68 THE ROOT. 



in Natural Philosophy, discovered by Dutrochct, which bears so strong a resem-' 

 blance to ahsoii^tion in Pliysiology, that late writers are generally agreed in ex- 

 plaining the latter by the former. It is, briefly, as follows : 



a. Let the broad end of a tunnel-sliaped glass be finnly covered with a piece of 

 bladder, and the cavity within be filled witli a solution of gum or sugai-. If now 

 the outer surface of the bladder be immersed in water, a passage of fluid will take 

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

 will be much increased, while at the same time there will be a current 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 

 current 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 

 inferred, tliat tlie conditions requisite for the action of these two currents are, two 

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

 these conditions exist, the current exists also. 



a. Now these conditions exist in the root. The spongiole is the porous sep- 

 tum ; the water around it is one of the fluids, and the other is the 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. 



160. The use of absorption in the vegetable economy is not merely 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 former will select the 

 sUex along with the water which it absorbs, for the construction of the more solid 

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

 the calcareous matter which the water holds in solution. 



