RETURN OF THE SAP. 39 



economy, where the channels of distribution, 

 and the organs of propulsion are plainly obser- 

 vable, afford but imperfect analogies to guide us 

 in this intricate inquiry : for although it is true 

 that in the higher classes of animals the circula- 

 tion of the nutrient fluid, or blood, through dis- 

 tinct vessels, is sufficiently obvious, yet in the 

 lower departments of the animal kingdom, and 

 in the embryo condition even of the more perfect 

 species, the nutritious juices are distributed with- 

 out being confined within any visible vessels ; 

 and they either permeate extensive cavities in 

 the interior of the body, or penetrate through 

 the interstices of a cellular tissue. That this latter 

 is the mode of transmission adopted in the vege- 

 table system has been considered probable, from the 

 circumstance that the nutritious juices are diffused 

 throughout those plants w hich contain no vessels 

 whatsoever with the same facility as throughout 

 those which possess vessels ; from which it has 

 been concluded that vessels are not absolutely 

 necessary for the performance of this function. 

 The nature of the forces which actuate the sap 

 in its descent from the leaves, and its distribu- 

 tion to different parts, is involved in equal ob- 

 scurity with the nature of the powers which 

 contribute to its motion upwards along the stem, 

 from the roots to the leaves. In endogenous 

 plants the passage of the sap in its descent, is, in 

 like manner, through those parts which have 



