200

ENDOSMOSE-EXOSMOSE.

osmotic action may be easily observed, by filling a bladder with colored syrup, attaching to its open end a glass tube, and then immersing it in a vessel containing water (Fig. 236). Under such circumstances the volume of the denser fluid in the interior of the bladder becomes increased (as will be at once seen by its rise in the tube), by the more rapid passage through the membrane of the thinner fluid than of the thicker, though at the same time a less portion of the syrup passes out into the water or thinner fluid, as may be proved by the sweet taste and color which the latter gradually acquires. This double current will continue as long as there is any material difference of density between the two liquids. The stronger ingoing current is termed endosmose, and the weaker outgoing current exosmose. If the position of the liquids be reversed, the currents will be reversed in like manner, the preponderating current, in almost all cases, being that which sets from the thinner to the denser liquid.

The absorption and transmission of liquid matters through cells is now easy to explain, for, as the fluid contents of the cells of the roots of plants are denser than the water contained in the media in which they grow, they will continually absorb the latter by endosmose; and, as the changes which are going on in the cells of plants by evaporation, assimilation, and other processes, tend to thicken their contained liquids, there will also be a constant passage of the absorbed fluids from cell to cell toward those parts where such processes are taking place. The laws of ordinary adhesive or capillary attraction and of the diffusion of fluids also regulate the flow of the juices, which in certain cases may be even set in motion by either force. The action, however, of the intervening membrane (cell-wall), in greatly modifying or even overcoming osmotic action, is evidenced by the numerous cases in which neighboring cells contain different substances without their intermixture. In cellular plants, such as Algæ and

MOVEMENTS OF CELL-CONTENTS.

201

Fungi, absorption may take place

at any part of the thallus; while in vascular plants it occurs principally through the roots, though all the green parts may contribute to it (see page 204), and that, too, probably independently of the presence or absence of stomata.

(3) Movements of the Cell-contents.-In many cells, and probably in all at a particular period of their life, when they are in a vitally active state, a kind of movement of a portion of their contents takes place. This movement is due to a circulation of the protoplasm, which is rendered visible by the opaque granular particles which it contains

다

FIG. 238.-Cells of the leaf of Vallisneria spiralis, showing the cir

(Figs. 237 and 238).

FIG. 237.-Hair on calyx of flowerbud of Althea rosea. The streaming of the protoplasm is indicated by the arrows. (After Sachs.)

circulating does not

The protoplasm thus pass from one cell to another, but is strictly confined to the cell in which it originates. This kind of movement has been termed Rotation, Gyration, or Intracellular Circulation; it ceases, in the generality of cases, in cells when they have attained a certain size, but in those of many aquatic plants it continues throughout their life.

The appearances presented by these culating current movements vary in different cases. Thus, with its granular contents, passing in the cells of many hairs, as in those of

up one side of

each cell, across, Althea rosea (Fig. 237), the protoplasm be

and down on the

direction of the

other side. The comes hollowed out, and the motion is in currents is indi- reticulated currents, radiating apparently from, and returning to, the nucleus; to

cated by the ar

rows.

202

MOVEMENTS OF CELL-CONTENTS.

this action the term circulation is applied. In the cells of the leaves of the Vallisneria (Fig. 238) and Anacharis, and in those of other parts of the same plants, intracellular movements may be readily observed when they are submitted to a moderate microscopic power; here, however, the protoplasm does not become hollowed out, but with its granular contents will be seen to pass round the interior of the walls of each cell, retaining its activity permanently; this movement is called rotation. In the Characea, however, and especially in the Nitella, the moving

FIG. 239.-A small portion of a species fied. The branches

of Chara, magni

are arranged in a

whorled manner. The contents of each cell exhibit a

kind of circulation. The direction of this circulation is indicated by the ar

rows.

protoplasm does not rotate round the walls, nor in reticular currents, but passes obliquely up one side of the cell (Fig. 239) until it reaches the extremity, and then flows down in an opposite direction on the other side.

No satisfactory explanation has yet been brought forward to account for this movement, but it is unquestionably connected with the vitality of the cell-contents, and all agents that actually injure the cell will generally stop it at once, and permanently, though in some plants (as Chara) a large cell may be tied across the middle with the effect of stopping the circulation temporarily; but after a short time it will recommence in each half.

(4) Elaboration of the Cell-contents. All cells exposed to light, heat, and air which contain protoplasm have the power of producing in their contents the different organic compounds which are concerned in the development of new tissues, and in the formation of others which have been termed secretions. (See page 217.) In old cells the secretions of the plant are also, in part, deposited.

2. Functions of Prosenchymatous Cells.-Pros

PROSENCHYMATOUS CELLS-VESSELS. 203

enchymatous cells are especially adapted, by their construction and mode of combination into a tissue, for giving strength and support to plants. In a young state also, before their walls are thickened, they appear to be the main agents by which the fluids absorbed by the roots are carried upward to the leaves and other external organs, to be elaborated by the agency of heat, light, and air.

3. Functions of Vessels.-The functions of the spiral, annular, reticulated, pitted, and scalariform vessels have been a subject of much dispute from an early period, and have been repeatedly investigated. Hales, Bischoff, and others came to the conclusion that these vessels were carriers of air, and it is certain that air alone is found in old vessels; while Dutrochet, Link, Rominger, etc., believed that their essential function was to carry fluids from the root upward, which views from recent observations appear to be correct. The experiments of Herbert Spencer, conducted with great care, tend to show that, in young plants at all events, the vessels are the chief sap-carriers, whence the fluid exudes into the surrounding prosenchyma.

Functions of Laticiferous Vessels.-The physiological importance of these vessels has given rise to much discussion, and is still involved in obscurity. But it would appear that they act as reservoirs of nutrient fluids, and also as carriers of such fluids to those parts of plants where they are required.

4. Functions of Epidermal Tissue.—The special functions of epidermal tissue are: To protect the tissues beneath from injury, and from being too rapidly affected by atmospheric changes; to regulate the transpiration of watery fluids; to absorb and exhale gaseous matters; and probably, to some extent, to absorb water. The epidermis itself is specially designed to prevent a too ready evaporation of fluid matters from the tissues beneath the stomata, facilitate and regulate the passage of fluid matters, and,

204

EPIDERMAL TISSUE-STOMATA.

in proportion to their number will be the exhalation from them.

It is also through the cells of the epidermis, and more especially through the stomata, that certain gaseous matters are absorbed from, and exhaled into, the atmosphere, in the processes of Respiration and Assimilation. (See pages 213 and 217.)

It has long been a disputed question whether the epidermal tissue and its appendages have the power of absorbing liquids, such as water. But the recent researches of Henslow seem to prove that leaves can absorb moisture. (See page 213.) Indeed, it is very difficult to account for the immediate recovery of drooping plants in a greenhouse when water is sprinkled upon the floors, or the revival in nature of vegetation when a mist follows a long succession of dry weather-except on the supposition that watery vapor is taken up by the epidermal tissue and its appendages, unless the presence of moisture acts only in the way of checking transpiration.

Origin and Development of Stomata.-A stoma is formed by the division of an epidermal cell (the mother cell) by a partition which extends across and divides the two daughter or sister cells (Fig. 240, s); this partition then becomes thickened, especially at the angles where it

FIG. 240.-, . Parenchyma of the leaf. e, e. Epidermis cells. s. Stoma. i. Air cavity. In these figures the development of the stoma of Hyacinthus orientalis is represented from the first division of the mother-cell in A into two daughter cells, to the complete separation shown in D. (After Sachs.)

joins the walls of the parent cell. ened partition becomes laminated,

After a time the thickwhen a cleft appears in