PSALLIOTA CAMPESTRIS 389 



gills of Psalliota campestris are extremely thin, and it is doubtful 

 if they could be appreciably thinner and yet continue to facilitate 

 the free escape of the spores. A Mushroom was gathered from a 

 field near Birmingham, England, and some tangential sections were 

 cut through the pileus so as to pass vertically downwards through 

 the region of the long gills at their deepest part, which happened to 

 be 7 mm. deep. In cutting the sections it was impossible to avoid 

 bending the gills somewhat, so that their median planes came to be 

 no longer straight. Two of these sections, which were of course 

 still living, were very carefully sketched with a magnification of 

 sixty with the help of a camera lucida. Fig. 139 shows these 

 camera-lucida drawings after they had been adjusted to the extent 

 of straightening the gills so as to give these the alignment which 

 they had in nature. With the aid of this Figure let us now con- 

 sider the thickness of a typical long gill. The thinnest part of the 

 gill is at its free edge. Just above this edge the actual thickness is 

 reduced to 0- 15 mm., and it could scarcely be thinner, owing to the 

 fact that the gill-substance is bound to include two hymenial layers, 

 two sub hymenial layers, and at least some tramal hyphae. A 

 quarter of the way up the gill, the thickness becomes increased to 

 O 1 25 mm., half way up to 0' 3 mm., and just below the interlamellar 

 sinuses at the top to 43 mm. The two sides of the gill converge 

 from above downwards at a very acute angle which increases in 

 size from above downwards. This angle in the upper part of the 

 gill is only about 1, but toward the middle of the gill it becomes 

 increased to about 2, whilst in the lower part of the gill it becomes 

 4. The convergence of the two sides of every gill is of the greatest 

 importance, for it is owing to its existence that every part of the 

 hymenium on the under side of a normally adjusted pileus comes 

 to look more or less downwards toward the earth, with the result 

 that every small portion of the hymenium (every square mm.) can 

 produce and liberate spores simultaneously during the whole period 

 of spore-discharge. 



In Marasmius oreades, many Hygrophori, Russulae, etc., the 

 gills are much thicker relatively to their depth than in Psalliota 

 campestris, or, to state the same thing in another way : the angle 

 of convergence of the sides of the gills is considerably greater in 



