under the given difference in density of the outer and
inner solutions, the water from the outside tends to
enter the cell). Such a water-distended cell is said to be
turgid or in a state of turgor. The pressure within it
may equal several atmospheres. Jost gives this pressure
for some desert plants as equalling one hundred atmos-
pheres, i.e. about 1500 pounds per square inch.
107. If a cell be in contact with a plentiful water
supply, it will become as turgid as the difference in
osmotic pressure outside and inside will permit. If a
cell adjacent to it is not in contact wdth the external
water, there will be a passage of water from one cell to the
other, the direction depending upon which cell has the
denser solution in its cell sap. Thus, in a plant with one
part exposed to evaporation into the air and with
the other part in water there will be a constant passage
of water into the plant and up through it from cell to
cell, by osmosis, and out into the air by evaporation from
the wet surface of the cell walls.
108. In larger land plants, however, this rather slow
passage of water from one cell to another b}^ osmosis is
too slow to supply the aerial parts with the requisite
amount of water. Such plants possess special elongated
cells no longer living and often with the separating
partitions dissolved out, viz. : the tracheae and tracheids.
(See paragraphs 46 to 49.) These serve as tubes
through whi(^h the water rises, not as a simple diffusion
of molecules but with a mass motion, i.e. as a definite
current carrying with it whatever miiy be dissolved.
109. In these plants then we can trace the water
through the following steps of progress. It enters the
root hairs by osmosis from the surrounding soil where it
74 PLANT PHYSIOLOGY
is present in thin or thick films around the soil particles,
the entry being molecule by molecule. It passes by
osmosis from cell to cell through the cortex of the root
until the tracheary tissue of the vascular bundle is
reached. It enters these vessels (just by what force is
not clear) and ascends through them (also by what force
is uncertain). Some of it is taken out
by osmosis, by various parenchyma
cells (e.g. medullary rays) bordering the
tracheary tissue and passed osmot-
ically to the various tissues at that ap-
proximate level, but the bulk passes
on out into the leaves w^here it is taken
Fig. 39. — Course of , • • ^ n i n
w a t e r into, and by osmosis mto the parenchyma cells.
through a land plant. -^^ ,, n i i • ii i
From the cells bordermg the larger air
spaces, it evaporates into these and passes as vapor out
through the stomata.
110. The evaporation of water from a wet membrane
(e.g. cell wall) makes available a large amount of energy
for lifting up water to replace that evaporated. It has
been shown that the energy thus available in a leaf is
many times more than that necessary to lift the water
up to the tops of the highest trees (150 meters). How-
ever, though the energy is ample, the air pressure at sea
level is only sufficient to lift water not quite ten meters
into a vacuum. The osmotic pressure developed in
roots that are rapidly absorbing water is enough oc-
casionally to lift water to a height of eleven meters in the
grape and even twenty-five meters in the Birch (Betula
In tea). The distance that this root pressure will lift
water plus the height air pressure will lift water into a
vacuum fall far short of the distance water must be
lifted in tall trees. It has been suggested that perhaps
the cohesion that exists in water in narrow vessels
PATH OF WATER 75
(e.g. in tho trachoaiy tissues) is sufficient to pull the
water u]) from tlio lowest roots. Other investigators
have suggested that some of the living parenchyma cells
which accompany all water-conducting tracheids and
tracheae are concerned in the lifting of the water (or
ascent of sap as it is often called).
111. Path of the Water. This is chiefly in the cavities
(lumina) of the tracheary tissue. It is also not to be
denied that the w^ater will pass upw^ard slowly from the
roots, passing from cell to cell in the parenchyma by
osmosis, for the tissues above ground have more con-
centrated solutions, and so bring about osmosis from the
root cells with their less concentrated solutions. This is,
however, not sufficient to supply an ordinary plant.
Within the tracheary tissue, the lumen contains not only
water but some bubbles of air, past which the water flows
in a thin film next to the cell wall. In trees the central
wood after a number of years suffers deposition of resins
or other insoluble substances within the cell cavities and
possibly walls as well, so that w^ater conduction is no
longer possible. Such wood is often different in color
and is called heart wood and contains no living cells.
The unchanged wood around it, the sap wood, contains
dead water-conducting tracheary tissue, dead fibrous
tissue and living wood parenchyma.
