the different solutions, the size and color of the leaves, etc.
Note when growth ceases and to what stage of development
the plant proceeds before its death.
(c) Bring some Spirogyra into the laboratory and place
in a dark room (not too cold) for twentj'-four to thirt^^-six
hours or until on testing some of the plants with iodine solution
no starch is found. Bring the dish into the sunlight and with
iodine solution test some of the plants for starch after five
minutes, ten minutes, half an hour, etc.
(d) In a rather broad, deep glass dish (e.g. a wide battery
jar) place some actively growing Spirogyra. Put a bit of wire
netting (iron, not copper nor brass) into the bottom of a short-
tubed funnel and invert over the Spirogyra submerging the
LABORATORY STUDIES 99
funnel and its tube completel}'. Over the latter invert a test
tube filled with water. Now raise the funnel as high as it will
go without lifting the edge of the test tube above the surface
of the water, supi)orting it on a small block. Place the whole
in the sunlight. As photosynthesis goes on the oxygen given off
by the pond scum collects in the test tube and may
be tested in various ways, e.g. by carefully re-
moving the test tube, inverting it and inserting
a glowing splinter which will burst into flame if
sufficient oxygen is present. The diameter of the
funnel must be considerably less than that of the
jar or no CO2 can reach the Spirogyra and photo-
synthesis will soon cease. If CO 2 is passed into
the water occasionally, taking care not to let any
bubbles enter the funnel, the activity of the process
is increased.
(e) In a similar way the oxygen evolved in photosynthesis by
Philotria (Elodea) may be collected by inserting the cut ends of
several plants into the mouth of an inverted test tube filled
with water and placing the whole dish in the sunlight. Care
must be taken, however, not to confuse two phenomena here, viz.
the rapid outflow of bubbles at first, due to the expansion of the
gas already present in the stem as it heats, and the much slower
evolution of oxygen by photosj-nthesis. After the first outrush
of gas due to the expansion by heat is past the relative
amount of photosynthesis can be told with a fair degree of
accuracy by counting the number of bubbles of oxygen evolved
per minute under the different conditions. Be sure, however,
to keep the water well supplied with CO2. Test now the effect
of placing glass plates of difi"erent colors in front of the dish
containing this j)lant, in all cases waiting long enough to
avoid the effect of the changing volume of the enclosed gas due
to changes of temperature.
(/) Place two potted geranium (Pehirgonium) ]ilants. prefer-
ably with plain, not variegated leaves, in the dark until their
leaves contain no starch. Now place them under bell jars,
sealing one air tight with sealing wax or by other means, first
placing under the jar a dish containing a strong solution of
KOH to absorb all CO2. Leave a small air space under the
edge of the other bell jar so as to permit the entry of air
containing CO2. After an hour or so place both plants in the
100 PLANT PHYSIOLOGY
sunlight and after three or four hours test their leaves for the
presence of starch as follows: Remove a leaf, immerse it in
hot alcohol for a few minutes to extract the chloro])hyll and then
cover with a strong solution of iodine which will color the leaf
blue or not according as the starch is present or absent. To
avoid rupture of the sealing by the expanding air it is well to
use a bell jar with an opening at the top into which is placed a
cork through which a glass tube passes. This tube should be
bent so that its other end is immersed in a dish of mercury.
As the air expands it passes out through this tube and escapes
through the mercury but the air and carbon dioxide from out-
side cannot enter.
(g) On a large leaf of geranium (Pelargonium), or other
plant which produces starch in abmidance in its leaves, clamp
on the upper side a flat cork and on the lower side a httle box
(a wooden box such as cover glasses come in will be satis-
factory) blackened inside and whose sidea
I r-T-x-r — , have been pierced from the outside by
^^' ^V' . ^ ^ ^ ^ ii'"^'^w^ â– â– ^''->|v>'^^>^ . â– a" > numcrous Small holes running obliquely
away from the leaf. These holes admit air
(and CO2) but as thej^ point awaj^ from the
Fig 45 — D" ^^^^ ^^^ ^\^\^ admitted through them is ab-
pearance of starch sorbed by the blackened inner surface of the
^^' box. Set the plant in the sunlight for sev-
eral hours then remove the leaf and treat
with alcohol and iodine as in (/). The spot protected from
hght by the cork and the httle box will show no starch.
