94 



LETTERS ON TREES. 



growing. It is directly connected with the vital pro- 

 cesses going on there during the spring and summer 

 — nay, it is exclusively dependent on these processes. 

 The first or earliest movement of the sap in spring is 

 in the immediate vicinity of the buds. The fluid 

 there, previously at rest, is the first to be set in motion, 

 and its movement is determined by the act of vegeta- 

 tion beginning in the buds under the quickening 

 influence of the sun. The subsequent and very rapid 

 increase in the activity of that process necessitating 

 additional and proportionably larger supplies of sap, 

 an agency is exerted which operates downwards in 

 the direction of the soil, and causes the nourishing 

 fluid to ascend. And it is further of consequence 

 to remark, that the whole season through, the amount 

 of sap drawn from the soil, and the rapidity of its 

 ascent through the trunk to the parts above where 

 vital actions are undoubtedly going on, are entirely 

 regulated by the activity of these actions. The sup- 

 plies furnished, and the times and rates of their delivery 

 to the living and growing parts above, are in the exact 

 measure and proportion of the demands they make. 

 They are large or small, slow or rapid, just as the vege- 

 tation is scant or luxuriant, languid or energetic. 



4. Of all this we have several decisive proofs. If 

 any single branch of a tree, standing in the open air, 

 be carried through and led into a hot-house hard by, 

 at a time when no vegetation is going on in any part of 



