PSALLIOTA CAMPESTRIS 385 



less indeed than do the longer gills at their mid-lengths where they 

 are deepest and thickest. The interlamellar spaces, as one can see 

 from a glance at Fig. 137 (p. 383), are actually wider at the bottoms 

 of the gills half-way between the stipe and pileus-periphery than at 

 the periphery itself ; but at the tops of the gills where these adjoin 

 the flesh they are all of approximately the same width. 



The Depth of the Gills. Let us now enquire why it is that the 

 gills of a mushroom are so organised that they have a certain depth 

 which varies but little about a mean. Why should not the gills 

 be twice or three times as deep, or only one-half or one-third as 

 deep ? What factors have led to the evolution of a depth such 

 as we are now able to observe ? I am inclined to believe that the 

 present gill-depth of Psattiota campestris has been gradually attained 

 as a result of variations which have tended toward the production 

 of the greatest amount of gill-surface with a given expenditure of 

 mushroom substance. The considerations on which this view is 

 based will now be elaborated. 



Let us imagine a large wedge made of steel with the width and 

 depth shown in the cross-section illustrated in Fig. 138 at ABC. 

 Now let us suppose that we replace this wedge by two others which 

 resemble the first in form but are only one-half as deep and wide 

 (ADE + EFB). It is evident that the area of the lateral surface 

 of the two new wedges taken together is the same as that of the 

 first wedge but that their total volume is only one-half. Let us 

 replace the two wedges by four others which resemble the two 

 in form but are only one-half as deep and wide (AGH -f- HIE 

 + EJK + KLB). The area of the lateral surface of the four wedges 

 taken together is still equal to that of the original wedge, but the 

 total volume has been reduced to a quarter. If we again replace 

 the four wedges by eight, we shall find that, while once more there 

 is no reduction of the original lateral surface, yet the volume is 

 reduced to one-eighth. This process of substitution could be 

 continued theoretically indefinitely, each substitution involving a 

 reduction of the original volume to one-half but no alteration of 

 the total lateral surface. Now let us suppose that it is desired to 

 have the wedges of such a depth that with the least expenditure 

 of material the surface of the lateral sides shall be collectively as 



VOL. II. 2 C 



