-32 THE VITAL FUNCTIONS. 



imbibition, in contradistinction to that by circu- 

 lation; a term, which, as we have seen, implies, 

 not merely a system of canals, such as those ex- 

 isting in Medusae, where there is no evidence of 

 the fluids really circulating, but an arrangement 

 of ramified vessels, composed of membranous 

 coats, through which the nutrient fluid moves in 

 a continued circuit. 



The distinction which has thus been drawn, 

 however, is one on which we should be careful 

 not to place undue reliance ; for it is founded, 

 perhaps, more on our imperfect means of investi- 

 gation, than on any real differences in the proce- 

 dures of nature relative to this function. When 

 the juices, either of plants or of animals, are trans- 

 parent, their motions are imperceptible to the eye, 

 and can be judged of only by other kinds of evi- 

 dence ; but when they contain globules, differing 

 in their density from that of the fluid, and there- 

 fore capable of reflecting light, as is the case 

 with the sap of the Chara and Caulinia, we have 

 ocular proof of the existence of currents, which, 

 as long as the plant is living and in health, pur- 

 sue a constant course, revolving in a regular and 

 defined circuit ; and all plants which have milky 

 juices exhibit this phenomenon. Although the 

 extent of each of these vegetable currents is very 

 limited, compared with the entire plant, it still 

 presents an example of the tendency which the 

 nutrient fluids of organized structures have to 



