union of two distinct cells (or at least their nuclei) to
form a single cell, the zygote. This may develop
directly into a new plant or into a mass of cells (the
spore fruit), which produces onlj^ eventually the repro-
ductive cells, which give rise to the new plants. The
no PLANT PHYSIOLOGY
uniting cells (gametes) may come from the same or
from different plants, indeed they may be sister cells,
i.e. formed by the division of one cell, but this is not
common. They may be alike (isogamous) or unlike
(heterogamous).
158. As we proceed from the simple to more complex
plants in the study of sexual reproduction we find entering
in, the principle of "increased parental care." In the
lowest plants with sexual reproduction the gametes
unite outside of the parent plant, at a higher stage one
gamete (the egg) is retained in the parent plant and is
fertilized by the motile sperm. Still higher the egg is
surrounded by special protective structures (cystocarp,
archegone, etc.) and produces no longer a simple zygote
but a spore fruit w^hich may also be included in the pro-
tective envelope. A still higher stage is whei'e the
spore fruit is so highly differentiated that it becomes a
separate generation (sporophyte), capable of separate
existence, similar to or differing in appearance from the
parent generation (gametophyte). Highest of all we
find the sporophyte becoming the prevalent generation,
the gametophyte being retained within its protective
tissues and only developing far enough to permit sexual
reproduction to occur.
159. Each gamete of the same species has the same
number of chromosomes in its nucleus. The cell re-
sulting from their union, the zygote, has double this
number (diploid number). Where a zygote is formed
which gives rise directly to a plant like the original one,
the reduction in the number of the chromosomes from
the diploid to the haploid number (see paragraphs 35
and 160), occurs with the germination of the zygote.
Where a spore fruit or sporophytic generation occurs its
cells retain the diploid number and the reduction divi-
REDUCTION OF CHROMOSOMES
111
sion does not enter in until tlie spores are being produced,
which give rise to the sexual generation (gametophyte).
This latter has the haploid number of chromosomes in
its nuclei. We must thus distinguish carefully between
typical asexual reproduction, where the resulting plant
is, as it were, but a separated part of the mother plant,
and the formation of a gametophytic generation from
the spore produced in the sporophytic generation. In-
deed each of these generations may have typical asexuiil
reproduction leading simply to the formation of other
plants of the same generation.
160. After the union of gametes the chromosomes
from the two gametes remain separate, but usually the
corresponding chromosomes from each gamete lie close
together. In the reduction division the chromosomes
gather at the equator of the spindle as double chromo-
somes, in all probability representing the two corre-
sponding chromosomes from the two gametes. Before
this stage is reached, and while the chromatin matter
is found on fine
threads, there is a
characteristic bunch-
ing together of these
threads (called the
synapsis) in the course
of which it is sup-
posed that certain
characters become ex-
changed in the corres-
ponding c h r o m o -
somes. These double chromosomes split apart and as
single ones go to the opposite poles. There are thus
entering into each daughter nucleus only as many chromo-
somes as were originally present in the gametes. These
Fig. 49. — Reduction division (diagrammatic).
112 PLANT PHYSIOLOGY
chromosomes do not, however, correspond exactly to the
originals, for in the synaptic stage there has been an
exchange of some characters. At the next division the
nuclear phenomena are like those of the ordinary
vegetative division.
161. These peculiarities of haploid and diploid chro-
mosome number, reduction division, and ordinary (so-
matic) division of the nuclei, as well as other observed
phenomena of heredity, have led most investigators to
conclude that the chromosomes are the chief bearers of
heredity. In sexual reproduction, then, is found a means
of combining in the most complicated ways the minute
or larger differences found in the different parents.
162. Variations. Hardly any two plants are exactly
alike. The differences are of two kinds: (1) a response
of the plant to slightly or greatly different environ-
mental conditions, and (2) a difference in the constitu-
tions of the plants that leads them to respond somewhat
differently in morphological or physiological characters
when exposed to the same conditions. These latter
are the only ones that demand attention here. They
may be slight differences that are apparently not inherit-
able (i.e. although the somatic or vegetative cells are
somewhat different the sexual cells are not so), or there
may actually have taken place a change in the constitu-
tion of the protoplasm that affects also the reproductive
cells, so that the heredity carriers (probably the chromo-
somes) are slightly different in the different plants.
