122 RESEARCHES ON FUNGI 



The Effect of Gravity. Gravity is the chief orienting stimulus 

 acting on the fruit - bodies of Hymenoruycetes. In Polyporus 

 squamosus, for instance, as we have already seen from the dis- 

 cussion in Chapter IV., it causes : (1) The stipe to turn the 

 rudimentary and terminal pileus into a horizontal position, (2) the 

 pileus to grow with a symmetry suited to the position of the 

 stipe, (3) the pileus flesh to grow parallel to the earth's surface, 

 and (4) the hymenial tubes to grow towards the earth's centre. 

 The stiped Agaricinese usually have stipes which are negatively 

 geotropic and gills which are positively geotropic. 



Although the stimulus of gravity is of the greatest importance 

 in orienting the hymenium and the basidia which it contains, 

 it appears to have no direct effect on the process of spore-dis- 

 charge. When a hymenium has once started its development, 

 ripe spores continue to be developed and separated from the 

 basidia, independently of the direction of gravitational attraction. 

 Thus, if a gill be placed so that the basidia on one side look 

 upward or downwards or at any angle with the vertical what- 

 soever, spore - discharge takes place equally well in all cases. 

 Evidence of this was obtained by watching spores leave their 

 sterigmata with the microscope, and will be given in the next 

 chapter, which deals with the violent projection of spores from 

 the hymenium. 



The Effect of the Hygroscopic Condition of the Air. It has 

 been mentioned already * that for Polyporus squamosus, so far as 

 it was possible to judge from the spore-clouds seen by daylight, 

 the liberation of spores takes place equally well both in a dry 

 and in a saturated atmosphere. For this species, therefore, 

 ordinary variations in the hygroscopic state of the atmosphere 

 do not appear to appreciably affect the rate of discharge of the 

 spores. 



When a small portion of a pileus of Polyporus squamosus, 

 PsaUiota campestris, or Amanitopsis vaginata, &c., was placed in 

 a vertically disposed compressor cell (cf. Fig. 58, p. 167, and Plate IV., 

 Fig. 29), so that the fall of spores could be watched with a hori- 

 zontal microscope, it was found that spores fell continuously when 

 1 Chap. VI. 



