HISTORY OF PILOBOLUS 41 



Allen and Jolivette remark : " Whatever the mechanism in 

 Pilobolus for the perception of hght may be. it is certainly efficient. 

 For example, in the white hght 95 per cent, of the sporangia struck 

 a four-centimeter (circular) opening (in the dark chamber) when the 

 culture was twenty centimeters distant from the light ; and, with 

 one or two exceptions, the remaining 5 per cent, struck within one 

 or two centimeters of the opening. It is plain that the aiming has 

 been done with remarkable precision." 



Among the conclusions of AUen and Jolivette are the following. 

 (1) Pilobolus aims point blank at a hght and makes no allowance 

 for the distance through which the sporangia must travel. (2) Pilo- 

 bolus fires its sporangia very accurately toward white and blue 

 hght, much less accurately toward yellow hght, and very inaccurately 

 at red light. 



Allen and Johvette also made the following interesting, un- 

 expected, and important discovery : when a culture is exposed to 

 two eqvAil beams of white light coming from two sufficiently different 

 directions (angle between the two beams greater than about 10°), 

 the sporangium of each Pilobolus fruit-body is aimed at one or the 

 other source of light and the aim is as accurate at the source of light 

 chosen as if the other source did not exist [cf. Fig. 15). The authors 

 were at a loss to explain their discovery and remark "apparently 

 there is nothing to prevent these simultaneous stimuh from acting 

 together to produce a resultant reaction. But this does not occur. 

 The visible reaction is to one and one only of the two possible sources 

 of illumination." That a Pilobolus fruit-body aims at one or the 

 other of two equal well-separated sources of light and not midway 

 between them is due, as we shall see shortly, to the pecuhar properties 

 of the subsporangial swelling. 



The Subsporangial Swelling as an Ocellus. — In 1921, 1 ^ published 

 a short paper entitled " Upon the Ocellus Function of the Sub- 

 sporangial Swelhng of Pilobolus " in which it was pointed out that 

 the subsporangial swelling acts not only as part of a squirting 

 apparatus but as a lens and thereby plays an important part in 

 heliotropic response. A model of a Pilobolus fruit body, which can 



1 A. H. R. Buller, " Upon the Ocellus Function of the Subsporangial Swelling 

 of Pilobolus," Trans. Brit. Myc. Soc, Vol. VII, 1921, pp. 61-64. 