112. The evaporation of water from the leaves and
stems is often given the name transpiration. It is an
unavoidable loss since the plant must have openings,
the stomata, through the epidermis, for the purpose of gas
exchange and when these are open the loss of water can-
not be jH-evented. The thickening of the cuticle in
plants of dry regions, the depression of stomata in the
pits to provide dead air spaces outside, the formation of
thick layers of hairs, etc., all indicate that it is not to the
76 PLANT PHYSIOLOGY
advantage of a plant, to have transjnralion taking place
but just the contrary.
113. The amount of water given off by transpira-
tion is very large. The water loss from a Birch tree,
standing alone and estimated to have 200,000 leaves was
calculated by von Hohnel at about 500 liters on a very
hot dry day and about 60 to 70 liters on average days.
An acre of hops will evaporate three million to four
million liters of water in a season. Dietrich estimates
that for every gram of dry substance found in a plant,
from 250 to 400 grams of water have been evaporated.
In twelve hours, a grape leaf evaporates as much water as
would form a film 0.13 mm. deep over the whole leaf,
while for cabbage and apple leaves in the same length of
time the figures are respectively 0.31 and 0.25 mm.
In one season, an oak tree, during the time it holds its
foliage, evaporates an amount equivalent to 33 mm. over
all its leaves. An open water surface would evaporate,
in the same time, 500 to 600 mm., showing that the
evaporation (transpiration) is far less from the leaves
than from a free surface.
114. It has been show^n that an impermeable surface
with very numerous openings, as for example, the
epidermis with its numerous stomata, evaporates nearly
as much water as if it were a free water surface. The
stomata, however, are capable of closing and thus almost
wholly preventing water loss for such periods of time as
they may remain closed. At night they are nearly
closed. When the plant begins to wilt, it has been
shown that they also close automatically through re-
duced turgor of the guard cells thus preventing too great
a loss of water. All physical phenomena which increase
evaporation also increase the water loss from the leaves
as long as the stomata remain open, e.g. increased
GUTTATIOX 77
temperature and dryness of the surrounding air, sun-
shine, etc.
115. Many plants exude water from specially modified
stomata (the so-called water pores) at the edges of the
leaves when the movement of water upward has been
strong and then, by increase of the humidity of the air,
the evaporation has been checked rather suddenly.
This may take place in the form of drops or even as a
fine stream. It is called guttation. Its mechanics and
use are not clear.
Laboratory Exercises. Note : In a large class, many of these
experiments cannot be performed by every student. In that
case the instructor should assign some experiments to one
student, others to another throughout the class or should set
up the experiments himself before the class. In either case,
every student should make complete notes upon the experiment
for himself.
(a) Weigh a handful of freshly picked leaves quickly before
they have begun to wilt. Place them in an oven at the
temperature of about 110° C. and dry them for twelve to
twenty-four hours. Now weigh them and note the loss in
weight. This is almost entirely due to the evaporation of the
water in the leaf. Calculate the percentage of water in the
original weight. Repeat the experiment with various parts of
the same plant such as stems, roots, flowers, fruit, seeds, etc.,
and compare the amount of water in these different parts as
well as with the corresponding parts of other plants.
(b) To demonstrate imbibition by cell walls, take a measured
block of wood 5 or 6 cm. long and 3 or 4 cm. square. Measure
it when perfectly dry, i.e. after having been kept a day or two in
an oven at 110° C. Then soak it in water (preferably warm or
hot, to hasten the process). Now measure accurately. The
piece will be found to have become perceptibly larger owing to
the imbibition of water by the cell walls. Probably the first
entrance of water into dry seeds is also due to imbibition of
water by the cell walls and protoplasm. As soon, however, as
the latter has imbibed enough to become hquid, osmosis
begins to act also in the taking in of water.
78 PLANT PHYSIOLOGY
(c) Osmosis may be demonstrated by tying a piece of fresh
bladder securely across the mouth of a thistle tube which is
inverted and filled with a strong solution of sugar up to a mark
on the stem. The larger end with the bladder is now placed
in a dish of water so that the water outside stands at the same
height as the water inside. The water will enter through the
bladder by osmosis and ascend the stem, perhaps reaching a
height of a meter or more above the level of the water outside.
