770. Office of the root. The absorption of liquids, containing in solution the food of the plant, is the peculiar and indispensable office of the root, as may be shown by an
771. Experiment. Take a small growing plant from the earth and immerse it by its roots, which should be nearly or quite entire, in a cup containing a definite quantity of water. Place near it another cup with a like quantity of water to indicate the amount of evaporation. The difference of the diminution in the two caps will be the amount of absorption. A plant of spearmint has thus been found to absorb more than twice its own weight per day. Every one is familiar with the rapid disappearance of water from the roots of potted plants, as Hydrangea, Oleander.
772. The absorbents. An impervious epidermis destitute of sto-mata everywhere clothes the roots, excepting its fibrillae and the tender extremities of the rootlets. No part, therefore, is capable of absorption except the latter. But these, by their multiplied numbers, present an adequate absorbing surface to the soil.
773. Experiment. Let a growing radish be placed in such a position that only the fibrils at the end may be immersed in water; - it will continue to flourish. But if the root be so bent that the fibrils shall remain dry while the body of the root only is immersed, the plant will soon wither, but will again revive if the fibrils be again immersed.
774. Inference. Hence, in transplanting a tree almost the only danger to its life arises from the difficulty of preserving a sufficient number of these rootlets.
775. The force with which plants absorb fluids by their roots is surprisingly great, as shown by
776. Experiment. If the stem of a grape-vine be cut off when the sap is ascending, and a bladder be tied to the end of the standing part, it will in a few days become distended with sap even to bursting. Dr. Hales contrived to fix a mercurial gauge to a vine thus severed, and found the upward pressure of the sap equal to twenty-six inches of mercury, or thirteen pounds to the square inch.
777. But what causes this absorption of fluids in a direction contrary to gravitation? In explanation of this phenomenon reference has been made to two well-known principles in physics, viz., to capillary attraction by the tubular vessels and to endosmose by the closed cells, which are far more numerous.
778. Experiment. Invert the end of several open thermometer tubes in a colored liquid. It will be seen rising in the tubes above its level, to various heights - highest in the smallest calibre.
779. Exp. Suspend a napkin in such fashion that its lowest corner shall dip into a cup of water. In a few hours the water will have ascended into the napkin. These are results of capillary attraction.
780. Exp. Throw dried prunes, currants, or raisins into water. After a while they will have become swollen and distended with fluid Now place them in strong syrup; they will again shrink.
781. Exp. Attach a bladder filled with syrup to a long glass tube, and immerse in water. The water flows in and the mixture arises slowly but forcibly in the tube. Reverse the liquids. Pure water from within the bladder will flow into syrup without. The former is a case of endosmose (εvδov, inwards, µω, to seek), the latter of exosmose (εξω, outwards).
782. Direction of the currents. The flow will continue until the two fluids are equal in density. In both cases there is also a flowing of syrup into the water, but the greater flow is always from the lighter into the denser fluid.
783. The force of endosmose is found to depend upon the excess in density of the inner fluid Syrup, with the density of 1.3, caused a flow of water with an upward pressure of 4 1/2 atmospheres (Dutrochet). The great force with which the capsule of the squirting cucumber (§ 606) bursts shows the power of endosmose. But a more probable theory is stated in § 791.
784. The use of absorption in the vegetable economy is not merely the introduction of so much water into the plant, but to obtain for its growth the elements of its food held in solution, whether gaseous or earthy. In attaining this object, the roots seem to be endowed with a certain power of selection or choice which we can not explain. Thus, if wheat be grown in the same soil with the pea, the former will select the silica along with the water which it absorbs in preference to the lime; the pea selects the lime in preference to the silica. Buckwheat will take chiefly magnesia, cabbage and beans, potash. This fact shows the importance of the rotation of crops in agriculture.
785. Other means of absorption. The office of absorption is not performed by the root alone. Every green part, but especially the leaf, is capable of absorbing gases and watery vapor.
786. Proofs. Every one knows how greatly plants, when parched and withered by drought, are revived by a shower which sprinkles their leaves without reaching their roots. Air plants or epiphytes (§ 143), such as the long-moss and Epidendrum, must rely on this source chiefly for the supply of their food; and when the dissevered stems of such plants as the houseleek grow without roots, suspended by a thread in air, it is evident that all their nourishment comes through their leaves.