the portion of the white light that strikes the chlorophyll
and is reflected back or passes through it without being
absorbed. The raw materials are carbon dioxide and
water, the energy is derived from the absorbed rays of
light and the end products are carbohydrates and oxygen.
125. The exact steps in photosynthesis are not
certainly known but the following seems to be the
probable course of events:
C02+H20 = H2C03 (water, plus carbon dioxide, equals
carbonic acid).
H2C03 = H2CO + 02 (carbonic acid acted on by the
energy derived from light by the cholorophyll is changed
into formaldehyde and oxygen) .
6H2CO = C6Hi206 (formaldehyde, probably by the
aid of more energy derived from the light is polymerized
into glucose).
It \\411 thus be seen that for every molecule of carbon
dioxide used up one molecule of oxygen (O2) will be set
free. Glucose is the carbohydrate first formed in most
cases but as this accumulates in the chloroplasts and
cell sap it is often transformed rapidly into the insoluble
starch (C6Hio05)n which becomes stored up in large
quantities in the chloroplasts. Sometimes instead of
86 PLANT PHYSIOLOGY
starch, drops of oil are produced in the cytoplasm and
cell sap, or cane sugar (C12H22O11) or some other
carboliydratcs.
126. The further fate of the carbohydrates formed in
photosynthesis is varied. The excess of glucose or other
sugars in the chlorophyll-bearing cells in addition to
what is put aside temporarily in insoluble form as starch
diffuses through the adjacent cells and finally reaches
the vascular bundles where it enters the parenchyma cells
bordering the sieve tubes. It probably diffuses through
these into the latter in which it diffuses and is probably
also carried by streams of protoplasm to those parts of
the plant where the tissues contain less glucose.
Here it diffuses out into these tissues. Besides passing
in the sieve tissues diffusion doubtless occurs from cell
to cell throughout the parenchyma of the cortex espe-
cially in those cells bordering on the sieve tubes. Dur-
ing the night the starch grains that have accumulated
in the chloroplasts in day time are transformed into
glucose which diffuses in the manner just described.
127. The carbohydrates transported in this manner
may be stored up as reserve food in various forms. Thus
they may be transformed into starch in the leucoplasts of
the storage organs, e.g. tubers of potato, roots of sweet
potato {lyomoea batatas), pith of various twigs such
as apple, sassafras, etc., medullary rays of many trees,
endosperm or cotyledons of seeds, etc. Cane sugar may
be found in many plants such as beets, maple, sugar cane,
etc. Inulin is found in the roots of many plants par-
ticularly those belonging to the order Asterales. Trans-
formed into fats they are found in many seeds, e.g. flax,
cotton, peanut, castor bean, as well as in the bulb scales
of onion, leaves of cabbage, etc. In the seeds of many
palms, e.g. date, and the wood of many trees, e.g. elm and
PROTEIN SYNTHESIS 87
mulberry, the reserve carbohydrate is in the form of a
thick deposit on the inner surface of the cell wall. This
is a substance closely related to cellulose, one of the hemi-
celluloses. The sugars in fruits perhaps belong in the
category of stored foods although they serve rather as
a bait for animals which on eating the fruit aid in the
distribution of the seeds.
128. The carbohydrates produced, whether first
stored up or used immediately, have for their ultimate
destination various functions. As building materials
they are used up in the formation of cell walls in the grow-
ing parts of plants. Whether they are thus used directly
or must first become a part of the protoplasm is uncertain.
The use of carbohydrates in furnishing energy to the
plant will be discussed under the topic Respiration.
129. A considerable portion of the carbohydrates
eventually becomes built up into those very complex
nitrogenous compounds called proteins. Whether the
carbohydrates are taken as such and combined with
nitrogen obtained from the nitrates and sulphur and
phosphorus from the sulphates and i:)hosphates re-
spectively, the product being proteins, or whether as
seems possibly may be the case part of them are broken
down and then combined with the nitrogen to form
hydrocyanic acid (HCN) this being polymerized and
combined with other carbohydrate molecules and with
sulphur and i)hosphorus, is not known. In any case
hydrocyanic acid is often formed in i:>lants in which active
protein production is taking i)lace.
