LETTER X. 



95 



the tree, and the sap is everywhere quiescent, the buds 

 of that branch will vegetate, and the sap circulate 

 through it, while as yet nothing of the sort is in pro- 

 gress in any of the other branches of the tree.* It 

 cannot surely be that the roots and stems should exert 

 an agency so exclusive, or have any share in the pro- 

 duction of a change so strictly local. Conversely, if 

 the buds be cut oii from a branch prior to the com- 

 mencement of the annual process of vegetation, no sap 

 will pass into that branch during the entire spring and 

 summer, although the other branches not thus muti- 

 lated will be full of it. Again, if at a later period in 

 the season the leaves be stripped from off a branch, 

 the flow of sap through it will speedily if not at once 

 be effectually and permanently arrested. 



5. What may be the nature of the agency thus 

 exerted in the living buds and leaves, or of the force 

 emanating from them, which thus causes the sap to 

 circulate, and which regulates the quantity of it pass- 

 ing through the old stems and roots, — and what the 

 mode or manner of its action, it is not easy to say. 

 It is clearly a force acting a f route, or in advance, and 

 attracting the sap, — in contradistinction to a force 

 acting a tergo, or from behind, and propelling the 



* " The excitement of vital action in a branch of a tree exclusively 

 exposed to the sun, is the cause, not the effect, of an exclusively 

 increased flow of the sap into it." — Alison, Outlines of Physiology y 

 3d ed. p. 70. 



