TYPES OF AGARICINEAE 239 



(2) Geotropism of the Gills. In addition to being wedge- 

 shaped in cross-section, the gills of fruit-bodies of the Aequi- 

 hymeniiferous Type are sensitive to the stimulus of gravity. During 

 development they are positively geotropic, and they grow in such 

 a way that their median planes are brought into vertical positions. 

 If the pileus, after becoming expanded, is tilted through an angle, 

 so that its orientation is permanently altered, in a short time the 

 gills exhibit a reaction to the stimulus of gravity : they execute 

 a turning movement about their lines of attachment to the pileus- 

 flesh until at least their lower parts have become vertical again. 

 The success of the re-adjustment depends upon the age of the 

 gills and their angle of tilt. The younger the gills and the less the 

 angle of tilt, the greater is the success which the gills attain in 

 bringing their median planes into a vertical position. 1 



(3) Position of the Hymenium in Space. Owing to the fact 

 that in the Aequi-hymeniiferous Type of fruit-body the gills are 

 wedge-shaped in cross-section and positively geotropic, it follows 

 that every part of the hymenium covering the gills in a normally 

 oriented fruit-body comes to look more or less downwards toward 

 the surface of the earth. 



(4) Development of the Hymenium. The Aequi-hymeniiferous 

 Type of fruit-body, as its name implies, bears a hymenium which 

 develops in an approximately equal manner all over the surface 

 of each gill : every small area of the hymenium (say every square 

 mm.) on every gill of a fruit-body produces and liberates spores 

 simultaneously during the whole of the spore-discharge period. 

 On each small hymenial area, the basidia come to maturity and 

 shed their spores in succession, so that the production and liberation 

 of spores frequently continue for a period of several days. 



(5) Discharge of the Spores. It was shown in Volume I that 

 the spores of the Hymenomycetes are shot out more or less 

 horizontally from the hymenium to a distance of 0*1-0' 2 mm. 

 and that, after being shot this distance, they curve in their 

 trajectory sharply downwards and subsequently fall slowly in a 

 vertical direction until they emerge from the interlamellar spaces 

 into the outer air, whence they may be carried away from the 



1 Cf. vol. i; 1909, pp. 50-52. 



