and torsion of the petiole, of the leaf blades, or of both. The blades 
often thus become so arranged that they do not greatly overlap 
or shade one another. Leaf mosaics ( 60) are produced in this 

In previous paragraphs it has been noted that light affects the 
direction of growth. That it also affects the rate of growth and the 
form of the plant becomes evident when bean or potato plants 
grown in darkness are compared with those grown in light](Fig. 110). 
The plants grown in darkness have very long, slender stems 
because of the greatly elongated internodes; their leaves are small, 
have long petioles, and show little differentiation in the internal 
structure of the blades; and because chlorophyll is not developed 
the plants have a pale yellowish appearance. The differences in 
these cases illustrate the general fact that the type of growth of 
plants is profoundly affected by the intensity of illumination. 
The precise effect of light stimuli varies greatly, however, between 
plants of different species and even between different organs of 
the same plant. For example, stems and branches are not always 
more elongated in weaker illumination; on the contrary, the 
branches on the shaded side of a tree are often shorter than those 
that are directly exposed to light. The effects of intense illumina- 
tion upon the growth of plants adapted to shady conditions are 
very different from its effects upon plants which ordinarily grow in 
sunny localities. 

Experiments suggest that the tips of phototropically responding 
organs form growth-regulating substances which are redistrib- 
uted under the influence of light, and that these substances cause 


a retardation of growth on the lighted side and a more rapid 
growth on the shaded side, the result being a curvature of the re- 
sponding organ. 

109. 4 * Long-day" and " Short-day "Plants. Both the intensity 
of the light to which a plant is exposed, and the length of time 
during which it is illuminated, materially affect its development. 
If a green plant is grown under conditions which are favorable 
except that illumination is feeble, the character of growth will 
be altered and the production of flowers will be delayed and may 
not occur at all. When such a plant is placed in light of greater in- 
tensity it produces flowers. 

If, on the other hand, the intensity of illumination is kept 
favorable, the length of daily exposure to light affects the amount 
and type of growth. Some plants flower and fruit much earlier 
when the period of exposure to light is less than 12 hours in each 
24. Among such " short-day " plants are ragweed and some 
varieties of soybean. Other plants flower and fruit earlier when the 
daily exposure to light is more than 12 hours. "Long-day" plants 
in this category include the tomato, radish, and lettuce and some 
biennials such as sweet clover. Still other plants, one of which is 
buckwheat, show little or no response in this respect to changes in 
the length of daily illumination. 

110. Responses to the Presence of Water. The direction of 
growth of young roots is often affected by the presence of a local 
supply of water in the soil. In such a case the distribution of 
water from the local source results in the presence of varied pro- 
portions of water in different parts of the soil, and the roots may 
grow toward the region in which water is most abundant. The 
effect of a stimulus of this nature on the direction of growth of 
roots may be shown by filling a large box with dry soil and imbed- 
ding in the soil near one end of the box a porous cup filled with 
water. The water will percolate into the soil, so that the soil 
near the cup is moister than that farther away (Fig. 129). If 
bean seeds germinate in the soil at some distance from the 
cup, their roots, when formed, will grow toward the cup until 
they reach a region in the soil containing a certain proportion 
of water. Here the primary root no longer responds to the stimulus 
supplied by the presence of water but, reacting to the stimulus of 
gravity, grows downward. The secondary roots usually continue 
their horizontal growth. It is evident from this experiment that 



both water and gravity play a part in determining the direction 
of growth of roots in soil. The stimulus supplied by the presence 
of water is, however, often the stronger of the two, as is illus- 
trated by the growth of roots along ditches, tile drains, and irriga- 
tion canals. 

That the proportion and distribution of water in the soil may 
have a formative influence is shown by the roots of many desert 

FIG. 129. Section of a box containing sand, with a plant near one end and a 
porous flower pot containing water near the other. The soil is moister 
near the pot, as indicated by the darker shading, and the unequal water 
content of the soil on different sides of the plant affects both the form 
and the direction of growth of the roots. 

plants. The root systems of such plants are in general relatively 
extensive, sometimes reaching to great depths. Many cacti have 
widely spreading root systems, extending laterally rather than 
deeply into the soil. Roots of plants growing in swampy soil are 
often shorter than those of plants of the same species growing in 
drier habitats. 

Water exerts an influence also upon the forms of aerial parts 
of plants, largely on account of differences in transpiration. 
Plants grown in damp air often have longer internodes and larger 
and thinner leaf blades than those of the same species grown in 
dry air. Leaf blades of nasturtiums growing in moist air and moist 
soil may be four or five times as large as those of nasturtiums 
living in dry air and dry soil. A frequent consequence of a change 



from an aerial to an aquatic environment or vice versa is that cer- 
tain organs, especially leaves, produced after the change differ 
in form from those previously produced. The leaves of the water 
crowfoot (Fig. 90) that develop in the air are very different in 
form and structure from those that develop under water. 

111. Responses to the Presence of Other Substances. Sub- 
stances other than water, such as oxygen, carbon dioxide, and 
illuminating gas in the air, or nutrient substances in the soil, may 
exert a formative as well as a directive influence upon plants. The 
presence of very small quantities of illuminating gas in the air of 
a greenhouse causes various abnormal developments and distor- 
tions of seedlings as well as the death of some plants. The 
direction of growth of roots is affected by the localized presence 
in the soil of mineral salts and other substances. 

112. Responses to Mechanical Stimuli. The responses of plant 
organs to the various kinds of stimuli thus far discussed are gener- 
ally visible only 

after the stimu- 
lus has been ap- 
plied for some 
hours. The fa- 
miliar sensitive 
plant, Mimosa 
pudica, a native 
of tropical South 
America, is es- 
pecially suitable 
for the study of 
the immediate 
responses of en- 
tire organs to 
stimuli, particu- 
larly to those of 
a mechanical na- 
ture such as a 
touch or a blow. 

The plant (Fig. 130) has an erect, thorny, more or less branched 
stem. The leaves have long petioles and are compound, each hav- 
ing two to four primary leaflets which in turn bear numerous 
pairs of secondary leaflets. 

FIG. 130. A sensitive plant (Mimosa pudica). 



If a single secondary leaflet of a Mimosa plant, under favorable 
conditions of light, moisture, and temperature, is touched very 
lightly, it reacts by bending upward and slightly toward the tip 
of the primary leaflet on which it is borne; if the stimulus is some- 
what stronger, one or more pairs of secondary leaflets may fold 

together. If the stim- 
ulus is still stronger, 
there may follow a suc- 
cessive closure of each 
pair of secondary leaf- 
lets from the apex to- 
ward the base of the 
primary leaflet, and 
then a slight drooping 
of the primary leaflet. . 
The secondary leaflets 
of the remaining pri- 
mary leaflets may then 
close successively from 
base to apex, the pri- 
mary leaflets droop 
somewhat, and the 
whole leaf droops. If 

the stimulus is very strong, its influence may be transmitted up 
and down the stem to other leaves and outward to their primary 
and secondary leaflets (Fig. 131). Some portions of a Mimosa 
plant are more sensitive than others to stimuli. The tips of 
secondary leaflets are^very sensitive, but nearly all the epidermal 
cells of aerial organs, except parts of the flower cluster, are like- 
wise sensitive to mechanical stimuli. 

Unless a certain pressure is applied when a leaflet is touched, 
no change occurs in its position. That is, a certain intensity of 
the stimulus is necessary in order to produce a visible response; 
the stimulus must be sufficiently intense to bring about physical 
and chemical changes within the cells of the leaf. 

The exact time interval between the application of a stimulus 
and the visible response can readily be determined. In Mimosa, 
the stimulus travels at the rate of from 8 to 20 mm. per second. 
The region in which the visible response occurs may be at a con- 
siderable distance from the region of perception of the stimulus. 

FIG. 131. A plant of Mimosa after leaflets and 
leaves have responded to a mechanical 



This visible response, which consists in a bending of a leaf or of a 
leaflet, is due to the action of a motor organ, the pulvinus. There 
is a pulvinus at the base of each secondary leaflet, one at the base 
of each primary leaflet, and one at the base of the petiole (Fig. 132). 
113. Structure and Action of a Pulvinus. A pulvinus is an en- 
largement at the base of a leaflet or of a petiole. In the center of 

FIG. 132. Leaf of Mimosa, showing the swellings (pulvini) at the bases of 
the petiole and of each primary and secondary leaflet. 

each pulvinus (Fig. 133) is a strand of vascular tissue, surrounded 
by a cylinder of living thin-walled cells; between these cells are 
fairly large intercellular spaces. When the effect of a stimulus has 
been transmitted to the pulvinus of the petiole, water exudes into 
the intercellular spaces from the thin-walled cells in the lower side 
of the pulvinus. These cells, therefore, lose their turgidity, and, 
since the turgidity of the cells in the upper side of the pulvinus is 



not diminished, the petiole is bent downward. That the cells in 
the upper portion of the pulvinus take an active part in the move- 
ment is shown by the fact that the petiole is bent upward against 

the force of gravity when the plant 
is inverted and the leaf is stim- 
ulated. The pulvini at the bases 
of primary leaflets behave in es- 
sentially the same way as does the 
pulvinus of the petiole, but the 
pulvini of the secondary leaflets 
behave in the opposite way, in the 
sense that these leaflets are bent 
upward instead of downward when 
they are stimulated. 

It is still uncertain exactly how 
the changes in permeability are 
brought about which result in a 
loss of water into the intercellular 
spaces from cells of a pulvinus. 
The stimulus will travel through a killed or girdled stem and is 
even transmitted through a severed stem if the two portions of 
the stem are connected by a glass tube filled with water. Some 

FIG. 133. Lengthwise section of a 
pulvinus (diagrammatic). 

FIG. 134. Oxalis plants; leaves in the characteristic positions by day (left) 
and by night (right). 

evidence suggests that the reaction is caused by a substance 
originating in the stimulated cells, which travels in the xylem 
from the stimulated region to the pulvinus and there brings about 
changes in permeability. 



114. " Sleep Movements." The same visible responses on the 
part of leaves and leaflets of Mimosa may be produced by other 
stimuli than that of contact. The change from light to darkness is 
a stimulus which causes the leaves and leaflets to change their 
positions. Leaves of many plants, including Mimosa and its 
relatives (peas, beans, alfalfa, clover, and other plants of the pulse 
family) as well as some members of other families, change their 
positions in late afternoon or evening (Fig. L34), thus reacting to 
variations in the degree of illumination. In the morning these 

FIG. 135. The flower of a tulip, open during the day (left) and 
closed at night (right). 

leaves return to the expanded position. The changes of position 
at the approach of night are often called " sleep movements," 
although they bear no relation to the sleep of animals. Other 
plant parts may respond to alternations of light and darkness; 
the flowers of many plants close at night and open in the morning. 
Such movements of floral parts, although induced by changes in 
illumination, are also influenced by temperature. Tulip and dan- 
delion flowers may remain closed even on bright days if the tem- 
perature is sufficiently low. In plant parts possessing pulvini, such 
as leaves and leaflets, the movements, as has been seen, are due 
to changes in the turgidity of certain cells. The movements of 
floral parts, however, often result from inequalities of growth on 
opposite sides. For example, the opening of the sepals and petals 
of a tulip (Fig. 135) is due to a greater growth on their inner than 
on their outer sides. In the closing of these floral parts, growth is 
greater on their outer than on their inner sides. 


115. Combined Effects of External Factors, -Xt&ny influences are 
constantly acting upon every plant. An ordinary land plant has 
its roots imbedded in the soil while its stem grows in the air. Among 
the factors affecting the roots of such a plant are the size and the 
chemical composition of the soil particles ; the amount and charac- 
ter of the soil water, which rains and other factors change from day 
to day and from season to season; and variations in temperature. 
The chemical and physical character of the soil is constantly 
changing, as well as its water content. The aerial portion of the 
plant is exposed to variations in temperature from hour to hour 
and from season to season; to variations in the duration of light 
and in its intensity; to variations in the amount of moisture, such 
as result from fogs, mists, rain, and snow; and to variations in 
many other factors. Thus the physical and chemical conditions 
prevailing in any particular region help to determine the type of 
plants found in that region. It is partly because conditions affecting 
the growth of plants vary at different parts of the earth's surface 
that the vegetation in each region differs more or less from that in 
any other region. 

However, there are great differences between plants of different 
kinds in the nature and degree of their responses to environmental 
conditions. Under the conditions characteristic of any particular 
locality, some plants grow more rapidly than others, or grow to a 
greater size, or produce flowers and fruits more abundantly. The 
adaptations of plants to particular external conditions determine 
which species will thrive in any region and which will not. The 
characteristic appearance of the flora of any locality, then, is de- 
termined, first, by the type of plants that are adapted to life in that 
locality, and second, by the ways in which the responses of those 
plants to local conditions affect their forms and habits of growth. 



128. Bases of Classification. Thus far, plants have been con- 
sidered from the standpoints of structure and of function without 
attempting to group them in an orderly manner. The earlier at- 
tempts of botanists to arrange plants in a system were entirely 
arbitrary and were based either upon the nature of the shoot, as 
the classification into trees, shrubs, and herbs, or upon a selection 
of certain floral features. Systems of classification based upon such 
arbitrarily selected characteristics are artificial. All present-day 
systems of classification are natural: that is, they attempt to indi- 
cate the actual relationships between plants of various kinds. 

The grouping of plants according to relationship is based upon 
the conclusion that different plants have been evolved one from an- 
other; that is, that the kinds of plants now living are descended 
from common ancestors more or less remote. The evidence used in 
establishing the evolutionary relationships between plants is of 
varied nature; it is derived from: (a) the comparison of vegetative 
structures; (&) the comparison of reproductive structures; (c) the- 
comparison of present-day plants with ancient plants (fossil plants) ; 
and (d) the study of the geographic distribution of plants now 
living. A more extensive discussion of the nature of this evidence 
will be found in Chapter XXXIII. 

129. General Course of Evolution. A study of present-day 
plants shows, among other things, that it is possible for a single 
cell to live alone as an individual plant. A cell which so lives 
alone has, in a very general way, the same structure as one of the 
cells found in a complex, many-celled plant; but, considered as 
an organism, it is less specialized, since this one cell must perform 
all the functions that are carried on by all the cells of a many- 
celled plant. 

All the available evidence indicates that the organisms which 
first appeared upon the earth were one-celled and that they lived 
in bodies of water. While these primitive organisms long ago 
disappeared, there are still many one-celled forms, some of which 



are certainly very different from their early ancestors; but some of 
those now living seem to possess relatively primitive character- 
istics. Many one-celled organisms formed, and many still form, 
temporary associations known as colonies. Others acquired the 
habit of remaining together in more persistent colonies. In some 
persistently colonial organisms, as will appear in later chapters, 
a diyision^ofjabor arose bet ween different groups of ce 
groups of cells became differentiated in ways that better fitted 
them to perform their particular functions. In this way tissues 
and organs appeared. In some lines of descent tissues became more 
varied and organs more highly specialized that is, each organ, 
each tissue, and each cell was limited more and more narrowly as 
to the particular work that it might perform. Thus in a very 
general way the course of evolution has been from simple to more 
and more complex. {However, it must not be overlooked that in 
many individual cases evolution has undoubtedly progressed in the 
opposite direction that is, from a complex to a simpler organ- 

The very first organisms, if they still existed, could not be classed 
either as plants or as animals. Many simple forms now living are 
likewise neither plants nor animals. Among one-celled organisms 
appeared some which, possessing chlorophyll and a cell wall, gave 
rise to the lines of descent which have led to the organisms that 
we call plants; whereas others, lacking a cell wall and lacking, or 
having lost, chlorophyll, became ancestors of what we know as 

Lf the course of time the habitation of some plants and animals 
was changed from water to land. Life on land presented new con- 
ditions, the adaptation to which led to the greater specialization 
of structures already present, as well as to the development of new 
tissues and organs. Hence, speaking very generally, the more com- 
plex and more highly specialized plants and animals live on land; 
the simpler ones, generally speaking, inhabit the water. 

130. Genealogy of Plants. The evolution of plants might $ 
represented by a line which branches early, and whose brandies 
themselves repeatedly branch. If all the species of plants that 
have ever lived were fully known, their relationships could perhaps 
be shown by a diagram having the form of a much-branched tree. 
The root of this genealogical tree would be among the primitive 
one-celled forms among which the genealogical tree of animals 


also arises; its highest branches would represent plants with the 
greatest complexity of structure. The branches would tend in 
all directions, including the downward one; for, as has been said, 
some plants have evolved from a complex to a simpler organization. 
The kinds of plants now living would be represented by the tips of 
some of the branches; and the task of one who studies the course 
of evolution is to reconstruct as large portions of this genealogical 
tree as the available knowledge of plants and of their relation- 
ships makes possible. 

131. Classification, Those individual plants which are alike in 
most or all of their structural jand .functional characters consti- 
The species is t!^ u^ Dif- 

ferent species whicrTresemble ea^l^eifasfty are grouped to- 
gether BS^STg^ms. Son^EImies a genus IncIuHes but one species; 
morlTcommonly it contains several*5r^anv "species ._ ~""~" 

The botanical^ namejof^^arit conslsts^oFtwo words, the first 
denoting the genus, the second the species. For example, all sun- 

commonly cultivated 

sunflowers constitute the species annuus; the name of the culti- 
vated sunflowers therefore (sTTelianffius o^rm/ims^ Similarly, the most 
widespread f orm of the^wateiFpest (Chap. II) belongs to the genus 
Elodea and the species canadensis; its name, therefore, is Elodea 

Just as species of plants are combined into genera, so genera 
in turn are grouped together in families; and families are com- 
bined to form successively orders, classes, and divisions. Follow- 
ing a widely used system of classification, all plants may be distrib- 
uted among the following four divisions: 

(a) Thallophytes are plants of relatively simple structure, not 
having stems, leaves, or roots like those of most ferns and seed 
plants. Thallophytes include algae and fungi. Typically, algae 
possess chlorophyll and can manufacture carbohydrates. Fungi 
lack chlorophyll and must obtain carbohydrates, and often other 
foods as well, from external sources. 

(6) Bryophytes (liverworts and mosses). Members of this divi- 
sion lack vascular tissues. In some, the structure is as simple as 
that of many thallophytes; others are differentiated into stem and 
leaves. Bryophytes characteristically have many-celled repro- 
ductive organs, the outer layer of cells of each organ being sterile* 
*(c) Pteridophytes (feftis and their allies). "A member of-*"! 


division has a plant body containing vascular tissues. Pteri- 
dophytes, like bryophytes, have many-celled reproductive organs, 
the outer cell layer of each such organ being sterile. 

(d) Spermatophytes (seed plants), which possess vascular tissues 
and which produce seeds. 

In the succeeding chapters a few types representing each of 
these divisions will be discussed, as nearly as possible in their 
evolutionary order; that is, following the simpler forms, others 
will be considered approximately in the order of their increasing 
complexity. It must be borne in mind that, since most or all of 
the ancestors of any particular form to be studied have disappeared, 
such a selected series of types may show the general course that 
evolution has taken, but it can not show the detailed history of the 
evolution of any one species. 



132. Nature. Members of this class have chloroplasts containing 
the pigments already mentioned ( 85). Their carbohydrate food 
is stored usually in the form of starch. Most of them at some stage 
produce motile cells whose movement is brought about by slender, 
whiplash-like structures (fiagella). The plant body (thallus) of a 
green alga may be a single cell or may be many-celled. In the lat- 
ter case, its structure is of less complexity than is found in certain 
other classes of algae. 

133. Distribution. Green algae, like other algae, are primarily 
aquatic and are widely distributed in both fresh and salt waters. 
Fresh-water members of the class occur in streams, lakes, and 
ponds. Some species grow attached to rocks and debris, some 
to larger plants. Others are unattached and constitute a large 
portion of the free-floating population^ of microscopic organisms 
known as plankton. At times the plankton algae of lakes and 
ponds are so^liumerous as to make the water appear colored. 
Such an abundance of plankton algae in reservoirs, followed by 
their death and decay, frequently gives rise to unpleasant tastes 
and odors which constitute a serious problem in connection with 
city water supplies. Green algae may grow also in places other 
than permanent bodies of water. They occur, for instance, in tem- 
porary pools, on moist, rocky cliff's, and on the shaded sides of 
trees and rocks. They grow on and in damp soils, where they some- 
times develop so luxuriantly as to form a distinct layer. Certain 
species can live arid multiply under conditions ordinarily thought 
of as unfavorable for the growth of plants. Examples of this na- 
ture are species living in brine lakes with a salt content of 15 to 30 
per cent, and others which grow in the semi-permanent snow fields 
of arctic regions or of high mountains. Algae of the latter type may 
be so abundant as to cause a green or red coloration of the snow. 

Many marine species live chiefly in shallow water along ocean 
shores, often being attached to rocks at levels that are exposed at 
low tide. Some attached forms, however, occur where the water is 




as much as 300 to 600 feet in depth. Other marine algae are free- 
floating and constitute a part of the ocean plankton. 

The green algae to be described in the present chapter are 
arranged approximately in their order of increasing complexity, 
either in structure or in methods of reproduction ; but it must be 
borne in mind that the forms selected show only the general 
course of evolution within the class. 


134. Structure. Chlamydomonas is representative of the more 
primitive green algae. It occurs in ditches, pools, and lakes, or on 
moist soil. Sometimes it is found in such quantity that the water 
appears green; but many other minute algae may occur with 
Chlamydomonas and help to give the water a green or greenish 

A cell of Chlamydomonas (Fig. 143, A) is typically egg-shaped. 
Like other cells that have been studied, it has a wall containing 




FIG. 143. Chlamydomonas. A, motile cell. B, cross section of a cell at the 
tevel of the nucleus. C, cross section at the level of the pyrenoid. D, 4- 
celled colony enclosed by the parent-cell wall E, many-celled colony en- 
closed in a gelatinous matrix formed from the parent-cell wall. 

eellulose. The most conspicuous portion of the protoplasm is 

^hloroplast. Each green cell of an Elodea or sunflower leaf cant 

numerous chloroplasts, but a Chlamydomonas cell has but t] 

This chloroplast, however, is large and very different 

from the small, rounded chloroplasts that have been 

It looks somewhat life a horseshoe when viewed latera.^^ ,___ 

sidered in three dimensions, it may be likened to a cup with v/ry 


thick bottom and sides. As in other cells, the film of the cyto- 
plasm, lying just within the wall, is the plasma membrane. The 
chloroplast lies next within the plasma membrane. Imbedded 
in the center of the thicker, posterior part of the chloroplast is a 
small, colorless, spherical pyrenoid. The pyrenoid is a specialized 
portion of the chloroplast whose particular function is starch- 
formation. Treatment of the cell with iodine shows the presence 
of minute, variously shaped starch granules surrounding the pyre- 
noid. In fact, it is usually the zone of starch granules about the 
pyrenoid that is seen in a living cell, rather than the pyrenoid it- 
self. The cell has no central vacuole, the central region inside the 
cup-shaped chloroplast being occupied by dense, cytoplasm in 
which lies a small nucleus. It is a noteworthy fact that the nucleus 
of Chlamydomonas is similar, except for size, to a nucleus of one 
of the more complex plants. It has a membrane, nuclear sap, a 
nucleolus, and a chromatic network. 

At the anterior end of the cell, two fine, thread-like extrusions 
of cytoplasm called flagella pass through the cell wall. Flagella 
are motile organs which, by lashing backward and forward, ffropel 
the cell through the water. The movement of the cell is not hap- 
hazard, but is a definitely directed response to stimuli. One, stim- 
ulus largely affecting its movement is light, and the mechanism 
for the reception of light stimuli is localized in a small orange-red 
pigment spot near the anterior end of the cell. The effect of the 
light stimulus, received by the pigment spot, is transmitted to the 
flagella, causing them to move the cell in a definite direction. This 
response is usually positive, the cell swimming toward the light. 
If, however, the intensity of the light stimulus passes a certain 
point, the response is negative that is, the plant swims away 
from the light. These responses can be shown by placing a dish 
containing many Chlamydomonas cells so that it is illuminated 
from only one side. In light of moderate intensity the cells collect 
as a green mass on the side of the dish toward the light (a positive 
response), but when placed in direct sunlight they frequently col- 
lect on the side away from the light (a negative response). 

In the dense cytoplasm near the base of the flagella are a variable 
number (commonly two) of small transparent contractik vacuoles. 
The size of these vacuoles is not constant; they gradually expand 
to a certain size and then contract, thus extruding their contenut 
Their function seems to be that of excretory organs. 


136. Reproduction. Sooner or later the cell ceases to move and 
draws in its flagella, and sometimes the wall becomes somewhat 
thicker. While the cell is in this quiescent state, it divides to form 
two daughter cells, both of which remain within the parent-cell 
wall. Cell division is preceded by a division of the nucleus, which 
goes on in substantially the manner described in Chapter XII. 
The division of the cell is by constriction. 

This division of the parent cell is usually followed by another 
nuclear division, and this by a division of each of the daughter 
cells, forming four (Fig. 143, Z>) ; and in some cases there is a third 
division, resulting in the formation of eight cells. The cells so 
formed by division, whether tw r o, four, or eight, remain for a time 
within the parent-cell wall and are not at first provided with in- 
dependent walls. A group of cells so held together may be called 
a colony; but this colony of Chlamydorrionas is but a temporary 
association. Sooner or later, each cell of the colony forms a wall 
of its own, produces flagella, and the parent-cell wall breaks down, 
allowing the young cells to become free. Each young cell is similar 
to the motile cell first described; and each, as it swims about, 
grows to approximately the size of the original cell. 

Under certain environmental conditions the cells of a colony do 
not develop flagella and become motile, but remain within a matrix 
formed by a gelatinization of the parent-cell wall (Fig. 143, E). 
Since each of the cells may in turn grow, divide, and form daughter 
cells, a colony is produced consisting of numerous cells, sometimes 
as many as 100 or more, all enclosed within a single gelatinous 
matrix. Eventually the cells of such a colony form flagella and 
become free. 

Thus, whatever the form of the resultant colony, it is during a 
quiescent stage in the life history of Chlamydomonas that an in- 
crease in the number of cells takes place. This increase in number 
is brought about by cell division. An increase in number of individ- 
uals is commonly spoken of as reproduction. In Chlamydomonas, 
therefore, as in all other one-celled organisms, reproduction and 
cell division are synonymous terms. 

136. Gametes and Their Union (Fig. 144). Under some condi- 
tions the division of a quiescent cell and of its offspring continues 
until 16, 32, or 64 cells are formed. Except for their smaller size, 
the appearance of these cells is exactly like that of the motile cells 
described above; but their function is different, and they are called 


gametes. The gametes are liberated by a dissolution of the parent- 
cell wall, and after swimming about for a time they come together 
in pairs. The cells of each pair are in contact at their anterior 
ends (Fig. 144, C). After two gametes have met, they begin to 
unite to form a single cell. The uniting gametes may be naked, or 
each may be surrounded by a wall. In the latter case the proto- 
plasts escape from the walls, which are left behind and take no 

FIG. 144. Gametic union in Chlamydomonas. A, gametes before their libera- 
tion from the parent-cell wall. #, free-swimming gametes. C, early stage 
in the union of gametes. D, zygote, just after the union of the gametes. 
E, zygote, flagella withdrawn. F f mature zygote. G, germination of a 

part in the process. In most species there is no visible difference 
between the two uniting gametes. However, in certain species, 
even though the gametes appear to be identical, union takes place 
only between two gametes which were produced by the division 
of different parent cells. Those species in which the gametes ap- 
pear to be exactly alike represent the simplest type of gametic 

The product of the union of two gametes is a zygote (Fig. 144, 
D-F). When first formed, the zygote has no wall and the two 
pigment spots, two chlofoplasts, and two nuclei derived from the 
respective gametes are 'still present. The flagella may disappear 
during the union; or they may persist, the young zygote in the 
latter case being motile for a time. A motile zygote is readily 
distinguished from a motile vegetative cell or gamete by its pos- 
session of four flagella, as well as of two pigment spots and two 
chloroplasts. Sooner or later the flagella disappear, and the zygote 
becomes rounded and begins to secrete a thick wall. While this 


wall is being formed, the two nuclei unite to form a single nucleus. 
The pigment spots, chloroplasts, and pyrenoids gradually become 
indistinguishable. In view of what is known of game tic unions 
in some other algae, it is probable that these bodies do not unite. 

The zygote of Chlamydomonas is a resting cell. Its resistant 
wall is especially adapted to withstand unfavorable conditions, 
such as a drying up of the body of water in which the plant lives 
a condition that would kill an ordinary motile cell. The contents 
of the zygote become red; the reserve starch disappears and is 
replaced by oil. Sooner or later, however, the protoplast of the 
zygote becomes green and divides, the daughter protoplasts divide, 
and each ultimate daughter cell, after developing flagella and a 
wall, is liberated by a rupture of the old wall of the zygote. The 
free-swimming daughter cells are then similar to the motile cells 
first described (Fig. 144, G). 

137. Relationships. Chlamydomonas seems to be related on the 
one hand to a widely distributed class of one-celled aquatic organ- 
isms known as flagellates, and on the other to many or possibly all 
of the plants classed as green algae. The flagellates are commonly 
considered to be the relatively primitive group from which most 
classes of plants and of animals have descended. Chlamydomonas 
and the order (Volvocales) of which it is a member belong equally 
among the flagellates and among the green algae. As will be noted 
in later chapters, many of the so-called higher plants seem to have 
been derived through the green algae from flagellates like Chlamy- 
domonas; others are probably independently 4esc,0filji*f from flagel- 
lates of different types. 

138. Sexual Differentiation. Many simple algae and other one- 
celled or colonial organisms, including many flagellates, form no 
gametes so far as is known. However, the majority of organisms, 
both simple and complex, produce gametes which unite in pairs. 
The establishment of the habit of gametic union so early in evolu- 
tionary history, and its general persistence, suggest that the union 
of gametes may bear an important relation to the welfare of plant 
and animal species. 

In such comparatively primitive organisms as Chlamydomonas, 
both gametes of any pair are motile and in most species are, so far 
as can be seen, alike in size and structure. But in various lines of 



ber of whose cells is some multiple of two. In one of the most 
widely distributed of these (Scenedesmus, Fig. 148, B-D), the 
somewhat elongated cells lie side by side to form a row of 2, 4, 8, 
or 16 cells. In another genus (Pediastrum, Fig. 148, A), the cells 
are usually arranged in concentric rings to form a flat plate one 
cell in thickness. In both Scenedesmus and Pediastrum, any cell 
of a colony may, by a series of divisions, form a daughter colony, 


143. Structure and Reproduction. Many colonial green algae 
have an indefinite number of cells attached end to end in an un- 
branched row or filament. Such an alga is Ulothrix (Fig. 149), 

FIG. 149. Ulothrix. A, filament, some of whose cells have 
swarm-spores. B, liberation of gametes. C, Z), union of 
ture zygote. 

ided to form 
E, ma- 

usual ly found attached to stones, sticks, or othei^fefolbcts in small, 
cool, swiftly flowing streams, or in pools or otlUk/tJ&dies of water 
which do not become warm and stagnant. Each filamentous colony 
is commonly attached to the substrate by a disk-like holdfast 
developed from the basal cell. Sometimes colonies are free-floating. 
The cells are cylinders of very unequal lengths. Each cell contains 
a single chloroplast having the form of a partial or complete girdle 
imbedded in the peripheral part of the dense cytoplasm. The 
chloroplast contains one, two, or several pyrenoids. Near the 
center of the cell is a large nucleus. 

Reproduction of cells takes place in Ulothrix, as in other plants, 
by means of cell division. The cells divide by constriction. Divi- 
sion increases the number of cells in the colony but not the number 


of colonies. The number of colonies may be increased, however, 
by an accidental breaking of the filaments. 

Reproduction of the colony (that is, the formation of now 
colonies) is brought about also by the production of motile cells 
(swarm-spores). With the exception of a few cells at the base, any 
cell of a filament may divide to form 2, 4, 8, 16, or 32 small naked 
cells. These daughter cells, at first angular, become rounded, form 
flagella, and, enclosed in a vesicle, escape through a pore in the 
side of the parent-cell wall (Fig. 149, A). These swarm-spores are 
similar to the motile cells of Chlamydomonas, each being ovoid 
and having a prominent chloroplast and a conspicuous pigment 
spot. Unlike the motile cells of Chlamydomonas, however, each 
swarm-spore has four flagella instead of two and is without a cell 
wall. After swimming for some time the spore comes to rest on 
some solid body, withdraws its flagella, forms a wall, and pushfcs 
out a protuberance which is the beginning of the formation of a 
holdfast. The growth and transverse division of this cell, of its 
daughter cells, and of their offspring, the cells always remaining 
in contact, give rise to a new filament. A cell of a colony may also, 
under some conditions, become an immobile rounded spore pro- 
vided with a wall; or it may first divide to produce two or more 
such spores. A spore of this type may develop directly into a new 
filament, or may be transformed into a ^warm-spore which will so 

144. Gametic Union. Ulothrix also produces gametes. These 
are formed in the same manner as are swarm-spores, and are 
similar to the latter except that they are frequently smaller and 
that each has two instead of four flagella (Fig. 149, B). After 
swimming for a time, the gametes unite in pairs (Fig. 149, C-fJ). 
In this union the flagella do not disappear, so that each zygote, 
having four flagella, continues moving about after its formation. 
Eventually it comes to rest, withdraws its flagella, secretes a wall, 
and, after a short period of rest, divides to form several (at least 
four) non-motile spores, each of which, like a swarm-spore, de- 
velops into a new filamentous colony. 

145. Ulva. Certain green algae with cells similar to those of Ulo- 
thrix differ from that alga in the structure of their colonies. One 
of the most striking of these is Ulva, the "sea lettuce" (Fig. 150). 



This plant commonly grows on rocks and wharves in brackish 
or salt water. The thallus, composed of Ulothrix-like cells, is an 
irregularly expanded sheet, often with a surface area of several 
square inches but only two 
cells in thickness. The plant 
is anchored at its basal end 
by a very irregularly shaped 
holdfast composed of elon- 
gated colls or rows of cells. 
Ulva produces four-flagellate 
swarm -spores and two- 
f 1 agellatc gametes . 


146. Structure and Repro- 
duction. Some filamentous 
green algae differ from Ulo- 
thrix both in cell structure 
and in colonial organization. 
An example is seen in Cla- 
dophora (Fig. 151, A), which 
grows attached to objects in 
streams arid in shallow water 
along the shores of lakes. 
The cylindrical cells are united 
end to end to form a branch- 
ing filament. Each cell is 
surrounded by^Jhicl^jvalL 
Within the wall is a layer of dense cytoplasm in which are 
imbedded many small disk-shaped chloroplasts. Some chloro- 
plasts contain one pyrenoid each, others lack pyrenoids. In some 
species the chloroplasts appear to be united into a continuous net- 
work. Cladophora differs from the algae previously described in 
that each cell contains many nuclei. These lie imbedded in the 
dense cytoplasm but farther inward than the chloroplasts. 

Reproduction of cells is brought about through cell division by 
constriction. New branches are usually formed only by cells near 
the upper end of a filament. A branch originates as a lateral out- 
growth from the upper end of a cell, and the first cross wall of the 
new branch is formed close to the point of origin of the outgrowth. 

FIG. 150. Ulva. A, the expanded leaf- 
like thallus. B y portion of thallus, show- 
ing cell structure. 



Reproduction of the colony results from the formation of one- 
nucleate, four-flagellate swarm-spores. These are produced by 
cells near the tips of branches, the protoplast of each such cell 
dividing to form many swarm-spores. The spores are liberated 
through a small pore near the upper end of the cell, or at its apex 

if it is the terminal cell of a 
branch. After swimming for 
a time a swarm-spore comes to 
rest upon some solid object, 
retracts its flagella, and se- 
cretes a wall. Growth and 
division of this cell, of its 
daughter cells, and of their 
descendants results in a new 
filament identical in appear- 
ance with that which produced 


147. Gametic Union. Gam- 
etes are formed and liberated 
in the same manner as are 
swarm-spores, and are similar 
to the latter except that each 
hasjteo. instead ~ of -four Jjjgtgelja 
FIG. 151. Cladophora. A, portion of a (Fig. 151, B). After swimming 

sw^m-swres^ I empt^walls from ^ or a ^ me > ^ e gametes unite 
which swarm-spores have escaped, in* pairs to form zygotes. A 
B formation and liberation of 

some species the zygote^develops immediately into a new filament 
which produces swarm-spores only. 

148. Life Cycle. In the species of Cladophora last referred to, 
but apparently not in all species, the life cycle includes two dis- 
tinct phases. Swarm-spores develop into plants which produce 
gametes. Each zygote formed by gametic union develops im- 
mediately into a plant exactly similar to that which formed the 
gametes; this plant, however, produces swarm-spores only. Since 
the spore-bearing plant (or generation) of Cladophora gives rise 
through spores to the gamete-bearing generation, and the latter 
generation through gametes Upd zygotes gives rise to the spore- 
bearing generation, there is Hibernation of generations. In most 

zygote becomes immobile and 
a wa j} l n 



green algae there is no comparable alternation of generations, 
although a few others are known to have an alternation essentially 
similar to that just described. 


149. Structure and Reproduction. Spirogyra, one of the free- 
floating plants commonly known as "pond scums," is a green alga 
whose cells form permanent filamentous colonies. It occurs in 
pools and other bodies of water and frequently forms masses of 
considerable sizelTlt may be distinguished from most other thread- 
like green algae by the slippery feeling of the threads, due to a 
gelatinous outer layer </f the cell walL) 

The cells of Spirogyra (Fig. 152, A) are cylindrical and attached 
end to end to form an unbranched thread. This arrangement 
results from the fact that all cell divisions take place in the same 
plane, namely, at right angles to the long axis of the cylindrical 
cells. A 

A thin layer of dense cytoplasm lies just within the wall of each 
cell. The most conspicuous feature of the cell, and the one from 

Chloroplast Pyrenoid Nucleus Central Vacuole Nucleus Pyrenoid 

I / 

Gelatinous Sheath Cell Wall A Dense Cytoplasm 

Gelatinous Sheath Central 

r Vacuole 

FIG. 152. A, cell of Spirogyra. B, cross section of a cell through the nucleus, 

which the name Spirogyra is derived, is the chloroplast. Each 
chloroplast is a trough-shaped ribbjoa. extending spiraUy JTCHBPL end 
to end of the cell; it is part of and contained in the dense cyto- 
plasm.TEach chloroplast contains several pyrenoids. Throughout 
the length of the chloroplast is a thick central strand connecting 
and surrounding the pyrenoids, the intervals between successive 
pyrenoids being approximately equal. A central vacuole occupies 
the greater portion of the space within the wall. In the center of 
this vacuole is the nucleus, surrojM&ed by a layer of dense cyto- 
plasm from which numerous cyt^pasmic strands extend to the? 


dense cytoplasmic layer at the periphery of the cell. Each strand 
usually joins the peripheral layer just beneath a pyrenoid. 

Reproduction of the cell takes place in Spirogyra in essentially 
the same manner as in other plants that is, by means of cell 
division. Cell division in Spirogyra, under ordinary conditions, 
occurs at night. It is preceded by a nuclear division similar to that 
already described for the cells of a root tip. The division of the 
cells increases the number of cells in the colony but not the number 
of colonies. There is usually no definite means for reproduction of 
the colony that is, an increase in number of colonies during 
the vegetative life of the plant. In most species of Spirogyra new 
colonies are formed only when a filament is accidentally severed. 
Since various aquatic animals feed upon the alga, the filaments 
are frequently cut, so increasing the number of plants. In certain 
species of Spirogyra, especially in some with small cells, the fila-. 
ments at times become separated into individual cells or short 
rows of a few cells each, which may then grow into long fila- 

160. Gametic Union. As a rule, the production of gametes by 
each species of Spirogyra occurs at a definite time of the year, 
commonly in spring or autumn. In preparation for this process the 
first step observed (in most species) is a pairing of the filaments 
so that the filaments of each pair lie side by side. Small dome- 
shaped protuberances now grow toward each other from opposite 
cells in the two filaments (Fig. 153, A, B), each protuberance in- 
creasing in size until it becomes a short tubular outgrowth. The 
outgrowths from opposite cells come into contact; the wall of each 
is digested at the point of contact, and thus a conjugation tube is 
formed. When the formation of the conjugation tube begins, the 
protoplasts (gametes) of the conjugating cells are similar in ap- 
pearance, but as the protuberances grow toward each other, one 
of each pair of gametes contracts from the wall and becomes 
rounded. This change in size is brought about by a loss of water 
from the protoplast. The contracted gamete soon migrates through 
the conjugation tube (Fig. 153, (7) toward the other, which at about 
this time also contracts and rounds up. The gamete which con- 
tracts first and which moves toward the other gamete is spoken 
of as male because of its greater activity; the passive gamete is 
female. Usually all the cells of a particular filament which function 
as gametes behave as gametes of the same sex, but at times some 



of the cells in a filament become male and others in the same fila- 
ment female gametes. 

In certain species of Spirogyra, conjugation takes place between 
adjacent cells of the same filament rather than between cells of 
separate filaments. The differentiation into male and female 
gametes and the formation of a zygote go on, however, in the same 
way as when cells of different filaments conjugate. 

After the male gamete has migrated into the cell cavity of the 
female gamete', the two unite to form a zygote (Fig. 153, D-F). 
Both a nuclear and a cytoplasmic union are involved. The cyto- 
plasm of the gametes seems to become intermingled; but the 

D F 

FIG. 153. Spirogyra; stages in the union of gametes and the maturing of a 
zygote (diagrammatic). 

chloroplasts do not unite, those of each gamete remaining distinct 
for some time. The subsequent behavior of the chloroplasts is 
difficult to follow, but the available evidence indicates that the 
chloroplast or chloroplasts contributed by the male gamete dis- 
integrate, so that the mature zygote contains only the chloroplast 
or chloroplasts derived from the female gamete. The zygote soon 
begins to secrete a wall which, when the nuclei have united and the 
paternal chloroplasts have disappeared, has become thick and re- 
sfctant. By this time the zygote lies at the bottom of the pool or 
other body of water, still enclosed by the old wall of the female, 
gamete. The zygote eventually becomes free, since both th^ *? 



which enclosed the female gamete and the empty wall that for- 
merly contained the male gamete disintegrate. 

When a zygote is newly formed it contains the nuclei derived 
from the male and female gametes; these unite to form a single 
nucleus (Fig. 154, A, 5). After a time this nucleus divides to 
form two daughter nuclei, and each daughter nucleus in turn di- 

Fio. 154. Diagrams showing stages in the history of a zygote of Spirogyra; 
AH in section; /, / in surface view. A, zygote just after the union of 
gametes; gamete nuclei still separate. B, male and female nuclei have 
united. C, first nuclear division in the zygote, forming 2 nuclei (D). E, 
second nuclear division, forming 4 nuclei (F). G, 3 nuclei beginning to dis- 
integrate. H, after the disappearance of 3 nuclei. /, J, stages in the 
germination of a zygote. 

vides (Fig. 154, C-F). The four nuclei now present in the zygote 
are similar when first formed, but three of them soon show signs 
of disintegration and eventually disappear. The fourth nucleus, 
however, persists and is the sole nucleus present in the mature 
zygote (Fig. 154, (?, H). The significance of this behavior of nu- 
clei in the zygote will become clear when certain corresponding 
processes in some of the more complex plants have been dis- 
cussed (see Chap. XXV). 

161. Germination of a Zygote. After the union of the gametes, 
the color of the zygote contents changes from green to orange-red. 
Shortly before the zygote is to germinate, its contents again be- 
come green. The interval between the union of gametes and the 
germination of the zygote may be a few weeks or a few months, or 
it may extend from one spring until the next. In germination the 
heavy outer layer of the zygote wall is broken, and the cell contents, 
surrounded by the inner layer of the zygote wall, form a short 


tubular outgrowth. The structures typical of a Spirogyra cell 
(chloroplast or chloroplasts, nucleus, and dense cytoplasm) are 
visible in this cell that lies partly within and partly without the 
broken portion of the wall of the zygote. A division of the nu- 
cleus is followed by a transverse division of the cell (Fig. 154, 7). 
The daughter cell that is now partly within the zygote wall does 
not divide, but from the outer daughter cell a new filament is 
produced by repeated cell division and growth (Fig. 154, J). 
This filament is similar to the parent filaments. 

When from any cause a cell that has prepared to function as a 
gamete does not unite with another, it not infrequently rounds up, 
secretes a thick wall, and so becomes, except for its somewhat 
smaller size, identical in appearance with a zygote. Such a resting 
cell (spore) can germinate in the same manner as a zygote to 
form a new filament. Thus it appears that any cell of a Spirogyra 
filament is capable of functioning either as a vegetative cell, as a 
spore which can grow into a new plant, or as a gamete. 


162. Structure. Almost every collection of algae from fresh- 
water pools or lakes contains members of the group known as 

It has been seen that the general course of evolution has been 
from simple to more complex forms. The filamentous colonies of 
Spirogyra and of Ulothrix represent, therefore, a more advanced 
as well as a more complex condition than does the one-celled 
Chlamydomonas. Desmids, on the other hand, are (chiefly) one- 
celled organisms whose immediate ancestors seem to have been 
filamentous^ algae closely , .related. M^kogyr^ They illustrate, 
therefore, the possibility that evolution may at times be from 
complex to simpler, instead of from simple to more complex. 

Most of the thousands of known species of desmids are dis- 
tinguished from other one-celled green algae by a conspicuous 
median constriction, each cell thus consisting of two symmetri- 
cal half-cells (Fig. 155). The cells of various species differ greaHfy 
in shape and frequently bear spines or other protuberances, 
Each half-cell, contains at least one chloroplast, often elaborately 
lobed, and within each chloroplast are one or more pyrenoids. 
A nucleus lies in the cytoplasm in the region of the median con- 



163. Reproduction. New individuals are formed by the division 
of a parent cell into two daughter cells (Fig. 155, (?). Before the 
cell divides the nucleus divides, each half-cell receiving a daughter 

A ' 

FIG. 155. Desraids. A, Closterium. B, Xanthidium. C, Staurastrum. Z>, Mi- 
crasterias. E, F, Cosmarium. G, division of a Cosmarium cell. 

nucleus. Nuclear division is followed by a transverse division of the 
cell in the plane of the median constriction. Each daughter cell 
&t first consists, therefore, of one half-cell and a portion of the 

FIG. 156. Staurastrum; stages 

in the union of gametes.^ 
De Bary. 

Adapted from 

median region of the parent cell. Later, by a growth of the con- 
stricted portion, each daughter cell develops a new half-cell. In 
most desmids the daughter cells become separated from each 
other as the new half-cells are forming, but in a few species the 
daughter cells remain united and by repeated division give rise 
to a filamentous colony. 



154. Gametic Union. Occasionally, when two mature cells come 
to lie close to each other, their walls break at the median con- 
strictions and their protoplasts function as gametes (Fig. 156), 
flowing out and uniting with each other to form a zygote. These 
non-flagellate gametes resemble those of Spirogyra except that 
they are not differentiated as male and female. When first formed 
the zygote is naked, but soon after its formation it secretes a thick 
wall. After a considerable period of rest the wall of the zygote 
breaks or becomes gelatinized and its contents develop into one, 
or (by division) into two or four vegetative cells of the form 
characteristic of the species. 


155. Structure and Reproduction. Oedogonium is another 
unbranched filamentous green alga of frequent occurrence, at- 

3 C D 


FIG. 157. Oedogonium. A, vegetative cell. B, swarm-spore before liberation, 
(7, liberation of a swarm-spore. D, young plant produced by the germina- 
tion of a swarm-spore. B-D after Hirn. 

tached or forming floating masses in pools and other bodies of 
quiet water. The cylindrical cells (Fig. 157, A) are joined end to 
end. Inside the wall of each cell are a layer of dense cytoplasm 
containing a single chloroplast, a nucleus, and a large central vac- 
uole. The chloroplast has the shape of a hollow cylinder with 
many irregularly shaped perforations. The large nucleus may lie 
toward one side of the cell, or may be suspended by cytoplasmip 


strands in the middle of the central vacuole much as is the nucleus 
of Spirogyra. 

Each cell of a filament may reproduce by division, the subsequent 
growth of the two daughter cells resulting, as in Spirogyra, in an 
increase in the length of the colony. As in Spirogyra, too, the num- 
ber of colonies may be increased by an accidental breaking of the 

Reproduction of a colony occurs also through the formation of 
swarm-spores (Fig. 157, Z?, C). The protoplast of a cell with- 
draws somewhat from the wall, becomes rounded, and develops 
a colorless area at one side. A circle of flagella is developed at the 
margin of this colorless area. The protoplast has thus been meta- 
morphosed into a swarm-spore. After the spore is mature, the old 
wall enclosing it splits transversely at one end, and the spore, 
moving slowly out through the opening in the wall, swims aw'ay 
by means of its flagella. 

After swimming for some time, the spore comes to rest with its 
flagellate end in contact with some solid body, often a filament of 
Oedogonium. Soon the spore withdraws its flagella and secretes a 
wall, and its colorless end becomes modified into a disk-shaped 
or root-like holdfast. The cell now increases somewhat in length 
(Fig. 157, D) and then divides transversely into two daughter 
cells. The lower daughter cell, that with the holdfast, does not 
divide again; the upper cell by repeated transverse divisions gives 
rise to a long filament, which becomes free-floating if accidentally 
broken from its attachment. 

156. Gametic Union (Fig. 158). Oedogonium forms gametes of 
two very different sorts. Any cell in a filament, except the basal 
cell, is capable of becoming an oogonium, but a cell which so de- 
velops is always one formed by a recent division. An oogonium 
becomes somewhat broader than a vegetative cell and spherical or 
ellipsoid in shape. Its protoplast (the female gamete or egg) 
shrinks and rounds up entirely within, and free from, the wall. 
As the egg approaches maturity, a small circular pore may be 
formed in the oogonial wall. In some species the wall cracks trans- 
versely instead of forming a pore. Since the oogonium and the egg 
are really the same cell, it is hardly necessary to apply both names 
in Oedogonium. But for the sake of harmonizing the use of terms 
in this and in some other algae in which an oogonium forms several 
eggs by division, it is customary to distinguish the egg of Oedo- 



gonium, which is a' protoplast only, from the oogonium, which is 
the protoplast plus the enclosing wall. 

Simultaneously with the development of oogonia, certain 
other cells of the same or of another filament by repeated trans- 







FIG. 158. Oedogonium. A, antheridia with antherozoids. B, oogonium in 
which an egg and an antherozoid are uniting. C, oogonium containing a 
mature zygote. After Him. 

verse division form a short series of disk-shaped cells, each of which 
is an antheridium. The protoplast of each antheridium either be- 
comes a male gamete (antherozoid) or divides to form two anther- 
ozoids. Except for their smaller size and their fewer flagella, anther- 
ozoids are similar in structure to swarm-spores. They are liberated 
from the walls enclosing them in the same manner as are swarm- 
spores. In certain species, antheridia are borne on very small, 
few-celled filaments which are attached to a filament bearing 
oogonia. An antherozoid swimming in the vicinity of an oogonium 
responds to a stimulus, exerted probably by a substance diffusing 
from the oogonium, swims through the pore or crack in the oogonial 
wall, and unites with the egg. The resultant zygote soon secretes a 
thick wall which often bears ridges, spines, or otKer "protuberances. 



The zygote is eventually liberated by the decay of the oogonial 
wall. After a period of dormancy it germinates (Fig. 159, A). 
The zygote wall breaks open and the protoplast by division forms 
four protoplasts (Fig. 159, B, C), each of which becomes a swarm- 
spore that may come to rest and develop into a new filament. 

In the organization of its colony, Oedogonium represents no 
advance over Spirogyra or Ulothrix. However, in the marked 



* Zygote Wall 

FIG. 159. Germination of the zygote of Oedogonium. After Juranyi. 

differentiation of its gametes it presents a condition far in advance 
of that found in either Ulothrix or Spirogyra. 


157. Structure and Reproduction (Fig. 160). Vaucheria com- 
monly forms a green, felt-like mass on damp soil or in shallow 
water. Each plant-is a sparsely branched thread that may attain 
a length of several inches but consists of onJy a single cell. Within 
the wall of this cell is a layer of dense cytoplasm containing small 
flattened, rounded chloroplasts. Imbedded in the dense cytoplasm 
are numerous nuclei. A noteworthy feature of the chloroplasts of 
Vaucheria is the absence^ of pyrenoids and of their accompanying 
starch granules, the reserve Food being^stpred^ in_the Jorm^ofjoil 
droplets. The central vacuole constitutes the greater part of the 
volume of the plant. 

A large swarm-spore is produced by a cell division near the end 
of a filament, followed by a rounding up of the portion of the 
protoplasm so cut off. Such a spore contains many nuclei and 
chloroplasts. Shortly before its liberation by a breaking of the 



cell wall, a pair of flagella are developed opposite each nucleus, 
so that the freed swarm-spore bears numerous flagella. After 
swimming for a time the spore comes to rest, withdraws its flagella, 

FIG. 160. Vaucheria. A, portion of a plant. B, end of a branch, showing the 
formation of a swarm-spore. C, swarrn-spore after its liberation. 

and grows into a new plant. Under certain conditions the spore 
does not form flagella. However, when such a non-motile spore 


i* *)} Oogonium 

/ ^ \ 


FIG. 161. Vaucheria. A, young sex organs. #, sex organs at a later stage; the 
antheridium empty, the oogonium containing a zygote. 

becoiriLes separated from the plant which produced it, it develops 
into a new plant in the same manner as does a swarm-spore. 

158. Gametic Union (Fig. 161). The gametes of Vaucheria 
are differentiated as male and female. The male gametes (an- 
therozoids) are produced within an antheridium which is at first 
a small terminal cell cut off by a cell division in a short cun$Sl 
branch. A young antheridium contains many nuclei and chloro- 


plasts, but later the chloroplasts disappear. Eventually the proto- 
plast of the antheridium is divided into a number of one-nucleate 
antherozoids which are liberated by a dissolution of the wall at 
the apex of the antheridium. Each antherozoid is small and spindle- 
shaped, lacks a wall, and has two lateral flagella. 

The female gametes (eggs) are borne in oogonia. Each oogonium 
in some species is borne on the same branch that bears an anther- 
idium; the oogonia of other species are borne on separate branches 
that arise near the antheridial branches. Like an antheridium, an 
oogonium is the end of a branch separated by a cell division and 
the formation of a cross wall. At first the young oogonium con- 
tains several nuclei, only one of which, however, persists; the 
protoplast of the oogonium, which includes the persistent nucleus, 
is now the egg. As the oogonium matures, a beak-like protuberance 
develops at one side and the wall disintegrates at this point. The 
antherozoids, which are liberated at this time, enter the oogonium 
through the opening, and one antherozoid unites with the egg. 
After this union the zygote develops a thick wall. It is eventually 
liberated by a disintegration of the oogonial wall and, after a 
longer or shorter period of rest, develops directly into a new fila- 
mentous plant. 

159. Sexual Differentiation. The green algae discussed in the 
present chapter illustrate several stages in the development of 
differences between gametes. Most species of Chlamydomonas form 
gametes that are visibly alike in all respects. The same is true 
of Ulothrix, although this alga has advanced much beyond 
Chlamydomonas in its colonial organization. In Spirogyra, the 
gametes have become differentiated with respect to motility, but are 
not greatly different in other ways. In Volvox, Oedogonium, and 
Vaucheria, male and female gametes have come to be very different 
in size, structure, and behavior. Since Spirogyra, Volvox, Oedogo- 
nium, and Vaucheria represent distinct lines of descent, it appears 
that sexual differentiation has proceeded on parallel courses in 
various independent lines. 



160. Nature. The members of this class contain a blue pigment, 
in addition to chlorophyll and the accompanying yellow pigments. 
Although the presence of blue, green, and yellow pigments typi- 
cally gives the organisms a blue-green color, variations in the pro- 
portions of these pigments cause the appearance of many shades 
and colors such as yellow, orange, pink, red, violet, purple, brown, 
and black. A preponderance of carotin, for example, results in a 
yellow or reddish color. Blue-green algae differ from green, brown, 
and red algae also in the absence of definite plastids, in the organ- 
ization of their nuclear substance, and in the complete lack of 
swarm-spores and gametes. 

Although included among the thallophytes, the blue-green algae 
are to be regarded as a class of this division of the plant kingdom 
that is not closely related to the other .classes, and one which has 
probably arisen independently. 

161. Distribution. Blue-green algae are widely distributed in 
both fresh and salt water. Fresh-water blue-green algae are of 
common occurrence in pools and ditches, especially in those con- 
taining stagnant water. Many of the plankton algae of fresh- 
water lakes and reservoirs are members of this class, and blue- 
green algae are chief among the organisms whose decay sometimes 
causes disagreeable odors and tastes in water supplies. The water 
of the Red Sea is at times colored by the presence of immense 
numbers of colonies of a species belonging to this class. 

These algae are prominent also among those which grow on 
damp soil and form papery layers on the surface of the earth. 
Almost all the algae of hot springs are blue-greens. Among the 
best-known instances of their occurrence in hot springs are in those 
of Yellowstone National Park. The brightly colored terraces of 
these springs result from a deposition by the algae of mineral sub- 
stances dissolved in the spring waters. 

162L/Gloeocapsa. The cells of certain genera of blue-green 
algae are solitary; those of other genera are united in colonies of 



definite or indefinite form. Gloeocapsa (Fig. 162) is a one-celled 
member of the class. Its solitary cells are spherical, each being 
surrounded by a thick gelatinous sheath which is often stratified. 
Frequently two or more daughter cells of Gloeocapsa remain within 
the 'sheath of the parent cell, forming a more or less persistent 

Within the thin wall of each cell is a protoplast, differentiated 
into an outer colored and an inner colorless region. The colored 

Central Body 

FIG. 162. Gloeocapsa. A , single cell. B, 3-celled colony. (?, cell killed and so 
stained as to show the central body. D t dividing cell. C, I) redrawn from 

portion of the protoplast contains the pigments already mentioned. 
The pigments appear to be evenly distributed throughout the 
peripheral portion of the protoplasm rather than located in definite 
plastids. In the colored Tegion of the protoplasm are also colorless 
granules, some of which are reserve foods. Probably many of 
these are composed of glycogen, a carbohydrate somewhat similar 
to starch. In the central colorless region of the cell is a relatively 
dense mass of material, the central body. This body is described 
by some observers as consisting in part of substances of the nature 
of those composing chromosomes, being therefore a nucleus of 
primitive type but without a nucleolus or a nuclear membrane. 
Other investigators question this conception and hold that a defi- 
nite nucleus is lacking. The material of the central body, however, 
takes the same stains as do chromosomes, and is commonly con- 
sidered to correspond in some measure to the substance of a true 



An increase in number of cells is brought about in Gloeocapsa, 
as in other algae, by cell division (Fig. 162, Z>). Before dividing, 
a cell elongates somewhat and the chromatic substance of its 
central body becomes divided into two masses. The cell is then 
divided transversely by a constriction of the plasma membrane, 
and each daughter cell becomes rounded and secretes a new gelat- 
inous sheath. 

163. Some Colonial Blue-green Algae. There are a considerable 
number of blue-green algae whose spherical cells are enclosed in 
a common gelatinous 

matrix. Cell division 

in these colonial algae 

may be in two or in 

three planes. In the 

former case, as in 

Merismopedia (Fig. 

163, A), the result is a 

flat, plate-like colony. 

Cell division in three B 

planes results in a FlG - 163 - Colonial blue-green algae. A, Me- 

more or less massive * dia - ^ Coelosphaerium. C, Aphano- 


colony, as in Coelo- 
sphaerium (Fig. 163, B) and Aphanocapsa (Fig. 163, C). Repro- 
duction of the colony results from its accidental rupture, or from 
the occasional freeing of a cell which then by division develops 
into a new colony. 

164. Oscillatoria. In the majority of colonial blue-green algae, 
cell division is always in the same plane. The result is a simple 

A B 

FIG. 164. Oscillatoria. A, portion of a living filament. B, cells killed and 
stained, showing the central body. B redrawn from Olive. 

unbranched row or filament of cells. One of the commonest fila- 
mentous forms is Oscillatoria (Fig. 164), which is often abundant 
in temporary pools or on damp soils. 

At each end of a filament of Oscillatoria is a hemispherical or 



conical cell whose free end is frequently expanded to form a 
button-like cap. The other cells of the filament are disk-shaped 
or cylindrical. Each cell is surrounded by a wall. The proto- 
plast, like that of Gloeocapsa, is differentiated into a colorless 
central region and a colored outer portion. A feature which 
distinguishes Oscillatoria from other blue-green algae, and which 
suggested its name, is its oscillating movement. A filament fre- 
quently waves back and forth, and occasionally moves longi- 
tudinally a short distance. The mechanism of these movements is 

As in other filamentous algae the number of cells in a filament is 
increased by cell division, the central body dividing before, or at 
the same time as, the cell divides. Oscillatoria has a definite means 
also of bringing about the reproduction of filaments. Frequently 
gelatinous disks are formed between adjacent cells at certain 

points in a filament; later 
the filament breaks at 
these points into several 
short parts each composed 
of a few cells, which may 
then, by division and 
growth, develop into longer 

Yyl66. Anabaena. An- 
other filamentous blue- 
green alga of frequent oc- 
currence in pools, ditches, 
and the plankton of fresh- 
water lakes is Anabaena 
(Fig. 165, A). The cells of 



FIG. 165. A y filament of Anabaena with a Anabaena are spherical 
spore and a heterocyst. B, development of j attached to onp an- 

n T j x oilivl cttlittiL-llCvl vU Ullt; ctll 

a spore of Cylmdrospermum mto a new 

filament. other so as to appear like 

beads in a necklace. The 

filaments may be straight or very much bent and contorted. 
Surrounding each filament is a thick, very transparent gelatinous 
sheath. The cells have much the same structure as those of 
Gloeocapsa or of Oscillatoria. 

An occasional cell of an Anabaena filament enlarges greatly, 
Becomes filled with reserve foods, and develops a thicker wall. 


Such a spore eventually becomes separated from the parent fila- 
ment and may develop into a new colony (Fig. 165, J5). 

Here and there in a filament are cells with much thicker 
walls and with transparent contents. These are heterocysts. 
Heterocysts are spore-like in nature, but they are spores whose 
capacity to develop into new colonies has been almost com- 
pletely lost. The filaments of some blue-green algae having 
heterocysts regularly break into shorter filaments at the points 
where the heterocysts occur; but this is not so generally the case 
in Anabaena. 

166. Nostoc. Other filamentous blue-green algae with heter- 
ocysts differ from Anabaena in the shape of the colony or in the 

FIG. 166. Colony of Nostoc. 

arrangement of their cells. Nostoc (Fig. 166), which grows both 
on damp soils and in water, resembles Anabaena in cell and colo- 
nial structure and in spore-formation. A Nostoc colony is sur- 
rounded by a gelatinous sheath much firmer and tougher than 
that of Anabaena* 

A colony of Nostoc may become separated into daughter col- 


onies which remain within the original tough sheath. By the 
growth and breaking of the daughter colonies a mass of consider- 
able size is often formed. Such a mass contains numerous filaments 
imbedded in a gelatinous matrix, and may be considered a com- 
pound colony. 



