RETURN OF THE SAP. .33 



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 alburnum, or 

 outermost layer of the wood. With regard to the 

 exact channels through which it passes, the same 

 degree of uncertainty prevails as with regard to 

 those which transmit the ascending sap. De Can- 

 dolle maintains that, in either case, the fluids find 

 their way through the intercellular spaces : other 

 physiologists, however, are of opinion, that parti- 

 cular vessels are appropriated to the office of trans- 

 mitting the descending sap. The extreme mi- 

 nuteness of the organs of vegetables has hitherto 

 presented insuperable obstacles to the investigation 

 of this important question ; and consequently our 

 reasonings respecting it can be founded only on 

 indirect evidence. The processes of the animal 

 economy, where the channels of distribution, and 

 the organs of propulsion are plainly observable, 

 afford but imperfect analogies to guide us in this 

 intricate inquiry ; for although it is true that in the 

 higher classes of animals the circulation of the 

 nutrient fluid, or blood, through distinct vessels, is 

 sufficiently obvious, yet in the lower departments 

 of the animal kingdom, and in the embryo condi- 

 tion even of the more perfect species, the nutritious 

 juices are distributed without being confined within 



VOL. II. D 