130. Certain bacteria, chiefly parasitic in the roots
of plants of the bean family (Fabaceae), are capable,
when supplied with carbohydrates and the necessary'
mineral salts, of using the atmospheric nitrog(>n (as dis-
solved in the soil water) in building up protein com-
88 PLANT PHYSIOLOGY
pounds. These bacteria form galls on the roots of the
host plants. As they grow old the host plant digests
them and is thus able to thrive in a soil free from nitrog-
enous compounds. Thus if the bacteria are present,
crops of beans, clover, alfalfa, etc. will actu-
ally increase the amount of nitrogenous
compounds in the soil instead of decreas-
ing it.
131. The proteins formed may be stored
up as such for future use by the plant (e.g.
aleuron in seeds) or may be transported to
those parts of the plant where new cell
â– ^SdulermcL') * production and growth are taking place.
Here it is built up into protoplasm. How
this is accomplished we do not know. The path of
transportation seems to be in the sieve and possibly
laticiferous tissues. The form in which protein matters
are transported may be either as simple proteins or as
amids.
132. Hysterophytic plants, i.e. plants that lack chloro-
phyll, must obtain their organized food (carbohydrates,
proteins, fats, etc.) from sources outside of themselves.
We find all degrees of ability to make use of various
food sources. Some hysterophytes simply require
carbohydrates and mineral salts and can produce their
own proteins, others must have special, and in the case
of parasites, living forms of proteins. Some even are
able to use simpler carbon compounds than carbohy-
drates such as some of the simpler organic acids, glycer-
ine, etc. In general, however, the nutrition of hystero-
phytes differs but little from that of holophytes (i.e.
plants containing chlorophyll) except in their inabihty
to manufacture their own carbohydrates.
133. The means by which hysterophytic plants
NUTRITION OF HYSTEROPHYTES 89
obtain their food supplies are quite varied. One-celled
plants like yeasts and bacteria absorb the organic sub-
stances directly, or often decompose them to the appro-
priate form by means of digestive ferments called
enzymes, which are organic compounds of complex
structure whose exact action is not clearly known. Fungi
consist of long filaments of cells which either pass
through the substances to be absorbed or send little
suckers, called haustoria, into the cell of the host, the
latter being often the case with fungi i)arasitic upon
living plants. Among the hysterophytic flowering plants
some feed on decayed organic matter in the soil, others,
e.g. dodder, send haustoria into living plants, and take
organic substances directly from them. Some of the
mistletoes which possess chlorophyll take little else than
water and mineral salts. Of especial interest are the
insectivorous plants which catch and digest insects by
means of special structures. The digested insects are
the source of their nitrogen for many of these plants that
hve where nitrogen compounds are lacking in the soil.
Some plants have fungous hyphae growing partly within
and partly outside of some or all of their roots. Such roots
are often of peculiar shape and are known as mycorrhiza.
The fungi absorb water and mineral salts from the soil
and deliver them to the root from which in turn they
take organic foods. Some of these fungi are said to be
able to make use of the atmospheric nitrogen as do the
bacteria in the root tubercles of the bean family.
134. All the foregoing processes, e.g. transformation of
carbohydrates from one form to another, their trans-
portation and storage, their ])uilding uj) into proteins,
the transportation and storing away of the latter and
their building up into protoplasm, require the expenditure
of a considerable amount of energy. This must be
90 PLANT PHYSIOLOGY
available in every living cell and not confined to any
definite locality in the plant. This is made available by
the process known as respiration.
135. Respiration. With the exception of a few
bacteria and low fungi to be mentioned later all living
cells absorb oxygen and give off carbon dioxide, the
process being accompanied by a loss in weight. In
green plants in the light the absorption of carbon dioxide
and giving out of oxj^gen are so much greater than this
other process that for years it was not known that the
latter takes place. It is not dependent upon the
presence of light nor are chloroplasts necessary for its
occurrence. It takes place more rapidly the higher the
temperature until an optimum temperature is reached
which is sometimes perilously near to the death point of
the cell.
136. The oxygen is taken from the air (which contains
nearly 20 per cent, of oxygen) by the aerial parts of the
plant. It passes through the stomata and lenticels and
also to some extent through the cuticle into the inter-
cellular spaces and from thence is absorbed by the
cells. The roots whose outer walls are only slightly
cutinized and whose root hairs are practically free from
cutin absorb the oxygen which is dissolved in the soil
water and which is present in the air spaces between
the soil particles. Submerged plants, e.g. algae, absorb
the oxygen dissolved in the water. Many trees which
grow in swamps where the soil lacks oxygen send up
peculiar vertical branches from their roots out to the
surface and up into the air, these serving as aerating
organs for the roots. Such are the ''knees" of the
bald cypress {Taxodium distichum) when the latter
grows in wet places (and which are lacking when it grows
in well aerated soil) and the aerial roots of sotne of the
RESPIRATIOX 91
mangroves (e.g. the black mangrove of Florida, Avicen-
nia nitida).