167. Nature. The brown algae are almost exclusively marine and 
are most abundant along the shores of colder portions of the oceans. 
Their cells contain galden-brown plastids which owe their color to 
a pigment chemically similar to carotin, that masks the chloro- 
phyll which is likewise present. Brown algae are distinguished 
from algae of other classes also by the structure of their swarm- 
spores and motile gametes. For this reason, particularly, the brown 
algae are considered not to be directly related to the green algae, 
but in all probability to have arisen from unicellular flagellates 
very different in structure from Chlamydomonas. 

Evolution within the class of brown algae has produced plants 
with much greater complexity of external form than has been at- 
tained by any of the green algae mentioned in an earlier chapter. 
Many brown algae have developed also a considerable degree of 
tissue-differentiation, and some grow to a very large size. Al- 
though in size and in complexity of structure some of them are 
fairly comparable with many seed plants, brown algae are to be 
included among the thallophytes because of their simple spore- 
and gamete-producing organs. 


168. Structure and Spore-formation. Ectocarpus (Fig. 167, A) 
has a much-branched filamentous thallus which grows attached to 
stones, rocks, and other objects, particularly those that are exposed 
by tidal action. The cells are cylindrical. Within each cell wall 
is a one-nucleate protoplast containing numerous small golden- 
brown plastids. 

In some plants, terminal cells of the main branches, or the 
terminal cells of short lateral branches, develop into sporangia 
(Fig. 167, B). A sporangium begins its development as a one- 
nucleate cell with numerous plastids. After a series of nuclear 
divisions its protoplast becomes divided into swarm-spores which 
are liberated by a breaking or dissolution of the wall at the upper 




end of the sporangium. Each spore (Fig. 167, C) is pear-shaped, 
has one nucleus and one plastid, and bears two lateral flagella of 
unequal length. After swimming for a time the spore comes to 
rest and develops into a new plant. 

169. Gametic Union. Other plants of Ectocarpus produce 
gametes. These are formed in multicellular organs (sex organs) 

FIG. 167. Ectocarpus. A, portion of a thallus bearing sex organs. B, sporan- 
gium. C, swarm-spore. 

which, like sporangia, are terminal in position. Each of these or- 
gans consists, unlike a sporangium, ol many transverse layers of 
small cubical cells separated by walls (Figs. 167, A; 168, A). The 
protoplast of each cell is metamorphosed into a motile gamete 
which is liberated by a breaking of its enclosing wall. The gametes 
(Fig. 168, B) are similar to, though usually smaller than, the 
swarm-spores, and they are capable, under favorable conditions, 



of uniting in pairs to form zygotes (Fig. 168, C-F). Before their 

union occurs some of the gametes become motionless.^ These are 

considered female gametes. Other (male) gametes remain active. 

One male gamete 

unites with each fe- 

male gamete. If con- 

ditions are not favor- 

able to their union,\ 

the gametes of Ecto- 

carpus, like those of 

Spirogyra, may func- 

tion as spores, devel- 

oping directly into new 



170. Structure. Lam- 
inaria } the 
kelp, grows 


in com- 

paratively shallow 
water near rocky 
ocean shores but just 
below the lower tide 
level. Like other large 
brown algae, it is most 
abundant in relatively 
cool waters. At one 
time kelps and rock- 
weeds ( 174) were of 
considerable impor- FlG 168 Ectocarpus . A, sex organ, gametes being 
tance as sources of liberated. B, gamete. C, female gamete (above) 
potassium and iodine. whose flagella have been withdrawn; below, 3 

TU^ Ai^^r^r ~f ; male gametes. Z>, E, stages in the union of 
The discovery of mm- gametes ^ zygote ^ re ^ awn f rom Berthold< 

eral deposits contain- 

ing these elements has, however, made their recovery from algae 


A plant (Fig. 169, A), which may be six feet or more in length, 
is attached to the rock by a holdfast that superficially resembles 
a much-branched root system. Above the holdfast is a short 
central axis which is continuous with a large flattened blade; in 



certain species the blade is divided lengthwise into several seg- 
ments. The central axis consists of a tough outer cortical zone 
and a loose inner region. The blade, likewise, has compactly ar- 
ranged cells next the surface 
and loosely arranged cells in 
the interior. 

171. Reproduction. During 
certain seasons of the year 
some of the surface cells of the 
blade of Laminaria develop 
into sporangia which are very 
similar in structure" and de- 
velopment ta the sporangia of 
Ectocarpus. The sporangia of 
Laminaria occur in closely 
packed groups in the blade, 
intermingled with elongated v 
hair-like cellsjFig. 169, B). The 
swarm-spores, similar in ap- 
pearance to those of Ectocar- 
pus, are liberated by a break- 
ing of the sporangial walls. 

Contrary to what might be 
expected, a swarm-spore de- 
velops, not into a plant sim- 
ilar to that which formed it, 
but into a very small branched 
filamentous plant: Further- 
more, the plants that develop 
from swarm-spores are of two 
sorts, producing respectively 
female and male sex organs. 
The male plants (Fig. 170, A) 
FIG. 169. Laminaria. A, thallus. B, bear many small antheridia 
cross section of a portion of a thallus near the tips of their branches, 
bearing sporangia. B after Oltmanns. r x i j. * i- 

Ihe protoplast of each an- 

theridium becomes a motile antherozoid. On female plants 
(Fig. 170, B) oogonia are developed, usually from terminal cells 
but sometimes from other cells of the branches. An oogonium 
is longer and thicker than other cells of the plant; its protoplast 



becomes an egg containing numerous plastids. The mature egg 
is extruded through, and remains attached to, the tip of the 

After an antherozoid has united with an egg the resultant zygote 
divides transversely. Further transverse divisions produce a short 


FIG. 170. Laminaria. A, male plant. B, female plant. CE, young plants 
produced by the germination of zygotes. A and B redrawn from Miss 
Myers: C-E redrawn from Kylin. 

filament of 6 to 10 cells (Fig. 170, C, D). Later divisions are in 
three planes, and continued growth and division lead to the de- 
velopment of a mature plant like that first described, with hold- 
fast, central axis, and blade. Under certain conditions an egg may 
develop into a new plant without uniting with an antherozoid. 

172. Life Cycle. There are two distinct phases in the history 
of Laminaria. Swarm-spores develop into small filamentous 
plants which produce gametes:. The union of gametes results in 
the formation of a zygote, from which develops a large plant that 
produces spores. Since the gamete-bearing plant (or generation) 
of Laminaria gives rise through the zygote to the spore-bearing 
generation, and this generation through the spores gives rise to 
the gamete-bearing generation, Laminaria has an alternation of 

A similar alternation of generations appears to characterize 
other genera of the brown algae. It is probable that the union of 



gametes in Ectocarpus (at least in some species) produces zygotes 
which develop into spore-bearing plants; and that the spore-bear- 
ing plants, in turn, give rise, through the spores, to gamete-bear- 
ing plants. If this is true, the two generations of Ectocarpus are 
identical in appearance, except for their reproductive structures ; v 
in Laminaria, on the contrary, the two generations are very differ- 
ent in size and structure. 

173. Other Kelps. Many of the kelps of the Pacific coast are 
notable both for their complexity of external form and for their 

habits of growth. One of the 
most remarkable is Postelsia 
(the "sea palm/' Fig. 171), 
which grows only at the water 
line on rocks exposed to the fun 
pounding of the surf. This alga 
has a much-branched holdfast 
which anchors it firmly to the 
rocks, and a stout, flexible axis 
a foot or more in length, bear- 
ing at its apex a crown of leaf- 
like blades. 

Many of the "giant kelps" 
of the Pacific coast are annu- 
als, and some of them grow to 
a length of 100 feet or more in 
a single season. Nereocyslis 
(Fig. 172), one of these giant 
kelps, grows in water 20 to 40 
feet in depth. It is anchored to 
the rocky bottom by a holdfast 
from which arises a long, slen- 
der axis. The upper portion of 
the axis is expanded to consti- 
tute a gas bladder that floats on 
the surface of the water and 
bears several long, strap-shaped blades. Another giant kelp (Macro- 
cystis), growing in similar locations, bears many small blades along 
its branching axis and a gas bladder at the base of each blade. 

FIG. 171. The "sea palm," Postelsia. 
Photograph by Lewis Josselyn. 




174. Structure. The rockweed (Fucus, Fig. 174, A) is a com- 
mon inhabitant of the sea coasts of all temperate regions. This 
alga grows most 

abundantly in 
the upper limits 
of the areas that 
are temporarily 
exposed by the 
tides. The leath- 
ery, flat, ribbon- 
shaped thallus is 
attached to rgcks 
by the develop- 
ment of its basal 
end into a hold- 
fast. The thallus 
forks at inter- 
vals, the two 
prongs of each 
fork being usu- 
ally of the same 
length.. Here and 
there along the 
thallus in certain 
species are large * 
hollow, bladder-* 
like expansions containing gases, chiefly carbon dioxide, that help 
buoy the plant when it is submerged."~"~Growth occurs at the free 
end of each branch, where new cells are formed by the repeated 
division of a single apical cell. 

175. Gametic Union. The free ends of the branches are often 
somewhat swollen, and in the swollen portions are numerous 
approximately spherical cavities each with a pore-like opening at 
its apex. It is in these cavities that the sex organs (oogonia and 
antheridia) are produced. In some species of Fucus, oogonia and 
antheridia are borne in the same cavity; in other species, the two 
kinds of organs are produced in separate cavities; in still other 
species, they are borne on separate plants. The cavities in which 

FIG. 172. Nereocystis, a giant kelp. 
Lewis Josselyn. 

Photograph by 



antheridia are produced (Fig. 174> B) are lined with numerous 
branching many-celled hairs, the terminal cells of whose lateral 
branches become antheridia. The protoplast of each young an- 

theridium divides, forming ul- 
timately 64 small pear-shaped 
antherozoids each of which 
bears two unequal lateral fla- 
gella. The whole antheridium 
may be liberated and may ooze 
out through the pore of the 
cavity into the surrounding 
water, where its wall dissolves 
and the antherozoids become 
free. Sometimes the anthero- 
zoids are freed from the an- 
theridium while the latter is 
still in place on the branch that 
produced it. 

The oogonia (Fig. 175, A, B) 
are borne at the ends of short 
stalks. The protoplast of each 
oogonium divides to form eight 
large eggs. When the oogonia 
are mature they break away 
from their stalks and float 
through the pore of the cavity 
into the surrounding water. 
Here the eggs are liberated by 
the rupture and dissolution of 
the oogonial wall. The eggs, 
being non-flagellate and there- 
fore non-motile, become spheri- 

FIG. 173. Egregia, the "feather-boa" 
kelp, attached to a large stone by 
its holdfasts. Photograph by Lewis 

cal when they are freed. An- 
therozoids that come into the 
neighborhood of an egg swim 
toward it, apparently in con- 
sequence of a chemical stimulus, and numerous antherozoids 
become attached by their flagella to the egg (Fig. 175, C). Even- 
tually one antherozoid makes its way into the egg, the cytoplasm of 
the egg and that of the antherozoid unite, and their nuclei unite. 



FIG. 174. Fucus. A, portion of a thallus, showing gas-containing vesicles 
and branches with swollen ends. B, cross section of a cavity containing 
anthericlia. (7, antheridial branch. D, antheridium at the time of the 
liberation of antherozoids. B-D redrawn from Thuret. 

A ^**^ B 

FIG. 175. Fucus. A, cross section of a cavity containing oogonia. B, oogonium 
that has burst and liberated eggs. C, gametic union. D, young plant pro- 
duced by the germination of a zygote. All redrawn from Thuret. 



After this union the zygote secretes a wall, and when it settles 
to the bottom of the water and comes into contact with some 
solid substance it develops into a new plant (Fig. 175, D). Un- 
der certain conditions an egg can 
develop into a new plant without 
uniting with an antherozoid. 

176. Sargassum. Among other 
brown algae which, like Fucus, 
form small motile antherozoids 
and large non-motile eggs is the 
gulf weed (Sargasstfm, Fig. 176). 
It has essentially the same type 
of flat, branching thallus as Fucus, 
but commonly with more marked 
differentiation between stem-like 
and leaf -like branches. Having 
more numerous gas bladders than 
Fucus, it floats freely when de- 
tached from the substrate. Float- 
ing plants of Sargassum are par- 
ticularly abundant in the warm 
waters of the Gulf Stream, and 

FIG. 176. Portion of a thallus of the 
gulf weed, Sargassum. 

the presence of masses of these plants in the Atlantic gave rise 
to the fable of the " Sargasso Sea." Whether the floating plants 
have become detached, or are able to perpetuate themselves in- 
definitely in the floating condition, is still uncertain. 


177. Nature and Distribution. On the basis of the color of their 
plastids the diatoms seem to be distantly related to the brown 
algae, but they are considered as constituting a distinct class. 

Diatoms occur in both fresh and salt water. They compose an 
important part of the plankton of the ocean, and in early spring and 
late fall make up the major portion of the plankton of fresh-water 
lakes. Other diatoms of both fresh and salt water grow intermin- 
gled with, and attached to, algae of other classes, or upon rocks and 
other solid bodies in the water. 

The siliceous wall of a diatom does not decay after the death of 


the cell, and great numbers of the walls accumulate at the bottom 
of any body of water in which diatoms live. Layers of fossil diatom 
walls deposited in former arms of the ocean are known as "dia- 
tomaceous earth." Some deposits of this nature in the western 
part of the United States are over 1,000 feet in thickness. Dia- 
tomaceous earth is of considerable economic importance as a heat- 
insulating material; it is also a source of fine abrasive substances. 
The abrasive qualities of some silver polishes and tooth pastes are 
due to fossil diatom walls. 

178. Structure. Diatoms are usually unicellular; but in some 
species the cells are united into filamentous or branching colonies. 
The shape of the cell differs greatly in various species, but in all 
cases the wall consists of two overlapping halves that fit together 
as do the two parts of a candy box. The wall is strongly impreg- 
nated with silica. Cell walls of diatoms are characteristically 
marked by minute pores or short lines. The markings either are 

FIG. 177. A diatom (Stephanodiscus) with radial arrangement of cell-wall 


radially arranged with reference to a central point (Fig. 177); or 
their arrangement is bilaterally symmetrical with respect to the 
long axis of the cell (Fig. 178). There is a layer of dense cytoplasm 
just within the wall ; within this layer is a central vacuole. Included 
in the dense cytoplasm are one, two, or several golden-brown plas- 
tids. The single nucleus is imbedded either in the outer dense cyto- 



plasmic layer or in a strand of dense cytoplasm that cuts across the 

central vacuole. 
179. Reproduction. Nuclear division is followed by a division 

of the whole protoplast, each of the two daughter protoplasts 

remaining within one of the 
closely fitting halves of the 
parent-cell wall. The develop- 
ment of a new half-wall over 
the naked face of each daughter 
protoplast completes the en- 
closure of the daughter cell by 
a typical two-parted wall. The 
daughter cells in most species 
separate; but in some species 
they remain attached. Of the 
daughter cells formed by divi- 
sion, one is of the same size 
as the parent cell, the other 
slightly smaller. In conse- 
quence of a continuation of re- 
production by cell division, 
4 r most of the cells in time are 

FIG. 178. Diatoms with bilaterally appreciably smaller than the 
symmetrical arrangement of mark- original parent cell. The pro- 
ings. A, B, top and side views of a gress i ve diminution in size does 
Pinnularia. C, Surirella. . ,. . -, ~ ., , 

not continue indefinitely, since 

a small cell may undergo changes in form and condition in the 
course of which it grows to the size of the original parent cell. In 
some cases such a rejuvenescent stage involves a rounding up of 
the protoplast within the siliceous half-walls. In other cases the 
rejuvenescent stage is* a zygote formed by the union of two proto- 



180. Nature. The characteristic color of red algae is due to 
the presence of a pigment in the plastids, which masks the 
chlorophyll. Color, however, is not a certain criterion, since 
the thalli of certain species of this class are olive-green, golden- 
brown, deep olive-black, or purplish black. Among the structural 
features which distinguish red algae are the occurrence of non- 
fta&ellate male as well as female gametes and, in most species, the 
presence of broad cy^plasmic connections between adjoining cells 
of the thallus. 

The union of non-motile gametes of different sizes characteristic 
of the red algae is so unlike gametic unions in other algae that the 
present group appears to have arisen entirely independently. 
Primitive one-celled organisms from which the red algae may have 
been derived are, however, unknown, and the origin of the class is 

181. Distribution and Structure. Red, like brown algae, are 
almost exclusively marine. They are most abundant in the warmer 
waters of the oceans, although by no means absent in cooler re- 
gions. Red algae always grow attached to some solid object, and 
usually below the levels exposed by tides. In cooler waters the 
lowest depths at which algae, chiefly reds, occur are 150 to 180 feet, 
but in regions somewhat nearer the equator, such as the Mediter- 
ranean, where a larger proportion of the days are sunny and where 
the sun's rays penetrate the water directly during a greater part 
of the year, red algae have been dredged from depths of 300 to 600 
feet. A few red algae, including members of the widespread genus 
Batrachospermum, live in fresh water. 

Although no red algae attain to so great a size as do some brown 
algae, the numerous species of this class show much variation in 
both size and form (Fig. 179). In some, the thallus is a much- 
branched, feathery structure; in others it is flat and leaf-like, thin 
and delicate, or tough, leathery, and compact. The thalli of some 
red algae (Corallines) become incrusted with lime; these species 




are of great importance in the formation of the " coral " reefs, atolls, 
and islands of tropical seas. 

Of the two genera discussed in the following paragraphs, one 
is typical of those red algae whose structure and life cycle are 


FIG. 179. Types of thalli found among red algae. A, thick branching thallus 
of Chondrus. J5, feathery thallus of Polysiphonia. C, thin, leaf-like 
thallus of Grinnellia. 

relatively simple; the other is representative of those with a more 
elaborate organization and a more complicated life cycle. 


182. Structure. The plant (Fig. 180, A) is a long, slender, 
sparingly branched cylinder made up of a rather dense mass of 
interwoven, much^branched filaments imbedded in a gelatinous 
matrix. The central porJjpa of the thallus consists chiefly of fila- 
ments which run lengthwise; numerous lateral branches, lying 
mostly at right angles to the central mass, form the dense outer 
portion. Adjacent cells within each filament are not completely 
separated by walls; there is a central pore of varying size in each 



cross wall through which the protoplasts are in contact. The cells 
are one-nucleate, and each cell in the outer portion of the thallus 
contains a single star-shaped plastid, within which is a pyrenoid 
surrounded by starch granules. 

183. Gametic Union. Male gametes (spermatia) are produced 
in dense clusters at the ends of short branches (Fig. 180, B). Each 






FIG. 180. Nemalion. A, thallus. B, portion of a branch bearing spermatia. 
C y , portion of a branch bearing a carpogonium. /), cluster of carpospores 
borne on branches that have grown from the zygote. 

spermatium is a small, non-motile cell. The spermatia become dis- 
lodged and are carried in all directions by water currents. 

A female organ (carpogonium) is the terminal cell of a short 
lateral branch (Fig. 180, C). The free end of the carpogonium is 
prolonged into a long hair-like outgrowth, the trichogyne. The 
basal portion of the protoplast of the carpogonium, including its 
nucleus, is the egg. Spermatia carried by water currents may lodge 
against tjie trichogyne. When spermatia come into contact with 
the trichogyne they adhere to it; the walls of one spermatium and 
of the trichogyne break down at the point of contact, and the pro- 
toplast of the spermatium moves into the trichogyne. Shortly 
after entering the trichogyne, the nucleus of the spermatium di- 
vides; the two male nuclei so formed move toward the base of the 
carpogonium , and one of them, reaching the enlarged basal por- 


tion, unites with the egg nucleus. After this nuclear union, numer- 
ous short branches develop from the zygote, the terminal cell of 
each brancE becoming a carpospore. The carpospores, which are 
borne in a dense cluster (Fig. 180, Z>), eventually become separated 
from the branches, float away, and develop into new plants. 


184. Structure.'The feathery thallus of Polysiphonia (Fig. 181, A) 
consists of a much-divided system of relatively large branches 
which bear many smaller branches. Each of the larger branches 
consists of several superimposed tiers of cells which are elongated 
in the direction of the length of the branch. Each tier consists of a 
central cell and a surrounding jacket layer one cell in thickness. 
At each end of the central cell is a thick cytoplasrnic strand con- 
necting it with the central cell of the adjoining tier. The jacket 
cells of each tier are similarly connected with the central cell of the 
same tier. Each of the smaller branches of the thallus consists of 
but one row of cells, connected by cytoplasrnic strands. All cells 
are one-nucleate, and each contains many small disk-shaped 

185. Gametic Union. Male and female gametes are borne on 
separate plants. The spermatia are densely crowded on club- 
shaped lateral branches (Fig. 181, B). When mature, the spermatia 
become separated from the branches bearing them and are trans- 
ported by water currents. 

A carpogonium (Fig. 182, A) is the terminal member of a short 
lateral branch of five cells. It is similar to a carpogonium of Ne- 
malion, consisting of a hair-like terminal trichogyne and an en- 
larged basal portion. " 

As in Nemalion, spermatia carried by water currents may lodge 
against, and become attached to, a trichogyne. The walls between 
a spermatium and the trichogyne break down at the point of con- 
tact, and the protoplast of the spermatium migrates into the 
trichogyne. The spermatium nucleus moves down the trichogyne 
until it comes into contact with the nucleus of the egg. There is 
no division of the spermatium nucleus, as in Nemalion. The union 
of the gamete nuclei forms a zygote nucleus. The five-celled branch 
bearing the carpogonium has by this time produced additional 
cells, and the zygote nucleus migrates through one of these new 
cells into the basal cell of the branch, which now is very irregular 



203. Filamentous Fungi. There are many simple plants which, 
like bacteria, do not contain chlorophyll; all these plants are classed 
together as fungi. The bacteria are the simplest fungi. The great 
majority of fungi are more complex than bacteria in the fact that 
their bodies, whether one- or many-celled, are composed of branch- 
ing filaments. One large class of (chiefly) filamentous fungi which, 
particularly in their methods of formation and union of gametes, 
are more or less like certain algae are known as Phycomycetes 
(algal fungi). In some phycomycetes there occurs a union of 
gametes that are nearly or quite alike in size and structure. Others 
are characterized by the union of very unlike gametes. 


204. Structure. Among phycomycetes with like gametes is 
included an order known as " black molds." One of these is the 

FIG. 193. Stages in the germination of a spore of Rhizopus and the develop- 
ment of a mycelium. 

common bread mold (Rhizopus nigricans), which forms an abun- 
dant soft, white, cottony growth on moist bread. The plant body 
is a filamentous, much-branched structure, each branch (hypha) 
being a slender thread. The whole complex of hyphae is a mycelium. 
The dark-colored spores produced by the bread mold are vari- 
able in size and shape, though usually ovoid (Fig. 193, A). When 
a spore comes in contact with water, it soon enlarges and becomes 




^Mature Sporangium /-~x 

Q >Spore ( ) 

Columella Wall _ 4 \ ) 
.. f Immature \ / 

angia I 

spherical; its wall, formerly wrinkled, becomes smooth. These 
changes result from the fact that the protoplasm absorbs water, 
swells, and exerts pressure on the wall. A little later, if the tem- 
perature is favorable, the outer layer of the wall breaks (Fig. 193, 
B-E) and a short hypha, surrounded by the innermost layer of 
the wall, protrudes. This hypha elongates rapidly, branches, and 
so gives rise to a young mycelium. If a spore is sown in water, 
the mycelium growing from it soon dies because the only available 
food is the small amount present in the spore. On the other hand, 
if a spore germinates in a nutrient liquid or on a piece of moist 
bread or other similar source of foods, growth continues until a 

much-branched my- 
celium is formed. 

A young hypha has 
a cell wall, within 
which is a granular 
cytoplasm; this con- 
tains many vacuoles 
of varying sizes, drop- 
lets of oil, and glyco- 
gen (a carbohydrate 
similar to starch), and 
includes numerous 
small nuclei. The 
protoplasm is in con- 
tinuous movement, 
chiefly toward the 
tips of the various 
branches. The entire mycelium is as yet but a single undivided 
cell. An older plant (Fig. 194) is composed of hyphae of three 
types. Hyphae of one sort (rhizoids) anchor the plant and pene- 
trate the substrate. The rhizoids, and a few other hyphae that 
come into contact with the bread or other substrate, secrete 
enzymes which digest the foods there present. The digested 
foods are absorbed by the mycelium and used in its growth. 
Certain other hyphae (stolons), usually larger than the rhizoids, 
grow approximately parallel to and above the substrate for a 
distance and then, bending downward, each stolon develops 
another group of rhizoids. Hyphae of a third type grow up- 
ward from the stolons at points where the rhizoids are formed. 

FIG. 194. Portion of a mycelium of Rhizopus. 



Each of these erect hyphae (sporangiophores) bears a sporan* 

205. Spore-formation. A young sporangiophore elongates consid- 
erably. Into its enlarging tip, now 
the sporangium, protoplasm mi- 
grates containing much food and 
many nuclei (Fig. 195). As this 
protoplasmic movement continues, 
the portion of the protoplasm con- 
taining most of the nuclei and food 
aggregates in the outer part of the 
enlarging sporangium, leaving the 
center occupied by protoplasm with 
many large vacuoles and few nuclei. 
Some of the vacuoles become ar- 
ranged in a dome-shaped layer 

between the outer, denser and 

4l , , , | FIG. 195. Apex of a young spo- 

the inner, less dense protoplasm rangiophore of Rhizopus. 

(Fig. 196,^4). These vacuoles soon 

become flattened, and as they come into contact with one 

another they unite into larger vacuoles until finally their union 

FIG. 196. Stages in the development of a sporangium of Rhizopus. A, just 
before the cell division separating the columella (inner part) from the 
spore sac proper. B, division of the protoplast of the spore sac into spores. 

forms a cleft separating the outer from the inner part of the 
sporangium. The united vacuolar membranes form a plasma 


membrane on each side of the cleft. The cleft is completed by a 
furrowing of the original plasma membrane about the base of the 
sporangium (Fig. 196, ). Between the two new plasma mem- 
branes a wall is secreted, which thus separates the dome-shaped 
central part of the sporangium (the columella) from the outer 
part, the spore sac proper. The latter is now a separate cell pro- 
vided with a continuous plasma membrane. This membrane be- 
comes furrowed in numerous places, both on the side next the 
outer wall and on that next the columella wall. The furrows cut 
into the protoplasm, branching, and dividing the contents of the 
spore sac into smaller and smaller protoplasts of irregular shape. 
The small cells ultimately produced by this process of progressive 
cleavage are the spores, each containing a variable number (2-10) 
of nuclei. This method of division by means of furrows which 
progressively divide a many-nucleate cell into smaller and smaller 
cells occurs also in some other fungi, as well as in a few algae. 
It is very different from division by means of a cell plate or by con- 

Finally the newly formed spores become rounded and each 
secretes a wall. The outer wall of the spore sac dries and becomes 
fragile when the spores are mature, and any slight disturbance 
breaks it, liberating the spores. The columella persists as a dome- 
shaped structure at the end of the sporangiophore (Fig. 194). 
A mycelium usually remains one-celled until columella-formation 
takes place in the sporangia, but after this time cell division by 
constriction may occur and cross walls be formed in various por- 
tions of the mycelium. At any stage of development, however, 
under unfavorable environmental conditions, divisions may occur 
in the hyphae. Even after such divisions, each of the cells that 
constitute the plant, except the spores, is comparatively large and 

206. Formation and Union of Gametes (Fig. 197). Gametic 
union in Rhizopus resembles the corresponding process in Spiro- 
gyra in that the two gametes are alike in size and are without 
flagella. When two hyphae of separate Rhizopus plants of dis- 
tinct strains (see next paragraph) come into contact, a short side 
branch (progamete) may be produced by each hypha at the point 
of contact. The terminal portion of each progamete becomes 
swollen. Within this swollen portion a transverse division occurs 
and autoes wall is secreted, the many-nucl^kte end cell so formed 



being a gamete. The basal portion of each progamete, which 
connects the gamete with the mycelium, is the suspensor. In 
time the walls between the two gametes, where they are in contact, 
dissolve, and the gametes unite to form a zygote. The zygote wall 
becomes very thick and black, and has a rough outer surface. 

1 FIG. 197. Rh'izbpus. A y B, stages in the development of progametes. (7, after 
the division of the progaraetes to form gametes and suspensors. D, E, 
young and mature zygotes. F, germination of a zygote. 

The thick-walled resting zygote contains an abundance of reserve 
foods, largely in the form of fats, as well as many nuclei derived 
from each of the gametes. The subsequent history of the nuclei 
in the zygote is not fully known, but it is very probable that some 
of them at least unite in pairs. After a period of rest the zygote 
germinates (Fig. 197, F), giving rise to a hypha which soon forms 
a sporangiophore and a sporangium. This sporangium, except for 



its smaller size, is similar to the sporangia produced on the ordi- 
nary mycelium. 

The plants of Rhizopus belong to strains of two different sorts, 
referred to as plus and minus. No gametes are formed unless a 
-Spwangium hypha of a plus mycelium comes into contact 
with one of a minus mycelium. In Spirogyra, 
the two uniting gametes are conspicuously 
different in behavior, so that they may be 
designated as male and female. In Rhizopus 
the two gametes are often different in size; 
but such differences are too variable to make 
it certain that they represent a sexual differ- 
entiation. A few black molds, however, have 
Sporangiophore distinctly different male and female gametes. 

207. Relatives of Rhizopus. While Rhizo- 
pus is the one most commonly found in the 
household, several other black molds are wide- 
spread. One of these (Phy corny ces) has spo- 
rangiophores which often reach a height of 
several inches. Pilobolus (Figs. 198, 199), 
frequently found in barnyard refuse, has a 
mechanism for throwing the entire sporangium 
to a distance* of more than six feet. Al- 
though the majority of black molds are sapro- 
phytic, a few are parasitic on other fungi, and 


FIG. 198. Portion of 
the mycelium of 

Pilobolus, a black some cause important diseases of both plants 
mold, with spo- 
rangiophore and 


and animals. 

Among other phycomycetes with like gam- 
etes, which are possibly related to the black 
molds, the best known are species of Empusa. One member of 
this genus is a parasite of the common house-fly and kills great 
numbers of flies. Its mycelium is composed of many short cells, 
each containing one to several conspicuous nuclei. 


208. Nature and Structure. Saprokgnia (Fig. 200) is one of the 
commonest of those phycomycetes that produce male and female 
gametes which differ greatly in size, structure, and behavior. 



Its species and those of other members of the same order are com- 
monly found in water, growing on the living or dead bodies of 
insects and fishes and often on other plant and animal substances. 
For this reason these fungi are called " water molds." However, 
many if not all of them are abundant in surface soils. Saprolegnia 
and its immediate relatives are usually saprophytic; but under 
some conditions they become parasitic upon fish, probably gaining 


FIG. 199. A culture of Pilobolus placed in a dark box. The sporangiophores 
bend toward the small window through which light is admitted, and the 
sporangia are discharged toward the window. 

entrance to the bodies of the hosts through wounds, and producing 
a destructive epidemic disease. Root diseases of many plants 
also are caused by fungi closely related to Saprolegnia. 

The mycelium consists of hyphae, some oi wlaidi are short and 
penetrate the substrate, while others are long and extend in all 
directions from the material upon which the fungus is growing. 
Like that of Rhizopus, the mycelium is, while young, a single 
much-branched, many-nucleate cell. 

209. Spore-formation (Fig. 200, A, J3). The protoplasm of the 
long external hyphae, at first vacuolate, gradually becomes denser 
and finally granular, especially at the tips. The tip of each hypha, 
becoming separated by cell division, forms a terminal many- 
nucleate sporangium. By means of furrows in the plasma mem- 
brane and of a flattening and branching of vacuoles, the proto- 
plasm of the sporangium is divided into many small spherical 



one-nucleate spores. The tip of the sporangium breaks, and 
through the opening so formed the spores emerge; each spore 
develops two flagella and finally swims away. After the swarm- 
spores have escaped, the basal wall of the sporangium becomes 
softened and is pushed up by the protoplasm below into the cavity 




FIG. 200. Saprolegnia. A, sporangium containing spores. JB, sporangium 
from which spores are emerging. C, oogonium surrounded by antheridial 
branches. D, after the formation of antheridia by the antheridial branches 
and the development of fertilization tubes. 

of the old sporangium, where a new sporangium is formed. By a 
repetition of this process as many as three or four sporangia may 
be formed successively, each within the wall of the next older. 
A swarm-spore may swim about until it comes into contact with a 
source of food, when it comes to rest, withdraws its flagella, 


secretes a cell wall, and develops into a slender hypha. This hypha 
penetrates the substrate, where it branches and develops into a 

210. Gametic Union (Fig. 200, C, D). Under some conditions 
certain hyphae produce sex organs instead of sporangia. The 
many-nucleate end of a hypha, cut off as a separate cell, develops 
in this case into an oogonium, which enlarges and becomes spheri- 
cal. By a process of cleavage somewhat similar to that which 
occurs in a sporangium, the protoplasm of the oogonium is di- 
vided into a variable number (4 32) of female gametes (eggs). 

Slender branches arising, some just beneath the oogonium, 
others on neighboring hyphae, grow toward, and become closely 
applied to, the oogonium. The slightly enlarged terminal portion 
of each of these hyphae is cut off as a many-nucleate antheridium. 
From the antheridium grows a slender fertilization tube which 
penetrates the oogonium and comes into contact with one or more 
eggs. In some species of Saprolegnia a nucleus and some cyto- 
plasm, together constituting a small male gamete, pass from the 
antheridium through the fertilization tube into each egg. The 
zygote formed by union of the gametes secretes a thick wall. 
In other species, antheridia and fertilization tubes are not pro- 
duced, or if produced do not function. Even in such cases, however, 
the eggs become thick- walled and have the appearance of zygotes. 
Under these conditions an egg functions as a spore, just as a gamete 
of Spirogyra sometimes does. The zygote or thick-walled spore 
usually enters upon a period of rest, and may retain its vitality 
for many months. The method of its germination is not well 

The immediate relatives of Saprolegnia, members of the same 
order, resemble it in structure and in the general course of their life 
history. There are important differences between members of the 
order, however, in the structure of sporangia and in the methods 
of liberation and germination of swarm-spores. 


211. Nature and Structure. All species of Albugo are parasitic. 
A very common one infects the radish, cress, mustard, shepherd's 
purse, and related plants. The portions of the host plant containing 
the fungus, which may be leaves, stems, branches, flowers, or 
fruits, are often discolored or enlarged and markedly distorted 



(Fig. 201). In time, white, mealy patches develop on the infected 
parts. Because of the appearance of these spots the disease is often 
called the " white rust." The mycelium of Albugo, like that of 
phy corny cetes previously mentioned, is one-celled when young, 

many-nucleate, and composed of 
many hyphae which grow between 
the cells of the host. Foods are 
obtained by means of short, knob- 
like lateral branches that pene- 
trate the walls of the host cells. 

212. Spore-formation. A time 
comes when, in consequence of a 
specially rapid branching of the 
mycelium, dense masses of hyphae 
are formed at various places be- 
neath the epidermis of the host. 
Each hyphal mass gives rise to a 
layer of parallel stocky, thick- 
walled cells whose long axes are 
perpendicular to the epidermis 
(Fig. 202, A). Each cell of this 
layer elongates at its tip, and cell 
division cuts off a small, many- 
nucleate spore (conidium; Fig. 
202, B). The cell below the spore 
elongates, and a second conidium 
is formed just beneath the first. 
By repetition of this process a chain 
of conidia is produced. The pressure caused by the elongation of 
numerous parallel spore chains finally breaks the epidermis of the' 
host. It is the exposure of a mass of conidia that causes the mealy 
appearance characteristic of the fungus. The conidia are now easily 
detached from one another and scattered. When they germinate, 
each divides to form six or more two-flagellate swarm-spores which 
in turn can infect the host plant. The disease is spread throughout 
the growing season of the host by the production of successive 
crops of conidia. 

213. Gametic Union (Fig. 203). After the formation of conidia, 
certain hyphae penetrate the deeper tissues of the host plant, 
especially those of the petioles or stem. In the intercellular spaces 

FIG. 201. Young radish plant in- 
fected by Albugo. The white 
spots are masses of spores. 



of these tissues the tips of the hyphae enlarge, some becoming 
spherical and filled with a dense cytoplasm containing many 
nuclei. A cell division occurs which separates the enlarged tip 

Young Spore 

Basal Cell 

Cell Wall 

FIG. 202. Albugo. A, cross section of an infected radish leaf, showing the 
development of spores (conidia) beneath the epidermis (diagrammatic). 
B, enlarged view of the end of a hypha, showing the method of spore- 

(now an oogonium) from the rest of the hypha. Another cell 
division within the oogonium in time separates a single, usually 
one-nucleate, central egg. from the surrounding many-nucleate 


FIG. 203. Gametic union in Albugo. A, young oogonium and antheridium. 
B, oogonium in which an egg has been formed; the antheridium has de- 
veloped a fertilization tube. C, old oogonium containing a zygote. 

protoplasm. The egg is bounded by a plasma membrane, but no 
wall is formed between it and the peripheral cell. By this time the 
slightly enlarged tips of other hyphae have come into contact with 
the oogonium ; the tips of these hyphae then by cell division become 


antheridia. A slender fertilization tube from one antheridium 
pierces the wall of the oogonium and grows until it reaches the egg. 
A male gamete passes from the antheridium through the fertili- 
zation tube, enters the egg, and the male and female nuclei unite. 
The zygote formed by this union becomes surrounded by a thick 
wall. The cytoplasm and nuclei of the peripheral cell gradually 
disintegrate. Repeated nuclear divisions occur in the zygote, 
which at maturity contains typically 32 nuclei. After several 
weeks' rest the zygote may germinate. In germination, water is 
.absorbed and cell and nuclear divisions occur, so that finally more 
than 100 two-flagellate, one-nucleate swarm-spores escape, each 
capable of infecting the host. 

214. Relatives of Albugo. The "damping-off "fungus (Pythium), 
which causes wilting; and decay in seed beds of various cultivated 

FIG. 204. Early and late stages in the late blight of potato, caused by 
Phytophthora. Photograph by I. E. Melhus. 

plants, the " downy mildew " (Plasmopard) , which causes great 
losses to the grape industry in the United States and Europe, and 
Phytophthora, which causes the destructive "late jalight" of the 
potato (Fig. 204), are all closely related to Albugo, differing from 
it mainly in the manner of production of their spores. 

215. Relationships of Phycomycetes. The members of this class 
show considerable resemblances to certain algae. Rhizopus and 



the other black molds are thought by some to have arisen from 
algae more or less like Spirogyra, because of the presence of similar 
or not greatly differentiated gametes and because of resemblances 
in methods of gamete-formation. The many-nucleate, undivided 
mycelium of the black molds renders this hypothesis questionable. 
It has been suggested, however, that there may be a connection 
through Empusa and some of its relatives, whose mycelia are made 
up largely or entirely of short cells each containing one large, 
centrally placed nucleus like that of Spirogyra. 

Saprolegnia and its immediate relatives may have come from 
algal ancestors similar to Vaucheria. Vaucheria and Saprolegnia 
are alike in their un- 
divided thalli with 
many small nuclei; in 
possessing motile 
spores; and in the dif- 
ferentiation between 
oogonium and anther- 
idium, and between 
male and female gam- 


A large order of 
fungi (Chytrids), 
mostly aquatic and in 
large part parasitic, 

FIG. 205. Successive stages in the development 
of Rhizophidium, a chytrid parasitic upon 
Spirogyra. The Spirogyra cell shown is in very 
abnormal condition in consequence of infec- 
tion by the parasite. 

and very much simpler than either Rhizopus or Saprolegnia, 
should be mentioned here because they are usually listed with 
the phycomycetes. Some chytrids have naked many-nucleate 
protoplasts that give rise by progressive cleavage to spores. 
In other species the greater portion of the plant body is sur- 
rounded by a wall, but naked protoplasmic threads extend from 
this central portion into the host cell (Fig. 205). At maturity the 
walled portion functions as a sporangium. In still other species 
the entire protoplast is surrounded by a wall. Varying degrees of 
sexual differentiation between gametes appear among the chytrids. 
Many chytrids give evidence of a not very remote relationship 
with flagellates. It is not unlikely, therefore, that included among 
the phycomycetes are forms descended from flagellates, as well as 
others, like Rhizopus and Saprolegnia, that have been derived 
from algae. 



216. Nature. Members of this, the largest class of fungi, vary 
greatly in form and structure. Some are saprophytic, some par- 
asitic. All ascomycetes at some time in their history produce spore 
sacs (asci), within which are formed ascospores. The number of 
ascospores in an ascus is variable; in a large majority of species the 
number is eight. In addition to ascospores, various ascomycetes 
produce spores of one or more other types, some species having, 
including ascospores, as many as four spore forms. 


217. Structure and Reproduction. A yeast is a fungus consisting 
of a single ovoid cell, or of a colony of two or more such cells. The 
most conspicuous feature of a yeast cell (Fig. 206, ^L) is a large 

Cell Wall 


FIG. 206. Yeasts. A, mature yeast cell B, colony formed by repeated cell 
division. C-E, stages in cell division. F, ascospores. A, C~E, from 
stained preparations. #, F, from living material. 

vacuole lying in a finely granular dense cytoplasm. In the dense 
cytoplasm are also reserve food particles of varied shapes and sizes. 
Some of these are rounded masses of glycogen or globules of fat; 
others, more or less angular, consist of proteins. In specially stained 
cells it is possible to distinguish a nucleus with a chromatic net- 
work and a nucleolus. A cell wall which probably consists, in part 
at least, of chitin encloses the cell. 

In a culture containing actively growing yeasts, cells of varying 
sizes are often united in colonies (Fig. 206, B). The cells composing 



n* s 

each colony have been derived from a single cell as a result of 
division and growth (Fig. 206, C-E). In the reproduction of a yeast 
under ordinary conditions, the nucleus divides in essentially the 
same manner as do the nuclei of more complex plants. A small 
localized area of the wall, usually at or near one end of the cell, 
becomes softened, probably as a result of enzymatic action. The 
wall bulges in the region of softening, a swelling or bud thus being 
produced. As this bud is forming, some of the cytoplasm and one 
daughter nucleus pass into it. A constriction of the plasma mem- 
brane in the plane of origin of the bud brings about a division of the 
cell into two daughter cells of very unequal size. The smaller of 
these daughter cells (the former bud) grows rapidly, and soon it 
also may produce a bud arid divide in the same manner as the 
parent cell. The cells may remain attached or may separate. 

Under conditions unfavorable for the ordinary development 
just described, such as a scarcity of food or water, a yeast cell 
often divides to produce a limited number, typically four, of one- 
nucleate cells which remain within the wall of the parent cell 
(Fig. 206, F). In this case the parent cell functions as a spore sac, 
(ascus), and the cells formed by division within the old cell wall are 
ascospores. With the return of conditions suitable for ordinary 
growth, the ascospores absorb water, grow, burst the wall of the 
spore sac, and develop into cells of the usual type. In many yeasts, 
including those of economic importance, no union of gametes 
occurs. The cells of a few yeasts, however, unite in pairs before 
forming ascospores, and thus function as gametes. This fact 
is considered by some investigators as evidence that the yeasts are 
descendants of ascomycetes that had a more complex life cycle. 

218. Fermentation. It was known to the ancients that if a 
mixture of flour and water (dough) was allowed to stand, it would 
make a leavened bread very different from the unleavened bread 
baked immediately after the dough had been prepared, and that 
leavening would proceed more rapidly if sugar was added. It has 
long been a matter of general knowledge, too, that if fruit juices 
are exposed to the air and left undisturbed for a time, the liquid 
becomes cloudy, gases are given off, and, as the sugar disappears, 
the liquid becomes alcoholic. But not until European wine-makers 
and brewers became interested in an effort to control the flavors 
of wines and beers was something definite learned regarding the 
agencies concerned in the fermenting of fruit juices and in the 



making of bread. Pasteur showed that these processes are due to 
the activities of yeasts and of other microorganisms. 

Most yeasts can live and grow only in a solution containing a 
sugar, or substances that may readily be changed to sugars. 
Yeasts can not grow and multiply, however, unless other substances 
are present, because the carbon, hydrogen, and oxygen of a sugar 
are not the only elements necessary for the building up of living 

matter. If the so- 
lution containing 
the necessary food 
substances is in a 
thin layer, so per- 
mitting access to 
an abundant sup- 
ply of air, the 
yeast cells will 
grow and divide 
rapidly, using a 
considerable por- 
tion of the sugar 
as a source of 
both building ma- 
terials and energy. 
If, on the other 
hand, little oxygen 
is available, as 
when the yeast 

FIG. 207. Evolution of carbon dioxide during fer- cells are so deeply 
mentation. The closed arm of the tube at the left . . \ [ 

is filled with a sugar solution containing yeast; the immersed in trie 
tube at the right shows the effect, after a few hours, solution that most 
of the production of carbon . dioxide which has o them are cut 
forced the liquid out of the closed arm. 

off from the air, 

the majority of the cells settle to the bottom and there live as 
anaerobes. Under this condition the cells secrete an enzyme 
(zymase) which breaks down the sugar into alcohol and carbon 
dioxide, and the yeast makes use of some of the energy so re- 
leased. Thus alcoholic fermentation is a type of respiration which, 
in the absence of free oxygen, replaces ordinary respiration as a 
source of energy for the yeast. A single species of yeast can ap- 
parently break down only a certain sugar or certain sugars, and 



some few rare yeasts are not known to cause alcoholic fermenta- 
tion under any conditions. 

The yeasts which bring about the fermentation of fruit juices 
in the making of wines and ciders are largely wild species. These 
yeasts live in or on the soil of vineyards and orchards and are 
carried with dust to the skins of the fruits. When the fruits are 
crushed, the yeasts are brought into contact with the fruit juices 
and fermentation ensues. The yeasts that ferment grape juice are 
of various species, and the characteristic flavors of different wines 
are due in large part to differences in the yeasts as well as in other 
organisms present, which cause, in addition to alcoholic fermenta- 
tion, the formation of sub- 
stances that modify the flavor 
of the wine. 

Yeasts used in brewing and 
in bread-making are cultivated 
yeasts. Cultures of these yeasts 
are grown and kept pure with 
the greatest care in order to 
prevent their contamination 
by wild yeasts and other or- 

Some bacteria and several 
fungi other than yeasts are ca- 
pable of inducing alcoholic fer- 
mentation. The production of 
alcohol by these organisms is 
encouraged by growing the 
bacteria and fungi under an- 
aerobic conditions. A few 
black molds closely related to 

FIG. 208. Lilac leaf infected by a 
powdery mildew. 

Rhizopus, as well as some spe- 
cies of Penicillium and Asper- 
gillus ( 221), readily form alcohol, and the commercial production 
of alcohol on a large scale is due in considerable measure to the 
activities of such fungi. 


219. Structure and Reproduction. These parasitic fungi appear 
during the summer and fall, giving to the leaves or young stems of 



their various host plants a whitish mealy or powdery appearance. 
The mycelium, composed of short one-nucleate cells, grows on the 
surface of a leaf or stem. Short branches from some of the cells of 
the mycelium, which act as absorbing organs, pierce the walls of 


B C 

FIG. 209. Powdery mildews. A, Microsphaera; J5-D, Sphaerotheca; E, F y 
Erysiphe. A, portion of a mycelium growing on the surface of a leaf and 
forming conidia at the end of a hypha. B, gametes. C, D, young zygotes. 
E, young fruiting body in which the cells derived from the zygote (shaded) 
are enclosed in a mass of cells from vegetative hyphae. F, cross section 
of a fruiting body at the time of the formation of asci. B-F redrawn from 

the epidermal cells of the host, or, growing through stomata, 
pengtrate the walls of the cells of underlying layers. 

A common powdery mildew (Microsphaera) lives on the leaves 
of the lilac (Fig. 208). As soon as the mycelium has become well 
established on the host, some of its cells grow outward, perpen- 
dicularly to the surface of the leaf. The terminal portion of each 
of these elongated cells is separated by a cell division (Fig. 209, A) 
and becomes a short cylindrical spore (conidium). Other divisions 
occur successively below the first, so that a row of conidia is pro- 
duced. The conidia are easily detached and separated from one 
another, and, since they can germinate immediately, are responsible 
for the rapid spread of the fungus to other leaves throughout the 
growing season 



220. Gametic Union. Sex organs are formed at the ends of spe- 
cial branches of the mycelium which grow so as to come into contact 
in pairs (Fig. 209, B~D). The slightly enlarged terminal cell of one 
branch of each pair is an antheridium; the much more enlarged ter- 
minal cell of the other branch is an oogonium. The one-nucleate 
protoplast of the oogonium functions as an egg; that of the anther- 
idium as a male gamete. The walls at the point of contact between 

Microsphaera. A, ascus forming ascospores. B, mature ascus. 
C, mature fruiting body. 

oogonium and antheridium are dissolved, and through the opening 
so formed the male gamete passes into the oogonium. The nucleus 
of the male gamete and that of the egg unite, and the opening be- 
tween antheridium and oogonium is later closed. The zygote 
formed by the union of egg and male gamete now divides, giving 
rise to a row of three to five cells. Branches arise from some of the 
cells of this row. Certain cells of these branches, containing two 
nuclei each, develop into asci. The two nuclei in each young ascus 
unite (Fig. 209, F), and, as soon as this nuclear union has taken 
place, the ascus begins to enlarge rapidly. During its enlargement 
the single nucleus now present divides, its daughter nuclei divide, 
and their daughter nuclei divide, so that the ascus contains eight 
nuclei distributed throughout its cytoplasm. By a process of cell 
division peculiar to the ascomycetes (free-cell formation),* in. which 


not all the cytoplasm of the i^enfr'^HI^tte ascus) is included 
within the daughter cells, eight or fewer t one-nucleate cells are 
formed inside the ascus (Fig. 210, A, B). These cells are asco- 

After the union of egg and male gamete, the zygote becomes sur- 
rounded by hyphae which grow from the cells immediately be- 
neath. While the asci are enlarging, the surrounding hyphae form 
a structure, black and almost spherical, in whose central portion 
the asci are enclosed (Fig. 210, C). Certain superficial cells of this 
fruiting body develop into greatly elongated appendages which 
(in Microsphaera) fork repeatedly toward their outer ends. The 
numerous fruiting bodies formed on an infected leaf become de- 
tached as the mycelium decays, and are scattered by winds. In 
the spring the outer layers of each fruiting body break down, ex- 
posing the asci, from 
which the ascospores 
escape. An ascd|pore 
falling on a lilac leaf 
may grow into a new 


221. Blue and Green 
Molds. Certain asco- 
mycetes form tangled 
mycelial masses on sur- 
faces of decaying fruits, 
vegetables, and meats, 
as well as on damp 
leather and on a great 

FIG. 211. A, lengthwise 8ection of a conidium- variety of plant and 
bearing branch of Aspergillus. B, conidium- animal substances. The 
bearing branch of PeniciUium. (Both figures characteristic flavor 
diagrammatic.) , ... . 

and mottled appear- 
ance of Roquefort and other cheeses are due to some of these 
fungi. Because of the general appearance of their mycelia, ascomy- 
cetes of this type are commonly spoken of as "molds." They differ 
from the black molds described in Chapter XIX in that their 
branching mycelium consists of relatively short one-nucleate cells, 
and also in their production of. asci. Most of the ascomyeetous 


which appear to be the same as the rust upon the wheat and which 
are for convenience called by the same name (Puccinia graminis). 
But the uredospores of the stem rust of wheat will infect the oat 
only with difficulty and will not produce on the oat a serious 
disease. In the same way, uredospores from Puccinia graminis on 
oats, rye, or barley will not readily infect wheat. Both the rust on 
wheat and that on oats pass their aecidial stages upon the barberry. 
Such cases illustrate the fact that it is possible to differentiate 
races or species on the basis of their function (in this case of their 
ability to infect different hosts), although no distinction can be 
made on the basis of structure. Just as there are different strains 
of Puccinia graminis on wheat, oats, rye, and barley, so there are 

A B 

FIG. 225. Apple rust. A, apple leaf, showing groups of 

aecidia on its lower surface. B, enlarged view of an 
area bearing aecijiia. 

many distinct physiological races of thif fungus each of which is 
limited to particular varieties of wheat. Similar conditions exist 
in other cereal rusts. 

234. Other Rusts. The apple rust forms spermatia and aecid- 
iospores on leaves and fruits of the apple (Fig. 225) and of some 
of its relatives. On the red cedar it produces swellings of the 
branches (so-called " cedar apples/' Fig. 226) in which teleuto- 
spores are formed. A teleutospore, still attached to the host, ger- 
minates, producing a basidium and basidiospores. The basidio- 
spores infect the apple. This rust forms no uredospores. 

Some rusts, differently from the wheat and apple rusts, complete 
their life cycles on a single host. Examples of this sort are the rose 



rust, the asparagus rust, the hollyhock rust, and the orange leaf 
rust of blackberries and raspberries. Many rusts have a shorter 
life cycle than the wheat rust. For example, the apple rust just 
mentioned produces no uredospores. The hollyhock rust represents 
the shortest known type of life cycle among the rusts; it produces 
only teleutospores and basidiospores. Although various rusts omit 
the formation of aecidiospores, spermatia, or uredospores, no rust 

A B 

FIG. 226. Apple rust. A , " cedar apples " or! the red cedar. 
B, a cedar apple producing horn-like gelatinous pro- 
jections on which teleutospores are formed. 

is known which does not produce both teleutospores and basidio- 


236. Field Mushroom. Numerous basidiomycetes, including 
many of the most conspicuous ones, are saprophytic. One of the 
commonest saprophytic basidiomycetes is the field mushroom 
(Psalliota campestris), which often grows in the rich soils of fields 
and open woods. This is the one mushroom that is extensively 
cultivated. Its vegetative body consists of colorless or whitish 
branching, short-celled hyphae which live for the most part under- 
ground. The cells of this mycelium are at first one-nucleate; but 
after a certain stage two-nucleate cells are formed, probably in 
consequence of a union of one-nucleate cells which therefore func- 
tion as gametes. Some of the hyphae are interwoven into thicker 



strands, but these strands, as well as the separate hyphae, are 
easily broken when the soil is disturbed. After the mycelium has 

FIG. 227. Young fruiting bodies of the field mushroom (Psalliota) 
arising from the mycelium. 

been developing for some time, compact, rounded masses of inter- 
woven hyphae appear here and there on the underground strands. 
At first such a mass is almost ^microscopic ; as it matures, it 

develops into the fruiting 
body commonly called a 
" mushroom " (Fig. 227). 
This body becomes differ- 
entiated into a stalk and 
a cap. The margin of the 
cap is attached to the stalk 
by a thin membrane which 
is broken as the cap en- 
larges. A portion of the 
membrane remains at- 
tached to the stalk in the 
form of a ring. Before the 
breaking of the membrane, 
the lower portion of the 
cap, extending from the 
stalk to the outer edge, has become transformed into many 
thin plates (gills), each free at its lower edge but attached 
above to the more compact portion of the cap. As the cap grows, 
it becomes much flattened so that the gills are fully exposed. 

FIG. 228. Mature fruiting body ot the field 
mushroom. Photograph by B. M. Duggar. 



The mature cap (Fig. 228) is two to five inches in diameter; its 
top is white, cream-colored, or brownisjj; it bears many fine, silky 
hairs, and often some brownish scales. The flesh is white, turning 
to pink if broken. The gills are at first flesh-colored or pink, grad- 
ually changing, as the fruiting body grows older, to dark brown. 
The terminal cells of many of the hyphae which compose a gill 



FIG. 229. A y diagram of 'a cross section of a portion of a gill of Psalliota. 
B, young 2-nucleate basidium. C, basidium after the union of the 2 nuclei. 
Z>, after the division of the zygote nucleus and of its daughter nuclei to 
form 4 nuclei. E, F, stages in spore-formation. 

form a layer on eabh side of the gill (Fig. 229). The cells of this 
layer are parallel to one another and perpendicular to the surface 
of the gill. Most of the cells of the surface layer become much 
enlarged basidia. Each basidium is at first, like other cells of the 
mycelium, two-nucleate. The two nuclei in a basidium unite; this 
union is followed by two nuclear divisions, so that the basidium 
contains four nuclei. From the free end of each basidium grow two 
or four slender projections; the outer end of each projection swells, 
into it passes one of the nuclei of the basidium, and the enlarged 
end is cut off as a basidiospore. When a basidiospore germinates, 
it gives rise to a mycelium which may in time produce fruiting 



236. Other Mushrooms. The "shaggy mane" (Fig. 230), which 
grows singly or in clusters on soil rich in humus, has a structure 
much like that of the field mushroom. The cap is long and narrow, 
and its upper surface bears numerous patches of hyphae interwoven 
in the form of strands and plates. As basidia and spores mature, 

FIG.. 230. The "shaggy 
mane," Coprinus. 

the cap darkens ; its lower edge 
and eventually the whole cap 
softens and breaks down into 
black, slimy droplets. The 
shaggy mane and the closely 
related "inky cap" are edible. 

Among the most beautiful as 
well as the most dangerous 
mushrooms are the deadly Amanita (Fig. 231) and the "fly mush- 
room/' also an Amanita. At the base of the stalk of each of these 
mushrooms is a cup or bulb, from whose center arises the stalk 
with its conspicuous ring. The deadly Amanita has a pure white 
cap; that of the fly mushroom is reddish or orange-colored. 
Scattered over the upper surface of the fly mushroom are some- 
times white wart-like elevations. The gills and the spores of both 
species are white. " 

The "honey mushroom" commonly occurs in clusters about 

FIG. 231. The deadly Amanita. 



FIG. 232. Bracket fungi (Fomes) on 
the trunk of a birch. 

trees or stumps during late summer and autumn. Both stalk and 
cap are yellow or brownish. The stalk is tough and commonly 
bears a definite ring. Near the center of the cap are usually a 
number of erect dark scales. The " oyster mushroom " forms large 
clusters on trunks of dead or dying trees. The stalk is short and 

very thick and bears a large 
cap, often six inches in diame- 
ter. The stalk and cap are 
white. The name "oyster 
mushroom" was suggested by 
the shape of the cap, which is 
commonly much more devel- 
oped on one side than on the 
other. Both the honey mush- 
room and the oyster mush- 
room are edible. 

There are no general rules 
for distinguishing between edible mushrooms and those which are 
unfit for food or poisonous. The one safe rule is to eat only mush- 
rooms identified by an expert. 

237. Bracket Fungi. Another considerable group of basidio- 
mycetes live as saprophytes or parasites on various trees and, 
shrubs. The mycelium 
penetrates the wood 
and on the external 
surface of the tree or 
shrub produces fruit- 
ing bodies of various 
forms. One of the sim- 
plest types of these 
fruiting bodies is com- 
posed merely of a crust- 
like layer of hyphae 
bearing basidia. Those 
of another type grow erect and are variously branched. In a third 
type, the branches of the fruiting body are covered with teeth or 
spines which project downward and whose outer surfaces bear 
basidia. The fruiting bodies of a fourth type are the so-called 
" brackets " (Fig. 232) that appear on stems and branches. Some 
of these brackets are soft, expanded outgrowths in whose lower 

FIG. 233. Lengthwise section 

a bracket 



surfaces are innumerable fine pores lined with basidia. Most of 
such fleshy forms live for but a single season. The fruiting bodies 
of other bracket fungi are firm, hard, sometimes almost woody in 
texture, and grow in size from year to year, forming each year a 

FIG. 234. Disintegration of wood caused by a bracket 

new pore-containing layer below,, and extending beyond, that of 
the previous year (Fig. 233). The fruiting bodies of Fomes applana- 
tus, one of the commonest bracket fungi, are often 12 inches or 

more in diameter and may live for 
10 years or longer. 

The bracket fungi and their near 
relatives cause immense losses through 
the decay of the wood of living trees, 
as well as of logs and timber in lumber 
yards and of lumber in factories and 
warehouses. It is estimated that these 
''wood destroyers" cause each year 
more damage than forest fires. 

238. Puffballs. The basidia and 
spores of another group of basidiomy- 
cetes are enclosed, often in special 
chambers, within the fruiting body. 
The spores of some species escape through special pores; those 
of others are set free only when the fruiting body decays or 
is accidentally broken. The best-known members of this group 
are the common "puffballs" of pastures and woods (Fig. 235)* 

FIG. 235. A puffball. 



The mycelium of a puff ball grows in the soil or in rotting wood; 
on it are developed spherical bodies, comparable at first to an 
early stage of the field mushroom. These bodies grow, those of 
some of the giant puffballs reaching a diameter of 12 to 15 inches 
and a weight of several pounds. The interior of a puffball remains 
white until its full size is reached, when numerous scattered areas 
in the upper portion darken. This color change occurs at a time 

FIG. 236. The "earth star" (Geaster), a puffball. 

when the ends of some hyphae have formed basidia. Other dark- 
ened areas appear successively within the fruiting body until finally 
the greater portion of its interior is filled with basidia and spores. 
Some of the hyphae surrounding the masses of spores disintegrate; 
other hyphae, whose walls become thickened, form a spongy net- 
work or serve as the boundaries of special chambers. The outer 
layer of the fruiting body in some species now opens by a definite 
pore, but in most puffballs it breaks irregularly or simply decays, 
and the spores escape. 

239. Relationships. Smuts and rusts have not always been 
classed with the "true" basidiornycetes, such as the mushrooms. 
While these three groups are now usually treated as constituting a 
single class of which the smuts are most primitive, some writers 
still consider the smuts unrelated to the others. 

As to the possible origin of basidiomycetes, little can be said. 
There is no group of algae from which they can readily be imagined 
to have been derived. The basidiospores have been likened to 
conidia, and some basidiomycetes produce true conidia that re- 


semble those of some phycomycetes. The spermatia of rusts are 
similar to those of some ascomycetes. Basidiomycetes resemble 
ascomycetes also in possessing a mycelium composed at different 
stages of one- and two-nucleate cells. In other respects the mycelia 
are similar enough in the two classes to render plausible the con- 
ception of a relationship, perhaps through a remote common an- 
cestry, between basidiomycetes and ascomycetes. 



240. Nature and Forms. A lichen is peculiar in being formed 
by the intimate association of two very different plants, one of 
which is a filamentous fungus, the other in almost all instances an 
alga. The two organisms seem in most cases to derive mutual ad- 
vantage from their association, the alga making carbohydrate foods 

I<IG. 23V. A crustose lichen (Uraphis scnpta). The dark linear and 
curved structures are fruiting bodies. 

and the fungus absorbing and retaining moisture for the partner- 
ship. In temperate regions, the fungal component is always an 
ascomycete; in a few lichens of warmer regions it is a basidiomycete. 
In the majority of lichens the other component is a one-celled green 
alga; in many, however, it is a blue-green alga. One lichen has been 
reported in wilich a filamentous fungus is associated with a bac- 
terium rather than with an alga. The bacterium is of a species that 




FIG. 238. Parmelia, a foliose lichen. 

produces a red pigment. Lichens may be divided according to their 
forms into three principal types (Figs. 237-241): crustose, forming 
crusts on trees, rocks, or soil; foliose, with leaf-like thalli whose 
upper and lower surfaces are 
different; and fruticose, which 
are pendent or erect. 

