ABSORPTION. 153 



780. EXP. Thrbw 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 with- 

 out. The former is a case of endosmose (ivdov, inwards, /z<D, to seek), the latter of 

 exosmose (e&, 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 up- 

 ward pressure of 4J atmospheres (Dutrochet). The great force with which the cap- 

 sule 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 dissev- 

 ered 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. 



CIRCULATION. 



787. TENDENCY OP THE FLOW. The fluids which are thus taken 

 into the system by absorption can not remain inactive and stagnant. 

 As their inward flow is regular and constant in its season, so must be 

 their upward and outward flow, in a course more or less direct, toward 

 the parts where they find an outlet or a permanent fixture. 



788. IN THOSE CRYPTOGAMS which are composed of cellular tissue 

 alone the circulation of the sap consists only of a uniform diffusion 

 from cell to cell throughout the mass, as through a sponge. 



789. IN THE HIGHER PLANTS, the different tissues perform appropriate 



