104 BIOLOGY. [BOOK n. 



the leaves is necessary to this aqueous aspiration, as the following 

 fact proves. 



Dutrochet having plunged into water a leaved stalk of Pisum 

 sativum, deprived it of its interstitial air by means of a pneumatic 

 pump. The aerian cavities, once purged of air, were promptly 

 invaded by water. The plant was then taken from the water 

 and the extremity of its stalk was alone kept in it. It was, in 

 this state, exposed to the diffused light, but it had become inca- 

 pable of aspiring water, and was asphyxiated, or rather it was 

 incapable of aqueous aspiration because it was asphyxiated. 



From the preceding facts it clearly results that the ascension 

 of the sap is bound in a certain measure to the functionment of 

 the leaves, and that this functionment, which comprehends simple 

 aqueous exhalation, is subordinated, like every physiological act, 

 to the absorption of oxygen by the anatomical elements, to 

 respiration. 



We must note a fact, whose explanation is however easy, 

 namely, that the absence of light has not on the green corollse 

 the action which it exercises on the chlorophyllian leaves. 

 As these corollse respire simply after the manner of the animal 

 anatomical elements, by absorbing oxygen and without acting on 

 the carbonic acid, the absence of light does not hinder them from 

 living, from aspiring sap, and so on. 



Once elaborated and concentrated by the special action of the 

 leaves, the sap comes forth from the cells by exosmosis, and just 

 as the endosmotic action of the radical spongioles has driven the 

 lymphatic sap from below to above, toward the branches and the 

 buds, the exosmotic impulsion drives back the elaborated sap 

 from above to below, from the leaves to the roots. We have 

 seen that the ascension was especially accomplished through the 

 ligneous centre, or at least through its youngest, peripheric part ; 

 the descent of the sap, on the contrary, is more habitually 

 achieved through the bark. In ascending it enriches itself more 

 and more, either carrying along with it the substances contained 



