VEGETABLE SAP. 129 



intelligent practical men into error ; they conceiving 

 that instead of the sap rising, as it is known to do, 

 it actually descends I Now, a moment's considera- 

 tion might convince them of their mistake. The 

 sap undoubtedly begins to be fluid and first in motion 

 at the top of the tree ; that immediately below is next 

 liquefied, followed by that lower still, and so down 

 lower and lower till the whole that in the root also 

 is in full motion. We have elsewhere ( Gard. Mag. 

 vol. vi. p. 214), compared this circumstance to the 

 sudden opening of a full canal at one end : thus opened, 

 the water flows oat ; the current begins at the outlet, 

 and is generated backward till the whole is flowing. 

 This is exactly similar to the first spring movement 

 of the sap of trees, and presents the appearance of a 

 motion directly contrary to its true course. 



Whether the maturation (if we may use the term) 

 of the cambium into alburnum, in the latter end of 

 summer, takes place in the same order, that is, begin- 

 ning at the top and ending at the root, remains to be 

 proved. The fact of roots being more enlarged 

 during autumn than at any other period of the year, 

 is some corroboration of the supposition ; still the 

 question is not free from doubt ; because the idea 

 presupposes that this change of cambium into albur- 

 num is only a descending effect, not a positive pro- 

 gression downward of any actual member or compo- 

 nent of the tree, and therefore unsatisfactory, unless 

 we could prove that the full expansion of every 

 inferior cell depends on that of the one immediately 



K 



