68 Tllii ILOOT. 



iu Natural Philosophy, discovered' by Dutrochct, whicli beai-s so stiong a resem- 

 blance to absorption in Physiology, tliat late writers are generally agreed in ex- 

 plaining die latter by the former/i It is, briefly, as follows : 



a. Let the broad end of a tunnel-shaped glass be firmly covered with a piece of 

 bladder, and the cavity Avithin be filled with a solution of gum or sugar. If now 

 tlie outer surface of the bladder be immd'scd in water, a passage of fluid will take 

 place through the membrane into the glass, so that the volume of the solution 

 ^^'ill be much increased, while at the same time there will be a cun-ent in the 

 opposite direction, the solution within passing into the water without, but in a 

 much smaller quantity. If, on the other hand, the glass be filled with water and 

 immersed in the solution, it will be partly emptied by this action. ) The principal 

 cuiTcnt is termed endosmose (flowing inwards), and the other exosmose (flow- 

 ing outwards). 



159. From the above experiment, and others of a similar nature, it is justly 

 inferi'ed, /that the conditions requisite for the action of these two cuixents are, two 

 fluids of ^different densities, separated by a porous septum, or partition. Wherever 

 these conditions exist, the cun-ent exists also. ] 



a. Now these conditions exist in the root. ' The spongiole, is the poroiis sep- 

 tum ; the water around it is one of the fluids, and the other is tlie fluid within, 

 rendered dense by the admixture of the descending sap elaborated by the leaves. ') 

 Now if the absorption be the endosmose resulting from these conditions, there must 

 be the counter current, the exosmose, also. That this is actually the case, is proved 

 by the fact that the peculiar products of the species may always be detected in 

 the soil about the roots of the plant, and also, that a plant grown in water, always 

 communicates some of its peculiar properties to the fluid in which it is im- 

 mersed. 



IGO. The use of absoi-ption in the vegetable economy is not mei'cly the intro- 

 duction of so much water into the plant, but to obtain for its growth those min- 

 eral substances held in solution by the water, which constitute an important part 

 of its food. 



a. Now in accomplishing this object, the roots seem to be endowed with a cer- 

 tain power of selection or choice, which has not been satisfactorily explained. 

 Thus, if wheat be grown in the same soil with the pea, the fonner wiU select the 

 silex along with the water which it absorbs, for the constiiiction of the more solid 

 parts of its stem ; while the latter Mill reject the silex, and appropriate to its use 

 the calcareous matter which the water holds in solution. 



b. The flomng of the sap from incisions, in early spring, depends upon the 

 excess of absorption over exhalation. After the decay of the leaves in autumn, and 

 the consequent cessation of exhalation, — the roq|;lets, being deep in the ground, 

 below the influence of the frost, continue their action for a time, and an accumu- 

 lation of sap in the vegetable takes phice. Also, in early sjjring, before the leaves 

 are developed, this action recommences, and the plant becomes gorged with sap, 

 so that it will flow from incisions, as in the sugar-maple. But this flowing ceases 

 as soon as the buds expand into leaves and flowers. 



