PSALLIOTA CAMPESTRIS 425 



with one another in endlessly varied ways. One can find out 

 only empirically the directions of the waves on any gill-surface, 

 and it is impossible to predict their course theoretically. Even 

 the waves which are opposite to one another on the two sides of 

 the same gill go their ways quite independently of one another. 



The very numerous small irregular waves of development 

 which we find in all representatives of the Panaeolus Sub-type, 

 including the Mushroom, may be contrasted with the single large 

 wave which, in all species of the genus Coprinus, passes from below 

 upwards on each gill. A black-spored panaeolate fungus with a 

 many- waved hymenium on each gill may well have been the 

 ancestor of the genus Coprinus in which the hymenium on each 

 gill is one-waved, the many small irregular waves having become 

 reduced in the course of evolution to one grand wave always 

 moving in an upward direction. 



We shall now proceed to a study of the hymenium of the Cul- 

 tivated Mushroom in cross-section, so that we may gain an insight 

 into the structure and mutual relations of the hymenial elements 

 when these are seen in lateral view. Fig. 147 represents a cross- 

 section of part of a living gill and shows semi-diagrammatically the 

 structure of the hymenium h, the subhymenium s, and part of the 

 trama t. The drawing is essentially synthetic and exhibits all the 

 histological details which were made out by the study of numerous 

 particular sections cut with a hand-razor from living gills and 

 mounted in water. Parts of some of these sections were drawn 

 with the camera lucida. Similar sections for the wild form of the 

 Mushroom will be represented in subsequent illustrations. The 

 gill in Fig. 147 is supposed to be taken from a mushroom which 

 has been shedding spores for about 24 hours and which therefore 

 will continue to shed spores for several more days, i.e. until the 

 end of the spore-discharge period, which terminates only with the 

 complete exhaustion of the hymenium. The stage of development 

 of the hymenium is exactly the same as that for the hymenium 

 represented in surface view in Fig. 146 (p. 416). 



In Fig. 147, a dark area of the hymenium is passed through pro- 

 ceeding to the right from element no. 8 to element no. 34, a light area 

 from element no. 34 to element no. 59, an intermediate area from 