137. Respiration consists primarily in the breaking up
of the complex molecules 'of certain organic compounds
(chiefly car])oh3'drates or even the carbohydrate portions
of protoplasm molecules) into simpler compounds. This
releases a large amount of energy much of which becomes
available for the use of the plant. Since all living parts
of the plant require energy, respiration will be found to
take place in all parts. The intensity of the respiration
varies with many factors, viz. the amount of food avail-
able that can be broken down into simpler compounds,
the availability of oxygen, the amount of water, the
temperature, etc. To what extent the protoplasm itself
can regulate the occurrence of this process, if the other
conditions are fulfilled, is uncertain.
138. Part of the energy set free in respiration is
exhibited in the form of heat. This is especially notice-
able where rapid gro^\i3h and rapid respiration are oc-
curring as in large flower buds, fruiting bodies of large
fungi, etc. In ordinary parts of plants the radiating
surface is great enough to keep the plant cool so that the
heating is not noticeable. In the case of wet leaves, hay,
manure, etc., the heat produced by the respiratory proc-
esses of the fungi and especially the bacteria present
leads in some cases to the kindling of some of the easily
inflammable substances produced so that it is a frequent
occurrence for hay, especially moist alfalfa hay, and
manure to catch fire.
139. It has been shown that there are two distinct
stages in respiration which follow one another so closely
in most cases that they a])pear as one. These are the
anaerobic and aerobic stages. Certain bacteria and
yeasts show only the first stage. In this stage no oxygon
92 PLANT PHYSIOLOGY
is required from outside the ceU. By the aid of certain
enz3^mes produced by the cell the carbohydrates or other
substances used in respiration are started in their disin-
tegration and proceed in it until simpler compounds and
some carbon dioxide are produced. Thus glucose is usually
decomposed into alcohol and car])on dioxide, the end
results being in accordance with the following formula:
C6H12O6-2C2H5OH+2CO2.
It is probable that the reaction is not as simple as this,
but that there are many steps in the process. This proc-
ess sets free a certain amount of energy. In the produc-
tion of alcohol and carbon dioxide from sugar by the yeast
plant it is this anaerobic stage of respiration that takes
place. Corresponding decomposition processes occur in
various kinds of bacterial fermentation and decay, the
intermediate and end products varying with the com-
position of the substance fermented and the kind of
organism.
140. The aerobic stage consists usually of the oxid-
ation of the rather complex compounds produced in the
anaerobic stage to simpler compounds, this also being
accompanied by the liberation of energy in large
amounts. This process also is probably carried on by
the aid of enzymes and it may be that the use of the
oxygen is rather to get rid of harmful products instead
of being the agent which sets free the energy. Taking
the case illustrated in the preceding paragraph the
alcohol is broken down and combined with oxygen to
form carbon dioxide and water. The final results, but
not the intermediate stages, are shown by the following
formula
C2H5OH+6O = 2CO0+3H2O.
Alcohol + oxygen = carbon dioxide + water.
RESPIRATION 93
By comparing the final results of the anaerobic and aero-
bic respiration of glucose with the steps in the photo-
synthetic production of glucose we realize that the proc-
esses are the reverse of one another. It is reasonable
to suppose then that the amount of energy set free in
the processes of respiration will equal that required to
build up the same amount of glucose in photosynthesis.
Viewed from this standpoint respiration is the process
by which the plant obtains at the places where it is needed
the energy taken in from the light by the chloroplasts.
The manufacture by photosynthesis of an excess of
carbohydrates over that used each day by the plant in
respiration enables the plant to store up a large amount
of energy for the winter season when photosynthesis
cannot occur or for the rapid grow^th of new organs
another season. With all the processes of respiration
the protoplasm, the living part of the cell, is intimately
connected. It is to it that the energy set fr^e is probably
transferred. It is apparently the protoplasm that regu-
lates the amount and location of the respiratory activi-
ties. How all this is brought about is still unknown as
is the relation of the structure of protoplasm and the
energy used to what we call ''life."
