36 On the MO T 1 N of 



THE afcent of the fap was afcribed by LUDWIG *, to the expan- 

 fion of the air with which it is impregnated ; and, with more 

 reafon, by others, to the expanfion of the air contained in the 

 trachea, or air-veflels of the plant. As thefe veffels exifl only 

 in the wood, and as it is by the wood chiefly that the afcending 

 fap is conveyed, this, no doubt, forms a prefumption, that they 

 contribute, in fome degree, to the elevation of the fap. But, 

 as we have found, that the fap moves with more force upwards 

 than in any other direction ; and that, in certain circumftances, 

 it is made to flow and afcend by cold as well as by heat, the 

 expanfion of air cannot, therefore, be admitted, in any fhape 

 whatever, as the caufe of its afcent. 



THE above experiments leave us flill in the dark, as to the pre- 

 cife caufe of the afcent of the fap. They fhew, indeed, upon 

 many occafions, that heat is the prime agent in producing this 

 effecT: ; and that, probably, by the expanfion of the fap itfelf, 

 rather than of any air, either contained in it, or in the trachea. 

 The incifions upon the birch ran freely in the day-time, efpeci- 

 ally during funfhine, but dried up regularly, as the cold of the 

 evening advanced. With a few exceptions, we find the afcent 

 of the fap conflantly promoted by heat, but retarded and even 

 arrefted by cold : Yet the precife manner in which heat and cold 

 produce thefe effects does not appear. It is likely that there are 

 other caufes which co-operate. Thefe probably are lodged in 

 the ftruclure of the plant, and to difcover them, would require 

 a more minute examination of that ftruclure, than has as yet 

 been beftowed upon it. 



5- 



FROM the preceding experiments, we may now attempt a fo- 

 lution of that curious queftion in vegetation, why the termina- 

 ting buds of branches are the firft which difdofe themfelves in 

 the fpring ? This phenomenon has been often imagined, and 



particularly 



* LUDWIG Inftitutiones Regni vegetabilis, Lipf. i"JS7> 8vo. p. 183. 



