104 RESEARCHES ON FUNGI 



possibly all, of the strongest rays of light which come to it through 

 the subsporangial swelling. Assuming that the pigmented proto- 

 plasm of which the septum is composed is photochemically sensitive, 

 this arrangement provides us with another beautiful example of 

 the way in which structure and function are correlated. 



When bright sunlight strikes a Pilobolus fruit-body head on 

 and forms a concentrated spot of light at the base of the sub- 

 sporangial sweUing in the manner shown in Fig. 46, the protoplasmic 

 septum, owing to its high content of carotin particles, glows with 

 a rich orange-yellow light. The glow can be readily seen with the 

 naked eye when a fruit-body which faces the sun is viewed from 



the side. 



If, when a fruit-body is facing the sun as shown in Fig. 46, one 

 looks at the top of the fruit-body so as to see it almost in face view, 

 one observes that the fruit-body has the appearance of a lurid red 

 disc with a black spot in the centre. The black spot is formed by 

 the intensely black sporangium and the red zone which surrounds 

 it is formed by the subsporangial swelling at the surface of which 

 red light, emitted by the glowing carotin in the protoplasmic septum 

 and passing upwards through the swelHng's great vacuole, is refracted 

 to the eye. At first sight such an apical view of a Pilobolus fruit- 

 body as that just described reminds one of the pink eyes of certain 

 albino animals. 



We have seen that, when the incident rays of light strike the 

 subsporangial swelling transversely, the spot of light is oval ; and 

 that, when they strike the sporangium and the subsporangial swelling 

 head on and are parallel to the long axis of the swelling, the spot of 

 light is round. It remains to add that, when the incident rays of 

 Hght strike the subsporangial swelling obliquely, say at an angle of 

 19° as shown in Fig. 47 (p. 92), the spot of light formed on the back 

 wall of the swelling is neither oval nor round but, owing to the shadow 

 cast by the black sporangium, convexo-concave or crescentic with 

 the two horns looking upwards toward the sporangium (Fig. 50, 

 B, p. 101). Such a convexo-concave spot of light can be perceived 

 when one looks down a microscope at a fruit-body which is directed 

 obliquely downwards in the air and is illuminated from below \vith 

 parallel rays coming from a plane mirror. 



