254 RESEARCHES ON FUNGI 



can readily escape from the pilei even when the gill-planes are tilted 

 in respect to the vertical at angles of appreciable size. 



(c) The zone of spore-free surface The spore-free zone of the 

 hymenium comes into existence owing to the fact that the zone of 

 spore-discharge moves from below upwards on each gill and thus 

 leaves behind it a zone from which the spores have been discharged. 

 This zone is shown at c in Figs. 104 and 105 (pp. 242 and 249). It is 

 parallel to the other zones and has a width of about 0-08 mm. All 

 the elements within it — long basidia /, short basidia 5, and para- 

 physes f — are still living and turgid, and even the sterigmata still 

 retain their original form. In Fig. 105, which shows a cross-section 

 of the zone, the long basidia can readily be distinguished from the 

 short basidia by their difference in length ; whilst, in Fig. 104, which 

 shows a surface view of the zone, the long basidia project toward 

 the reader farther than the short ones and therefore can be identi- 

 fied by their more prominent appearance as indicated by heavier 

 shading. Within the spore-free zone, generally speaking, all the 

 basidia have lost their spores and present to the eye nothing but 

 vacant sterigmata. However, owing to inevitable imperfections 

 which introduce themselves into the process of manufacturing and 

 discharging the millions of spores which are developed even on a 

 single gill, one may occasionally observe within the spore-free zone 

 a basidium which has failed to discharge some or all of its spores 

 at the proper time. One such basidium is shown at i in Fig. 104 : 

 it possesses four undersized colourless spores which, as we shall see, 

 are destined to become waste spores and to join the other waste 

 spores at the gill-edge. 



{d) The zone of autodigestion. The zone of autodigestion lies 

 just below, and parallel to, the zone of spore-free surface and is about 

 0-04 mm. wide. The spore-free zone is constantly extending above 

 owing to the discharge of spores in the zone of spore-discharge, but it 

 is constantly contracting below owing to the progressive encroachment 

 of the zone of autodigestion. The upper and lower boundaries of 

 the spore-free zone move upwards on a gill at exactly the same 

 rate. Hence the spore-free zone does not appreciably alter in width 

 whilst travelling up a gill from the bottom to the top. This con- 

 stant width, as we have seen, is about 0-08 mm. 



