THE CHANGES WHICH TAKE PLACE IN THE FOOD OF PLANTS DURING THEIR GROWTH.
The simple compounds which the plant absorbs from the atmosphere and
soil are elaborated within its system, and converted into the various
complex substances of which its tissues are composed, by a series of
changes, the details of which are still in some respects imperfectly
known, although their general nature is sufficiently well understood.
They may be best rendered intelligible by reference, in the first
instance, to the changes occurring during germination, when the young
plant is nourished by a supply of food stored up in the seed, in
sufficient quantity to maintain its existence until the organs by which
it is afterwards to draw its nutriment from the air and soil are
sufficiently developed to serve that purpose.
_Changes occurring during Germination._--When a seed is placed in the
soil under favourable circumstances, it becomes the seat of an important
and remarkable series of chemical changes, which result in the
production of the young plant. Experiment and observation have shown
that heat, moisture, and air, are necessary to the production of these
changes, and though probably not absolutely essential, the absence of
light is favourable in the early stages. The temperature required for
germination varies greatly in different seeds, some germinating readily
at a few degrees above the freezing point, and others requiring a
tolerably high temperature. The rapidity with which it takes place
appears to increase with the temperature; but this is true only within
very narrow limits, for beyond a certain point heat is injurious, and
when it exceeds 120 deg. or 130 deg. Fahrenheit, entirely prevents the process.
The presence of oxygen is also essential, for it has been shown that if
seeds are placed in a soil exposed to an atmosphere deprived of that
element, or if they be buried so deep that the air does not reach them,
they may lie without change for an unlimited period; but so soon as they
are exposed to the air, germination immediately commences. Illustrations
of this fact are frequently observed where earth from a considerable
depth has been thrown up to the surface, when it often becomes covered
with plants not usually seen in the neighbourhood, which have sprung
from buried seeds. When all the necessary conditions for germination are
fulfilled, the seed absorbs moisture, swells up, and sends out a shoot
which rises to the surface, and a radicle which descends--the one
destined to develop the leaves, the other the roots, by which the plant
is afterwards to derive its nutriment from the air and the soil. But
until these organs are properly developed, the plant is dependent on the
matters contained in the seed itself. These substances are mostly
insoluble, but are brought into solution by the atmospheric oxygen
acting upon the gluten, and converting it into a soluble substance
called diastase, which in its turn reacts upon the starch, converting it
first into dextrine, and then into cellulose, and the latter is finally
deposited in the form of organised cells, and produces the first little
shoot of the plant. At the first moment of germination, the oxygen
absorbed appears simply to oxidize the constituents of the seed, but
this condition exists only for a very limited period, and is soon
followed by the evolution of carbonic acid, water being at the same time
formed from the organic constituents of the seed, which gradually
diminishes in weight. The amount of this diminution is different with
different plants, but always considerable. Boussingault found that the
loss of dry substance in the pea amounted in 26 days to 52 per cent, and
in wheat to 57 per cent in 51 days. Against this, of course, is to be
put the weight of the young plant produced; but this is never sufficient
to counterbalance the diminished weight of the seed, for Saussure found
that a horse bean and the plant produced from it weighed, after 16 days,
less by 29 per cent than the seed before germination. The same
phenomenon is observed in the process of malting, which is in fact the
artificial germination of barley, the malt produced always weighing
considerably less than the grain from which it was obtained. It was
believed by Saussure, and the older investigators, that the carbonic
acid evolved was entirely produced from starch and sugar; and as these
substances may be viewed as compounds of carbon and water, the change
was very simply explained by supposing that the carbon was oxidised and
converted into carbonic acid and its water eliminated. But this
hypothesis is incapable of explaining all the phenomena observed; for
woody fibre, which is one of the chief constituents of the young plant,
contains more carbon than the starch and sugar from which it must have
been produced, and we are, therefore, forced to admit that the action
must be more complicated. There is every reason to believe that the
nitrogenous constituents of the seed are most abundantly oxidized, for
they are remarkably prone to change; but the action of the air is not
confined to them, and it appears most probable that all the substances
take part in the decomposition, and the process of germination may, in
some respects, be compared to decay or putrefaction, which, like it, is
attended by the absorption of oxygen and evolution of carbonic acid; but
while in the latter case the residual substances remain in a useless
state, in the former they at once become part of a new organism.
