96 RESEARCHES ON FUNGI 



the median cross-section had a width g-h (corresponding to o-d in 

 the constructional diagram below) and the lateral dark parts had 

 widths of e-g (corresponding to a-c) and h-f (corresponding to d-h). 

 It will be seen by inspection that the actual illuminated central part 

 of the cross-section, namely, k-l, is approximately equal to the 

 calculated, namely, g-h ; and this equaUty may be taken as justifying 

 the assumption that the refractive index from air into cell-sap is 

 approximately the same as that from air into water, namely, 1-34. 



The subsporangial swelling has the optical properties of a bi- 

 convex lens. When sunlight strikes upon one side of it, the rays 

 are refracted through it and converge so as to form a spot of light 

 on the opposite side. The spot can be seen under the microscope 

 in living fruit-bodies which are unilaterally illuminated and its mode 

 of formation can readily be inferred from a study of the diagrams 

 reproduced in Figs. 46, 47, and 48. 



When the incident rays of Ught strike the sporangium and sub- 

 sporangial swelling head on and are exactly parallel to the long axis 

 of the sweUing as shown in Fig. 46, the spot of Ught which is formed 

 by the rays entering that part of the swelling which bulges out 

 beyond and around the sporangium is symmetrically placed at the 

 base of the swelling. Under these conditions the sporangiophore 

 is in physiological equihbrium and no heliotropic reaction is possible. 

 When, however, the incident rays of light strike a sporangium and 

 subsporangial swelling obHquely, say at an angle of 19° as shown 

 in Fig. 47, the spot of Ught is formed on one side of the wall of the 

 swelling in a manner which is asymmetrical for the swelling as a 

 whole. Under these conditions the sporangiophore is in physiological 

 inequilibrium, in consequence of which it reacts heliotropically : 

 the top of the stipe bends toward the source of the hght and swings 

 the subsporangial sweUing and sporangium round until they face 

 the light head on. 



The heliotropic reaction just described may be explained as 

 follows. The patch of protoplasm which is strongly illuminated 

 by the spot of light [st in Fig. 47) undergoes a photochemical change, 

 in consequence of which it sends out a stimulus which travels down 

 the protoplasm Uning the wall of the base of the subsporangial 

 swelhng to the protoplasm surrounding the top of the stipe. The 