Crustose species vary greatly 
in form, color, and thickness. 
The body of such a lichen (Fig. 
242, .4) usually consists of an 
upper layer of closely packed 
and interwoven fungal hyphae, 
beneath this a layer of algal cells 
intermixed with hyphae, and 
finally a region of loosely woven 
hyphae which are intimately in- 
termingled with the substrate. An additional lower closely packed 
layer is present in some species, especially in those which develop 
free lobes at the margin of the thallus. The bodies of some crus- 
tose lichens are partly or wholly Imbedded in the bark, disinte- 
grating rock, or soil upon which 
they grow. In the latter case, 
all that appears above the sur- 
face of the substrate may be 
the fruiting bodies of the lichen. 
Graphis scripta (Fig. 237) is a 
crustose lichen growing upon 
and partly imbedded within 
the smooth bark of some trees. 
Superficially it appears as an 
ashy or whitish crust on the 
surface of the bark, marked by black linear, curved, or branched 
fruiting bodies. Because of the resemblance of the fruiting bodies 
to hieroglyphic writing, this lichen has been observed and re- 
corded since very ancient times. 

A foliose lichen consists of one or more flat lobes which usually 
adhere more or less firmly to the substrate by means of strands of 
hyphae. The structure of the thallus is similar to that of a crustose 
lichen except that there is in all cases a well-developed lower closely 
packed layer from which grow holdfasts strands of hyphae which 

FIG. 239. Gyrophora, a foliose lichen. 



attach the thallus to the substrate. Some foliose lichens, such as 
Gyrophora (Fig. 239), are attached to the substrate each by a small 
central holdfast. On the lower sides of certain large foliose lichens 
are depressed, light-colored areas. In these areas the lowermost 

layer is lacking, and its absence al- 
lows a free passage of air to the algal 

The body of a fruticose lichen 
varies in shape from flat to cylindri- 
cal. Commonly it is much branched 
(Fig. 240). There is a central region 
of hyphae, surrounded by a zone con- 
taining algal cells, and this in turn 
by an outer zone of compact hyphae. 
There are no clearly differentiated 
upper and lower surfaces. The lichen 
is attached to the substrate by a 
definite basal portion composed of 
strands of densely packed hyphae. 
In some lichens, such as Cladonia 
(Fig. 244), the body is a combination 
of a crustose or foliose part with erect 
(fruticose) stalks. 

241. Vegetative Multiplication. 
Any portion of the body of a lichen 
that is broken off may, under suit- 
able conditions, develop independ- 
ently. The commonest method of 
vegetative multiplication, and one 
found in most lichens, is by the de- 
velopment on the upper surface of minute bud-like outgrowths 
(soredia, Fig. 242, B). A soredium is composed of one or more 
algal cells surrounded by fungal hyphae. Soredia are formed at 
points at which the outermost layer of the thallus is interrupted, 
and are sometimes so abundant as to appear like dust on the sur- 
face of the thallus. Each soredium is pushed outward by the 
elongation of the hyphae to which it is attached; other soredia 
are formed below it, and later they too are pushed out. In cer- 
tain species of lichens this is the only known method of repro- 
duction of the thallus. Many lichens bear on their surfaces also 

FIG. 240. "Old man's heard 
(Usnea), a fruticose lichen. 


larger branching outgrowths, which likewise are composed of both 
fungal and algal elements. These outgrowths are easily broken 
off when dry, and under suitable conditions they may develop 

242. Spore-production. Minute dark pores appear on the sur- 
faces of many lichens. Each such pore opens into a small cavity 

FIG. 241. A fruticose lichen (Evernia), with saucer-shaped fruiting 
bodies containing asci. 

(pycnidiwri). At the tips of hyphae lining the cavity, spores (pyc- 
nidiospores) are formed. The spores are extruded and, if they ger- 
minate, produce new fungal hyphae. Experiments indicate that 
when these hyphae are grown in association with the appropriate 
algal cells, a lichen thallus results. In some lichens pycnidia are 
the only spore-forming organs known. 

In case the fungal component is an ascomycete, asci are borne 
either in saucer-shaped (Fig. 243) or elongated (Fig. 237) structures 
on the surface of the lichen or in approximately spherical structures 
that are partly or entirely imbedded in the thallus. In a few lichens, 
the formation of asci has been shown to be preceded by a gametic 
union. Whatever the form of fruiting body, the included asci are 
intermingled with slender sterile hyphae (Fig. 242, C). Both asci 
and sterile hyphae grow approximately at right angles to the inner 



surface of the fruiting body, constituting a fairly definite lining 
layer. Each ascus, in most lichens, contains eight ascospores. 
The spores are one-, two-, several-, or many-celled, their shapes 

varying with the genus and 

An ascospore, or each cell 
of an ascospore, may develop 
into a hypha which branches 
and elongates until its food 
supply is exhausted. If the 
hypha does not come into con- 
tact with algae of the species 
with which it is ordinarily as- 
sociated, it :lies; but if the 
appropriate algae are encoun- 
tered, the fungus grows about 
the algal cells to form a lichen. 
243. Practical Significance. 
Lichens play an important 
part in the formation of soil. 
Many crustose lichens grad- 
ually dissolve and disintegrate 
rocks to which they are at- 
tached. Lichens may be al- 
most wholly imbedded in such 
rocks, the rock particles being 
held together by the gelati- 
nized walls % of the hyphae. 
When the lichens die they 
form, together with the disin- 
tegrated rock, a substrate for 
the growth of other lichens or for that of mosses, ferns, and seed 

The " reindeer moss" (Cladonia mngiferina, Fig. 244) is of con- 
siderable importance as a food for reindeer and cattle. It forms 
dense tufts sometimes twelve inches in height, and is abundant in 
extremely cold regions where other vegetation is practically non- 
existent and where it may be buried in snow for long periods with- 
out injury. It grows equally well on sand, moist turf, or soils other- 
wise barren. " Iceland moss," another lichen, is similarly useful 

FIG. 242. A, cross section of the 
thallus of a lichen. B, a vege- 
tative bud (soredium) which when 
broken away is capable of repro- 
ducing the thallus. ~<7, cross section 
of a portion of a fruiting body, 
showing the formation of asco- 



in Iceland. The "rock tripe " of northern countries has been eaten 
by travelers when in danger of starvation. Another lichen (Leo 

FIG. 243. Cross section of a fruiting body of a lichen. 

anora esculenta) that has been used for food grows in the deserts 
of northern Africa. It is thought to have been the "manna" of 

&W ''!'' : ; ; i(l i 1 '' f ' ; 

FIG. 244. The "reindeer moss" (Cladonia rangiferina). 

the Israelites, and is still called the "bread of heaven." Many 
lichens were used by the ancients in the treatment of disease. The 


"dog lichen" (PeUigera canind) was used as a cure for hydrophobia, 
and the "lungwort" (Ldbaria pulmonarid) in the treatment of 
diseases of the lungs. The last-mentioned lichen has been used also 
in tanning, and as a substitute for hops in brewing. 

The cell walls of the fungi in a number of lichens contain color- 
ing substances. The most important of these coloring matters is 
orchil or cudbear, which is abundant in Rocella tinctoria. In ex- 
tracting this pigment, the lichen is soaked in an alkaline solution 
until the latter attains a purple color. Orchil was formerly ex- 
tensively used in the dyeing of woolen and silk^ *abrics. 



244. Nature. Liverworts are green plants, constituting one of 
the two classes of bryophytes. Their vegetative structure is in 
general more complex than that of any thallophyte. Most members 
of this class gi$>w prostrate upon the surface of the substrate, 
although many of them produce branches or other organs that 
tend to grow upright. Some are strictly thallose; in others the 
plant body is differentiated into stem (and branches) and leaves, 
which organs, however, are almost or quite without distinction of 
tissues. Growth of the thallus or of a stem or branch is chiefly at 
one end the anterior end or apex. Tjjvgrwnrf^ ^A sharply dis- 
tinguished from thaUophytes by the nature of their sex organs. 
These organs are always many-celled; some of their cells, at least 
those of the outermost layer, are sterile that is, they do not 
develop into gametes. 

The remote ancestors of this group must, it would seem, have 
been green algae; but whatever speqies 01 plants may~have con- 
stituted links between algae and liverworts have long since dis- 
appearedT JViost liverworts are terrestrial that is, they live on 
soil, rocks, decaying wood, or the bark of trees. A few are aquatic 
(living in or on water), but these are clearly descended from ter- 
restrial species. Since algae ^r^ chfl.rRntfiyifitifiallyfl.qiifl.tifi T many 
of the features that distinguish liverworts from green algae are 
probably fo HP thought of sw adaptations to a land habit. However, 
liverworts are still dependent upon an abundant supply of water, 
although some, such as certain species living on tree trunks, can 
withstand long periods of desiccation. 


245. Gametophyte. Members of the genus Rictia are among 
the simplest liverworts. Most of the 100 or more specif of this 
genus live on moist soil, or rarely on rocks. Two of the most 
familiar species, however, Riccia natans and Riccia fluitans, often 
occur floating in pools, ponds, and lakes. In case the body of water 




in which they are living partly or entirely disappears, the plants 
may be left upon the mud where they will continue to live and 
grow. A plant, especially one of Riccia natans (Fig. 245), growing 

on soil assumes a very dif- 
ferent habit of growth from 
that which characterized it 
on the water. Riccia natans 
will be particularly de- 
. B scribed in the following 

FIG. 245. Gametophytes of Riccia natans. paragraphs. 

A, a plant growing on land. B, a floating A spore of Riccia, under 

p favorable conditions, ger- 

minates by swelling and pushing out at one side a filamentous 
outgrowth (Fig. 246). In this process, the outer layers of the 
spore wall are broken and the protruding filament 
is surrounded by an extension of the innermost 
layer of the w|01 The dense cytoplasm and chlo- 
roplasts of the spore aggregate chiefly in the outer 
end of the filament ; at this end, by a series of cell 
divisions, a small mass of cells is formed. By fur- 
ther growth and cell division this group of cells 
develops gradually into a mature plant. 

The vegetative body is a thallus. When living 
on land, it is flat and at first approximately 
ribbon-shaped; it is thickest in the middle and 
gradually thinner toward the edges. The surface 
is more or less distinctly marked off into small 
rhomboidal areas~ While still comparatively short 
the thallus forks at its growing end, producing two 
similar branches; in time each branch forks, and 
the process is indefinitely repeated. The result 
of this method of branching is the formation of 
a rosette-like plant. When it grows on water 
the thallus is thicker, and each branch grows but 
little in length before it in turn branches. The 
water form, consequently, has a more compact 
appearance than the land form. The apical por- 
tion of each branch can live through the winter and resume growth 
the following spring.!/ 

On the upper surface of a branch is a median longitudinal furrow. 

FIG. 246. A 9 ger- 
mination of a 
spore of Riccia. 
B, early stage 
in the devel- 
opment of a 
thallus 'from a 
spore. Modified 
from Campbell. 


This furrow is often less conspicuous, except at the apical end of 
the branch, in the water than in the land form. At the apex of the 
branch is a notch, and at the base of ...this notch a small group of 
embryonic cells. It is by the division of the cells of this group, and 
by"the re^ateH"3IvjiS^ them, tEat all the 

._ceH^of"tEe"thailus are formed. The apices of the branches, there- 
fore, are the regions of growth. Occasionally some cells in the cen- 
ter of an apical group of embryonic cells cease -dividing. The apical 
group thus becomes separated into two groups of embryonic cells. 
In consequence of the formation of daughter cells which lie between 
these groups, the two embryonic regions gradually diverge, and 
ultimately the thallus forks, each fork or branch now having its 
own group of embryonic cells. In time the older parts of the thallus 
begin to undergo progressive death and decay. When decay reaches 
a point at which branching occurred, the surviving parts constitute 
two separate plants. Therefore, as a result of apical growth, 
branching, and the progressive death of older portit^s, the number 
of plants is from time to time increased. Adventitious buds are 
sgnietimes produced on the lower surface of the thallus which, if 
they become separated, may grow into new plants. It appears, too, 
that any cell or group -of cells may, in response to an effective 
stimulus such as that supplied by a wound, develop an outgrowth 
tnat will become a new thallus. 

In the apical region small intercellular spaces (air chambers) 
appear, which later extend to the surface of the thallus and in- 
crease in size with the growth and division of surrounding cells. 
The external openings of these chambers (Fig. 247) are narrow 
pores, each surrounded by a ring of five or six small cells. The air 
chambers become divided by partitions, each a single layer of 
cells; the interior chambers so formed are sometimes connected 
with one another and with those of the outermost layer by pores. 
In a mature part of the thallus, the greater proportion of its thick- 
ness is occupied by the numerous air chambers. The upper surface 
of the thallus is formed by a single layer of cells which bounds the 
outermost air chambers. The cells of the surface layer, as well as 
those constituting the partitions between chambers, contain many 
small chloroplasts; these cells, having access through air chambers 
and pores to the gases of the atmosphere, can carry on photosyn- 
thesis. The part of the thallus below the air chambers varies in 
thickness from one layer of cells at the margins to several layers 


in the median portion; indeed, in the median region air chambers 
may be entirely lacking. The cells of this lower tissue contain few 
or no chloroplasts; some of the cells are filled with masses of oil 
mixed with other substances. From the lower surface of Jhe thallus 
grow many long, narrow scales, each a single layer of ofefls. Some 
cells of the lower surface grow out into long, slender rhizoids which 

Air Pore 

Air Chamber 




FIG. 247. Portion of a cross section of a thallus of Riccia, bearing archegonia. 

attach the plant tothe soil or extend into the water. Some rhizoids 
are smooth-walled; the walls of others have peg-like internal thick- 
enings. The rhizoids, in their mode of origin and in their functions, 
resemble the root hairs of such a plant as the sunflower. Scales 
are more abundant on the water form, rhizoids on the land form. 
246. Sex Organs. The female gamete (egg) is produced in an 
archegoniwn, the male gamete (antherozoid) in an antheridium. 
The archegonium of a liverwort is very different from any struc- 
ture found among thallophytes. The antheridium differs fromj j 
organs of the same name borne by some algae and fungi in that ilj 
possesses an outer layer of sterile cells cells, that is, which never 
develop into male gametes. These organs may appear on plants of 
Riccia living either on the water or on land. Both archegonia and 



antheridia may be borne by the same plant and even by the same 
branch. They are produced in three to five rows on the upper 
surface in or near the median line (Fig. 247), and when mature 
are nearly or quite imbedded in the thallus in consequence of the 
division apd growth of neighboring cells. 

Antheridia appear first on young plants; after a varying number 
of antheridia have been formed, the development of archegonia 

B ' /v -* C 

FIG. 248. Sex organs, of Riccia. A, nearly mature archegonium with sur- 
rounding &|tt|jW> mature archegonium. C, antheridium with surround- 
ing tissue. *jPJP^ 

begins* JtCach sex organ originates in the apical region of the thallus; 
hence, in a branch bearing organs of both kinds, the antheridia 
are on the older portion and the archegonia are nearer the apex. 

An antheridium (Fig. 248, C) consists of a short, few-celled 
stalk and an ovoid body; the latter is composed of an outer layer 
or jacket of j^terile cells and numerous internal cells which, while 
the anffieTrTdium is growing, undergo repeated divisions. After 
these -divisions cease, each of the hundreds of internal cells now 
present develops into an aQtiiiii2fiy . An antherozoid has a slender, 
somewhat coiled body, and two long flagella attached near its 
anterior end. The mature antheridium is entirely enclosed within 
a cavity of the thallus; this cavity opens by a narrow pore at its 
upper end. After antherozoids are formed, if water penetrates the 
cavity, the sterile cells constituting the upper end of the anther- 



idial jacket become softened and disintegrate, and a viscous fluid 
containing the antherozoids oozes out of the antheridium and 
through the neck of the cavity to the upper surface of the thallus. 
Here, if sufficient water is present, the antherozoids swim freely 

An archegonium (Fig. 248, A) also has a short stalk; its body 
is composed of an enlarged basal venter and a slender neck. (Both 
neck and venter consist of a single outer layer of jacket cells and 
an inner axial row typically of six cells of which the lowest and 
largest, lying within the venter, is the egg^When the archegonium 

Jacket of 

FIG. 249. Stages in the development of a sporophyte of Riccia within the 
venter of an archegonium. A, 2 cells, formed by division of the zygote. 
B 9 somewhat later stage. C, stage at which spore mother cells are 

is mature, all the cells of the axial row, except the egg, degenerate 
into a mucilaginous mass; the cells at the distal end of the neck 
become spread apart; and a canal filled with the mucilaginous 
substance is thus formed, which extends from the open end of 
the neck of the archegonium to the egg (Fig. 248, B) f The arche- 
gonium, like an antheridium, is enclosed in a cavity, but the end 
of its neck protrudes slightly above the surface of the thallus and 
into the median furrow. 

247. Gametic Union. When the plant is floating, some of the 
freely swimming antherozoids are sure to come into the immediate 
vicinity of mature archegonia. If the plant is on land, a film of* 1 
water must be present on its upper surface, as at the time of a 
rain or of a heavy dew, in order to make possible the approach of 
an antherozoid to an archegonium. In either case the antherozoid, 



coming near the mouth of an archegoniumjresponds to a stimulus, 
probably of a chemical nature, by swimming directly toward and 
into the archegonium and down its neck toward the eg| Several 
or many antherozoids 
may thus enter an 
archegonium. One of 
them (and usually, at 
least, only one) unites 
with the egg,- 

248. Sporophyte. 
The zygote formed by 
the union of egg and 
antherozoid secretes 
a new wall and begins 
to grow almost 

mediately (Fig. 249). 
The first division of 
the zygot ia^&ppim- 
imat ely horizontal . As 
a result of succeeding 
divisions and further 
growth, the zygote 
develops into a spheri- 
cal mass of cells; the 
cells of the outer layer 
of this mass, becom- 
ing large and flat, con- 
stitute a sterile jacket. 
These outer ceUs con- 
tain chloroplasts and 
carry on some photo- 
synthesis. The cells 
within the jacket con- 
tinue to divide until 
there are present a 
large number of spore 
Another cells. These become more or less separated and rounded, and 
each undergoes two divisions to form four spores (Fig. 250). The 
spores in turn become separated and each secretes a thick wall. 
The simple spherical structure developed from the zygote, although 

FIG. 250. A, nearly mature sporophyte; the mass 
of spores is immediately surrounded by the outer 
layer of cells of the venter, which is enclosed 
within the tissue of the gametophyte. B, spore 
mother cell. C, young spores (only 3 of the 4 
visible) formed from a single spore mother celL 
D, mature spore. 



very small and entirely different from the plant that bore the 
gametes, is nevertheless a distinct plant. Since this small plant 
produces spores and therefore reproduces asexually, it is the sporo- 
phyte or asexual generation of Eiccia, as distinguished from the 
much larger green thallose plant which bears the gametes and is 
therefore the gametopfiyte or sexual generation. The sporophyte, 
being enclosed within the tissues of the gametophyte, although 



r .~<y/ 


FIG. 251. Diagram of the life cycle of Riccia (or other liverwort). 

its cells carry on a limited amount of photosynthesis, is largely 
parasitic upon the gametophyte. 

The development of the zygote into a sporophyte goes on within 
the venter of the archegonium. As the sporophyte grows the ven- 
ter also grows, continuing to enclose the sporophyte, while the 
neck of the archegonium withers. Very soon after the union of 
the gametes, the venter, at first composed of one layer of cells, 
in consequence of cell divisions becomes two cells in thickness. 
After spores are formed, the cells of the inner layer of the venter, 
as well as those of the jacket of the sporophyte, disintegrate. The 
rounded mass of spores, now surrounded only by the outer cell 
layer of the venter, remains imbedded in the gametophyte until 


the spores are liberated by the death and decay of that part of 
the thallus. Each spore may then develop into a gametophyte in 
the manner already described. 

249. Alternation of Generations. The life cycle of Biccia in- 
cludes two distinct phases (Fig. 251). The germinating spore 
develops into a gametophyte, which bears sex organs in which 
gametes are produced. The union of gametes forms a zygote. The 
zygote develops into a sporophyte whose characteristic function is 
the production of spores. Each spore produced by a sporophyte 
may in turn develop into a gametophyte. These facts may be 
expressed in the following formula: Gametophyte Gametes Zy- 
gote Sporophyte Spores Gametophyte Gametes, etc. Each 
generation produces by means of its reproductive cells the other 
generation, hence there is an alternation of the two generations. 

A fundamentally similar alternation of generations (that is, of 
gametophyte and sporophyte) characterizes the life cycles of all 
liverworts and of all the plants that stand above them in the evolu- 
tionary scale, as well as of many algae and fungi. 


260. Gametophyte. From a condition, somewhat like that in 
Riccia, of a simple thallose gametophyte and a very simple sporo- 
phyte, evolution among liverworts seems to have taken place in 
several divergent directions. In one line of descent, beginning 
with forms more or less like Riccia and culminating in Marchantia, 
both gametophyte and sporophyte became progressively larger and 
more complex. The gametophyte of Marchantia presents, so far 
as we now know, the highest degree of complexity (though not 
the greatest size) ever attained by a thallose plant. 

The gametophyte of Marchantia polymorpha (Fig. 252), one of 
the most widely distributed liverworts, grows on moist rocks or 
soil. It resembles that of Riccia in general form, as well as in its 
method of development from a spore, in apical growth by means 
of a group of embryonic cells, and in method of branching. It is, 
however, broader and thicker than the thallus of Riccia, and has 
a rather conspicuous midrib marked above by a shallow groove 
and below by a projecting ridge. As in Riccia, the upper surface 
of the thallus is divided into small rhomboidal areas (Fig. 253, A), 
each area indicating the position of an air chamber just beneath 
the uppermost layer of cells. The air chambers are in a 



layer (Fig. 253, J5) ; each chamber opens externally by a pore which 
is surrounded by a chimney-like structure composed of four ver- 
tical rows of cells. In each air chamber are branching filaments 
of cells, growing upward from the layer of cells that compose the 
floor of the chamber. The cells of these filaments contain chloro- 
phyll and constitute the chief photosynthetic tissue of the plant; 

FIG. 252. Thallus of Marehantia bearing cupules in which gei 



but there are many chloroplasts also in the cells of the lepers 
bounding each chamber above and below, and in those of the par- 
titions between the chambers. The portion of the thallus below 
the air chambers consists of several layers of cells possessing few 
or no chloroplasts. Many of these cells contain leucoplasts which 
form storage starch; in some of them are oil bodies like those of 
Riccia; a few large cells contain mucilage. The cells of this part 
of the thallus are parenchymatous, except that in the midrib are 
elongated cells with locally thickened walls, constituting probably 
a rudimentary conductive tissue. From the lower surface of the 



thallus grow scales and rhizoids. Some rhizoids are smooth-walled; 
the inner surfaces of the walls of others are marked by localized 
thickenings of varied form. 

251. Vegetative Multiplication. In consequence of apical growth 

and branching and of the progressive death of the older parts of 

the thallus, the number of plants is increased, just as is the case in 

Upper Epidermis Pore Air Chamber 


Lower Epidermis 



FIG. 253. Marchantia. A, surface view of a portion of a thallus showing 
rhomboidal areas, each marking the position of an air chamber. B, cross 
section of a portion of a thallus. 

Riocia. Adventitious branches also may develop from almost any 
part of the thallus in consequence of wounds or possibly of other 
stimuli; and these branches, if separated by any means, become 
new plants. In addition, Marchantia has a means of vegetative 
multiplication by the formation of lens-shaped structures (gemmae) 
which are produced in great numbers in shallow cups (cupuks, 
Fig. 252) on the upper surface of the thallus. Each gemma 
(Fig. 254) is attached to the thallus by a single-celled stalk which 
is easily broken. If the gemma, freed by the breaking of its stalk, 
comes to lie upon the soil, rhizoids develop from certain special 
cells of the surface in contact with the soil, and two groups of 



cells located in two notches on opposite edges of the gemma begin 
to divide. By the division of the cells in these groups, and by the 
growth and division of their daughter cells, the gemma in time 
develops into a new plant. 

252. Sexual Branches. The sex organs of Marchantia are 
similar in structure to those of Riccia. They are borne, however, 
on special upright branches (Fig. 255), each composed of a stalk 

and a terminal horizontal 
Mucilage Cells ^^^ RMzoidalCell digk Male ftnd female 

branches (in Marchantia 
polymorpha, though not in 
some other species of Mar- 
chantia) are borne on dis- 
tinct plants. It is worthy of 
note that the sexual distinc- 
tion between plants of this 
species is so sharply fixed 
that gemmae from a male 
plant give rise always to 
male plants and those from 
female plants develop al- 
ways into female plants. 
The upright sexual branches 
are direct continuations of 
the horizontal branches of 
the thallus; and a cross sec- 
tion of the stalk of a male or female branch shows the presence 
of tissues corresponding respectively to those of the upper and lower 
surfaces of an ordinary branch of the thallus. 

The disk borne by a male branch (Fig. 256) is eight-lobed, and 
imbedded w in the upper surface of each lobe are many antheridia, 
the oldest raptest the center, the youngest toward the outer ex- 
tremity of fSBb lobe. Between the cavities containing antheridia 
are air chambers with pores. When an antheridium is mature, 
Contact with a drop of water causes some of the sterile cells in the 
upper part of its jacket to disintegrate, and the mass of anthero- 
zoids (Fig. 257) oozes out to the surface of the disk. 

A female disk (Fig. 258) is inconspicuously eight-lobed, a group 
of archegonia being borne in an inverted position, not imbedded, 
on the lower surface of each lobe. However, the first-formed 


FIG. 254. A gemma of Marchgntia. 





264. Gametophyte. In another line of descent, beginning ap- 
parently with species whose gametophyte was even simpler than 
that of Riccia, evolution has resulted in an external differentiation 
of the thallus into stem and leaves, accompanied by little if any 
internal differentiation of tissues. The leafy liverworts thus pro- 
duced, of which Porella platyphylloidea (Fig, 260, A, B) is a common 


FIG. 261. Porella. A, male plant; antheridia are borne on the short side 
branches. B, female plant with short archegonial branches. C, female 
plant bearing sporophytes; at the upper right, a sporophyte still enclosed 
in a sheath; lower right, a sporophyte whose capsule has opened some- 
what prematurely, freeing spores and elaters; upper left, a fully matured 
sporophyte with elongated stalk and emptied capsule. 

example, typically have three rows of leaves. The leaves of two 
rows seem to have been developed in the course of evolution from 
lateral lobes of the thallus, the divisions between which extended 
almost to the median line, leaving as a central axis only a midrib 
or stem; the leaves of the third row seem to correspond to the 
scales borne on the under surface of the thallus of such a form as 
Riccia or Marchantia. 

Porella grows most commonly on the bark of trees and on rocks, 
the branching plants forming close green mats. It can withstand 
drying, at least for several months, without apparent injury. At 
the growing end of the stem is a single apical cell (Fig. 260, C) 
instead of a group of embryonic cells such as occurs in Riccia or 



Marchantia. An apical cell of Porella has the form of a triangular 
pyramid whose base is the free (anterior) face of the cell. From 
each of the three lateral faces of this cell daughter cells are formed 
in regular sequence, which by their division followed by divisions 
of their daughter cells give rise to all the cells of the plant. Each 

row of leaves, as well as a part 
of the stem, arises from the 
cells thus cut off from one face 
of the apical cell. Since cells 
are cut off from three faces, 
three rows of leaves are pro- 
duced. The stem is a continuous 
central axis which gives rise, 
from time to time, to lateral 
branches a method of branch- 
ing very different from that 
which characterizes Riceia or 
Marchantia. Each branch has 
an apical cell, and its develop- 
ment (including the produc- 
tion of leaves and sometimes of 
secondary branches) repeats 
the development of the main stem. Scattered smooth-walled 
rhizoids, which attach the plant to the substrate, grow from the 
lower surfaces of stem and branches. 

Each lateral leaf has a large upper lobe and a smaller lower one; 
the lower lobe appears like a flap attached to the stem and to the 
posterior edge of the upper lobe, and turned forward under the 
latter. Each lobe consists of one layer of cells. When a branch is 
formed, it replaces the lower lobe of a leaf. On the lower side of 
the stem is a row of smaller leaves, also one cell in thickness. 
There is no differentiation of tissues in stem, branches, or leaves. 
255. Sex Organs. These arise on special lateral branches, 
antheridial and archegonial branches being borne (in Porella plat^ 
yphylloidea) on separate plants (Fig. 261). The male plants 
(those bearing antheridial branches) are in general smaller than 
the female (those bearing archegonial branches), but the differ- 
ence is not great enough to make it easy always to determine the 
sex of a plant that is not producing sexual branches. 
An antheridia! branch is comparatively short, and its leaves are 

FIG. 262. A, portion of a branch of 
Porella bearing antheridia. B, 



Jacket of Capsule 


Spvre Mother Cell 


very close together. In the axil of each leaf is an antheridium 
(Fig. 262, A) differing from an antheridium of Riccia only in that 
it has a long stalk and that its wall, except at the outer end, is 
composed of more than one layer of cells. It is not enclosed in a 
cavity. The antherozoids are like those of Riccia, and, as in th$jb 
plant, gametic union 
depends upon the 
presence of sufficient 
water to enable an 
antherozoid to swim 
and to be carried to 
the neighborhood of 
an archegonium. 

An archegonial 
branch is shorter 
than an antheridial 
branch; it bears only 
two or three leaves 
and, at its end, a 
group of a few arche- 
gonia. One of the ar- 
chegonia is developed 
from the apical cell 
of the branch, and 
thus further growth 
of the branch is pre- 
vented. Each arche- 
gonium (Fig. 262, B) 
resembles one of 
Riccia except that the 
venter is little broader than the neck. After the archegonia reach 
maturity, the whole group is surrounded by a thin, cup-like sheath 
that has developed from the archegonial branch just below the 

256. Sporophyte. The sporophyte of Porella (Fig. 263) is 
similar to that of Marchantia in being composed of foot, stalk, and 
capsule. As in Marchantia, the stalk elongates when the spores 
are mature, pushing the capsule well out beyond the enclosing 

The jacket of the capsule consists of two or more layers of cells. 

"IG. 263. Lengthwise section of a developing spo- 
rophyte of Porella, with adjacent parts of the 
parent gametophyte. 



In the interior of the capsule are produced, as in Marchantia, 
spores and elaters. When the elongating stalk has pushed the 
capsule beyond the sheath, the capsule wall splits from its apex 

to near its base jnto four parts, liberat- 
ing the spores. Each spore may develop 
into a new gametophyte. 


* 267. Gametpphyte. A third type of 
evolutionary development among iiver- 
wort^ i illustrated by Anthoceros 
(Fig. 264). In this plant it is the sporo- 
phyte which shows the most marked 
advance over a primitive condition. The 
gametophyte is small and irregularly 
FIG. 264. Gametophyte of and inconspicuously branched. It has 
Anthoeeo8 bearing spo- no differentiated tissues and no air cham- 


t bers, but has intercellular spaces open- 

ing to the lower surface of the* thallus. Some of these spaces 
are filled with a mucilage-like substance; in others are colonies 
of a blue-green alga (Nostoc). Antheridia (Fig. 265) develop in 

groups, each group in an 
internal cavity beneath the 
upper surface of the thal- 
lus. The layers of cells form- 
ing the roof of this cavity 
are finally broken by the 
growing antheridia. Arche- 
gonia (Fig. 266) develop 
separately rather than in 
groups and are closely im- 
bedded in the upper surface FIG. 265. 
of the thallus, only the ex- 
treme ends of their necks 

Antheridium Antheridial Chamber 

Portion of a thallus of An- 
thoceros in vertical section, showing 2 
enclosed antheridia. 

protruding. The venter and neck of each archegonium are con- 
tinuous with the surrounding cells of the thallus. 

268. Sporophyte. A young sporophyte consists of a foot, a very 
short stalk, and a capsule. The capsule is characterised by the 
presence of an embryonic region at its base, in which cell division 
continues for a long time. Consequently, the capsule grows into 



the bogs in which Sphagnum grows, these dead organic substances 
are not completely decomposed by the bacteria that cause decay. 
Chemical ctianges other than decay result in the foltaatian of the 
spongy, dark-colored substance known as peat. Further changes 
in the peat may in the 
course of long periods 
of time lead to the pro- 
duction of certain types 
of coal. 

At the growing er d of 
the stem arid of each 
branch is an apical cell, 
resembling in shape arid 
function the apical cells 
of the mosses already 
described. As in other 
mosses, leaves are 
formed in three rows; 
but as the stem and 
leaves grow, the latter 
become displaced and 
lose their three-ranked 
arrangement. While 
very young, a leaf con- 
sists of a single layer of 
cells, all similar (Fig. 
280, A); but as the leaf 
matures, these cells be- 
come differentiated (Fig. 
280, B-D). Alternate 
cells grow both in length 
and in breadth and ulti- 
mately die, leaving only 
their walls. These large 
cells are frequently char- 

FIG. 280. Development of a leaf of Sphagnum. 

A, tip of a young leaf, its cells still embryonic. 

B, C y portions of leaves at later stages (C at 
maturity), showing the differentiation of 2 
types of cells. I), portion of a cross section of 
a mature leaf; small green cells alternate with 
large dead ones. 

acterized by spiral and ring-shaped thickenings on the inner sur- 
faces of their walls, and often the walls are perforated, the pores 
being variable in size and shape. Between these large cells and 
forming a network are other cells which grow chiefly in length, 
remain alive, and retain chlorophyll. Large dead cells like those 



in the leaves occur in the cortices of the stems and branches 
of a few species. Cells of this character in leaf and stem play an 
important part in the absorption and retention of water. The 
stem and branches, except in species that grow submerged in 

water, possess mechanical tis- 
sues also. 

266. Sex Organs. Anther- 
idia and archegonia (Fig. 
281) are produced usually in 
late summer and early fall on 
short branches borne near 
the apex of the stern. The 
antheridial and archegonial 
branches may, according to 
the species, be borne on the 
same plant or on separate 
plants; but antheridia and 
archegonia never occur on the 
same branch. The leaves on 
an antheridial branch are of- 
ten brown, purple, or red, 
even in those species the 
leaves on whose other 
branches are green. An an- 
theridium resembles one of 
Porella (Fig. 262, A), and as 
in Porella each antheridium 


is borne in the axil of a leaf. 

001 a f , . The very short archegonial 

Fia. 281. Sex organs of Sphagnum. .4, , . J . . , j 

portion of an antheridial branch with branches are closely crowded 

an antheridium. B, apex of an arche- at the apex of the stem. One 

genial branch; the archegonium .en- archegonium is developed 

closed by leaves. f i n r u 

from the apical cell of each 

branch, and several other archegonia may be formed about the 
base of the first. In cold countries the sex organs pass the winter 
under the snow, and gametic union occurs in the spring at the time 
of the melting of the snow and ice. 

267, Sporophyte. A mature sporophyte of Sphagnum (Fig. 282) 
consists of a bulb-like foot which is imbedded in the tissues of 
the branch beneath the archegonium; a terminal capsule, al- 




most spherical in shape, within which are borne spores; and a very 
short stalk that is hardly more than a constriction between foot 
and capsule. The spores are formed in a relatively tliin, dome- 
shaped zone in the upper part of the capsule. When the spores 
are nearly mature, a portion of the branch beneath the foot of the 
sporophyte elongates and carries the sporophyte beyond the en- 
veloping leaves. A dome-shaped lid is formed at the apex of 
the capsule. The lid is 
thrown off by an ex- 

, . ,. c ,, SporophytiA 

plosive action of the Tissue/ 
capsule, which also 
ejects the spores. 

268. Uses of Sphag- 
num. The part played 
by Sphagnum plants 
in the formation of 
peat and of some kinds 
of coal has already 
been mentioned. Dried 
peat is used in various 
countries as a fuel. 
During the World 
War, Sphagnum came 
into wide use as a ma- 
terial for surgical dress- 
ings. It had, indeed, 
been used for centuries 





1- _ 

FIG. 282. A, diagram of the apical portion of a 
leafy shoot of Sphagnum bearing a sporophyte. 
Note especially the elongation of the upper 
portion of the gametophytic tissue (shaded) 
that pushes the sporophyte (unshaded) above 
the surrounding leaves. B t lengthwise section 
of a nearly mature sporophyte. 

in the dressing of wounds in Scotland, Ireland, and some parts of 
northern Europe, and was extensively employed for first-aid pur- 
poses ty the Japanese during the Russo-Japanese War. The great 
advantage of Sphagnum for this purpose is its capacity for absorb- 
ing liquids ; a mass of Sphagnum of one of the species best adapted to 
surgical use may absorb from 15 to 20 times its own weight of water. 
Sphagnum is much used in the packing of live plants which are 
to be shipped. Its value for this purpose lies also in its ability to 
absorb moisture and to retain it for a considerable time. 

269. Distinctive Features of Bryophytes. Liverworts and mosefes 
together constitute the division known as bryophytes. About 9,000 


species of liverworts and 13,500 species of mosses are now recog- 

Although conspicuous differences distinguish various liverworts 
and mosses, their relationship is clearly shown by a close similarity 
throughout the division in the form and structure of sex organs 
and gametes. Both antheridium and archegoriium are character- 
ized by the presence of an outer layer of sterile cells. In this respect 
the gamete-producing organs of a bryophyte differ from the cor- 
responding organs of any thallophyte. 

Liverworts and mosses are alike also in having an alternation 
of generations ; the gametophyte is, in every case, the larger, longer- 
lived, independent generation; the sporophyte is smaller, shorter- 
lived, and parasitic upon the gametophyte. While an alternation 
of generations occurs in many thallophytes (algae and fungi), in 
no alga or fungus are the relations between gametophyte and spo- 
rophyte closely comparable to those existing between the two gen- 
erations of a bryophyte. 

The bryophytes display a high degree of sexual differentiation, 
marking a great advance over the primitive form of gametic union 
which occurs in Chlamydomonas, in which the two gametes that 
unite are, to all appearances, alike. In various lines of evolution 
among plants, a differentiation has appeared between the gametes. 
Some of the steps in this type of evolutionary development are 
illustrated by algae and fungi described in previous chapters. One 
type of gamete performs especially the function of storing food to 
be used by the zygote and, in many-celled forms, by the young 
plant (embryo) which will develop from the zygote; in adaptation 
to its function, this gamete, now called the female gamete, has 
become larger and has* lost the power of movement. The male 
gamete, on the other hand, retains the power of movement, which 
is essential to its union with the female gamete; and, relieved of 
the necessity of food-storage, it has become smaller and better 
adapted to rapid movement. The female gamete (egg) and the 
male gamete (antherozoid) of a bryophyte have thus come to be 
very different in size and structure, the antherozoid being reduced 
to little more than a nucleus and a pair of flagella. 

This differentiation of gametes into two sorts, each adapted to a 
particular function, is the basis of what is commonly known as sex 
in both plants and animals. Sexual differentiation extends in the 
bryophytes also to the production of distinct organs (archegonium 


and antheridium) in which the respective gametes are produced; 
and in a number of liverworts and mosses it has extended to a differ- 
entiation in size or in external form between the female gameto- 
phyte, which produces only eggs, and the male gametophyte, 
which produces only antherozoids. In some species the male game- 
tophyte is much smaller than the female gametophyte. 



270. Chromosome Numbers and the Alternation of Genera- 
tions. The number of chromosomes in each cell of the gametophyte 
of a liverwort or of a moss may conveniently be represented as n. 
The numerical value of n is different for different species; for 
example, in some common mosses n equals 6, that is, each cell of 
the gametophyte contains six chromosomes. When any nucleus 
of the gametophyte divides, by the process described in Chap- 
ter XII, each parent chromosome is divided and its daughter chromo- 
somes pass to the respective daughter nuclei; hence the chromo- 
some nuAber in each daughter cell is the same as that in the parent 
cell. Every cell throughout the life of the gametophyte, then, 
contains n chromosomes, and consequently each gamete (egg or 
antherozoid) which is produced by this plant has n chromo- 

The union of the gametes involves a union of their cytoplasm 
and of their nuclei, but not of their chromosomes. Therefore the 
zygote nucleus contains n chromosomes that were contributed by 
the egg plus n chromosomes contributed by the antherozoid in 
all, 2 n chromosomes. 

The zygote, with 2 n chromosomes, is the starting-point of the 
sporophytic generation. When the zygote nucleus divides, each 
of its chromosomes divides; each daughter cell, therefore, formed 
by the division of the zygote receives 2 n chromosomes. In the 
nuclear divisions which follow during the development of the spo- 
rophyte, each parent chromosome is divided and each of its daugh- 
ter chromosomes passes to one daughter nucleus; hence each cell 
of the sporophyte has 2 n chromosomes. Therefore, one fundamen- 
tal difference between gametophyte and sporophyte is the presence 
in the two generations of different numbers of chromosomes 
respectively n and 2 n. In a moss each cell of whose gametophyte 
contains 6 (n) chromosomes, each cell of the sporophyte would 
contain 12 (2 n) chromosomes. Among the values of n found in 



various species of mosses are 6, 8, 10, 12, 16, 20, and 32. In Riccia 
natans, n equals 9; in Marchantia polymorpha, n likewise equals 9. 
In plants of other groups the value of n ranges from 3 (2 n being 6) 
to as high as 100 (2 n being 200). 

271. Reduction of the Chromosome Number. The doubling 
of the number of chromosomes each time two gametes unite would 
result, if nothing occurred to prevent, in a continuous increase in 
the chromosome number. It is clear that such an accumiilation of 
chromosomes from generation to generation could not long con- 
tinue. As a matter of fact, there is no such accumulation, because 
in each life cycle, at some point between one gametic union and the 
next, the chromosome number is reduced by one half (that is, from 
2 nto n). In liverworts and mosses and in the plants above them 
in the evolutionary scale, the reduction of the chromosome number 
is brought about in the two successive nuclear divisions that occur 
when a spore mother cell is divided to form four spores (Fig. 276, 
D-~F). The spore mother cell, when it was formed, like Any other 
cell of the sporophyte received 2 n chromosomes from its parent 
cell; but in the divisions of the spore-mother-cell nucleus and of its 
daughter nuclei, the chromosome number is reduced to n. These 
two nuclear divisions are different, therefore, from all other divi- 
sions in the life of the plant. Since each spore, possessing n chro- 
mosomes, is the starting-point of a gametophyte, each cell of the 
gametophyte has n chromosomes. 

This history of the chromosomes may be jmjmmed up by saying 
that in the union of gametes the point of transition from game- 
tophyte to sporophyte the chromosome number is doubled (from 
n to 2 n); and that in spore-formation the point of transition 
from sporophyte to gametophyte the chromosome number is 
reduced (from 2 n to n). 

272. First Reduction Division (Fig. 283). In all cases in which 
the process has been fully studied, reduction is brought about in 
the course of two successive nuclear divisions, often called reduction 
divisions. The history of these divisions seems to be similar, in es- 
sential respects, in all plants and animals in which a reduction in 
chromosome number occurs. 

When the first reduction division begins (for example, in the 
spore mother cell of a moss), the nucleus possesses the 2 n chromo- 
somes which it received from its parent nucleus. But in the earjy 
stages of this division an event occurs which does not take place 



in any other division namely, the chromosomes present in the 
nucleus come into contact in pairs, side by side (Fig. 283, C). 

FIG. 283. Diagrams illustrating the formation of spore mother cells and the 
first reduction division. A, division by which spore mother cells are 
formed; 2 n chromosomes pass to each daughter nucleus (n being con- 
sidered as 3). B y spore mother cell; nucleus in the resting stage. C, pairing 
of chrojnosomes; n pairs are present. 7), the chromosomes of each pair 
in intimate contact. E, paired chromosomes are again visibly separate. 
F, the chromosomes have contracted. G, the chromosomes are now 
plainly split. //, separation of whole (but split) chromosomes after the 
equatorial-plate stage. /, the 2 daughter cells, each containing n split 

It will be recalled ( 119) that in an ordinary nuclear division 
the chromosomes appear to be double at an early stage, and that 
there are indications that the double condition is the result of a 


splitting which occurred late in the course of the preceding division. 
There is evidence, also, that at a very early stage in the first reduc- 
tion division each chromosome is similarly double (or split). The 
split nature of the chromosomes, however, can be made out in the 
early stages only in specially prepared sections of the dividing 
nuclei and at very high magnifications (see Fig. 397, A, B). Con- 
sequently the usual appearance, as shown in Figure 283, C-F, is 
that it is whole (that is, unsplit) chromosomes which are pairing. 
In these early stages, at least in many plants, the chromosome pairs 
are arranged end to end. 

For a time the pairing of the chromosomes is so close that they 
often seem to be completely united; the appearance then is that 
of a series of single chromosomes (Fig. 283, D). Later the paired 
chromosomes separate slightly (Fig. 283, E). They become shorter 
and the end-to-end arrangement, if previously present, disappears 
(Fig. 283, F). 

Sooner or later each chromosome of every pair seems to become 
split lengthwise (Fig. 283, (?). From what has already been said, 
it is evident that this apparent splitting is really a reappearance 
of the doubleness which characterized each chromosome at a much 
earlier stage. The nucleus now contains what appear to be n 
chromosomes each split into four parts, but are in reality 2 n 
chromosomes (in pairs) each split into two parts. 

After the spindle is formed, the pairs of split chromosomes be- 
come so arranged in the equatorial plate that when later they are 
divided into two groups, one split chromosome of each pair is drawn 
to one pole of the spindle and the other of the pair is drawn to the 
opposite pole. In consequence, each daughter nucleus receives n 
longitudinally split chromosomes ; the number of chromosomes has 
actually been reduced. This nuclear division is followed (in a moss) 
by a division of the spore mother cell. 

273. Second Reduction Division (Fig. 284). The lengthwise 
split that appeared in each chromosome in the early stages of the 
first reduction division was in preparation for the second division. 
This division follows very soon after the first; so soon, in many 
cases, that the daughter nuclei of the first division have not time 
to pass into a resting condition. A new spindle is formed for the 
division of each of these two nuclei. The chromosomes of each 
nucleus become arranged in an equatorial plate on one spindle, 
and the halves of each split chromosome pass to opposite poles. 


Thus four daughter nuclei are formed, each with n chromosomes. 
Nuclear division is again followed By cell division, as a result of 
which each of the four nuclei formed by the second division be- 
comes the nucleus of a spore. Therefore, by means of two divisions, 
four spores each with n chromosomes have been formed from the 
spore mother cell which had 2 n chromosomes. 

274. Some Effects of the Reduction Divisions. One obvious 
effect of these two nuclear divisions is to reduce the number of 
chromosomes from 2 n to n. Perhaps more important, however, 


FIG. 284. Second reduction division. A, equatorial-plate stage. B, comple- 
tion of the second division; each of the 4 spores contains n chromo- 
somes. C Y , separation and rounding up of spores. 

is the fact that during the reduction divisions new combinations 
of chromosomes are brought about. 

The nucleus of a moss zygote (or of the zygote of any other plant 
or animal) contains 2 n chromosomes : n chromosomes which are 
maternal because they were contributed by the egg ; and n paternal 
chromosomes which were contributed by the male gamete. When 
the zygote nucleus divides, each of its daughter nuclei receives one 
half of each maternal, and one half of each paternal, chromosome; 
each daughter nucleus, therefore, like the zygote nucleus, has n 
maternal and n paternal chromosomes. Since a splitting and a 
division of each chromosome occur in the course of each succeeding 
nuclear division, the nucleus of every cell of the sporophyte, to and 
including the spore mother cells, has n maternal and n paternal 

When the chromosomes become paired in the early stages of the 
first reduction division in a spore mother cell, each pair is composed 
of one maternal and one paternal chromosome which in some way 
correspond each to the other. When the chromosomes of anv pair 


pass to the respective daugh|pr nuclei, it is a matter of chance 
which daughter nucleus receives the maternal, and which the 
paternal, chromosome of that pair. If one daughter nucleus re- 
ceives the maternal chromosome of one pair and the paternal chro- 
mosome of a second pair, the other daughter nucleus will receive 
the paternal chromosome of the first pair and the maternal chromo- 
some of the second. Hence new chromosome combinations result. 
If the parents differed in numerous characters, as is ordinarily the 
case, then, since chromosomes carry substances concerned in 
inheritance, each maternal chromosome is likely to differ somewhat 
in constitution from the paternal chromosome of the same pair. 
In such a case the nuclei formed by the first reduction division, 
and ultimately the spore nuclei formed by the second division, will 
be of different kinds with respect to the inheritance which they 
will pass on to the next generation. The processes that occur in 
the course of the reduction divisions help, therefore, to explain 
how it comes about that offspring differ from their parents, and 
that offspring of the same parents differ from one another. 

Not only are new combinations of whole chromosomes brought 
about in the way just described; it appears that in the course of 
the reduction divisions new combinations of parts of chromosomes 
may be effected. Present evidence indicates that during the close 
side-by-side association of paired chromosomes in the early stages 
of the first reduction division, an interchange of material may 
occur (see also Chap. XXXII). Therefore, when the paired chro- 
mosomes separate, the maternal one of any pair may contain some 
substances of paternal origin, and the paternal chromosome may 
contain some of maternal origin. Such interchanges between 
chromosomes evidently greatly increase the possibility of the ap- 
pearance of different combinations of characters in the spores and 
in the plants to which the spores will give rise. 

275. Chromosome Reduction in Thallophytes. In bryophytes 
(liverworts and mosses) and in the plants that stand above them 
in the evolutionary scale (pteridophytes and seed plants), the re- 
duction of the chromosome number occurs regularly at the same 
stage in the life cycle namely, in the division of the nucleus of a 
spore mother cell. The plants of these higher groups agree, there- 
fore, in possessing a sporophytic generation characterized by the 
presence of 2 n chromosomes and a gametophytic generation each 
of whose cells has n chromosomes. 


Among thallophytes, however, tjjere are great differences be- 
tween different, classes with respect to the stage at which the 
chromosome number is reduced. In several green algae, for ex- 
ample, it has been shown that chromosome reduction occurs in 
the division of the zygote nucleus and of its daughter nuclei. 
This is the case in Volvox, Ulothrix, and Oedogonium, and almost 
certainly in Chlamydomonas. In such an alga the zygote is the 
only cell possessing 2 n chromosomes, and there is no genera- 
tion corresponding to the sporophyte of a moss. In Spirogyra, 
similarly, chromosome reduction is effected in the two divisions 
that occur shortly after gametic union. Of the four nuclei, each 
with n chromosomes, formed from the zygote nucleus by these di- 
visions, it has been seen that three nuclei degenerate. Hence the 
single nucleus present in the zygote of SpirogjTa during its later 
history has but n chromosomes. The filament resulting from the 
germination of the zygote consists of cells with n chromosomes 
each that is, it is a gametophyte. Spirogyra, like Volvox, there- 
fore, has no sporophytic generation. In some other green algae, 
including certain species of Cladophora and Ulva, there is a true 
alternation of generations. 

The brown algae possess an alternation of generations corn- 
parable with that in bryophytes and higher plants. In some 
brown algae, including Ectocarpus, the gametophyte and sporo- 
phyte are alike except for their reproductive structures. In other 
brown algae the two generations are very dissimilar in size and 
structure. For example, the sporophyte of Laminaria is a large, 
complex plant and the gametophyte is small and simple. Reduc- 
tion of the gametophyte is carried to an extreme in Fucus, whose 
sporophytic (2 n) generation is practically the whole plant. 
Chromosome reduction in Fucus occurs in the first two nuclear 
divisions in oogonium and antheridium respectively; the gameto- 
phyte is represented, therefore, only by the four- and eight-nucle- 
ate stages in the oogonium and by the four-nucleate and succeed- 
ing stages in the antheridium. 

In the majority of red algae also, including Polysiphonia, there 
is an alternation of generations. The plant of Polysiphonia which 
bears gametes is a gametophyte, with n chromosomes in each cell; 
the sporophytic generation, beginning with the zygote, includes 
the branches that give rise to carpospores, the carpospores them- 
selves, and the plant that bears tetraspores. Chromosome reduc- 


tion occurs in the divisions that form the tetraspore nuclei. On 
the other hand, Nemalion |nd some other red algae possess only 
a gametophytic generation ; as in some green algae, the chromosome 
number is reduced in the first and second nuclear divisions after 
gametic union. These divisions, in Nemalion, occur when the 
zygote germinates. 

Differences appear among fungi as great as those among algae 
with respect to the time of chromosome reduction. 

In Rhizopus, chromosome reduction is probably effected during 
some of the nuclear divisions that occur at the time of, or shortly 
after, the germination of the zygote. The particular divisions 
with which chromosome reduction is connected have not, how- 
ever, yet been certainly recognized. 

In the ascomycetes (with perhaps some exceptions) the chromo- 
some number is reduced by the first two nuclear divisions in the 

Among basidiomycetes, chromosome reduction is brought about 
in the rusts (again with possible exceptions) by the two nuclear 
divisions in the germinating teleutospore; in smuts, probably by 
two divisions in the germinating winter spore; and in mushrooms, 
by the two nuclear divisions in the basidium. 

276. Relation of Chromosome Reduction to Gametic Union. 
A knowledge of the steps in the reduction of the chromosome num- 
ber throws some additional light upon the real nature and signif- 
icance of gametic union. The pairing or conjugation of chromo- 
somes that takes place early in the first reduction division is the 
final step in a history that began with the union of gametes. Every 
case of gametic union, then, involves three steps: i , 

(a) The union of cells. 

(6) The union of nuclei. 
(c) The pairing or conjugation of chromosomes. 

Since the conjugation of the chromosomes may result in a 
redistribution of chromosomes, and to some extent in a redis- 
tribution of parts of chromosomes, one of the important conse- 
quences (if not the important consequence) of a gametic union 
is this ultimate redistribution, with the result that new combina- 
tions of inherited qualities may appear. In other words, the union 
of gametes, because it results finally in chromosome conjugation, 
is a means of securing variation (in the sense of a new grouping of 
inherited qualities) in plants and animals. 


In the different plants thus far described, steps a, 6, and c are 
separated from one another in different degrees. In Spirogyra, 
the union of cells (step a) is followed closely by the union of nuclei 
(step 6), and this very soon by the conjugation of chromosomes 
(step c). In the wheat rust, there is a long period (represented by 
the aecidiospores, the mycelium in the wheat, and the uredo* 
spores) between steps a and b; step 6, however (taken during the 
maturing of the teleutospore), is followed as soon as the teleuto- 
spore germinates by step c. In bryophytes and higher plants, 
steps a and 6 are close together in time, and a long gap (repre- 
sented by the sporophytic generation) occurs between steps b and c. 



277. Ferns and Their Distri- 
bution* The division pterido- 
phytes includes the ferns, to- 
gether with the plants of 
certain other classes which 
stand more or less nearly at a 
common level of develop- 
ment. Like mosses, ferns 
are widely distributed. Some 
grow in the crevices of rocks 
and on the faces of cliffs 
where they find a scanty foot- 
hold; others in fields and open 
woods; but most of them thrive 
best in damp, shady places. In 
the tropics ferns are often par- 
ticularly abundant, both in 
number of individuals and in 
number of species. It is in 
the tropics also that the larg- 
est ferns are found. These 
are tree ferns (Fig. 285), with 
erect cylindrical stems which 
may reach a height of 40 feet 

or -more, each bearing at its apex a crown of widespreading com- 
pound leaves. In temperate regions most ferns have underground 
stems which bear slender roots and aerial leaves. 


278. Sporophyte. Among the common ferns living in temperate 
regions is a group of very similar species all generally knoWh as 
the brake or bracken. The bracken of eastern North America is 
Pteridium latiusculum (Fig. 286). It grows in woods or clearings 


G. l^Sf>. Tree terns (Alsophitii glauca) 
in the Taiping Hills, Federated 
Malay States. After Campbell. 



and is particularly abundant in sandy regions. In some parts of 
the world brackens form dense undergrowths. The plant which con- 
sists of stem, leaves, and roots is the sporophyte. Since the stem 
is entirely underground, the only parts that appear above the soil 
are leaves. In most regions of the United States the leaves of 

brackens are relatively 
small, rarely attaining 
the height of a man; 
but those of the bracken 
growing in the moist, 
rich soils of the forests 
of western Washington 
and Oregon may reach 
twice that height. In 
Australia, bracken 
leaves commonly grow 
10 to 12 feet high, and 
some 14 feet in height 
have been reported 
from the Andes. 

279. Stem. The stem 
of Pteridium is long 
and slender, branching 
occasionally, and grow- 
ing horizontally a few 
inches beneath the sur- 
face of the ground. 
Growth occurs at the 
anterior end, new cells 
being formed by divi- 
sion of a single apical 
cell; the older tissues 
at the posterior end in 
time die. When the 
progressive death of the older parts of the stem extends to a point 
at which branching has occurred, the branch becomes separated 
from the main stem and continues its development as an inde- 
pendent plant. This process is one of the means by which the 
number of plants is increased. 

The outermost layer of the stem (Fig. 287) is an epidermis of 

FIG. 286. The bracken. 


thick-walled cells, next within which is a sheath of mechanical 
tissue several cells in thickness. The greater part of the interior 
of the stem consists of parenchymatous cells which often contain 
an abundance of starch grains. Near the center of a section cut 
through an internode are two well-defined strands of mechanical 
tissue. Between these strands, in the central part of the stem, are 

Mechanical Tissue ^^tfflHBBIHB^ Parenchyma 


FIG. 287. Cross section of the underground stem of the bracken. 

usually two vascular bundles; and in a zone outside the strands of 
mechanical tissue are a variable number of vascular bundles some 
of which are relatively small. Each vascular bundle (Fig. 288) 
is surrounded by an endodermis, just within which are one or two 
layers of pericyclic cells. The phloem lies next within the pericycle 
and 'entirely surrounds a central xylem. There is no cambium be- 
tween xylem and phloem. The vascular bundles are approximately 
parallel through the internodes, but at each node some of them 
unite and new branch bundles are given off. Most of these branch 
bundles extend through the next internode, but some of the 
branches from the outer bundles pass into roots, and some from 
both outer and inner bundles pass into leaves. Thus the system of 
vascular bundles forms a network connecting all parts of the plant. 
. 280. Roots. The roots of the bracken are small, slender, and 
sparingly branched. The growing end of each root is covered by a 




root cap, and, as in the case of the stem, new cells arise at the 
growing end by division of a single apical cell. A short distance 
back from the root cap is a region in which root hairs occur. A 
cross section of a root in the mature region shows tissues generally 
similar in structure and arrangement to the primary tissues in a 

root of the sunflower 
(Fig. 17). However, 
fern roots are incapable 
of secondary thickening 
since no cambium is 

tfrt^ 281. Leaves. On a 
mature plant, each leaf 
begins its development 
as a small swelling of 
the embryonic region at 
the apex of the stern. 
The early development 
of a young leaf, there- 
fore, occurs under- 
ground ; in time, an elon- 
gation of the petiole 
pushes the coiled up- 
per portion of the leaf 
through the soil into the air. The leaf then, in a manner charac- 
teristic of fern leaves, unrolls from base to tip. A fully developed 
leaf (Fig. 286) consists of a slender petiole and a much-divided 
blade. Borne upon the central axis of the blade are two rows of 
primary leaflets, the basal pair being much the largest. Several 
pairs of the lower primary leaflets may themselves be divided, 
but the smaller upper primary leaflets are usually undivided. 
The internal structure of a leaflet resembles in most respects 
that of a leaf of the sunflower, having an upper and a lower 
epidermis, a palisade layer, spongy tissue, and veins. Stomata are 
abundant in the lower epidermis. 

282. Sporangia. All the leaves of the bracken are green and 
carry on photosynthesis. Many of them also bear sporangia. A 
leaf bearing sporangia, whether or not it manufactures foods, is 
a sporophyll. Although all the leaves of the bracken are similar 
in general appearance, some of them are not sporophylls, that is, 

Endodtrmis Cortical p arenchyma 


FIG. 288. Cross section of a vascular bundle of 
the bracken stem. 



they do not produce sporangia. On the under side of each leaflet 
of a sporophyll and near each edge a narrow ridge develops from 
whose surface grow many sporangia (Fig. 289). This ridge and 

Cuticle Spongy Tissue 


Immature Sporangium' 

FIG. 289. Cross section of a portion of a bracken leaf, showing sporangia 
covered by the curved margin of the leaf. 

the sporangia that it bears are covered by the curved margin of 

the leaflet. Each sporangium (Fig. 290) consists of a slender stalk 

and a capsule. The outer layer of cells of the capsule constitutes 

a jacket. Within the 

jacket, as a result of a 

series of divisions, spore 

mother cells are formed. 

Each spore mother cell 

by two further divisions 

produces, as in a moss, 

four spores. In these 

last two divisions, as in 

the corresponding divi- ~ f^l B 

sions in a moss, the FlG - 29 - Mature sporangia, closed (A) and 

chromosome number is open 

reduced. All the cells of the jacket are thin-walled except those 

of one row. Each of the cells composing this row has thick walls 

on all sides but the outer one. This row of cells, extending from 

the base of the capsule up one side, over the top, and partly down 




the other side, is the annulus. When the spores are mature, the 
cells of the jacket are dead and dry. The cell walls of the annulus 
are sensitive to changes in moisture. As a result of such changes 
the annulus straightens, breaking open the capsule, and then snaps 

forward. In this lat- 
ter movement most 
of the spores are 
thrown out. 

283. Gametophyte. 
The spores of the 
bracken ripen and are 
shed in late summer. 
Each spore is approx- 
imately tetrahedral in 
shape, and its wall has 
two layers: the inner 
one thin, the outer 
hard, brown, and ir- 
regularly thickened. 
When a spore germi- 
nates, the thick outer 
layer of the wall 
breaks, and the pro- 
FIG. 291. A, B 1 early stages in the development toplast, surrounded 
of a prothallium from a fern spore. C, half- ^y ^e j nner layer of 
grown prothallium. / . , 

the wall, lorms a short, 

green outgrowth from which a colorless projection, the first rhizoid, 
grows. As a result of growth and of cell divisions in one plane, 
the green outgrowth becomes, usually, a row of three or four cells 
(Fig. 291, A). By subsequent growth and by cell divisions in two 
planes, the young plant, except for the few cells nearest the old spore 
wall, is transformed into a flat, green plate one cell in thickness. 
If this small prothallium is not crowded during its further growth, 
it develops typically into a heart-shaped plant with a shallow 
notch at its anterior end (Fig. 291, C). A mature prothallium 
(Figs. 292, 293) is one cell in thickness, except that in a region 
back of the apical notch a cushion several cells thick is formed. 
From various cells of the under surface of the plant, and partic- 
ularly in the older portion (that farthest from the notch), slender, 
colorless rhizoids grow out which anchor the plant and absorb 



water and other materials from the soil. Prothallia may reach 
maturity in a few months, but they remain so small that they are 

A rchegonia A pica I Notch 

Anther idia 

fihizoid -/ 

I i 

FIG. 292. Mature fern pro thallium viewed from below. 

rarely observed in nature unless sought for. Fully grown prothal- 
lia are often not more than a quarter inch in diameter. 

The prothallium is the sexual generation or gametophyte of a 
fern, and like the gametophyte of a moss it produces gametes. In 




FIG. 293. Vertical lengthwise section of a fern prothallium. 

some species of mosses, gametes of both kinds are borne by the 
same gametophyte; in other species, antherozoids and eggs are 
borne on separate plants. The bracken is like the mosses of the 
former type, in that antherozoids and eggs may be produced by 
the same gametophyte. If prothallia are small and poorly nour- 



ished, they often form only antheridia and antherozoids, but such 
prothallia may develop archegonia and eggs if they are placed 
under better conditions for food-making. Antheridia may occur 
on almost any part of the plant but are most numerous on the 
under surface, particularly on the posterior portion of the pro- 



Jacket . ^-^ ^ B 

FIG. 294. A, antheridium of a fern. B, antherozoid. 

thallium, where rhizoids are abundant. Archegonia are borne 
also on the under surface, but only on the cushion of cells back of 
the apical notch. 

