306 NECESSITY FOR A CIRCULATION OF AIR. 



Some plan must be pursued from infancy which 

 shall provide against these evils ; this will be de- 

 tailed hereafter. 



Sixth, the importance of a circulation of air through 

 the head of the tree is closely associated with that of 

 light. As the latter is composed of rays of different 

 colors, acting chemically upon vegetation, some 

 absorbing one ray and some another, so the rust- 

 ling of every leaf is necessary to secure to it those 

 atmospheric substances which aid it in the elabora- 

 tion of its sap. To produce this the same means 

 are employed as with the last, — a loose, open head. 



Seventh, the formation of an equal system of 

 branches. All trees throw most of their vigor into 

 a strong central shoot. This is while they are 

 young ; with increased age it is generally lost in 

 the mass of vertical branches which form the head. 

 The horizontal limbs, which were produced at the 

 outset, receive so small a share of sap, that they 

 become insignificant. By pruning, this energy and 

 vigor should be equally distributed. The tendency 

 upward is so great that in those trees which have 

 been trained horizontally for a long time, a single 

 bud, if allowed to escape the knife of the gardener, 

 would soon turn the balance of strength in its favor. 

 All systems of pruning contain this principle to a 

 greater or less extent. 



To accomplish it one must be aware of the nature 

 of the buds upon a shoot, and the difference in 



