PLANT 583 terial in solution, and exhalation of watery vapor goes on rapidly in growing plants. This vapor is mainly exhaled through the stomata or leaf openings, and its amount is governed by the hygrometric condition of the atmos- phere ; in a damp day but little transpires ; in a long continued drought evaporation might continue greatly to the detriment of the plant were not the stomata so constructed (see LEAF) as to shut off nearly all communication with the interior of the leaf. The common agricul- tural plants during five months of growth ex- hale 200 times their dry weight of water. The tissues of the plant being gorged with liquid, when evaporation takes place in the leaves there is a demand upon the parts below to sup- ply this loss, and thus an upward flow is estab- lished from the roots toward the leaves, and the amount of water taken up by the root is in direct ratio to that given off by the leaves. If the demand made by the leaves is greater than the root can supply, the plant droops. It was formerly thought that the leaves absorbed water from the atmosphere, but experiment has shown this to be erroneous. Though the cells and ducts differ greatly in length, the tissues of the plant are really made up of closed cavities, the walls of which, though no open- ings have been detected, are permeable by liquids, and the movement of sap within the plant consists of a transference from one cell to another. Several physical forces are prob- ably concerned in the movement, specially that known as osmose. "When two liquids of different densities are separated by a per- meable membrane there is a tendency to inter- change, a small quantity of the denser liquid passing into the lighter, and a much larger quantity of the lighter liquid passing through the membrane into the heavier; this force, called osmose, explained at length in works upon physics, and easily demonstrated (see ENDOSMOSE), is sufficient to account for much of the sap movement and transference of the fluids from cell to cell; if the contents of the leaf cells are made denser by evaporation, the less dense liquids of the cells near by will flow toward them, and the movement be com- municated from cell to cell down to the roots. Some of the phenomena of the movement of liquids are attributed to capillary or surface at- traction, and the power of membrane to sepa- rate solutions of different kinds, discovered by Graham (see DIALYSIS), is evidently concerned in the nutrition of the plant. While there are no regular ascending and descending currents of sap, as was formerly supposed, the liquids in the different tissues of highly organized stems are not all alike ; the ascent of water or crude sap from the root is mainly through the newer wood, and the movement of formative materi- al or elaborated sap to contribute to the growth of parts below the leaves is through the young bast or inner bark. But growth is taking place in other parts of the plant, new stems and leaves are forming beyond the assimilating leaves, and in most cereal grains the growth of the fruit is quite above all the foliage and consequently not to be reached by descending sap. The motion of the assimilated material is toward all points in which growth is taking place, or to those in which it is to be stored as a reserve ; and whenever it is appropriated, either by forming new cells or stored up as starch, these points become centres of attrac- tion, and the movement of the material is in their direction. Automatic Movements and Irritability. The movements of plants may often be readily observed; many leaves take on a different position at nightfall, as may be seen in the common locust and wood sorrel (see OXALIS), and many flowers which close at night or in dull days ; in the evening prim- rose (see (ENOTHEKA) the petals open with a sudden jerk. The "movements of climbing plants " are admirably described by Darwin in a memoir with that title; the free end of a twining stem makes a rotary movement with a rapidity differing with the species, the hop on a warm day describing a circle in one to two hours ; when the stem comes in contact with a support, it winds around it; the phenomenon can only be observed when such stems extend above the support and are free to move, in which case the upper internodes are seen to revolve as if in search of something to twine upon. Tendrils move in a similar manner; when the hooked end of a tendril catches a twig or some other object, the tendril twists into a coil, as if by shortening itself to draw the plant nearer to the support, one half of the coiling being in an opposite direction to the other half ; if the tendril fails to catch hold of some object, it coils after a while, but in this case the spiral runs all in one direction. Ten- drils are irritable, and coiling may be induced by slight friction. These voluntary or rather automatic movements are very slow when com- pared with those of some of the lower algse, which like oscillaria are constantly writhing. In the higher plants this motion is seen in the flowers of some orchids, in which the lower petal or lip is raised and lowered with a regu- lar movement ; desmodium gyrans, sometimes seen in greenhouses, has trifoliate leaves, of which the two lateral are constantly in mo- tion, moving by a series of visible jerks day and night. These movements are independent of any external agency, while others are due to irritability, as may be conspicuously seen in the sensitive plant (see MIMOSA) and some oth- ers ; the irritability of tendrils is above referred to ; another striking instance of this property, apparently connected with something corre- sponding to a nervous system, is presented by the Venus's fly-trap. (See DION^EA.) Nor is this irritability confined to leaves; an Australian orchid shuts up its lip when an insect alights upon it and encloses the captive, and the sta- mens of the common barberry move if touched. Propagation of Plants ly Division. In many plants there is ample provision for multipli-