An antheridium (Fig. 294, A) is dome-shaped and much smaller 
than an antheridium of a moss. The few cells of its outer layer 

FIG. 295. A, nearly mature archegonium of a fern. B, archegonium at the 
time of the entrance of antherozoids. 

constitute a jacket. After a series of cell divisions, each interior 
cell develops into an antherozoid (Fig. 294, B) which is larger than 
the antherozoid of a moss and has the form of a short spiral. Borne 



on its slender anterior portion are many flagella by means of which 
the antherozoid swims rapidly. 

An archegonium (Fig. 295) has essentially the same structure 
as an archegonium of a moss, but is smaller and composed of fewer 
cells. Its venter is imbedded in the cushion of the prothallium. 
Its neck is short and usually curves backward from the notch 
toward the older portion of the prothallium. At maturity, the 
cells of the canal row disintegrate and the cap cells of the neck 
break apart, leaving a passage-way to the egg. Although sex organs 

'Remains of 

* Root 


FIG. 296. A t early stage in the development of a fern embryo within the 
venter of an archegonium. B, an embryo, still partly within the arche- 
gonium, differentiated into foot, primary root, primary leaf, and stem. 

of both kinds are produced on the same prothallium, most of the 
antheridia usually develop and discharge their antherozoids before 
the archegonia on the same plant have matured. Hence the union 
of gametes from different plants, rather than from the same plant, 
is probably the rule in the bracken. Such union is made possible 
by the fact that the prothallia grow in groups in moist places. 

284. Development of an Embryo (Fig. 296). Though many 
antherozoids may reach the mouth of an archegonium and enter 
the neck, only one unites with the egg. The zygote, like that of 
a moss, germinates within the venter of the archegonium. After 
a few divisions, forming a small mass of cells, the young sporo- 
phyte (embryo) becomes four-lobed. By further division and 
growth, one lobe develops into a foot, a small organ imbedded in 
the prothallial cushion; from this cushion the foot absorbs food 
for the embryo. Another lobe develops into a primary root, which 



Blade of 
Primary Leaf 

Petiole of 
Primary Leaf 

pushes downward through the surrounding tissues and grows into 
the soil. A third lobe gives rise to a primary leaf which, growing 
outward and forward beneath the prothallium, turns upward at the 
notch and develops a green blade much simpler in form than the 
blades of the leaves to be produced later. The stem develops slowly 

from the fourth lobe. Until 
the time of the production of 
the primary root and primary 
leaf, the embryo (young spo- 
rophyte) has been parasitic 
upon the gametophyte (Fig. 
297). With the full develop- 
ment of the primary leaf and 
primary root, however, the 
sporophyte becomes an inde- 
pendent plant, and somewhat 
later the gametophyte dies. 
The stem grows slowly into 
the soil, producing secondary 
leaves and secondary roots. 
After the formation of several 
secondary leaves and second- 
FIG. 297. Young sporophyte still attached ary roots, the primary leaf 

Jhyte d parasitic upon ' the gameto " and the P rimar y root die - 

Thus the mature sporophyte 

has been derived from only one lobe of the embryo, namely, that 
which developed into the stem. 

285. Life Cycle (Fig. 298). The life cycle of the bracken, like 
that of a moss, includes "two distinct phases. A spore gives rise to 
a minute green plant, the gametophyte, which forms sex organs 
bearing gametes. The union of gametes (antherozoid and egg) 
forms a zygote, which on germination produces an embryo para- 
sitic upon the gametophyte. By further growth the embryo de- 
velops into a large, independent plant, the sporophyte, consisting 
of stem, roots, and leaves. On some of the leaves are borne sporan- 
gia which contain spores, completing the cycle. 

While the histories of moss and fern are alike in general outline, 
there are important differences. The gametophyte of a moss is 
relatively large and may live for a number of years, whereas its 
sporophyte is relatively small and short-lived. In a fern the game- 





tophyte is very small and comparatively short-lived, while the 
sporophyte is large and may live for many years. The conspicuous 
moss plant is the gametophyte; the conspicuous fern plant is the 


286. Leaves. Although all ferns are alike in the general char- 
acteristics that indicate their relatively close relationship, different 

FIG. 298. Life cycle of a fern. 

species vary markedly in form and structure. The differences 
are evident chiefly in the sporophytic generation. The game- 
tophytes of most ferns are essentially like the gametophyte of the 

One conspicuous feature in which ferns differ from one another 
is in the form of the leaf blade. The " walking fern' 7 represents a 
type whose blades are not lobed or divided. A leaf blade of this 
fern has the shape of a greatly elongated triangle whose slender 
tip grows until it bends over and comes into contact with the soil. 
When the tip touches the soil it develops a small bud that gives 
rise to roots and a stem, and so produces a new plant. A repetition 



of this process by the successively formed new plants explains the 
name given this fern. Some ferns, such as the common polypody, 
have simple but very deeply lobed leaf blades. Others, including 
the royal fern, the lady fern, and the male fern, produce pinnately 
divided leaves, each primary leaflet being also pinnately lobed or 
divided. The petiole of the maidenhair fern is forked at the sum- 
mit, each of the two divisions so formed bearing on one side several 
spreading, pinnately divided leaflets. 

287. Sporangia. The sporangia of most ferns, unlike those of the 
bracken, are produced in rounded or linear groups (sori) on the 

"V,/' ^ ri '^r^^'^'ts'A 

t j < 't ,t ';' r ,,'''.' 

. .Lett, tne interrupted tern, itignt, trie grape tern. 

under surfaces of leaf blades. Each sorus is borne upon an elevated 
cushion of tissue, and in some ferns is covered by a variously 
shaped outgrowth of the leaf. 

In the bracken, as has been seen, many leaves bear sporangia 
that is, are sporophylls; those leaves which are sterile otherwise 
resemble the sporophylls. In the interrupted fern (Fig. 299) the 
production of sporangia is confined to several pairs of leaflets near 
the middle of the blade. These leaflets are small and brown, and 
on their margins are borne numerous sporangia; the other (sterile) 



leaflets are larger and green. After the spores are shed in the early 
summer the spore-bearing leaflets wither. The sporangia of the 
royal fern are borne on a few leaflets at the apex of the leaf blade. 
A leaf of the grape fern (Fig. 299) consists of two distinct parts: 
one is a flat, much-divided blade which performs most of the photo- 
synthetic work; the other part of the leaf has as its function only 
the production of spores. In a few species, including the cinnamon 
fern, the sensitive fern (Fig. 300), and the ostrich fern, there are 
two different kinds of leaves. Those of one sort, the sterile leaves, 
are broad and green and are photosynthetic organs; those of the 


FIG. 300. Fertile (A-C) and sterile (D) leaves of the sensitive fern. 

other sort, the sporophylls, are brown at maturity and function 
only in spore-production. 

288. Roots (Fig. 301). The roots of all ferns are essentially 
similar in the structure and arrangement of their tissues. In the 
mature region of a root a relatively thick cortex encloses a small 
central stele. On its outer side the cortex is bounded by an epider- 
mis one cell in thickness. The cortical cells are for the most part 
thin-walled and often contain numerous starch grains. The cells 
of the endodermis are distinguished by thickenings on their radial 
walls. Next within the endodermis is the pericycle, consisting of 


one or two layers of thin-walled cells. At various points just 
within the pericycle are strands of xylem separated from one an- 
other (in a young portion of the mature region) by thin-walled 
cells. While the first-maturing portions of these xylem strands, 
consisting of small cells, are adjacent to the pericycle, the later- 
maturing portions extend the strands inward toward the center of 
the stele so that in most cases a central solid mass of xylem is 
eventually formed. Just within the pericycle and alternating with 




First-Matured Xyletn Later- Matured Xylem 

FIG. 301. Cross section of a portion of the stele and cortex in a root of 


the first-maturing strands of xylern are small strands of phloem. 
Such a stele, in which the xylem forms a more or less solid central 
column with the phloem on its outer side, is a protostele. 

The arrangement of xylem just described is an exarch arrange- 
ment. Exarch xylem is characterized by the fact that the first- 
matured xylem elements are at the outer margin of the stele adja- 
cent to the pericycle, the later-maturing xylem elements lying 
inward from those first matured. A protostele with such a xylem- 
arrangement is an exarch protostele. All roots, both in pteridophytes 
and in seed plants, have exarch steles; in many cases these are 
protosteles; but in others, the center of the root is occupied by pith, 

289. Stems. The stems of ferns, unlike their roots, vary greatly 
in external form and in internal structure. Many ferns growing 
in temperate regions, like the bracken, have underground stems. 
In the tropics, however, there is greater diversity. Here some 


species grow perched on the limbs and branches of trees. The 
stems of other species are prostrate on the ground or clamber 
upon other plants. Still other species have stems beneath the 
soil. In the tropics also are found tree ferns (Fig. 285). 

An exarch protostele seems to represent the most primitive 
type from which all other types of stele have been derived. As 
has been seen, the primitive exarch protostele is still characteris- 
tic of roots; in the stems of most pteridophytes and seed plants, 
however, other stelar types derived from the exarch protoetele are 
found. The stems of only a few ferns have protos teles; one of 
these is Gleichenia flabellata, which grows chiefly in the tropics and 
subtropics. The stem of Gleichenia has a thick-walled epidermis, 
beneath which is a cortex some of whose cells have relatively thick 
walls. A single-layered endodermis encloses the pericycle, which 
may be several cells in thickness. Within the pericycle is a thin 
cylinder of phloem; and entirely enclosed by the phloem is the 
xylem which fills the center of the stele. A few parenchymatous 
cells are scattered through the xylem. This protostele differs from 
that of a root in the course of development of its xylem. The 
first xylem elements to mature are, as in a root, groups of small 
cells developing at various points within the pericycle. However, 
the later-maturing xylem not only develops to the center of tiie 
stele but also surrounds the first-matured strands. This arrange- 
ment, in which the later-maturing xylem surrounds the first- 
matured strands, is a mesarch arrangement. Whereas the root of 
a fern contains an exarch protostele, the stem of Gleichenia has 
a mesarch protostele. 

The maidenhair fern (Adiantum pedatwri) illustrates a still 
more advanced type of stem structure. Its stele encloses a central 
pith. In the stele, midway between pith and cortex, is a continuous 
cylinder of xylem. On both inner and outer faces of the xylem are, 
successively, phloem, pericycle, and endodermis. This type of 
cylindrical stele with a central pith is a siphonostele. The siphono- 
stele of Adiantum has both internal and external phloem; in most 
siphonosteles, however, phloem is present only on the outer side of 
the xylem. 

The xylem of Adiantum, like that of Gleichenia, is mesarch. 
Adiantum, therefore, has a mesarch siphonostele. Just as the 
mesarch condition has been derived from the exarch, so an endarch 
condition, in which the first-matured xylem elements are on the 



FIG. 302. Diagrams of various types of fern steles; the, first-matured xylem 
in all cases is in black, the later-matured xylem diagonally shaded, the 
phloem stippled; pith and cortex are unshaded. A, exarch protostele. 
B, mesarch protostele. (7, D, mesarch siphonosteles with (C) phloem on 
both sides, and (D) on the outer side only, of the xylem. E t endarch 
siphonostele. F t dictyostele. 


inner face of the xylem zone, has been derived from the mesarch. 
The endarch condition is characteristic of the stems of nearly all 
living seed plants. 

As a young leaf of Adiantum grows and develops, a vascular 
strand is formed, extending outward from the stele through the 
cortex of the stem and into the petiole. Beyond the junction of 
leaf trace and stele there is in the stele of the stem a long, slender 
area in which parenchymatous cells are formed instead of xylem 
and phloem. This elongated interruption of the stele just ahead of 
the junction of each leaf trace,, in which only parenchymatous cells 
are formed, is a leaf gap. In many ferns leaf gaps are long and nu- 
merous, giving to the stelar cylinder the form of a network of 
bundles with elongated meshes. In cross section such a network 
has the appearance of separate bundles arranged in a circle about a 
central pith. This type of stele is a dictyostele. Such a stele occurs 
in the stem of Polypodium. The arrangement of bundles in the 
stem of the bracken represents a modified type of dictyostele. 

290. Bryophytes and Ferns. About 150 genera and more than 
6,000 species of ferns are known. They constitute a class stand- 
ing conspicuously higher than the bryophytes in the sense that 
they have advanced further from a primitive condition. Like the 
bryophytes the ferns have a distinct alternation of generations". 
The sporophyte of a bryophyte is small, relatively simple, and 
always attached to and largely dependent upon the gametophyte. 
The sporophyte of a fern, on the other hand, is a relatively large, 
complex plant differentiated into stem, leaves, and roots, and 
therefore independent of the gametophyte. The fern gameto- 
phyte, however, does not show a corresponding development. 
Although an independent green plant, it is always very small 
and simple in structure. In spite of the radical change in relative 
size and complexity whereby the sporophyte has become the 
large, conspicuous generation, the gametophyte still retains the 
function of producing gametes and the sporophyte continues to 
produce spores. 



291. Nature. The few living species of Equisetum (" horsetails ") 
are related to a group of plants which, during one period of the 
earth's history, formed a conspicuous feature of its vegetation. 

Some of these ancient 
plants developed into good- 
sized trees, but the present- 
day species of Equisetum 
are mostly small. In tropi- 
cal South America, the 
stems of one species grow 
to a height of more than 
30 feet. Its stems are, how- 
ever, very slender and lean 
upon the shrubs and trees 
among which they grow. 

Equisetum is almost 
world-wide in its distribu- 
tion and thrives in a vari- 
ety of habitats. Certain 
species grow in ponds and 
in swamps ; others in mead- 
ows and in damp, shaded 
places; and still others in 
relatively dry and exposed 

FIG. 303. Sporophyte of Equisetum. ,4,ster- situations Slich BS sandy 
ile aerial branch. ,B, fertile aerial branch. 
C, sporophyll. Z>, E, spores with spiral 
bands uncoiled (D) and coiled (E). 



292. Sporophyte. Equi- 
setum arvense (Fig. 303) is 
common in habitats of the type last mentioned. The sporophyte 
of this species is composed of a horizontal, branching underground 
stem and of aerial branches, some sterile and some fertile, which 
grow upward from the nodes of the stem. The underground stem 



when dry and coiling about the spore when moist. The bands of 
several spores may become entangled; the spores, therefore, are 
shed in small clusters and m&y germinate to form groups of game- 

294. Gametophyte and Embryo. The germination of a spore 
results, as in a fern, in the formation of a small green prothallium 
(Fig. 305) which bears sex organs. A prothallium of Equisetum 
differs from that of a fern in its form and in the location of the 
sex organs. When mature, it is usually a disk-shaped cushion 
several cells in thickness, from whose upper surface arise irreg- 
ularly lolbed, flattened branches each one cell thick. Rhizoids 
grow from the lower surface of the cushion. Antheridia are borne 
usually near the apices of the vertical branches; archegonia, on the 
upper surface of the cushion at the bases of the branches. An 
antherozoid resembles one of a fern in having many flagella. 

An embryo in its early stages of development is in most re- 
spects similar to that of a fern. Lobes of a young embryo develop 
respectively into a foot, a primary stem bearing two to four pri- 
mary leaves, and a primary root. The primary stem remains very 
small. At its base a bud arises that grows into a larger branch. In 
like manner at the base of this first branch a second branch arises, 
and the process may be repeated. Eventually one of the later- 
formed branches grows downward, penetrates the soil, and de- 
velops into the characteristic stem from which sterile and fertile 
aerial branches subsequently arise. 


295. Sporophyte. The club mosses also are related to an old 
group of plants once very abundant. Some of the ancient club 
mosses were tree-like. The present-day members of the group are 
all small. The plants familiarly known as "club mosses," "ground 
pines/' and "Christmas greens" are members of the genus Lyco~ 
podium (Fig. 306). The various species of this genus occur in 
tropical as well as in temperate regions. Some tropical and sub- 
tropical species grow on trunks and branches of trees. Those of 
temperate regions grow on the ground. The form of the sporophyte 
differs somewhat according to the species. Often it has a branch- 
ing stem which creeps over the surface of the ground or lives within 
the soil, producing slender roots and sending up aerial branches. 
The aerial stems and branches are usually well covered with small, 



narrowly triangular, sessile leaves. The leaves are relatively ( 
simple in structure, being only a few cells in thickness. Stomata 
are present; the internal cells of a leaf are all similar excepting 
those of the phloem and xylern, which compose an unbranched 

vein or midrib extend- 
ing from the base of 
the leaf part-way to- 
ward the apex. The 
stem has an epidermis, 
a thick cortex, and a 
stele. The xylem and 
phloem of the stele 
are in plates whose ar- 
rangement varies with 
the species as well as 
with the direction of 
growth of the stem. 

296. Spore-forma- 
tion. On the inner side, 
and near the base, of 
each of certain leaves 
is a small sporangium 
(Fig. 307, C). Such 
leaves are sporophylh. 
In some species of 
Lycopodium, the spo- 
rophylls are not read- 
ily distinguishable, 
either by their appear- 
ance or by their posi- 
tion, from the sterile 
(foliage) leaves. In 
other species the sporophylls are borne, more or less compactly 
grouped, on the terminal portions of some of the upright branches, 
which thus constitute strobili (Fig. 307, A, B). In such a case the 
sporophylls are smaller, and contain a lesser proportion of chloro- 
phyll, than the foliage leaves. Each sporangium has a short stalk, 
and a jacket several cells in thickness. Within the sporangium 
are developed many spore mother cells, each of which finally divides 
to form four spores. 

FIG. 306. Sporophyte of Lycopodium. 



297. Gametophyte and Embryo. The spores seem to lie dormant 
for several years after being shed. In the majority of species, a 
spore on germination forms a small subterranean, saprophytic 
gametophyte (Fig. 308), on whose upper portion are borne anther- 
idia and archegonia. In general plan the sex organs resemble those 



FIG. 307. Lycopodium. A, branch bearing 2 strobili. B, lengthwise section 
of the tip of a strobilus (diagrammatic). C, sporophyll bearing a spo- 

of a fern, differing, however, in the fact that they are more or less 
imbedded in the gametophyte. An antherozoid, unlike one of a 
fern or of Equisetum, has two flagella, resembling in this respect 
the antherozoids of liverworts and mosses. 

The zygote divides by 'a transverse wall into an outer and atl 
inner cell. The outer cell ordinarily elongates, becoming a 



pensor. The inner cell, which has been pushed by the elongation 
of the suspensor deeper into the tissue of the gametophyte, de- 
velops into an embryo. The suspensor is therefore only a tem- 
porary organ which functions in bringing the embryo into a better 

nutritive relation. The em- 
, . , ~ , , , , 

bryo is at first dependent 

upon the gametophyte, se- 
curing its food by means of 
a foot. Later, by the growth 
of stem and leaves above 
the soil, and by the develop- 
ment of a first root under 
the soil, the embryo be- 
comes an independent 


FIG. 308, Underground gametophytes of 
Lycopodium, the 2 at the right bearing 


298. Sporophyte. The 

members of this genus are 
mostly tropical plants, although a few grow in temperate regions. 
Some species of temperate regions live on rocks and on dry, sandy 
soil; others thrive best in more moist and shaded habitats. 

The conspicuous plant, as in the case of a fern, is the sporophyte 
(Fig. 309). Its branching stem bears many small, simple leaves. 
The stem, in some species, grows along the ground and bears two 
rows of small leaves and two rows of larger leaves. In other 
species the branches of the stem grow more or less upright, and 
the leaves are uniform in size. Roots develop directly from the 
stem in certain species; in others, they arise from short, leafless 

In some species the stem contains a small, centrally placed stele. 
The xylem of the stele is surrounded successively by phloem, peri- 
cycle, and endodermis, the latter in niany species being peculiar 
in that it is interrupted by large intercellular spaces. The inner 
part of the cortex is commonly composed of thin-walled cells, but 
the cells of the outer portion are usually thick-walled. An epider- 
mis encloses the cortex. 

The leaves are small, unlobed and undivided, narrowly tri- 
angular, and pointed. A leaf has an upper and a lower epidermis, 



between which is spongy tissue containing a single vascular butidle 
or midrib. Stomata occur chiefly in the lower epidermis. 

299. Spore-formation. The leaves on the terminal portions of 
many of the branches are more or less compactly arranged in four 
rows. At the inner side of each of these leaves, near its base 
(Fig. 310), is a small, short-stalked sporangium. A leaf that bears a 
sporangium is a sporophyll, and the limited portion of a branch 
which bears sporo- 
phylls, together with 
the sporophylls them- 
selves, is a strobilus. 
The sporangia borne on 
different sporophylls in 
the same strobilus are 
of two distinct kinds 
(Figs. 311, A; 312, A). 
Each sporangium of one 
kind contains com- 
monly four relatively 
large spores; sporangia 
of the other kind con- 
tain many very small 
spores. The large spores 
are macrospores, the 
sporangium which con- 
tains them is a macro- 
sporangium, and the 
leaf on which this spo- 
rangium is borne is a macrosporophyll In like manner, the small 
spores are microspores y the sporangium which contains them 
is a microsporangium, and the leaf on which this sporangium is 
borne is a microsporophyll. The distribution of micro- and macro- 
sporophylls upon the axis of the strobilus differs in different species; 
in some species (Fig. 3 10), the lower sporophylls are macrosporo- 
phylls, the upper are micros^arophylls ; in others, one side of the 
strobilus bears microsporophylls, the other side macrosporophylls; 
in still other species, the two types of sporangia are intermingled. 

A mature sporangium of either type has a short stalk and a 
jacket three cells in thickness. A macrosporangium is larger thai? 
a microsporangium and is generally lobed, the lobes corresponding 

FIG. 309. 


Portion of a mature sporophyte of 



Apical Cell 

to the positions of the macrospores within. Both macro- and 
microsporangia develop alike to the spore-mother-cell stage. In 
some sporangia, most of the numerous spore mother cells divide 
to form four spores each. The result is the production of a large 
number of small spores (microspores) in such a sporangium. In 
other sporangia, usually all but one of the spore mother cells dis- 
integrate. From this remaining spore mother cell, by two suc- 
cessive divisions, four spores (macrospores) are formed, some or 

all of which increase 
greatly in size and de- 
velop thick, corrugated 

The difference in size 
between the two kinds 
of spores is associated 
with a difference in their 
function. A macrospore 
develops into a macro- 
gametophyte (female 
garnetophyte) ; a micro- 
spore into a microgame- 
tophyte (male gameto 
300. Macroganieto- 




Central Axis 

phyte (Fig. 311, B). A 
macrospore has some- 
what the shape of a 

FIG. 310. Diagram of a lengthwise section of a J QW b roa( j pvramid 
portion of a strobilus of Selaginella. ' , V f 

with a rounded base. It 

possesses a single nucleus. While held closely within the macro- 
sporangium, the macrospore develops into a macrogametophyte. 
Its development begins with a series of nuclear divisions, not im- 
mediately followed by cell divisions. The free nuclei are more or 
less evenly distributed in the dense cytoplasm lining the inner 
surface of the spore wall. After a number of nuclear divisions, 
cell divisions occur between those nuclei lying near the pointed 
end of the macrospore wall. The cells so formed may undergo 
further growth and division, and on the cushion of cells thus pro- 
duced archegonia develop. 

In many species of Selaginella. at some stage in the development 



of the macrogametophytes as described above, the macrospo 
rangia mature and break open and the macrogametophytes fall 
to the ground. If conditions are favorable, macrogametophyte- 
development continues. At about the time that archegonia are 
being formed, the wall of each macrospore cracks open at its pointed 
end, exposing the slightly protruding portion of the macrogame- 






- Hf 

Waerosporophyll \^t|H 

/H T^ 
-4 Central Axis 



FIG. 311. Selaginella. A, section through a macrosporangium. B, macro- 
gametophyte; redrawn from Miss Lyon. 

tophyte, which bears archegonia. The partly imbedded archegonia 
resemble those of the bracken but are fewer-celled. Although 
the protruding portion of the macrogametophyte may develop 
chlorophyll, the macrogametophyte depends chiefly upon stored 
food supplied during its development by the sporophyte. 

301. Microgametophyte. A microspore (Fig. 312, B) y except for 
its much smaller size, closely resembles a macrospore in shape 
and structure. It germinates (Fig. 312, C~E) while still within the 
thicrosporangium. Its division results in the formation of a large 
and a small cell, both of which are wholly within the microspore 
wall. The smaller is the prothallial cell, so called because this 
single cell is thought to correspond to the vegetative tissue of a 
fern pro thallium. From the larger cell, by further, divisions, is 
developed a central group of cells surrounded by a single layer of 
jacket cells. Each cell of the central group is finally transformed 
into a spirally coiled antherozQid with two flagella^ the jacket 
cells meanwhile having disintegrated. 



In many species of Selaginella, the microsporangia at maturity 
break open and the gametophytes at some stage in their develop- 
ment fall to the ground. Here their further development continues. 
After antherozoids have been formed, the layers of the spore wall 
break open and the antherozoids are liberated. If the micro- and 

Microsporangium B 
$_ Microsporophyll 

Jacket Cells 
Central Art's 

Young Antherozoids 

Prothallial Cell 

FIG. 312. Selaginella. /I, section through a microsporangiurn. B, mierospore. 
C, microgametophyte after the formation of the prothallial cell. I), nearly 
mature microgametophyte. E, mature microgametophyte with an- 
therozoids. B-E redrawn from Miss Lyon. 

macrogametophytes lie near together on the ground, a film of 
water connecting them will enable the antherozoids to swim to the 
archegonia. As in mosses and ferns, one antherozoid unites with 
each egg. 

302, Embryo (Fig. 313, A). Shortly after gametic union, cell 
divisions occur in the previously undivided basal portion of the 
macrogametophyte. The zygote divides into two cells. The daugh- 
ter cell nearer the neck of the archegonium elongates, forming a 
suspensor that pushes the other daughter cell farther into the tissufc 
of the macrogametophyte. The latter daughter cell develops into 
an embryo, consisting of a foot, a stem bearing tw r o primary leaves, 
and a primary root. The primary root and the stem bearing the 
primary leaves grow outward from the tissue of the macrogameto- 
phyte, and the young sporophyte becomes independent (Fig. 313, 
B). As the stem continues its growth, it develops secondary leaves 
and secondary roots; the primary root and primary leaves even 
tually disappear. 



Although the history of the gametophytes recited above is 
characteristic of many species of Selaginella, a modification of the 
story is found in certain species. This latter account is of interest 
because these particular species closely approach the seed habit 
characteristic of sperrnatophytes. In these species, macrogameto- 
phytes develop in the manner already described. When a macro- 
sporangium is mature it cracks open, but not sufficiently to permit 






Photosynthetic Tissue 
O/" Macrogametophyte 


Storage Tissue 
Of Macrogametophyte 

FIG. 313. Selaginella. A, macrogametophyte with a young sporophyte; re- 
drawn from Bergen and Davis. #, macrogametophyte with an older 
sporophyte which is still parasitic upon the gametophyte. 

the escape of the developing macrogametophytes. The micro- 
sporarigia, however, when mature burst open and the developing 
gametophytes are thrown out. Some of th ,m, sifting down be- 
tween the sporophylls, fall by chance into the partly opened 
macrosporangia. Lying now in the same sporangium, the two 
kinds of gametophytes complete their development. If sufficient 
water is present, gametic union occurs. At some time later, the 
opening of the macrosporangium becomes larger and the macro- 
gametophytes with the developing embryos may fall to the ground. 
303. Life Cycle (Fig. 314). A strobilus produces two kinds of 
sporophylls, two kinds of sporangia, and two kinds of spores. A 
macrospore develops into a macrogametophyte which forms arche- 



gonia. Each archegonium contains an egg. A microspore develop* 
into a microgametophyte which produces antherozoids. The union 
of antherozoid with egg forms a zygote. The zygote grows into 
an embryo parasitic on the macrogametophyte. By further growth 
and development the embryo becomes a mature sporophyte aa 

FIG. 314. Life cycle of Selaginella. 

independent plant composed of stem, roots, and leaves and bearing 

As in a moss or fern, the chromosome number is doubled in 
gametic union; it is reduced in the two successive divisions by 
which four macrospores are formed from a macrospore mother cell, 
and likewise in the divisions by which four microspores are formed 
from each microspore mother cell. In Selaginella, therefore, as 
in mosses and ferns, the gametophytic generation (here including 
macrogametophyte and microgametophyte) is marked by the 
presence in each of its cells of n chromosomes; and the sporophytic 
generation by the presence in each of its cells of 2 n chromosomes- 

304. New Features in Selaginella. From an evolutionary stand- 
point, Selaginella shows certain marked advances over the ferns. 


One notable advance in Selaginella is in the production of two 
kinds of spores, each of which develops into a specific kind of 
gametophyte. A microgametophyte is a greatly reduced plant 
in that it consists of relatively few cells. In the same sense, a 
macrogametophyte also is greatly reduced as compared with the 
gametophyte (prothallium) of a fern. A microgametophyte devel- 
ops to maturity within the wall of the microspore; a macrogameto- 
phyte develops chiefly within the wall of the macrospore and barely 
protrudes from this wall when mature. The male (micro-) and 
female (macro-) gametophytes are markedly different. But in 
Selaginella sexual differentiation is not limited to the gametophyte, 
as it is in bryophytes and ferns. Instead, this differentiation has 
been pushed back, as it were, to the sporangia, which are structures 
belonging to the sporophyte. Hence the difference between the 
sporangia, although these are strictly asexual reproductive struc- 
tures, is nevertheless actually a sexual difference. 

Another important difference concerns the nutrition of the 
gametophytes. A microgametophyte, having no chlorophyll, is en- 
tirely dependent upon foods received from the sporophyte. That 
is, it has become indirectly parasitic upon the sporophyte. A 
macrogametophyte also is chiefly dependent upon foods derived 
from the sporophyte; although, in its later development, that part 
of a macrogametophyte which is exposed by the cracking of the 
macrospore wall may develop chlorophyll and carry on a very 
limited amount of photosynthesis. Thus the nutritive relation- 
ships which existed between gametophyte and sporophyte in the 
mosses and liverworts have been virtually reversed in Selaginella. 
In a moss or liverwort, the sporophyte is largely parasitic upon 
the gametophyte. In Selaginella, the gametophytes have become 
in effect parasitic upon the sporophyte. 

A third important characteristic of certain species of Selaginella 
is that a young microgametophyte, on being discharged from its 
sporangium, may sift into a partly open macrosporangium and 
there, in close proximity to a developing macrogametophyte, 
continue its development. 

A fourth new feature in the species just referred to is that the 
macrogametophyte, surrounded largely by the macrospore wall, 
may remain within the partly open macrosporangium during 
gametic union and the early stages of embryo-development. Hence 
these species of Selaginella approach closely the formation of a 


seed, the production of which is one of the outstanding features 
of the seed plants. 

306. Pteridophytes. The ferns and their allies constitute a 
division, pteridophytes, whose members are approximately at a 
common level of development in the sense that they stand higher 
in the evolutionary scale than do biyophytes, but have not attained 
the seed habit characteristic of spermatophytes. 

In general, the sporophyte of a pteridophyte is a relatively large, 
independent plant differentiated into stem, leaves, and roots. 
Sporangia are usually borne on leaves. The sporophylls are there- 
fore an important feature. In some pteridophytes the sporophylls 
bear sporangia in which the spores are all alike; in others there are 
two sorts of sporophylls, of sporangia, and of spores. In certain 
pteridophytes sporophylls are grouped together, forming cones 

The gametophytes of pteridophytes, always small, are variable 
from genus to genus, both in structure and in method of nutrition. 
Some gametophytes, such as those of most ferns and of Equisetum, 
are green, independent prothallia; others, including those of Lyco- 
podium, grow underground and are saprophytic. 

Those pteridophytes with two sorts of spores form simpler game- 
tophytes which, developing almost entirely within the old spore 
walls, are extremely small and indirectly parasitic upon the old 
sporophytes. The antherozoids, however, are motile, and the 
presence of water is still essential to gametic union. A sporophyte 
begins its existence as a parasite, but eventually, through the 
production of stern, roots, ^and leaves, becomes an independent 



306. Seed Plants. The spermatophytes or seed plants constitute 
the highest division of the plant kingdom. In this division the 
sporophyte reaches its greatest complexity; the gametophytes are 
reduced to minute plants parasitic upon the sporophyte. Seed 
plants are divided into gymnosperms, whose seeds are not enclosed, 
and angiosperms, which have enclosed seeds. 

The geological record shows that during certain periods of the 
earth's history gymnosperms were relatively abundant; indeed, 
some orders are known only in the form of fossils, and one order is 
now represented by but a single surviving species. The largest 
orders of living gymnosperms are the Cycadales and the Conifer ales. 
Cycadales are all tropical or subtropical ; Conif erales, which include 
the pines, spruces, and related trees and shrubs, are mainly in- 
habitants of temperate regions. 


307. Sporophyte. The Cycadales are the most primitive of 
living seed plants and in certain respects show greater similarities 
to pteridophytes than do any other existing seed plants. Zamia 
(Fig. 315), a member of this order, grows extensively in Florida. 
The sporophyte, which is the conspicuous generation, rarely attains 
a height of more than four feet. Its stern is short, thick, and erect, 
frequently with its greater portion underground. At its center 
(Fig.- 316) is a large pith, surrounded when the stem is young by a 
cylinder of vascular bundles. Each bundle consists, like a bundle 
of a dicotyledonous stem, of xylem, cambium, and phloem. As 
the stem grows older, only small amounts of secondary xylem and 
secondary phloem are formed by the activity of the cambium. The 
xylem so formed is not differentiated into annual rings; it is inter- 
rupted by relatively wide medullary rays. In the thick cortex 
great amounts of starch are stored, the quantity being so large 
that the plant was used as a source of food by the Seminole Indians 
under the name of "conti." 




The foliage leaves arise in a crown near the apex of the stem, 
a cluster of new leaves being produced from year to year. The 
leaves are leathery in texture and resemble those of many ferns in 

being pinnately divided. 
The tip of each young 
leaf unrolls slightly, in 
this respect somewhat re- 
sembling a fern leaf. The 
individual leaflets, how- 
ever, do not unroll, as is 
the case in a pinnately 
divided fern leaf. A leaf 
of Zamia may live for 
several years. As the 
older leaves die and 
wither, their bases re- 
main attached for a short 
time to the stern. 

Zamia usually forms a 
relatively large tap root 
from which a few slender 
branching roots arise, A 
blue-green alga gains en- 
trance into the cortex of 
some of the smaller roots ; 
the stimulus resulting 
from its presence causes 
each invaded root to 
change its direction of 
growth and to produce a 
compact cluster of small 
tubercular branch roots 
at or near the surface of 
the soil. 

A tap root or one of 
FIG. 315. Mature sporophyte of Zamia bearing ., , hranr-h rnnt* 

a carpellate strobilus. tne Iarger biancn roots 

commonly possesses a 

siphonostele. The exarch primary xylem and the primary phloem 
alternate in the radial arrangement characteristic of roots. 
Secondary xylem and phloem are formed by cambial activity. 



In the younger portions of a tap root the cortex is relatively 
thick and stores abundant foods. In older portions a cork cam- 
bium is formed in the pericycle. After this cork cambium has, 

FIG. 316. Cross section of a portion of the stem of Zamia. 

produced cork cells on its outer side, the cortex and epidermis are 

sloughed off. 

308. Spore-formation. The spore-bearing leaves (sporophylls) of 

Zamia differ greatly in appear- 
ance and structure from foliage 
leaves. "They are borne com- 
pactly arranged on the termi- 
nal portions of short branches 
growing from the apical region 
of the stem. Zamia has two 
kinds of spores distinguished, 
on the basis of their size, as 
macrospores and microspores. 
The sporangium that contains 
the larger spores (macrospores) 
is a macrosporangium, the leaf 
on which this sporangium is 
borne is a macrosporophyll, and 
the terminal portion of the 
branch on which macrosporo- 
phylls are borne is a carpellate 
strobilus. In like manner, the 
sporangium that contains the 
smaller spores (microspores) is 

a microsporangium, the leaf on which this sporangium is borne 

is a microsporophyllj and microsporophylls are borne on a stamiwate 

FIG. 317. A, staminate strobilus of 
Zamia. B, microsporophylls bearing 
on their lower surfaces many micro- 



strobilus. Any one plant produces only carpellate or only staminate 

A staminate strobilus (Fig. 317) bears on its central axis many 
horizontally placed, closely packed microsporophylls. Each mi- 
crosporophyll is almost scale-like and bears on its lower surface 
from 30 to 40 small microsporangia. Within each microsporangitim 
(Fig. 318), as within the sporangia of ferns, spore mother cells are 
produced, and the division of each microspore mother cell gives 

Microspore Mother Ceils 
Microsporangium Wall 


FIG. 318. A, vertical section of a young microsporangium of Zamia contain- 
ing microspore mother cells. B y spores resulting from the division of a 
microspore mother cell. C, mature microsporangium. 

rise to four microspores. Since each microsporangium produces 
approximately 500 microspores, and since there are, on an average, 
perhaps 35 microsporangia on each of the 200 or more microsporo- 
phylls, the number of microspores produced on a single plant is 

A carpellate strobilus (Fig. 319) is larger than a staminate stro- 
bilus, and consists of a central axis bearing macrosporophylls. 
A macrosporophyll is larger and fleshier than a microsporophyll. 
When mature, it consists of a stalk and an expanded outer portion; 
to the inner side of the latter that is, to the side toward the cen- 
tral axis of the strobilus are attached two ovules. 

An ovule (Fig. 320, A) begins its development as a bluntly 
conical protuberance, the nucellus (or macrosporangium), on the 
inner surface of the expanded portion of the young macrosporo- 
phyll. From its base an enclosing integument grows upward and 



about the nucellus but leaves a small tubular opening, the micro- 
pyle, leading to the outer end of the nucellus. The integument 
seems to be a distinct organ, the sporangium proper being only the 
nucellus. But since nucellus and integument are closely combined, 
the two together are commonly treated as a single structure, the 

A D 

FIG. 319. A, carpellate strobilus of Zamia. B, macrosporophyll bearing 2 
ovules. C, lengthwise section of a young macrosporophyll, showing an 
early stage in the development of macrogametophytes. D, similar section 
at a later stage; macrogametophytes fully developed. 

ovule. As development continues, a small depression (pollen 
chamber) is developed at the end of the nucellus next the 

Only one macrospore mother cell becomes differentiated within 
the nucellus, and so but four macrospores are formed. These macro- 
spores lie in an axial row in the central part of the nucellus. 

309. Macrogametophyte (Fig. 320, B, C). Two fundamentally 
important features of seed plants are that the macrospore is firmly 
and permanently enclosed within the macrosporangium, and that 
the macrogametophyte develops to maturity entirely within the 
macrospore wall. Although four macrospores are formed within 
the macrosporangium, but one, usually that farthest from the 



micropyle, develops into a macrogametophyte; the other three 
soon disintegrate. 

The development of the macrogametophyte begins with a series 
of nuclear divisions. Later, by cell division it becomes many- 
celled, and by further cell division and growth the garnetophyte 
increases in size. Meanwhile the nucellus and integument are 

Pollen Chamber /Integument 

Mocrogam etoph yte 

Archegonial Chamber 
Pollen Oiamber 

Nucellus Macrosporei) ( 

n .. -^ Integument 

PoUen Chamber ^^ ~" 



Microgam etophyte Macrogam doph yt, e 

Microgam etophyte 

JFlcshy Layer 

FIG. 320. Zamia; stages in the development of a macrogametophyte. A, 4 
macrospores are formed, ft, 1 macrospore has geminated to form a macro- 
gametophyte. C, just before gametic union. 

growing; but the macrogametophyte is developing at the expense 
of the adjoining cells of the nucellus, some of the nucellar tissue 
being digested and broken down. As the macrogametophyte 
develops further, a small depression, the archegonial chamber, 
appears in the end toward the micropyle. Two to six archegonia 
are formed at this end of the macrogametophyte, each opening 
into the archegonial chamber. An archegonium consists of two 
neck cells and a very large egg, the latter imbedded in the tissue 
of the macrogametophyte. 

310. Microgametophyte (Fig. 321). The development of a 
microgametophyte from a microspore begins while the latter is 
still within the microsporangium. The microspore divides to form 
two daughter cells of unequal size. The smaller is the prothallial 
cell, so called because this single cell is thought to correspond to 
the vegetative tissue of a fern prothallium; the other and larger 
cell soon divides into a small generative cell and a large tube cell. 
Both these divisions occur within the microspore wall. The devel- 
opment of the microgametophyte now ceases for a time. This 
three-celled immature microgametophyte is a pollen grain. 



The microsporangium now breaks open, and the pollen grains 
are distributed by winds. Some of the dust-like grains may be 
blown to a carpellate strobilus. At this time, in consequence of 
an elohgation of the central axis of the carpellate strobilus, the 
macrosporophylls are not closely pressed together. Some pollen 
grains, therefore, may sift between the macrosporophylls and 

Prothallial Cell 



V-v.^'Ht. / 

Tube Cell 

Prothallial Cell 


FIG. 321. Zamia; stages in the development of a rnicrogametophyte. A, B, 
stages passed through in the microsporangium; C-F, while the pollen tube 
is growing through the nucellus. A, microspore. B, 3-celled microgame- 
tophyte (pollen grain) at the time of its liberation from the microspo- 
rangium. C, early stage in the " germination " of a pollen grain. D, the 
generative cell has divided into stalk and body cells. E, the body cell has 
divided to form 2 antherozoids. F 9 mature antherozoid. 

lodge in i^he vicinity of the ovules. At the outer end of the micro- 
pyle of each ovule is a drop of a sticky liquid in which some pollen 
grains become caught. Later this liquid, with the imprisoned 
pollen grains, is withdrawn through the micropyle to the pollen 
chamber (Fig. 320, B). 

The transportation of young microgametophytes from the 
microsporangium to a specific place in the vicinity of the macro- 
gametophyte (a process known as pollination) is one of the fea- 
tures especially characteristic of seed plants. The dependence of 
Zamia upon wind pollination necessitates the production of a 
very large number of pollen grains, since the vast majority of 
grains will not be carried to a place where they can function. 

After the pollen grains reach the pollen chamber, their develop- 
ment is resumed. This further development (the "germination" 
of a pollen grain) begins with an elongation of the tube cell into 
a cylindrical pollen tube which grows into the nucellus and ab- 
sorbs food materials for the further growth of the microgameto- 
phyte. Several pollen grains may germinate in the pollen chamber 



and develop tubes. The generative cell divides into a stalk cell 
and a body cell. The body cell in turn divides to form two cells 
that ultimately become antherozoids. An antherozoid has approx- 
imately the shape of a top ; beginning at its pointed end is a spiral 
groove of several turns, and from the base of this groove grow a 
great number of flagella. 

311. Gametic Union. At the time of pollination the rnacrogame- 
tophyte is still in an early stage of development. The completion 

Gamete Nuclei 

Cotyledon EjncotyL 
-ra - ' 

r ntegument 

Young Foliage 
" Leaf 

Seed Coat 

-primary Root 

FIG. 322. Zaraia. A, apical portion of a macrogametophyte; gamete nuclei 
uniting at the left; at the right a young embryo. B, lengthwise section of a 
seed. C, young sporophyte developing from a germinating seed. 

of its development requires several months, during which time the 
pollen tube is growing in the nucellus. Finally, when the macro- 
gametophyte and eggs are mature, the basal end of the tube 
that is, the end still in trie pollen chamber grows through the 
nucellus directly to the archegonial chamber, and the end of the 
tube bursts. Since several tubes usually reach the archegonial 
chamber, a number of antherozoids may be discharged into, and 
swim about in, the liquid of the chamber. These antherozoids, or 
some of them, make their way through the necks of the archegonia 
and unite with the eggs (Fig. 322, A) only one antherozoid 
uniting with any one egg. 

312. Development of a Seed. The nucleus of the zygote now 
divides, and the daughter nuclei by repeated divisions give rise 
to a many-nucleate proembryo. Still later, cell divisions occur, 
chiefly between those nuclei of the proembryo which lie in its 


basal part. Some of the uppermost of the cells so formed elon- 
gate and push the basal portion of the proembryo deep into the 
tissue of the macrogametophyte. The elongating cells form a 
long, slender, much-coiled suspensor. The cellular mass which 
is thrust, by the growth of the suspensor, deep into the macro- 
gametophyte and which feeds upon the cells of the macrogameto- 
phyte is the embryo. The proembryo has, in this manner, become 
differentiated into embryo and suspensor. The embryo continues 
to grow and develop slowly. 

The whole structure ultimately developed from the ovule and 
its inclusions is a seed (Fig. 322, B). It consists of: 

(a) The embryo the new sporophyte which becomes differ- 
entiated into two large primary leaves (cotyledons) and a central 
axis. The part of this axis below the level of attachment of the 
cotyledons is the hypocotyl; the part above, a small mass of embry- 
onic tissue, is the epicotyl. The suspensor is still discernible, 
attached to the end of the hypocotyl. 

(6) A large mass of nutritive tissue or endosperm filled with 
reserve foods. The endosperm is the persisting tissue of the macro- 

(c) A seed coat, composed of an outer fleshy layer and an inner 
stony layer, both developed from the integument. A thin, papery 
layer immediately about the endosperm is derived chiefly from 
the nucellus. The integument and the nucellus are parts of the 
old sporophyte. The seed, therefore, consists of structures belong- 
ing to three distinct generations the new sporophyte, the macro- 
gametophyte, and the old sporophyte. 

313. Germination of a Seed (Fig. 322, C). By the time the seeds 
are mature, a shriveling of the macrosporophylls makes it pos- 
sible for the seeds to drop from the strobilus and to fall to the 
ground. Under suitable conditions they may later germinate. In 
germination, a seed absorbs water and the embryo contained in 
it continues growth. The hypocotyl elongates and pushes out 
through the micropylar end of the seed coat, bending, if neces- 
sary, in order to grow downward into th. soil. The terminal por- 
tion of the hypocotyl forms the primary root. Although the 
cotyledons remain partly within the seed coat, they elongate 
sufficiently to free their basal portions and the epicotyl. The 
stem and leaves eventually develop from the epicotyl, except that 
the lowermost portion of the stern is derived from the hypocotyl, 



Until this time the young sporophyte has been unable to manu- 
facture its own carbohydrates, and has been dependent upon 
reserve foods stored in the endosperm (macrogametophyte) 
foods that were derived by the macrogametophyte from the cells 
of the parental sporophyte. As soon as the epicotyl has de- 

FIG. 323. Life cycle of Zamia. 

veloped chlorophyll-containing leaves, the young sporophyte is 

314. Life Cycle (Fig. 323). Two sporophytic plants are neces- 
sary in the life cycle of Zamia. One sporophyte bears staminate 
strobili, microsporophylls, microsporangia, and microspores. An- 
other sporophyte bears carpellate strobili, macrosporophylls, 
ovules (macrosporangia with integuments), and macrospores. 
A microspore develops into a microgametophyte which produces 
antherozoids; a macrospore into a macrogametophyte which 
produces archegonia and eggs. The union of an antherozoid with 
an egg forms a zygote. After gametic union a proembryo is formed 


whose apical portion develops into an embryo. The embryo and 
the surrounding structures of the ovule mature into a seed. On 
germination the em- 
bryo within the seed 
develops into either a 
staminate or a car- 
pellate sporophyte. 

315. Stem. The 

Coniferales, which in- 
clude the pines, are 
the largest order of 
gyrnnosperms both in 
number of species and 
in number of individ- 
uals. In the temper- 
ate regions of the 
northern hemisphere, 
Coniferales form for- 
ests of vast extent. 
Most of the lumber 
sold in the United 
States is sawn from 
trunks of coniferous 
trees. In fact, the de- 
mand for this type of 

lnmhor hn<* so fir nut FlG * 324 ' Pine ; a mature sporophyte. Photo- 
iumber lias so lar out- gmph by L g Cheney 

stripped the supply 

that there are but small remnants of available coniferous tim- 
ber left in the north central and north Atlantic states. There 
are still extensive coniferous forests in the Pacific states and in the 

The pine tree (Fig. 324) is the sporophytic generation. Since 
its terminal bud grows more rapidly than the terminal buds of the 
branches, a conspicuous central trunk is formed. The lateral buds 
which are to develop into long branches are borne in whorls. The 
gradual transition in length of these branches from the lowermost 
and longest to the uppermost and shortest gives the tree as a whole, 
when it stands in the open, a conical form. If it grows in a dense 



stand, however, the tree bears branches in its upper portion only, 
the lower branches having died and fallen early. 

In addition to long branches, the pine has branches of another 
sort (spur branches, Fig. 325) which, although they may live for 
a number of years, remain very short and slender. It is at the 

ends of these spur branches 
Foliage Leaves that the needle-shaped fo- 
liage leaves are borne. 

The primary tissues of 
a young pine stern (Figs. 
326, 327) consist of stele, 
cortex, and epidermis. At 
the center of the stele is a 
pith, surrounded by a cyl- 
inder of vascular bundles 
separated from one an- 
other by medullary rays. 
The conducting elements 
of the primary xylem are 
tracheids. The pine, like 
most other gymnosperms, 
produces no vessels. The 
phloem consists of thin- 
walled sieve tubes inter- 
mingled with a few shorter 
but broader thin-walled cells. Between the xylem and the phloem 
of each bundle is a cambium. The cortex is composed of rounded 
thin- walled cells which, at^ least in the outer portion of the cortex, 
frequently contain chloroplasts. Here and there in the cortex and 
xylem are longitudinal resin canals. The outermost layer of the 
stem is a heavily cutinized epidermis. 

Growth in thickness of the stem is due mainly to the formation 
of secondary xylem and secondary phloem by cambial activity. 
The tracheids produced by the cambium are of much the same size 
as those in the primary xylem. Since the tracheids produced at 
the beginning of each growing season are somewhat larger than 
those developed later in the year, there are well-defined annual 
rings in the secondary xylem (Fig. 40). It is chiefly because of the 
approximate uniformity in size of the tracheids and because of the 
hardness of their walls that the pine is so valuable as a source of 

; - Long Branch 

FIG. 325. Apical portion of a stem or branch 
of the white pine. 


lumber. The original medullary rays are continuous through the 
secondary xylem. The cambium gives rise to new medullary rays 
from time to time, so that the later-formed rings of xylem contain 
many more rays than do those parts of the xylem formed earlier. 
The portion of a medullary ray within the xylem contains rel- 
atively short living thin-walled cells which at certain seasons are 
filled with reserve foods; in some species it contains also thick- 

FIG. 326. Cross section of a young pine stem. The 
primary bundles are evident, although cambial activity 
has begun. 

walled tracheids whose function is the lateral transport of liquids. 
In the portion of a ray within the phloem, all the cells are living 
and thin- walled. Resin canals are formed here and there in the 
secondary xylem, extending for considerable distances up and down 
the stem. These canals appear in cross section as large pores bor- 
dered by thin-walled cells, and are connected by other resin canals 
that run approximately horizontally in the medullary rays. The 
secondary phloem cells ordinarily remain functional for only a 
year, the phloem of previous years persisting as a crushed mass 
of cells outside that formed later. 

At about the time that cambial activity begins in the stele, a 
cork cambium is developed in the cortex just beneath the epi- 
dermis. This cork cambium functions for a time; later, additional 



cork cambiums are developed, as in dicotyledonous stems. By the 
activity of the various cork cambiums and by the formation of new 
phloem, a thick bark is formed that surrounds the xylem. 

316. Roots. In a young portion of the mature region, the stele 
of a pine root includes two xylem and two phloem strands. These 
xylem and phloem strands alternate, forming an interrupted cyl- 
inder. Later, a central strip of thin-walled cells between the two 

Cork Camttum 
Cork | Cortex 


Rwin Canal 

Resin Canal* 

FaUWood Spring Wood 
* Annual Ring 

PIG. 327. Cross section of a portion of an older pine stem, showing its com- 
ponent tissues. 

xylem strands also becomes xylem. The young stele now includes 
a thin plate of primary xylern a few cells in thickness flanked on 
either side by a plate of primary phloem, the region between pri- 
mary xylem and primary phloem being occupied by thin-walled 
cells. The outer portion of the stele is a pericycle, several cells in 
thickness. The cortex and epidermis resemble those of a dicotyle- 
donous root. 

As the root grows older, certain strips of thin-walled cells be- 
tween primary xylem and primary phloem begin to function as a 
cambium; still later, cambial activity extends to the cells of the 
pericycle just outside each edge of the xylem plate. The primary 
xylem is now completely surrounded by a cylinder of cambium. 
This cambial cylinder develops secondary xylem on its inner and 
secondary phloem on its outer side (Fig. 328), as does the cambium 
of the stem ; an old root, therefore, has much the same appearance 
in cross section as an old stem. A cork cambium is formed from 
the outermost cells of the pericycle; after this cork cambium has 



foliage leaves. They are borne compactly arranged on the ter- 
minal portions of short branches. The pine has two kinds of spores 
distinguished, on the basis of their size, as macrospores and micro 
spores. The sporangium which contains 
the larger spores (macrospores) is a macro- 
sporangium, the leaf on which this spo- 
rangium is borne is a macrosporophyll, and 
the terminal portion of the branch on 
which macrosporophylls are borne is a 
carpellate strobilus. In like manner, the 
sporangium which contains the smaller 
spores (microspores) is a microsporangium, 
the leaf on which this sporangium is borne 
is a microsporophyll, and microsporophylls 
are borne on a staminate strobilus. Any 
one sporophyte may produce both carpel- 
late and staminate strobili. 

Staminate strobili (Fig. 330) are pro- 
duced in clusters near the ends of long 
branches. Each strobilus is comparatively 
small, rarely more than a half inch in 
length. It consists of a central axis bear- 
ing many horizontally disposed scale-like 
microsporophylls (Fig. 331). On the un- 
der side of each sporophyll, and with 
their long axes parallel to the long axis 
of the sporophyll, are two ovoid micro- 
sporangia. Within each microsporangium 
are produced many microspore mother cells, each of which by 
division gives rise to four microspores. 

Carpellate strobili (Fig. 332) are much larger than staminate 
strobili. A young carpellate strobilus has a central axis bearing 
numerous bracts. As the strobilus grows older (Fig. 333), a scale- 
like structure, several times the size of a bract, develops in the 
axil between each bract and the central axis. This scale bears two 
ovules on its upper surface. Opinions differ as to whether the scale 
is a macrosporophyll or whether it represents a reduced branch. 

An ovule (Fig. 333, C) begins its development as a mass of em- 
bryonic tissue the nucellus or macrosporangium on the surface 
of a scale. From the base of the nucellus an enclosing integument 

FIG. 330. Branch of a 
pine bearing stami- 
nate strobili. 



grows up and about it, leaving 
an opening, the micropyle. The 
integument seems to be a dis- 
tinct organ, the sporangium 
proper being only the nucellus. 
The two together constitute an 
ovule. A very small depression 
(pollen chamber) is formed at 
the end of the nucellus next 
the micropyle. 

A single macrospore mother 
cell becomes differentiated near 
the center of the nucellus. 

From this cel1 ' ^ di > is 

crosporophylls viewed from the side later formed an axial row of 
and from below. f our macrospores. 

319. Macrogametophyte. Only one macrospore in a nucellus, 
usually that macrospore farthest from the micropyle, develops into 
a macrogametophyte; the other three macrospores soon disinte- 
grate. The development of 
the functional macrospore 
into a macrogametophyte 
begins with a series of 
nuclear divisions (Fig. 
334, A). Later, by cell di- 
vision, a many-celled mac- 
rogametophyte is formed 
which, by the repeated 
growth and division of its 
cells, increases in size 
(Fig. 334, B, C). As a 
rule, two or three arche- 
gonia are formed at the 
micropylar end of the 
macrogametophyte. Each 
archegonium consists of 
four or eight neck cells 
and a very large egg, the 

latter being imbedded in FlQ 332 Branch of a pine bearing 
the macrogametophyte. pellate strobili. 






Mother Cell 

320. Microgametophyte and Gametic Union. The development 
of a microgametophyte from a microspore (Fig. 335, A, B) begins 
while the latter is still 
within the microspo- 
rangium. The rnicro- 
spore divides into a 
small prothallial cett 
arid a much larger ap- 
ical cell; a similar divi- 
sion of the apical cell 
forms a second small 
prothallial cell and a 
large cell, the latter 
still called an apical 
cell. Both prothallial 
cells begin to disinte- 
grate soon after they 
are formed. The apical 
cell in time divides into 
a small generative cell 
and a large tube cell. It 
is at this four-celled 
stage (that of the tube 

cell, generative cell, 

j . j , ,. 
and two disintegrating 

prothallial cells) that the partially developed microgametophyte 
(now a pollen grain) is shed. The pollen grain of the pine has two 
lateral inflated appendages ("wings") that give it buoyancy. 
These wings were developed from the wall of the microspore be- 
fore it germinated to form a microgametophyte. 

The transfer of pollen from the opened microsporangia to the 
carpellate strobili (pollination) is brought about by winds. At 
about the time that pollen is being shed, the central axis of the 
carpellate strobilus elongates, separating the macrosporophylls 
which until this time had been closely pressed together. In con- 
sequence, some pollen grains may sift between the macrosporo- 
phylls and lodge in the micropyle. A growth in thickness of the 
apical portion of the integument now closes the micropyle and 
imprisons these pollen grains in the pollen chamber. 

After the young microgametophyte reaches the pollen chamber, 

333 pine A ^ carpellate stro bilus. B, scale 
bearing 2 ovules. C, lengthwise section of an 
ovule; the macrospore mother cell is differ- 

entiated. A after Bessey. 



its development is resumed. In this " germination" of the pollen 
grain (Fig. 335, C, D), the tube cell elongates into a pollen tube 
which penetrates the nucellus. The generative cell divides into a 
stalk cell and a body cell. By this time the prothallial cells have 




FIG. 334. Pine. A, lengthwise section of an ovule, showing an early stage in 
the development of the macrogametophyte; a microgametophyte growing 
through the nucellus. #, a young macrogametophyte, shortly after the"* 
formation of archegonia. C, apical portion of a mature macrogametophyte, 
showing 1 archegonium. 

completely disintegrated. The stalk and body cells now leave the 
old wall of the pollen grain and migrate slowly down through the 
pollen tube. During this migration the body cell enlarges, and 
its nucleus divides to form two male gamete nuclei; it is not yet 
certain whether this nuclear division is followed by a division of 
the body cell. The end of the pollen tube grows through the nucel- 
lus to the macrogametophyte, and then to and through the neck 
of an archegonium. Finally the end of the tube bursts, and the 



contents of the end of the tube (including the tube nucleus, the 
stalk cell, and the cytoplasm of the body cell containing the two 
male gamete nuclei) are discharged into the apex of the egg. One 
male gamete nucleus migrates to the egg nucleus (Fig. 336, A); 

Stalk Cell Body dell Tube Cell 



Second Male First Male 
Nucleus Nucleus 

Stalk Cell Cells of 



Tube Cell 

Tube Cell 


FIG. 335. Pine. A, microsporc. #, microgametophyte (pollen grain) at the 
time of its liberation from the microsporangium. C, the generative cell 
has divided to form stalk and body cells. D, distal end of the pollen tube, 
shortly before gametic union. C and D after Miss Ferguson. 

the remaining structures from the pollen tube, including the other 
male gamete nucleus, remain in the cytoplasm near the apex of 
the egg and eventually disappear. 

321. Seed-development. After gametic union the zygote nu- 
cleus divides and the daughter nuclei divide. The four nuclei now 
present migrate to the base of the cytoplasm of the old zygote 
(now a proembryo), and come to lie in a plane at right angles to 
its long axis (Fig. 336, C, D). The four nuclei divide, their divi- 
sion being followed by a cell division that forms four basal one- 
nucleate cells; the four nuclei above this tier of cells are in what 
may be called compartments, each of which is delimited by walls 
at its lateral and basal sides, but is continuous above with the 
cytoplasm of the proembryo (Fig. 336, E). By another division 
of the four cells and four compartments the proembryo comes to 
consist of four tiers, three tiers of four cells each and one tier of 
four compartments (Fig. 336, F). 

The proembryo now undergoes differentiation. The four cells 



farthest from the neck of the archegonium (apical cells) develop 
into the embryo (or embryos) ; those of the next tier elongate 
greatly to form a suspensor that pushes the apical tier of cells deep 
into the macrogametophyte; the tier of four cells next above 
(rosette cells) forms a brace for the suspensor in the pushing of the 

Second Male.. t \^ ' 

A First Male 

Igg Nucleus 





Rosette Cells 

^Apical Cells 



Rosette Cells 

Cytoplasm of 



Cytoplasm* ^ G o^,r ^ Embryo 


FIG. 336. Pine; gametic union and embryo-development. A, union of gamete 
nuclei. B, 2 nuclei, resulting from the division of the zygote nucleus, are 
now present. C, D, 4-nucleate proembryos (only 2 nuclei showing in D). 
E, proembryo with 4 cells and 4 "compartments." F, after the next cell 
division. G, the suspensor cells have elongated and the apical cells have 
divided. //, a single embryo at a later stage; 1 of 4 initiated by the 
separation and division of the apical cells of preceding stages. 

apical cells into the macrogametophyte (Fig. 336, G, //). The 
compartments forming the uppermost tier eventually disintegrate, 
together with the remaining unused cytoplasm of the proembryo. 
Very commonly the whole structure splits vertically at an early 



stage into four parts, each consisting of a vertical row of three cells 
(one apical, one suspensor, and one rosette cell). Thus four em- 
bryos may be developed from each zygote, but ordinarily only 
one reaches maturity, the other three embryos ceasing after a time 
to develop. The young sporophyte therefore can be traced back to 
the four apical cells, or to one of the four apical cells, differentiated 
from a proembryo. From the apical cell or cells of any embryo 






FIG. 337. A, carpellate strobilus (cone) of a pine at the time seeds are shed. 
#, scale with 2 seeds. C, lengthwise section of a seed. D, E t germination 
of a seed. 

there is formed by division a many-celled mass which even- 
tually becomes differentiated into hypocotyl, epicotyl, and coty- 
ledons. The fully developed embryo (Fig. 337, C) is imbedded in 
the center of the macrogametophyte (now called the endosperm) , 
and is nearly as long as the endosperm. The hypocotyl is con- 
spicuous, and there are several cotyledons, the number varying 
with the species of pine; the epicotyl is still very small when the 
seed is mature. 

The development of an embryo is accompanied by certain 
changes in the structures surrounding it. The endosperm (macro- 
gametophyte) of the mature seed is larger and contains more re- 


serve food than it did at the time of the union of gametes. The 
endosperm is surrounded by a thin layer, the remains of the nucel- 
lus. In the development of the seed the integument also is modified, 
its cells becoming stony and forming a hard seed coat. In many 
species of pine, a portion of the scale remains attached to each 
seed, forming a wing that assists in the dispersal of the seed by 
winds (Fig. 337, B). 

After pollination, and until seeds are fully developed, the scales 
of the strobilus are closely appressed. When the seeds are mature, 
however, the scales again become separated from one another 
in consequence of the growth of the central axis of the strobilus 
(Fig. 337, A), the seeds then being free to fall from the strobilus. 