To clamp two corks together on both sides of the leaf is un-
satisfactor}^, as in that case not only is the light cut off but the
CO2 as well, so that the reason for the lack of starch in that case
is two fold.
(A) Reserve carbohydrate in the form of starch may be
demonstrated in the tubers of potatoes, root of sweet potatoes
(Ipomoea batatas) , seeds of corn (Zea 7/ia?/s), wheat, beans, etc.
In the form of cane sugar it is present in the root of the beet
(especially in the sugar beet), in the stem of corn and sugar cane,
etc. As hemicellulose it is present in the wood of mulberry
(Morus) and elm where it ma}' be demonstrated by treating a
section with sulphuric acid followed by iodine solution. Food is
stored up in the seeds of cotton, castor bean (Ricinus), flax,
etc., and in the scales of onions, leaves of cabbage, etc., as fats.
LABORATORY STUDIES 101
It may be demonstrated by treating with dilute osmic acid
solution which turns fats black, or withalkannin solution, which
stains the fat drops red.
(t) Place a geranium (Pelargonium) plant in the light for
several hours until starch has been produced in quantity in the
leaves. On two or three leaves cut one or two of the main
radial veins leaving the other veins intact. Cover the whole
plant loosely with a bell jar to prevent these injured leaves from
drying out too much and place in the dark for from twelve to
twenty hours. Treat these leaves with alcohol and iodine
solution as in (/) to determine the location of the starch. It
will be found to have disappeared except from the portions
bordering on the cut veins, showing that it is through these
veins (vascular bundles) that the carbohydrates are transported.
(j) Reserve protein in the form of aleuron in the seeds of
beans, peas, etc., was studied in connection with cell inclusions
(paragraph 24). It will be worth while to repeat these
observations.
(k) Examine one of the powdery mildews (Erysiphaceae) as
an example of a hysterophytic lower plant that obtains its
food from living plants (i.e. is parasitic). Take a bit of infected
leaf and moisten with alcohol, then mount in water or dilute
potassium hj-drate solution wdth the infected side uppermost.
By careful focusing the filaments of the fungus may be dis-
tinguished and here and there may be seen the haustoria
("suckers") which are sent into the epidermal cells of the
leaf. Better developed haustoiia can sometimes be found on
making cross-sections of leaves or stems affected by downy
mildew (Peronosporaceae) or wliite rust (Albugo). In these
cases the whole fungus except certain reproductive ])arts is
within the host plant, growing interccllularly and sending well
developed haustoria into the cells between which it passes.
In both cases note the lack of chlorophyll in the fungus.
(/) Examine a dodder plant (Cuscuta) as an example of a
higher plant that is parasitic. No leaves are to be found and
in most cases no chlorophyll, and the plant carries on no
photosynthesis. The original root which penetrated the soil
dies as soon as the plant has attached itself to its host or even
before. Note the roots by which it obtains its food from the
host. Sections of the stem will reveal vascular bundles, epi-
dermis, etc., but usually no chlorophyll-bearing cells.
102 PLANT PHYSIOLOGY
(m) Place a number of fresh leaves or a short shoot with
leaves in the large end of a retort with a little water and place
the small end under a surface of mercury to prevent the
entrance of gases. Keep in a dark moderately warm place for
from twelve to twenty-four hours. Note tiiat the volume of
the gas does not seem to be changed. Carefully without allow-
ing any air to enter run a pipette full of strong KOH solution
into the small end of the retort or introduce a small piece of
stick potash (KOH) with a few drops of water, these rising to
the surface of the mercur}'. As the CO2 is absorbed the
mercury rises. When the ascent ceases (i.e. all the CO2 has
been absorbed) introduce a strong solution of pyrogallic acid.
This has the property when mixed with alkaline solutions of
absorbing oxygen. Note w^hether the mercury rises any
further. If it does so it shows that some oxygen was present.
Repeat the experiment using a retort without any leaves in it.
It will be found that about as much COowas produced by the
leaves (as shown by the height to which mercury rose with the
KOH alone) as oxygen was present (as shown in the control
experiment by the distance the mercury rose with the KOH
and pyrogallic acid). If this can be done with graduated cylin-
ders the amounts can be measured more accurately.