The more impermeable the membrane is to the substance in
solution while still remaining permeable to water, the greater
the difference in level and the higher the pressure
that can be obtained. The latter can be measured
roughly by connecting the stem of the thistle tube to
a mercury manometer.
(d) The relation of osmosis to turgor may be demon-
strated by making an ''artificial cell." Fill a test
tube with a strong sugar solution and tie a piece of
bladder firmly over the open end. Place in a dish of
water. The water that passes into the tube by osmo-
sis through the bladder causes the latter to be
stretched and to bulge out. On removing the tube from the
water, and pricking the bladder with a pin, the pressure
developed by the stretching of the bladder will force the water
out in a stream.
(e) Mount one or two filaments of Spirogyra in water and
examine. Measure the length of a portion including a definite
number of cells. Now draw a 2 per cent, potassium nitrate
solution or a 5 per cent, sugar solution under the cover glass by
adding it at one side and withdrawing the water from the
other side with a piece of filter paper. Measure the filament
again. Add increasingly strong solutions and when the right
strength is reached, the cytoplasm will be found to be drawing
away from the corners of the cell wall, i.e. plasmolysis has
begun. This indicates that with the withdrawal of water by
the solution outside, the much stretched cell walls have lost
their tension until they have reached a state in which they are
not at all stretched. As the water is still withdrawn from the
cell, the cytoplasm is pulled further and further away from the
wall. At this stage, again measure the fdament and calculate
the amount that the turgid filament was stretched.
(/) To demonstrate that evaporation from a membrane filled
LABORATORY STUDIES 79
with water has a strong Hfting power, cover the end of a thistle
tube tightly with a piece of bladder or fill the mouth with a
tightly fitting thin layer of plaster of Paris. Invert the tube
and fill completely with water that has been boiled to remove
the air so that bubbles will not be produced in the tube. Invert
again with one end of the tube in a dish of mercury. Wet the
bladder or plaster of Paris plug externally. As evaporation
progresses, the mercury will be drawn up into the tube until a
point is reached where the pressure of air on the outside of the
bladder or plaster of Paris is sufficient to force the water
back out of it so that it is no longer wet. It then permits air
to pass through rapidly and the mercur}'- soon recedes to its
original level. Similarly, it is assumed that the
evaporation of water from the wet cell walls into the O
intercellular spaces of the leaves exerts a strong lift-
ing power on the water in the stem of the plant.
This will be shown by the following experiment.
(g) Cut a leafy twig and fasten it, without allow-
ing the cut end to dry out, into a glass tube filled fig. -ti.
with water and with its lower end in mercury. This — Evapora-
/•I'lii • e ''''"^ experi-
expernnent, if successful, will also show a rise of mer- mem (/).
cury in the glass tube as in the preceding one.
(h) Place the cut end of a stem (preferably a herbaceous one)
in a strong aqueous solution of safranin. After an hour or so,
make cross-sections at various points. The colored solution
will be found in the tracheary tissue (and after longer standing
also in some of the immediately surrounding tissues, especially
in wood fibers).
(i) Place a branch which has been girdled (i.e. the bark
removed to but not including any of the wood) with its lower
end in water, the girdled area being protected from drying out
by coating with grafting wax or paraffin. Compare with a
similar branch not girdled. Take a third branch and through
a small slit in the bark cut off the wood entirely with as little
injury to the bark as possible. Place it in water like the other
two. Note the differences in the rapidity of wilting in the
different cases.
(j) Take a potted plant, e.g. a geranium or begonia, and
after watering it well, envelop the pot in a sheet of rubber,
tying the rubber firmly about the stem of the plant. Instead
of using the rubber, the outside of the pot and the top of the
80 PLANT PHYSIOLOGY
soil may be made practically water proof by means of melted
paraffin whose melting point is sufficientl}'' low so as not to
injure the stem when applied to the top of the soil in a melted
condition. Weigh the pot and place in a dry room for an hour
and weigh again. Calculate the loss of water per square
centimeter of leaf surface. Place in a moist room under the
same light conditions as before and note the loss of weight in an
hour. Such experiments are not accurate as many factors
enter in to interfere, but they give an idea of the approximate
amount of water evaporated. The experiment may be
continued a long time by providing an opening in the rubber or
paraffin through which a thistle tube passes and adding every
twenty-four hours as much water as was lost in the preceding
2-4-hour period. By keeping a record in this way, the amount
of water lost in a week can be determined roughly. (Of course
the increase in weight of the plant itself as it grows is a factor
not taken into consideration in the foregoing nor the effect
upon the roots of the partial exclusion of the air by the rubber or
paraffin.)
