SECT* II. ASCENT OF THE SAP. 107 



cision is made in the bark, a fluid is found to exude 

 from the lower lip, and that consequently the sap 

 ascends by it. But in order to make this argument 

 good, the fluid must first be proved to be sap, or at 

 least to afford the presumptive evidence of continu- 

 ing to flow for a considerable length of time ; as it 

 is known that the vessels of plants will empty 

 themselves at both ends when cut horizontally, as 

 any one may see merely by cutting in two the stem 

 of any species of Spurge ; so that the mere exuda- 

 tion of a fluid from the lower lip, is no proof that 

 it proceeds from the ascending sap since the vessels 

 might thus empty themselves if they but contained 

 even a fluid descending. 



It was further contended that old Willows and 

 several other sorts of trees will still continue to 

 vegetate even when the whole of the woody part of 

 the trunk is decayed, and nothing but bark remain, 

 ing. But this is not exactly the fact ; for in the 

 case alluded to, there will always be found to be 

 more or less of wood immediately under the bark; 

 so that the ascent of the sap through the channel of 

 the bark is by no means established. 



The second opinion does not seem to have ,been 

 entertained by any very distinguished phytologists 

 namely, that of its ascending between the bark 

 and wood ; but it seems to have been entertained 

 by those who held it, because much juice is found 

 there ; because the wood is formed there ; and be* 

 cause the graft takes effect there. 



