POSITION OF THE HYMENIUM 21 



Position of the Hymenium. — Excepting a few gelatinous species 

 which require further investigation, it is a general rule that in 

 Hymenomycetes the hymenium is situated on the underside of 

 the fruit-bodies. Encrusting forms, developing on logs and twigs, 

 usually produce their hymenium on the under or lateral surfaces of 

 the substratum. 1 That the hymenium should not be developed on 

 a surface looking upwards is of great importance for spore-liberation. 

 It was found with the beam-of-light method that, if a fruit-body of 

 a Polyporus, Polystictus, Lenzites, Psalliota, Stereum, &c, is turned 

 on its back, it is unable to liberate its spores into the air. It has 

 been determined that, if the hymenium on the gill of a Mush- 

 room, &c.j is made to look directly upwards, the spores can be 

 shot upwards about 01 mm. above the basidia. 2 This does not 

 seem to be high enough to permit of the spores, which fall at the 

 rate of 1-5 mm. per second, being carried off by moderate air- 

 currents. Hence, when a hymenial surface looks upwards, the 

 spores shot upwards from it fall back again immediately on to 

 the hymenium and adhere there. Even when a fruit-body is set 

 in its natural position once more, such spores never regain their 

 freedom. 



In the great groups of the Agaricineae and the Polyporere, 

 the fruit-bodies are characterised by having the greater part of 

 the hymenial surfaces disposed in almost vertical planes. In the 

 Agaricineae the hymenium is situated on the surfaces of wedge- 

 shaped gills (Figs. 2 and 3; also Plate I., Fig. 4); and in the 

 Polyporere it lines the inner sides of cylindrical or slightly conical, 

 vertically-placed tubes (Fig. 7, p. 33, and Fig. 66, p. 189). From 

 observations on the paths and rates of fall of individual spores, as 

 well as by direct beam-of-light studies of spore-clouds produced 

 from fruit-bodies when tilted at various angles, I have come to the 

 conclusion that it is only when the hymenium is vertical or looking 

 downwards at a greater or less angle that successful spore-liberation 



1 I have noticed the fruit-bodies of Irpex obliquus growing on the upper side 

 of an inclined tree, but the hymenium appeared to be irregular. Falck (he. cit.) 

 grew abnormal fruit-bodies of Porta vaporaria and Merulius lacrimans on the upper 

 surfaces of wooden blocks in the laboratory. 



2 Vide infra, Chap. XI. 



