PSALLIOTA CAMPESTRIS 391 



these fungi than in the Common Mushroom. On the other hand, 

 there are some Agaricineae which have thinner gills than" Psalliota 

 campestris, namely, the Coprini. In these, however, the gills are 

 parallel-sided or subparallel-sided, and associated with this parallel- 

 sidedness we find a special arrangement for the production and 

 liberation of spores which is quite different from that in the Mush- 

 room. If a Mushroom had parallel-sided gills with a thickness say 

 of 0*2 mm., and its hymenium continued to be active everywhere 

 at one and the same time, then, unless the gills could be kept practi- 

 cally quite vertical during the whole period of spore-discharge, 

 a great many spores would be wasted. Suppose that a gill of a 

 Mushroom were parallel-sided and 7 mm. deep and that the spores 

 were shot out from the gill-sides to an average distance of 0* 1 mm. 

 Then, if the median planes of the gills were to diverge from the 

 vertical by an angle of only 0*8, which represents a slope just 

 exceeding one in seventy, most of the spores on the upward-looking 

 side of the gill would fail to escape from the pileus. It is evident 

 that the margin of safety necessary for compensating for irregu- 

 larities in development in such a case would be too small. In Mush- 

 rooms, as we actually find them in the field, the lower part of the 

 gills can be bent from the vertical by an angle exceeding 2 and still 

 all the spores can escape. In a former investigation, of which an 

 account was given in Volume I, the critical angle of tilt for the gills 

 of a particular Mushroom was found to be 2'S . 1 The discussion 

 just given may be taken as supporting the view that in Psalliota 

 campestris the gills, provided they are to continue to liberate their 

 spores successfully, have probably become reduced to their minimum 

 thickness. 



The Ends of the Gills. We have now to account for the fact 

 that the gills, when observed in lateral view (Fig. 134, p. 377), are 

 deepest toward the middle and become narrowed down to a vanishing 

 point at each end. 



The narrowing of the gills toward the pileus-periphery, from the 

 mechanical point of view, has the same significance as the corre- 

 sponding thinning out of the pileus-flesh, i.e. it is in accordance with 

 the mechanical requirements of the fruit-body as a whole. From 

 1 Vol. i, p 40. 