(k) To show that it is mainly through the stomata that
evaporation (transpiration) occurs, take three lilac leaves of as
nearly equal size as possible. Coat the ends of the petioles of
each and the under surface of one and the upper surface of
another leaf with a varnish made of equal parts of
beeswax and lard or ordinary grafting wax if some-
what softened. Both surfaces of the third leaf are
to be left uncoated. The stomata are found only on
the lower surface and it will be found that the leaf
with this surface coated, thus covering the stomata,
remains fresh for a long time while the other two
wither quickly.
Fig. 42. (A 'p^g Icaves of the Cottonwood (Populus, vari-
— R oot •\i ii-iVki
pressure ous spccics) havc stomata on both sides. Repeat the
(Ji^f"™®'^^ foregoing experiment with leaves of this and com-
pare with the results obtained with the lilac,
(m) Root pressure may be demonstrated by cutting off the
stem of a rapidly growing sunflower or other rather large
plant (e.g. tomato, geranium, castor bean, etc.) and slipping a
heavy rubber tube over the cut stump, connecting this with a
narrow glass tube. If the soil be kept warm and wet water will
soon begin to escape from the cut surface and will rise to a
ENTRY OF SOLUTES 81
considerable height in the tube. If the latter be connected with
a mercury manometer the pressure can be measured.
116. Nutrients Other than Water. All other sub-
stances can enter the plant only in solution in water.
This is true of the gases as well as of mineral salts, for
although a gas may enter the air spaces of a leaf in the
gaseous state, it cannot penetrate the wet cell walls in this
state but must go into solution. It is then subject to the
same physical laws of diffusion as the other solutes.
117. The wet cell wall presents no (at least marked)
obstacle to the diffusion of any solute. The plasma
membrane, however, is impermeable for some, difficultly
permeable for others, and easily permeable for still other
substances. Accordingly the molecules of the substances
in solution outside of a cell will penetrate into the cell
with different degrees of rapidity and independent of the
direction that the water is passing. The result will be
that the solution inside of the cell may have its compo-
nents in entirely different proportions from the solution
outside.
118. The process by which solutes pass into the cell
and from cell to cell is diffusion. This is the molecular
passage of a solute from that part of a solution where the
concentration of that particular solute is greater to where
it is less. As long as the plasma membrane is easily
permeable for the particular solutes they have no osmotic
effect and may diffuse in the same direction with or
counter to the osmotic stream. Thus the dissolved salts
that enter a plant do so independently of osmosis and
diffuse toward those parts of the plant where these
particular salts are less abundant. They will not
become more concentrated anywhere in the plant than
outside of it as long as they retain their same composition
and the permeability of the plasma membrane remains
82 PLANT PHYSIOLOGY
the same. Frequently, however, they are changed chemi-
cally after they enter the plant and then are no longer able
to pass through the external plasma membrane. In
such a case the plant may be able to take in large amounts
of one substance from a dilute solution. Certain sea-
weeds, for example, accumulate large amounts of iodine
compounds from the sea water which contains iodides
only in very great dilution.
119. Water consists of hydrogen and oxygen (H2O).
Besides these two elements eight others are ordinarily
necessary to plant life. They are carbon (C), which
chiefly enters the plant in the form of carbon dioxide
(CO2) (see paragraph on photosynthesis), nitrogen (N)
in the form of nitrates or ammonium salts, calcium (Ca),
magnesium (Mg) and potassium (K), these mostly oc-
curring as phosphates, nitrates, sulphates or carbonates,
iron (Fe) in very small amounts as salts of various acids,
sulphur (S) almost entirely as sulphates (except in those
plants that feed on organic food where it may be taken up
from the proteins and a few lower plants which use
H2S or even free sulphur) and phosphorus (P) as various
phosphates. In addition to these, sodium (Na) is re-
quired by some plants, while on the other hand calcium
(Ca) is not required by certain fungi. Of the ten
elements first mentioned the last seven are usually taken
in as mineral salts from the water in which they are
dissolved. The oxygen is taken in, in the acid radical of
the sulphates, nitrates, carbonates and phosphates, in
combination with hydrogen in water, and in combination
with carbon in carbon dioxide as well as in the elementary
form directly from the air or in solution in the water.