163. Gregor Mendel, in 18G6, published a paper in
which he pointed out that certain characters that differed
in the two parents and that are mutually exclusive
(i.e. that allow of no intermediate form) would appear in
the second generation in a pure form in some of the
plants. This is now explained by the phenomena taking
VARIATIONS 113
place in connection with the reduction division, where
during synapsis certain character-determining units in
the chromosomes may become exchanged, so that the
chances are about equal whether one or the other char-
acter from respectively one or the other parent will be
present in the new cell. Mendel found that about one-
fourth of the second generation plants show a given char-
acter from one of the original plants and one-fourth the
character from the other plant, while one-half still re-
tains (at least potentially) both characters, although only
one is visible, it being ''dominant" over the other char-
acter which is ''recessive.'^ That both characters are
present is shown by the fact that seeds from this half
produce plants which divide up again into one-fourth,
one-fourth, and one-half, etc.
164. In sexual reproduction the various differences
borne by the different chromosomes, or perhaps more
accurately by the unit structures of the chromosomes,
will be redistributed among the daughter and grand-
daughter plants in new combinations. Some of these
will be advantageous to the plant, and it will succeed
better and be able to reproduce more freely; other com-
binations may be less favorable, and the plants with
such combinations will have a poorer chance in the
struggle for existence, and will not reproduce so freely.
As a result, ''Natural Selection'^ sorts out those whose
combinations are most favorable. Thus we see that
sexual reproduction forms a means by which the con-
stantly arising individual differences (and why they arise
we do not know) are made use of in the most manifold
combinations, the most favorable of which are perpet-
uated. This is what was called by Darwin "The
survival of the fittest."
165. These inheritable variations may be slight or
lU PLANT PHYSIOLOGY
they may be strongly marked. They are often called
"mutations" to distmguish them from the non-in-
heritable variations. If the plants showing them are
considerably better able to exist, they will rapidly crowd
out the less favorably constituted plants, and thus a
new species will replace the old. Under other environ-
mental conditions this new feature may be less favorable
and so the older form will persist. Thus we find plants
with all sorts of differences or what we call ''species,"
all over the world. Some plants have changed but little
apparently from their primitive structure, as they were
able to persist in that form under certain conditions,
while some of their descendants, it may be, have pro-
gressed far along the evolutionary line. Thus we find
the Vegetable Kingdom made up not only of the ends of
long evolutionary branches but also of stragglers that
have progressed only a very little way, and of those that
have grown further before branching out in some other
direction. It is this fact that enables us to attempt to
show the probable course of evolution (phylogeny) of the
Vegetable Kingdom in our arrangement of the plants now
existing.
166. The conditions that favor reproduction have
been worked out for a good many plants, but are un-
known for the vast majority. It seems that those con-
ditions that favor continued vegetative growth, such as
an abundance of water and all foods, tend to delay or
prevent reproduction. On the other hand, there must
usually be a certain amount of food stuffs stored up.
If these can be prevented from accumulating, or can be
used up by promoting vegetative growth, reproduction
will be held back. In many cases, however, the repro-
ductive stage comes on in spite of all efforts to keep it
back, showing that not all the factors are known.
PLANT BREEDING 115
167. The breeding of plants is an application of the
principles of reproduction and heredity to the production
of plants with certain desirable characteristics. In-
stead of waiting for the chance production of a desirable
type of plant, the plant breeder either grows many plants
in conditions under his control and selects for further
propagation those he deems most desirable (method of
selection), or he takes two distinct plants, each with
certain characters that he desires, and crosses them, and
grows the progeny in large numbers for several generations
until by the laws of chance in the distribution of the
unit character determinants there appears a plant
combining the desirable characters of the two parents.
This is the method of hybridization or crossing. The
discovery by Mendel of the segregation of characters by
definite laws of numbers (see paragraph 165) has given a
great impetus to this line of work.
Laboratory Studies. Not much can be done in the way of
laboratory work on this subject. In the study of the different
forms of plants in the later chapters of the book, the points
emphasized in the foregoing paragraphs should be borne in
mind. A few suggestions are made for observations on the
part of the student.
