

NUTRITION OF PARASITES AND SAPROPHYTES 139 



decaying organic matter. But there are many saprophytic 

 fungi which grow on plant remains which are not in the condi- 

 tion to which we apply the term humus. 



217. The wood destroying fungi which are so common on 

 dead logs, stumps, branches and even some species on the living 

 trees are also saprophytic fungi. Many of those which grow on 

 living trees are not parasites since they cannot attack a sound 

 tree. They can only enter the tree 

 when it has been injured so that 

 the living cambium layer (see para- 

 graph 1 06) is destroyed at a given 

 point or has been broken through, 

 i.e., at wounds in the tree. The 

 wounds are produced in a variety 

 of ways; by wind, heavy snows, 

 the felling of timber, etc., branches 

 are broken off, or the cambium is 

 broken through; or by fire which 

 kills the cambium. The heart 

 wood which is therefore sound, but 

 dead, is thus exposed. The germs 

 (spores, see Chapter XXXI) car- 

 ried by the wind, lodge on these 

 wounds, germinate and form the 

 fungus threads which grow into the 

 heart wood and thus gain access 

 to the heart of the tree trunk. 



The threads of mycelium are enabled to perforate the cell 

 walls by the excretion of a ferment or enzyme (cytase) which 

 dissolves an opening in the wall. Here they cause "heart 

 rot" of the tree and render the tree unfit for timber. The 

 fungus lives here for years, and now and then during certain 

 seasons the mycelium develops to the outside through the 

 wounds and forms the well-known bracket fungi so common 

 in the forests, or in the case of other species forms the toad- 

 stools or mushrooms often seen growing from the wounds of trees. 



Fig. 101. 



A wound parasite (Polyporus bore- 

 alis) causing heart rot of the hemlock 

 spruce. The fruit bodies are shelving, 

 white and overlap each other. The 

 mycelium extends through the heart 

 wood to the topmost branches and out 

 into the roots. 



