CHAP. II.] VEGETAL NUTRITION. 105 



in the cells which it bathed in its passage, or by the special action 

 of the green leaves ; l in descending it is impoverished, on the 

 contrary, more and more, for in going along, either it relinquishes 

 organisable substances at the expense of which anatomical 

 elements are formed, or it simply deposits in already prepared 

 cavities masses of nutrimentary or organisable matters. 



We haye just described, in a fashion, the schematic mode of the 

 circulation of the sap ; doubtless there are numerous irregularities, 

 and the sap undergoes in its course diverse impulsions, diverse 

 deviations. The exosmotic impulsion is not the sole cause of 

 the descendent propulsion. We must likewise take into account, 

 as for the ascending sap, the relative void, made by the fixation 

 of a duliquid part, by the nutrition and the formation of the 

 tissues, and lastly, mechanical influences. For instance, it has 

 been remarked that the agitation of the stems by the wind 

 favours the growth of plants. Knight made on this subject 

 precise experiments by immobilising certain stems. We are 

 justified in thinking that the waving and the bending of the 

 stems have as a result local pressions which aid the march of the 

 sap in the canals. This descendent march of the elaborated sap 

 must be slower than that of the aqueous sap : for this time we 

 have before us a liquid, denser, more viscous, even sometimes 

 differentiated. In effect there are assuredly diverse species of 

 sap ; for we must consider as such the milky liquid called latex, 

 existing in great abundance in the cortical system of lettuce, of 

 the fig-tree, of the euphorbise, and so on. The gum of trees of 

 the Pinus kind, and so on, the resin of the conifers, are probably 

 also sap residua. 



1 Knight collected in the spring at various heights the sap of the sycamore 

 and of the birch. These are the variations of specific weight which he 

 observed on the sycamore : 



Level of the soil 1,004 



At seven feet 1,008 



At twelve feet 1,102 



In the last case the sap had moreover a sugary savour. 



