i 4 8 RESEARCHES ON FUNGI 



the formation of fruit-bodies takes place much more readily and 

 perfectly from secondary mycelia produced by the union of two 

 mycelia of opposite sex than from individual primary mycelia, 

 each of which has originated from a single spore and is unisexual. 1 

 Some wood-destroying fungi, e.g. Schizophyllum commune, Stereum 

 purpureum and Armillaria mucida, are known to be heterothallic, 2 

 and it may well be that heterothallism prevails in the genus Fomes. 

 It is therefore not unlikely that Fomes applanatus is hetero- 

 thallic. If it is, the production by its fruit-bodies of vast numbers 

 of spores would not only increase the chance of multiple infection 

 of a wound-surface on a tree, but also make it possible for spores of 

 opposite sex to germinate near to one another and thus produce 

 secondary mycelia of full fruiting capacity. 



From the foregoing discussion we may conclude that the daily 

 production of thousands of millions of spores by the fruit-bodies of 

 Fomes applanatus and of other wood-destroying fungi is not essen- 

 tially a waste of reproductive energy, but is necessary and distinctly 

 advantageous to the species concerned, owing : (1) to the fact that 

 the probability of any particular spore being carried by the wind to 

 substrata suitable for germination rather than to substrata un- 

 suitable for germination is excessively small ; (2) to the fact that, 

 in the struggle for the occupation of new substrata suitable for 

 growth and reproduction, each plant must compete with other 

 fungus plants of the same species and of other species, and (3) to the 

 likelihood that the fungi are heterothallic and that only secondary 

 mycelia produced by the union of two primary mycelia of opposite 

 sex can produce fruit-bodies in a normal manner. 



Finally, for the Hymenomycetes in general, it seems probable 

 that the number of spores produced by the fungus plants is corre- 

 lated roughly : (1) with the nature of the difficulties encountered 

 by the spores in passing from their parent fruit-bodies to suitable 

 substrata, (2) with the amount of competition from other organisms 

 which must be met with in the struggle for existence, and (3) with 

 the requirements of sex. 



1 Irene Mounce, loc. cit. 2 H. Kniep, loc. cit. 



