PSALLIOTA CAMPESTRIS 387 



is rounded both at the free edge and on each side where it adjoins 

 its two neighbouring gills. Let A'B'C' be the shape of a cross- 

 section of a gill taken where the gill is deepest. Now let us replace 

 this gill by two others which resemble the first in form and are of 

 equal size (A'D'E' + ET'B'). The two gills wiU not meet at an 

 acute angle, as did the two steel wedges, but the interlamellar space 

 between them must be rounded as shown, so as to permit of the 

 violent discharge of the spores for a distance of 1-0 2 mm. from 

 the hymenium even at the narrowest part of the space, and so as 

 to allow in addition for a margin of safety for the adjustment of the 

 gill-planes in vertical directions even when the pileus happens to 

 be slightly tilted (cf. Fig. 139, p. 390). Let us suppose that the 

 radius of the rounded top of each interlamellar space, for the reasons 

 given, is not to be reduced beyond 25 mm. Then we can speak of 

 the narrowest portion of the interlamellar space as having a minimum 

 width of 5 mm. Let us now replace the two gills by four. The 

 new interlamellar spaces must be as wide as the old ones, i.e. 0-5 mm. 

 wide at the top. Again, let us replace the four gills by eight. Here 

 again, the seven interlamellar spaces all have a minimum width at 

 the top of 5 mm. We thus see that, as the gills are progressively 

 diminished in size, the minimum width of the interlamellar spaces 

 remains constant. The gills, as they become more and more 

 numerous and shallower, tend to be less and less like the original 

 gill and more and more like rounded ridges, their exterior reminding 

 one of the waves on the surface of water. If the process of sub- 

 stitution were to be continued, the gills would eventually disappear 

 and the hymenium become perfectly flat. It is evident, therefore, 

 that, unlike what was found to happen with the steel wedges, 

 reduction of gill-depth with increase in gill-number leads to a very 

 considerable reduction of the absolute amount of gill-surface. The 

 greatest amount of gill-surface is possessed by the original gill 

 A'B'C', a slightly less amount by the two gills A'D'E' + ET'B', 

 and progressively less amounts with every increase of gill-number 

 until the gill-substance has been reduced to zero. We may regard 

 two tendencies as having had effect in leading to the establishment 

 of the particular gill-depth which we find in Psattiota campestris 

 at the present day : (1) a tendency to increase the absolute amount 



