SAP in TREES. 35 



fap is confined to the wood of the tree, and that no part of it 

 moves between the wood and the bark. In many of the above 

 experiments, I was convinced, that this opinion has arifen from 

 fome miftake. The flow of the fap, between the wood and 

 bark, was daily and evidently perceived \ likewife, that it was 

 there always in an afcending (late ', and that it even moved more 

 freely, and with greater rapidity, in this channel, than in any 

 part of the fubflance of the wood. 



4. 



THE caufe of the afcent of the fap in trees, is a curious and 

 important point in the hiflory of vegetation, but ftill involved in 

 obfcurity. Many opinions have been formed on the fubjecT:, 

 but thefe only at random, or, at beft, only from a general view 

 of vegetation. The caufe ftill remains fecret ; nor is it likely 

 that it will ever be brought to light, but by means of minute 

 and accurate experiments. 



THE afcent of the fap has been afcribed, by fome, to fermen- 

 tation *, and, by others, to a certain force communicated to it 

 from the root ; but without any evidence to mew, that fuch 

 caufes even exifl. 



THE afcent of the fap by filtration was an opinion rather more 

 plaufible j but it is oppofed by fome of the principal phenome- 

 na. Did the fap afcend like water in a fponge, or in capillary 

 tubes, why mould its motions be affected, nay even almoft en- 

 tirely regulated by heat and cold ? Or why, as has been found 

 in thefe experiments, fhould its motion be accelerated by placing 

 a branch in an inverted petition ? 



FROM Dr HALES'S experiments, it has been concluded, that 

 the perfpiration of the leaves is the great agent in the motion 

 and elevation of the fap. But, from the experiments here re- 

 lated, it is evident, that the fap afcends with great vigour in 

 trees, even when they are deftitute of leaves. 



E 2 THE 



* HENCKEL. Flora Saturnizans, cap, 4. 



