CHAPTER II 



THE EXTENT OF THE HYMENIUM— PRINCIPLES UNDERLYING THE 

 ARRANGEMENT OF GILLS AND HYMENIAL TUBES— THE MARGIN 

 OF SAFETY— THE GENUS FOMES 



The Hymenomycetes are classified in subdivisions corresponding 

 in the main with the manner in which the pileus is arranged in 

 relation to the hy menial surfaces. Only in the Thelephoreae, some 

 Tremellinese, and the Exobasidiinese is the hymenium smooth and 

 flat, whilst in the Agaricinea* it is arranged upon gills, in the 

 Polyporea? in tubes, in the Hydnese upon spinous prolongations, 

 and in the Clavarieae upon the exterior of more or less numerous 

 branches of the fruit-body. 



The various forms of fruit-bodies may be explained in their 

 evolutionary aspect on the supposition that a chief factor in their 

 survival has been the advantage arising from the production of 

 a relatively large number of spores with a relatively small expendi- 

 ture of fruit-body material and energy. The gills, spines, tubes, &c, 

 all have the same significance, namely, that of increasing the extent 

 of the hymenium which a fruit-body may bear. The same end has 

 been attained by different means. One can easily imagine how, 

 beginning with the Thelephoreas with smooth and flat hymenial 

 surfaces, the more highly complex fruit-bodies of the Agaricinete, 

 the Polyporere, the Hydnese, and the Clavarieae have been evolved. 

 The principle of folding to increase surface is well illustrated in 

 these four groups. Perhaps every possible means of economically 

 increasing hymenial surface, consistent with the liberation of the 

 spores, has been exhausted by them. 



In order to obtain more precise information with regard to the 

 advantage obtained by the production of gills, spines, tubes, &c, a 

 number of calculations have been made. 



Let A be the area of the flat surface on the underside of a 



