What we call individual plants are complex communities of real
microscopic individuals, which biologists call cells. These are
associated in numerous sub-communities differing from one another in
structure and function. Their specialization results in a division
of labor and a correspondingly large total efficiency, much as
specialization and division of labor lead to efficiency and productive
possibilities in a nation consisting of states, and these of smaller
communities made up of trades, guilds and professions, which in
co-operation follow the manifold activities that characterize a nation
and collectively constitute the national life of its individuals, which
is far more effective and greater than the individual life of any one
person or class.
The active, living part of a cell is its protoplasm--the physical basis
of life, as Huxley called it in animals and plants alike. Commonly this
protoplasm encloses itself by a wall of cellulose, an organic substance
manufactured by the protoplasm. Where two cells are in contact, they
are usually flattened against one another. When men first began to use
the microscope, only a little over two centuries ago, it was the walls
and shapes of cells that attracted attention, and the resemblance to
honey-comb on a small scale was so striking that the cavities were
naturally called cells.
Protoplasm itself is a very complex substance, chemically, and even the
much simpler cell-wall is far from being always really one identical
substance. A considerable part of the thickening of matured cell-walls
has been laid down on the original partition between two cells, and not
only differs from this but is not alike in different kinds of cells,
and in structures like wood and cork it is impregnated with other
materials that affect the cell-wall very greatly in such respects as
hardness and permeability to water.
The shells of nuts, for instance, are so impervious that they are
commonly “stratified” by planters so that their hard shells may
disintegrate more or less as a preliminary to germination, a process
that not infrequently requires more than a year unless hastened by some
expedient like that of passing haw fruits through the digestive mill
of poultry as a means of softening their boney cores, of filing the
hard envelope, which is a favorite trick of gardeners with nut-like
fruits of the lotus or with canna seeds. (This is similar to the
scarifying of sweet clover seed.--Editor.) This is the reason that
several times as much clover seed--even good seed--must be used on an
acre as seems necessary for securing the desired number of plants. One
of these modifications is usual in the outer layers of cell-walls on
the surface, and it is called cuticularization. Cuticularized walls
are more or less completely water-proofed. When the cells that produce
nectar are at the surface, their outer walls are cuticularized in this
way; when they are within the nectary and the nectar passes out through
stomata, this is scarcely, if at all, the case.
The greater part of nectar is water, which reaches the surface from
within the plant cells. To do this it must pass through walls that
are little, if at all, cuticularized, or it must break through the
cuticle. This does not mean that it must break through the entire
cell-wall; a small part if this is modified by the protoplasm into a
gum or mucilage, or some similar substance, and the water accumulates
in this layer and swells it until the overlying cuticle is burst. Some
form of sugar is a frequent result of this disintegration of cellulose.
Dissolved sugars pass through the ordinary cellulose wall, but they do
not pass through the ordinary surface layer of protoplasm in the outer
cells.
When water is separated from a solution like that of sugar by a filter
of this sort, which allows water to pass but is not permeable to the
dissolved substance, the action is set up that physicists call osmosis,
and water accumulates on the side of the dissolved substance until it
exercises a very considerable pressure. This action not only bursts
the cuticle, when it starts beneath it, but results in a flow of water
within the plant at that point.
The absorbing roots of plants show another result of this physical
property, osmosis. They are not waterproofed; water is continuous
through them, from the thin layer in which it occurs about particles
of the soil, to the water which composes a great part of the weight of
the protoplasm within the cells. This sap of the root cells contains
dissolved sugar and other osmotic substances. Osmotic absorption by
the roots results in a pressure of several atmospheres. This pressure,
passed from cell to cell, gives the crispness to fresh celery. Its
loss, through evaporation from the leaves, results in the loss of this
crispness, or wilting.
When evaporation is slight, as in a saturated atmosphere, water exudes
at the surface through pores such as occurs at the tip of a young grass
or clover leaf. Water pores of this sort are common. They are regarded
as pressure-valves by many botanists. The water that they eliminate is
usually filtered by the protoplasm that it passes through, which does
not allow the passage of substances dissolved in the cell sap; but some
plants which grow where they absorb very “hard” water pass lime salts
out through their water pores to such an extent that they are encrusted
with lime as the water evaporates.
[Illustration: Part of flower of _Geranium pyrenaicum_. (_n_)
nectaries; (_et_) stamens. Greatly magnified. Copied from Bonnier’s
“Les Nectaires.”]
The safety-valve elimination of water under strong internal pressure
and lessened normal evaporation is hardly to be called excretion or
accretion; the extruded-water is neither by-product nor manufactured
output. The elimination of lime appears to be on the border line of
excretion.
Nectar is not merely water; if it were its production would be more
easily understood. To the taste, it is sweet; to the sense of smell,
it is often fragrant; occasionally it is poisonous; often it is
somewhat colored. Commonly it is very fluid, but in the nectar-cups
of poinsettia it becomes very gummy. These properties come from
substances--sugars, volatile oils, poisonous organic compounds--that
were made by and in the plant, and they differ in different kinds of
plants. Whatever bees or ants may do in changing nectar into honey,
they do not entirely change or remove these substances, and the rank
brown honey of the drug store is as easily run to its source as the
popular white clover honey, the daintily flavored product of western
alfalfa, the aromatic acid honey of the red raspberry, or the greenish
product of the sweet clover with its delicate vanilla-like aroma, the
cumarin source of which shows itself in an occasional head-ache, much
as the minor organic constituents of some honeys derived from the heath
family now and then prove seriously poisonous.
A fluid that contains these organic substances necessarily falls into
the category of excretions or secretions, according as it represents
waste or usable material. As either excretion or secretion it is the
product of specialized organs, glands, and its appearance marks these
glands as in action or performing their function. Whatever else may be
involved, this depends upon the activity of their protoplasm, or is
controlled by it. When this is killed, secretion or excretion stops.
[Illustration: All parts of plants are composed of individual cells
flattened against one another. This figure copied from Bonnier’s “Les
Nectaires,” exhibits a longitudinal section of a stamen in _Colinsia
bicolor_. Magnified.