FOMES APPLANATUS 141 



The foregoing calculations are in conformity with the view that 

 there is no fundamental difference in the structure of the hymenium 

 of a Mushroom and of Fomes applanatus, and that the chief reason 

 why a Fomes applanatus fruit-body produces some three hundred 

 and thirty times as many spores as a large Mushroom is that the 

 hymenial area of the former is about 165 times greater than that 

 of the latter. I have observed that, in Panaeolus campanulatus, 

 Stropharia semiglobata, Psalliota campestris, and in the Hymeno- 

 mycetes generally, the hymenium contains, from its first origin and 

 before a single spore is developed, a definite limited number of 

 rudimentary basidia destined to produce and liberate spores during 

 the course of the spore-discharge period. 1 The basidia ripen and 

 shed their spores in succession, and spore-discharge ceases as soon 

 as the pre-formed basidial elements have become exhausted. Each 

 square millimetre of any hymenium can develop a certain number 

 of spores only and no more. The exact number varies with each 

 species and is affected by the size and packing of the basidia, 

 species having large basidia and correspondingly large spores pro- 

 ducing per unit of hymenial area fewer spores than species having 

 small basidia with correspondingly small spores. 



The Cause of the Long- Spore-discharge Period in Fomes 

 applanatus. Granted that the structure of the hymenium of 

 Fomes applanatus does not differ essentially from that of the 

 Agaricineae, how comes it, one may ask, that, whereas the 

 Agaricineae shed all their spores in about six to eighteen days, 

 a Fomes applanatus fruit-body goes on shedding spores for about 

 six months ? It seems to me that the answer to this question is 

 to be sought in the difference in the mode of development of the 

 structures which support the hymenium, i.e. of the gills in the 

 Agaricineae and of the hymenial tubes in Fomes applanatus. In 

 what follows let us think of the Common Mushroom, Psalliota 

 campestris, as representing the Agaricineae. 



After a Mushroom has developed its gills up to the stage when 

 the pileus is just beginning to expand, no new basidial elements 

 are added to the hymenial layers. As the pileus expands, it is 

 true that the two hymenial layers covering the two sides of each 



1 Vide injra. 



