go RESEARCHES ON FUNGI 



subsporangial swelling, and stipe, as they grow larger, come to 

 touch the crystals in the manner shown in Fig. 45, B. 



The Two Functions of the Subsporang-ial Swelling. — The sub- 

 sporangial swelling of the Pilobolus gun functions in two entirely 

 different ways : (!) as an ocellus which receives the heliotropic 

 stimulus which causes the stipe to direct the free end of the gun 

 toward the source of the brightest light ; and (2) as part of a squirting 

 apparatus which, by violently expelling cell-sap, shoots away the 

 sporangium from the sporangiophore. 



The squirting function of the subsporangial swelling was recog- 

 nised by Link ^ as long ago as 1809, but the ocellus function 

 remained unknown until I ^ called attention to it in a brief paper 

 published in 1921. In what follows these functions will be treated 

 of in detail. 



The Heliotropism of the Sporangiophore with Special Reference 

 to the Ocellus Function of the Subsporangial Swelling. — When a 

 Pilobolus fruit-body is exposed to unilateral light, the upper part 

 of its stipe just beneath the subsporangial swelling makes a positively 

 heliotropic curvature, with the result that, within about an hour, 

 the free end of the fruit-body, i.e. the subsporangial swelling and 

 sporangium, comes to point in the direction of the brightest incident 

 rays of light. The heliotropic reaction of the stipe, as we shall 

 see, is controlled by the subsporangial swelling which, in its mode 

 of refracting and collecting light, acts as a lens. 



One morning in February, 1919, on looking at a large number of 

 fruit-bodies of Pilokolus longipes which were growing on horse dung 

 in a culture chamber, I observed that, although they all pointed 

 toward a distant window so that their free ends were parallel to the 

 incident rays of light (c/. Fig. 46), the top of each stipe glowed with 

 a reddish light. It at once became obvious in respect to each 

 fruit-body : (1) that, but for the existence of the subsporangial 

 swelling, the top of the stipe would be in the shadow of the black 

 sporangium directly in front of it ; and (2) that the glow must be 



^ H. F. Link, " Observationes in Ordines plantarum naturales," Magaz. d. Ges. 

 naturf. Freunde, Berlin, Bd. Ill, 1809, p. 32. 



2 A. H. R. BuUer, " Upon the Ocellus Function of the Subsporangial Swelling 

 of Pilobolus," Trans. Brit. Myc. Soc., Vol. VII, 1921, pp. 61-64, no illustrations. 