Carbon in addition to being taken in as carbon dioxide
exists in the carbonates and in the case of hysterophytes,
also in various organic substances taken in by the plant.
ADDITIONAL NUTRIENTS 83
The use of free nitrogen by certain bacteria, will be
discussed further on.
120. In addition to the substances mentioned in the
preceding paragraph, silicon (Si) is taken up by many
plants (as silicates of various kinds) and adds to their
hardness but can be dispensed with except by the
diatoms whose cell walls are composed largely of silica.
Sodium can take the place of potassium for many pur-
poses, e.g. neutralizing acids, but cannot be substituted
for it entirely. Similarly an excess of calcium can replace
part but not all of the magnesium, while barium (Ba) and
strontium (Sr) can replace part of the calcium. Chlorine
(CI) in the form of chlorides is useful to many plants but
apparently can be dispensed with by almost all. The
various other salts present in the soil solution may be
taken up by the plant in greater or less degree, but
appear either to have no use whatever or to be used only
incidentally without being indispensible. Such are salts
of copper (Cu) aluminum (Al) manganese (Mn) zinc
(Zn), etc.
121. The role that the various substances mentioned
in the foregoing paragraphs play in the plant economy
is not certain in all cases. It is probable that calcium
and potassium, perhaps also magnesium and iron, are
essential parts of the protoplasm molecule. Sulj)hur is a
component of proteins while phosphorus is found in some
proteins, especially in the nucleus. Carbon, hydrogen
and oxygen are the components of the carbohydrates
which are the chief building materials of the plant (e.g.
cellulose) and of the proteins out of which protoplasm is
built up. In the absence of iron the chlorophyll seems
impossible of formation although it does not contain iron
itself. Mention must be made of the principle of
antagonistic action by various salts. Thus it has been
84 PLANT PHYSIOLOGY
shown that solutions of certain salts poisonous to plants
become innocuous upon the addition of certain other
salts which of themselves may also be poisonous. This
discovery has thrown doubt upon many of the con-
clusions of earlier botanists as to the functions of salts
that are supposed to be essential to plant life.
122. So far we have merely considered what sub-
stances are required by the plant and something of the
form in which the plant takes them in. Before they can
be used they must undergo various decompositions and
recombinations; in other words after absorption there
must be assimilative processes. Perhaps the most funda-
mental of these processes is that by which the carbon
compounds are built up by green plants, a process called
photosynthesis.
123. Photosynthesis. The green parts of all chloro-
phyll-bearing plants absorb carbon dioxide from the
surrounding water if aquatic plants, or from the air, which
contains about three parts of it to ten thousand. This
absorption goes on only when the plant is exposed to the
light. At the same time there is an increase in the
amount of carbohydrates often manifesting itself to the
eye by the formation of starch grains in the chloroplasts,
but also demonstrable chemically by the increased
amount of sugars (chiefly glucose C6H12O6) in the cell
sap. At the same time it can be demonstrated that
oxygen is given off by the plant. It is this process, the
manufacture of carbohydrates by green plants in the
presence of light, that has received the name photo-
synthesis (from the Greek meaning ''putting together
by light").
124. Careful experiments have shown that this
process cannot occur in the absence of any one of the
factors mentioned in the preceding paragraph. Thus a
PHOTOSYNTHESIS 85
plant growing in the light in an atmosphere free from
carbon dioxide cannot manufacture carbohydrates any-
more than if it were in the dark. A plant lacking chloro-
plasts, e.g. the fungi, cannot manufacture carbohydrates
from carbon dioxide even if light be present (excepting cer-
tain bacteria, the so-called nitrite and nitrate bacteria).
The process takes place in the chloroplasts apparently.
The light rays most effective in photosynthesis seem to be
those in the red part of the spectrum while those at the
violet end also have some value. Those lying between
seem in the main to be useless. The green color represents