376 RESEARCHES ON FUNGI 



efficient possible, if one considers the conditions necessitated by the 

 one great function to be performed, namely, the production and 

 discharge of millions of basidiospores. A structure which must 

 spread widely in a horizontal direction, so as to provide as great 

 an area as possible on its lower surface for the hymenium, must 

 be held firmly at a distance of several inches above the surface of 

 the ground, so as to bring into existence a free space below the 

 hymenium through which the wind may sweep ; and the support- 

 ing structure must be slender so as to economise fungus-substance 

 as much as possible, so as to offer the least possible obstacle to 

 the stream of escaping spores, and so as not to occupy a greater 

 area of the valuable under-surface of the structure to be supported 

 than is absolutely necessary. The structure to be supported is, of 

 course, the pileus, and the supporting structure the stipe. There 

 are various forms which one can think of as possible for both stipe 

 and pileus, and various ways in which these bodies might be 

 attached to one another. By mathematical calculations, the 

 nature of which is well known to mechanical engineers, it can be 

 shown that the fruit-body as a whole is most efficiently constructed, 

 i.e. brings into play the smallest stresses and strains in its various 

 parts, when : (1) the stipe has the form of a hollow cylinder, (2) 

 the pileus has the form of a cone, (3) the axes of the stipe and of 

 the pileus are vertical, and (4) the stipe is attached immediately 

 under the centre of the lower side of the pileus. Since these 

 characteristics are all found' more or less perfectly embodied 

 in the Mushroom, we must conclude that the general shape of 

 this fruit-body approximates to the ideal. 



In Lepiota procera, as shown in Figs. 14 and 15 in Volume I 

 (pp. 44 and 45), the very large pileus remains in the form of a low 

 upright cone during the whole period of spore-discharge. In Psalliota 

 campestris the pileus, during expansion, also assumes the form of 

 a low upright cone or plano-convex structure (Fig. 134, A), but 

 subsequently its edges turn up so that finally the top of the pileus 

 becomes flattened ; and then the cone is, as it were, turned upside 

 down (Fig. 134, B). The turning up of the edge of the pileus causes 

 the gills to be raised in such a way that their long axes are inclined 

 upwards through a considerable angle. The disadvantage accruing 



