146 PHYSIOLOGICAL BOTANY. 



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. 



461. An impervious epidermis destitute of stomata every- 

 where clothes the roots, excepting its fibrillce and the tender 

 extremities of the rootlets. No part, therefore, is capable of 

 absorption except the latter. But these, by their multiplied 

 numbers, present to the soil an adequate absorbing surface. 

 Hence, in transplanting a tree, almost the only danger to its life 

 arises from the difficulty of preserving a sufficient number of 

 these rootlets. 



462. The force with which plants absorb fluids by their roots is surprisingly great. If 

 the stem of a Grapevine 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. Da. 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. 



463. 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 en- 

 dosmose by the closed cells, which are far more numerous. 



464. The effects of capillary attraction are seen in experiments like the following. In. 

 Bert 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. Or sus- 

 pend 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. 



465. Eiidosmose is thus illustrated: 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. The flawing in, as in the first case, is called 

 endosmose ; the flowing out, as in the second case, is called exosmose. (See Class Book of 

 Botany, 781-783.) Another explanation of absorption is given in 471. 



466. Other means of absorption. The office of absorp- 

 tion is not pei-formed by the root alone. Every green part, but 

 especially the leaf, is capable of absorbing gases and watery vapor. 



Every one knows how greatly plants, when parched and withered by drought, are re- 

 vived by a shower which sprinkles their leaves without reaching their roots. Air-plants, 

 oi epiphytes ( ?08), 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 a? the 

 Ilouseleek grow without roots, suspended by a thread in air, it is evident that all their 

 nourishment comes through their leaves. 



467. The use of absorption in the vegetable economy is not 

 merely r the introduction of so much water into the plant, but to 