(a) Find and compare carefully a dozen different plants of
timothy {Phleinn prateiisc), red clover {Trijolmm pratense),
ribbed plantain (Plantogo lanccoJata), etc. Select those
plants of the same age and from as ncarlj^ as possible the same
soil and growing under the same environmental conditions.
Note how thc}^ differ in height ; number, size and sluii)e of leaves;
size of flower heads; number of flowers in the head; amount of
hairiness of various parts, etc.
(6) Compare plants of the same kind grown in sun and shade,
in dry and moist soils, in barren and on fertile ground, for
differences due largely to the environment. Note the (Hffcr-
enccs in the times of flowering and of ripening of seeds, as well
as the structural differences.
116 PLANT PHYSIOLOGY
168. Movements. Plant movements are of four
kinds: (1) hygroscopic, (2) protoplasmic, (3) turgor^
and (4) growth movements. The first is a strictly
physical phenomenon of dead cells, the last three are
functions of living cells or tissues.
169. Hygroscopic Movements. Cell walls have a
great power of iml^ibition of water, and when filled with
water have a greater volume than when dry. In many
plant organs certain cell walls have a greater power of
imbibition than others, or in some cases certain tissues
on one side prevent the organs from elongating or con-
tracting on that side. The result in either case is that
as the cell walls absorb water or give it up a curvature
takes place. This may be a simple bending or may consist
of twisting. Mostly the organs straighten out on becom-
ing wet and curve or twist as they dry. In some cases the
differences in the moisture content of the air are sufficient
to produce movements. These movements are of value to
the plant in opening reproductive organs (sporangia, seed
capsules, etc.) or in enabling seeds to penetrate the ground
(twisting of the long awn of porcupine grass, Stipa).
170. In the case of the sporangia of the common ferns
(Potypodiaceae), the cell lumen as well as the walls is
filled with water. As the water evaporates through the
cell wall, the cell
Ob, O,
- o^
'^^^ <>
contracts to compen-
?=?^^I'VOo sate for the water
lost. As the walls
are thin and collap-
sible on one side
Fig. 50.-Dispcrsal of fern spores. Ouly, and thick but
flexible on the
others, the cell contracts more and more toward the thin
side until the row of cells instead of being in a nearly
PROTOPLAS]\IIC MOVEMENTS 117
complete circle with the thin wall at the outside, is bent
back into almost a reverse circle, the whole row being now
under high tension. As the evaporation proceeds, further
contraction becomes impossible, and the collapsed thin
cell walls become dry in spots. These dry spots are per-
meable to air, which rushes into them and permits the
whole ring to snap back with extraordinary violence,
flinging the spores a comparatively long distance.
171. Protoplasmic Movements. We may distinguish
two types of these, the movements of the cytoplasm
within the cell and the movement of the cell as a whole,
due to the motion of the cytoplasm or special parts of it
(cilia or flagella).
172. The motion of cytoplasm within the cell seems
to be a normal phenomenon in all living cells whose
protoplasm has imbibed enough water to make it rather
liquid, i.e. in all active cells. It is probably
entirely absent in so-called dormant cells, such ^
as the cells of dry seeds, etc. In many cells it
cannot be distinguished except by special methods.
The motion may consist of a rotation of all the
cytoplasm of the cell except a thin layer against
the cell wall (as in Chara and Nitella), or of
large streams in which chloroplasts and cell inclu- pio. 51.
sions are swept along (as in Philotria), or in cur- i^nTpfo-
rents in the parietal cytoplasm and delicate (tEIS
strands crossing the vacuole (as in Tradescantia) , *'^°***^-
or it may consist of rather local disturbances.
173. Of especial interest are those movements by
which the nucleus is carried from one part of the cell to
the other. Thus in a cell that is growing rapidly on one
side or secreting abundantly at one side, the nucleus
is often carried to the point of activity. The chloroplasts,
too, change their position with reference to the light. If
Q>
118 PLANT PHYSIOLOGY
tlie light is dim, they are carried to the top or bottom
of the cell, where they will get the strongest light broad-
side. If the light is too strong, they are carried to the
sides of the cell, where the light will only strike them
edgewise.
