94 



ABSORPTION AND EXHALATION. 



LECT. IV. 



Fig. 6. 



Let us now examine whether the absorption of the nutri- 

 tive juices, which takes place by means of the roots, and 

 movement of these juices in the plants themselves, should 

 be considered as simple phenomena of capillarity, or im- 

 bibition. 



At the commencement of spring, the sap rises from the 

 roots to the leaves, through the central part of the trunk ; 

 and at this time a liquid, called the succus proprius, or 

 proper juice, whose composition is different from that of the 

 sap (or succus communis,) passes in a contrary direction ; 

 that is, from the leaves through the cortical tissues to the 

 roots. If we bore a hole to the centre of the trunk of a 

 growing plant, we can collect a considerable quantity of 

 sap, which is denser in proportion as it is taken higher up 

 towards the leaves. If, on the contrary, we pass a ligature 

 around the trunk, or if we re- 

 move a circular layer of the 

 bark, the swelling which we 

 shall soon see formed above 

 the knot or ring on the side of 

 the leaves, will prove the ex- 

 istence of the descending cur- 

 rent of the proper juice. Hales 

 has shown that the quantity of 

 liquid absorbed by a plant in 

 vegetation, increases in pro- 

 portion with the superficies of 

 its leaves ; a fact which he ex- 

 plains by ascribing it to evapo- 

 ration. 



This double movement of 

 juices in the interior of vege- 

 tables, is a thing inexplicable 

 by the mere forces of capil- 



Hales's experiment to ascertain the 

 force of the sap of the vine. 



