34 THE VITAL FUNCTIONS. 



especially in the returning sap, of all known plants, from its 

 bland and unirritating qualities, from its great solubility in 

 water, and from the facility with which other vegetable 

 products arc convcrtai)le into tliis product, Gum may be 

 fairly assumed to be the principal basis of vegetable nutri- 

 ment; and its simjilo and definite composition points it out 

 as being the immediate result of the chemical changes 

 which the sap experiences in the leaves. During the de- 

 scent of the sap, however, this iluid undergoes, in various 

 parts of the plant, a farther elaboration, which gives rise 

 to other ])roducts. We arc 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 ex- 

 act channels through which it passes, the same degree of un- 

 certainty prevails as with regard to those which transmit 

 the ascending sap. I)e Candollc maintains that, in either 

 case the fluids find their way through the intercellular 

 spaces: other physiogists, however, are of opinion, that par- 

 ticular vessels are appropriated to the ofTice of transmitting 

 the descending sap. The extreme minuteness of the organs 

 of veo-etables has hitherto presented insuperable obstacles to 

 the investigation of this important question: and consequent- 

 ly our reasonings respecting it can be founded only on in- 

 direct evidence. The processes of the animal economy, 

 where the channels of distribution, and the organs of pro- 

 pulsion 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 sufficient- 

 ly obvious, yet in the lower departments of the animal king- 

 dom and in the embryo condition even of the more perfect 

 species, the nutritious juices are distributed without being 

 confined within any visible vessels; and they either perme- 