174. The locomotion of cells is accomplished mostly
by the lashing movements of slender cytoplasmic pro-
jections from the surface of the naked cell. If few in
number and long, they are usually called fiagella. If
numerous and rather short, they are called cilia. When
single or few, they are usually attached at the anterior
end of the cell. A few plant cells
move by amoeboid motion, i.e.
send out processes or lobes into
which the whole protoplasm flows.
The cells of diatoms (Bacillario-
ideae) are provided with cell walls
of cellulose so filled with silica as
Fig. 52.— Flagellate cells, , . , ,. i i -,,1
(uiothrix, pieurociadia. to bc nou-clastic and brittle.
JMarchantia, Struthiopteris, ^ ,. , ,, , ,
Zainia). lu some diatoms the protoplasm
comes to the surface through a
longitudinal slit, the raphe, and its longitudinal motion
in this slit is probably the cause of the motion of the cell.
Finally, must be mentioned the motion of some diatoms
as well as desmids, and some of the blue-green algae
(e.g. Oscillatoria) which is ascribed to the secretion of a
slime through the cell wall. The bending of the
Oscillatoria filaments, however, may be due to proto-
plasmic contraction.
175. All of these movements are dependent on an ample
supply of oxygen, and cease very quickly in its absence.
The usual cardinal points of temperature can be found
for these as well as for other functions of the cell. Ap-
parently the movements within the cell are of use in
LOCOMOTION OF CELLS 110
distributing various food products as well as other sub-
stances throughout the cell.
176. In motile cells there is observable a response
in direction of the movements to various external stimuli.
Thus many cells swim toward the light, or away from it
(positive and negative phototaxy). Others swim to-
ward or away from various chemical substances (e.g.
food matters, acids, etc.) diffusing through the water,
this being called chemotaxy. In many cases a degree
of light or of concentration of a chemical that causes
positive reaction, when increased beyond a certain point
repels the cell. It is not always the case that harmful
chemical substances (poisons) repel the cell, although
usually this is the case.
Laboratory Studies, (a) Insert the point of the fruit of
porcupine grass (Stipa) into a cork or fasten the fruit of cranes-
bill (Erodium) to a cork with a drop of seahng wax, with the
main shaft of the fruit upright, and place a marker opposite
the tip of the bent portion. Place a bell jar partially lined with
wet filter paper over it and note how it changes its position and
the direction of the motion. Remove the bell jar and note the
change in the direction of motion. By spraying a fine mist on
the specimen a lively movement will be obtained.
(b) Mount several ripe sporangia of a fern in a very little
water without a cover glass and watch the motion as the water
dries out.
(c) Examine some of the end cells of Chara or Xitella for
rotatory movement of cytoplasm, the leaf of Philotria for large
streams of cytoplasm carrying the chloroplasts with them, the
stamen hairs of Tradescantia or the stem hairs of petunia,
tomato or watermelon for more delicate strands of streaming
cyto])Iasm.
(d) With some of the foregoing test the effect on the move-
ment of cold (laying on a block of ice) and heat (up to 40° or
45° C), examining again at room temperature.
(e) Place some green felt (Vauchoria) that has been growing
on the surface of the ground in a dish of water. Often this will
120 PLANT PHYSIOLOGY
cause it to form its multiciliate zoospores. Study their motion.
Study also zoospores of Ulothrix, Chaetophora or Draparnaldia
which can often be obtained by bringing these algae into the
laboratory and leaving them over night in a dish of water.
Often they will collect at the side of the glass next to the hght.
(/) With sharp scissors cut off as much as possible of the
mycelium (fungous threads) of Saprolegnia growing on a fly or
piece of meat thrown into a dish of algae. Place it in a dish of
clean water and after a few hours hang a small piece of meat in
the water at one side of the dish. After a comparatively short
time the zoospores produced will be found congregated around
the meat (chemotaxis).
