THE INTERCELLULAR SYSTEM.

205

it, narrower in the middle, wider without and within, which unites the intercellular space (Fig. 240, D, S, t) with the external air. Before the parent cell divides, a cuticularization of its surface also takes place, the cuticle extending over the apposed surfaces of the sister cells, and the adjoining cells of the epidermis. Even when the division is complete, a portion (if the leaf is examined in a superficial position) still remains as a simple lamella. These two sister or daughter cells are called guard cells, and further differ from the rest of the epidermis in containing chlorophyl and starch.

5. Functions of the Appendages of the Epidermis.-Hairs and their modifications appear to be designed to protect the epidermis and parts beneath from injury due to cold and other external influences, hence we find young buds (page 54), etc., frequently coated with hairs. Hairs also appear in certain instances to absorb fluid matters from the atmosphere, and to assist the epidermis in restraining exhalation. Plants which are densely coated with them are well adapted to grow in very dry situations, and to sustain without injury a season of drought.

6. Functions of the Intercellular System.-The intercellular canals, except at those times in which the tissues of the plant are gorged with sap, as in the spring of the year, are filled with air, and the special function which they perform is to allow a communication between the external air and the contents of the internal tissues by virtue of the laws regulating the diffusion of gases. They likewise facilitate exhalation of liquid matters by their connection with the stomata. The intercellular spaces are also, in most cases, filled with air; while the air-cells and aircavities, as their names imply, are in like manner filled with aëriform matters, and in water-plants are especially designed to diminish the specific gravity of the parts in which they are found, and thus to enable them to float readily, or to be suspended in the water.

206

ABSORPTION BY THE ROOT.

SECTION 2. PHYSIOLOGY OF THE ORGANS OF NUTRI

TION.

1. Of the Root or Descending Axis.-The offices performed by the root are: 1. To fix the plant firmly in the earth or to the substance upon which it grows, or, in some aquatic plants, to float or suspend it in the water. 2. To absorb liquid food. 3. According to some authors, to excrete into the soil certain matters which are injurious, or at least not necessary for the healthy development of the plant, though in the earth they may assist subsequent absorption by dissolving substances which could not otherwise pass into the plant. 4. To act as a reservoir of nutri

ment.

Absorption by the Root.-The function which the root performs of absorbing food for the uses of the plant, from the materials in or upon which it grows, is not possessed by its whole surface, but is confined to the cells and root-hairs (Fig. 109) of the newly developed portions and young parts adjacent to them; and even these parts can only absorb when they are in the closest contact with the particles of soil by the root-hairs. Hence in the process of transplanting, it is necessary to preserve the young growing rootlets as far as possible, otherwise the plants thus operated upon will languish or die, according to the amount of injury they have sustained.

This absorption of food by the youngest rootlets is due to osmose taking place between the contents of their cells and the fluids of the surrounding soil. (See page 200.)

Roots absorb more water than the plant requires, and this excess of fluid exerts a pressure up the stem called Root pressure, which may be measured by cutting off the upper part of the stem of a growing plant and attaching a manometer to the cut end. (See pages 233 and 234). substances in a liquid state,

Roots can only absorb

therefore the different inorganic substances which are de

SELECTION BY ROOTS.

207

rived from the soil, and which form an essential part of the food of plants, must be previously dissolved in water. If the roots of a freely growing plant be placed in water in which charcoal in the most minute state of division has been put, as that substance is insoluble in the fluid, it will remain on the surface of the roots, and the water alone will pass into them.

Selection of Food by Roots.-Numerous experimenters have proved that when the roots of living plants are put into mixed solutions of various salts, some will be taken up more freely than others. Again, though the seeds of the common Bean and Wheat be sown in the same soil, and exposed, as far as possible, to the same influences in their after-growth and development, yet chemical analysis shows that the Wheat stalk contains a much larger proportion of silica (which it must have obtained from the soil) than that of the Bean.

