88 ENDOSMOSE AND EXOSMOSR. 



action which takes place through it. Half-baked porcelain, and 

 a peculiar porous limestone, have been shown to have the same 

 properties ; so that it is evident, that this curious result is not 

 dependent upon any modification of vital power. It is, in fact, 

 but a peculiar form of capillary attraction.* 



119. Now the conditions requisite for this action, are two 

 fluids of different densities, separated by a septum or partition of 

 a porous character. This we find in the roots. The fluid in 

 their interior is rendered denser than the water around, by an 

 admixture (as will be hereafter explained, . 140) of the de- 

 scending sap ; and the spongiole supplies the place of the par- 

 tition. Thus, then, as long as this difference of density is main- 

 tained, the absorption of fluid may continue. But if the rise of 

 the sap is due to the action of Endosmose, there ought also ta 

 be an Exosmose. This is found to take place ; for if a plant be 

 grown with the roots in water, the fluid surrounding them is 

 soon found to contain some of the peculiar substances they form, 

 and which are contained in the descending sap ; thus a Pea or 

 Bean would disengage a gummy matter, a Poppy would com- 

 municate to the water an opiate impregnation, and a Spurge 

 would give it an acrid taste. 



120. Thus we see how beautifully and how simply this 

 action, extraordinary as it seems, is accounted for, when its 

 whole history is known, on principles which operate in other 

 departments of Nature. It has been asked, why should not 

 Endosmose take place, when the roots of a dead plant are put in 

 water ? This question may be answered by another ; why 

 does the wick of a lamp suck up oil, only when it is lighted ? 

 The answer to both is, that it is only when the fluid already 

 absorbed is in some way removed, that absorption can go on. 

 In the latter case, as fast as it is withdrawn by combustion, the 

 lower part of the wick raises it by capillary attraction. In the 

 former, as fast as the fluid is got rid of by exhalation from the 

 leaves (Chap, vin.), the Endosmose below will keep up the 

 supply ; but if the demand is suddenly checked, (as when the 

 plant is withdrawn from the influence of light, or its vitality be 



* See MECHANICAL PHILOSOPHY, . 28. 



