. Botany for high schools. Botany. NUTRITION OF PARASITES AND SAPROPHYTES 13I 314. Saprophytic fungi.—The humus saprophytes mentioned in the preceding paragraph are of course saprophytic fungi, and the term saprophytic fungi appHes 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 sa
. Botany for high schools. Botany. NUTRITION OF PARASITES AND SAPROPHYTES 13I 314. Saprophytic fungi.—The humus saprophytes mentioned in the preceding paragraph are of course saprophytic fungi, and the term saprophytic fungi appHes 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 w^hen it has been injured so that the living cambium layer (see paragraph 100) is destroyed at a given point or has been broken through, , 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 m\'celium extends through the heart wood to the topmost branches and out into the Please
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