238 PROCESS OF DEVEXOPEMENT. CHAP. IV. 



and if it does prove the root to be the organ of 

 elaboration, why does the sap in general ascend the 

 stem unaltered ? 



In proof of the fact of the circulation of the 

 sap it was said that it is impossible to conceive how 

 the important and complicated operations of the 

 secretion of the peculiar juices of the plant could 

 be effected all at once ; and that, therefore, it was 

 reasonable to suppose them to be effected by re- 

 peated efforts, which the circulation of the sap, as 

 of the blood in the animal system, was the best 

 fitted to produce. But this is obviously an argu- 

 ment of convenience, which proves nothing. For 

 if the cause of vegetable developement should 

 remain unexplained, we are not to assume that of 

 the process of circulation without any adequate 

 proo merely because it seems to account for it 

 the most easily. But it was added that different 

 sorts of vessels are distinguishable in the structure 

 of the plant, some for conducting trie sap in its 

 ascent, and others for conducting it in its return. 

 This it was admitted might be the fact, without 

 proving the one set to be arteries and the other 

 veins. It was then said that they must still be 

 supposed to exist, though we should not be able to 

 distinguish or ascertain them ; in the same manner 

 as we must suppose their existence in the wing of 

 a butterfly without being able to distinguish them. 

 This is no doubt possible; but it is contradicted 