141. In place of the type of respiration described
above a few bacteria obtain their energy in other ways.
Thus the nitrite bacteria oxidize the ammonia of am-
monium salts to nitrites and the nitrate bacteria oxidize
the nitrites to nitrates, each of these processes setting
free a small amount of energy which is made use of by
the bacteria. In both cases the energy thus obtained is
sufficient to enable the cells to build up from carbon
dioxide and water the carbohydrates needed in the
cell's growth and further to combine these with the nec-
essary substances to form proteins and protoplasm.
94 PLANT PHYSIOLOGY
Still other bacteria inhabiting sulphur springs or places
where sewage is abundant obtain the necessary energy
by oxidizing US to SO2, sulphur frequently being stored
up as a reserve food supply. It is held by some investi-
gators that other bacteria obtain their energy by oxi-
dizing certain iron compounds, others by oxidizing
methane and still others hydrogen.
142. In the foregoing processes of photosj^nthesis
and respiration (including fermentation) many other
substances are produced besides those mentioned. Some
of these are perhaps nothing more than waste products,
or at least by-products, but others are reserve food of
various kinds. Still others perhaps serve for special
functions such as protection of plants from attacks of
insects, covering of wounds, etc. Among the substances
thus produced and whose functions are not certainly
known, are the alkaloids of which a great many have been
studied, e.g. caffein, nicotine, etc. Besides these may be
mentioned resins, rubber, gutta-percha, glucosides, etc.
Many of these are of great use to man. Many are very
poisonous. The organic acids mostly stand in another
category. They are either directly reserve stuffs, re-
placing carbohydrates, or are stages in the respiration
of carbohydrates, or in many cases are the substances
which produce the requisite osmotic pressure within the
cell. The commonest organic acids are the following:
maUc, (C4H6O5) found in the apple and many other
fruits as well as in the leaves of many succulent plants,
citric (CeHsO?) in the fruits of lemon, orange, etc.,
tartaric (C4H6O6) in fruit of grapes, oxalic (C2H2O4)
in the leaves of many plants, e.g. Oxalis, Rumex, etc.,
and tannic acid (C14H10O9) and its derivatives which ap-
pear to play a very important but little understood part
in the energy relations of the plant. ]\Iany of these
TEMPERATURE 95
acids are present in the free form but some of them
appear mostly as the acid salts of various metals.
143. Temperature. The relation of the plant to
temperature will be discussed here as it is chiefly a ques-
tion of the effect of temperature upon the nutritive
functions. Five cardinal points for temperature can be
distinguished for these different processes. They are:
death point from cold, death point from heat (points
which are the same whatever the process and mentioned
here simply because when reached the process cannot
be resumed when normal temperatures are again re-
gained), minimum, optimum and maximum. The last
three are quite different for different life processes.
Thus the optimum and maximum for respiration are
usually much higher than for photosynthesis, in fact
they often lie close to the death point from heat. Be-
tween the death point from cold and the minimum for
various processes may be a small range or sometimes
a great range of temperature. Usually the minimum
point is a little above or not much below 0° C. The
maximum temperature for the various functions lies
usually between 36° and 43° C. and the death point be-
tween 50° and 55° C, but in a few plants of hot springs
as well as some bacteria causing the heating of manure,
etc., the optimum temperature may be about 60° and
the death point even as high as 75° to 85° C.
144. The death of plants by heat appears to be due
to the coagulation of some of the protein constituents of
the protoplasm. Since this coagulation cannot occur
unless a certain amount of water is present we find that
some nearly water-free structures are able to endure
rather high temperatures. Thus the spores of some
bacteria can be boiled for several hours before they are
killed and some seeds can endure a dry heat exceeding
96 PLANT PHYSIOLOGY
100° C. Similarly dry plant parts can endure very low
temperatures. Many seeds are not killed by an ex-
posure for several hours to the temperature of liquid
hydrogen (below — 250° C). The latter is also true for
many single-celled water plants that must contain plenty
of water, e.g. diatoms, bacteria, etc. On the other hand
many watery tissues are killed by a temperature that does
not reach the freezing point. Just the reason for this is
unknown. It is here suggested that at these low
temperatures certain processes continue which result in
the accumulation of poisons, while the processes that
would usually destroy these poisons, are prevented by the
low temperature so that in reahty the death of the plant
would be due to poisoning.
