96 ABSORPTION AND EXHALATION. L.ECT. IV. 



however, that the identity of these phenomena should be 

 better demonstrated than it has been by Dutrochet's obser- 

 vations. Be it as it may, the explanation given by this 

 author is one which, in the present state of science, is the 

 least improbable. 



How does the juice ascend in a plant whose roots have 

 been removed, and whose lower extremity has been 

 plunged in water? The great height to which the liquid 

 can ascend in the trunk of a tree, appears to be opposed to 

 the explanation of the phenomenon by considering it as an 

 effect of imbibition or of capillarity; phenomena which, 

 we know, act within much narrower limits than those ob- 

 served in the trunks of trees. Hales, finding that the quan- 

 tity of sap which rises in a plant, was proportional to the 

 superficies of its leaves, concluded that in consequence of 

 the evaporation of the liquid contained in the superficial 

 cells of the leaves, these absorbed, by capillary attraction? 

 more liquid from the cells beneath, and that in this way a 

 kind of suction gradually became propagated to the cut ex- 

 tremity. 



By drying in different degrees some plants of the herb 

 mercury (Mercurialis annun^) Dutrochet ascertained that 

 they did not absorb proportionally to their degree of dry- 

 ness ; for one of the plants which had lost a third of its 

 weight by evaporation, absorbed much less than another 

 which had only lost a tenth. Notwithstanding that the de- 

 gree of dryness was greater, the absorption was less ; and 

 yet the plant had not been sufficiently dried for its texture 

 to have become altered. 



Evaporation or transpiration by the leaves is not then the 

 cause of the ascent of a liquid in the stem of a plant 

 plunged in water ; or, what amounts to the same thing, it 

 is not the cavities of the superficial cells which occasion the 

 ascent of the juice. The latter is not effected without the 



