100 RESEARCHES ON FUNGI 



observed to be 0-43 mm., 0-76 mm., and 0-16 mm. respectively. 

 A simple calculation based on these data shows that the sporangium, 

 when head on to a beam of parallel light rays, casts a shadow which 

 has an area 7* 2 times that of a cross-section of the stipe. If there 

 were no subsporangial swelling, the shadow of the sporangium 

 would cut off the light from the top of the stipe before the ortho- 

 heliotropic position had been completely attained. This would 

 prevent the gun from being accurately directed toward the source 

 of the strongest light. Evidently, the difficulty of supplying the 

 stipe with the delicate heliotropic stimulus which it requires has 

 been overcome in the course of evolution by the intercalation, 

 between the sporangium and the stipe, of the large light-collecting 

 subsporangial swelling. That part of the swelling which bulges 

 out laterally beyond the black sporangium receives the light and, 

 by refraction, concentrates it upon the base of the swelling. The 

 asymmetrical position of the spot of light so produced provides the 

 condition for the despatch of a stimulus from the swelling to the 

 motor region of the stipe, and to this stimulus the motor region of 

 the stipe can react with great precision, even when it lies in the 

 shadow of the sporangium and subsporangial swelling and is receiving, 

 as shown in Fig. 47, no direct rays of light whatever. 



The spot of light which is formed within the subsporangial 

 swelling by sunlight and is of so much physiological importance for 

 the heliotropic reaction of the sporangiophore is not constant in 

 size, shape, and brightness, but varies in all these qualities with 

 the position in which it happens to be formed. A study of this 

 variability will now be attempted. 



When the incident rays of light strike the subsporangial swelling 

 transversely, i.e. perpendicularly to its long axis, the spot of light 

 formed by the convergence of the rays on the back wall of the 

 swelling is oval in outline, of maximum size, and of minimum 

 brightness (Figs. 50, A, and 51, p. 105). Its oval shape is due to 

 the fact that the main mass of the swelling is oval like an egg, its 

 relatively large size to the fact that the swelling is not nearly so 

 wide as it is long, and its paleness to the fact that the rays of light 

 fall on the back wall of the swelling at a considerable distance in 

 front of their focal point. As may be seen by reference to Figs. 48 