145. Freezing of plants may cause death in several
ways: (1) the ice crystals formed may rupture the
cells or disrupt the tissues; (2) the water may escape
into the intercellular spaces and be frozen there and on
thawing rapidly may remain outside the cells filling up
the intercellular spaces and cutting off the air supply;
(3) the withdrawal of water from the protoplasm by freez-
ing may so increase the concentration of certain sub-
stances dissolved in the cell sap that the cells are killed.
Upon the whole subject considerable uncertainty rests.
146. Effect of Poisons. Many substances are poison-
ous to living plant cells. The effects are almost as varied
as the types of poisons. Some, like the strong acids,
simply decompose the protoplasm and cell walls and so
destroy life; others, Hke the salts of the heavier metals,
coagulate the protoplasm; others even in minute quanti-
ties interfere with the nutrition of the cell in a manner
not understood, and kill it. Thus one part of copper in
ten million parts of water will kill certain algae and fungi.
Hydrocyanic acid acts apparently by preventing the
EFFECT OF POISONS 97
taking in or using of oxygen in respiration. IMany
parasitic plants, e.g. bacteria and fungi secrete poisons
or induce activities in the cells of the host that lead to the
accumulation of poisons that may destroy the life of a
cell or lead it to abnormal growth or functioning.
Laboratory Studies, (a) Take a piece of the root of a living
red beet. Cutout a cube a centimeter or so in diameter. Wash
off the colored cell sap that has escaped from the cut cells and
place the cube in a test tube of water. So long as the cells are
alive their plasma membranes prevent the colored solute in the
cell sap from escaping. Gently heat the test tube. When the
death point of the beet tissues is reached (below G0° C.) the
plasma membranes are no longer impermeable and the color
diffuses out into the surrounding water. This experiment also
shows that the cell walls themselves are but slight obstacles
to diffusion. Instead of by heating, similar results may be
obtained by using certain poisons such as strong alcohol, etc.,
but care must be taken not to choose a substance that will
destroy the coloring matter.
(b) Set up a series of water cultures as follows : Take glass
jars (]\Iason jars will do) and to keep the contents dark encase
each with a cylinder of pasteboard which can be removed to
permit of observation. Fill these jars nearly full of the solution
to be tested, leaving a small air space between the water and
the cork. The cork should have at the center a hole 5 or
6 mm. in diameter. Germinate some peas, corn, buckwheat or
mustard seeds. When the radicles are 2 to 3 cm. long, fasten
one seed to each cork in such a way that the root just enters the
solution and the plumule is in a position to pass uj) through the
hole in the cork (or the seed can be fastened outside with the
root passing through the hole). Instead of a cork the jars may
be nearly filled with water and melted parafhn poured upon it ;
after the paraffin has hardened several holes may be made
through it by means of a hot metal rod. The water can now
be poured out and the desired liquid poured in, nearly up to the
under side of the paraffm. The germinated seeds can be set
upon this paraffin cap in such a way that the radicles will pass
throu2;h the holes. Expose all the jars to the same light and
temperature so that as far as possible the only differences will
7
98 PLANT PHYSIOLOGY
be those of the composition of the solutions. Make up the
following solutions and fill into the jars:
1. Distilled water
2. Complete culture solution (Sachs)
3. Complete culture solution, omitting the KNO3
4. Complete culture solution, omitting the ]\IgS04
5. Complete culture solution, omitting the KXO3 and
K2SO4 and adding Ca(N03)2 in place of the first.
6. Complete culture solution, omitting theCa3(P04)2
and adding an equal amount of Ca(N03)2
7. Complete culture solution, omitting theK2S0i and
MgS04 and replacing by an equal amount of
Mg(N03)2
8. Complete culture solution omitting the Ca3(PO.i)2
and substituting K2HPO4
9. Complete culture solution omitting the FeCU.
The Sachs' solution consists of:
Distilled water 1000 cc.
KNO3 1 gm.
K2SO. 0.5 gm.
MgS04 0.4 gm.
Ca3(PO02 0.5 gm.
FeCls trace.
Let the plants grow for several weeks, rej^lacing the old
solutions by fresh ones of the same composition every week or
so. Compare the amount of growth of both roots and stems in