322. Germination of a Seed (Fig. 337, D, E). A pine seed may 
germinate in the spring following its maturation; or it may remain 
dormant for several years if conditions are not favorable for ger- 
mination. When conditions are suitable, the seed absorbs moisture 
and the embryo resumes growth. The end of the hypocotyl pushes 
its way through the seed coat, bending if necessary in order to 
grow downward into the soil. The cotyledons now emerge from 
the seed coat, and the portion of the hypocotyl adjacent to the 
cotyledons bends and grows upward. When they emerge from the 
seed coat the cotyledons spread apart and become green. Growth 
to this stage has been accomplished by the absorption of water 
and the use of foods stored in the endosperm; after the cotyledons 
develop chlorophyll, the young sporophyte, being able to manu- 
facture foods, becomes independent of the endosperm. The foods 
manufactured by the cotyledons are used in the development of 
the epicotyl and the adjoining part of the hypocotyl into a stem 
and in the development* of the terminal portion of the hypocotyl 
into a root. After the portion of the stem derived from the epicotyF' 
has developed secondary leaves, the cotyledons eventually dis- 
appear, the secondary leaves now performing the photosynthetic 
work of the plant. The young plant may now, by the growth and 
development of its parts, and by the formation of new branches, 
leaves, and roots, become a mature sporophyte or pine tree. 

323. Life Cycle (Fig. 338). The sporophyte (pine tree) produces 
staminate and carpellate strobili. A staminate strobilus bears 
many microsporophylls on each of which are two microsporangia. 
In a microsporangium are formed many microspore mother cells. 
Each microspore mother cell divides to form four microspores. 



A microspore develops within the sporangium into a young male 
gametophyte (pollen grain). 

A carpellate strobilus bears many macrosporophylls each pro- 
ducing two ovules. An ovule is a macrosporangium covered by an 
integument. Within a macrosporangium (nucellus) one macrospore 
mother cell is differentiated; this divides to form four macrospores. 

FIG. 338. Life cycle of a pine. 

Of these, three degenerate. The persisting macrospore grows into a 
macrogametophyte which bears archegonia. 

A pollen grain reaching the nucellus develops a pollen tube 
within which two male gamete nuclei are formed. The pollen tube 
finally penetrates an archegonium and liberates the male gamete 
nuclei. After garnetic union a cellular proembryo develops from 
the zygote and becomes differentiated into embryo and suspensor. 
By further development the embryo forms epicotyl, hypocotyl, 
and several cotyledons. The ovule has become a seed, in which the 
embryo lies enclosed by the endosperm (macroffametophyte), 


which in turn is covered by the remains of the nucellus and by a 
seed coat derived from the integument. 

When the seed germinates, the embryo resumes growth, develop- 
ing into a long-lived pine tree. This produces branches, leaves, 
roots, and eventually staminate and carpellate strobili. 

324. New Features in Gymnosperms. Gymnosperms stand 
higher in the evolutionary scale than do pteridophytes. Important 
new features developed by such gymnosperms as Zamia and pine 

(a) The production of two kinds of strobili which bear respec- 
tively microsporophylLs and macrosporophylls. 

(6) The retention for a time of the developing microgameto- 
phytes (pollen grains) in the microsporangium. 

(c) The permanent retention of the rnacrospore and the macro- 
gametophyte within the macrosporangium (nucellus). 

(d) The development of a covering (integument) about each 

(e) A further simplification of the microgametophyte. 
(/) Pollination and the formation of a pollen tube. 

(g) Direct parasitism of both gametophytes upon the sporo- 

(h) The establishment of the seed habit. 



325. General Characteristics. Angiosperms are still more highly 
specialized than are gymnosperms. They present, therefore, the 
highest evolutionary development reached, to the present time, 
by any plants. Fossil remains of angiosperms are found only in 
the later geological formations, and it seems clear that as a group 
they are more modern than are bryophytes, pteridophytes, or 
gymnosperms. Angiosperms include most of the familiar cultivated 
plants, "wild flowers, " and weeds; and, with the exception of 
conifers, almost all trees and shrubs. 

Angiosperms are distinguished from gymnosperms by the fact 
that their seeds, instead of being exposed, are produced within an 
enclosing organ (the fruit). The general structure of the vegetative 
parts of the sporophytic generation of angiosperms has been dis- 
cussed in Chapters IV VII. There remain for consideration the 
gametophytes, and those portions of the sporophyte namely, 
the flower, fruit, and seed which are intimately associated with 
the gametophytes. 

326. Structure of a Flower (Fig. 339). A flower, like a strobilus, 
is a branch (or the terminal portion of a branch or stem) which 
bears sporophylls. The flowers of all angiosperms are alike in that 
each possesses one or more sporophylls. There are great differences 
between the flowers of different angiosperms, however, in number, 
size, and arrangement of sporophylls, and in number, size, shape, 
and color of other lateral appendages (floral leaves) if these are 
present. In nearly all flowers the lateral structures (including 
sporophylls) are arranged in spirals or concentric circles (whorls) 
upon a shortened, somewhat flattened axis. 

In a flower, such as one of the apple, primrose, strawberry, 
violet, or trillium, which has all the characteristic parts, the 
following structures are present: 

(a) An outer set of green floral leaves (sepals) which enclose the 
other parts of the flower until these are nearly mature. The sepals 
together comprise the calyx. 




(6) An inner set of showy colored or white leaves (petals), con- 
stituting the corolla. In many flowers the petals aid in attracting 
the attention of insects which assist in pollination. 

(c) A set of stamens (microsporophylls) within the petals. 

(d) At the center of the flower, one or more carpels (macro- 

r-, t, . Stigma 
Pollen Grain 



Filament * 


FIG. 339. Diagram of an angiosperm flower in vertical section. 

sporophylls), constituting a pistil or pistils. Some flowers, such 
as that of the strawberry, have many pistils. 

(e) A receptacle, the terminal portion of the branch or stem, 
which bears the calyx, corolla, stamens, and pistil or pistils. 

327. Evolution of Flowers. While angiosperms, as a group, are 
the most highly specialized of all plants, yet within the group there 
are important differences in degree of specialization. Especially 
is this true with respect to the organization of the flowers of various 

It is probable that the flowers of primitive angiosperms bore 
a general resemblance to gymnosperm strobili; each consisting 


of an elongated central axis bearing many spirally arranged sporo- 
phylls and having nothing closely corresponding to sepals or petals. 
It is uncertain whether the primitive angiosperm flower bore both 
macrosporophylls and microsporophylls, or whether each flower 
bore sporophylls of but one kind. Beginning with such a strobilus- 
like structure, the following general tendencies seem to have 
marked the evolution of flowers in different families of angio- 

(a) The differentiation of accessory leaves sepals, or sepals 
and petals borne below the sporophylls. 

(6) An advance from a spiral arrangement of each set of floral 
parts (sepals, petals, stamens, and carpels) to a cyclic arrangement 
of one or more sets. 

(c) A reduction of the large and indefinite numbers of floral 
parts to smaller and definite numbers. 

(d) A shortening of the receptacle, accompanying steps b and c. 
The receptacle became broadened and flattened, or in some lines 
of descent concave. 

(e) An advance from a condition in which all the members of 
any particular set of floral parts are alike and symmetrically ar- 
ranged around a central axis (a condition of radial symmetry) to 
one in which the members of at least one set differ among them- 
selves in size and shape, so that there is only one plane in which 
the flower can be divided into two equal parts (a condition of 
bilateral symmetry). 

(/) An advance from a condition in which all the members of 
each set of floral parts are distinct and separate to one in which 
they are united in varying degrees with one another or with mem- 
bers of another set or sets. 

The advances in these different lines have often not been at the 
same rate in the ancestry of any particular family or order, and 
related species in any such group may show very different degrees 
of advancement in floral structure. It is true also that in any single 
line of descent advance has gone on at different rates with reference 
to different sets of floral members; so that the flowers of any par- 
ticular species may be advanced in one respect and primitive in 

328. Stamens. A stamen (microsporophyll) consists usually of 
a more or less elongated stalk-like filament and an enlarged lobed 
anther which is borne at the apex of the filament. Within the anther 


are a variable number of pollen sacs (microsporangia). Very com- 
monly the young anther is two-lobed and each lobe contains two 
pollen sacs. Within each young pollen sac of an angiosperm, as 
within a microsporangium of Selaginella or of a gymnosperm, are 
produced a number of microspore mother cells (Fig. 340). These 
are surrounded by a conspicuous layer of nutritive cells. Each 


Microspore Mother Cells Nutritive Pollen Sacs * 

Layer (Microspor&ngia) 

FIG. 340. Diagrammatic cross section of an anther before the division of 
microspore mother cells. 

microspore mother cell, by two successive divisions (reduction 
divisions), gives rise to four microspores. While the microspore 
mother cells are dividing, the cells of the nutritive layer and some 
of the adjoining cells of the anther disintegrate. If there are two 
pollen sacs in each lobe, the tissue between them also often dis- 
integrates. While the microspores are developing into pollen grains 
(331), a longitudinal strip of special tissue is, in many angio- 
sperms, differentiated on the outer face of each lobe of the anther. 
The lengthwise splitting of the anther along this strip (Fig. 341) 
permits the escape of the pollen grains from the pollen sacs. In 
some plants, as in the potato, the anther opens by a terminal slit 
or pore. 

329. Pistils. A pistil is usually differentiated into three regions 
(Fig. 339): a swollen, hollow basal portion, the ovary; a narrow, 



more or less elongated portion, the style; and, at the apex or along 
the side of the style, the stigma. In certain flowers the pistil has no 

Mechanical Tissue 

Region of 

Pollen. Grains 
(Young "Micrggametophytes) 

Pollen Sacs 

FIG. 341. Diagrammatic cross section of an anther at the time of the libera- 
tion of pollen grains. 

style, and the stigma is attached directly to the upper portion of 
the ovary. 

A pistil consisting of one carpel (macrosporophyll) only is a 
simple pistil (Fig. 342, A). In such a pistil the margins of the carpel 


FIG. 342. Cross sections of ovaries, the united margins of the carpel or carpels 
being indicated by dotted lines. A, may apple; a simple ovary. B, violet; 
a compound ovary with a single cavity. C, lily; a compound ovary with as 
many cavities as carpels. 

are usually so united as in the bean or pea as to enclose a single 
cavity. A pistil may, however, be formed by the union of two or 



more carpels (Fig. 342, B, C). The ovary of such a compound pistil 
may enclose one or more than one cavity. For instance, the pistil 
of a lily or of a hyacinth is made up of three carpels united in such 
a way as to form three cavities within the ovary. In the pistil of 

E F G H 

FIG. 343. Diagrams showing, in lengthwise section, the development of an 
ovule (including macrosporangium) and macrogametophyte. A, very 
young ovule; the inner integument has appeared. B, both integuments 
are present; the macrospore mother cell is differentiated. C, the nucellus 
is nearly enclosed by the integuments; the macrospore mother cell has di- 
vided to form 4 macrospores. D-H, successively later; the stages of macro- 
gametophyte-development in />, E, F, (7, and H respectively correspond 
to those shown in A, B, C, D, and E, Figure 344. 

the violet, three carpels are so united that there is but a single 

Within the cavity or cavities of the ovary are one or more 
otiUles. Since the ovules are enclosed, pollen grains can not come 
into direct contact with them. This condition is very different 
from that in gymnosperms, whose pollen grains eventually reach 
the ovules. The difference in this respect is a fundamental one 
between gymnosperms and angiosperms. 

An ovule (Fig. 343) may arise from the base of the ovary or 
from tjie inner surface of a carpel. As in gymnosperms, the nucellus 
(macrbsporangium) is the first part of the ovule to develop. From 



the basal portion of the nucellus one or two integuments grow up 
and around it, leaving a passage-way (the micropyle) at the apex. 
Each ovule has a distinct stalk, the funiculus. 

330. Macrospores and Macrogametophyte. The most frequent 
course of events within an angiosperm ovule is the following. A 
single macrospore mother cell is differentiated within the nu- 
cellar tissue (Fig. 343, B). This cell by two successive divisions 



FIG. 344. A, beginning of the development of a macrospore (still 1-nucleate) 
into a macrogametophyte. J5-D, 2-, 4-, and 8-nucleate stages in the de- 
velopment of a macrogametophyte. E, mature macrogametophyte. All 
figures diagrammatic. 

(reduction divisions) gives rise to four macrospores which lie in an 
axial row within the nucellus (Fig. 343, (7). One macrospore, 
usually that farthest from the micropyle, develops into a macro- 
gametophyte; the other three macrospores disintegrate. The 
functional macrospore enlarges greatly (Fig. 344) and its nucleus 
divides; the two daughter nuclei eventually lie near the opposite 
ends of the cell; each of these daughter nuclei divides, and their 
daughter nuclei in turn divide. The macrogametophyte is now a 
large eight-nucleate cell, four of its nuclei lying in the micropylar 
end of the cell and four at the opposite end. One nucleus from each 
group of four now moves to the center of the macrogametophyte; 
and cell division occurs, the macrogametophyte being divided to 
form seven cells (Fig. 344, E). At each end are now three c^lls, 
each with a single nucleus; and in the central part is a large 0011 
containing two nuclei. The cells at the micropylar end ot* the 


macrogametophyte are the egg and two synergids; at the opposite 
end are three antipodal cells; and the large two-nucleate cell is 
the primary endosperm cell. 

From this history of macrospores and macrogametophyte there 
are, among the 150,000 or more known species of angiosperms, 
many variations. One of the more striking of these variations is 
found in the evening primroses, in which the mature macrogameto- 
phyte contains but four nuclei and three cells, instead of eight 
nuclei and seven cells. Another variant condition occurs in species 
of Peperomia, belonging to the pepper family; here the macro- 
gametophyte has sixteen instead of eight nuclei; the number of 
cells varies with the species. 

331. Microgametophyte. The development of a microspore 
into a microgametophyte begins w^hile the microspore is still 
within the pollen sac. The microspore divides to form a rela- 
tively large tube cell and a smaller generative cell (Fig. 345, B). 
Since a prothallial cell is usually not produced, the history of an 
angiosperm microgametophyte is shortened as compared with 
that of a microgametophyte of a gymnosperm. In most angio- 
sperms the two-celled microgametophyte (now a pollen grain) 
is liberated, and by the aid of winds, insects, or other agencies it 
may be carried to the stigma of the same or of another flower. 

After the pollen grain lodges on a stigma, the tube cell grows out 
as a pollen tube (Fig. 345, C-E). The tube elongates rapidly, 
growing through the style to the ovary, and finally to an ovule. 
In some plants the pollen tube may grow from stigma to ovule 
within a few hours. In the bean, for example, this growth requires 
eight to nine hours. The nucleus of the tube cell is located at this 
time a short distance from the growing end of the tube. The gener- 
ative nucleus divides into two male gamete nuclei. In some cases 
this nuclear division is followed by a division of the generative 
cell into two male gametes; in some angiosperms, however, the 
cell division is apparently omitted. The division of the generative 
nucleus, whether or not it is followed by a division of the genera- 
tive cell, occurs in some species within the pollen grain before the 
latter is liberated from the pollen sac; but more commonly this 
division occurs after the beginning of the growth of the pollen tube. 
In either case, the male gametes or gamete nuclei move down the 
pollen tube and come to lie a short distance behind the tube nu- 
cleus.* When a pollen tube reaches an ovule, it usually grows 



through the micropyle to the 
nucellus, through the nucel- 
lus, and into the micropylar 
end of the macrogameto- 

332. Pollination. Pollina- 
tion in most angiosperms is 
brought about by winds or by 
insects. Wind-pollinated spe- 
cies are chiefly thoee that 
grow in localities exposed to 
the wind, or those which grow 
close together in large num- 
bers. Among the smaller an- 
giosperms the grasses, which 
form extensive stands of one 
or a few species, as in mead- 
ows where the wind has a 
free sweep, are largely wind- 
pollinated. Many trees, 
whose flowers are high above 
the ground and are thus ex- 
posed to the wind, also are 
wind-pollinated. Wind-pol- 
linated flowers produce rela- 
tively large amounts of pollen 
and have large, and fre- 
quently rough or hairy, stig- 
mas on which pollen grains 
may lodge. 

Insect pollination has cer- 
tain advantages over wind 
pollination. It is more eco- 
nomical because the pollen- 
carrying insect commonly 
travels from flower to flower 
of the same species. Conse- 
quently, a particular pollen 
grain has a better chance of 
reaching a stigma, and it is 


Tube Cell 

Male Gamete 



FIG. 345. A microspore and its devel- 
opment into a microgametophyte. A, 
microspore. B, pollen grain (young 
microgametophyte) ; the microspore 
has divided into a generative and a 
tube cell. C, " germination" of a 
pollen grain; development of a pollen 
tube. D, the generative cell has 
moved into the pollen tube. E, the 
generative nucleus has divided into 2 
male gamete nuclei. All figures 


unnecessary for the plant to produce such enormous amounts of 
pollen as characterize wind-pollinated plants. Insect pollination 
is a method especially suitable for those species whose individual 
plants are more or less isolated. Another advantage is the greater 
opportunity offered for cross-pollination. 

Insects visit flowers to gather food, or in special cases to deposit 
their eggs. Certain insects, including many moths, gather nectar 
only; others, such as bees, collect and utilize as food both nectar 
and pollen. The structure of a flower is often such that, when an 
insect visits it, portions of the insect's body become dusted with 
pollen which may rub off against the stigma of the next flower 
visited. The relations of their flowers to insects have apparently 
been an important factor in the evolution of angiosperms; and one 
reason why angiosperms as a group have been so successful is the 
fact that many of them have secured the help of insects in pol- 
lination. The relations of insects to flowers have likewise been an 
important factor in the evolution of insects. These statements 
imply, not that the advantage of a particular structure of a 
flower or of an insect has been the cause of the appearance 
of that structure; but rather that, when a particular structure 
has once appeared and has proved useful, it has persisted. Hence 
the present great variety in form and structure among both 
flowers and insects may be in part accounted for by the interre- 
lations between insects and angiosperms, especially in respect to 

Salvia (Fig. 346) illustrates a rather high degree of structural 
correlation between flower and insect. The corolla of Salvia is 
tubular below; its upper portion is divided into two lips. The 
lower lip of the corolla serves as a landing-stage for insects visiting 
the flower; the upper lip constitutes a protective shield for the 
stamens and stigma. There are two stamens. The basal part of 
each filament is fixed; jointed to the upper end of this short basal 
part is a curved lever whose two arms are unequal in length. The 
shorter arm is sterile; the longer arm bears an anther and extends 
within the curved upper lip of the corolla. When a bee alights upon 
the lower lip and attempts to thrust its nectar-collecting append- 
ages into the corolla tube, its head presses against the short arm 
of the lever. When this sterile arm is depressed, the basal part of 
the filament serving as a fulcrum, the longer arm swings into 
contact with the hairy surface of the back of the insect. If the 



pollen in the anther is mature, it is dusted upon the bee's body. 
Smaller insects attempting to secure nectar are not strong enough 
to depress the short arm of the lever. In a young flower of Salvia* 
the style lies within the concavity of the upper corolla lip. As the 
flower matures, the style elongates and curves so that the stigma 
is midway between the upper and lower lips of the corolla. If a bee 
sprinkled with pollen 
visits such a flower in 
search of nectar, its 
pollen-dusted back rubs 
against the stigma and 
there deposits some of 
the pollen. 

333. Gametic Union. 
As the tip of a pollen 
tube enters a macro- 
gametophyte, it en- 
larges somewhat, its end 
bursts, and some of its 
contents, including the 
male gamete nuclei, are 
discharged into the mac- 
rogametophyte (Fig. 
347). One male nucleus 
enters the egg and unites 
with its nucleus, so 
forming a zygote with 
2 n chromosomes. The 
other male nucleus 
passes to the primary 
endosperm cell and 
unites with the two 

FIG. 346. Pollination of Sal via. A, flower into 
which a bee has entered. B, C, lengthwise sec- 
tions of a flower before and after the bee 
enters, showing the effect of its entrance upon 
the position of a stamen; arrows indicate the 
direction taken by the bee. After Kerner. 

nuclei of that cell; sometimes these two nuclei have united before 
the male nucleus reaches them. The union of three nuclei in the 
primary endosperm cell is a feature peculiar to the angiosperms. 
Since these nuclei are gametophytic, the nucleus formed by their 
union contains 3 n chromosomes. On the basis of its chromosome 
number, therefore, the primary endosperm cell, after this union of 
three nuclei, is neither gametophytic (with n chromosomes) nor 
sporophytic (with 2 n chromosomes). 



334. Development of a Seed. Just as in a gymnosperm, the 
structure from which the seed of an angiosperm is matured (the 
ovule) is well^ along in its development at the time of gametic 
union. As in a gymnosperm, also, the zygote of an angiosperm 
develops into the embryo of the mature seed, and the integument 
(or integuments) mature into the seed coat (or coats). Differ- 
ently from the condition in gymnosperms, the macrogametophyte 



'Endosperm Cell 


Male Gamete 
Nucleus Uniting 
With Egg Nucleus 

Male Gamete 

Nucleus Uniting 

With Primary 

Endosperm Nudeus 


FIG. 347. Mature macrogametophyte of prickly lettuce. One male gamete 
nucleus is uniting with the egg nucleus, 1 with the primary endosperm 
nucleus (the latter previously formed by a union of 2 nuclei). Drawing by 
K. L. Mahony. 

(the ^-chromosome generation) does not persist as a tissue in 
which most of the reserve food is stored. Instead, the primary 
endosperm cell, now containing, as has just been seen, a nucleus 
with 3 n chromosomes, develops into a temporary or permanent 
tissue of the maturing seed. The development of a seed shows 
considerable variation in different angiosperms, although certain 
features of this development are common to all species. 

The shepherd's purse (Capsella) represents a type of seed- 
development commonly found in dicotyledons. Before the gametes 
unite, the ovule of Capsella contains an elongated, somewhat 
crescent-shaped seven-celled macrogametophyte within a thin 
nucellus (macrosporangium), which in turn is surrounded by 
two integuments. Shortly after gametic union the ovule grows 
rapidly, its growth being due mainly to an extraordinary increase 
in size of the primary endosperm cell. This enlargement of the 
primary endosperm cell involves the development of a large central 
vacuole, the dense cytoplasm thus being limited to a peripheral 





layer which is thickest in the regions at the ends of the cell. As 
a result of the division of the primary endosperm nucleus and of 
the repeated division of nuclei derived from it, the cell finally 
contains many nuclei, each with 3 n chromosomes, which are 
fairly evenly distributed throughout the dense cytoplasm (Fig. 348). 
Considerably later, by cell 
division, this large many- 
nucleate cell becomes a 
many-celled endosperm. 

This tissue should not 
be confused with the endo- 
sperm of a gymnosperm, 
which is the macrogame- 
tophyto and each of whose 
cells has n chromosomes. 
The endosperm of an an- 
giosperm is a new structure 
developed after gametic 
union, whose cells are 
ordinarily marked by the 
presence of 3 n chromo- 
somes. Despite these 
differences, the endosperm 
has the same function in 
both gymnosperms and an- 
giosperms that of provid- 
ing for the nutrition of the 
young sporophyte. 

The endosperm of Cap- 
sella is in time almost en- 
tirely digested and ab- 
sorbed by the developing 
embryo. In some angio- FIG. 348. Capsella; an immature seed con- 
sperms, the endosperm per- tainin s a y un s embr y- 

sists to form a relatively large portion of the mature seed ; in these 
cases the endosperm constitutes a tissue filled with reserve foods 
which will be used by the embryo during the germination of the 

While the endosperm is being formed, the zygote of Capsella 
develops into a many-celled structure, the embryo or young sporo 



Basal Cell 



phyte. The development of the embryo begins with the division 
of the zygote and of its daughter cells to form a short, few-celled 
filament (Fig. 349, A). The cell of this filament farthest from the 
micropyle is the one which will form most of the body of the em- 
bryo; the other cells of the filament constitute a suspensor whose 


Basal Cell 

FIG. 349. Development of an embryo of Capsella. A, B, early stages. C, D, 
later stages, showing the differentiation of hypocotyl and cotyledons. 
E, F, other embryos of the dicotyledonous type. 

growth pushes the terminal cell toward the center of the primary 
endosperm cell. The suspensor is never more than a few cells in 
length; its basal cell is always larger than its other cells and is 
imbedded in the micropylar end of the nucellus. The terminal 
cell of the filament gives rise by divisions to two tiers of four cells 
each (Fig. 349, B). It is at this stage that differentiation of 
the parts of the embryo takes place the tier of four cells nearer 
the suspensor by repeated division arid growth developing into the 
major portion of the hypocotyl, the four cells farther from the 
suspensor similarly forming the cotykdons and epicotyl. 

In the terminal portion of the developing hypocotyl the dif- 
ferentiation of tissue regions occurs that is characteristic of a 
root tip. A root cap which covers, and is continuous with, the 
end of the hypocotyl is formed from the adjacent cells of the sus- 



The group of cells from which the cotyledons and epicotyl are 
to develop is for a time a hemispherical mass. At a later stage, 
cell division and growth go on most rapidly at two points near the 
apex of this mass. As a result, two projections appear which are 
the primordia of the cotyledons 
(Fig. 349, C). The apex of the 
cell mass between the young coty- 
ledons remains substantially un- 
changed for a considerable time. 
Later, however, when the cotyle- 
dons have become comparatively 
large, cell division and growth are 
resumed at this point, and so a 
small projection, the epicotyl, later 
to produce most of the aerial por- 
tion of the plant, is formed be- 
tween the bases of the cotyledons. 
When young, the two cotyledons 
are widely divergent; as the de- 
velopment of the embryo continues 
they become more nearly parallel; 
still later, inequalities of growth 
on their respective sides cause both 
cotyledons to fold over on one side 
of the embryo, and to grow parallel 
to the hypocotyl (Fig. 349, F). 
During the later stages in the development of the embryo, its cells, 
particularly those of the cotyledons, become filled with reserve 
foods. In some dicotyledons, such as the bean, the cotyledons 
become conspicuously enlarged. 

A type of embryonic development characteristic of many mono- 
cotyledons is found in the arrow-head (Sagittaria, Fig. 350). Here 
the zygote develops into a row of three cells, the basal one of which 
is much the largest and does not divide; the median cell by division 
and growth develops into a suspensor, a hypocotyl, and a lateral 
epicotyl; and the terminal cell into a single terminal cotyledon. 

During the development of most angiosperm seeds, while the 
endosperm and embryo are growing, the nucellus, antipodal cells, 
and synergids disappear, and the integument or integuments be- 
come modified to form the protective seed coat or coats. All these 

FIG. 350. Development of a mon- 
ocotyledonous embryo (Sagit- 
taria). A-D, early stages; 
redrawn from Schaffner. E, 
mature embryo. 



changes by means of which an ovule and its inclusions develop 
into a seed take place within the ovary. 

335. Fruits. Angiosperms differ from gymnosperms in the de- 
velopment of a carpel (macrosporophyll) or carpels into a pistil 
whose lower portion, the ovary, encloses the ovules. Another 

FIG. 351. Life cycle of an angiosperm. 

feature peculiar to the angiosperms is the development of the 
ovary and its contents into a fruit. 

Pollination and gametic union influence the subsequent devel- 
opment both of the ovules and of the ovary enclosing the ovules. 
Maturation of the tissues of the ovary into a mature fruit may or 
may not involve a very considerable increase in size. The different 
parts of the ovary in different species mature into tissues and 
structures of varied nature. Different angiosperms, therefore, dis- 
play great variation in the size, form, and structure of their mature 
fruits. Development of a fruit often involves other parts of the 
flower than the ovary, such as the sepals and the receptacle. A 


true fruity however, includes only structures derived from the 

When fruit and seed are mature the seed is shed, either sepa- 
rately or still enclosed by the fruit. Under appropriate conditions 
the seed may germinate and the embryo resume development into 
a mature sporophyte. 

336. Classes of Angiospenns. Angiosperms are divided into 
two classes, dicotyledons and monocotyledons. These classes are 
named from the numbers of cotyledons which respectively char- 
acterize their embryos; the embryo of a dicotyledon (such as 
Capsella) having ordinarily two cotyledons, that of a monocotyle- 
don (such as Sagittaria) ordinarily only one. Attention has already 
been called (Chaps. IV -VII) to certain characteristic differences 
between these two classes in the structure of the plant body (the 
sporophyte). These and other features which chiefly distinguish 
dicotyledons and monocotyledons may be summarized as follows: 

(a) The embryo of a dicotyledon has typically a terminal epi- 
cotyl and two lateral cotyledons; the embryo of a monocotyledon 
has typically a lateral epicotyl and a single terminal cotyledon. 

(6) Most dicotyledonous stems are characterized by the pres- 
ence of a single cylinder of vascular bundles; in most monocotyle- 
donous stems the bundles are scattered throughout the stele. 

(c) Each vascular bundle in a dicotyledonous stem possesses a 
cambium; that of a monocotyledonous stem lacks a cambium. 

(d) A dicotyledonous root has, as a rule, but few ridges radiating 
from the central mass of primary xylem ; a monocotyledonous root 
usually has numerous xylem ridges. 

(e) A dicotyledonous root develops a cambium; a monocotyle- 
donous root rarely forms one. 

(/) The leaves of dicotyledons are netted-veined; those of mono- 
cotyledons are usually parallel-veined. 

(g) The parts of a dicotyledonous flower (sepals, petals, sta- 
mens, and carpels) are very commonly in fours or fives or in mul- 
tiples of four or five; in monocotyledons, the floral parts commonly 
occur in threes or in multiples of three. 

There is no characteristic difference between dicotyledons as a 
class and monocotyledons as a class in the structure or the develop- 
ment of the gametophytes. 

337. Angiosperms and Gymnosperms. Angiosperms (including 
dicotyledons and monocotyledons) and gymnosperms are alike in 


the essential points that distinguish the seed plants from plants 
below them in the evolutionary scale. Angiosperms, however, 
differ from gymnosperms in the following respects: 

(a) In the presence of sepals, petals, or both, in addition to 

(6) In the development of the macrosporophyll or macrosporo- 
phylls into a pistil. 

(c) In some further reduction of the microgametophyte. 

(d) In the lodging of the young microgametophyte (pollen 
grain) on the stigma at some distance from the macrosporan- 

(e) In a marked reduction of the macrogametophyte to a few- 
celled structure. 

(/) In the functioning of a male gamete in initiating the develop- 
ment of the endosperm. 

(g) In the development of a fruit. 

(h) Typically, in the presence of vessels in the xylem. 

338. Progress from Bryophytes to Angiosperms. In the evolu- 
tionary series leading from bryophytes through pteridophytes to 
seed plants, there has been no modification of the most fundamen- 
tal features of the life cycle. Throughout this series the gametes 
unite to form a zygote that develops into a sporophyte whose dis- 
tinctive function is the production of spores. A spore, in turn, 
develops into a gametophyte whose distinctive function is the 
production of gametes. Throughout the entire series, also, the 
chromosome number is doubled when gametes unite, and halved 
when spores are formed. * 

Despite these basic similarities, however, the series from bry- 
ophytes to seed plants displays certain evolutionary tendencies 
that culminate in the angiosperms. 

(a) The gametophyte of a bryophyte is the larger, independent 
generation; in a fern it is small but still independent; in Selagineila 
and a few other pteridophytes it is much reduced and essentially 
parasitic upon the sporophyte; in seed plants it is still more reduced 
and entirely parasitic, reaching the extreme in these respects in 
angiosperms. The sporophyte has followed the opposite evolution- 
ary course; small and largely parasitic in bryophytes, in ferns it 
is the larger generation, parasitic only in an embryonic stage; in 


Selaginella it is but briefly parasitic upon the very small macro- 
gametophyte; in gymnosperms the same is true; but in angiosperms 
the nutritive function of the macrogametophyte has been trans- 
ferred to a new 3 n-chromosome structure, the endosperm. In 
addition, in both gymnosperms and angiosperms the old sporo- 
phyte has become partially responsible for nourishment and pro- 
tection of the young sporophyte. 

(b) In bryophytes and pteridophytes gametic union depends 
upon the presence of water to enable male gametes to reach the 
eggs. In seed plants the male garnetophyte has developed a new 
structure, the pollen tube, which insures the transport of male 
gametes to the neighborhood of an egg. Male gametes, still motile 
in the more primitive gymnosperms such as Zamia, in most gymno- 
sperms and in angiosperms have lost their flagella and much of 
their independent motility, depending upon the pollen tube to 
supply a pathway to the female gametophyte. 

(c) In bryophytes and most pteridophytes all spores are sub- 
stantially alike in size. In certain bryophytes spores are sexually 
differentiated in the sense that some are destined to give rise to 
male, some to female, gametophytes. This is not the case, how- 
ever, in many bryophytes or in most ferns. In Selaginella and a 
few other pteridophytes, spores are of two distinct sorts: large 
(female) arid small (male). Sexual differentiation in Selaginella 
does not, however, begin with the spores; it has been pushed back 
to the sporangia, structures of the sporophyte. Although the 
sporangia are asexual reproductive structures, they are sexually 
differentiated. In gymnosperms sexual differentiation extends 
back to the sporophylls and strobili. In angiosperms with sep- 
arate pistillate and starninate flowers this differentiation affects 
all the structures of the flower; and in some angiosperms with 
separate pistillate and staminate plants the whole sporophyte is 
sexually differentiated. 

(d) In most bryophytes and pteridophytes a spore develops into 
a gametophyte after it has left the sporangium. In some species 
of Selaginella the production of two kinds of spores is correlated 
with the fact that a spore of the larger type develops into a macro- 
gametophyte entirely within the macrosporangium. Shortly after 
gametic union the macrogametophyte is liberated. In a gymno- 
sperm the macrogametophyte is not similarly liberated. Con- 
sequently, during its early development the sporophyte of a gymno- 


sperm is surrounded both by the macrogametophyte and by 
structures of the old sporophyte. 

Permanent retention of macrogametophyte within sporangium, 
the beginning of the development of the zygote into a new sporo- 
phyte while still enclosed within both structures, and the matura- 
tion of the integument, or of nucellus and integument, into pro- 
tective tissues mark the appearance of a wholly new structure 
the seed. Another novel feature connected with seed-development 
is the temporary cessation of growth of the new sporophyte at a 
certain stage. The seed of an angiosperm shows a further advance 
in that the macrogametophytic tissue is obliterated after gametic 
union, reserve foods for the young sporophyte being stored in a 
newly developed tissue, the endosperm, or in the young sporo- 
phyte itself. 



339. Nature of Seeds. A seed is a matured ovule. During its 
maturation, the zygote develops into an embryo, the primary endo- 
sperm cell into a nutritive tissue (the endosperm); and the integu- 
ment or integuments develop into a seed coat or seed coats. 
Development of the embryo is initiated by the union of a male 
nucleus with the egg nucleus; that of the endosperm, by the union 
of a male nucleus with the two nuclei of the primary endosperm 
cell. Development of an integument into a seed coat is a second- 
ary phenomenon resulting from a stimulus arising in the develop- 
ing embryo, endosperm, or both. The endosperm may persist in 
the seed, or it may be absorbed by the developing embryo. In 
the seeds of a few plants a portion of the nucellus persists as a 
food-storage tissue. 

Seeds of angiosperms differ widely in structural details. The 
two classes of angiosperms, dicotyledons and monocotyledons, are 
named from a striking difference between their respective seeds. 
The embryo of a dicotyledon has two approximately equal, later- 
ally placed cotyledons, between which is a terminal epicotyl; the 
embryo of a monocotyledon typically has one large cotyledon, 
apparently borne terminally, and a laterally placed epicotyl. 
Many angiosperm seeds, such as those of corn, contain a consider- 
able amount of endosperm when mature. In the development of 
the seeds of other angiosperms, such as the shepherd's purse and 
bean> the endosperm, although formed, is quickly absorbed, and 
reserve foods are stored in the cotyledons instead of in an endo- 
sperm. In general the mature seed of a monocotyledon contains 
an endosperm; the mature seeds of some dicotyledons possess 
endosperms, those of others do not. 

340. Structure of Seeds. On the concave edge or face of a bean 
seed (Fig. 352) is a fairly large scar, the hilum, marking the former 
point of attachment of the seed to the short stalk (funiculus) 
which connected the ovule with the edge of the carpel. Near one 
end of the hilum is the micropyle. There are two seed coats de- 




veloped respectively from the two integuments, the inner coat 
being somewhat thicker and heavier than the outer, and the two 
more or less firmly united. The embryo occupies all the space 
within the seed coats. It has two large, thick, firm cotyledons, 
closely appressed and enclosing the epicotyl which bears two op- 
posite overlapping immature foliage leaves. The hypocotyl lies 
outside the cotyledons and is bent backward along the line of 


FIG. 352. Seed of bean. A, side view. B, as seen from the inner (attached) 
edge. C, from the outer edge. />, embryo; seed coats and 1 cotyledon 

meeting of the cotyledons on the concave edge of the seed. The 
cotyledons contain large reserves of starch and proteins, as well 
as some sugars and fats. 

A seed of a lily is broad and flat. Its seed coats are thin and 
membranous, sometimes forming a narrow, wing-like expansion 
about the entire circumference of the seed. Within the seed coats 
is a firm, starchy endosperm, in whose center the relatively small 
embryo is imbedded (compare the iris seed, Fig. 353). The embryo 
is long and narrow, nearly cylindrical, and slightly curved. The 
hypocotyl is near the micropylar end of the seed and frequently 
projects a short way beyond the surrounding endosperm. There 
is a long, massive cotyledon, partly surrounding a small epicotyl. 
The embryo contains some fats and proteins. The endosperm is 
rich in starch. 

341. Power of Seeds to Germinate. One great difference be- 
tween a seed and most other plant organs, such as root, stem, 
or leaf, is that in the seed the processes characteristic of living 
matter may go on very slowly. Dormant seeds respire, but res- 



piration in an air-dry seed is almost infinitesimal in amount as 
compared with respiration in a germinating seed or in a stem or 
leaf. The partial suspension of activity in a seed results primarily 
from its comparative dryness. During the maturing of a seed the 
greater part of its contained water has been lost. In most 
cases the seed coat is relatively impermeable to both water and 
oxygen, so that no new supply of water can enter. Although the 

Seed Coat 

FIG. 353. A, B, surface view and cross section of a fruit of iris. C, lengthwise 
section of an iris seed. 

amount of water in seeds is usually small, some water must be 
present if the embryo is to survive, and this small amount of 
water is tenaciously held. 

It is commonly said that a seed is ripe at its separation from the 
parent plant, but this ripeness is not necessarily coincident with a 
readiness to germinate. Many seeds germinate at maturity or 
shortly thereafter if conditions arc suitable. This is especially 
true of short-lived plants, including many crop plants, whose 
seeds usually germinate as soon as they are scattered. Such power 
of early germination is sometimes a disadvantage; in warm, moist 
autumns, for example, the seeds of corn may germinate while 
still upon the ear. 

However, many seeds must undergo after-ripening before ger- 
mination can occur. Among other things, after-ripening involves 
changes in the acidity of the seed contents, the formation of 
enzymes, and the digestion of stored foods. In some seeds these 
changes require days; in others, weeks, months, and even years 
are necessary. In certain cases the prolonged postponement of 


germination is due to the hardness and thickness of the seed coats. 
Variations in permeability of seed coats ma/ account for the fact 
that often some seeds germinate long before others of the same 
species. This latter condition, in annuals, insures the persistence 
of the species, even though a particular season proves unfavorable 
for germination. 

The length of time during which seeds remain capable of ger- 
mination varies greatly. At one extreme are those of willows and 
poplars, which must germinate within a few days or not at all. 
Some acorns will not germinate after a year; coffee beans, not 
after six months. Among common crop plants, tobacco has proba- 
bly the longest-lived seeds; tobacco seeds 20 years of age have 
germinated. Seeds of certain species from dated herbarium col- 
lections have germinated after 87 to 130 years. The seeds of the 
pulse family, whose seed coats are notably impermeable to w r ater 
and gases, are probably longest-lived, some retaining their vitality 
for 150 to 250 years. However, despite these extreme cases, no 
seeds can remain alive indefinitely. The stories told of seeds that 
germinated after being for thousands of years in Egyptian tombs 
are quite without foundation. 

The ability of a seed to remain capable of germination depends 
both upon the structure of the seed coat and upon the nature of 
the substances wdthin the seed. Seeds rich in enzymes quickly 
lose their power of germination. Those rich in fats do not survive 
as long as do those whose reserve foods consist largely of starch. 
A low water content, which reduces the rate of respiration and of 
other processes to a minimum, is responsible for the longevity of 
many seeds. 

Although the presence of water is essential to germination, 
submergence in water for any great length of time results in the 
death of many seeds, including those of rye, oats, and corn. Seeds 
of not a few water plants, however, can withstand submergence 
for years, probably because of the extreme resistance offered by 
their seed coats to the penetration of water. If deeply buried in 
soil, many seeds retain the needed water and the power of germi- 
nation almost indefinitely, Indian lotus seeds buried in the mud 
of Manchuria for 200 years have germinated after treatment with 
certain chemical substances which modified their permeability to 
water and gases. Many plants become weed pests because of the 
great longevity of their buried seeds. 


342. Germination (Fig. 354). Germination depends upon certain 
external factors; of these the most important are the presence of 
water and of oxygen and a suitable temperature. Water is essential 
to the expansion of certain parts of a seed, as well as to the initia- 
tion of activities within, including the digestion of stored food and 
its translocation to the parts of the embryo where it is to be 
utilized. When a bean seed is planted, water does not enter at 
equal rates over the entire surface of the seed, but enters most 
readily through the region of the hilum. This fact is shown by a 
wrinkling of the seed coat first in the neighborhood of the hilum. 
The entrance of water results from an actual imbibition by the cell 
walls of the seed coat in the region of the hilum, followed by a 
transfer of the water by imbibition and osmosis into interior cells. 

Temperature is an important factor, since up to a certain point 
the higher the temperature the more rapid are various processes 
within cells. Assimilation and respiration are much more active 
at 20 than at 5 C. Temperatures for the germination of most 
seeds range from 3 to 49 C., the optimum being about 33 C. 
The minimum temperature at which seeds will germinate varies 
greatly as between different species. 

Light has no direct or indirect influence upon the germination 
of most seeds, although some are sensitive to light. 

Oxygen is essential to the respiration of a developing seedling, 
both before and after it emerges from the seed coat. A germinating 
seed is the seat of a series of processes, all involving the expenditure 
of energy. Since the energy to be used in these processes must be 
released by respiration, and since, therefore, respiration is charac- 
teristically rapid in germinating seeds and seedlings, a considerable 
supply of oxygen is necessary. 

When germination begins, the imbibition of water by the em- 
bryo and endosperm causes a swelling of these structures, which 
expands and finally ruptures the seed coat. In some cases the 
breaking of the seed coat is irregular, as in the bean; in others it 
takes place along definite lines. The seed coat of a germinating 
squash seed is broken first at its narrow end. The earliest growth 
of the embryo involves chiefly an enlargement of already existing 
cells as a result of the intake of water, rather than a formation of 
new cells by division. Growth is at first largely localized in the 
hypocotyl, which elongates and soon emerges from the seed coat. 
Seeds of many plants are so constituted that the part of the seed 


containing the hypocotyl is that which is most likely to be turned 
toward the soil. For the majority of seeds, however, this is not 
the case, and it is a matter of chance whether the side of the seed 
from which the hypocotyl emerges is toward or away from the soil. 
In any case, the primary root, which constitutes the greater part 
of the hypocotyl, turns to whatever extent is necessary to enable 
it to grow downward. This bending is due to the strongly positive 
geotropism of the hypocotyl. 

At least after the first stages of germination, the growth of the 
hypocotyl and of other parts of the embryo involves the formation 
of new cells by division as well as the enlargement of already exist- 
ing cells. The formation and growth of new cells necessitate the 
use of foods. Since the seedling as yet has no chlorophyll-containing 
cells and hence can not carry on photosynthesis, it is dependent 
for the foods needed in its growth as well as for those utilized in 
respiration upon the reserves stored in the seed. If, as is frequently 
the case in dicotyledons, reserve foods are stored in the cotyledons, 
these foods are digested by enzymes produced in certain parts of 
the embryo. The digested foods are then translocated to the grow- 
ing portions of the seedling where they arc to be utilized. In seeds 
containing endosperm, the secretion of enzymes and the absorption 
of digested foods from the endosperm are brought about largely 
or entirely by the cotyledon or cotyledons. This is especially true 
of members of the grass family such as wheat and corn, in which 
the cotyledon is a digestive and absorptive organ that never 
emerges from the seed coat. 

During its further growth the hypocotyl frequently becomes 
arched in such a way as to pull the cotyledons out of the seed coat. 
This arching is well illustrated in the seedlings of the common bean 
(Fig. 354, A). In the development of the seedlings of the squash 
and of some of its relatives, the removal of the cotyledons from 
the seed coat is assisted by a peg-like outgrowth from the hypocotyl 
(Fig. 354, B). Sometimes, as in the castor bean, the arching and 
elongation of the hypocotyl carry upward into the air the cotyle- 
dons, still enclosed by the seed coat ; later the hypocotyl straight- 
ens, and the seed coats are removed in consequence of the growth 
of the cotyledons themselves. The arching of the hypocotyl and 
its later straightening result from a negative rather than a positive 
geotropism of the portion of the hypocotyl in the vicinity of the 
cotyledons. Sometimes, as in the pea and the scarlet runner bean, 



the hypocotyl remains short and unarched; the cotyledons, like 
the single cotyledon of the corn, never emerge from the seed coat, 
and the shoot which issues above ground and produces foliage 
leaves is developed entirely from the epicotyl. 

In those cases in which the cotyledons are withdrawn from the 
seed coat and pushed above the soil, they form more or less chloro- 
phyll arid to some extent function as foliage leaves. Often, however, 
as in the common bean, the cotyledons are thick and soon shrivel 

FIG. 354. A, stages in the germination of a bean seed. B, germinating seed of 
squash; the emergence of the embryo from the seed coat is assisted by a 
peg-like outgrowth of the hypocotyl. 

and are dropped off. Cotyledons of other seedlings, such as those 
of the castor bean, become flat, expanded leaves which persist and 
function for some time in photosynthesis. As a rule, however, 
cotyledons are different in form from the leaves developed by the 
epicotyl, often being smaller and simpler. 

If the cotyledons emerge from the seed, the epicotyl is brought 
out with them. If, as in the corn and the pea, the cotyledon or 
cotyledons do not emerge, the epicotyl is the last part of the em- 
bryo to be freed from the seed coat. When the epicotyl emerges, 
its structures, which are to develop into all or nearly all the aerial 



parts of the plant, are in a very immature state. The stored foods 
in the seed must therefore be chiefly relied upon by the seedling 

until the stem and its leaves 
have developed sufficiently to 
make the plant independent. 

343. True and False Fruits. 
The changes by which an ovule 
and its inclusions develop into 
a seed within an ovary are ac- 
companied by a metamorpho- 
sis of the ovary into a fruit. 
A true fruit, in the strict in- 
terpretation of the term, is a 
structure developed solely from 
an ovary containing one or 
more seeds. Often, however, 
the development of a fruit in- 
volves parts of the flower other 
than the ovary, such as the 
sepals, the receptacle, and the 
floral axis. A structure of this 
nature, not derived in its en- 
tirety from the ovary, is called 
a false fruit. The apple is a 
false fruit, since its fleshy por- 
tion is derived mainly from the 

344. Simple Fruits. When 
but a single ovary, with or 
without surrounding struc- 
tures, develops into a fruit, 
whether true or false, the fruit 
is simple. At the time of 

pollination and of gametic union the ovary walls of all species 
are essentially alike in that they consist of tissues which, except 
for vascular bundles and provascular strands, are homogeneous in 
structure. The growth and maturation of the ovary wall after 
pollination result, however, in the development of fruits which 
differ markedly with respect to the tissues that compose them. 
Simple fruits are either fleshy or dry. 

FIG. 355. A, the tomato, a berry 
whose ovary wall has become greatly 
thickened and juicy. B, the same in 
cross section. The fruit here shown 
was developed from 5 carpels. 



346. Fleshy Fruits. If the fruit into which an ovary matures is 
one in which the ovary wall (at least its inner portions) and the 
interior structures of the ovary are enlarged and juicy, the fruit 
is a berry. The seeds, each with a hard coat of its own, are imbedded 
in the juicy flesh of the fruit. A berry may develop from an ovary 
composed either of a single carpel or of more than one carpel. The 
tomato, orange, and grape are berries. The tomato (Fig. 355) is 
a large berry consisting in primitive forms of two carpels, although 
in many cultivated varieties there are twelve or more. The fleshy 
portion of the fruit is developed from the qvary wall, the very 
greatly enlarged ovule-bearing ridges, and the partitions between 
the carpels. Citrus fruits, including orange, lemon, and grapefruit, 


FIG. 356. The plum, a stone fruit (immature). 

are berries of a type with a tough, leathery rind. Each section of 
an orange or of a grapefruit represents a carpel, the carpels being 
firmly attached to one another at their outer surfaces but readily 
separable along their lateral faces. Except for the space occupied 
by the seeds, each carpel is filled with many small hair-like out- 
growths arising from its inner surface. As the fruit matures, these 
outgrowths become filled with juice. The date is a berry with a 
single hard seed, the fleshy part having developed from the ovary 

Several false fruits also are classified as berries. The currant 
and gooseberry are of this sort. The fleshy part of each of these 
fruits is developed largely from the receptacle. In a blueberry or 
cranberry the calyx tube forms a part of the juicy portion of the 
fruit. A banana is a berry whose "peel" has been developed from 





the receptacle. Melons and squashes are false fruits of the berry 
type. In these, the receptacle is closely united with the ovary wall. 

In a stone fruit (drupe), such as a plum 
(Fig. 356), peach, or cherry, the outermost 
layers of the ovary wall form the skin; the 
layers next within become fleshy or fibrous; 
and the innermost layers, becoming hard 
and stony, form the "pit" which encloses 
the comparatively soft, thin-coated seed. 
The walnut and coconut are stone fruits, 
the hard shell of the so-called "nut" in 
each case being formed from the inner 
layer of the ovary wall. 
During the maturation of the apple type of fruit (pome), the 
petals and stamens wither and fall away; the upper portion of the 
style withers also but 
may remain attached. 
In the apple the car- 
pels are more or less Fruit Coat- 
firmly united with one 
another, at least at 
their inner edges, and 
are completely sur- 
rounded by, and united 

FIG. 357. Cross section 
of a fruit of buckwheat. 

~ -Cotyledon 


with, the receptacle. 

The outer portion of 
the ovary wall consti- 
tutes part of the pulp 
of the apple; the inner 
part of the ovary wall 
is leathery, and each 
division of the core, 
corresponding to one 
carpel, usually con- 
tains two, sometimes 
more, seeds. The 
greater part of the 
fleshy tissue is devel- 
oped from the receptacle. The sepals frequently remain at- 
tached at the upper end of the mature fruit. 



Root Cap 

FIG. 3,58. Lengthwise section of a fruit (grain) 
of corn. 



FIG. 359. 

Fruits (samaras) of 

346. Dry Fruits. Instead of becoming fleshy, an ovary may 
mature into a dry, more or less hard fruit. There are two general 
types of dry fruits: indehiscent fruits which do not split open at 
maturity, and dehiscent fruits which regularly split at maturity 
and expose the contained seed or seeds. As a general rule, an in- 
dehiscent dry fruit contains a single seed and a dehiscent dry 
fruit contains more than one seed. 
If an indehiscent dry fruit contains 
but one seed, the seed being attached 
to the fruit wall at only one point, 
the fruit is an achene. Achenes are 
produced by the buckwheat (Fig. 
357), sunflower, and buttercup. Since 
the fruit wall of an achene is merely 
a thin, dry layer enclosing the seed, 
the fruit is seed-like in appearance. 
The achenes of many plants are com- 
monly called "seeds." 

If the thin, transparent fruit wall 
is attached at all points to the seed 
coat, the dry fruit is a grain. Fruits of this type are produced by 
many members of the grass family, including the corn (Fig. 358), 
wheat, arid other cereals. In a corn grain an abundant endosperm 
completely fills the seed coat except for the space at one side that 
is occupied by the embryo. The greater part of the embryo con- 
sists of a broad cotyledon whose infolded 
edges almost completely enclose the epi- 
cotyl. The small epicotyl bears several 
immature leaves, the oldest one of which 
constitutes a sheath that encloses the 
younger leaves and the epicotyl. Extend- 
j n g j n a direction opposite to that taken by 
the epicotyl is a small hypocotyl, sur- 
rounded by a special sheath and partly 
surrounded by the large cotyledon. The endosperm consists of an 
opaque starchy and a translucent horny portion, the latter con- 
taining the major part of the protein foods. The embryo contains 
reserves of fats, sugars, some proteins, and small amounts of starch. 
In some cases the margin or the apex of an indehiscent dry fruit 
develops into a wing-like structure. A fruit of this type is known 

FIG. 360. Fruits (nuts) 
of hickory, each con- 
taining 1 seed. 



as a samara (Fig. 359). The maple, ash, and elm bear samaras. In 
the flower of a maple two ovaries are borne side by side, a com- 
pound style arising at the midpoint of the upper edge. The outer 
upper angle of each ovary is extended and flattened into a wing. 
After the union of gametes, the ovaries enlarge greatly and the 
wings grow in length and breadth, finally becoming dry and papery. 

FIG. 361. Fruit (legume) and embryo of pea. A, immature fruits. B, surface 
view of a mature fruit. C, mature fruit split open. D, embryo, with 1 
cotyledon removed. 

The two ovaries (now fruits) fall from the tree still attached to 
each other, or they may split apart and fall separately. 

A nut is similar in structure to an achene, but it is of larger size 
and it frequently develops from an ovary containing more than one 
ovule. In the latter case, only one ovule matures into a seed. In 
nuts, as in the hickory nut (Fig. 360), chestnut, hazelnut, or acorn, 
the shell is the fruit coat and the softer edible portion within is the 
seed. Some structures commonly known as "nuts" are something 
less than the whole fruit. An almond corresponds to the stone of a 
drupe, the outer fibrous part of its fruit coat having dried and fallen 
off. A coconut resembles an almond in that the hard shell is the 
inner part of the fruit coat. Some true seeds are also commonly 
called "nuts." A Brazil nut is a seed, 18 to 24 such seeds being 
borne in a single fruit. A horse-chestnut is also a seed; the fruit, a 
prickly capsule, contains 1 to 3 seeds. 

A compound pistil, each carpel enclosing a single ovule, may 
mature into an indehiscent fruit whose parts (the carpels) separate 
from one another at maturity. The fruits of mallow and geranium 



and of members of the parsley family are examples of this type, 
the schizocarp. 

Dehiscent dry fruits may be developed from either simple or 
compound pistils. One type of dehiscent dry fruit is the legume, 
characteristic of the bean, pea (Fig. 361), and other members of 
the pulse family. A legume develops from a pistil consisting of a 
single carpel whose edges are united. Seeds are borne attached 
alternately to the two edges; but, since the edges are united, the 
seeds lie in a nearly straight line. However, when a legume splits 
open, some of the seeds re- 
main attached to one edge 
and some to the other. The 
fruit of the peanut forms an 
exception in that it does not 
open at maturity. It is, how- 
ever, a legume and not a nut. 

A follicle is like a legume 
except that it splits longitu- 
dinally along but one edge, 
whereas a legume splits along 
two opposite edges. The 
fruits of milkweed and col- 
umbine are follicles. 

A capsule also is a dry fruit 
which cracks or breaks open 
at maturity; differently from 
a legume or follicle, it is de- 
veloped from an ovary com- 
posed of more than one carpel. 
Capsules open in various ways when the seeds are mature; most 
commonly they split lengthwise into a definite number of seg- 
ments, the number corresponding to the number of carpels. In 
the lily the split is along the line corresponding to the midrib of 
each carpel; in some other members of the lily family, the split 
occurs along each line of juncture between adjacent carpels. In 
a poppy capsule, the opening is by means of a circle of pores in 
the upper edge. 

The fruit characteristic of the mustard family is the silique. 
It consists of two many-seeded carpels which separate at maturity, 
leaving between them a thin partition. 

FIG. 362. Aggregate fruit of raspberry. 



347. Aggregate Fruits. In contrast with a simple fruit is an 
aggregate fruit a structure consisting of several or many closely 



FIG. 363. Aggregate false fruit of strawberry, bearing many true fruits 
(achenes) on its surface. A, surface view. B, lengthwise section. 

adherent fruits all developed from a single flower. Aggregate fruits 
include both true and false fruits. A raspberry (Fig. 362) is an 
aggregate of true fruits. The flower 
contains many pistils borne on the 
conical terminal portion of the re- 
ceptacle. The ovary of each pistil 
develops into a small stone fruit. 
However, the many stone fruits 
are densely crowded and become 
so firmly attached during matura- 
tion that they adhere in a single 
mass when separated from the re- 
ceptacle. A blackberry is also an 
aggregate fruit of this type, but the 
receptacle becomes softened and 
juicy and breaks off with the 

A strawberry (Fig. 363) is an 
aggregate false fruit developed 
from a single flower. The flower, 
like that of the raspberry, has a conical receptacle bearing many 
pistils, During maturation, the receptacle becomes greatly en- 
larged both beneath and between the pistils, the pistils so being 


FIG. 364. A, multiple false fruit 
of mulberry. B, surface view 
of an individual false fruit. C, 
the same, with the fleshy sepals 
bent back, exposing the true fruit. 


commercial varieties of pineapple do not contain seeds. At the 
apex of the whole multiple fruit are many greenish bracts in whose 
axils no flowers were borne. 

The fig (Fig. 366) resembles a multiple fruit in including many 
individual fruits, each developed from a single flower. It differs 
in the fact that the individual fruits are not adherent. The many 
flowers of a fig are borne within the enlarged hollow, flask-shaped 
end of a branch. At the apex of the flask-shaped cavity is a small 
opening surrounded by several bracts. Some flowers are staminate, 
some pistillate, the distribution of the two kinds varying as between 
different varieties and species of fig. Each pistillate flower may 
produce a true fruit which is an achene similar to a true fruit of 
the strawberry but usually smaller and more nearly spherical. The 
end of the branch bearing the flowers becomes enlarged and is the 
fleshy part of the "fruit." 

349. Classification of Fruits. 

1. Simple: 

(a) Fleshy: 

Berry (grape). 

Drupe or stone fruit (plum). 

Pome (apple). 
(&) Dry: 

Indehiscent : 

Achene (sunflower). 

Grain (corn). 

Nut (acorn). 

Samara (maple). 

Schizocarp (geranium). 

Legume (pea). 

Follicle (milkweed). 

Capsule (poppy). 

Silique (mustard). 

2. Aggregate (raspberry). 

3. Multiple (pineapple). 

350. Dispersal of Seeds and Fruits. In a seed plant the seed 
represents the point in the life cycle at which a wider distribution 
of the species may chiefly be brought about. It follows that means 
for the dispersal of seeds to a greater or less distance from the plant 



portions of its fruit, which are spongy and especially resistant to 
salt water. In consequence, the fruit may float for a long time 
without injury and may be carried by ocean currents to great dis- 

Animals, too, play an important part in the dispersal of seeds 
and fruits. The fruits of many common weeds bear hooks or 
barbs (Fig. 368) by means of which they become attached to the 

FIG. 368. Fruits which become attached to coats of passing animals. A, B, 
stick-tight (Hedysarum). C, cocklebur. /), beggar-tick. After Kerner. 

coats of passing animals or to the clothing of human beings, being 
then carried to varying distances. Such outgrowths, in the cases 
of stick-tights, beggar-ticks, cocklebur, and stickseeds, are develop- 
ments from the fruit coat; those of the burdock are the developed 
bracts of the floral head. The mud collected by the feet and legs 
of wading birds often contain seeds, which are thus distributed. 
Some fruits and seeds have sticky coverings by means of which 
they may adhere to the bodies of animals. 

A very common means of dispersal is the production of edible 
seeds or fruits. Fruits of such trees as the walnut, hickory, and 
oak are carried away and hidden by squirrels, often in places 
where, if not eaten, the seeds may germinate. Other animals, 
especially birds, eat such edible fruits as berries. In such a case the 
seeds are usually swallowed; but these seeds are commonly pro- 
tected by their coats from the action of the animal's digestive 
juices, and hence pass uninjured through its alimentary tract and 
are deposited at a distance from the plant that produced them. 

Man has played a larger part in the distribution of seeds and 
fruits than have any of the lower animals. His part has consisted 
both in the intentional extension of the range of numerous cul- 



tivated plants and in the accidental dispersal of seeds. Weed 
seeds are carried with the seeds of cultivated plants as well as in 
packing materials, in dust, and in other accidental ways, and 
are distributed by means of shipping 
lines and railways. Many weeds, like 
the Russian thistle and the Canada 
thistle, appear along railways, where 
their seeds have dropped from passing 
trains and thence spread to surrounding 
regions. Many troublesome weeds rep- 
resent species that are not objectionable 
in their native lands but that, when 
carried to other countries, find favorable 
conditions for rapid multiplication. 

Many plants have means by which 
their seeds, when mature, are explosively 
discharged from the fruits. The seeds 
of a violet are squeezed out by a con- 
traction of the sectors into which the 
fruit coat splits. The fruits of vetches 
(Fig. 369) and of the witch hazel open 

suddenly so as to shoot out the seeds FlG - 369 - Legume of a vetch, 
T .IT i i j. ,1 . i which opens suddenly and 

by a method comparable to that found hurlg ^ geeds 

in the violet. In the cranesbill, the 

fruit coat splits suddenly and its parts curl in such a way that 
the seeds are discharged. In touch-me-nots, the explosion of the 
fruit is brought about by the pressure of turgid tissues; and the 
seeds of the "squirting cucumber/ 7 with a juicy pulp in which 
they are imbedded, are ejected through an opening produced in the 
base of the mature fruit by its separation from the flower stalk. 



portions of its fruit, which are spongy and especially resistant to 
salt water. In consequence, the fruit may float for a long time 
without injury and may be carried by ocean currents to great dis- 

Animals, too, play an important part in the dispersal of seeds 
and fruits. The fruits of many common weeds bear hooks or 
barbs (Fig. 368) by means of which they become attached to the 

FIG. 368. Fruits which become attached to coats of passing animals. A, B, 
stick-tight (Hedysarum). C Y , cocklebur. D, beggar-tick. After Kerner. 

coats of passing animals or to the clothing of human beings, being 
then carried to varying distances. Such outgrowths, in the cases 
of stick-tights, beggar-ticks, cocklebur, and stickseeds, are develop- 
ments from the fruit coat; those of the burdock are the developed 
bracts of the floral head. The mud collected by the feet and legs 
of wading birds often contain seeds, which are thus distributed. 
Some fruits and seeds have sticky coverings by means of which 
they may adhere to the bodies of animals. 

A very common means of dispersal is the production of edible 
seeds or fruits. Fruits of such trees as the walnut, hickory, and 
oak are carried away and hidden by squirrels, often in places 
where, if not eaten, the seeds may germinate. Other animals, 
especially birds, eat such edible fruits as berries. In such a case the 
seeds are usually swallowed; but these seeds are commonly pro- 
tected by their coats from the action of the animal's digestive 
juices, and hence pass uninjured through its alimentary tract and 
are deposited at a distance from the plant that produced them. 

