ADJUSTMENTS OF FRUIT-BODIES 59 



developed sterile fruit-bodies were growing. After varying the 

 intensity of the light considerably and extending the experiment 

 over many days, I was unable to detect any change in the direction 

 of growth of the branches in response to the illuminating rays. 

 In being unprovided with heliotropic properties, the sterile fruit- 

 bodies of Polyporus squamosus present a marked contrast with 

 those of Lentinus lepideus. 



The formation of the pileus is brought about entirely by a 

 morphogenic stimulus of light. To this kind of stimulus the 

 young fruit-bodies (although not the older sterile ones) are re- 

 markably sensitive, for, when some of them were temporarily 

 exposed to daylight for only a single hour and then replaced 

 in the dark-room, they subsequently produced pilei. 



In the development of a pileus, light is only of real importance 

 in the very initial stages. Some fruit-bodies were grown on a 

 log in ordinary daylight. When they were about two days old and 

 the pileus on each was yet in a most rudimentary condition, the 

 log was placed in the dark-room. The fruit-bodies, thus shut 

 off from all illumination, continued their development, attained 

 a considerable size, produced hymenial tubes, and liberated millions 

 of spores. Light therefore appears to give a sufficient morphogenic 

 stimulus to a fruit-body within a comparatively few hours after 

 the latter has come into existence. It unlocks developmental 

 forces which, when once set free, become independent of the 

 liberating agent. 



In the light, the ends of the conical processes after flattening 

 or becoming depressed, quickly expand to form small pilei (Fig. 21, 

 B and C; Plate V., Figs. 32 and 33). That part of a process which 

 bears the pileus we shall now refer to as the stipe. 



The stipe owes its position in the first instance to accident. 

 However, it quickly becomes negatively geotropic and makes an 

 upward curvature so as to bring the top of the pileus into a 

 horizontal plane (Fig. 21, D). When this has taken place the 

 pileus becomes diageotropic, and now expands rapidly in a direction 

 parallel to the earth's surface (Fig. 21, E; Plate V., Figs. 3o and 34). 

 Subsequently hymenial tubes develop on its under surface ; they 

 are positively geotropic and grow vertically downwards. These 