The experiments of others, again, appear, on the contrary, to indicate that roots absorb all substances presented to them indifferently, and in equal proportions. But the simple fact, as just mentioned, which is easily proved by chemical analysis-that the ashes of different plants contain different substances or in very different proportions-seems to prove incontestably that roots have a power of selecting their food. In using the term selecting, we do not, however, intend to imply that roots have any inherent vital power of selection resembling animal volition, but only to express the result produced by virtue of the mutual actions of the root and the substances which surround it in the soil. This power or property of selection is without doubt due to some at present little understood molecular relation which exists between the membranes of the cells of different plants and the substances which are taken up or rejected by them, different roots possessing different osmotic action for the same sub

stances.

208

DEVELOPMENT OF ROOTS.

Excretion by Roots.-Roots seem to have no power of getting rid of excrementitious matters like that possessed by animals; but that they do throw off into the soil a portion of their contents by a process of exosmose, which appears to be an almost necessary result and accompaniment of the endosmose by which absorption takes place, is most probable. Carbon dioxide, and possibly other acid substances, are parted with by roots in this way; and thus assist subsequent absorption by dissolving substances which could not otherwise pass into the plant.

Storing of Nutriment by Roots.-Roots frequently act as reservoirs of nutriment in the form of starchy, gummy, and similar matters for the future support of the plant. The tubercles of the Dahlia and Orchis, and the roots of the Turnip, Carrot, and other biennials, are familiar illustrations.

Development of Roots.-The growing part of the root is called the growing point (punctum vegetationis). It is commonly spoken of as the apex of the root, but is not really so, since it is covered with a cap of cells, the pileorhiza. (See page 62.) The cells composing it consist of primary meristem ;* they are thin-walled, filled with protoplasm, and are capable of division. Here, as in stems, and unlike leaves, the last formed part is toward the apex; hence the growth in length is indefinite, the difference between the growing part or so-called apical cell in roots and stems being that, in the former case, it or they (for there is frequently a group of apical cells) are covered by a cap of cells (Fig. 241, k, l, m, n) formed from the distal or apex end of the so-called apical cell, v; whereas in stems there is no such cap. (See page 211 and Fig. 241.)

*This name is given to that kind of meristem which forms the whole tissue of very young organs or parts of organs, in order to distinguish it from another kind of meristem, termed secondary meristem (page 44), which occurs in organs along with permanent tissue, or that tissue in which the cells are no longer capable of division.

THE STEM-WOOD.

m

v

n

ས

209

2. Of the Stem or Caulome.-The offices performed by the stem and its ramifications are: To form a support for the leaves and other appendages of the axis, which have but a temporary existence, and thus enable them to be freely exposed to the influences of light and air, which are essential for the proper performance of their functions and development; 2. To convey air and fluid matters upward,

с

FIG. 241.-Longitudinal section through the end of the root of Pteris hastata, showing apical region. v. Apical cell, from which are developed the tissue of the substance of the root, o, c, and the root-cap or pileorhiza, k, l, m, n. (After Sachs.)

downward, and inward, to those parts of plants where active chemical and other changes are going on; and 3. To act as a reservoir for the so-called secretions of the plant.

Special Functions of the different Parts of the Stem.-(1) The Medulla or Pith.-Various functions have at different times been ascribed to the pith. In the young plant, and in all cases when newly formed, the cells of the pith are filled with a greenish fluid containing nutrient substances in a state of solution; but as the pith increases in age it loses its color, becomes dry, and is generally more or less destroyed. The pith, therefore, would appear to serve the temporary purpose of nourishing the parts which surround it when they are in a young state; and in some cases it seems also to act as a reservoir of the so-called secretions of the plant.

(2) The Wood. The wood, when in a young and pervious condition (alburnum), is the main agent by which the crude sap is conveyed upward to the external organs to be aërated and elaborated; but whether the passage is primarily by the vessels or the prosenchymatous cells is