33^ ECOLOGY. 



619. Wood-destroying fungi. Several thousand different 

 species of mushrooms are known in different countries. A large 

 number of them grow in the soil, deriving their nutriment from 

 decaying organic matter in the soil. Others grow in decaying 

 logs and plant parts. Still quite a large number of the mush- 

 rooms and their relatives are able to grow in the woody portions 

 of the trunks of living trees, causing decay of the trunks. Still 

 others are parasitic. The wood-destroying fungi not only do 

 great damage in destroying the usefulness of some timber trees 

 for lumber, but they often so weaken the tree trunk or roots of 

 the tree that the trees are broken down during gales. 



620. The mycelium enters the tree at wounds in the trunk, 

 limbs, or roots. A limb of a tree broken during a heavy wind, 

 or by falling trees, or by the weight of snow, makes an infection 

 court for the mycelium. A falli-ng tree may bruise and knock off 

 the bark from a sound standing tree and thus open a way for the 

 entrance of the wood-destroying mycelium. The roots of trees 

 are sometimes injured by the wheels of passing vehicles. In some 

 cases I have known fungi to enter through such injuries. Shade 

 trees are also similarly injured as well by the gnawing of animals 

 when allowed to stand near them. Severe pruning of many large 

 limbs of trees often renders them liable to injury from the attacks 

 of wood-destroying fungi, since the small amount of leaf surface 

 remaining is too little for the manufacture of the necessary plant 

 food for repair of the wounds. A few limbs should be taken off 

 in a single season when necessary to prune, and extend the proc- 

 ess over several seasons, rather than to prune so severely in a 

 single season. 



621. From our studies on the growth in thickness of woody 

 stems we know that the living and growing part of a tree trunk 

 is confined to a layer just underneath the bark. So when a 

 bruise or break passes through this layer (cambium) and exposes 

 the wood within, the mycelium of the wood-destroying fungi can 

 easily enter. From this point it spreads for long distances in the 

 interior of the tree, causing decay. Trees thus often become 

 " hollow." Some of the topmost branches die. The mycelium 



