.18 THE VITAL FUNCTIONS. 



to be the principal basis of vegetable nutriment ; 

 and its simple and definite composition points 

 it out as being the immediate result of the che- 

 mical changes which the sap experiences in the 

 leaves. During the descent of the sap, however, 

 this fluid undergoes, in various parts of the plant, 

 a further elaboration, which gives rise to other 

 products. We are now, therefore, to follow it in 

 its progress through the rest of the vegetable 

 system. 



The returning sap descends from the leaves 

 through two different structures : in exogenous 

 plants the greater portion finds a ready passage 

 through the liber, or innermost layer of bark, 

 and another portion descends through the albur- 

 num, or outermost layer of the wood. With re- 

 gard to the exact channels through which it 

 passes, the same degree of uncertainty prevails 

 as with regard to those which transmit the as- 

 cending sap. De Candolle maintains that, in 

 either case, the fluids find their way through the 

 intercellular spaces : other physiologists, how- 

 ever, are of opinion, that particular vessels are 

 appropriated to the office of transmitting the des- 

 cending sap. The extreme minuteness of the 

 organs of vegetables has hitherto presented 

 insuperable obstacles to the investigation of this 

 important question ; and consequently our rea- 

 sonings respecting it can be founded only on 

 indirect evidence. The processes of the animal 



