HISTORY OF PILOBOLUS 43 



and Czurda ^ repeated the experiments of Allen and Jolivette and 

 confirmed their statement that, when the fruit-bodies of Pilobolus 

 are exposed to two beams of light coming from sufficiently different 

 directions, the sporangia are shot to one or other of the two sources 

 of light and not in an intermediate direction. They also observed 

 that, before the development of the sporangium and the sub- 

 sporangial swelling, the young pointed stipe, when subjected to two 

 beams of light coming from different directions, is affected by both 

 beams and takes up a resultant phototropic position. In attempt- 

 ing to explain the non-resultant response of a mature fruit-body 

 to two beams of light, Pringsheim and Czurda made two assumptions : 

 (1) light here hinders growth, and (2) the refraction of the light 

 that enters the subsporangial swelling at a small angle on one side 

 is such that the waU on the same side at the base of the swelling is 

 brightly lighted. As Van der Wey has rightly pointed out, the 

 first of these assumptions is unjustifiable and the second is contrary 

 to facts. 



Van der Wey,2 dissatisfied with the conclusions of Pringsheim 

 and Czurda, re-investigated the response of mature fruit-bodies of 

 Pilobolus Kleinii to two beams of light and, in 1929, in a paper 

 containing numerous statistical observations, graphs, reproductions 

 of photographs, and a construction diagram very similar to one of 

 my own {vide infra. Fig. 59, p. 124), gave an explanation of the 

 phenomenon which seems to be very satisfactory. He states that 

 we know : (1) that the orientation of the ripe sporangiophores does 

 not follow the resultant law (Fig. 15), and (2) that light falhng at an 

 acute angle on the subsporangial swelling has a greater heliotropic 

 influence the smaller the angle ; and he assumes that the pigment 

 zone at the base of the subsporangial swelling is the region of the 

 protoplasm where perception of the light takes place. He then 

 presents a picture of the course of the reaction of Pilobolus to two 

 beams of light as follows. 



" Let us suppose that a sporangiophore during its first 



1 E. G. Pringsheim and V. Czurda, " Phototropische und ballistische Probleme 

 bei Pilobolus," Jakrh. f. tviss. BoL, Bd. LXVI, 1927, pp. 863-901. 



2 H. G. Van der Wey, " Ueber die phototropische Reaktion von Pilobolus," 

 Proceedings, Koninklijke Akademie van Wetenschappen te Amsterdam, Vol. XXXII, 

 1929, pp. 1-13. 



