NUTRITION OF PARASITES AND SAPROPHYTES 131 



214. Saprophytic fungi. The humus saprophytes mentioned 

 in the preceding paragraph are of course saprophytic fungi, and 

 the term saprophytic fungi applies not only to these humus sapro- 

 phytes but to all fungi which grow on dead and decaying organic 

 matter. But there are many saprophytic fungi which grow on 

 plant remains which are not in the condition to which we apply 

 the term humus. 



215. 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 paragraph 



100) 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 XXIX) carried 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 



Fig. 99- 



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. 