Man has played a larger part in the distribution of seeds and 
fruits than have any of the lower animals. His part has consisted 
both in the intentional extension of the range of numerous cul- 



tivated plants and in the accidental dispersal of seeds. Weed 
seeds are carried with the seeds of cultivated plants as well as in 
packing materials, in dust, and in other accidental ways, and 
are distributed by means of shipping 
lines and railways. Many weeds, like 
the Russian thistle and the Canada 
thistle, appear along railways, where 
their seeds have dropped from passing 
trains and thence spread to surrounding 
regions. Many troublesome weeds rep- 
resent species that are not objectionable 
in their native lands but that, when 
carried to other countries, find favorable 
conditions for rapid multiplication. 

Many plants have means by which 
their seeds, when mature, are explosively 
discharged from the fruits. The seeds 
of a violet are squeezed out by a con- 
traction of the sectors into which the 
fruit coat splits. The fruits of vetches 
(Fig. 369) and of the witch hazel open 
suddenly so as to shoot out the seeds FlG - 369 - Legume of a vetch, 
by a method comparable to that found ^Ss ^ seeds ^^ ^ 
in the violet. In the cranesbill, the 

fruit coat splits suddenly and its parts curl in such a way that 
the seeds are discharged. In touch-me-nots, the explosion of the 
fruit is brought about by the pressure of turgid tissues; and the 
seeds of the " squirting cucumber/' with a juicy pulp in which 
they are imbedded, are ejected through an opening produced in the 
base of the mature fruit by its separation from the flower stalk. 



351. Arrangement of Flowers. The flowers of some plants are 
borne singly, as in the trillium arid the may apple, at the end 
either of the stem or of a branch. In the latter case the branch, 
like most branches, commonly arises in the axil of a leaf. The 
stalk of the flower is a peduncle. Sometimes, as in Fuchsia, pedun- 
cles arise at various points along the stem, in the axils of foliage 
leaves. Often the leaves in whose axils peduncles and flowers are 
borne are small and sessile. Such leaves are known as bracts. 

More commonly, however, flowers are borne in a cluster. In 
such a case, the stalk of the cluster is a peduncle, and the indi- 
vidual stalk of each flower is a pedicel. Pedicels, like peduncles, 
may arise in the axils of bracts. Two main types of flower cluster 
are recognized, depending upon the relative times at which dif- 
ferent flowers of the cluster mature. 

If the flowers which open first are those attached nearest the 
base of the peduncle, the cluster is indeterminate. It is so called 
because the floral axis can continue indefinitely to grow and to 
produce new flowers. 

The form of an indeterminate cluster depends largely upon the 
relative lengths of pedicels and peduncle. If, as in the lily of the 
valley, the currant, and the choke cherry, both peduncle and 
pedicels are fairly long- and all pedicels are of about the same 
length, the cluster is a raceme (Fig. 370, A). If the flowers are 
arranged as in a raceme, but the pedicels of the lower flowers are 
longer than those of the upper ones, so that the flowers are borne 
at nearly or quite the same level and the cluster is approximately 
flat-topped, it is a corymb (Fig. 370, B). In an umbel (Fig. 370, C) 
the pedicels arise at approximately the same level on the peduncle. 
Since the pedicels are all of the same or nearly the same length, an 
umbel, like a corymb, is flat-topped. If the peduncle is elongated, 
the intervals between the pedicels are short, and the pedicels them- 
selves are short, the flowers therefore being borne close to the pe- 
duncle and to each other, the flower cluster is a spike (Fig. 370, D). 




The common mullein and the dooryard plantain bear spikes. A 
catkin (Fig. 370, E), such as is borne by a willow or a poplar, is 
a spike with scaly bracts. If the peduncle is so shortened and 
thickened, as in the red clover or sunflower, that the cluster is 
more or less round- or flat-topped, it is a head (Fig. 370, F). 

In a determinate flower cluster the central or terminal flower is 
the first to open, later flowers arising below the first one. The up- 


FIG. 370. Types of flower clusters. A, raceme (diagrammatic); B, corymb 
(diagrammatic); C, umbel (diagrammatic); Z>, spike of plantain, after 
Bailey; E, catkin of willow; F, head of clover, after Smalian. 

ward growth of the main floral axis is terminated by the develop- 
ment of the central flower. A cluster of this type is called a cyme 
(Fig. 371, A). Cymes may resemble in form either racemes or 

Apart from the simple types of cluster already mentioned, 
there are many types of compound flower clusters. In these 
either the peduncles are branched, each branch bearing pedicels; 
or the pedicels are branched; or both peduncle and pedicels are 
branched. A common compound type is the panicle (Fig. 371, C) 
such as is borne by the oat and by many other grasses; other types 



are compound corymbs, compound umbels, and compound cymes 
(Fig. 371, B). 

352. Classification of Angiosperms. The division of angio- 
sperms into dicotyledons and monocotyledons has already been 
mentioned. In arranging the members of these classes into orders, 
families, genera, and species, the structure and arrangement of 
the parts of flowers and fruits are chiefly used as bases of clas- 

FIG. 371. Types of flower clusters. A, cyme (diagrammatic); B, compound 
cyme of Saponaria, after Rusby; C, panicle (diagrammatic). 

sification. Monocotyledons include 45 families, divided into 
1,500 genera and about 25,000 species. Dicotyledons include 240 
families, 7,300 genera, and over 100,000 species. On account of 
the large number of angiosperms, only a few representative families 
can be described on the following pages. These families are se- 
lected either because of their large numbers of species or because 
they include especially well-known plants. 


353. Willow Family. A number of common trees and shrubs 
belong to a group of small families which are considered to be 
among the more primitive dicotyledons. One of these is the wil- 
low family, to which belong the willows (Fig. 372) and poplars. 
In all members of this family, flowers are borne in catkins of two 
kinds: one kind composed of pistillate flowers, the other of stami- 
nate flowers. Pistillate and staminate catkins are borne on sepa- 
rate plants. The flowers are very simple; a pistillate flower of the 
willow consists of one pistil borne in the axil of a hairy, scale- 
like bract. The pistil is composed of two united carpels, and the 
ovary contains a large number of ovules. A staminate flower, 



borne likewise in the axil of a hairy bract, consists of two or more 
stamens, the number varying with the species. 

The flowers of poplars are similar in general structure to those 
of willows. The fruit is a capsule which opens when the seeds are 
mature by a separation of its two constituent carpels. Each of the 
many seeds bears a circle of hairs at its base, forming a parachute- 
like structure that facilitates the carrying of the seed by winds. 




FIG. 372. Flowers, fruit, and seed of willow. A, pistillate catkin. B, pistillate 
flower. C, fruit. D, seed. E, staminate catkin. F, staininate flower. 

Cottonwoods are species of poplar in which the hairs borne by 
the seeds are especially long and silky. 

Among the relatives of the willow family are the walnut family, 
which includes the hickory, pecan, and walnuts; the birch family, 
to which belong hazels, alders, and birches; and the beech fam- 
ily, including chestnuts, beeches, and oaks. 

354. Nettle Family. Among the many members of this family 
are a number of trees as well as herbaceous plants. The flowers 
are still simple but somewhat more complicated, especially by the 
presence of a calyx, than those of the willows. In most species 
the flowers containing stamens and those containing pistils are 
separate, the two kinds of flowers being borne either on the same 
or on different plants. The stamens are most commonly equal in 
number to the sepals; there is a one- (rarely two-) chambered ovary 
which forms a one-seeded fruit. The fruit is a samara, an achene, 
or a drupe. 

The great majority of this family are tropical. The leaves of 
some members, including the nettles from which the family is 
named, have hairs that secrete an irritating acid. Among the trees 
of the family are elms (Fig. 373), hackberries, and mulberries. 



The leaves of the white mulberry, which has been cultivated in 
Mediterranean countries since the twelfth century and in its native 
country, China, much longer, are used as food for silkworms. The 
fruits of this and of other mulberries are edible. Closely related 
to the mulberries is the Osage orange. Other members of the fam- 
ily are the hop and hemp. From hemp are obtained the drugs 

known as hashish and 
cannabis. Hemp is cul- 
tivated largely also for 
its bast fibers which are 
used in making ropes and 
fabrics. The family in- 
cludes some other fiber 
plants. The bread-fruit 
tree of the tropics is a 

Plants of this family 
frequently contain a 
milky juice (latex). The 
latex of the South Ameri- 
can cow tree furnishes a 
nutritive beverage. The 
latex of several tropical 
members of the family is 
a source of rubber. The 
" India-rubber tree," the 
best-known rubber-yield- 
ing plant native to the 
eastern hemisphere, is a 
species of Ficus. Small 
specimens of this tree are 
grown as house plants in colder climates under the name of 
"rubber plant." (The tree most largely cultivated in plantations 
for rubber in various parts of the world is a member of the spurge 
family.) To the genus Ficus belong also the cultivated figs and 
the banyan tree (Fig. 31). 

355. Pink Family. The families thus far mentioned are char- 
acterized for the most part by inconspicuous flowers, borne usually 
in close clusters, which either are naked (that is, without sepals 
or petals) or have sepals only. Many members of the pink family 

FIG. 373. Elm twig bearing flowers and 
fruits. The flowers of the elm, differently 
from those of most members of the nettle 
family, are often " perfect" that is, the 
same flower contains both a pistil and 



have large, showy flowers borne singly or in small clusters and 
provided with both sepals and petals. They are mostly herbs, 
whereas the more primitive families include a large proportion of 
trees. The flowers in this family have usually five (sometimes four) 
sepals, as many petals if petals are present, and not more than 
twice as many stamens as sepals. The family includes some plants 
commonly cultivated for their flowers, the best known of which 
are the carnations and the related pinks and sweet williams 

(Fig. 374). The carnations are 
descendants of a European spe- 

FIG. 374. Flower cluster of sweet 
william, a member of the pink 

FIG. 375. A buttercup. 

cie's that has long been cultivated. Among common wild plants 
of the family are the chickweeds, catchflies, campions, bouncing 
bet, and corn cockle. 

356. Crowfoot Family. This, like the pink family, includes many 
species with showy, often solitary flowers having either a con- 
spicuous calyx or a green calyx and a showy corolla. 

The flower of a buttercup (Fig. 375) is fairly illustrative of the 
characteristics of this family. The receptacle is dome-shaped. 
The parts of the flower sepals, petals, stamens, and pistils are 
arranged spirally upon this receptacle, the sepals being lowest and 


each succeeding set of parts arising from the receptacle above the 
set just outside it. The sepals are typically five, although there 
are variations from this number. Next within are five, or occa- 
sionally more, almost circular yellow petals, each bearing on its 
inner side at its base a small scale. Within the petals are an in- 
definite, rather large number of stamens, and within these a like- 
wise indefinite number of pistils. Each pistil consists of a single 
carpel, arid its ovary contains one ovule. A flower, therefore, pro- 
duces a considerable number of achenes. 

The flowers of most other members of the crowfoot family are 
similar in general plan to that of the buttercup. The numbers of 
the floral parts vary considerably. In a few species, including the 
larkspur, the flowers are irregular and bilaterally symmetrical, in 
consequence of the fact that the sepals of any flower are not all 
of the same shape, the same being true of the petals. Among the 
many familiar wild plants belonging to this family, in addition to 
various species of buttercup, are the anemones, hepaticas, marsh 
marigold, baneberry, clematis, meadow rues, and columbines. 
Some of the cultivated members of the family are the peony and 
species of columbine, clematis, and larkspur. 

357. Mustard Family. The great majority of plants in this 
family are herbaceous; their roots, stems, or leaves in many cases 
contain sharp-tasting substances that make them valuable as con- 

The flowers of the familiar shepherd's purse (Fig. 376) illustrate 
structures characteristic of the family. These flowers are borne 
in a long raceme. All the parts of each flow r er arise from a flat- 
tened receptacle. There are four green sepals; four small white 
petals, arranged in the" form of a cross; and six stamens, of which 
two are shorter than the other four. The four long stamens seem 
really to represent two, each of which is branched close to its base. 
In the center of the flower is a single pistil composed of two united 
carpels. The ovary is divided by a partition into two chambers in 
each of which are many ovules. The fruit (silique) is flattened, 
approximately triangular in shape, and notched at the apex. Like 
the ovary from which it developed, the fruit is divided by a par- 
tition; at maturity the sides of the fruit separate from the parti- 
tion, allowing the seeds to be scattered. 

The flowers of members of the mustard family are all so similar 
to that of the shepherd's purse, being marked especially by the 



cross-shaped corolla, that they are readily distinguished from those 
of other families. Members cultivated as sources of food are the 
turnip, rutabaga, radish, horse-radish, garden cress, and mustard. 
A very important species of the family is Brassica oleracea 
(Fig. 413), which by variation has given rise to the cabbage, cau- 
liflower, kohlrabi, Welsh cab- 
bage, and Brussels sprouts. 
Members grown for their 
showy flowers are the stocks 
or gilly-flowers, sw r eet alyssum, 
and candy-tuft. The water 
cress belongs in this family. 

358. Rose Family. This is 
one of the best-known families, 
because it includes a very large 
proportion of the common cul- 
tivated fruits as well as many 
plants with showy flowers. 
Among its members are herbs, 
shrubs, and trees. 

The receptacle of the flower 
is either cup-shaped, bearing 
the carpels on its inner sur- 
face, as in roses and plums, or 
a cone-like protuberance as in 
the strawberry and raspberry. 
The sepals, petals, and stamens 
are borne on the outer portion 
of the receptacle. In one sec- 
tion of the family that which 
includes the apple the tube-like receptacle is united with the 
ovary, so that the outer parts of the flower stamens, petals, and 
sepals seem to be borne above the ovary. 

The family is divided into seven sections, each characterized by 
its special type of flower and fruit. One section includes the spi- 
reas; another, the apple, hawthorn, and serviceberry; a third, the 
strawberry and cinquefoil; to a fourth belong raspberries and 
blackberries; to a fifth, the agrimony; the sixth section includes 
roses; and the seventh, the plum, cherry, peach, apricot, and al- 

FIG. 376. A plant of shepherd's purse, 
and a raceme bearing flowers on its 
upper portion and fruits below. 



The flower of the strawberry represents a comparatively simple 
type. The flower cluster is a few-flowered cyme. At the outside of 
each flower are five small green bracts which are not strictly parts 
of the flower. Next within is a whorl of five wedge-shaped green 
sepals ; next, five rounded white petals ; then three cycles of stamens 
(ten in the outer whorl and five in each of the inner two whorls) ; 



FIG. 377. Members of the rose family. A, a wild rose. B, flower cluster of 
plum. C, lengthwise section of a plum flower. 

and finally, on the conically elongated central portion of the recep- 
tacle are many pistils, each consisting of one carpel, which are spi- 
rally arranged and closely packed together. Some cultivated vari- 
eties of strawberry have more than twenty stamens ; other varieties 
have no functional stamens. The ovary of each pistil contains a 
single functional ovule; the style projects upward from the side of 
the ovary. After pollination and the union of gametes, the petals 
fall away and the stamens wither. Each ovary develops into an 
achene while the receptacle enlarges greatly both beneath and 
between the ovaries, forcing them apart. The enlarged receptacle 
becomes soft and pulpy, constituting the juicy portion of the 
edible " strawberry." 

The flower of a wild rose (Fig. 377, A) is especially distinguished 
from that of the strawberry by the fact that the tube formed by 
the receptacle has the shape of an urn with a comparatively narrow 


mouth. The tube becomes fleshy after gametic union, forming a 
rounded structure within which many dry fruits, each containing 
one seed, are enclosed. The flower has usually five petals and a 
large and indefinite number of stamens. Sometimes there are addi- 
tional petals, making the number more than five; usually the addi- 
tional petals replace stamens. By the selection of occasional plants 
with larger numbers of petals, and by a repetition of the selection 
when another similar variation occurred, cultivated varieties of 
roses have been developed with many petals and with few or no 

A plum (Fig. 377, B, C) bears its flowers either singly or in 
small clusters. The tube formed by the receptacle only partly en- 
closes the single pistil. There are five green sepals, five white petals, 
and numerous (usually 15 to 20) stamens. The ovary contains 
two ovules, only one of which develops into a seed. After the 
gametes have united, the outer parts of the flower fall away and 
the ovary develops into a drupe. The outer portion of the ovary 
wall forms the skin and the fleshy part of the fruit; the inner por- 
tion of the ovary wall forms the hard stone. The soft structure 
within the stone is the seed. The closely related cherry, apricot, 
and peach have flowers and fruits of the same type as the plum. 
The same is true of the almond, but the outer layer of its fruit, 
which corresponds to the skin and flesh of the plum, dries and is 
split off; the inner part of the fruit, corresponding to the stone of 
the plum, is the almond of commerce. 

359. Pulse Family. This, comprising more than 12,000 species, 
is, with one exception, the largest family of seed plants. Its mem- 
bers, distributed throughout the world, include herbs, shrubs, and 
trees. A great majority of the species have bilaterally symmetrical 
(irregular) flowers of the type illustrated by the bean and pea, 
although some have regular or nearly regular flowers. All of them 
bear fruits of the kind known as a legume, developed from the 
ovary of a simple pistil. 

The flowers of the sweet pea (Fig. 378) are borne in loose, open 
racemes. The peduncle arises from the axil of a leaf, and each 
pedicel from the axil of a minute bract. The five pointed sepals of 
a flower are united by their basal parts to form a cup, the three 
lower sepals being longer than the two upper ones. There are 
five white or colored petals, although there appear to be but four 
because two are intimately united. The upper petal (standard) 



is broad and upright; the two lateral petals (wings) are borne one 
at either end of the standard ; the two lower petals are united and 
their free margins are rolled inward to form a trough-like keel. 
The keel almost completely encloses the ten stamens; nine stamens 
are united by the expanded bases of their filaments into a sheath 
surrounding the ovary; the tenth (upper) stamen is separate. The 
pistil consists of one carpel whose structure suggests that of a 
leaf folded on its midrib so that its edges are brought together 

and united. The ovary contains 
several ovules, borne in two rows 
(apparently one) along the in- 
folded and united edges of the 
carpel. The style curves upward 
nearly at right angles to the ovary. 
The stigmatic surface is along one 
face or edge of the style. This 
type of flower shows advances 
over the primitive condition in 
the union of sepals, the union of 
two petals, the union of the fila- 
ments of nine stamens, and in its 
bilateral symmetry. 

After gametic union the petals 
and stamens fall off, and the ovary 
enlarges greatly as the seeds de- 
velop. When mature, the fruit 
formed by the growth of the ovary 
becomes dry and opens along two lines, one corresponding to the 
midrib of the carpel, the other to the line of junction of its two 
united edges. 

As mentioned in Chapter XVIII, many members of the pulse 
family are characterized by a peculiar relation to certain bac- 
teria which enables them indirectly to use the nitrogen of the air. 
In consequence of this relation, several of them are widely used 
forage plants, and their cultivation plays an important r61e in 
conserving and adding to the supply of nitrogenous food materi- 
als in the soil. Plants extensively grown for this purpose are the 
clovers, alfalfa, vetches, cowpea, and soybean. Another impor- 
tant characteristic of members of the family is the habit of storing 
reserve proteins in their seeds. It is because these contain a much 

FIG. 378. Sweet pea. The small 
bracts in whose axils the pedicels 
arose have disappeared. 



larger percentage of proteins than most other seeds as well as 
large carbohydrate or fat reserves that the seeds of the pea, 
bean, and lentil are important as human foods. A peculiar feature 
of the peanut is that after pollination its pedicels turn and grow 
downward, pushing the fruits into the soil where they ripen. 
Other well-known members of the family are the honey locust, 
black locust, wistaria, and mimosa. Among the woody tropical 
and subtropical species are many that supply lumber, resins, 
gums (including gurn 
arabic), dyes (especially 
indigo), and drugs. 

360. Parsley Family. 
One general characteris- 
tic of this family is the 
arrangement of the flow- 
ers in umbels (Fig. 379). 
The individual flowers 
are small and usually 
white or yellow. Each 
has five sepals, five pet- 
als, and five stamens, all 
of which parts seem to 
be borne above the ovary. 
This appearance is really 
due to a union of the 
tube-like receptacle with 
the ovary. The single 
pistil is composed of two carpels united to form a two-chambered 
ovary, the two styles, however, being separate. The ovary devel- 
ops into a hard, dry, two-parted fruit (schizocarp), each part con- 
taining one seed. When ripe, the two parts of the fruit separate. 
The family is characterized also by hollow internodes, by vari- 
ously lobed or divided leaves with sheathing petioles, and by the 
secretion of volatile oils and resins which impart characteristic 
odors and flavors. 

The leaves, fruits, and other organs of such species as parsley, 
celery, anise, dill, fennel, and coriander are used as foods 
or condiments because of their aromatic flavor. The carrot 
and parsnip are members of this family; so are several poi- 
sonous plants, including the water hemlock and poison hemlock, 

FIG. 379. Wild carrot; a compound umbel 
and (below) one of the simple umbels of 
which the compound umbel is composed. 



some weeds such as the wild carrot, and several plants that supply 

361. Mint Family. This family, with over 3,000 species, includes 
plants (mostly herbs) with usually four-sided stems and opposite 

leaves. In most species, each 
flower has five sepals which 
are united except at their 
tips; five petals united to form 
a more or less two-lipped 
corolla, the upper lip com- 
posed of two petals, the lower 
of three; four stamens, of 
which two are longer than the 
other two; and a pistil consist- 
ing of two two-lobed carpels 
surrounding a central style. 
At maturity four nutlets are 
formed, one from each carpel 

The leaves of most species 
bear small glands containing 
a volatile oil which makes 
many of them useful as sources 
of flavors, perfumes, and 
drugs. Among cultivated 
members of the family are 
horehound, rosemary, laven- 
der, sage, peppermint, and 
spearmint (Fig. 380). Coleus 
is cultivated because of its 
ornamental variegated leaves, and some species of Salvia are 
, grown for their flowers. Horse mint and catnip are familiar weeds. 

362. Nightshade Family. To the nightshade family belong many 
cultivated plants, of which the best known are the potato, tomato, 
and tobacco. The members are nearly all herbaceous with regular 
(radially symmetrical) flowers. A flower (Fig. 381) has five sepals 
united for a varying distance from their bases into a tube; five 
petals similarly united; five stamens which are united with the 
bases of the petals, and a pistil composed of two carpels. The 
fruit is a two-chambered capsule or berry, each chamber contain- 

FIG. 380. Spearmint. 



FIG. 381. Apical portion of a plant of tomato, 
a member of the nightshade family. 

ing numerous seeds. The fruits of many species contain poisonous 
substances which are used in such drugs as belladonna, hyoscya- 
mus, and stramonium, or narcotics such as characterize the to- 
bacco. Even the tuber of the potato contains a small amount of a 
slightly poisonous sub- 
stance. The large genus 
Solarium to which the 
potato belongs includes 
also the black night- 
shade, eggplant, horse 
nettle, and buffalo bur. 
The tomato, red pep- 
pers, ground cherry, and 
petunia are other mem- 
bers of the family. 

363. Gourd Family. 
The plants of the gourd 
family are mostly herbs 
with thick, juicy stems 
that bear tendrils. The flowers (Fig. 382) are of two sorts: one 
with a pistil and rudimentary stamens, the other having stamens 
and a rudimentary pistil. Thus, in a family which stands relatively 
high in the evolutionary scale, the same characteristic of separate 

staminate and pistillate 
flowers appears that is 
found in the very primi- 
tive willow family. In some 
members of the gourd fam- 
ily both kinds of flowers 
are borne on the same 
plant; in other species, 
some plants bear usually 
only staminate, others usu- 
FIG. 382. Portion of a plant of cucumber, a a lly on ly pistillate flowers, 
member of the gourd family, with flowers rpv n c i ,1 . 

and a young fruit. The flowers f both ^P 68 

are marked by a consider- 
able degree of union of their parts. The sepals are united into a 
tube, and the petals are likewise united. The receptacle is com- 
pletely united with the large ovary, which is thus distinctly below 
the levels of insertion of the other floral parts. The stamens also 


are often united by their anthers or by both anthers and filaments. 
The fruit is developed from the ovary together with the surround- 
ing tissues of the receptacle, some of whose outer layers form a 
hard rind; many seeds are imbedded in the pulpy interior tissues. 
Most species are tropical or subtropical, and those cultivated in 
temperate regions have been introduced from warmer climates. 
Familiar members are the cucumber, pumpkin, squashes, water- 
melon, muskmelons, and gourds. 

364. Composite Family. Not only is this family the most highly 
developed, it is also the largest family of angiosperms, containing 
some 23,000 species. Some composites, including the thistles, 
dandelion, and other very common weeds, have such efficient 
methods of distribution of their fruits, arid produce fruits in so 
great numbers, that it is almost impossible to exterminate them. 
The name " composite " is given because the individual flowers are 
grouped closely together in a head which has the general appear- 
ance of a single flower, the more so because just below the head 
are green bracts that look like sepals. The tip of the peduncle is 
thickened and flattened into a broad, disk-like or cone-shaped 
flower-bearing surface. 

The sunflower (Fig. 383) illustrates the floral organization 
typical of the family. At the edge of the flower-bearing disk are 
two or more cycles or very close spirals of overlapping green bracts. 
Just within these, and on the face of the disk, the flowers are 
borne closely packed together, each in the axil of a small bract. 
These latter bracts are arranged in incomplete open spirals. There 
are two types of flowers: the ray flowers are borne in a single or 
double row near the edge of the disk; the disk flowers cover the rest 
of the disk's surface. The receptacle of each disk flower is a hollow, 
wedge-shaped structure standing almost perpendicularly to the 
surface of the disk. It partly encloses, and is completely united 
with, the ovary. The pistil probably consists of two carpels, al- 
though the ovary contains but one functional ovule. Just within 
the sepals is a long, flaring tube having a conspicuous inflation 
about one fourth way up from its base. The portion of the tube 
below this inflation is formed by the united bases of the petals 
and stamens; the part above consists only of the united petals, 
which are separated at their tips into five blunt teeth. Above the 
level of their separation from the petals, the filaments of the sta- 
mens are separate from one another, but the anthers are united 



by their edges into a long tube. The top of the ovary extends 
slightly above the top of the receptacle and completely fills the 
space at the center of the flower. The style, extending up through 
the corolla tube and the anther tube, terminates in two relatively 
large stigmas. A ray flower differs from a disk flower in having 
one side of its corolla greatly extended into a broad, flat struc- 
ture. Frequently, also, in a ray flower the inflation near the base 
of the corolla tube is lack- 
ing ; the stamens and style 
may be abortive, and 
there may be three sepals 
instead of two. 

After the union of gam- 
etes, the style, stamens, 
petals, and calyx are shed, 
and the united receptacle 
and ovary enlarge greatly 
and become dry and some- 
what hard. The single 
seed fills the space within, 
but is united to the ovary 
wall only over a very 
small area. 

The composite type of 
flower cluster is the most 
specialized among the di- 
cotyledons. The union, in FIG. 383. The composite flower cluster (head) 

_ , . , . . . of a sunflower, 

eiiect, oi the receptacle 

with the ovary, causing the outer parts of the flower to be borne 
above the ovary, is an advanced feature; the union of the petals 
into a corolla tube and that of the anthers into an anther tube 
also are advanced characters. The occurrence of flowers of two 
distinct types in the same head is likewise a highly specialized 
condition. In this latter respect, however, the sunflower is not 
typical of all composites. In the dandelion, as in a number of 
related genera, the head contains flowers of only one type which 
are similar in corolla form to the ray flowers of the sunflower. 

Among the comparatively few members of the family that sup- 
ply food for man are the lettuce, endive, chicory, salsify, arti- 
choke, and Jerusalem artichoke. The sunflower is used as food 



for livestock. Drugs are obtained from some composites, including 
camomile, calendula, arnica, tansy, and wormwood. Among orna- 
mental plants of the family are the daisies, sunflower, dahlia, 
asters, and chrysanthemums. Some of the commonest wild plants 
and weeds, among them being conspicu- 
ous members of the autumnal flora, are 
asters, golden-rods, ragweeds, and thistles, 
the sagebrush, dandelion, beggar-ticks, 
yarrow, cocklebur, and burdock. 


365. Cat-tail Family. The monocoty- 
ledons seem to have arisen from some 
very primitive dicotyledon or dicotyle- 
dons. Within the class of monocotyledons, 
the course of evolution has substantially 
paralleled that which has marked the 
history of dicotyledons. In consequence, 
while preserving the characteristics that 
distinguish them from dicotyledons 
( 336), monocotyledons show very much 
the same steps in the evolution of floral 
structures that have been described for 
dicotyledons. The small cat-tail family 
is one of the most primitive among living 
monocotyledons, and may be thought of 
as holding much the same position in 
this class that the willow family occupies 
An "among dicotyledons. 

The characteristics of the family are 

FIG. 384. Cat-tail, 
aerial branch, a single 

leaf, and a flower clus- essen ti a lly those of the common cat-tail 
er (Fig. 384), growing abundantly in wet, 

marshy places. This plant has a branching horizontal stem 
that lives in the mud from year to year, and each spring sends 
up aerial branches. Each such branch bears at its base long, 
sheathing leaves. At the upper end of an aerial branch is a long 
cylindrical spike of flowers. The central axis of a close cylin- 
drical spike of this nature is a spadix. The flowers on the spadix 
are partly covered while young by long, thin, sheathing bracts 
(spathes); one spathe arises from the base of the spike, and 




others may appear higher up, interrupting the cylindrical mass of 

The flowers in the upper part of the spike are staminate, those 
in the lower part pistillate. Each staminate flower consists of two 
or three stamens borne on a short pedicel from whose lower part 
arise a number of hair-like outgrowths. A pistillate flower has a 
single pistil consisting of one carpel borne, like the stamens, upon 
a short, hairy pedicel. The ovary contains one ovule. After pol- 
lination, which is brought about by winds, the staminate flowers 
wither and disappear, leaving 
the upper part of the spadix 
bare. Each ovary may de- 
velop into an achene; the 
pedicel with its many hairs 
remains attached to the fruit 
when the latter is shed, and 
the hairs assist in the distri- 
bution of the fruit by winds. 

366. Grass Family. Here 
belong about 4,500 species 
which, like the cat-tails, have 
small, simple flowers and one- 
seeded fruits. In various re- 
spects, however, grasses show 
a considerably greater degree 
of specialization than do cat- 
tails, and they are very much 
more widely spread, different 
species being adapted to very 
different habitats. Like most 
monocotyledons they are herbaceous, although the tall, almost 
tree-like bamboos have more or less woody stems. The stems of 
grasses are jointed, the internodes being commonly hollow, and 
the leaves are alternately arranged in two vertical rows. Eco- 
nomically the most important grasses are the cereal grains, which 
include wheat, oats, barley, rye, corn, rice, and millet. 

The flower of wheat (Fig. 385) may be taken as typical. The 
compound flower cluster, commonly called a head or spike, is 
made up of many small spikelets. Beginning at the base of a 
spikelet, and alternating on opposite sides of its central axis, are 




FIG. 385. Wheat. A, spikelet. B, single 



two rather large bracts (empty glumes), and successively above 
these a few progressively smaller glumes (lemmas), each with a 
flower in its axis. A lemma has its concave face toward the axis 
of the spikelet, and the lower lemmas may bear long, stiff bristles 

(awns). Partly enclosed by 
each lemma is a thin bract 
(pakl) which envelops the 
flower proper. The flower 
includes a pistil with a short 
ovary, and two short styles 
each terminating in a long, 
feathery stigma; three sta- 
mens with long anthers and 
thread-like filaments ; and 
two small scales (lodicules) 
which may correspond to 
sepals. The ovary with its 
single ovule develops into a 

In the corn and a few re- 
lated grasses, stamens and 
pistils are borne in separate 
flowers; the staminate 
flower cluster of the corn is 
the tassel; the pistillate 
flower cluster is the ear. 

In addition to cereals, the 
grasses of most practical in- 
terest are the sugar cane, 
sorghum, and broom corn; 
Fio. 386. A sedge. the bamboos, which in their 

native countries are used for a great variety of purposes; and many 
species which, like red top and timothy, are used for forage. The 
value of wild grasses for pasturage results in large part from their 
habit of growing together in great numbers, so that a considerable 
area may be covered by one or a few species. Their power of 
rapid multiplication by means of seeds as well as by the growth and 
branching of their underground stems makes some of the grasses, 
like so many of the composites, troublesome weeds. Some familiar 
weeds of this family are the wild oat, quack grass, and chess. 



367. Sedge Family. Very similar to grasses in general appear- 
ance and in many characteristics are the sedges (Fig. 386). Most 
of them have three-sided solid stems, bearing leaves in three rows. 
The fruits are nut-like and one-seeded; the embryo, instead of 
being at one side of the seed as in a grass, is near the base and is 
entirely surrounded by endosperm. Some "rushes" and so-called 
"marsh grasses" belong to this family; so do the umbrella plant, 
and the papyrus which was used in ancient times in the manu- 
facture of paper and from whose 

name the word paper is derived. 

368. Palm Family. This is 
distinguished from other fami- 
lies of monocotyledons by the 
fact that most of its members 
have woody stems. Many of 
them are trees, each bearing at 
its tip a crown of large leaves. 
Some palms, such as the rattan 
palm, are climbing plants. 
Practically all palms are tropi- 
cal or subtropical. 

In many species the flowers 
are borne on a spadix that is 
enclosed in a spathe. Some 
have branching flower clusters 
(Fig. 387). A single flower or- 
dinarily has six perianth leaves 
in two whorls of three each, 

the outer whorl often being distinguished as a calyx, the in- 
ner as a corolla; there are usually six stamens in two whorls 
of three each, although in some species the stamens are fewer or 
more numerous than six; there are three carpels, forming either 
three separate pistils or one compound pistil. In many species 
staminate and pistillate flowers are separate and borne either on 
the same or on distinct plants. The fruit, usually one-seeded, is 
either a stone fruit as in the coconut, or a berry as in the date. 
The embryo is at one side of the seed; the seed contains also an 
abundant endosperm which is often hard. The hard part of the 
fruit of the date palm is the endosperm; the endosperm of another 
palm furnishes "vegetable ivory," used in the making of buttons. 

Fio. 387. Flower cluster and fruits of a 
fan palm (Washingtonia) of southern 



The endosperm of the coconut, instead of being hard, constitutes 
most of the "meat" of the nut. 

Coconut oil is made from copra, which is the dried meat of the 
coconut. Palm oil is derived from the fruits of certain species of 
western Africa and eastern South America. The betel nut, exten- 
sively chewed by natives of the East Indies, is the fruit of a palm. 
Sago is made by washing out the starch which is present in great 

quantities in the piths of some 
palms. Among the many other 
products of palms are fibers 
of various sorts, such as those 
from the petioles of the raffia 
palm; building materials, soap, 
wax (from the surfaces of 
stems), and various alcoholic 
drinks including arrack. 

369. Arum Family. Mem- 
bers of this family are charac- 
terized by having their flowers 
crowded on a spadix which is 
subtended or enveloped by a 
relatively large, persistent 
spathe; the spathe is often 
white or conspicuously col- 

A familiar native member 
of the family is the jack-in- 
the-pulpit or Indian turnip 
FIG. 388. A, flower cluster of jack-in- (Fig. 388). The flowers of this 
the-pulpit. B, staminate flower. pl an t are of two sorts, the 
C, pistil (constituting a pistillate , . , a , . , 

flower) staminate flowers being borne 

on the upper part of the spa- 
dix, the pistillate flowers on the lower part. Not infrequently 
the flowers of one type abort, so that the functional flowers borne 
by a particular plant are all staminate or all pistillate. Each stam- 
inate flower consists of a varied number of short stamens; a pis- 
tillate flower is but a single simple pistil whose one-chambered 
ovary contains five or six ovules. The fruit is a scarlet berry with 
one or two seeds. The aerial shoot which terminates in the spathe 
and spadix, and which usually bears also two three-parted leaves, is 



a branch growing from an underground stem. This stem is thick 
and approximately spherical; like various vegetative parts of many 
other members of the family, it has an intensely acrid taste. 

Among familiar plants of the family are the skunk cabbage, 
sweet flag, and water arum. The arum family is most largely 
represented in the tropics, and many of the tropical species 
with showy or oddly shaped 
spathes are grown in green- 
houses and as house plants. 
Among these are the calla lily, 
caladium, dracontium, and an- 

370. Lily Family. The flower 
of the hyacinth (Fig. 389) is 
fairly representative of the 
flowers of this family. The 
hyacinth has a raceme with a 
thick peduncle, each of the 
spirally arranged flowers being 
borne in the axil of a small 
bract. The perianth consists 
of two whorls of three leaves 
each, alike in color and shape 
and united at their bases to 
form a tube. The outer whorl 
of perianth leaves may be con- 
sidered a calyx, the inner whorl 
a corolla. Near its middle the 
perianth tube is considerably 
constricted. Below the con- 
striction the bases of the fila- 
ments of the six stamens are FlG - 389 - A6rial portion of a hyacinth 

united with the perianth tube, 


but above the constriction each stamen is separate and distinct. 
Within the perianth tube but entirely separate from it is the 
pistil. This consists of three carpels and has a three-chambered 
ovary, a single style, and a three-lobed stigma. Along each line 
of junction of adjacent carpels their edges are much swollen, 
and each swollen edge bears a vertical row of ovules. Hence 
there are six rows of ovules extending through most of the length 


of the ovary, and the ovules, together with the edges of the 
carpels on which they are borne, nearly fill the cavities of the 
ovary. The hyacinth flower represents a considerably advanced 
type in the partial union of the staminal filaments with the perianth 
tube, and in the complete union of carpels. After gametic union, 
the perianth and stamens are shed; the ovary enlarges greatly and 
becomes dry, splitting into three compartments each of which con- 
tains many seeds. Thus the fruit of the hyacinth is a capsule. 

The flowers of other members of the family are in general similar 
to that of the hyacinth; the fruits of some are capsules, of others 
berries. Of the true lilies (members of the genus Lilium), some, 
such as the Easter lily, tiger lily, and Turk r s-cap lily, have long 
been cultivated for their flowers. The same is true of many other 
plants of the family, including the lily of the valley, tulip, orange 
day lily, and yellow day lily. The greenhouse "smilax" and other 
species of asparagus are grown for ornamental purposes. Familiar 
wild plants are the trilliums, Solomon's seal, false Solomon's seal, 
dogtooth violet, and bellwort. Plants cultivated for food purposes 
are the asparagus and various members of the genus Allium, in- 
cluding onions, garlic, chives, and leeks. A few members of the 
family, including a species of Yucca and the dragon tree (Dra- 
caena), have a special method of secondary thickening, referred to 
in 46. The family includes also several drug plants and some 
plants which yield fibers. 

371. Orchid Family^In number of species this family, with over 
9,000 members, is the largest among monocotyledons; few of its 
species, however, are abundant and some are very rare. Orchids 
are characterized by their remarkable bilaterally symmetrical 
flowers which show the greatest degree of union of floral parts 
found among monocotyledons. They occupy, therefore, a position 
among monocotyledons somewhat similar to that of the composites 
among dicotyledons. 

The great variety of floral forms in the family seems to represent 
so many adaptations to insect pollination often to pollination 
by insects of a certain size and even perhaps of a particular species. 
The flower of a lady's-slipper (Fig. 390) well illustrates the pos- 
sibilities of development of an insect-pollinated flower. It has three 
sepals of which the two lowermost are united, and three petals, 
one of which, much larger than the other two, has the form of a 
slipper-like sac open at the top. The opening is partly closed by a 



The edges of the opening in front of the flap are curved in- 
ward. At the bottom of the sac on the inside are juicy hairs that 
are eaten by insects which make their way into the sac. On the 
lower side of the flap is the stigma; at either side of the stigma is 
an anther, whose pollen re- 
mains together as a sticky 
mass. A third stamen has no 
anther. Insects, if not too 
large, can make their way into 
the sac in front of the flap, 
but because of the curved 
edges of the opening they can 
not readily escape at the same 
place. If, however, like some 
bees, they are sufficiently 
strong, they can push out 
through the opening at either 
side of the flap. In such a case 
the insect brushes against the 
anther on that side and carries 
away its pollen mass. The 
same insect, entering another 
flower, brushes against the 
stigma, where the pollen mass 
may lodge. 

Many orchids are grown be- 
cause of their rarity or for the 
beauty of their flowers. Not 
many are otherwise useful, although the fruit of the vanilla, a 
tropical American orchid, supplies a well-known flavoring extract, 
and the dried tubers of some old-world orchids are used, under 
the name of salep, both as a food and as a drug. 

Many tropical orchids are epiphytes living high up on the 
trunks of trees. A few, including the coral root, possess no chloro- 
phyll; with the aid of fungi in their underground parts they lead a 
saprophytic life. 

FIG. 390. 

Lady's-slipper, a member of 
the orchid family. 



372. Inheritance. It is a fact of common observation that off- 
spring in most respects are similar to their parents, as well as to 
more distant ancestors. The general rule of resemblance between 
parent and offspring holds for one-celled as well as for many-celled 
plants and animals. This rule is implied when it is commonly said 
that offspring have inherited from their parents such characters as 
height, color of flower, or a tendency to respond in certain ways to 
stimuli. It will appear later that the statement that characters 
such as these are inherited is not strictly accurate. It would be 
more nearly correct to say that characters possessed by parents 
have reappeared in their offspring. 

373. Independence of Characters. To a considerable extent, 
distinct characters behave independently in inheritance. That is, 
an individual plant may show one character that was present in a 
parent such as tallness, but may not display another character of 
the same*parent such as flower color. The behavior of characters 
as something like separate units was demonstrated by the classical 
experiments of Gregor Mendel (1822-1884). While Mendel was not 
the first to observe such behavior, he devised a most important 
method of investigation of the subject in his studies on the common 
garden pea, the results of which were published in 1866. 

Mendel first tested -varieties of pea to determine whether they 
were pure-bred that is, whether they regularly produced offspring 
like themselves. From among the varieties which proved pure in 
this sense he selected some which differed one from another in one 
or more sharply marked characters. These varieties were then 
crossed ; that is, pollen from flowers of one variety was transferred 
to the stigmas of another variety. 

In one experiment a tall variety of pea (six to seven feet high) 
was crossed with a short variety (% to 1% feet). All the offspring 
of the cross were tall plants (Fig. 391). These immediate offspring 
constituted the first filial (Fi) generation. Mendel spoke of the tall 
character displayed by all the plants of the FI generation as dom- 




inant. The short character, possessed by one parent but not by 
any of the FI generation, was recessive. 

When plants of the FI generation, all tall, were self-pollinated, 
three fourths of their offspring in the second filial (F 2 ) generation 
showed the dominant character of tallness, but one fourth dis- 
played the recessive character of shortness. The recessive charac- 
ter, which seemed lost in the FI generation, had reappeared. 

When plants of the F 2 generation were self-pollinated, the short 




F s Generation 

J?IQ. 391. Diagram illustrating the inheritance of tallness and shortness in 
Mendel's experiments with peas. 

ones produced only short offspring (of the F 3 generation); one 
third of the tall F 2 plants produced only tall offspring; the remain- 
ing two thirds of the tall F 2 plants produced, like their parents of 
the FI generation, offspring of which three fourths were tall and 
one fourth were short. 

In another series of experiments Mendel crossed a variety with 
purple flowers and one with white flowers. The purple-flower 
character proved dominant over the white-flower character. In 
the FI, F 2 , and F 8 generations this pair of characters behaved just 
as had tallness and shortness in the previous experiments. In all, 



seven pairs of characters found in different varieties of pea were 
similarly tested. 

Mendel studied also the simultaneous inheritance of two pairs 
of contrasting characters. When a tall variety with purple flowers 
and a short variety with white flowers were crossed, all the Fj. 
generation possessed both dominant characters purple flowers 
and tallness (Fig. 392). The plants of the F 2 generation displayed 
all possible combinations of the two pairs of characters, and these 

Tall, Purple-, 



_ Short, White- 
\j Flowered 

Fj Generation 

TaU. Purple. 



TaU, Purple 


I I I 



F 2 Generation 

FIG. 392. Diagram illustrating the inheritance of tallness and shortness, 
purple and white.flower color, in Mendel's experiments. Solid black rods 
represent purple-flowered plants; white rods, white-flowered plants. 

combinations appeared in the proportions that would be expected 
if the characters of one" pair (purple and white flowers) were trans- 
mitted independently of those of the other pair (tallness and short- 
ness). The F 2 generation consisted, therefore, of plants of four 
classes in the proportions: nine tall with purple flowers; three tall 
with white flowers ; three short with purple flowers ; one short with 
white flowers. 

Crosses between varieties differing in three pairs of characters 
for example, purple and white flowers, plump and wrinkled seeds, 
yellow and green cotyledons gave corresponding but of course 
more complicated results. The distribution of the characters of 
any pair among plants of the F2 generation bore no relation to the 
way in which the characters of any other pair were distributed; 



consequently there appeared in this generation different classes of 
individuals, in proportions that could be calculated in advance, 
possessing every possible combination of the characters of the 

Menders description of his experiments was almost completely 
overlooked until about 1900. Since its rediscovery, studies similar 

FIG. 393. Results of crossing a red- and a white-flowered four o'clock (Mi- 
rabilis). All the FI plants had pink flowers. In the F 2 generation J^ had 
red, % pink, and }^ white flowers. 

to his have been made upon very many plants and animals. These 
studies have shown that in large measure characters behave in 
inheritance as though they were transmitted separately being, 
as it were, reshuffled and arranged into varying combinations in 
each succeeding generation. Often, as in the peas studied by Men- 
del, a particular character of one parent is dominant in the Fj 


generation over a contrasting character of the other parent. For 
example, red, blue, or yellow flower colors are in most cases domi- 
nant over white. Hairiness of stems is dominant over smoothness. 
Brown eye color in man is dominant over blue or gray. 

In other instances, however, dominance does not appear; in- 
stead, the character that appears in the FI generation is in some 
degree intermediate between the characters of the parents. A 
case of this nature is that of a cross between a red and a white 
four o'clock (Fig. 393), which yielded offspring with pink flowers. 
When these FI pink-flowered plants were self-pollinated, their 
offspring in the F2 generation were: one fourth red-flowered, one 
fourth white-flowered, and one half pink-flowered. The latter pink- 
flowered plants, like their parents, displayed the hybrid character. 
But whether the FI generation shows complete dominance of one 
parental character, partial dominance, or intermediacy, it is still 
true that both parental characters are in effect separately trans- 
mitted. In the case in question, the pink-flowered plants trans- 
mitted to some among their offspring the capacity to produce 
flowers like those of the original parents namely, red and white. 

374. Inheritance and Chromosomes. It was pointed out in 
Chapter XII that all inheritance must be by means of structures 
or substances that are transmitted from parent cell to daughter 
cell in the course of nuclear and cell division. It follows that off- 
spring do not literally inherit characters from their parents ex- 
cepting, of course, the structural and functional characters of the 
spore or gametes which gave rise to the offspring. Apart from 
characters belonging to such cell or cells received from the parent 
or parents, all that the new generation inherits is certain substances, 
the presence of which In the cells of the offspring makes possible 
the development of characters like those of the parent. A pine 
tree does not literally inherit tallness; it inherits certain substances 
which give it the ability to grow tall. 

It has appeared also that in the main the substances concerned 
in inheritance are carried in the chromosomes. There is evidence 
that substances or bodies (such as plastids) present in the cyto- 
plasm play a part also in the transmission of hereditary possibil- 
ities. But the role of cytoplasmic structures in this respect seems 
so clearly to be subordinate to that of the chromosomes that the 
chromosomes are considered to constitute the essential mechanism 
of inheritance. 


The fact is to be emphasized that chromosomal and cyto- 
plasmic substances together do not finally determine the charac- 
ters of a plant or animal. These substances endow the organism 
with certain possibilities of development; whether, or to what 
extent, those possibilities are to be realized depends upon the 
environment that surrounds the organism while it is developing. 
The actual characters that the mature plant or animal displays 
are therefore the result of an interplay of inherited tendencies 
and environmental influences. The pine tree above referred to 
can by appropriate treatment such as keeping it in a pot too 
small to permit the free development of its root system be in- 
duced to grow very slowly and to remain a dwarf throughout a 
long life; but the inherited capacity for tall growth remains and 
may be passed on to its descendants. 

375. Characters and Genes. The fact that characters appear 
and reappear independently in large measure of one another has 
led to the assumption that characters are in some way represented 
by small portions or units of the hereditary substance. These 
units, called genes, are, if they exist, too small to be visible under 
any power of the microscope. Genes are thought of as borne in or 
upon the chromosomes or, together, as perhaps constituting the 
whole chromosome substance. It is considered that the presence 
of particular genes in the chromosomes of a pea makes possible 
the development of tall or of short plants, of yellow or of green 
cotyledons, of purple or of white flowers. 

Any pea plant, according to this conception, may possess in 
each of its nuclei two genes concerned with the appearance of 
tallness, having received one such gene from each parent; or two 
genes for shortness; or one gene for tallness derived from one parent 
and one for shortness coming from the other parent. If two genes 
for tallness are present, the plant is tall (external conditions being 
favorable); if two genes for shortness, the plant is short. If one 
gene for tallness and one for shortness are present, only the domi- 
nant gene expresses itself and the plant is tall. For the most part, 
other genes which affect various characters of the pea seem like- 
wise to belong to contrasting pairs; as the genes for yellow and for 
green cotyledons, or those for purple and for white flowers. A 
similar statement can be made as to the genes that influence the 
characters of other species of plants and animals. In the pea more 
than 50 pairs of genes are recognized; in corn between 200 and 


300, the largest number yet known for any species of plant. In a 
small fruit-fly (Drosophila melanogaster) the number is approxi- 
mately 500 the largest known for any animal. 

What has been said may seem to imply that each character is 
influenced by but one pair of genes, and that each pair of genes is 
concerned in the production of a single character. The facts, how- 
ever, are by no means so simple. In general, any gene affects not 
one character alone, but several or many characters. For example, 
the gene in the pea which causes the production of a purple flower 
color also influences the color of the seed coat and that of the stem 
in the leaf axils. Conversely, each character is the result of the 
activity of several or many genes. Three pairs of genes are recog- 
nized which affect flower color in the pea; Mendel's work dealt 
with one of these pairs. Similarly, three pairs of genes, of which 
one pair was involved in Mendel's experiments, influence the color 
of cotyledons. Eye color in Drosophila is said to be affected by at 
least fifty genes. The total constitution of a plant or animal, then, 
depends upon the complicated interaction of many genes, whose 
effect in turn is conditioned by the environment. 

Even though a character is influenced by several genes, it may 
be inherited as though it were represented by but one pair. This 
is explained by the fact that two races or varieties of a species may 
be alike with reference to all save one pair of the genes that notice- 
ably affect the character in question. For example, if, as in Men- 
del's work, plants are crossed that differ with respect to only one 
pair of genes that may influence height, then tall and short plants 
will appear among the later generations in the same proportions 
as though only that one pair of genes affected height. For the pur- 
poses of the experiment these may be referred to as a gene for tall- 
ness and a gene for shortness. 

376. Genes and Chromosomes. It has been seen (Chap. XXV) 
that each cell of a sporophyte contains 2 n chromosomes, of which 
n are of maternal and n of paternal origin. Each maternal chromo- 
some corresponds to a particular paternal chromosome in the sense 
that the two bear the same or corresponding genes. 

Suppose that two pure-bred plants, one tall and one short, dif- 
fer, like those Mendel worked with, in one pair of genes. The 
chromosomes of one pair in each cell of the tall plant carry each a 
gene for tallness. Each chromosome of the corresponding pair in 
the cells of the short plant carries a gene for shortness. When the 


reduction divisions occur in the tall plant, each macrospore and 
each microspore receives one chromosome bearing a gene for tall- 
ness. Since the spores give rise to corresponding gametophytes, 
each cell of a macrogametophyte (including the egg) and each cell 
of a microgametophyte (including the male gametes) contains in 
its single set of chromosomes one which carries a gene for tall- 
ness. Similarly, each spore, gametophyte, and egg or male gamete 
produced by the short plant has a chromosome bearing a gene for 

Suppose now thai/ the two plants are crossed. An egg from the 
tall plant, one of whose chromosomes carries a gene for tallness, 
unites with a male gamete from the short plant, one of whose 
chromosomes bears a gene for shortness. The zygote so formed 
has among its 2 n chromosomes one pair bearing respectively a 
gene for tallness and a gene for shortness (Fig. 394). A similar 
pair of chromosomes is present in each cell of the FI sporophyte 
that develops from this zygote. Since the gene for tallness is domi- 
nant, the sporophyte is tall. 

When the reduction divisions occur in this sporophyte, the 
chromosomes of the pair in question . (like those of other pairs) 
conjugate and separate. Hence half the macrospores, and the 
macrogametophytes and eggs to which they give rise, receive each 
a chromosome bearing a gene for tallness, and half receive each 
a chromosome carrying a gene for shortness. Similarly with the 
microspores, microgametophytes, and male gametes; half receive 
a gene for tallness, half a gene for shortness. If eggs of the two 
classes unite indiscriminately with male gametes of the two classes, 
the result will be (as shown in Fig. 394) that one fourth of the 
zygotes formed, and of the F 2 sporophytes developing from them, 
receive each two genes for tallness; these will be tall plants. One 
half of the F 2 sporophytes receive each one gene for tallness and 
one for shortness; these also will be tall plants. The remaining 
fourth of the F 2 sporophytes receive each two genes for shortness; 
they will be short plants. 

A comparison of Figure 394 with Figure 391 shows how the 
behavior of the chromosomes and of the genes they carry explains 
MendePs results in this and similar experiments. 

If the parents of the cross differ in two genes instead of one 
(for example, in genes for tallness and shortness, for purple and 
white flowers), these genes being borne on chromosomes of dif- 



ferent pairs, the story will be as shown in Figure 395. In this case 
the FI sporophytes will give rise to four kinds of macrospores, 
macrogametophytes, and eggs; and to four kinds of microspores, 

Egg from ( T) Cs 
Tall Race V_y S- 
\ * 

)MaU Gamete 
from Short Race 

Zygote ( Ts ) 


Ft Sporophytes 
(All Tall) 

(2 Types] 


R Sporophytes 
( 3 / 4 Tall 
SS I y 4 Short) 

FIG. 394. Diagram illustrating the transmission of genes for tallness and 
shortness, borne on chromosomes of 1 pair. Each original parent con- 
tributed, through its gamete, 1 chromosome of the pair. The 2 chromo- 
somes are separated in the reduction divisions, half the macrospores and 
half the microspores receiving the chromosome bearing a gene for tallness, 
the other half of the macrospores and microspores receiving the chromo- 
some bearing a gene for shortness. These spores give rise to gametophytes, 
and these to gametes, containing corresponding chromosomes; hence 
there are 2 kinds of eggs and 2 kinds of male gametes. The result of hap- 
hazard unions between eggs and male gametes is the production of zygotes, 
and hence of sporophytes, of which J^ have 2 genes each for tallness, j^ 
have each 1 gene for tallness and 1 gene for shortness, ^ have each 
2 genes for shortness. Sporophytes of the first 2 classes are tall, tallness 
being dominant; those of the third class are short. 

microgametophytes, and male gametes. The indiscriminate union 
of eggs of four kinds with male gametes of four kinds will result in 
the formation of nine types of zygotes, and hence of nine types 
(although in appearance only four) of sporophytes, in the propor* 



tions shown in Figure 396. The behavior of chromosomes and 
genes as illustrated in Figures 395 and 396 explains Mendel's 
results when he crossed plants differing in two characters (Fig. 392). 
377. Linkage. In the case last referred to of a cross between 
parents differing in two pairs of characters, the different combina- 

Tall Race 

with Purple 

Male Gamete 

fr<m Short 

Race with 

White Flowers 



v / 

J F t Sporophytes 
1 All Tall with 
1 Purple Flowers 

f X (2) (D < 
1 1 

I 1 
) < 




/T\ Microspores 
W a7|ff>) 

/T\ Afwro- 
\ W J gametophytes 



(4 Types) 



FIG. 395. Diagram illustrating the transmission of 2 pairs of genes, borne on 
different pairs of chromosomes. Each original parent contributed, through 
its gamete, 1 chromosome of each pair. Chromosomes of 1 pair bear re- 
spectively genes for tallness (T) and for shortness (s); those of the other 
pair, genes for purple (P) and for white (w). The FI sporophyte produces 
4 types of macrospores and 4 types of microspores. These spores give rise 
to macrogametophytes of 4 types, producing each its own type of egg; 
and to rnicrogametophytes of 4 types, producing each its own type of 
male gamete. The result of haphazard unions between different types of 
eggs and male gametes is shown in the following figure. 

tions of genes occurring in the F 2 generation result from the fact 
that all the genes are borne on separate chromosomes. If, how- 
ever, two genes (A and B) are carried on the same chromosome 
and genes a and b (respectively recessive to A and B) on the other 
chromosome of the same pair, a very different result would be 
expected. Since the chromosomes of this pair conjugate and sepa- 
rate in the reduction divisions in an FI plant, only two types of 
macrospores, and hence of eggs, would be produced. Half the eggs 
would possess each a chromosome bearing genes A and B, half 
would possess a chromosome bearing genes a and 6. The micro- 


spores, and hence the male gametes, would likewise consist of 
two classes: half having genes A and B, half having genes a and 
b. In other words, the spores and gametes would be expected to 
possess the same combinations of genes that were present in the 
original parents. The indiscriminate union of eggs and male gam- 



ITT fjrr I 
Pw IT | 

l I 

TS Ts 


PJ ~~* \PP \Pw 




FIG. 396. Diagram showing the result in the F 2 generation of haphazard 
unions between eggs and male gametes produced by the FI generation of 
a cross between a tall^ purple-flowered and a short, white-flowered plant 
(see preceding figure). Nine different combinations of genes occur in the 
F 2 zygotes and resultant sporophytes: TTPP, TTPw, TsPP, TsPw, 
TTww, Tsww, ssPPy ssPw, and ssww. Since T is dominant over s, and 
P over w, plants of the first 4 of these classes are tall and purple-flowered ; 
those of the next 2 classes, tall and white-flowered; those of the next 2 
classes, short and purple-flowered; and those of the last class, short and 
white-flowered. Hence in appearance there are 4 classes of F 2 sporophytes, 
in the proportion 9:3:3:1. 

etes would then result in the production of but three types of 
zygotes (and F2 sporophytes) instead of nine. 

If the original parents differed in three, four, or more genes, 
all these genes being borne on the same chromosome, a similar result 
would be expected. The genes which distinguish each parent, 


being carried on the same chromosome, would tend to remain 
together from generation to generation; they would be linked, or 
would constitute a linkage group. 

If the original parents differed in many genes, some borne on 
the chromosomes of each pair, those carried on any particular 
chromosome would tend to pass together from generation to gen- 
eration. There would be as many linkage groups as chromosome 
pairs. It has in fact been found that some of the genes of the pea 
tend to pass from generation to generation in groups. The ques- 
tion has not yet been tested for all the recognized genes; because 
to determine with what other genes a given gene is linked requires 
experiments carried on for some time and on a large scaH| Thus 
far six linkage groups of genes are recognized in the pea of the seven 
groups expected. The corn has ten pairs of chromosomes; ten link- 
age groups are recognized. The species of Drosophila already men- 
tioned has four pairs of chromosomes. Of approximately 500 pairs 
(or series) of genes known for this fly, the linkage relations of a 
large proportion have been determined. All are found to be ref- 
erable to four linkage *^>ups. In no species of plant or animal 
have the genes been found to constitute a number of linkage groups 
greater than the number of chromosome pairs. 

378. Crossing Over. Each of the linkage groups just referred to 
consists of genes which tend to remain together. Although cases 
of real or apparent complete linkage are known, it is the general 
rule that any two linked genes now and then become separated. 
For example, a gene for plump seeds in the pea (dominant over 
wrinkled seeds) is linked with one for the presence of leaf tendrils 
(dominant over the absence of tendrils). If a plant with plump 
seeds and tendrils is crossed with one having wrinkled seeds and 
no tendrils, and if the genes concerned were completely linked, the 
plants in the F 2 generation would be of two classes: some with 
plump seeds and tendrils, some with wrinkled seeds and no ten- 
drils. As a matter of fact, the 2 generation produced in one such 
experiment consisted of 319 plants with plump seeds and tendrils, 
four with round seeds and no tendrils, three with wrinkled seeds 
and tendrils, 123 with wrinkled seeds and no tendrils. The occur- 
rence of small numbers of plants of the second and third classes in- 
dicates that, in about one of every 64 spore mother cells in the FI 
plants, genes which were linked, and therefore were presumably 
borne on the same chromosome, became separated. 



Such an occasional separation of two linked genes seems to 
take place in that early period in the first reduction division at 
which the chromosomes of each pair are closely associated. At 
this stage, as was pointed out in 274, there is a possibility of 
some interchange between the chromosomes of each pair. Under 
very favorable conditions of fixation and staining, chromosome 

FIG. 397. Crossing over. A and B, chromosome pairs at an early stage of the 
first reduction division in a macrospore mother cell of lily. Interconnec- 
tions between half-chromosomes. Drawn by D. C. Cooper. C, diagram 
illustrating a single crossing over. D t double crossing over. E y crossing 
over (above) between half-chromosomes, and (below) between the other 
halves of the same chromosomes. 

arrangements like those shown in Figure 397, A, B y have been 
seen in a few plants and animals. This figure represents a stage 
approximately corresponding to that shown diagrammatically in 
Figure 283, C. Each chromosome of the pair is double, and at 
certain points one of the halves of each chromosome is intercon- 
nected with one half of the other chromosome of the pair. Appear- 
ances of this nature in pairing chromosomes, together with the 
facts of partial linkage, indicate that in the early stages of the 
first reduction division parts of two paired chromosomes may be 

Figure 397, C, shows diagrammatically how, in the simplest 
possible case, such crossing over would bring about new combi- 
nations of genes. The dominant genes A and B are borne on one 
chromosome and the recessive genes a and 6 on the other chromo- 


some of the same pair. A crossing over occurs between one half 
of each respective chromosome at a point between A and B (and be- 
tween a and b). When the separation of the four half -chromosomes 
is finally effected by the first and second reduction divisions, one 
of the four nuclei formed by these divisions will contain a chromo- 
some bearing genes A and B; one nucleus, a chromosome bearing 
A and b; one nucleus, a chromosome beariixg a and B; and one a 
chromosome bearing a and b. With respect to the genes on this 
pair of chromosomes, the macrospore or microspore nuclei formed 
would be of four different kinds. 

Figure 397, D and E, show some of the more complicated cases 
of crossing over which have been found to occur. 

In the study of linked genes in the pea and other organisms, 
various proportions of crossing over are found. It is considered 
that the proportion of crossing over occurring between any two 
linked genes is some measure of the distance between them on the 
chromosome those genes showing the larger proportion of cross- 
ing over being farther apart than those between which crossing over 
is relatively rare. 

The frequency with which crossing, over occurs is affected by 
temperature, as well as by X-rays and radium emanations. 

379. Variation. While resemblance between ancestors and 
offspring is general, it is not universal. Offspring, though like their 
parents in most respects, always differ, usually in minor ways, 
from the parents as well as from other members of the same fam- 
ily. The appearance of such differences is variation. The characters 
of an individual vary in some measure from the characters of the 
parent or parents. 

In most cases a character in respect to which an individual 
differs from its parent is not transmitted to future generations; the 
change that has appeared is not permanent. Variation of this 
non-heritable type seems in general to result from the development 
of the new individual under an environment different from that 
which surrounded its ancestors. Since under natural conditions 
no two environments are exactly alike, it is not surprising that no 
two wheat plants, however similar in inheritance, are precise dupli- 
cates. The effect of environment in causing variations may, there- 
fore, be observed in any field of wheat or corn or in any community 
of human beings; in each of these cases many differences between 
individuals result from differences in inheritance; but many result 



from differences in environment (see, for an extreme example of 
variation due to environment, Fig. 398). 

