By MAXWELL T. MASTERS, M.D.
A living plant feeds, breathes, grows, develops, multiplies, decays, and
ultimately dies. In so doing it receives, it spends, it accumulates, it
changes. Some of these processes are always in operation, very generally
more than one is going on at the same time, and the action of one is
modified by and controlled by that of another. Some circumstances and
conditions favor these operations, others hinder them.
The nutritive process has to be entered on the creditor side as a
receipt. The plant will indeed feed upon itself for a time, or rather it
will feed upon what its predecessor left it as an inheritance for this
very purpose, or upon the stores accumulated in the plant itself during
the preceding season; thus, when a seed, or rather the young plant within
the seed, begins to grow, it is at first unable to forage for itself, but
it depends for its sustenance on the materials laid up for its use during
the preceding season by the parent plant. So the bud of a tree awakening
into life, and beginning its career as a shoot which is to bear leaves
and flowers, derives its first meals from the reserves accumulated the
autumn previously in the parent branch. Very generally a little water,
supplied from without, is required before the plant can avail itself
of these stored-up provisions, but this is not always indispensable.
Potatoes begin to sprout in their cellars or pits, as growers know to
their cost, before they can have obtained a drop of water from without.
In this latter case there is water enough already in the tuber to allow
of food being utilized.
A certain degree of useful heat is, of course, quite indispensable.
Practically, no plant will feed when its temperature is reduced as
low as the freezing point, and in most cases the heat requires to be
considerably greater. Each kind of plant, each individual plant, and
indeed each part of a plant, feeds, and performs each item of its
life-work best at a certain temperature, and ceases to work at all
when the temperature falls below or rises above a certain point. The
particular degree, whether most or least favorable, varies according to
the plant, its age, stage of growth and various external circumstances,
which we need only mention, as their effects will be readily understood
without the necessity of explanation.
Leaving, however, on one side, the temperature, we have to consider the
water which is so essential, not only in the feeding processes with
which we are now concerned, but with every other action of plant life.
Fortunately there is, in general, no lack of it; the earth and the air
contain their shares of this elementary compound in varying proportions
and varying modifications as liquid or gaseous. Besides, the plant itself
has so much of it that even at the driest condition compatible with life,
it still constitutes a very large proportion of the entire weight. Now,
it is as a rule when the plant, the seedling, or the bud is at its driest
that growth begins, the necessity for food first manifests itself, and
the demand for a further supply of water becomes imperative. How is the
demand supplied? We have seen that there is no lack of that fluid. How is
it to get into the plant?
When one liquid, say spirit, is poured into another, say water, the
two gradually mix. If we suppose these liquids to consist of a number
of molecules, then, mixture may be taken to be the result of the
displacement say of one molecule of water by one molecule of spirit,
and so, throughout the whole quantity of liquid, there is displacement
and replacement till at length equilibrium is restored and a thorough
diffusion results. This power of diffusion does not always exist. The
molecules of water and of oil will not mix or diffuse freely through each
other. Water containing carbonic acid gas will not mix, in this sense of
the term, with water containing acetate of lead.
It may be a truism to say, that for the process of diffusion the liquids
must be diffusible, but the fact must be carefully borne in mind in all
questions relating to the feeding of plants. In the case of plants, the
phenomenon of diffusion, or the gradual admixture of two liquids of
different natures, is complicated by the presence of a membrane in the
shape of the cell-wall. The water from the outside has to pass through
the membrane to reach the protoplasm on the other side. Speaking broadly,
there are no holes in the membrane through which the water can pass.
Ingress is secured by that process of diffusion to which reference has
just been made, and by virtue of which the molecules of the membrane
and the molecules of the water shift and change places; the space that
was occupied by a molecule of membrane is now occupied by a molecule
of water, and _vice versa_. The access, therefore, of water into the
interior of a closed cell is the result of the process of diffusion.
Where two liquids mix without any intervening membrane, the mixture is
called diffusion simply; where there is an intervening membrane, the
diffusion process is known as “osmosis.”
The raw material (the term is not quite accurate, but for illustration
sake it may pass) is that very marvelous substance now called
“protoplasm.” We must leave it to chemists and microscopists to explain
its composition and indicate its appearance.
Diffusion is not equal or alike in all cases; it depends upon the extent
to which the two liquids are diffusible, upon their different densities,
upon temperature, and a variety of other conditions. So, in the case of
osmosis, we have not only the nature of the two fluids to consider, but
their relation to the membrane that separates them. The membrane may be
much more permeable to one of the two fluids than to the other. Thus, in
the case of a living cell, the membrane or wall is much more permeable
to water than it is to protoplasm; and so it happens that, while water
readily penetrates the membrane and diffuses itself in the protoplasm,
protoplasm does not nearly so readily permeate the membrane as the water.
Ingress of water is easy and of constant occurrence, egress of protoplasm
is rare and exceptional.
Pure water or weak saline solutions, such as are generated in the soil
under certain circumstances, pass readily through membrane—that is,
the molecules of the one shift and change places with those of the
other—while those of gummy or albuminous substances like protoplasm do
not. After a time, if there is no outlet for the water absorbed, or if it
is not utilized within the plant in some way, absorption and diffusion
cease, the cell becomes saturated with water, and until something happens
to disarrange the balance, no more is absorbed. But, even in the case
where the cell is saturated with water, it may still take up other
liquids, because the diffusive power of those other liquids, in relation
to the cell-wall and to the protoplasm, is different from that of water,
and this absorption may go on in its way till saturation point is reached
for each one of them, just as in the case of water. On the other hand,
it may happen that the plant may be saturated with other substances, and
incapable of taking up more of them, while at the same time pure water
may be freely taken up.
Just so much and no more of each particular substance is absorbed, the
exact quantity of each being regulated in all cases by the condition and
requirements of the cells, their membranous walls, and their contents.
Thus it happens that some particular substances may be found by the
chemist to exist in large relative proportions in the plant, while the
quantity in any given sample of the soil from which it must be derived
is sometimes so small as to elude detection. The plant in this case, or
some part of it, is so greedy, if we may so say, for this particular
substance, that it absorbs all within its reach, and stores it up in its
tissues or uses it in some way, the demand ensuring supply. On the other
hand, the soil may contain a large quantity of some particular ingredient
which is incapable of being absorbed, or which the plant does not or can
not make use of, and, in consequence, none is found within the plant. The
supply is present, but there is no demand.
The different physical requirements of the plant supply also the
explanation of the fact that different plants, grown in the same soil,
supplied with the same food, yet vary so greatly in chemical composition.
Thus, when wheat and clover are grown together, and afterwards analyzed,
it is found that while lime is abundant in the clover, it is relatively
in small quantity in the wheat; and silica, which is abundant in the
wheat, is absent from the clover. Poisonous substances even may be
absorbed, if they are of such a nature as to be capable of absorption;
and so the plant may be killed by its own action—by suicide, as it were.
The entrance of water into the plant and the entrance of those soluble
materials which a plant derives from the soil are therefore illustrations
of the process of osmosis, and are subjected to all the conditions under
which osmosis becomes possible, or under which it ceases to act.
One thing we must strive to impress forcibly on the reader, because, if
the notion is well grasped, it will enable him to understand plant life
so much more vividly. We allude to the continual changes that are going
on throughout the whole living fabric of the plant while in its active
condition. Cell membrane, the protoplasm, the entire mass of liquid
and solid constituents of which the plant consists, are, as we have
seen, made up of molecules, each, as it were, with a life of its own,
undergoing continual changes according to different circumstances, acting
and reacting one upon another so long as any active life remains.