44 



RESEARCHES ON FUNGI 



development has grown in a resultant direction between two direc- 

 tions of light, A and B. What must happen in the second period of 

 reaction ? Let us assume that the subsporangial swelling has then 

 beome fully developed. So long as the sporangiophore stands in the 

 resultant direction, the distribution of light is quite symmetrical ; 



the light cannot therefore cause a 

 reaction. The sporangiophore is in 

 a condition of equilibrium which, 

 however, is unstable. For, as soon 

 as it inclines towards A, it receives 

 the light of A at a smaller angle 

 and that of B at a greater angle, 

 in consequence of which the in- 

 fluence of A becomes greater and 

 that of B smaller. As a result of 

 this the sporangiophore will bend 

 toward A until finally a position 

 of stable equilibrium has been 

 attained. This final position is 

 expressed in illustrations of the 

 results of shooting-experiments 

 {Schuss-hilder) and in Figs. 8 

 and 9 (graphs) can be read for 

 every angle." 



Van der Wey has also shown : 

 (1) that, when the angle between 

 the two beams of light is small 

 (about 7 °- 1 ° ) , the sporangiophores 

 take up a position which indicates that they are influenced strongly 

 by one beam of light and also to a shght degree by the other beam, 

 and (2) that, as the angle becomes very small (2°-4°), the effect of 

 the two beams becomes more and more nearly equalised. Evidence 

 of the almost equal effect of the two beams converging on the fruit- 

 bodies at an angle of about 4-5° is shown in the photograph 

 reproduced in Fig. 16. 



Already, in 1920, I had solved the problem of the non-resultant 

 reaction of Pilobolus to two beams of white light in essentially the 



Fig. 16. — Pilobolus Kleinii. Result of 

 another heliotrojiic experiment in 

 which tlie sporophores were illu- 

 minated with two equal beams of 

 white light, but here the beams 

 converged on each fruit-body at a 

 relatively narrow angle (about 

 4-5^). Conditions otherwise the 

 same as given in the description of 

 Fig. 15. The sporangia are most 

 densely disposed between the two 

 windows, from which it is evident 

 that the sporangia directed them- 

 selves not toward the one or the 

 other source of light but in 

 directions between the two sources of 

 light. Photograph by Van der Wey 

 (loc. cit.. Fig. 2). 