(n) That CO 2 is given off by a hving plant may be demon-
strated in the following waj^ also. Place a potted plant under
a bell jar with a dish of Ba(0H)2 solution or (less preferably)
Ca(0H)2 solution. Put in a dark place. The CO2 given off
forms a crust of BaCOc (or CaCOs) on the surface of the liquid
while in a control experiment with no plant under the bell jar
the amount of CO 2 in the air (3 parts in 10,000) produces only
a very small precipitate.
(0) Soak some peas over night and then place them in a
tall glass jar filling it about half full, and cover with a vase-
lined glass plate. After a few hours remove the plate and
lower a burning taper into the cyHnder. It is extinguished
by the CO2 which has replaced the oxygen. If the air is
very still it is more striking to place a small lighted taper in
the bottom of anotherjar and topour the CO2 from the jar of
peas into this jar, extinguishing the light.
ip) vSoak some peas over night. Fill a test tube with mer-
cury and invert over a dish of mercury. Force three or four
peas under the mercury so that they come under the edge of the
LABOUATORY STUDIES 103
test tube, when the}- will rise to its closed end. Respiration
in its first (anaerobic) stage will go on and gas will
be formed, oftentimes driving nearly all the mercur}^ |
out of the tube. Introduce a strong KOH solution L
or a piece of stick KOFI and a little water under Hffl
the edge of the test tube and the gas will all be I^uts-
absorbed, showing that it is CO2 that was produced, pirution
, GXpG r 1-
(q) Yeast plants ordinaril}^ carrj' on only this first ment
stage of respiration (called fermentation in this case). ^ ^'
To potato water (made by grating up a potato and boiling it in
a little water and expressing the latter) add about 5 per cent,
glucose. Place in a flask with a cork and a glass tube bent so as
to lead the gas produced under water. Break up part of a cake
of compressed yeast in a little water and add it to tlie solution in
the flask and insert the cork and glass tube. In a short time
gas will begin to escape in bubbles from the end of the tube.
Collect some in a test tube and test in various ways such as for
inflammabiUtj^, absorption by KOH, etc. It will be found to
be CO2. Note what large amounts are produced. After the
evolution of gas has ceased the proper chemical tests will show
the presence of alcohol in the liquid. Distill the latter and
collect the first part that comes over. Add to it some strong
KOH solution and some flakes of iodine, and heat. If alcohol is
present a strong odor of iodoform will be produced and if much
is present this will show as a yellow precipitate.
(r) The liberation of heat during resj^iration can be demon-
strated by placing a quantity of soaked peas or a number of
mushrooms just expanding in a flask with an accurate chemical
thermometer bulb in their midst and placing this flask in a
mass of cotton in another vessel and covering all with several
layers of cloth, leaving only the thermometer tube exposed.
Often the temperature within tlie flask will rise 3 or 4 degrees
or more above that of the surrounding air. Of course this
experiment must be carried on in a room where the temperature
is fairly constant. If a Dewar bulb or a Thermos bottle is used,
these being double walled with a vacuum between so that the
loss of heat is very small, the difference of temperature is
much more marked.
(.s) Without special thermostats where temperatures can be
controlled exactly, satisfactory ex])eriments as to the cardinal
points of temperature cannot be made. However, it will be
104 PLANT PHYSIOLOGY
helpful in the autumn to list the plants most susceptible to
injury and those that suffer least from frost.
147. Growth. In the one-celled plants, or plants
made up of undifferentiated cells, growth is a function of
every cell. It enlarges up to a certain point and then
divides into two cells which enlarge and divide, etc.
In some cases the cell divides internally into many small
cells which enlarge until they reach the size of the parent
cell and repeat the process. The growth of a cell in-
volves a number of factors. Among these are the in-
crease in the amount of cytoplasm and sometimes a great
increase in the amount of cell sap, also the enlargement
of the cell wall in area and frequently also in thickness.
These cells are meristematic in many features. In such
plants we can hardly dissociate growth from reproduction.
148. In the more complex plants we find some parts
that are the seat of the growth, the growing points and
adjacent region and cambium layers, while the rest of the
plant practically ceases to grow. The reproductive
functions are carried on by special parts of the plant
which have nothing to do with its ordinary growth.
The growth in such plants takes place still by the
process of cell growth and division, but we find that these
differ considerably from the case in one-celled plants.
