382 KKSKAHCHKS OX FUXCil 



larger mushrooms were twice, or somewhat more than twice, as 

 numerous as those in the very small mushroom. The larger mush- 

 rooms had a gill-system which was everywhere made up of gills of 

 at least three lengths : long, short, and shorter, while between the 

 two last types of gills very short gills were frequently interpolated. 



Other factors, besides the diameter of the pileus, which affect 

 the number of the gills, are : the diameter of the stipe and the 

 thickness of the gills where they adjoin the pileus-flesh. The 

 thickness of the gills depends on their depth, for the deeper a gill 

 is, since it is wedge-shaped in cross-section, the thicker it must be 

 where it is attached to the pileus. With a given diameter for a 

 pileus, therefore, the deeper the gills are, the greater space will 

 each occupy below the pileus and the fewer there can be of them. 

 I carefully compared two sets of fruit-bodies : (1) wild mushrooms 

 gathered from a field near Birmingham, England, and (2) culti- 

 vated mushrooms of a particular variety grown artificially on a 

 mushroom-bed. I found that, with fruit-bodies of the same 

 diameter, the wild mushrooms had deeper and thicker but less 

 numerous gills than the cultivated ones. The depth of the middle 

 of the long gills in the wild mushrooms was 9 mm., whereas the 

 corresponding depth in the cultivated mushrooms was only 6 mm. 

 A wild mushroom with a diameter of about 8 cm. possessed about 

 413 gills and a cultivated one with an equal diameter about 600, 

 the count in each case having included all the very smallest gills 

 which could be clearly discerned at the pileus-periphery. It 

 became obvious from this comparison and from others that, not 

 merely theoretically but also actually, there is a relation between 

 gill-depth and gill-number. 



The close packing of the gills in one of the cultivated mush- 

 rooms is obvious from a glance at Fig. 137. This particular 

 pileus was 6 cm. in diameter, but it was not yet quite fully ex- 

 panded. It might perhaps have increased its diameter to 6-5 cm. 

 if it had been left to continue its development upon the mush- 

 room-bed. By throwing a photograph of this pileus on a screen 

 from a lantern-slide, it became possible to count the gills at the 

 stipe aiid at the pileus-periphery very exactly. The number of 

 gills at the stipe was 132 and the number at the pileus-periphery 



