PEHNING. 85 



pends upon the elaborating j^rocess carried on in the 

 leaves, and that this j)rocess can be maintained only in a 

 free exposure to the sun and air. This being the case, it 

 is obvions that any part of the tree excluded from the 

 action of these agents, cannot keep pace in growth with 

 other parts to which they have full access. In nursery 

 rows, as trees are usually j)l anted, the stems, after the 

 first year's growth, are, to a great extent, excluded from 

 the light, consequently the buds and leaves on them can- 

 not perform their parts in the creation of new wood. The 

 top of the tree, however, is fully exposed, and, conse- 

 quently, it makes a rapid growth towards the free air and 

 light. "When this is continued for two or three years in 

 succession, the tree becomes top-heavy ; the quantity 

 of woody fibre at the top is as great as, and it may be 

 greater than, at the bottom ; and hence it bends under its 

 own w^eight. 



2d. The Tendency of tlie Sap to the Growing Points at 

 the Top of the Tree. — Growth is always the most active 

 andvigorous, w^hen trees are in a natural condition, at the 

 newly-formed parts. The young buds are the most excit- 

 able, and the more direct their communication with the 

 roots, the more rapid will be their growth. Hence it is 

 that a yearling tree furnished wdth fifteen to twenty buds 

 or more, from its base to its top, frequently produces a 

 shoot from its terminal bud only, and seldom more than 

 three or four shoots from the whole number of buds, and 

 these at the top. This natural tendency, and the exclusion 

 of light from the stems of nursery trees, by their closeness 

 to one another, are the chief causes of weak and crooked 

 trees, to counteract which we resort io pruning. 



In " heading down?'' a young tree^ we cut aw^ay one 

 third or one half of the length of the stem, and this removes 

 the actively growing parts ; the sap must then find new 

 channels. Its v/hole force is directed to the buds that were 