But sometimes a new character, once it has appeared, is passed 
on to later generations in the same way as are older characters. 
While variation producing such heritable changes is much less 
frequent than variation of the non-heritable sort, it occurs often 
enough to play an important part in giving rise to new kinds of 

plants and animals. 
Examples of heritable 
variation are the ap- 
pearance of a tree bear- 
ing smooth peaches 
(nectarines), whose an- 
cestors bore peaches of 
the downy type; the 
occurrence of a white- 
flowered plant whose 
ancestors were red- or 
blue-flowered; or of a 
beardless wheat plant 
in a regularly bearded 
variety. Such an in- 
dividual may become 
the starting-point of a 
new race; if the new 
race differs sufficiently 
from the older race 
from which it arose, it 
may be considered a 
new variety or even a 
new species. Innumer- 
able varieties of cul- 
tivated plants and of domestic animals have originated with 
individuals in which, in consequence of variation of this nature, a 
new heritable character or a new combination of such characters 
appeared. Doubtless many wild varieties and species have had a 
similar origin, although the fact can be known only when, as in 
cultivated plants and domestic animals, the first appearance of a 
new character or character-combination is actually observed. 
As has been seen, variation of the non-heritable type is largely 

FIG. 398. Influence of environment upon ex- 
ternal form (producing non-heritable varia- 
tion). A y dandelion plant grown at a low alti- 
tude. B, plant of the same species grown at a 
high altitude. Both to the same scale. Re- 
drawn from Bonnier. 



if not entirely due to the environment. It is not so clearly true 
that heritable variation is similarly caused; because the environ- 
ment seems not, as a rule, to affect the hereditary substances in 
the chromosomes. It is true, as will appear hereafter, that under 
experimental conditions certain environmental factors may cause 
changes in the number and constitution of chromosomes and in 

A B 

FIG. 399. Oenothera Lamarckiana (A) with 14 chromosomes, and a variant 
form, Oenothera gigas (B) with 28 chromosomes. After De Vries. 

the constitution of genes. But to what extent, if at all, such fac- 
tors produce similar effects in nature is still unknown. 

While the underlying causes of chromosomal and genie changes 
which result in heritable variation are yet obscure, much has 
been learned regarding the nature of these changes. A few types 
of change now fairly well known will be mentioned. 

380. Changes in Number of Chromosome Sets. In 1895 there 
appeared in De Vries' cultures of an evening primrose (Oenothera 
Lamarckiana) a plant which differed from the parent in being 
larger in most of its parts, including stem, leaves, and flowers 
(Fig. 399). Whereas the parent species has 14 (2 n) chromosomes, 


this giant plant possessed 28 (4 n). Giant forms with 4 n chromo- 
somes have appeared suddenly in cultures of various plants, in- 
cluding two other species of evening primrose, the tomato, and 
tobacco. Many cases are known in nature of closely related species, 
one of which has twice as many chromosomes as the other. There 
is little doubt that in many of these instances a sudden doubling 
of the chromosome number occurred. 

It has many times been observed that a nuclear division, either 
vegetative or reductional, was not concluded ; instead, the chromo- 
somes, which had begun to separate, were all brought together in 
a newly organized nucleus. The new nucleus then possessed a 
doubled complement of chromosomes. Such stoppage of nuclear 
division, with consequent doubling of the chromosome number, 
has been brought about experimentally in some organisms; notably 
in Spirogyra by cooling or anesthetizing the filaments, and in 
onion root tips by treatment with chloral hydrate and with other 
poisons or narcotics. A similar disturbance of division evidently 
occurs not infrequently in nature, although from unknown causes. 
In all probability many occurrences of giant forms, as of the eve- 
ning primrose, are to be traced back to an uncompleted nuclear 
division; although this is not the only way in which the chromo- 
some number may be doubled or otherwise increased. 

Other observed occurrences, among the offspring of parents with 
2 n chromosomes, have been the appearance of plants with 3 n, 
5 n, 6 n, and other multiples of n chromosomes. It has happened 
too in a number of genera, likewise including evening primroses, 
the tomato, and tobacco, that a plant (sporophyte) appeared with 
only n chromosomes. Such a plant seems to have developed from 
an egg which had failed to unite with a male gamete and which 
had, therefore, but n chromosomes. 

Any sporophyte with an even number of chromosome sets (as 
2 n or 4 n) may conceivably produce offspring substantially like 
itself. This is because when the reduction divisions occur in such 
a plant all the chromosomes can arrange themselves in pairs of 
corresponding chromosomes; hence functional spores and ulti- 
mately functional gametes may be produced. If, however, a plant 
has an odd number of chromosome sets (as norSri), regular pair- 
ing can not take place. The spores and gametes of such a plant, 
if spores and gametes are produced at all, will receive variable 
numbers and combinations of chromosomes. Most of the gametes, 



sometimes all, will not function; if any do function, the resultant 
offspring are highly variable. It follows that a sporophyte with 
n or 3 n chromosomes can not give rise to a constant variety or 

FIG. 400. Varieties of wheat representing 3 classes; below, a microspore 
mother cell from each variety. A, a wheat with 14 chromosomes (7 pairs). 
B, one with 28 chromosomes (14 pairs). C, one with 42 chromosomes (21 
pairs). Figures of microspore mother cells from Sax. 

species, unless the ordinary methods of reproduction are replaced 
by some form of multiplication (vegetative or other) which does 
not involve the union of gametes. Exactly this seems to have 
happened in some of the roses mentioned below. 

In many genera, series of species are found whose chromosome 
numbers are multiples of a common basic number. One such genus 



is that of the wheats. All known species and varieties of wheat 
fall into three classes (Fig. 400) . Those of one class have 14 chro- 
mosomes; those of another class 28; those of a third, 42. Those 

with 28 and those with 
42 chromosomes seem 
to be descended from 
plants with like num- 
bers that arose as va- 
riants from races with 
14; although there are 
suggestions that hy- 
bridization with plants 
of a related genus hav- 
ing likewise 14 chromo- 
somes may have been 
concerned in the pro- 
duction of the species 
with higher numbers. 
The roses provide a 
similar but more exten- 
sive series. Some spe- 
cies and varieties have 
14 chromosomes; others 
have respectively 21, 
28, 35, 42, and 56. 

381. Other Changes 
in Chromosome Num- 
ber. Another mode of 
departure from the or- 
dinary course of nuclear 
division consists in the 
FIG. 401. Shoots of Oenothera Lamarckiana (A) failure of sister chro- 
and of 2 variant forms derived from it: (B) , ,, 

Oenothera nanetta, a dwarf form having 14 niOSOines (or, in the 
chromosomes (2 n) like the parent species, first reduction division, 
and (C) Oenothera lata, with 15 chromosomes nf thp rhrnmnQrmu>a nf 

/rt , ^ v -pv . ,, _- __ . VJJ. t>U.v/ Vylll \Ji.i.l\Jo\jmXsQ VJA 

(2 n + 1). Redrawn from De Vnes. . x . , , 

a pair) to separate and 

to pass to opposite poles. If both pass to the same pole, one 
daughter nucleus receives one more, the other daughter nucleus 
one less, than the usual number. If this occurs in the direct 
ancestry of spores or gametes, some of these reproductive cells 




>,' inj^ v r f( ^ f j ih rV t j ; it', 

[ it 1 If/ if^lrWf *- 'i " I I V 1 ' 

,'i r vte" j iv r ^,., i ,v r , 1,1 ' ' j, 1 ;, " 

FIG. 402. Above, a 1'ruit of the Jirnson weed (Datura Stramonium), which has 
24 chromosomes (2 ri). Below, the fruits of 12 variant forms that have 
arisen from this species, each variant having 25 chromosomes (2 n + 1). 
After Blakeslee, in the Journal of Heredity. 


will possess n + 1, others n 1 chromosomes. If then a plant 
produces some gametes with each of these numbers, together with 
other gametes having n chromosomes, haphazard union of such 
diverse gametes might result in the appearance of offspring having 
variously 2 n 1, 2 n, and 2 n + 1 chromosomes. 

The evening primrose already referred to has produced a number 
of forms having 15 (2 n + 1) chromosomes (Fig. 401, C). Fifteen- 
chromosome plants differ markedly from the parent form in such 
characters as form of leaf and habit of growth. Each such variant 
plant seems to have been produced by the union of a gamete with 
7 (n) and one with 8 (n + 1) chromosomes. The Jirnson weed 
(Datura Stramonium) also has given rise to a series of forms with 
2 n + 1 (in this case 25) chromosomes (Fig. 402). In both of these 
as well as in other genera, the theoretically possible forms with 
2 n 1 chromosomes do not occur. Apparently gametes with 
n 1 chromosomes do not function or, if they do, the resultant 
zygotes with 2 n 1 chromosomes do not develop into new 

As a result of other irregularities in nuclear division more or less 
similar to that just described, many different chromosome group- 
ings may arise. Evening primrose plants have appeared with 
chromosome numbers ranging from 7 (n) to 30 (4 n + 2) . A corre- 
sponding series in the Jimson weed runs from 12 (n) to 51 (4 n + 3). 
Evidently many plants with variant chromosome numbers can 
not, for reasons already stated, perpetuate themselves. Others 
may do so. For example, if a plant has 2 n + 2 chromosomes, the 
two extra chromosomes representing the same pair, a regular 
pairing may conceivably occur and functional spores and gametes 
may be produced. There are genera whose species growing in the 
wild show series of chromosome numbers which may well have 
arisen in consequence of irregular behavior on the part of individual 
chromosomes. An example is Carex (the sedges), whose various 
species have such chromosome numbers as 30, 32, 36, 38, 48, 50, 52, 
and up to 112. 

As to the causes which in nature modify the usual course of 
division, affecting either single chromosomes or chromosome sets, 
nothing is certainly known. There is evidence that high or low 
temperatures may play a part. Experimentally, the proportional 
occurrence of such irregularities has been increased by treatment 
with X-rays or with radium. 


382. Changes Affecting Parts of Chromosomes. Apart from 
cases of unusual behavior of whole chromosomes, irregularities 
or accidents sometimes cause transverse breaks in chromosomes. 
Such breaks may take place during nuclear division, and also, 
apparently, at other times as well. They occur under natural as 
well as under cultural conditions. The proportion of their occur- 
rence, like that of the deviations previously discussed, is increased 
by treatment with X-rays or radium. 

A chromosome once broken, various fates may overtake its 
fragments. They may remain separate; in such a case one part 
is often lost: but apparently it sometimes persists, so increasing 
by one the chromosome number of the nucleus containing it and 
of the descendants of that nucleus. One fragment may be inverted 
and reattached to the other; or it may become attached to a differ- 
ent chromosome. If two breaks occur in one chromosome, the mid- 
dle fragment may be inverted and reattached, or it may become 
separated and the end portions reunited. If breaks occur in two 
chromosomes, sometimes there is an exchange of segments and a 
reattachment in new combinations. 

Varied effects upon inheritance result from changes such as 
these. The permanent loss of a part of a chromosome seems not 
necessarily to be fatal to the cells of the individual in which that 
loss occurs; but it may result in a failure to produce either func- 
tional gametes or functional zygotes. If, on the other hand, both 
or all fragments of a chromosome are retained, the consequence 
may be the appearance of a race with an increased chromosome 
number. It is possible that such a series of chromosome numbers 
as characterizes the sedges may have resulted in part from chromo- 
some-fragmentation. An interchange of segments between two 
different chromosomes leads to changes in the arrangement of 
linkage groups, and to some degree of cross-sterility between the 
new form and the old. Such segmcntal interchanges have occurred 
in the establishment of races of certain plants, including corn and 

383. Changes in Nature of Genes. These, according to current 
views, explain the majority of cases of heritable variation. In 
many instances a red- or blue-flowered species has been observed to 
give rise to a white-flowered race. In Drosophila a red-eyed species 
occasionally produces individuals whose eyes are vermilion or 
white. The evening primrose has now and then produced a dwarf 



plant (Fig. 401, B), and at other times a plant with especially 
short styles (Fig. 403). In such cases as these, if the new form is 
crossed with the parent form, the offspring show 
a distribution of the pair of characters concerned 
similar to that which appeared in Mendel's ex- 
periments. That is, the offspring behave as though 
the new race differed from the old in a single gene. 
It is assumed in each >such case that a gene of the 
parent has become changed and that the change 
in the gene caused the appearance of the new 
character. It is assumed also that, when two races 
occurring in nature are crossed, if the character or 
characters that distinguish them are distributed 
among the offspring in a Mendelian fashion, then 
one of those races was derived from the other (or 
each from a common ancestor) in consequence of 
changes in one or more genes. As yet nothing is 
known about the factors which in nature cause 
such changes. It is known, however, that the pro- 
portion in which they occur can be increased by 
radiation (X-rays and radium) and by high tem- 

Changes involving genes may conceivably con- 
sist in changes in the nature of old genes, in their 
complete loss, or in the appearance of entirely 
new genes. It is an interesting fact that in the 
experimental study of variation a great number of 
variations have been -observed which can be explained as due to 
changes in, or loss of, genes, whereas comparatively few seem prob- 
ably to have resulted from the appearance of new genes. 

Changes in number of chromosomes and those due to the frag- 
mentation of chromosomes can be directly observed. Changes in 
genes can not, since the genes themselves are too small to be seen. 
The occurrence of such changes, therefore, can only be inferred 
from the indirect evidence supplied by breeding experiments. 

384. New Races Resulting from Variation. While individuals 
produced by the methods of heritable variation just discussed 
differ greatly in their ability to survive and reproduce, and in the 
constancy with which their characters are transmitted to offspring, 
many such variant plants and animals have proved vigorous and 


A B 

FIG. 403. Pistils 
of Oenothera 
(A) and Oeno- 
thera brevisty- 
lis (B), the 
latter a vari- 
ant form with 
short styles. 
Redrawn from 
De Vries. 


capable of giving rise to offspring like themselves. Hence these 
methods of variation are likewise methods by which new, stable 
races arise. It is worth noting, however, that races coming into 
being in consequence of changes in the distribution of chromosomes 
or of parts of chromosomes possess characters which, however new 
in appearance, are the expression of previously existing genes now 
present in changed numbers, changed proportions, or changed 
combinations. Real progress in an evolutionary sense can ap- 
parently come about only through the development of new genes; 
and this, so far as can be seen at present, is a rare occurrence. 

385. New Races Produced from Crosses. When a stable new 
race has appeared differing from its parent race in one or more 
genes, it will usually interbreed with the old race as well as with 
other new races derived from the same parent race. That is, 
crosses will occur, or can be brought about, between individuals 
differing with respect to one, two, or several pairs of genes. If two 
individuals differing in two or more pairs of genes are mated, their 
descendants in the F 2 generation, in consequence of recombinations 
of chromosomes and of crossing over between chromosomes, will 
possess varied combinations of grandparental genes. Among these 
combinations will be some that differ from the combination pos- 
sessed by either grandparent. Individuals with these new combina- 
tions of genes will usually present new combinations of visible 
characters; and if they are able to live and to reproduce, they may 
give rise to new races. 

Such production of new races by the crossing of related forms, 
making possible varieties with new combinations of desirable 
qualities, is largely used by plant and animal breeders. Many 
varieties of useful plants and animals have resulted from crosses 
between different varieties or different species for example, nu- 
merous cultivated apples, potatoes, roses, and orchids. New 
races, varieties, and species are likewise constantly arising in 
nature from accidental crosses between related but distinct forms. 
In each case, whether in cultivation or in nature, the varieties 
or species to be crossed must previously have originated by vari- 
ation from a common source. This method of producing new races 
likewise gives rise to no new genes; some of the old genes are com- 
bined in new ways. 



386. The Facts of Evolution. In the preceding chapter was 
given a brief outline of what is now known regarding the ways 
in which new races and species of plants and animals come into 
existence. Observation shows that new races and species do thus 
appear from time to time, and that some of them increase in num- 
bers and in the extent of territory occupied, and become estab- 
lished among older species. It is observed too that some older 
species are growing fewer in number of individuals and are occupy- 
ing gradually smaller areas, and that from time to time a species 
disappears altogether. The plant and animal population of the 
earth is therefore constantly changing in consequence of the ap- 
pearance of new species derived from older ones, of changes in the 
proportional numbers of different species and in the area they 
occupy, arid of the disappearance of some of the older forms. 
This continuous process of change in the make-up of the earth's 
population is referred to as evolution, or organic evolution. 

387. The Evolutionary Generalizations. The constant and 
gradual change in the population of the earth, now seen to be in 
progress, has been going on from as early a time as history and 
archeology record. The process is a slow one, and individual 
species may continue iii existence for thousands or even millions 
of years. Nevertheless, the present condition of the earth's sur- 
face, as concerns the species of plants and animals inhabiting 
it and their distribution, is, taken as a whole, different from the 
condition that existed when the Egyptian pyramids were built, 
and still more widely different from that which prevailed in the 
days of the Cro-Magnons. Upon this well-established fact of con- 
stant change is based a generalization which says that a similar 
process of gradual change has been continuously in operation 
since living organisms first appeared upon the earth. 

While the constancy of change in population over a very long 
period of time is established by direct evidence, it can not be 
directly demonstrated that new species which have appeared from 



time to time have invariably descended from older species. Such 
an origin is absolutely shown for a new species only when the 
origin of that species is actually observed. However, the fact that 
new species are seen to arise from older ones, and the further fact 
that variation is universally characteristic of living organisms, 
have led to a second generalization, namely: that all species now 
living arose by descent from older species, those from still older 
ones, and so on back to the organisms that w r ere first to inhabit the 

These two generalizations, supported as will be seen by an im- 
mense mass of evidence derived from many independent sources, 
constitute foundation stones of present-day thought. It was 
Darwin who first brought together much of the evidence in their 
favor. No one who, since the publication of his " Origin of Species " 
in 1859, has impartially investigated this evidence has questioned 
the validity or the usefulness of the idea of continuous evolution. 
But it must be remembered that the second generalization in par- 
ticular is not itself, and probably never can become, an established 
fact. While all the available evidence tends to confirm the concep- 
tion, it can not be absolutely proved that all species have arisen, 
as species are now seen to arise, by descent from older species. 

The evidence upon which these important generalizations are 
based is of six general sorts: the observed origin of new races; 
the facts of classification; the facts of underlying similarity in 
structure; the facts of similarity in function and development; 
the facts of geographic distribution; and the fossil record of ex- 
tinct species. Of these six classes of evidence, the first-mentioned 
has been sufficiently discussed. 

388. Evidence from Classification. When a considerable bulk 
of information accumulates upon any subject, it becomes neces- 
sary to classify that information in order that it may readily be 
utilized. The Greek and Roman observers who, so far as is known, 
were the first to preserve in writing extended observations upon 
plants, saw that many individuals possess much the same charac- 
teristics; they grouped together all the plants that seemed to 
them substantially alike under one name. So arose the conception 
of species, each species including many individuals. As the num- 
ber of known species increased, those which seemed more or less 
alike were grouped together in such larger units as are nowadays 
called genera and families. The first classifications of this nature 


were artificial. The classification of Linnaeus, published in 1753, 
which was by far the most important up to its time, was strictly 
artificial; it was based, so far as seed plants were concerned, 
primarily upon the number of stamens in the flower of each species. 

As knowledge of plants increased, it was seen more and more 
clearly that species fall naturally into larger groups, the sim- 
ilarity between the species within any group being indicated, not 
by one character alone such as number of stamens, but by many 
characters of flowers, fruits, and vegetative parts. Most classifi- 
cations since Linnaeus' time have attempted to take into consid- 
eration these numerous similarities between species; present-day 
classifications are, therefore, as far as available information per- 
mits, natural instead of artificial. A natural classification expresses 
the fact that some species are so closely similar that they belong 
together in a group called a genus ; that several genera, while show- 
ing somewhat wider differences than those between closely related 
species, are sufficiently alike to be grouped in a family; and, like- 
wise, that related families belong together in an order, related 
orders in a class, and related classes in a division. A natural clas- 
sification is an expression of various degrees of likeness that actu- 
ally exist among plants. Among species of animals, also, varying 
degrees of similarity appear, and the classification of animals 
like that of plants has progressed from the stage of an artificial to 
that of a natural system. 

It seems impossible to explain the occurrence of such a scale of 
similarities between species except by supposing that each degree 
of similarity represents a comparable degree of relationship. 
Relationship implies that all the species of a genus are descended 
from a single species, that all the genera of a family are descended 
from a single but more remote source, and so on for the origin 
each from a single source of the families of an order, the orders of a 
class, and the classes of a division. 

389. Evidence from Structure. The classification of plants and 
of animals is based in the main upon details of structure; but in 
classifying, ordinarily only those structural features are taken into 
account which are found especially useful in making the clas- 
sification. When, however, a study is made of all the elements 
that enter into the structure of particular species, further evidence 
appears as to the relationships between species, and more espe- 
cially between larger groups. For instance, the presence of leaves 


borne on a stem belonging to the sporophytic generation is a 
character practically universal throughout pteridophytes and seed 
plants. Although leaves show the greatest diversity in form and 
function for example, foliage and scale leaves, spines and ten- 
drils their manner of origin and their development show them to 
be all of fundamentally the same nature. A like statement may be 
made of the various forms presented by stems and by roots. That 
is, the same general plan of structure characterizes the sporophytes 
of all the species that are grouped together as pteridophytes and 
seed plants. The universality of a general plan of structure 
throughout these two divisions seems explainable only by suppos- 
ing that all pteridophytes and seed plants are descended from a 
common ancestry. 

Confirmatory evidence is afforded by the occurrence of strobili 
of similar general plan in various orders of gymnosperms, and of 
flowers, likewise of similar general plan, throughout the angio- 
sperms; and, among internal structures, by general likeness in 
the vascular systems of pteridophytes and seed plants. The pres- 
ence of archegonia, again of the same general plan, in all bryophytes 
and pteridophytes points, with other similarities in the gameto- 
phytic generation, to a relationship .between the members of these 
two divisions. Perhaps the most far-reaching evidence of rela- 
tionship is offered by the regular recurrence of flagellate cells, 
in the form of swarm-spores in algae and fungi, and in that of 
gametes, especially male gametes, in algae, fungi, bryophytes, 
pteridophytes, and the more primitive seed plants (such as Zamia). 
The widespread power of forming flagellate cells seems to point 
to the descent of the plants of all these groups from flagellates. 

390. Evidence from Similarity in Function and Development. 
The study of the functions of plants, like the study of their struc- 
ture, shows likenesses in important respects between the members 
of each major group, as well as similarities of less fundamental 
character within the limits of smaller groups. Indeed, certain 
characteristics are common to all living organisms, such as the 
essential structure of living matter itself and its organization into 
cells, the power of responding in varied ways to stimuli of many 
sorts, and the ability to carry on both constructive and destructive 
metabolism. The universal possession by all plants and animals 
of these powers and characteristics unavoidably suggests their 
descent from a common ancestry. On the other hand, many func- 


tions are peculiar to, or especially characteristic of, certain groups 
and, taken together with other similarities, indicate a relationship 
between the members of each such group. The plants of some 
groups form cell walls largely by the secretion of cellulose; those of 
other groups, by the secretion of chitin. The typical reserve food 
manufactured by the organisms of some groups is a sugar; in other 
groups it is starch; in others, glycogen; in still others, a fat. An 
illustration of a function characteristic of a relatively small group 
of plants is furnished by members of the pulse family; the reserve 
foods which most of them store in largest amount in their seeds are 
proteins, whereas starch is the most abundant reserve food in the 
seeds of most other families of angiosperms. 

The light thrown upon evolution by the development of indi- 
vidual organisms may be considered with that supplied by other 
functions, since development is itself a function of the developing 
organism. The history of all many-celled plants and animals is 
alike in that each individual begins its existence as a single cell, 
and that its development to maturity consists in a series of cell 
divisions together with an increasing differentiation of cells into 
what are, in the more complex species, tissues and organs. In 
addition to this general resemblance between all organisms, more 
detailed similarities appear in the development of individuals of 
separate divisions, classes, and orders. For instance, the seedlings 
of most if not all conifers bear needle-like leaves on long branches; 
although mature plants of different species, such as pines and arbor 
vitae, differ greatly in the form and arrangement of their leaves. 
The structure of the embryonic sporophyte of the bracken, con- 
sisting of root, leaf, foot, and stern, is characteristic of the corre- 
sponding stage in the sporophytic development of most other ferns. 
Other pteridophytes, such as Equisetum and the club mosses, have 
embryos similar to that of the bracken in general plan, although 
differing in important respects. The differences indicate a more 
remote relationship between Equisetum, club mosses, and ferns 
than that among the ferns themselves. As has been seen, dicotyle- 
dons and monocotyledons are distinguished in several ways which 
indicate that these two classes of angiosperms have long been 

391. Evidence from Geographic Distribution. The distribution 
of plants also furnishes much evidence as to relationships between 
species as well as between genera, families, and orders. In general, 


where a large area of land exists, sufficiently uniform as to climate 
and soil and not broken by barriers which interfere with the migra- 
tion of plants, its native flora is likewise uniform ; that is, its whole 
extent is inhabited by the same or closely similar species. Instances 
of such large areas in North America occupied by uniform floras 
are to be found in the tundra, the northern evergreen forest, and 
other regions to be described in the following chapter. On the 
other hand, if an effective barrier exists, such as a high mountain 
range, an extensive desert, or a large body of water, the floras on 
opposite sides of the barrier are likely to be very different. For 
example, the flora of the region west of the Rocky Mountains 
differs greatly from that of the region to the east. Much of this 
difference, to be sure, is due to climatic differences; but when, as 
is frequently the case, two species of the same genus occur on 
opposite sides of the mountains, they are usually so different as to 
suggest that they have been separated and have undergone evolu- 
tionary changes in divergent directions during a considerable time. 
The flora of Madagascar and that of the neighboring coast of 
Africa are very distinct. Marked differences exist also between the 
floras of Australia and of the Asiatic mainland. 

The degrees of similarity or difference between the floras of 
separate bodies of land, such as islands or continents, furnish in- 
dications as to whether or not such land areas were at one time 
connected; and conclusions upon such points, based upon the 
characteristics of floras, agree in general with conclusions founded 
upon geological study. There is sufficient likeness between the 
floras of eastern North America and western Europe although 
the native species of these regions are as a rule distinct to render 
it probable that at a not very distant geological period the two 
continents were connected by land that has now disappeared. 

392. Evidence from the Fossil Record. In the sedimentary 
rocks which were formed from deposits at the bottoms of bodies 
of water in past ages are many remains of plant parts. Some of 
these remains are impressions or casts, which show only the gen- 
eral form and the surface structure of the plants or organs that 
formed them. Others are petrifactions, resulting from the gradual 
replacement of the materials of plant bodies by mineral substances 
deposited from a solution with which the plants or plant parts 
were impregnated. Sections of a petrifaction show much of the 
original structure of the plant, often to minute microscopic details. 


Necessarily, fossils, with rare exceptions, show only broken, 
often partly decayed, fragments of plants. The softer plants, 
such as algae and mosses, are less often preserved than are harder, 
more woody plants or those with silicified cell walls. In the time 
that has elapsed since fossil-bearing rocks were formed, these rocks 
have been subjected to great changes in consequence especially of 
heat and pressure; and in the course of such changes many of the 
fossils present were destroyed. For all these reasons, the fossil 
record of ancient plants is very fragmentary, with many large gaps 
which laborious investigation is but slowly closing. 

In spite of its incompleteness, however, the fossil record supplies 
much information regarding the nature of plants of past times. 
The evidence so obtained as to the general course of evolution 
agrees with that furnished by the structure and functions of liv- 
ing plants. The distribution of fossils through rocks of different 
ages indicates, for example, that the earliest plants were compara- 
tively simple water-inhabiting species. In later ages appeared 
pteridophytes, primitive seed plants, forms more or less similar 
to present-day gymnosperms, and finally aiigiosperms. 

393. General Course of Evolution. Six classes of evidence have 
now been cited as indicating that evolution has been a continuous 
process from the first appearance of living organisms upon the 
earth. All this evidence, except that belonging in the first category 
(the observed origin of new races) agrees in indicating that the 
course of evolution has been in general, though with many excep- 
tions, from simplicity to complexity. Primitive organisms seem 
to have been very simple, and larger and more complex ones to 
have come into existence step by step. So strongly is this concep- 
tion of the course of evolution supported by the available facts 
that it is virtually unquestioned. The series of types described 
in previous chapters, leading from Chlamydomonas to angio- 
sperms, illustrates the accepted notion as to the general course 
that the evolution of plants has followed. 

This conception implies that from time to time heritable vari- 
ations have occurred, each of which introduced a new character 
or a new group of characters. But it is notable that, so far as the 
actual origin of new races is now observed, such a race seems to 
arise in the great majority of cases in consequence of a recombi- 
nation of genes already existing, or as a result of a change in or 
loss of a gene. Only in rare cases is a new race observed to arise 


apparently because of the acquisition of a new gene. It is true, 
because of the complicated interrelations between genes and char- 
acters ( 375), that a new combination of genes, or even at times 
perhaps the loss of a gene, may result in the appearance of what 
must be considered a new character. Nevertheless, the progressive 
evolution of new species possessed of new possibilities would seem 
to have meant the acquisition from time to time of new genes. 
The seeming discrepancy between the genera] course that evolu- 
tion appears to have followed in the past and the course that it 
is observed to be following at present is not to be overlooked. 
This discrepancy may perhaps be explained by the consideration 
that the progress of evolution has been extremely slow. Even 
the rare development of new genes may, in the course of the hun- 
dreds of millions or billions of years during which organic evolu- 
tion has been going on, have brought into existence the diversified 
forms of plants and animals that now populate the earth. 

394. Survival and Extinction of Races. The evidence just out- 
lined indicates that, as new forms are now coming into being, so 
new races and species have arisen in the past. Of the plant and 
animal forms that have originated by variation, some have been 
very short-lived ; some have become well-established species which 
persisted during long periods, although the great majority of 
species that lived in previous ages sooner or later disappeared. 
Evolution the progressive change in the sum total of organisms 
inhabiting the earth depends not only upon the appearance of 
new races as a result of variation, but also upon the relative ability 
of new as well as of older races to perpetuate themselves. 

Whether or not a race shall persist, and if it does, how widely 
it shall become distributed, depends upon the interaction of many 
factors. These factors may be classed under four heads: the ability 
of the race to reproduce; the degree of its adjustment to the non- 
living environment; conflict of interests with other organisms; and 
cooperation with other organisms. 

395. Power of Reproduction. Reproduction includes all means 
by which the number of individuals of a species may be increased ; 
among them, cell division in one-celled organisms; in many-celled 
organisms, the formation of spores and other special reproductive 
bodies, gametic union, and varied methods of vegetative multipli- 
cation. Other things being equal, a species which multiplies rapidly 
is more likely to survive and spread than is one which multiplies 


slowly. The great success of weeds like the dandelion and Canada 
thistle is largely a result of their remarkable powers of reproduc- 
tion. Rapid multiplication is especially important to such sapro- 
phytic plants as the bread mold, or to parasitic plants like the 
wheat rust. Both are dependent upon the more or less accidental 
and temporary presence of the necessary substrate or host; and 
both produce immense numbers of spores, so increasing the chance 
that some spores may reach the host or substrate. 

396. Adjustment to the Non-living Environment. An organism 
is dependent for existence upon the conditions surrounding it. If 
it lives in water, it must be able to secure from the water the sub- 
stances essential to its metabolic processes. If, like most seed 
plants, it lives partly in the soil and partly in the air, its structure 
must enable it to secure from these two sources the necessary 
materials, such as carbon dioxide, oxygen, water, and inorganic 
salts, and must prevent the loss of water at a more rapid rate than 
that at which it can be obtained. If surrounding conditions are 
subject to periodic changes, the organism must be able to pass 
into stages in which its functions or structure, or both, are modified 
to correspond with the changes in environment. Thus Spirogyra, 
which often lives in ponds that become dry in summer, forms zy- 
gotes that can endure drying and can germinate when water is 
again present. Any perennial seed plant of a temperate or cold 
region has means of preserving alive through the winter, though in 
a dormant condition, either its whole body (as an evergreen tree) 
or a part (such as a tuber or underground stem) and of resuming 
vegetative activity upon the return of warmer weather. 

Now and then the environment changes in a way that affects 
all plants and animals over a large area. A low region is uplifted, 
becoming drier and perhaps colder; or marked climatic changes 
occur, such as led to the glacial period in the northern hemisphere, 
and later to the disappearance of glaciers and the restoration of a 
milder climate over large parts of Europe and North America. 
Such changes on a large scale profoundly affect the course of plant 
and animal evolution. Among the older organisms of the region, 
only those survive that are adjusted to the new conditions; and 
among new forms that may result from variation, especial op- 
portunities are offered, because of the disappearance of many older 
species, to those whose structure and functions fit them to the 
changed environment. 


397. Conflict of Interests. The interests of different individuals 
of the same or of different species come into conflict in a variety 
of ways. In general, as Darwin long ago pointed out, most species 
can give rise by reproduction to vastly more individuals than the 
available supply of foods could support. An illustration of such a 
possibility in the case of bacteria was given in 189. While bacte- 
ria reproduce more rapidly than do most other organisms, the same 
general principle applies to most species of plants and animals. 
Consequently, there occurs among the individuals of each species 
a competition for food materials; and those individuals that are 
best adapted by length of root, rapidity of growth, power of ab- 
sorption, or in any one of many ways to succeed in obtaining 
nutrients are those which will survive and will in turn produce off- 
spring. In so far as the advantages possessed by such individuals 
are heritable, their offspring will possess the same favorable char- 
acteristics. In this way the competition for food materials tends 
to select those strains within a species that are test fitted to secure 
nourishment, and so to improve the average of the species in this 
respect. Just as does a competition for food materials, so a com- 
petition for favorable conditions, such as a temperature suitable 
for growth, results from the presence of an increasing population. 
In various respects, therefore, the tendency to overpopulation 
brings about, through competition, an improvement- in the average 
capacity of each species to maintain itself. Similarly, there is com- 
petition Ixjtwoen different species for food materials and other 
necessities. The net result of the crowding of population and of 
the consequent competition is to select those species, and those 
strains or races within each species, that are best fitted to maintain 
themselves under the conditions surrounding them. 

The survival of strains, races, and species is affected by other 
forms of conflict which are not so obviously competitive. One is 
the preying of some organisms upon others; a particular form of 
this is parasitism. The relations between a parasitic fungus and 
its host plant favor, on the side of the parasite, those individuals 
best fitted to secure nourishment from the host; and on the side 
of the host, those individuals that are most effectively guarded 
from the attacks of the parasite, or that can best survive the in- 
juries which the parasite inflicts. Another illustration of conflict 
is that between the human species and weeds. Weeds conflict 
with man's practical interests when they interfere with the grow- 


ing of crops; and with his esthetic interests when they deface 
lawns and parks. 

398. Cooperation. Competition as an evolutionary factor is 
much discussed. Not so much is ordinarily said of the bearing upon 
evolution of cooperation between individuals of the same or of 
different species. Yet the part played by cooperation in evolution- 
ary development has perhaps been fully as great as that played by 
competition. Among very primitive organisms, cooperation is 
illustrated by the tendency in many lines of descent for one-celled 
organisms to come together or to remain together in colonies. 
Further steps in cooperation were taken when different cells of the 
same colony took on different functions and became differentiated 
in structure; this differentiation finally leading to the development 
of tissues. As between more complex plants arid animals of the 
same or of different species, there are innumerable illustrations of 
the tendency to cooperate. One type of cooperation is the establish- 
ment of a partnership, as between a leguminous plant and the 
bacteria in the nodules of its roots; between forest trees and the 
fungi whose mycelia become closely associated with their roots; 
or between the fungus and the alga in a lichen. Another type of 
cooperation is seen in the formation of plant associations, illus- 
trated by the relations between forest trees and the shrubs and 
herbaceous plants that grow in their shade. Another is illustrated 
by insect pollination and the accompanying interrelations between 
angiosperms and insects; still another by the cultivation by man 
of useful and desirable plants and animals. A very extensive piece 
of cooperation is involved in the nitrogen cycle ( 197), participated 
in by most of the many-celled plants and animals, some of the 
higher fungi, and many bacteria. 

399. Natural Selection. The factors belonging to the three 
classes last mentioned may be grouped together as involving the 
relations between an organism and its environment; since the en- 
vironment of any individual includes the other organisms, as well 
as the non-living things, with which it comes in contact. The 
effect of all these environmental factors, taken together, upon the 
course of evolution is often referred to as "natural selection," be- 
cause the net result of the influences at work is to preserve or select 
those individuals, races, and species that are best adapted to the 
environment. Differences in power of reproduction between 
different species may tend to the perpetuation and extension of a 


species which is also favored by natural selection. On the other 
hand, it may and often does happen that these two sets of selective 
factors (natural selection and differential powers of reproduction) 
work in opposite directions; so that a species that is favored by its 
powers of rapid reproduction is discriminated against by natural 
selection, or vice versa. Much the same idea as that involved in 
natural selection, but with emphasis upon the competitive factors, 
is expressed by the phrase "struggle for existence." The term 
"artificial selection " is sometimes applied to the conscious selec- 
tion of desirable races by man. But since man is one of the species 
that constitute a part of the environment of other species, the 
distinction between natural and artificial selection is meaningless. 



400. Factors Concerned in Distribution. Under natural con- 
ditions the distribution of plants over a given area is governed by 
a complex of factors which are in part hereditary and in part 
environmental. The nature of the hereditary factors, which affect 
in very important ways the ability of a plant to live in a particular 
environment, has been discussed in Chapter XXXII. The en- 
vironmental factors fall naturally into two groups: those related 

FIG. 404. Relation of soil moisture to the distribution of vegetation. The 
portion of the hill at the right is exposed to drying summer winds. The 
soil of the sheltered northern slope at the left retains sufficient moisture 
to permit the growth of a forest. 

to climate, such as temperature, moisture, light, and wind; and 
those related to the soil, including its physical make-up, its chem- 
ical composition, its slope and drainage, and the amount of avail- 
able water. 

The hereditary endowment of some plants is such that they can 
become adapted to a wide range of habitats. The common dande- 
lion, for example, thrives on a great variety of soils and ranges 
from lowlands to mountain tops. Most plants, however, can not 
become adapted to so wide a range of conditions; their distribu- 
tion, therefore, is dependent upon a more definite set of factors, 
the absence of any one of which from the environment makes the 
existence of the plant in that habitat impossible. Many species 



Northern Evergreen Forest 
Deciduous Forest 
Southern Evergreen Forest 

Tropical Forest 

FIG. 405. General regions of vegetation in North America. 



of tropical or subtropical plants, such as palms, oranges, lemons, 
and bananas, are badly injured or killed by freezing temperatures. 
Cacti, growing best on arid soils, are unable to live in wet, poorly 
aerated soils. Cranberries find their natural habitat in acid bogs 
and die quickly if transferred to neutral or slightly alkaline soil. 
Seedlings of hemlock grow best in dense shade, but those of some 
poplars require abundant light for growth and development. 

401. General Regions of Vegetation. In consequence of the 
interaction of the various factors concerned in the distribution of 

FIG. 406. The tundra. 

plants, North America may be divided roughly into four regions of 
vegetation : tundra, forests, grasslands, and deserts. Each of these 
general regions is of ceurse capable of further division and subdi- 
vision. In the following discussion the boundaries of the respective 
regions are given only in a general way, since they merge one into 
another, often with broad transitional zones; and the brief descrip- 
tions are of the vegetation as it existed before the extensive settle- 
ment of the continent. In the United States, particularly, man 
has destroyed or profoundly modified much of the native vegeta- 

402. Tundra. The tundra, in general, fringes the northern 
limits of the continent from Alaska to Labrador. Here the win- 
ters are long and cold, with relatively light snowfall. The air in 
winter is very dry, and often strong winds blow. The growing 


reason is of short duration. Only the upper portion of the soil 
thaws, to a depth of from a few inches to one or more feet, this 
depth varying chiefly with the direction of the slope; consequently 
the soil temperature is low and the ground water is cold. The 
plants that are able to thrive under these conditions include cer- 
tain mosses, lichens, grasses, and sedges, a few other herbs, and 
some shrubs. Many of the herbaceous species bear relatively 
large and brightly colored flowers, although their stems are for 
the most part very short so that they form rosettes or compact 
cushions. The shrubs of the tundra are likewise characteristically 
low; there are several species of willow that grow to only a few 
inches in height. 

403. Northern Evergreen Forest. In general, the northern ever- 
green forest stretches across the continent from the Atlantic to the 
Pacific. Its southern boundary extends from Vermont westward 
to the Great Lakes and, including the northern portions of Michi- 
gan, Wisconsin, and Minnesota, swings sharply northwest to the 
eastern slopes of the Rocky Mountains. Thence it extends north- 
ward to Alaska. A wide transitional belt of mixed type joins the 
tundra with the densely forested area. In this belt forests fringe 
the rivers, but over large areas trees are scattered singly or in 
small groups. The dense forest is composed for the most part 
of conifers. Among them are black spruce, white spruce, balsam 
fir, tamarack, arbor vitae, hemlock, white pine, red ("Norway") 
pine, and jack pine. Deciduous trees (belonging to the angio- 
sperms) occur among the conifers, and, especially where the origi- 
nal forest has been removed by cutting or burning, deciduous trees 
may form extensive pure stands. Prominent among them are the 
aspen, white birch, and balsam poplar. 

404. Deciduous Forest. Merging on the north with the ever- 
green forest, the deciduous forest occupies an area extending 
approximately from central New York southwest along the Ap- 
palachians to Louisiana and Texas, its western boundary stretch- 
ing from eastern Oklahoma to southern Wisconsin. This forest 
reaches its most characteristic development in the mountainous 
area of western North Carolina and eastern Tennessee. Among 
the common trees of this area are the white oak, black oak, scarlet 
oak, shagbark hickory, pignut hickory, sugar maple, red maple, 
chestnut, birch, ash, elm, walnut, and tulip tree. Associated with 
some of the deciduous trees, conifers such as the short-leaf pine, 



white pine, and hemlock occur in the mountainous regions or on 
high hills. Rhododendron and various other shrubby plants often 
form extensive undergrowths on the mountain sides. 

405. Southern Evergreen Forest. This forest area covers the 
coastal plains from eastern Virginia to Texas. The low, rolling, 

FIG. 407. A stand of white pine in the northern evergreen forest. 
After Moon and Brown. 

sandy land near the coast from South Carolina to Louisiana is the 
habitat of the long-leaf pine. In and about the numerous and 
extensive swamps are live oaks, water oaks, bald cypress, gums, and 
magnolias. These trees are often heavily draped with an epiphytic 
seed plant, Tillandsia, commonly called "gray moss" or "Florida 


moss." On the higher portions of the coastal plain, and more re- 
mote from the sea, are areas of short-leaf pine which merge into the 
deciduous forest of the Appalachian foothills. 

406. Tropical Forest. The tropical-forest area includes the 
southern quarter of the peninsula of Florida, most of the coastal 
margin of Mexico, all of Central America, and the islands of the 
West Indies. The type of tropical forest developed in southern 
Florida is meager, but the tropical-forest relationship is shown 

FIG. 408. A deciduous forest. Photograph by E. J. Kraus. 

by various palms and other tropical trees, by lianas (climbing 
woody vines, Fig. 409), and by tropical epiphytes such as brome- 
liads and orchids. Along the coast, and fringing the keys, are 
characteristic mangrove swamps such as are usually found on 
muddy tropical shores. 

The broad coastal plain of Mexico, except for the dry north- 
western portion, contains grassy savannas, broken by jungle; but 
the southern portion, in consequence of its warm, moist climate, 
possesses a luxurious tropical forest. Such a forest in its fullest 
development is remarkable for the great abundance and variety 
of its flora and fauna. Commonly the tall trees form so dense a 



FIG. 409. A liana-covered tree in the 
tropical forest of Florida. Photograph by 
E. J. Kraus. 

canopy as to shut off much 
of the light from the floor 
of the forest, resulting hi a 
sparse undergrowth and 
making the forest open 
and easily penetrable. The 
trunks and upper branches 
of the trees, however, are 
heavily populated with a 
great variety of epiphytes 
lichens, mosses, ferns, 
orchids, bromeliads, and 
shrubs. Lianas also are nu- 
merous, twining about the 
trunks of trees and push- 
ing their tangled branches 
into the forest canopy. 

In Central America and 
the West Indies much of 
the open forest has been 
destroyed by centuries of 
nomadic agriculture, and 
in its place, over large dis- 
tricts, has grown up a 
dense and almost impene- 
trable jungle. 

407. Grasslands. This 
great area, whose eastern 
boundary extends irregu- 
larly from central Texas 
to southern Manitoba, in- 
cludes the southern por- 
tions of Saskatchewan and 
Alberta and has its western 
limits along the foothills of 
the Rockies from Alberta 
to New Mexico. The name 
"prairie" is applied to the 
easternmost irregular strip 
of this grassland, reaching 


from Texas to Manitoba. The prairie region was formerly covered 
with a rich growth of various kinds of tall grasses, forming a 
characteristically dense turf. Growing with the grasses were many 
other herbaceous plants, such as blazing star, asters, golden-rods, 
and sunflowers. In general, the soil of the prairie is rich in humus, 
beneath which lies clay or sand. The nature of the soil seems, 
however, to have played little part in determining the absence of 
trees in this region. The treelessness of the prairie has been vari- 
ously accounted for, having been ascribed, for example, to frequent 
and extensive fires or to the grazing of vast herds of buffalo. What- 
ever minor part tnese factors may have played, it is probable that 
the prairie has remained treeless chiefly in consequence of an ex- 
cessive transpiration, in proportion to the amount of soil water 
available for the use of plants. 

That extensive area of the grasslands which lies west of the 
prairie constitutes the "Great Plains" and is the home of grasses 
which are characteristically short and grow in patches or tufts. 
Scattered over the Great Plains also are various cacti as well as 
other herbs and shrubs adapted to dry habitats. The western mar- 
gin of the plains passes into various types of scrub growth. Climatic 
conditions apparently are responsible for the characteristic vege- 
tation of the plains. The light annual rainfall arid the high rate 
of transpiration seem to make the development and growth of tree 
seedlings impossible under natural conditions. 

408. Deserts. The area extending south from eastern Oregon 
and western Idaho, embracing most of Nevada and Arizona, the 
southern portions of California and New Mexico, and including a 
part of southern Texas and northern Mexico adjacent to the Rio 
Grande, is largely made up of desert areas. Most of the peninsula 
of Lower California also is desert. This whole region is in general 
one of low rainfall and high evaporation. 

The extensive depression between the Sierras and the Rocky 
Mountains, often called the " Great Basin/ 7 is dominated by the 
sagebrush, a dusty-gray shrub with strongly scented leaves. 
Associated with sagebrush are a few other shrubs of similar ap- 
pearance. After the seasonal rains appears a sudden growth of 
small annual plants which flower and fruit and as quickly wither 
and disappear. The appearance of the desert therefore varies 
greatly with the time of year. 

To the south and southeast the Great Basin passes into a regioa 



of intense summer heat and scanty rainfall. Here is a remarkable 
development of plants peculiarly adapted to an arid habitat. The 
leafless creosote bush, cacti (Fig. 4.11) of weird shapes, yuccas 
(Fig. 50), and thick-leaved agaves are among the characteristic 
plants of this region. Bunch grasses are found in certain areas, 

FIG. 410. Vegetation of the semi-arid sand hills of eastern Colorado. The 
dominant plant of this region is the sagebrush (Artemisia). After Duggar 
(photograph by H. L. Shantz). 

and after the seasonal rains annual grasses and other small herbs 

409. Western Evergreen Forest. The western evergreen forest 
extends in general from Alaska to southern Mexico, and may be 
divided roughly into two areas: the Rocky Mountain forest and 
the Pacific Coast forest. Conifers are the chief forest trees in both 

The Rocky Mountain forest stretches along the Rockies from 
northern British Columbia to southern Mexico. This great sys- 
tem of mountains, extending nearly the whole length of North 


America, presents a wide range of climates which vary with the 
latitude, as well as with the elevations at any given latitude. In 
consequence of the diverse climatic conditions, not all of this 
great mountain area is covered with forests. Thus, within the 
United States, the eastern slopes of the Rockies, grading into the 
more or less arid plains, have a general level below which trees do 
not grow. This level lies roughly between 4,000 and 6,000 feet. 

FIG. 411. A desert region of Arizona, showing "giant cacti " (Carnegiea 
giganted}. Photograph by Frank N. Campbell. 

There is likewise a general level (the " timber line") above which 
trees do not grow. The height of the timber line also varies in dif- 
ferent localities; in the Rockies of the United States it ranges 
approximately from 9,000 to 11,000 feet. Above this timber line 
a low "alpine" vegetation occurs, resembling that of the tundra. 
Farther and farther north along the mountains, both alpine and 
forest belts appear at increasingly lower levels; consequently, in the 
Canadian Rockies the forests cover the lower mountain sides and 
the valleys. 

The dominant tree of the Rocky Mountain forest is the western 
yellow pine. The lodgepole pine also is widely distributed. Among 
other conifers are some of the true firs, the Douglas fir, the western 
larch, and the western hemlock. 

The Pacific Coast forest occupies the slopes of the coastal moun- 
tains from southern Alaska into California. The area from Alaska 
to southern British Columbia is dominated by the Sitka spruce. 


With this spruce occur other conifers, among them the western 
hemlock and Douglas fir. The coastal region of southern British 
Columbia, Washington, and Oregon has a mild winter climate and 
a heavy annual rainfall. Because of these favorable conditions the 

FIG. 412. The dense growth of trees characteristic of the western evergreen 

forest in Oregon. 

conifers here reach a luxuriance unequaled in any other part of 
the world. Many of them grow to heights of 200 feet or more, 
and the bases of their trunks often exceed 10 feet in diameter. 
Douglas fir and western hemlock dominate among the large 
species; associated with them are other conifers such as the west- 
ern white pine, Sitka spruce, white fir, and, western white cedar. 
As a rule, the forest can be penetrated only with difficulty on 
account of the dense undergrowth of ferns, shrubs, and low-grow- 
ing deciduous trees, including maples, poplars, alders, and birches. 
On the coastal range, and confined to a narrow belt extending 
from the southern edge of Oregon into central California, are the 
coast redwoods (Sequoia sempervirens) . Their even larger rela- 
tives, the "big trees " (Sequoia gigantea, Fig. 56), occur only in a 
few groves on the west slopes of the Sierras in central California. 



410. Why Plants Are Cultivated. The cultivation of plants for 
food and for other useful purposes has been carried on since very 
early times. The domestication of plants has been an important 
factor in the progress of the human race. When and by whom 
wild plants were first brought under cultivation, and when their 
selection and improvement began to give rise to the forms now 
chiefly cultivated, are not certainly known. Many common 
vegetables, fruits, and cereals have been cultivated for hundreds, 
and some for thousands, of years. It is known that rice has been 
grown for at least 5,000 years. The plants most largely cultivated 
for food by the ancients were those that produced edible seeds 
and fruits, particularly the cereal grains, leguminous plants, the 
apple, peach, fig, date, arid olive. The cabbage and onion also 
have long been grc TO. Other plants, such as flax and hemp, were 
cultivated for their fibers; some as sources of dyes; and still others, 
like the tea and the grape, for use in the preparation of beverages. 
The majority of the plants more recently brought under culti- 
vation are of less economic importance than those longer known, 
being used mainly for stock feeding or for medicinal purposes. The 
discovery of America and increased facilities for transportation 
and communication between the peoples of the world resulted in a 
more extensive use and distribution of existing varieties. Among 
the contributions of the Americas to the world's stock of culti- 
vated plants were the Indian corn, the true beans, the potato, 
tomato, and tobacco. Many valuable new races and varieties of 
species previously cultivated have been developed, and efforts 
are constantly being made to modify and improve existing vari- 
eties, especially with a view to increasing the yield of their useful 
parts. Great changes in various organs of plants are brought 
about by selection, particularly in roots, leaves, and flowers. 
With few exceptions crop plants are seed plants, arid the vast 
majority are angiosperms. 




411. Organs Used for Food. Various parts of plants are sources 
of food. Even different plants belonging to the same family may 
be grown for the food found in different organs. In the goosefoot 
family, the beet and the mangel are cultivated for the food stored 
in their fleshy roots whereas chard and spinach are grown for 
their leaves. Some members of the mustard family, including the 
radish, turnip, 4 and rutabaga, store food in .enlarged roots. Others 

of the same family, such 
as white and black mus- 
tard, produce useful seeds. 
Many representatives of 
the pulse family, including 
the pea, beanjtlentil, and 
vetch, arc cultivated for 
their seeds; others, such 
as alfalfa and clover, are 
important forage plants. 
Although the grass family 
supplies the cereal grains, 
many other grasses, some 
wild and some cultivated, 
are used for forage. The 
underground branches 
(tubers) of the potato are 
used for food; in the same 
family, the eggplant, to- 
mato, and peppers bear 
FIG. 413. The wild Brassica oleracea (a), e dible fruits, arid tobacco 
from which the following cultivated . , , ,,. , , 

plants seem to hava been derived: (6) 1S extensively cultivated 
kohlrabi; (c) cauliflower; (d) cabbage; for its leaves. 
(e) Welsh or Savoy cabbage; (/) Brussels Equally marked differ- 
sprouts. After Smalian. , . 

ences appear between va- 
rieties of the same species. Thus the edible parts of the common 
cabbage and of Brussels sprouts are buds; kale is grown for its 
leaves; kohlrabi for its enlarged fleshy stem; and the cauliflower 
which, like kohlrabi and kale, belongs to the same species as the 
cabbage, is grown for its abortive flower clusters. 

412. Other Uses of Plants. Apart from being cultivated as 
sources of food for man and domestic animals, many plants are 
grown or used for other purposes. Practically all the great variety 


of beverages, aside from water and milk, are derived from plants; 
among them various fruit juices and alcoholic drinks, coffee, 
tea, chocolate, and cocoa. Many plants supply stimulating or 
narcotic substances, such as tobacco, opium, morphine, and co- 
caine. Fats and oils stored in fruits and seeds are of great com- 
mercial value, many of them being used in soap-making and for 
various other purposes. Olive oil, cottonseed oil, peanut oil, corn 
oil, and coconut oil are used for human food. Linseed oil, obtained 
from flaxseed, enters into the manufacture of paints, varnishes, 
linoleum, and printers' ink. Many waxes, gums, and resins, such 
as Japan wax, gum arabic, gum tragacanth, balsam, and turpen- 
tine, are plant products of commercial value. Other products 
obtained %>m plants include spices, flavors, perfumes, and many 
medicinal substances. 

All the important textile materials, with the exceptions of silk, 
wool, and asbestos, are made from fibers derived from plants. 
Artificial silk should also be excepted. This is made from plant 
substances, but not necessarily from fibers. The flax plant pro- 
duces in its bark very fine, tough fibers from which linen thread 
and fabrics are made. In countries about the Mediterranean, flax 
has been cultivated for thousands of years. The most important 
of fiber plants is cotton, a member of the mallow family to which 
also belong the hollyhocks. Cotton fibers are hairs that grow out 
from the epidermal cells of the seed coat. Cotton has been grown 
from very ancient times; at present more than half the world's 
supply is produced in the United States. In the phloem of the 
stem of hemp, a representative of the nettle family, is a mechan- 
ical tissue composed of tough fibers from which sail cloth, sacking, 
binder twine, carpet yarns, thread, rope, and oakum are made. 
Jute, belonging like basswood to the linden family, is the fiber 
plant that chiefly competes with hemp. Jute fiber is extensively 
used in the manufacture of sugar sacking, gunny sacks, burlap, 
and wool sacking. Although jute is easily cultivated in most warm 
climates, it is grown most extensively in India. Sisal, largely used 
for binder twine, is obtained from the leaves of an agave, a plant 
of the amaryllis family related to the lilies. This agave is grown 
principally in Yucatan. The plants mentioned are the leading 
sources of plant fibers, but the fibers of many other plants also 
are of practical importance. 

Most of the coloring matters used as dye-stuffs were obtained 



from plants until the manufacture of aniline dyes resulted in their 
displacement; some vegetable dyes, however, are still much used. 
413. Crop-distribution. The climatic conditions that determine 
the geographic distribution of plants in nature affect also the dis- 
tribution of crop plants. Each crop grows best in certain regions, 
and this fact largely determines the location of industries depend- 
ent upon special plants. The climatic conditions most commonly 

FIG. 414. Chief crop areas of the United States arid Canada. 

affecting the distribution of crops, as well as of most wild plants, 
are moisture, temperature, and light. A study of the natural veg- 
etation of a region and of the conditions favorable for particular 
associations of wild plants often suggests the kind of crop best 
adapted to that region. Each crop plant has its own particular 
requirements, although the requirements of several species may 
be similar; climatic conditions largely determine, therefore, which 
crop or crops will be best suited to a given locality. 

The physical and chemical nature of the soil affects crop-pro- 
duction favorably or unfavorably. Every soil differs to some ex- 
tent from every other soil. Pure sandy soils contain almost no 
soluble materials; " alkali" soils are highly impregnated with sol- 
uble inorganic salts. In most ordinary soils the solutions present 


contain about the same kinds and proportions of soluble sub- 
stances, but such soils differ very greatly with respect to the in- 
soluble organic matter present. It follows that within any cli- 
matic region there may be many different soil habitats, in each 
of which some crops will grow better than others. 

The geographic distribution of important crop plants of the 
United States is found to correspond in general to the well-known 

FIG. 415. Distribution of the agricultural lands of the United States on the 
basis of the value of their products. Each dot represents $1,000,000. 
From the Yearbook of the U. S. Department of Agriculture. 

centers of natural vegetation. Timothy, spring wheat, rye, buck- 
wheat, and potatoes occupy the same region as the northeastern 
forest trees; corn, winter wheat, oats, red clover, and beans are 
the crops that dominate the central region; cotton, tobacco, sweet 
potatoes, cowpeas, and peanuts predominate in the southeastern 
forest region. 

In the New England states and New York, over half the cul- 
tivated land is devoted to the growing of hay and other forage 
crops, and much land is used for pasturage. The climatic con- 
ditions and the topography of this region render the production 
of cereals less profitable. Apples, grapes, and other fruits thrive, 
especially near Lakes Erie and Ontario. 

Cotton is the principal crop of the southern states. Although 


much cotton is shipped to New England and abroad for manu- 
facture, much is manufactured in the South. The production 
of cottonseed oil is an industry of the southern states. In North 
Carolina and Tennessee, tobacco occupies the same region as 
cotton. Tobacco is largely grown also from Kentucky northeast- 
ward into Virginia. In the cotton regions of Louisiana, sugar cane 
and rice also are grown. Rice is grown in the same region as to- 

FIG. 416. Acreage planted to cotton in the United States. Each dot represents 
10,000 acres. From the Yearbook of the U. 8. Department of Agricul- 

bacco in Texas. Sweet potatoes, cowpeas, and peanuts are raised 
in most of the cotton belt. 

The corn belt embraces the region from Ohio to eastern Kansas 
and Nebraska. This region produces more than half the corn crop 
of the country, eastern Illinois being the greatest productive cen- 
ter. The corn belt has a more fertile soil than any other region 
of similar extent, containing a larger proportion of humus, and 
has sunny summers, a relatively high summer temperature, and 
a comparatively heavy annual rainfall. The great cattle markets 
and packing industries are located in the corn belt, and various 
corn products are there manufactured. Other crops, such as wheat, 
oats, hay, and sweet corn, also are grown in the corn belt; but 
they are of secondary importance in this region. Sugar beets 
are raised in an area lying in general north and west of the corn 
belt, the centers of sugar-beet production being in Colorado and 

Extending from the Mississippi westward to the foothills of the 
Rocky Mountains, grasslands predominate. This region is char- 
acterized by pronounced climatic variations, especially with respect 



to rainfall and evaporation. The natural vegetation of the eastern 
portion, the prairies, differs from that of the western portion, the 
plains. The true prairies extend roughly from North Dakota to 
Texas and eastward to Indiana. It is in these central prairies that, 
as already mentioned, the bulk of the corn crop of the United 
States is produced. The northern portion of the prairies is the 
leading region for the production of spring wheat, which centers 

FIG. 417. Regions of the United States planted to corn. Each dot represents 
approximately 10,000 acres. From the Yearbook of the U. S. Department 
of Agriculture. 

in North Dakota; the center of the production of winter wheat 
is in Kansas. Because of the presence of water power and of its 
'nearness to the wheat-growing centers, Minneapolis is the leading 
flour-milling city of the world. Large quantities of flax and barley 
also are raised in the northern prairies. 

Between the prairies and the foothills of the Rockies are the 
Great Plains, extending from Saskatchewan on the north to Texas 
on the south. The plains, which were originally the grazing lands 
of the buffalo and later were occupied by cattle ranges, possess a 
drier soil than that of the prairies. The introduction of plants 
that can conserve moisture or utilize the available water has made 
it possible to grow certain crops in this area, so that, in addition 
to grazing, some portions of the plains are used for farming pur- 



poses, producing much alfalfa and such hardy wheats as durum. 
Farther south, especially in western Kansas, " Kafir corn" (a 
grain-producing sorghum) and millet (milo) are successfully grown. 
The production of broom corn, another variety of sorghum, cen- 
ters in Kansas, Oklahoma, and Texas. Grazing, pasturage, and 

Fia. 418. Wheat-growing areas of the United States. 
approximately 25,000 acres. 

Each dot represents 

the raising of forage crops are therefore the chief agricultural ac- 
tivities of the Great Plains. 

The native flora of the states of Washington, Oregon, and Cal- 
ifornia, because of the great variations in topography and climate, 
is extremely diverse. Even the valleys, coastal and intermountain, 
vary greatly in their native vegetation, and crops of wide diversity 
are produced, especially in the drier sections when water for irri- 
gation is available. The crops comprise forage plants, cereal 
grains, and fruits of many kinds both subtropical and tropical. 
In fact, some part of this region can be found that is adapted to 
the growing of almost any crop. The western portions of Wash- 
ington and Oregon have the greatest average annual rainfall of 
any section of the United States. 

Eastward from the Cascades, in the northern half of this region, 


the elevations are greater and in general the rainfall is less. Much 
of this portion of the territory is timbered to a greater or less ex- 
tent, but considerable parts are devoted to grazing and grain- 
growing. In eastern Oregon and Washington wheat-growing is 
important. Some of the intermountain valleys, where water is 
available for irrigation, are given over to fruit-raising. Apples are 
produced in large quantities in the moister valleys of Washington, 
Oregon, and Idaho. Other fruits of various kinds, such as peaches, 
plums, apricots, and berries, also are raised. Many of these crops 
are grown on the highly fertile irrigated soils of this region. 