Thus near the tips of the growing points the cells in-
crease their cytoplasm and cell wall area so as to become
perhaps twice as large, and then divide and form new cells
as is the case in one-celled plants except that the cells
remain attached. Gradually, however, some of these
cells that by the formation of new cells have come to lie
further from the tip increase more and more in size
and are not so active in their division. This increase in
size takes place largely by an increase in size of the
vacuoles so that the cells contain proportionally less and
less cytoplasm, although probably the amount of cyto-
GROWTH 105
plasm actually docs increase, or decreases but little. In
other words the growth of the cell is mainly accomplished
by absorbing large amounts of water, the cell wall being
increased in area so as to keep pace with the increase in
volume. It is possible that in some cases where the
growth of the cell is very rapid the total amount of cyto-
plasm in the cell may actually be reduced in manu-
facturing the additional cell wall substance required.
In this growth we can distinguish three phases which can
be more or less clearly set off, viz., formative phase, phase
of enlargement and phase of differentiation or maturation.
149. Thus it comes about that at the growing root tip
or tip of the stem we can distinguish an area near to the
tip where growth is not very rapid but cell division is
taking place abundantly (i.e. the cells are in the formative
phase of growth), and another area into which the first
grades, and a little distance back from it, where the cells
are enlarging very rapidly and but little cell division is
taking place (i.e. the cells are in the phase of enlarge-
ment). This gradually grades off into that portion of
the root or stem where growth in size. is no longer oc-
curring but where the various tissue differentiations are
taking place (i.e. the phase of differentiation). In the
root these zones are well marked, while in the stem the
elongation may persist for a long while and may become
localized in nodes while the internodes cease to grow.
In this case the nodes usually retain some meristem and
possess the power of producing new cells as well as in-
creasing in size.
150. There are several factors that influence plant
growth. There must in the first place be sufficient food
stuffs to enal)le the cells to manufacture the necessary
new cytoplasm and cell wall. Then there must be
sufficient organic substances to produce the osmotic
106 PLANT PHYSIOLOGY
pressure necessary to take in the requisite large quanti-
ties of water that increase the bulk of the cell so greatly
during the phase of enlargement. Then sufficient food
substances must also be present to supply in the process
of respiration the energy necessary for growth. Further-
more the water supply must be ample, for growth ceases
immediately if the cells of the plant are not kept strongly
turgid, hence the reason that in a dr}^ season a plant may
remain alive for months on a minimum of water, but
scared}^ grow at all. The temperature also has a
marked influence on growth. The cardinal points of
temperature for growth are often quite different from
those for photosjmthesis or respiration in the same plant.
In some plants that come up through the snow the
optimum temperature for growth may be but little
above 0° C, while in Indian corn, for example, the opti-
mum lies between 37° and 42° C.
151. The effect of light upon grow^th is noteworthy.
Careful records of the rate of growth with automatically
recording instruments show that, given constant tem-
perature, the growth is much more rapid in darkness
than in light. If the rays from the blue end of the
spectrum are excluded growth is scarcely if at all checked
by light. The absence of light, however, although favor-
ing the elongation of the plant, prevents the normal form-
ation of leaves. This is possibly due in part to lack of
food, but it seems probable that a definite stimulus on the
part of light is needed before leaves will be produced in
the normal form and size. Plants kept in the dark become
much elongated (remaining pale in color) with only rudi-
ments of leaves. Such plants are said to be etiolated.
To a certain degree this is useful to a plant in that a tuber
or seed buried too deep produces an abnormally elongated
shoot which may thus be able to reach the light.
CIROWTH 107
152. The amount of growth in a given length of time
varies with the plant. Sonic trees in dry regions, e.g.
Ccrcocarpus parvifolius, the mountain mahogany of
Colorado, may scarcely attain a height of two meters in
one hundred j-ears, while a morning glory vine (Ipomoea)
may grow 17 cm. per day, a bamboo shoot 60 cm. per
day and a stamen of Avheat 1.8 mm. per minute, i.e. at a
rate of over 25 meters a day (but of course this rate of
growth actually lasts only a few minutes).