_Changes occurring during the After-growth of the Plant._--When the
plant has developed its roots and leaves, and exhausted the store of
materials laid up for it in the seed, it begins to derive its
subsistence from the surrounding air, and to absorb carbonic acid,
water, ammonia, and nitric acid, and to decompose and convert them into
the different constituents of its tissues. These changes take place
slowly at first, and more rapidly as the organs fitted for the
elaboration of its food are developed. The roots and the leaves are
equally active in performing this duty, the former absorbing the mineral
matters along with the carbonic acid, ammonia, nitric acid, and moisture
in the soil, or the manure added to it; the latter gathering the gaseous
substances existing in the air. Each of these undergoes a series of
changes claiming our consideration.
_Decomposition of Carbonic Acid._--Carbonic acid, which appears to be
absorbed with equal readiness by the roots, leaves, and stems, undergoes
immediate decomposition, its carbon being retained, and its oxygen, in
whole or in part, evolved into the air. This decomposition occurs only
under the action of the sun's rays, and has been found to be
proportionate to the amount of light to which the plant is exposed. It
takes place only in the green parts of plants, for though the roots
absorb carbonic acid, they cannot decompose it, or evolve oxygen; and
the coloured parts, the flowers, fruits, etc., have an entirely opposite
effect, absorbing oxygen and giving off carbonic acid. The absorption of
carbonic acid and escape of oxygen has been proved by numerous direct
experiments by Saussure and others, in which both atmospheric air and
artificial mixtures containing an increased quantity of carbonic acid
have been employed. Saussure allowed seven plants of periwinkle (_Vinca
minor_) to vegetate in an atmosphere containing 7.5 per cent of carbonic
acid for six days, during each of which the apparatus was exposed for
six hours to the sun's rays. The air was analysed both before and after
the experiment, and the results obtained were--
Volume Carbonic
of the air. Nitrogen. Oxygen. Acid.
Before the experiment, 5746 4199 1116 431
After " 5746 4338 1408 0
---- ---- ---- ----
Difference, 0 +139 +292 -431
In this experiment the whole of the carbonic acid, amounting to 431
volumes, was absorbed, but only 292 volumes of oxygen were given off.
Had the carbonic acid been entirely decomposed, and all its oxygen
eliminated, its volume would have been equal to that of the acid, or
431, so that in this instance 139 volumes of the oxygen of the carbonic
acid have been retained to form part of the tissues of the plant. On the
other hand, the nitrogen is found to be increased after the experiment.
It might be supposed that the nitrogen evolved had been derived from the
decomposition of the nitrogenous constituents of the plant, but this
cannot be the true explanation, because in this particular case it
greatly exceeded the whole nitrogen contained in the plants
experimented on. Its source is not well understood, but Boussingault
supposes it to have existed in the interstices of the plant, and to have
escaped during the course of the experiment. Saussure found that the
oak, the horse-chesnut, and other plants, absorb oxygen and give off
carbonic acid in less volumes than the oxygen, while the house-leek and
the cactus absorb oxygen without evolving carbonic acid. The absorption
and decomposition of carbonic acid takes place only during the day, and
matters are entirely reversed during the night, when oxygen is absorbed
and carbonic acid eliminated from all parts of the plants.
Although the action occurring during the night is the reverse of that
which takes place during the day, it is in no degree to be attributed to
a re-oxidation of the carbon which had been deposited in the tissues of
the plant. It appears, on the contrary, to be a purely mechanical, and
not a chemical process. During the night the sap continues to circulate
through the vessels of the plant, and moisture, carrying with it
carbonic acid in solution, is absorbed by the roots; but when it reaches
the leaves, where the sun's light would have caused its decomposition
during the day, it is again exhaled unchanged. The oxygen absorbed
during the night must, however, take part in some chemical processes,
for if it were merely mechanical, the absorption would not be confined
to that gas alone, but would be participated in by the other
constituents of the air. Moreover, the amount of absorption varies
greatly in different plants--being scarcely appreciable in some, and
very abundant in others. Plants containing volatile oils, which are
readily converted into resins by the action of oxygen, or those
containing tannin or other readily oxidizable substances, take up the
largest quantity. This is remarkably illustrated by an experiment in
which the leaves of the Agave americana, after twenty-four hours'
exposure in the dark, were found to have absorbed only 0.3 of their
volume of oxygen, while those of the fir, in which volatile oil is
abundant, had taken up twice, and those of the oak, containing tannin,
eighteen times as much oxygen.