In northern California much grain is grown, and there is con- 
siderable range land in the drier parts of Oregon and California. 
Farther south, walnuts, plums, apricots, peaches, and grapes are 
important products. Irrigation has made the fertile SQilof south- 
ern California especially valuable for the production oF~citrus 


414. Early Interest in Plants. Since very remote times plants 
or plant parts have been used as medicines, either in the cure of 
disease or in the treatment of wounds. The earliest studies of 
plants appear to have been made with a view to discovering or 
recording their usefulness either as food or in medicine. The foun- 
dations of the sciences of botany and zoology were laid by Aristotle 
and Theophrastus, who lived in Greece in the fourth century B.C. 
Aristotle chiefly studied and wrote about animals; Theophrastus 
studied and described plants. The latter may be considered the 
first scientific botanist, since he attempted to investigate plant 
structure, growth, and distribution, and recorded the names of 
about 500 different kinds of plants. In the first century A.D., Dios- 
corides, a Greek physician and medical officer serving with Roman 
legions, wrote a " Medical Botany" in which he devoted a chapter 
to each plant, animal, or other product of nature useful in medi- 
cine. The chapters on plants contain brief comments on the hab- 
itat and general distribution of each plant considered, an account 
of its root, stem, leaf, flower, and fruit, and a description of medic- 
inal effects, methods of preparation, and usage. For more than 
1,000 years Dioscorides' volume was one of the chief sources of 
information in the training of physicians. 

In the first century A.D. also lived a popular Roman author, 



Pliny the Elder. His " Natural History " was widely read through- 
out the middle ages. This work consists of 37 " books," of which 
16 deal with plants and their agricultural, horticultural, and medic- 
inal uses. Pliny was a compiler of information from earlier writers 
rather than an original investigator. 

After the time of Pliny came a long period in which no addi- 
tions were made to the knowledge of plants. In the fifteenth 

and sixteenth centuries 
a number of medical 
men in European coun- 
tries published elabo- 
rate volumes called 
herbals. These books 
contMn extensive de- 
scriptions of plants and 
are often accurately il- 
lustrated, many of the 
drawings having been 
made from nature. In 
the seventeenth and 
eighteenth centuries 
botanical science as 
such came slowly into 

Botanical gardens 
were established at an 
early date. One of the 
earliest of these existed 
at Athens in the fourth 
century B.C. It was 
founded by Aristotle 
and after his death 
was carried on by The- 
ophrastus. There is a 
record also of a garden of medicinal plants at Rome during the 
first century A.D. 

415. Medicinal Plants and Drugs. In more modern times, at 
least until rather recently, the bulk of vegetable drugs has been 
obtained from wild plants; but this source has been in large meas- 
ure exhausted, so that at present the cultivation of drug plants 



FIG. 419. Figure of a poppy from the herbal of 
Leonhard Fuchs, first published in 1542. 


is carried on to an ever-increasing extent. Of the plant drugs 
formerly in use, a number have been replaced or supplanted by 
synthetic chemical compounds, many of which are prepared from 
coal tar. 

The substances contained in plant drugs which give them their 
medicinal properties are of varied nature; among these substances 
are alkaloids, glucosides, volatile oils, and oleoresins. The active 
constituents of a drug may occur throughout the entire plant or 
only in certain of its or- 
gans. Plant drugs are 
classed by pharmacists as 
herbs, barks, woods, 
leaves, roots, rhizomes 
(underground stems) , 
bulbs, tubers, flowers, 
flowering tops, floral parts, 
seeds, and fruits. 

A drug composed of the 
whole plant is called an 
"herb." Peppermint, 
spearmint, and lobelia are 
examples. The active con- 
stituent of an "herb" is 
often localized in certain 
parts of the plant; thus, 
peppermint and spearmint 
owe their medicinal prop- 
erties to a volatile oil pro- 
duced in glandular hairs 
which are found princi- 
pally on the leaves and 

One of the most valuable 

G. 42 - 

-Figure ot Veronica from 
Fuchs> herbaL 

barks is that of the cin- 
chona tree, found originally in Peru but now cultivated largely 
in the East Indies. From it is obtained the alkaloid quinine, a 
specific cure for malaria. The bark of Rhamnus purshiancij a 
tree growing in Oregon, Washington, and California, is' known 
as "cascara"; it supplies a valuable cathartic. From Quillaja 
saponaria, a tree of Peru and Chili, soapbark is obtained. Its 


principal constituent, saponin, is a mixture of glucosides. Saponin 
is used to some extent in medicine, but its greater use is as a foam- 
producer in beverages. 

One of the woods most used in medicine is sandajwood, coming 
from India and the East Indies. It contains a volatile oil, used as 
a perfume, and medicinally in diseases of the mucous membranes. 
Camphor is obtained by passing steam through the finely cMpped 
wood and bark of a tree growing in Formosa, China, arid Japan. 
It is used because of its stimulative properties. Oil of turpentine 
and rosin are derived from the wood of the long-leaf pine and other 
pines. Tar is obtained by the destructive distillation of pine wood. 

The leaves of the coca plant, grown in the mountains of Peru 
and Bolivia, contain cocaine, used as a local anesthetic in minor 
surgery. Cocaine is one of the most widely used habit-forming 
drugs. Other leaves much used in medicine are those of belladonna, 
digitalis, senna, and henbane. 

Among root drugs, ipecac from Brazil and Bolivia is one of the 
most important. It is used as an emetic and in cough mixtures. 
The dandelion and burdock are common weeds whose roots have 
medicinal qualities. Licorice is known to every child. Its use in 
medicine is principally to modify the taste of bitter drugs. 

Of the rhizomes, ginger is used both as a condiment and as a 
medicine. Golden seal, formerly abundant in the northern United 
States, has been in such demand on account of its tonic properties 
^hat it has almost disappeared from that region and is now being 
cultivated to some extent. The rhizome of valerian is a nerve 
stimulant. The rhizomes of wild geranium, bloodroot, and may 
apple are used as drugs." In the case of rhubarb, both rhizome and 
root are used. The medicinally valuable rhubarb comes from west- 
ern China and Tibet. 

Camomile, ai'nifili, .and santonica are drugs prepared from the 
flowers of members of the composite family. Cloves are dried 
flower buds and are useful because of their high percentage of a 
volatile oil. 

The part of the Indian hemp used in medicine is the whole flower- 
ing top of the pistillate plant. The important constituent is a resin, 
cannabin. Hemp is sold in the bazaars of India for smoking pur- 
poses under the name of "gunjah." When mixed with aromatic 
drugs it is called "hashish." It is a powerful narcotic. Most of 
the poisons of plant origin, like cannabin, are useful as drugs in 


small doses. Strychnine and brucine are alkaloids obtained from 
the seeds of Strychnos nux-vomicaj a small tree growing in India 
and the Philippines. These alkaloids are stimulants and nerve 
tonics. The Calabar bean or ordeal bean from western Africa is 
another drug plant whose seeds contain poisonous alkaloids. Its 
action is opposite to that of strychnine, and it is used as an antidote 
in cases of strychnine poisoning. The kernel of the seed of the 
bitter almond contains hydrocyanic acid (prussic acid) and is used 
in medicine as a sedative. 

Many fruits of members of the parsley family contain volatile 
oils used in medicine. Anise, fennel, caraway, and coriander are 
familiar examples. Vanilla "beans" are the long, slender fruits of 
a Mexican epiphytic orchid. 

In addition to the drugs already mentioned, which consist of 
whole plant organs, there are many medicinal substances composed 
of cell contents or of secretions. Opium is the thickened latex 
from the fruit coat of the poppy. It contains, besides morphine and 
codeine, a number of other alkaloids. The drug known as aloes 
is prepared by condensing the mucilaginous juice obtained from the 
fleshy leaves of a number of species of Aloe, a subtropical genus 
of the lily family. Gum arabic is an exudation from the wounded 
bark of Acacia Senegal and other species of Acacia. Myrrh is a 
gummy resin obtained from a shrub growing in northeastern 
Africa. It is used in incense as well as in medicine. 

Large quantities of volatile oils derived from plants are used 
in the manufacture of perfumes. These oils are mostly imported 
from England, Germany, France, and Mediterranean countries. 
Some of the best-known oils so used are those of rose, lavender, 
rosemary, rose geranium, bergamot and other citrus fruits, sandal- 
"wood, and bay. 


416. Forest Reserves. The early settlers in America used land 
mainly for hunting and pasturage; later, when agriculture became 
an important pursuit, it was assumed that timber was so abundant 
as to be inexhaustible, and forests were destroyed with no serious 
thought for the future. A similar reckless destruction of forests 
had taken place in Europe long before; but as early as the tenth 
century, laws were adopted by many German, Swiss, and French 
cities and states, controlling the cutting of timber and providing 


for the planting of new forests. In consequence, forest areas have 
been established under government control in some European 
countries, assuring a continuous supply of timber. 

Although sporadic efforts had been made at different times in 
the United States, both by the states and by the national govern- 
ment, to protect the timber supply, it was not until 1876 that def- 
inite plans were formulated on a national scale. In 1891 Congress 
authorized the President to set aside as forest reserves lands wholly 

FIG. 421. Pine seedlings in a forest nursery containing 5,100,000 trees. From 
the bare plots at the left 1,065,000 trees have been removed for trans- 
planting. Photograph from the New York Conservation Commission. 

or in part covered with timber. During the Harrison administra- 
tion reserves totaling more than 13,000,000 acres were established; 
during the Cleveland administration 22,000,000 acres were added. 
President McKinley established a few forest reserves; in the suc- 
ceeding Roosevelt administration more than 150,000,000 acres 
were reserved and a thorough-going forestry policy was established. 
Much land has since been added; some has been released or sold. 
There are now in the national forest reserves, including Alaska 'and 
Puerto Rico, approximately 200,000,000 acres. Many states have 
established forest reserves, and each year these reserves are being 
increased and so supervised that they will continue to furnish tim- 
ber for future generations. 


417. Lumbering. Because the supply seemed inexhaustible, 
only the largest trees were selected and only the better portions of 
felled trees were saved. The other portions left on the ground be- 
came dry, and in these " slashings" serious forest fires often started 
which destroyed much of the timber still standing. Sawmills, 
too, used only the better portions of a log, and it is estimated that 
not more than one fourth of a selected tree was finally made into 
lumber. The extensive forests of white and Norway pine which 

FIG. 422. Cut- and burned-over forest lands in Minnesota. Photograph by 
the U. S. Forest Service. 

covered great areas in the northern and eastern states were almost 
destroyed by these methods. Much southern yellow pine and large 
stands of hard woods throughout the country also have been de- 

At present lumbermen are using larger portions of the trees that 
are cut, the slashings are so handled as to minimize the fire risk, 
and sawmills are using much of the former waste material in the 
manufacture of by-products. Forest rangers, supported by the 
national government, states, and individual timber owners, are 
doing much to check forest losses. A policy of replanting cut-over 
areas is each year adding thousands of acres to the available timber 


418. Utilization of Forest Products. Although the greater part 
of the timber cut is manufactured into lumber, much is used for 
other purposes. Among articles made from timber are firewood, 
telegraph poles, railway ties, fence posts, mine timbers, barrels, 
and pails. Much material of the sort once wasted is made into 
spools, toys, and matches. Firs and spruces, which still form ex- 
tensive forests in the northern United States and Canada, furnish 

FIG. 423. A cultivated forest of Norway spruce in Europe. Photograph by 
the U. S. Forest Service. 

pulpwood for the manufacture of paper; other woods, both soft 
and hard, also are now utilized for this purpose. 

Oak and hemlock bark and the wood of the chestnut are the 
favorite materials for use in the tanning of leather. But since the 
supply is limited, various other barks and woods as well as chemical 
tanning agents are likewise used. 

Resin is an exudate secured from certain species of pine in the 
southern states, long-leaf pine furnishing the greater part of the 
supply. Distillation of resin yields turpentine ("oil of turpen- 
tine") ; the solid portion remaining is rosin. Turpentine is obtained 
also by dry distillation from the waste materials of sawmills, in- 
cluding slabs and sawdust. This latter process of distillation 
yields various other products, including tar, pyroligneous acid, 
crude oils, and charcoal. Charcoal is made also from various hard 


woods; additional substances obtained in its production are acetate 
of lime, acetone, and tar. Large quantities of sawdust and shavings 
are ground, chemically treated, and molded under pressure to make 
many articles formerly made of hard rubber. 

Veneers (thin slices) were formerly produced only from the more 
expensive woods; but the demand is now so great that the making 
of veneers has become an important industry. Red gum, yellow 
pine, maple, yellow poplar, cotton wood, white oak,, and birch are 
among the many timbers used in the manufacture of veneers for 
doors, furniture, and partitions. The slices average about 1/40 
inch in thickness. It is estimated that almost a half billion feet of 
wood are used each year in this industry. Basswood, cottonwood, 
yellow pine, and yellow poplar are made into excelsior, used for 
upholstering, packing, and various filtering purposes. 

The sap of the hard maple is obtained by boring holes into the 
trunk near its base. " Tapping " is done during late March and 
early April, and the sap is boiled down to make maple syrup or 
maple sugar. The maple syrup industry is confined chiefly to New 
Hampshire, Vermont, northern New .York, and some of the states 
bordering the Great Lakes. 


419. What Is a Weed? The term weed is applied to any plant 
that is growing where it is not desired. Most ordinary weeds are 
seed plants, although some ferns and horsetails are so considered. 
Some weeds belong to species that are or have been cultivated, 
but that become troublesome when they interfere with the growth 
of useful crops of other plants. Horse-radish and Johnson grass 
are examples of cultivated plants which have become injurious 
weeds in certain localities. Some plants are innocuous in their 
native habitats but become nuisances when they invade a new 
locality. In general, most of the aggressive weeds have been intro- 
duced from other countries; but some native plants under special 
circumstances may become weeds. Although in almost every 
locality many species of plants grow wild and many appear among 
crops, comparatively few are objectionable weeds. 

420. Dissemination. In order to compete with farm and garden 
plants, weeds must be able to survive unfavorable influences and 
to increase their numbers with great rapidity. Very commonly, 
weeds are distinguished by a marked power of vegetative mul- 


tiplication. Many possess deeply growing, tough, or extensive 
root systems; others have widespreading underground stems and 
branches which in some cases give rise to many aerial shqpts. If 
the underground parts are cut into pieces by farm implements, 
each piece may produce a new plant. The Canada thistle, for 
example, has a branching underground stem, the branches of 
which become separate plants by the destruction or death of the 
older parts. Some grasses which are weeds are multiplied in a 
similar manner. Certain plants, such as some of the hawkweeds, 
produce runners above the ground which, when separated, give 
rise to new plants. Other weeds, like nut grass, multiply by means 
of tubers. 

Most weeds are spread also by means of seeds which may be 
sown with the seeds of crop plants. Such weeds are introduced 
into fields together with grass, clover, or other commercial crops. 
Weeds are often unavoidably harvested with crop plants, and 
their seeds are then distributed with those of the crop; chess seed, 
for example, is distributed with wheat, wild oats with cultivated 
oats, arid dodder with clover and alfalfa seed. Weed seeds are 
spread also by threshing machines and other farm implements, as 
well as by railway trains and automobiles, so gaining a foothold 
along railway tracks and highways and spreading thence to neigh- 
boring farm lands. Seeds are frequently spread also in stable 
manure and in other litter of farmyards and stables. 

421. Weeds of Various Regions. Regions differing in soil and 
climate differ also with respect to their prominent weeds. A par- 
ticular weed may be troublesome in one part of the country and 
not in another. Qu&ck-grass, an annoying weed from Maine to 
Minnesota, is not so serious a pest in the southern states; on the 
other hand, Johnson grass, practically unknown in the North, 
has escaped from cultivation and become a weed in the South. 
Soil moisture is an important factor in determining the distribu- 
tion of weeds. Cacti are weeds from central Kansas westward and 
southward, but do not grow to any extent farther east and north. 
Some verbenas, the common mullein, and everlastings occur as 
weeds in dry pastures. Certain ferns, smartweeds, mints, and 
dock grow only in meadows and moist pastures. With the removal 
of forests, many weeds introduced from Europe found a favorable 
environment in the oil once occupied by less hardy native species 
and spread r-^idly as cultivation increased. As the area of cul- 


tivation extended westward beyond the forested regions, some 
native species of the prairies, such as cocklebur, verbena, and 
horse nettle, contributed to the weed flora. Once the black-eyed 
Susan grew only west of the Alleghenies, but it is now an abundant 
weed throughout the eastern states. 

422. Injuries Caused by Weeds. Weeds are harmful in various 
ways : 

(a) They absorb from the soil moisture and salts required by 
useful plants. 

(6) They crowd out useful plants because, as a rule, of their 
numbers and their rapid growth. Rapidly growing weeds also 
shade shorter plants and seedlings, so interfering with photosyn- 

(c) Some weeds, like the dodders, are parasitic on useful plants 
and rob them of their foods. 

(d) Some parasitic fungi, like the rusts, pass a part of their life 
upon weeds which thus provide an opportunity for the overwinter- 
ing of the fungi. Other weeds furnish food for insects injurious to 
useful plants. The potato beetle lives on many plants of the night- 
shade family, from which it migrates to potato plants. Stubble 
with which weeds are intermingled furnishes places for cutworms. 

(e) Some weeds are poisonous, injuring domestic animals; such 
weeds are lambkill (sheep laurel) and water hemlock. Hemp and 
the "loco weeds " produce symptoms of intoxication and poison- 
ing in horses and sheep. Many plants also contain poisonous sub- 
stances injurious to man. 

(/) The seeds and fruits of some weeds cling to domestic animals. 
Burs on sheep render their wool less useful. Some spiny plants 
injure stock; for example, thorny shrubs are a great source of 
trouble to wool-growers. The horny and barbed fruits of some 
grasses irritate or wound the mouths of grazing animals. 

(g) Certain weeds, such as wild garlic and stinkweed, when 
eaten by cows, taint their milk and render it unfit for human con- 

(h) Weed seeds diminish the commercial value of crop seeds 
with which they are mixed. 

423. Control. In order to control or exterminate weeds, a knowl- 
edge of their habits and reproductive methods is necessary. Annual 
weeds may be eradicated by any method that will hasten the 
germination of their seeds and then destroy *Uo young plants. 



Biennial weeds should be cut down or plowed under before they 
have an opportunity to produce seeds in their second year. Peren- 
nial weeds are most troublesome and most difficult to destroy. 
Cutting down the plants, plowing them under, and destroying 
their underground parts are methods employed for their eradica- 
tion. Seeds used for crops should be as free as possible from weed 
seeds, and care should be taken not to spread weed seeds in stable 
manure. All places favorable for the growth of weeds should be 
cultivated, or the weeds should be otherwise removed. 


424. Nature. When, as a result of external conditions, the 
ordinary or " normal " functions of a plant or of its parts are inter- 

FIG. 424. Mosaic disease of potato. At the left an uninfected leaf; 
at the right an infected leaf. Photograph from James Johnson. 

fered with or deranged, the plant is said to be diseased. The va- 
rious ways in which the structure and functions of the plant are 
thus caused to deviate from the normal condition are spoken of 
as symptoms. A very large proportion of plant, as well as of animal, 
diseases result from the attacks of other organisms. In such a 
case it is the parasitic organism that causes the disease and not 
infrequently the death of the host. 

Many parasitic animals are capable of entering and living in 
the tissues of plants; among the most common of these are species 



oi nematodes or "eel worms." Nematodes are often root parasites, 
their presence in roots resulting in the formation of galls. The 
wheat nematode and some others of the group can invade all por- 
tions of the host plant and entirely check the formation of normal 

Among parasitic plants are some that contain chlorophyll. Cer- 
tain algae, for example, live in the tissues of more complex plants, 
sometimes doing no in- 
jury to the host but at 
other times causing dis- 
ease. Of parasitic seed 
plants the best known 
are the mistletoes and 

425. Virus Diseases. 
There are a considerable 
number of plant diseases 
for which, as for such 
human diseases as scarlet 
fever and cancer, no 
causal organism has been 
found. Among the more 
common of these are the 
"virus" or mosaic dis- 
eases of potato (Fig. 424), 
tobacco, and cucumber. 
Each of these hosts may 
be attacked by several 
virus diseases. The 
mosaics of these and of 
some other plants can 
readily be transmitted 
from plant to plant. The 
mosaic diseases of the peach, including "yellows," are trans- 
mitted under natural conditions in a manner not yet understood. 
Artificially, peach yellows can be transmitted only by means of 
grafting or budding. Some virus diseases, like aster yellows and 
"curly top" of the sugar beet, are transferred only by specific 
insects. As in the case of certain animal viruses, the active agent 
of such a plant disease must have spent some time in the body of 

FIG. 425. Fire blight of pear. 



the insect carrier. The virus diseases of some members of the 
pulse family differ from most of this class in that they may be 
transmitted through seeds. 

The symptoms of virus diseases are variable; in most cases the 
leaves of the host are malformed, stunted, and wrinkled; often 
they are mottled in appearance. In a few cases the entire plant 

is badly stunted; in others the 
number of leaves is greatly in- 
creased but they are abnormal 
in size and color. \S 

426. Diseases Caused by 
Bacteria. One of the common- 
est bacterial plant diseases is 
the "fire blight" or "pear 
blight" (Fig. 425), which oc- 
curs on the apple, crab apple, 
pear, arid related plants. The 
bacterium causing this disease 
gains entrance to the host 
through wounds, or more often 
through the floral nectaries, 
and multiplies very rapidly 
killing the host cells so quickly 

FIG. 426. Crown gall of apple. 

that the affected portion ap- 
pears scorched. At the end 

of the growing season, the bacteria remain dormant at the edges 
of the diseased portion; when the host plant resumes growth the 
following spring, the bacteria again multiply rapidly and are often 
exuded in a viscous liquid. Insects visit this exudate and carry 
the bacteria to other plants. 

"Soft rot," caused by bacteria that enter the host through 
wounds, is responsible for the rotting of many vegetables in the 
field, and for still further losses if the diseased vegetables are stored 
in warm, moist places. 

Cabbage is attacked by a black rot caused by bacteria that enter 
the leaves of the host, usually through water pores. The bacteria 
travel through the conducting elements of the xylem, multiplying 
so greatly as to clog these elements. The water supply is thus cut 
off from the tissues of the plant, and if the main stem is invaded 
the whole plant may die. 


"Crown gall," so called because of the large galls formed on 
stems and roots at the surface of the soil, is a serious disease of 
apples (Fig. 426), and is abundant also on many other plants, 
including raspberries, grapes, and walnuts. The bacteria enter 
through wounds; the stimulus supplied by their presence causes 
a rapid division of the 
host cells in their vicinity, 
as well as a marked en- 
largement of many of 
these cells. 

427. Diseases Caused 
by Slime Molds. Slime 
molds have certain char- 
acteristics possessed by 
fungi and others common 
to some very simple ani- 
mals. When studied by 
botanists, slime molds are 
included among the lower 

The plant body of a 
slime mold is a naked 
mass of protoplasm (plas- 
modiwri) containing when 
fully developed very many 
nuclei. Most slime molds 
are saprophytic, but a few 
are parasitic in the tissues 
of higher plants. The 
saprophytic forms when mature develop fruiting bodies (sporangia) 
of various sizes and shapes, within which by cell division one- 
nucleate spores are formed. 

The strictly parasitic slime molds do not produce fruiting bodies. 
A plasmodium of one of these species divides within the host tissue 
into masses of spores, which masses may be irregular or may take 
on a characteristic form. One of the best known of these parasitic 
slime molds (Plasmodiophora) causes a disease (clubroot) of cab- 
bage and related plants (Fig. 427). The parasite enters a root of 
the host plant in the form either of an amoeboid cell or of a very 
small plasmodium, which enlarges in a cortical cell of the root. 

I V IG. 427. Ulubroot of cabbage, me swell- 
ings of the roots are caused by a parasitic 
slime mold (Plasmodiophora). 


As a result of the stimulus due to the presence of the fungus, the 
host cell grows and divides. The division of the host cell is often 
accompanied by a division of the plasmodium within it. In con- 
sequence of the repeated growth and division of the original host 
cell and of its offspring, large swellings appear on the roots of the 
host plant. Most of the cells of these swellings contain plasmodia. 
The plasmodia grow, consuming the contents of the host cells, 
until they fill or almost fill the spaces within the cell walls of 
the host. Finally, the plasmodia divide into one-nucleate spores 
which are liberated when the host tissue breaks down. 

Closely related to Plasmodiophora is an organism causing the 
" powdery scab" of potatoes. This disease has long been known in 
Europe, and in some localities is very destructive. It appeared in 
the extensive potato-growing areas of southeastern Canada and 
adjacent parts of the United States about 1910; here it has been 
practically eliminated by the sterilization of tubers before they 
are planted, and by the growing of r^isianVvarieties. The slime 
mold causes the formation of blister-like spots on developing tu- 
bers; the spots increase greatly in size and become filled with a 
brownish powdery substance composed of broken-down host tis- 
sues together with the spores of the fungus. 

428. Diseases Caused by Phycomycetes. The serious diseases 
due to Albugo, Pythium, Plasmopara, and Phytophthora were 
mentioned in Chapter XIX. The damage done by the "downy 
mil dew " of the grape (Plasmopara) and the "late blight " of 
potato (Phytophthora) has been greatly checked in the United 
States by systematic spraying of the host plants. Another phy- 
comycete (a chytrid, Urophlyctis) attacks alfalfa in the irrigated 
regions of the West. The fungus invades the young alfalfa buds 
at the surface of the soil, checks their development, and causes 
the formation of numerous galls. Aphanomyces, related to Sapro- 
legnia, attacks the roots of many leguminous plants and sometimes 
causes serious losses through the breaking down of the tissues of 
the roots, so preventing the transportation of water. 

429. Diseases Caused by Ascomycetes. Besides the powdery 
mildew of the lilac ( 219), a number of powdery mildews cause 
plant diseases some of which result in considerable damage. 

One of the simpler ascomycetes (Taphrina) causes "plum 
pocket." The younger branches, leaves, and fruits of the plum, in- 
vaded by this fungus, are stimulated to excessive growth. The 


fruit often becomes hollow, the pit being absent. After a period of 
rapid vegetative growth, the fungal hyphae grow to the outside 
of the part affected and on its surface form many asci. Peach, 
poplar, alder, hazel, and other hosts are attacked by various 
species of Taphrina. 

The apple and pear are often injured by species of Venturia, 
which produce a dark-colored mycelial growth on the leaves and 
fruits. The mycelium penetrates only the cuticle, but the growth 
of the epidermal and immediately underlying cells of the host is 
checked. Infected fruits are often very irregular in shape and 
sometimes display large cracks. 
Conidia, formed at the ends of 
protruding hyphae, spread the 
disease during the growing sea- 
son. After the leaves fall, the 
fungus penetrates them and 
continues to grow as a sapro- 
phyte. In the dead leaves, 
small rounded fruiting bodies 
containing asci are developed 
which mature the next spring 
and liberate ascospores that 
can infect new hosts. 

Cherries, plums, and related 
plants are attacked by species 
of Sclerotinia (Fig. 428); in- 
fected fruits decay and turn 
dark brown. Many conidia are 
produced by the aerial hyphae 
on the surfaces of the decay- 
ing fruits, and if there is ample moisture the disease is spread by 
the conidia from tree to tree. This " brown-rot" fungus remains 
alive in fruits that fall to the ground and in the following spring 
renews its growth, forming saucer-shaped fruiting bodies contain- 
ing many asci. The ascospores, liberated during spring rains, 
communicate the disease to the newly forming fruits. ^** 

About 1910, a disease was discovered on chestnut trees in the 
vicinity of New York City; during succeeding years it spread 
throughout the chestnut forests of New York and neighboring 
states. So serious was the disease and so rapid its spread that all 

FIG. 428. Brown rot of plum. 



efforts made by state and national governments failed to check it, 
and the chestnut forests in the northeastern United States have 

been almost entirely destroyed. 
The disease is caused by an 
ascomycete, probably intro- 
duced from Japan where it has 
long been known but does little 

430. Diseases Caused by 
Basidiomycetes. All cereal 
grains and many wild grasses 
are attacked by one or another 
of the smuts (Chap. XXI). 
Serious diseases of the onion, 
of rice, and of many leguminous 
plants also are caused by smuts. 
In addition to those dis- 
cussed in Chapter XXI, vari- 
ous species of rusts are parasitic 
upon the great majority of seed 
plants, both cultivated and 
wild. Some attack ferns as well. 
The "blister rust/' which 
threatens the existence of white 
pines in the United States and 
Canada, was imported from 
Europe on pine seedlings. The 
stage of this rust that produces 
uredo- and teleutospores is 
passed on currants and goose- 
berries. One means being used 
in the attempt to control the disease is the extermination of cul- 
tivated and wild currants and gooseberries in the neighborhood of 
white pine forests. 

431. Diseases Caused by Imperfect Fungi. Imperfect fungi are 
so called because they or many of them are thought to be asco- 
mycetes or basidiomycetes whose life cycles are only partly known. 
The part of the cycle which is known in each case produces spores 
of varied types, but does not include the ascospore- or basidio- 
spore-forming stage. Each year the study of these fungi reveals 

FIG. 429. Anthracnose of beau. 


the unknown stages in the life cycles of some of them, which are 
then transferred to the appropriate class (ascomycetes or basid- 
iomycetes). Many thousands of species of imperfect fungi are 
known, a large proportion of them being parasitic and causing 
diseases of bryophytes, pteridophytes, and especially of seed 
plants. Among the diseases that they produce are the early blight 
of potato, the leaf spot of beets, anthracnose of beans (Fig. 429), 
and cabbage yellows. 


References in bold face are to pages containing illustrations. 

Abscission layer, 75, 109 

Achene, 453, 470, 479 

Adaptation to environment, 5, 6 

Aecidiospores, 298 

Aecidium, 298 

Aerobes, 254 

Agricultural lands, 537 

Air chambers, 319, 320, 327, 331, 412 

Air pores, 320, 327, 331; see also 

Air space, 383 
Albugo, 275-278 
Alcohol, formation of, 281 
Aleurone grains, 154, 165 
Alfalfa, nitrogen fixation and, 259, 


root system, 42 
Algae, 199-249 

blue-green, 225-236, 310, 398 
brown, 231-243, 362 
colonial, 227 

distribution of, 199, 225, 231, 243 
green, 199-224, 243, 362 
lichens and, 310 
nature of, 197 
red, 243-249, 362, 363 
Alternation of generations, 212, 213, 
235, 236, 249, 325, 348, 349, 
362, 363 
Aluminium, 151 
Amanita, 306 
Ammo-acids, 149 
Ammonia, 150, 258 

formation of, 258 
Anabaena, 228, 229 
Anaerobes, 255, 282 
Angiosperms, 423-485 
advances in, 439 
classes of, 439 
classification of, 464-485 
evolution of, 432 
families of, 462-485 
nature of, 397, 423 
Animals, fruit dispersal and, 459 
seed dispersal and, 459 

Annual rings, 38, 39, 59-62, 72, 397, 

408, 410 
false, 62 

Annuals, 80 

Annulus, 369, 370 

Anther, 424, 425, 426, 427 

Antheridium, 221, 223, 234, 235, 238, 
239, 274, 277, 284, 285, 320, 329, 
334, 335, 336, 343, 362, 371, 385 

Antherozoids, 207, 221, 234, 238, 239, 
321, 330, 335, 343, 344, 345, 
372, 391, 392, 403, 404 
nature of, 207 

Anthoceros, 336-339 

Anthocyans, 161 

Antipodal cells, 429, 430, 434 

Antitoxins, 262 

Aphanocapsa, 227 

Apical cell, 237, 319, 332, 333, 337, 
342, 351, 366, 415, 418 

Apple, 452 

Archegonial chamber, 402 

Archegonium, 320, 321, 322, 323, 331, 
334, 336, 337, 343, 344, 362, 371, 
372, 384, 385, 391, 393 

Arum family, 482, 483 

Ascomycetes, 280-289, 309, 310, 313 
disease-producing, 556-558 
nature of, 280 
relationships of, 289 

Ascospores, 280, 286, 288, 314, 315 

Ascus, 280, 288, 313, 314 

Aspergillus, 286 

Assimilation, 156 

Atoms, 15 

Autumnal coloration, 160, 161 

Awn, 479, 480 

Azotobacter, 260 

Bacillus, 261 
Bacteria, 167, 250-266 

disease-producing, 262-264, 554* 

iron, 261, 266 

relationships of, 266 




Bacteria, sulphur, 261, 266 

Bacterin, 264 

Bacteriophages, 265 

Banyan, 46 

Barberry and wheat rust, 297-299 

Bark, 39, 63 

hard, 64, 65 

soft, 64, 66 

Basal cell, 277, 296, 298 
Basidiomycetes, 290-309, 310 

disease-producing, 558 

nature of, 290 

relationships of, 308, 309 
Basidiospores, 290, 291, 292, 293, 297, 
299, 301, 304 

formation of, 304 
Basidium, 290, 291, 292, 297, 301, 

304, 306, 307 
Bast fibers, 56, 57 
Berry, 460, 451, 482 
Beverage plants, 533, 535 
Biennials, 80 
Bleeding, 130 

Blue-green algae, see Myxophyceae 
Body cell, 403, 404, 416, 417 
Boron, 151 
Botulism, 262 
Bracken, 365-375 
Bracket fungi, 306, 307 
Bracts, 413, 457, 462 
Branches, 74, 75 

aerial, 383 

primordia of, 61, 74, 86 

spur, 408 

Brassica oleracea, 634 
Brown algae, see Phaeophyceae 
Bryophytes, 317-355 

nature of, 197, 353-355 

relation to pteridophytes, 381 
Budding, 72 

of spores, 291, 292 

of yeasts, 280, 281 
Buds, 74, 83-93, 341 

active, 85 

adventitious, 89, 319 

axillary, 83, 84, 91 

floral, 85, 88 

latent, 87 

lateral, 407 

leaf, 84, 85 

mixed, 85, 91 

naked, 83-86 

nature of, 83 

Buds, plant form and, 90-92 

position of, 83 

protected, 84, 86-89, 90 

terminal, 83, 84, 407, 408 
Bulbs, 89, 90 

medicinal, 543 

Cacti, 112, 631 
Calcium, 149 

rdle of, 151 
Callus, 71 
Calyx, 423 

Cambium, 36, 36, 37, 52, 64, 56, 67, 
69, 70, 71, 397, 410; see also cork 
cambium, secondary thickening 

formation of, 36, 52 
Canal, 322, 372 
Canal cells, 344 
Capsule, 331, 336, 346, 363, 455, 

Carbohydrates, 147, 148 

energy of, 147 

formation of, 140-148 

respiration of, 161, 162 

storage of, 153, 154 

utilization of, 161 
Carbon, 149 
Carbon dioxide, 140, 143 

formation of, 282 

in soil, 125 

liberation of, 163 

photosynthesis and, 143 

utilization of, 141 
Carotin, 137, 231 
Carpels, 427, 428, 451 
Carpogonium, 246, 246, 248 
Carpospores, 245, 246 
Cascara, 543 
Catkin, 463, 466 

pistillate, 465 

stain inate, 466 
Cat-tail family, 478, 479 
Cell division, 27, 184, 190-192, 218, 
226, 242 

by cell plate, 190 

by constriction, 191, 214, 252, 281 

by furrowing, 269, 270, 273 

in asci, 286, 286 

significance of, 183 
Cell plate, 190 
Cell sap, 11 
Cell walls, 9, 10 
Cells, discovery of, 7 



Cells, embryonic, 26, 27 

many-nucleate, 267, 273, 276, 402, 
414, 416, 429, 436 

maturation of, 28 

method of study, 12 

resting, 27 

structure of, 7-14, 213, 219, 251 

two-nucleate, 295, 302, 309 

formation of, 299 
Cellulose, 148 
Central body, 226, 227 
Centrifugal force, 171, 172 
Cereal grains, 533, 537, 539 
Characters, 486, 490 

dominant, 486 

genes and, 491, 492 

new, 500 

recessive, 487 

transmission of, 487, 488, 489, 494, 
495, 496 

variation in, 499 
Chemical elements, 149-151 
Chitin, 252 

Chlamydomonas, 200-204, 224 
Chlorophyceae, 199-224 
Chlorophyll, 136-139, 231 

formation of, 138, 139 

function of, 145-147 

photosynthesis and, 142 
Chlorophyll a, 137 
Chlorophyll 6, 137 
Chloroplasts, 10, 12, 136, 137, 200, 

209, 213 
Chlorosis, 151 
Chondriosomes, 13, 27 
Chondrus, 244 
Chromatic network, 12, 13 
Chromonerna, 185 
Chromoplasts, 140 
Chromosomes, 185 

changes in, 501-507 

combinations of, 361 

daughter, 187 

distribution of, 360 

formation of, 185 

genes and, 492-499 

in endosperm, 433 

inheritance and, 490-499 

interchange between, 361 

linkage in, 498 

maternal, 360, 492 

number of, 186, 502, 503, 504, 

Chromosomes, parent, 188 

paternal, 360 

persistence of, 192, 193 

reduction in number of, 356-364 

separation of, 187, 188, 368 
Chytrids, 279 
Cladophora, 211, 212, 362 
Classes, 197 
Classification, artificial, 195, 512 

evolution and, 511, 512 

natural, 195, 512 

plant distribution and, 522 
Clay, 47 
Climate, plant distribution and, 522 

rusts and, 296 
Clinostat, 171 
Closterium, 218 
Club moss, 385-388 

smaller, 388-39C 
Coagulation, 149 
Coccus, 251 
Coelosphaerium, 227 
Coleus, 46 
Colloids, 149 

diffusion of, 149 
Colonies, 200, 229, 253 

evolution of, 196 

nature of, 202 

reproduction of, 206, 214, 220 
Columella, 268, 269, 270 
Companion cells, 32, 33, 56, 67, 68 
Competition, 519, 520 
Composite family, 476-478 
Compounds, 15 

inorganic, 15 

organic, 15 
Conidia, 276, 277, 284, 286, 291, 


Conifers (Coniferales), 397, 632, 525 
Conjugation tube, 214 
Cooperation, 520 
Copper, 151 
Coprinus, 306 
Coral reefs, 244 

Cork, 39, 40, 64, 65, 66, 410, 411 
Cork cambium, 39, 40, 64, 65, 66, 
409, 410, 411 

origin of, 64 
Corn, 5 

leaf of, 4, 104 

root of, 34, 43 

stem of, 67 
Corolla, 424 



Cortex, 29, 30, 34, 63, 64, 383, 397, 

408, 410 

Corymb, 462, 463 
Cosmarium, 218 
Cotton, 535, 537, 638 
Cotyledons, 404, 405, 419, 420, 436, 

437, 444, 462 

Crop-distribution, 536-541 
Crop plants, 522-541 
Cross sterility, 507 
Crossing over, 496, 497, 498 
Crowfoot family, 467, 468 
Cup fungi, 287, 288 
Cupules, 326, 327 
Cuticle, 122, 412 
Cutin, 54 
Cuttings, 45, 46 
Cycads (Cycadales), 397 
Cytoplasm, 10, 27 

dense, 10, 12, 106 

movement of, 11, 167 

structure of, 10-12 

Damping off, 278 
Darwin, 511 

Datura, inheritance in, 506, 506 
Decay, 255 
Denitrification, 258 
Denitrifying bacteria, 258 
Deserts, 623, 529, 630 
Desmids, 217, 218 
Diastase, 155 
Diatomaceous earth, 241 
Diatoms, 240-242 
Dicotyledons, 36 

characters of, 439 

classification of, 464-478 

leaves of, 98, 99 

root of, 30, 36, 36 

secondary thickening in, 55, 66 

stem of, 52, 63 ,. 
Dictyostele, 380, 381 
Diffusion, 15-17 

of gases, 15 

of liquids, 16 

of solids, 16 
Digestion, 155, 156 
Diseases, 262-265 

nature of, 262, 552 

plant, 272, 278, 290, 295, 552-559 
Dispersal of plants, 197 
Divisions, 197 
Dominance, 486 

Drugs, 316, 466, 474, 475, 478, 541- 


Drupe, 452, 471 
Dyes, 316, 535, 536 

Ectocarpus, 231-233, 362 
Eggs, 207, 220, 221, 235, 238, 239, 
274, 275, 277, 285, 344, 372, 391, 
402, 416, 429, 430, 434 
liberation of, 239 
nature of, 207 

Elaters, 331, 332, 336, 337, 338 
Elements, 15 

essential, 149-151 
Elodea, 8 

leaf of, 9 
Embryonic region, 25, 26, 60, 61, 74, 

Embryos, 346, 373, 385, 387, 392, 

393, 404, 405, 435, 443, 451 
dicotyledonous, 436 
monocotyledonous, 437 
Endodermis, 33, 34, 35, 36, 40, 368 y 

378, 383, 412 
function of, 128 
Endosperm, 404, 405, 419, 435, 443, 

448, 451, 452, 481 
nature of, 435 
Energy, 145, 166, 282 
liberation of, 161, 163 
photosynthesis and, 145 
storage of, 161 
transformation of, 145 
utilization of, 163 
Environment, 163 
growth and, 182 
heredity and, 499, 600 
leaf structure and, 108 
plant distribution and, 522, 536 
plant form and, 75, 77, 123, 182, 


Enzymes, 255, 282, 448 
function of, 156 
nature of, 155 
Epicotyl, 404, 405, 419, 436, 437, 444, 


Epidermis, 30, 33, 34, 53, 64, 67, 101, 
102, 104, 320, 327, 383, 410, 412 
Epiphytes, 485 
Equatorial plate, 185, 188 
Equisetum, 382-385 
Etiolation, 139 
Evolution, 510-521 



Evolution, general course of, 195, 196, 
516, 517 

nature of, 510 

plant distribution and, 514 
Excretion, 159 
Extinction, 517 

reproduction and, 517 518 

Family, 197 
Fats, 148, 280, 535 

energy of, 148 

storage of, 154 
Female gamete, see egg 
Fermentation, 281-283 

nature of, 164 
Fern, grape, 376 

interrupted, 376 

sensitive, 377 
Ferns, 365-381 

tree, 365 

Fertilization tube, 274, 275, 277 
Fiber plants, 482, 484, 535 
Fig, 467 

Filament, 424, 425 
Filial generation, 486, 487, 488, 489 
Filicineae, 305-38 1 
Flagi-lla, 200, 201, 252, 274, 321, 344, 

373, 391, 404 
Flagellates, 204 
Flower clusters, determinate, 463 

indeterminate, 462, 463 
Flowers, 1, 423-439, 462-485 

advanced, 476 

arrangement of, 462-464 

composite, 476, 477 

compound, 463 

disk, 476 

evolution of, 424, 425, 432, 472 

irregular, 468 

pistillate, 465, 475, 479, 480, 482 

pollination of, 431-433 

primitive, 464, 478 

primordia of, 86 

ray, 476 

staminate, 465, 475, 479, 480, 482 

structure of, 423, 424, 464, 465, 467, 
468, 469, 470, 471, 473, 474, 
475, 476, 478, 479, 480, 481, 
482, 483, 485 

symmetry of, 425 

union of parts of, 425, 472, 475, 483 
Follicle, 455 
Foods, manufacture of, 136-151 

Foods, movement of, 152-155 

nature of, 147 

storage of, 48, 153-155, 405, 409, 

utilization of, 152-165 
Foot, 331, 335, 338, 346, 353, 373, 

374, 393 

Forage plants, 537, 538 
Forest nursery, 546 

products, 545-549 
Forestry, 545-549 
Forests, deciduous, 523, 525, 526 

evergreen, 623, 525, 526, 527, 530- 

Pacific Coast, 523 

Rocky Mountain, 623 

tropical, 623, 527, 528 
Fossil plants, 351, 382, 385, 397, 423, 

515, 516 

Free-cell formation, 285, 286 
Fruit coat, 452 
Fruiting body, 286, 286, 287, 303, 

313, 315 
Fruits, 1, 533, 450-458 

aggregate, 466, 466, 457 

classification of, 458 

dehiscent, 453, 455 

dispersal of, 458-461 

dry, 453-455 

edible, 533 

false, 450, 451, 456 

fleshy, 451 

indehisccnt, 453-455 

multiple, 457, 458 

nature of, 438 

simple, 450-455 

stone, 461 

true, 450 

Fucus, 237-240, 362 
Funiculus, 424, 436 
Fungi, 250-316 

cultivation of, 302 

imperfect, 558, 559 

lichens and, 310 

nature of, 197 

nutrition of, 269 

parasitic, 272, 275 

Gamete nuclei, 416, 417 

male, 245, 430, 431, 434 
Gametes, 202, 203, 209, 271; see also 
antherozoidj egg 

chromosome number in, 356 



Gametes, female, 205, 214, 220, 233 

male, 205, 214, 233, 245, 430 
Gametic union, 214-216, 218, 219, 
220-222, 223, 224, 232, 233, 237, 
238, 245, 246-248, 270-272, 275, 
276-278, 285, 286, 322, 323, 330, 
345, 373, 392, 404, 417, 418, 433 
chromosome number and, 356 
chromosome reduction and, 363, 


evolution of, 440-442 
Gametophytc, 317 323, 325-330, 333- 
335, 336, 340-345, 349-352, 370- 
373, 384, 385, 387, 388 
evolution of, 395, 440-442 
nutrition of, 320 
Gemmae, 327, 328 
Genealogy of plants, 196, 197 
Generative cell, 402, 403, 415, 417, 

430, 431 

Genes, 491, 493, 495, 509 
changes in, 507-509 
characters and, 491, 492 
inheritance and, 491, 492 
Genus, 197 

Geographic distribution, 522-532 
Geotropism, 168-172, 448 
negative, 168 
positive, 168 
transverse, 168 
Gills, 303, 304 
Gloeocapsa, 225-227 
Glucose, 141, 152 

formation of, 141 
Glume, 479, 480 

Glycogen, 148, 226, 252, 268, 280 
Gourd family, 475, 476 
Grafting, 71, 72 
Grain, 453 

Grass family, 479, 480 
Grasslands, 623, 528, 529, 539 
Gravity, 168-172; see also geotropism 
Green algae, see Chlorophyceae 
Grinnellia, 244 
Growth, 157-159 

by apical cells, 319, 334, 342 
cell division and, 194 
external factors and, 182 
light and, 138 
nature of, 157 
responses, 169 
Guard cells, 120 
changes in, 120, 121 

Guard cells, function of, 120, 121 

Gurns, 535 

Gymnosperms, 36, 75, 397-422 

evolution of, 422 

nature of, 397 

secondary thickening in, 55, 66 

stem of, 52 

Hairs, 63 

epidermal, 55, 101 

glandular, 101 

root, 30, 34, 126 
Head, 463 
Heart-wood, 63, 73 

Hepaticae, 317-339; see also liverworts 
Herbals, 542 
Herbs, 77 

medicinal, 543 
Heterocyst, 228, 229 
Hilurn, 444 
Hooke, 7 

Horsetails, 382-385 
Hosts, 254, 295, 297 
Humus, 47 
Hydrogen, 149 
Hydrophytes, 123 
Hyphao, 267, 273, 288, 291 
Hypocotyl, 404, 405, 419, 420, 436, 
437/444, 448, 449, 452 

Imbibition, 21, 22, 106, 122, 447 
Infection, 292, 296 
Inheritance, 486 509 

cell division and, 184 

chromosomes and, 490-499 

in peas, 486-490 

mechanism of, 490-499 

nuclear division and, 193 

ratios in, 487, 488, 489 
Integument, 400, 402, 404, 405, 413, 

415, 416, 419, 428, 435 
Internode, 2, 3, 51, 66 

dicotyledonous, 55 
Inulin, 148 
Invertase, 155 
Involution forms, 252, 261 
Iodine, 233 
Iron, 149 

rdle of, 151 
Irritability, 166-182 

nature of, 167 

Jimson weed, inheritance in, 606, 506 
Jute, 535 



Kelps, 233, 236, 237, 238 
Koch, 250 

Laminaria, 233-236 
Latex, 159, 466 
Leaf axil, 2 

blade, 2, 96, 97 

gap, 381 

mosaic, 96 

scar, 84, 85, 96, 110 

trace, 55 
Leaflets, 90 
Leaves, 94-113, 332, 375, 398 

abscission layer of, 109, 110 

aeration of, 118-120 

aerial, 108 

alternate, 96 

arrangement of, 94-96 

compound, 99, 100 

development of, 94 

dicotyledonous, 98 

external structure of, 96-100 

fall of, 109, 110 

food-storage in, 105 

immature, 61, 55 

insect-catching, 107, 108 

intercellular spaces of, 118-120 

monocotyledonous, 97 

netted-veined, 99 

opposite, 94, 96, 97 

parallel-veined, 98 

primary, 373, 374, 393 

primordia of, 61, 83, 86, 111 

scale, 86, 86, 90, 111,383 

sessile, 97, 386 

sheathing, 3 

structure of, 101-107, 110, 351, 
368, 388, 411, 412 

submerged, 108 

variation in same plant, 107, 108 

venation of, 21, 100, 102 

vertical, 105 

water-movement in, 119 

water-storage in, 106, 106 

whorled, 94 
Leeuwenhoek, 250 
Legume, 464, 455, 461, 471 
Lemma, 479, 480 
Lenticels, 66 
Leucoplasts, 27, 139, 153 
Lianas, 528 
Lichens, 289, 310-316 

crustose, 310, 311 

Lichens, foliose, 311 

fruticosc, 311, 312, 313 

nature of, 310 
Life cycle, 235, 236 

of angiosperm, 438 

of fern, 374, 376 

of liverwort, 324, 325 

of moss, 348 

of pine, 421 

of Selaginella, 393, 394 

of wheat rust, 300 

of Zamia, 406 
Light, energy of, 161 

nature of, 146 

photosynthesis and, 144 

plant form and, 174 

respiration and, 162 

responses to, 172-175, 180, 181 
Lily family, 483, 484 
Linkage, 495-497 
Linnaeus, 512 
Upases, 155 
Liverworts, 317-339 

ancestry of, 317 

evolution of, 317, 325, 333 

leafy, 333 
Living matter, 156, 157 

nature of, 13, 14 
Loam, 48 
Lodicule, 479, 480 
Long-day plants, 175 
Lumber, 548 

methods of sawing, 72, 73, 74 
Lumbering, 545 
Lycopodium, 385-388 

Macrocystis, 236, 238 
Macrogametophyte, 390, 391, 401, 

402, 414, 416, 424, 428, 429, 430, 

Macrosporangia, 389, 390, 391, 399, 

400, 428 

Macrospore mother cell, 428 
Macrospores, 389, 390, 401, 413, 428, 

Macrosporophyll, 389, 390, 391, 399, 

401, 413 
Magnesium, 149 

role of, 139, 151 
Manganese, 151 
Marchantia, 325-331 
Mature region, 26, 26, 52 
Mechanical stimuli, 177-180 



Mechanical tissues, 38, 52, 63, 54, 67, 
68, 383, 412 

Medicinal plants, 541-545 
Medullary rays, 37, 38, 63, 57, 68, 

59, 72, 409, 410 

Mendel, 486, 487, 488, 489, 495 
Merismopedia, 227 
Mesophytes, 123 
Metabolism, 165 

of bacteria, 254-262 

of yeasts, 282 
Micrasterias, 218 , 

MicroganftcitijJ?, 390, 391, 392, 

402, ar/tlK^, 427, 430, 431 

Micropyle, *)1, 402, 414, 416, 435, 


Microsphaera, 284-286 
Microsporangia, 389, 390, 392, 399, 

400, 413, 426, 427 
Microsporc mother cells, 400 
Microspores, 389, 390, 392, 399, 413 
Microsporophyll, 389, 390, 392, 399, 

413, 414 
Mildews, downy, 278 

powdery, 283-286 
Milk and bacteria, 256 
Mimosa, 177, 178 
Mint family, 474 
Molds, black, 272 

blue, 286, 287 

bread, 267-272 

green, 286, 287 

water, 273 
Molecules, 15 
Monocotyledons, 36, 94 

characters of, 439 

families of, 478-485 

leaves of, 97 

roots of, 34 

steins of, 52, 66-70* 
Morels, 288, 289 
Moss, Iceland, 314 

reindeer, 314, 316 

tree, 340 
Mosses, 340-355 
Motile cells, 200-207, 228; see also 

antherozoids, swarm-spores 
Movements of organs, 177-181 
Musci, 340-355 
Mushrooms, 302-308 
Mustard family, 468, 469 
Mycelium, 267, 268, 271, 273, 276, 
291, 293, 296, 308 

Myxophyceae, 225-230 
nature of, 225 

Natural vegetation, 522-532 
Neck, 321, 322, 344, 372 
Nectaries, 130 
Nemalion, 244-246, 363 
Nereocystis, 236, 237 
Nettle family, 465, 466 
Nightshade family, 474, 475 
Nitrates, 150 

formation of, 258 
Nitrification, 258 
Nitrifying bacteria, 258 
Nitrites, formation of, 258 
Nitrobacter, 258 _ 

cljopGO, 261, 263, 520 

fixlion of, 259, 260, 472 

role of, 139, 150 
Nitrosomonas, 258 
Node, 2, 3, 51, 383 

dicotyledonous, 55 

responses and, 170 
Nostoc, 229, 230 
Nucellus, 400, 402, 413, 416, 416, 

Nuclear membrane, 12, 13 

sap, 13 
Nuclei, 10, 12, 200 

daughter, 188-190 

division of, 183-194, 216; see also 
reduction divisions 

egg, 246 

gamete, 246 

primitive, 226 

structure of, 12, 13 

union of, 216, 297 

zygote, 246 
Nut, 463, 454 
Nutrition of plants, 136-165 

of bacteria, 254, 255 

of yeasts, 281, 282 

Oedogonium, 219-222, 224, 362 

Oenothera, 601, 504 

Oils, 268, 482, 535; see also fats 

volatile, 100, 159, 473, 545 
Oleoresins, 159 

Oogonium, 220, 221, 223, 234, 236, 
238, 239, 274, 275, 277, 284, 285 
Orchid, 48 

family, 484, 485 



Orders, 197 
Organs, 1 

functions of, 5 
Origin of new plants, 508, 509; see 

also evolution 
Osciliatoria, 227, 228 
Osmosis, 17-19, 152 

demonstration of, 18, 19 

significance of, 19-21 
Osmotic pressure, 131 
Ovaries, 426, 427 

compound, 427 

simple, 427 

Ovules, 400, 401, 413, 415, 424, 428 
Oxygen, 149, 282 

in soil, 125 

liberation of, 141 11 **^ 7 !TO 

respiration and, 161-165 '"'_;- 

seed germination and, 447* ' 

utilization of, 161-105 

Palet, 479, 480 
Palisade cells, 101 

tissue, 102, 104 
Palm family, 481-483 
Panicle, 463 
Paper, 548 

Parasites, 254, 287, 290, 306, 552-559 
Parenchyma, 33 
Parsley family, 473, 474 
Pasteur, 250, 256 
Pasteurization, 256 
Peat, 351 
Pediastrum, 208 
Pedicel, 462 
Peduncle, 462 
Penicillium, 286, 287 
Perennials, 80 
Pericycle, 30, 33, 36, 36, 40, 53, 64, 

368, 378, 411 
Peridium, 298, 299 
Permeability, 18 

differential, 18 
Petals, 424, 471 
Petioles, 2, 110, 111 

structure of, 103 
Phaeophyceae, 231-243 

nature of, 231 
Phloem, 30, 32, 102, 397, 410, 412 

function of, 152 

parenchyma, 56, 57 

primary, 32, 35, 36, 52, 53, 54, 67, 
368, 378, 383, 398, 411 

Phloem, secondary, 36, 87, 56, 67, 408, 

Phosphorus, 148, 149 

r61e of, 151 
Photosynthesis, 140-147, 295 

essential conditions, 141-145 

nature of, 140 

respiration and, 162 
Phototropism, 172-175 

negative, 172 

positive, 172 

transverse, 172 
Phycomycetes, 26 


Phytophthora, 278#5 
Pigment spot, 200, 201 
Pigments, 136-139, 160, 161, 316 
Pilobolus, 272, 273 
Pine, 407^21, 626 
Pineapple, 457 
Pink family, 466, 467 
Pistils, 424, 426-429 

compound, 428 

simple, 427, 471 
Pitcher plant, 107 
Pith, 52, 53, 64, 410 
Plankton, 199, 228 
Plant-breeding, see inheritance 
Plant diseases, 272, 278, 290, 295, 

Plasma membrane, 10, 11, 12 

formation of, 190, 191 

r61e of, 19-21 
Plasmolysis, 20, 21 
Plastids, 136, 139, 140, 153, 231 
Poisons, 473 
Polar caps, 186 
Pollen chamber, 401, 402, 414 

grain, 402, 403, 416, 424, 426, 427, 

germination of, 403, 416, 417, 431 

sac, 426, 427 

tube, 403, 416, 431 
Pollination, 403, 415, 431-433 

cross, 486 

insect, 431-433, 484, 485 

wind, 403, 415, 431 
Polysiphonia, 244, 246-249, 362 
Pome, 452 
Porella, 333-336 
Postelsia, 236 
Potassium, 149, 233 

r6Je of, 151 



Potometer, 116 

Primary endosperm cell, 429, 430, 
433, 434 

nature of, 433 
Primitive organisms, 195 
Proembryo, 404, 417, 418 
Progamete, 270, 271 
Progressive cleavage, 270 
Propagation of plants, 45, 71 
Proteases, 155 
Proteins, 148, 149, 261 

decay of, 255 

formation of, 149 

nature of, 148 

storage of, 154 
Prothallial cells, 391, 392, 402, 403, 


Prothallium, 370, 371, 384, 385 
Protococcus, 208 
Protonerna, 341, 349 
Protoplasm, 10 
Protoplast, 10 
Protosteles, 378 

exarch, 378, 380 

mcsarch, 379, 380 
Provascular strands, 52 
Psalliota, 302-304 
Pterfdium, 365-375 
Pteridophytcs, 365-396 

nature of, 197, 396 
Ptomaines, 262 
Puccinia graminis, 300, 301 
Puffballs, 307, 308 
Pulse family, 471-473 
Pulvinus, 179, 180 
Putrefaction, 255 
Pycnidiospores, 313 
Pycnidium, 313 
Pyrenoids, 200, 201, 209, 213 

Quarter sawing, 72, 7*5 
Quinine, 543 

Raceme, 462, 463 
Receptacle, 424 
Recessiveness, 487 
Red algae, see Rhodophyceae 
Reduction divisions, 356-364 

inheritance and, 493-499 
Region of elongation, 25, 26, 28, 29, 

50, 51, 74 
bending and, 169, 170, 173 

of maturation, 25, 26, 50, 51, 74 

Regions of vegetation, G23, 524-532 
Relationships of plants, 512 
Reproduction, 194; see also colonies, 
gametic union, spores, vegetative 

Resin canals, 408, 409, 410, 411, 412 
Resins, 535, 548 
Respiration, 161-164 
aerobic, 164 
anaerobic, 164 
of yeasts, 282 
Responses, 166-182 

nature of, 166 

Rhizoids, 268, 320, 327, 341, 371 
Rhizomes, 543, 544 
Rhizopus, 267-272 
Rhodophyceae, 243-249 
Ascomycetes and, 289 
nature of, 243 
Riccia, 317-325 
Root cap, 25, 26, 368 
drugs, 544 
hairs, 30, 34, 125 
formation of, 34 
function of, 34, 126 
nodules, 259, 260 
pressure, 127-129, 132 
system, 4 

Roots, 25-49, 377, 378 
adventitious, 4, 5, 45, 46, 79 

function of, 44 
aerial, 48 

environment and, 46-48 
fascicled, 44 
fibrous, 43 
functions of, 48 
organization of, 25, 26 
primary, 4, 373, 374, 393 
regions of, 25-29 
secondary, 4, 40-44 
conduction in, 41 
origin of, 41 
structure of, 29-40, 367, 368, 378, 

398, 410, 411 
tap, 42, 43 
Rose family, 469-471 
Rosette cells, 418 
Rubber plants, 466 
Rusts, 294-302 
apple, 301, 302 
blister, 558 
wheat, 293-301 
white, 276 



Thallus, branching of, 318 
Thorns, 112 
Tissues, 8 

primary, 35 

secondary, 35, 38 
Tobacco, 538 

bacteria and, 256 
Toxins, 262 
Tracheids, 31, 68, 132, 408 

formation of, 31 

water-conduction in, 131, 132 
Translocation, 152, 153 
Transpiration, 1 14-123 

amount of, 1 15-1 18 

checking of, 121 '23 

rate of, 115-118 
Trees, 75-77, 407, 525-528, 545-549 

form of, 90, 91 

Trichogyne, 246, 246, 248, 289 
Tropical plants, 528 
Tube cell, 402, 403, 415, 417, 430, 431 
Tubers, 154, 475 

medicinal, 543 
Tumble-weeds, 459 
Tundra, 623, 524, 525 
Turgidity, 20 
Turgor, 20, 21 

pressure, 21, 122, 126 
Turpentine, 548 

Ulothrix, 209, 210, 224, 362 
Ulva, 210, 211, 362 
Umbel, 462, 463 

compound, 473 

simple, 473 
Uredosorus, 295 
Uredospores, 296, 301 
Utilization of plants, 533-549 

Vaccination, 264 

Vacuolar membrane, 10, 11, 12 

Vacuoles, l^ 

central, 10, 11, 12 
formation of, 28 
contractile, 200, 201 
Variation, 486-509 

new races and, 508, 509 
Vascular bundles, 52, 63, 54, 68, 66, 
67, 68, 367, 368, 397; see also 
phloem, xylem 
arrangement of, 63, 67 
Vaucheria, 222-224, 279 
Vegetative multiplication, 312, 319. 
327, 347, 350, 366, 550 

Veneers, 549 

Venter, 321, 322, 344, 372 

Vessels, 31, 68, 132 
blocking of, 62, 63 
formation of, 31, 32, 37 
water-conduction in, 131, 132 

Vines, 78 

Virus diseases, 264, 265, 553, 554 

Viruses, 264, 265 

Volvocales, 204 

Volvox, 205-207, 224 

Water, amount of, in plants, 114 

balance, 134, 135 

capillary, 124 

exudation of, 129, 130 

gravitational, 123 

hygroscopic, 124 

importance of, 114 

intake of, 125-129, 447 

loss from plants, 114-123 

movement in plants, 119, 131-134 

plant-form and, 175, 176 

plants and, 114-135 

pores, 129 

responses to, 175-177 

seed-dispersal and, 459 

storage of, 351 

tensile strength of, 133, 134 
Water-storage tissue, 106, 106, 135 

function of, 135 
Water table, 123 
Waxes, 535 
Weeds, 474, 480, 518, 549-552 

control of, 551, 552 

dispersal of, 549, 550 

injurious, 551 

nature of, 549 
Wheat-growing areas, 640 
Willow family, 134, 135 
Wind, seed-dispersal and, 459 

spore-dispersal and, 300 
Wood, see also lumber, xylem 

destruction by fungi, 307 

fibers, 58, 59 

spring, 68, 59, 72, 410 

structure of, 72-74 

summer, 58, 59, 60, 61, 72, 410 

utilization of, 545-549 
Woods, medicinal, 544 
Wound tissue, 70, 71 

Xanthidiiun, 218 
Xanthophyll, 137 



Xerophytes, 123 

Xylem/30, 102, 397, 410, 412 

primary, 29, 36, 36, 62, 63, 64, 
67, 368, 378, 383, 398, 408, 

secondary, 36, 37, 56, 67, 68, 59, 
60, 61, 62, 398, 408, 411 

Yeasts, 280-283 
cultivated, 283 
wild, 283 

Zamia, 397-407 

Zinc, 151 

Zygote, 203, 216, 221, 223, 242, 271, 

275, 277, 284, 285, 345, 417 
chromosome number in, 350 
germination of, 203, 204, 207, 216, 
217, 222, 224, 236, 239, 271, 

inheritance through, 494, 496, 496 
nature of, 203 
Zymase, 282