THE PILOBOLUS GUN AND ITS PROJECTILE 97 



stimulus may be nothing more or less than a diffusible growth- 

 promoting substance ; but, whatever it is, in response to it the 

 top of the stipe (m in Fig. 47), which is the motor region of the 

 sporangiophore, reacts by growing in length and by growing in 

 length most rapidly on the side which is nearest to the spot of 

 hght, i.e. on the side which receives the strongest stimulus. As a 

 result of this differential growth-reaction of the stipe, the sub- 

 sporangial swelling is turned about its base through an angle, and 

 the spot of light gradually passes downwards on the wall of the 

 swelling until it comes to be symmetrically placed at the base of 

 the swelhng, as shown in Fig. 46. As soon as the spot of light 

 reaches this symmetrical position, a physiological state of equilibrium 

 becomes established in the sporangiophore and a further heliotropic 

 reaction of the stipe is impossible. At the end of the turning 

 movement the Pilobolus gun is directed toward the source of the 

 brightest light. 



In support of the explanation of the heliotropic reaction of the 

 sporangiophore just given, which involves the passage of a stimulus 

 from the rigid subsporangial swelling to the plastic motor region 

 of the stipe, may be cited the fact (1) that, under conditions like 

 those represented in Fig. 47, the motor region of the stipe, m, when 

 the heliotropic reaction begins, is in a shadow, s, and receives no 

 direct rays of light whatever, and (2) that the most strongly 

 illuminated protoplasm is the patch shown at st which receives the 

 light concentrated upon it by the body of the subsporangial swelling 

 which acts as a lens. 



We may noAv enquire to what degree, if any, the light falling on 

 the stipe acts as a stimulus which assists in bringing about the 

 heliotropic reaction of the stipe. It is true that in such a tilted 

 fruit-body as that shown in Fig. 47 the rays of light falling on the 

 stipe (rays 9-14) converge so as to form a bright band of light along 

 the middle of the back of the stipe ; but, as shown in Fig. 47, the 

 highest ray strikes the stipe at u some distance below the motor 

 region m ; and, as may be inferred by comparing Fig. 47 with Fig. 46, 

 as heliotropic bending takes place and becomes more and more 

 complete, the subsporangial swelling shades the upper part of the 

 stipe to a greater and greater degree from the direct rays of the sun, 



VOL. VI. H 