In the flowers, both by day and night, there is a constant absorption of
oxygen, and evolution of carbonic acid. In fact, an active oxidation is
going on, attended by the evolution of heat, which, in the _Arum
maculatum_ and some other plants, is so great as to raise the
temperature of the flower 10 deg. or 12 deg. above that of the surrounding air.
_Decomposition of Water in the Plant._--In addition to the function
which water performs in the plant, as the solvent of the different
substances which form its nutriment, and hence as the medium through
which they pass into its organs, it serves also as a direct food,
undergoing decomposition, and yielding hydrogen to the organic
substances. Its constituents, along with those of the carbonic acid
absorbed, undergo a variety of transformations, and form the principal
part of the non-nitrogenous constituents. It has been already observed
that starch, sugar, and the other allied substances, may be considered
as compounds of carbon with water; and they might be supposed to owe
their origin to the carbonic acid losing the whole of its oxygen, and
direct combination then ensuing between the residual carbon and a
certain proportion of water; but this would imply that the latter
substance undergoes no decomposition, and though undoubtedly the
simplest view of the case, it is by no means the most probable. It is
much more likely that the carbonic acid is only partially decomposed,
half its oxygen being separated, and replaced by hydrogen, produced by
the decomposition of a certain quantity of water into its elements.
Thus, for instance, sugar may be produced from twelve equivalents of
carbonic acid and twelve equivalents of water, twenty-four equivalents
of oxygen being eliminated, as thus represented:
12 equivalents of carbonic acid, C_{12}O_{12}O_{12}
12 " water, H_{12}O_{12}
1 " sugar, and 24 of ox. C_{12}H_{12}O_{12} + O_{24}
It must not be supposed that we are in a condition to assert that sugar
is really produced in the manner here shown, the illustration being
given merely for the purpose of pointing out how it may be supposed to
occur, and on a similar principle it is possible to explain the
formation of most other vegetable compounds; and this subject has been
very fully discussed by the late Dr. Gregory, in his "Handbook of
Organic Chemistry." That water must be decomposed, is evident from the
fact, established by analysis, that the hydrogen of the plant generally
exceeds the quantity required to form water with its oxygen, so that
this excess at least must be produced by the decomposition of water. The
hydrogen of the volatile oils, many of which contain no oxygen, and that
of the fats, which contain only a small quantity, must manifestly be
obtained in a similar manner.
_Decomposition of Ammonia._--The nitrogenous or albuminous compounds of
vegetables must necessarily obtain their nitrogen from the decomposition
either of ammonia or nitric acid, experiment having distinctly shown
that they are incapable of absorbing it in the free state from the
atmosphere. It has been clearly ascertained that the albuminous
substances do not contain ammonia, and it is hence apparent that a
complete decomposition of that substance must take place in the plant.
No doubt carbonic acid and water take part with it in these changes,
which must be of a very complex character, and in the present state of
our knowledge it seems hopeless to attempt any explanation of them.
_Decomposition of Nitric Acid._--Chemists are not entirely at one as to
whether nitric acid is directly absorbed by the plant, or is first
converted into ammonia. But there are certain facts connected with the
chemistry of the soil, to be afterwards referred to, which seem to us to
leave no doubt that it may be directly absorbed; and in that case it
must be decomposed, its oxygen being eliminated, and the nitrogen taking
part with carbon and hydrogen in the formation of the organic compounds.
It must be clearly understood that while such changes as those described
manifestly must take place, the explanations of them which have been
attempted by various chemists are not to be accepted as determinately
established _facts_; they are at present no more than hypothetical views
which have been expressed chiefly with the intention of presenting some
definite idea to the mind, and are unsupported by absolute proof; they
are only inferences drawn from the general bearings of known facts, and
not facts themselves. Although, therefore, they are to be received with
caution, they have advantages in so far as they present the matter to us
in a somewhat more tangible form than the vague general statements which
are all that could otherwise be made.