177. Turgor Movements. Many plant organs change
their position or become curved by the change in turgor
of the cells on one or both sides of the organ. Thus at
the base of the petiole of the leaf of the sensitive plant
{Mimosa pudicd) there is a strongly developed mass
of thin-walled cells, the pulvinus. When the cells on the
lower side are turgid the leaf is held out horizontally or
inclined upward. In response to various stimuli these
cells suddenly allow their water to escape into the
intercellular spaces, thus losing their turgor and contract-
ing considerably. Apparently the cells on the upper
side of the pulvinus take up this water very quickly,
thus becoming turgid in their turn. This process takes
place very rapidly and results in a quick dowmward
bending of the leaves. It is by a similar arrangement
that the two halves of the leaf of the Venus fly-trap
{Dionaea muscipula) snap together quickly enough to
catch insects lighting upon them, or that in the case of
the sundew (Drosera), when an insect is caught by the
sticky mass on one of the so-called tentacles, the ad-
jacent ones bend over until they too touch the un-
fortunate victim and the whole leaf gradually closes in
on it. The movement of the stamens in the flower of
barberry (Berberis) is also due to turgor changes as are
TURGOR MOVEMENTS 121
the constant movements of the lateral leaflets of the
leaves of the telegraph plant {Desmdoium gyrans).
178. Some turgor movements are so-called auton-
omous movements; i.e. they seem to be due to internal
causes and not caused by external stimuli. Such seems
to be the case in the movements of the leaflets of Des-
modium referred to above. The haflets of red clover
{Trifolium pratense) show a similar rising and falling,
but instead of requiring only a few seconds as is the case
with Desmodium, require several hours. It is possible
that these so-called autonomous movements are due to
external stimuli which have not yet been recognized.
179. Most turgor movements are in response to
some recognized stimulus. Whereas the hygroscopic
movements are the direct physical result of the in-
creased or decreased moisture in the surrounding air,
the movements in response to a stimulus are not the
direct physical effects of the energy exerted by the
stimulus but are due to energy stored up in the tissues
which is released by the stimulus as the energy of the
gunpowder is released by the chain of events between the
pulling of the trigger and the discharge of the gun. As
the strength with which the trigger is pulled has no
influence upon the energ}^ applied to the bullet, so the
intensity of the stimulus has no direct effect upon the
vigor of the movement resulting from it (except in so far
as a more vigorous stimulus may reach more cells and so
release more energy in that way).
180. The most frequent stimuli for turgor movements
are variations in temperature and light. Examples of
this are the so-called sleep movements of leaves of clover,
Oxalis, Mimosa, etc., and probably all leaves that have a
pulvinus at the base of the leaflets or of the petiole.
On the other hand the sudden movements of the stamens
122 PLANT PHYSIOLOGY
of barberry, the rapid closing of the leaf halves of
Dionaea, the closing of the leaflets and dropping down-
ward of the leaves of Mimosa are responses to the stimulus
of contact. In the case of the sundew the movement of
the tentacles may take place both in response to contact
or to the presence of certain chemicals such as ammonium
sulphate, proteins, etc. It is worthy of note that the
stimulus may be applied at a distance even of several
centimeters from the point where the change in turgor
occurs, i.e. the plant tissues are able to transmit a stimu-
lus for a considerable distance. Kone of these move-
ments will take place except under the proper degrees of
temperature, moisture, etc.
Laboratory Studies, (a) Observe a plant of Desmodium
gijrans at a temperature of between 20° and 30° C. The
rapidity of the rotation of the leaflets will be found to vary
with the temperature, degree of illumination and other factors.
(6) Observe clover and Oxalis leaves by night and by day.
Compare also the leaves of Mimosa, Robinia, etc., in light and
darkness.
(c) Touch one of the three bristles on the surface of a leaf
half of Venus fly-trap (Dionaea). Note the sudden closing of
the leaf. The temperature and humidity must be rather high
or it will not respond well.
{(I) Toucli a leaf of a sensitive plant {Mimosa pudica) at
the under side of the pulvinus. Touch or sHghtly pinch other
leaves of the same plant at various points. Apply the flame
of a match to the end of one of the leaflets. Note in this case
the progressive closing of the leaflets followed by the dropping
of the whole leaf and in many cases of the nearest leaves above
and below.
(e) Place a grain of sand on the tip of a tentacle of a leaf of
sundew (Drosera). Note the degree of movement in the sur-
rounding tentacles. On a tentacle on another leaf place a tiny
piece of meat or a very small crystal of ammonium sulphate and
note the movements of the adjacent tentacles.
181. Growth Movements. Many plant movements
are the result of unequal growth on opposite sides of an
NUTATION 123
organ. Here again can be distinguished autonomous
movements whose stimuU if external are not recognized