ADJUSTMENTS OF FRUIT-BODIES 63 



the effect of light. However, it was found that when very young 

 fruit-bodies which had just formed pilei in the light were placed in 

 various directions in the dark-room, they continued their develop- 

 ment and took up situations relatively to their substratum similar to- 

 those observed in nature (Plate V., Fig. 41). Both the position 

 assumed by, and the degree of symmetry of, a fruit-body are 

 governed by gravity alone. However, there can be little doubt 

 that light exerts a tonic influence upon the pileus during its develop- 

 ment. When a half-grown fruit-body which had been exposed to- 

 light in the open for about a week is placed in the dark so that the 

 plane of its pileus is vertical, the flesh grows sharply through a right 

 angle at its upper margin, and thus the new part takes up a position 

 parallel to the earth's surface. In this case the diageotropism of the 

 pileus is very marked. If a very young fruit- body which has 

 developed in the light for only two or three days (cf. Plate V., 

 Figs. 32 and 33) is set in darkness, the pileus often grows obliquely 

 upwards for some time and its diageotropism is but poorly displayed 

 (Plate V., Fig. 42, /). If, in response to a light stimulus of about 

 an hour's duration, a pileus develops upon a branch of a stag's-horn- 

 like structure which is still growing vigorously and is looking 

 upwards, it becomes trumpet-shaped in the dark (Plate IV., Figs. 40 

 and 42, d and e). If it responds to the stimulus of gravity at all, it 

 only gives an obliquely geotropic reaction. On the other hand, a 

 symmetrical pileus grown throughout in daylight has a very flat top 

 and is evidently strongly diageotropic (cf. Fig. 4, p. 28, and Plate V., 

 Fig. 40). It thus appears that the geotropic reaction of the pileus 

 flesh is partly determined by the amount of illumination to which 

 a fruit-body is exposed. 



When a stag's-horn-like structure has grown for two or three 

 weeks in the dark, and has almost attained its full extension, it can 

 be caused to form patches of hymenial tubes on its under surface by 

 exposure to sufficient daylight (Plate V., Figs. 38 and 39). The 

 branched fruit-body thus produced presents a remarkable contrast 

 with such normal ones as are found on trees. 



A feebly developed, trumpet-like fruit- body which came to- 

 maturity in weak light (cf. Plate V., Fig. 40) produced hymenial tubes 

 both on the lower and the upper surfaces of its pileus (Fig. 23). A 