153. As growth occurs in a stem or root various
tensions arise owing to the unequal amount of growth in
different parts. Thus the pith of many plants (especially
herbaceous ones) elongates considerabl}" when removed
from the stem and the surrounding portions shorten a
little. While they remain in the plant the result is that
certain parts of the plant are stretched and the pith
compressed, w^hich stiffens the plant just as in a turgid cell
the stretched cell wall pressing against the osmotic
pressure within the cell renders the cell stiff. Bark of
trees usually shows a circumferential stretching also
which helps to keep the stem rigid.
Laboratory Studies, (a) Examine plants of Protococciis
(one to few celled) or of Spirogyra (chain of cells). Cells of
different sizes will be found but the largest cells are
rarely more than twice as large as the smallest ones.
Here each cell grows and divides for itself and in the
case of the first the cells soon separate, forming new
plants.
(6) Take a germinated seed of Indian corn, sun-
flower or other plant and on a rapidly growing root,
using a thread dipped in India ink, mark lines 1 mm.
apart making the first mark 1 mm. back from the tij)
(special markers for this i)urpose may be bought, but
although more convenient are not indispensible). Place fu.. 47.
this seed on moist cotton with the marked root J~r^J,"„*i5i'
directed downward and cover with a bell jar to cxpcri-
prevent drying out. Examine at intervals of several
108 PLANT PHYSIOLOGY
hours to determine in what segment so marked the most
rapid growth occurs. It must be remembered that tliis zone
of most rapid growth is rapidlj^ passing down the root all
the time, keeping about the same distance back from the root
tip, so that the marked root must not be left too long before
examination or the conclusions will be faulty.
(c) Attach the thread of an auxanometcr (instrument for
measuring growth) to the tip of a leaf just growing out of an
onion or hj-acinth bulb or to the tip of the flower scape of such
a plant, or just below the cotyledons of a sunflower seedHng.
If possible have the plant in a situation where
it is almost equally lighted from all directions.
If the instrument is not self-recording readings
should be made every one or two hours during
the day and night. If the records are automat-
ically made the readings need not be taken during
the course of the experiment but the records can
be studied afterward. So far as possible keep the
nomete/Sr* temperature constant. Interesting results may
be obtained by varying the temperature while
keeping the intensity of the light the same or bj^ varjdng
the hght with constant temperature. The effect of keeping
the soil very wet and very dry may also be compared.
(d) Observe a potato that has started to grow in a dark
corner of a cellar and compare its growth with that from a tuber
that has been grown in full hght.
(e) Place potted plants under bell jars as follows: (1) clear
white glass, (2) double bell jar with space filled with saturated
K2Cr207 solution, (3) double jar with space filled with saturated
cuprammonia solution. Compare the growth. Note also the
differences in the color and development of the leaves. The
cuprammonia solution is prepared by carefully adding to a
copper sulphate solution sufficient ammonia to precipitate all of
the copper as copper hydroxide but not adding enough ammonia
to redissolve the precipitate. Filter and wash the precipitate
and then dissolve it in strong ammonia using only enough of
the latter to completely dissolve it. This must not be done
on the filter paper as the solution thus formed dissolves cellulose.
(/) The rate of growth under normal conditions can be meas-
ured by an auxanometer or with a horizontal microscope or in
the case of rapidly growing plants, such as Indian corn, morn-
REPRODUCTION 109
ing glory vine, bamboo, etc., it can be measured even^ day with
a ruler. ]\Iake and record such measurements night and morn-
ing for several kinds of plants.
154. Reproduction. This is the ultimate function of
all plants. For many it is the final function of hfe, the
death of the old individual occurring with the formation
of the new individual. It is perhaps to be considered as
the final act of growth toward which all development
of the plant has been leading.
155. In many of the lower plants, especially those
that are undifferentiated, reproduction is nothing more
than cell division followed by separation of the cells thus
produced. In the more differentiated plants, however,
we find certain cells set aside for reproductive purposes.
These may be at first ordinary vegetative "cells that
later take up the reproductive function, or they may be
destined for the latter from their beginning.
156. Very early in the vegetable and animal kingdoms
two types of reproduction become recognizable, the
asexual and the sexual. The former consists essentially
of the division of the plant, or of the separation from it
of single cells or groups of cells or even whole plant
members. By further growth these parts thus pro-
duced become like the parent plant. Not to be confused
with true asexual reproduction, is the development
of the gametophyte from the spores produced by the
sporophyte.
157. Sexual reproduction is fundamentally different
from asexual reproduction in that there is requisite the