[890.] 



NEW-YORK MICROSCOPICAL SOCIETY. 



Ill 



in the situation, and effectually, as a rule, do that which is nec- 

 essary to protect those already formed from the attacks of the 

 fungi, and build up and carry on the structure of the tree 

 unimpaired. 



It is now well known that it is 

 the function of many of the fungi by 

 growth to undo the structure of the 

 wood, cause it to decay, and reduce 

 it to elements which again can be 

 used by growing trees or plants. 



For the growth of fungi, three 

 conditions in combination are essen- 

 tial, viz.: ist. Moisture; 2ViA, Heat, 

 ranging from 35° to 140° Fahr. : 3d, 

 Air. 



Eliminate any one of these con- 

 ditions, and the growth of the fungi 

 is checked, the wood not decaying. 



To more readily trace the features 

 of the natural system of pruning, and 

 to see how the tree eliminates one 

 of the conditions for the growth of 

 the fungi, it will be well to recall to 

 our minds a few facts regarding the growth of trees. They 

 increase their height, year by year, by successive additions to 

 the length of their leaders or leading branches. 



When the lateral branches shoot off from the trunk, their height 

 remains practically the same distance from the ground, as the 

 tree grows. Whether all those branches will continue to develop, 

 or the lower ones die, depends largely upon the amount of light 

 the tree receives. Therefore, if the tree grows in an open field, 

 it will be surrounded by light on all sides, which, acting on the 

 chlorophyll in the leaves, will maintain the vigor and growth of 

 all the branches. The trunk of the tree will be comparatively 

 short, having many lateral branches. In this case, little if any 

 natural pruning has taken place. On the other hand, when trees 

 grow in the dense forests, only the uppermost branches receiv- 

 ing sufficient light for development, the lower ones being 

 shaded, become dwarfed, die and are attacked by fungi. The 



Fig. 1. 



