98 RESEARCHES ON FUNGI 



so that during the last stages of the bending, in such a fruit-body as 

 that shown in Fig. 46, the upper part of the stipe for some distance 

 back from the motor region receives no direct rays of light what- 

 ever. It is further to be emphasised that, as the heliotropic re- 

 action of the sporangiophore becomes more and more complete, 

 whereas the spot of light in the subsporangial swelling approaches 

 nearer and nearer to the motor region of the stipe until finally it 

 comes to rest upon the annular heap of protoplasm just above it, 

 the band of light in the stipe recedes more and more from the motor 

 region. 



The facts brought forward in the above discussion justify the 

 conclusion that the heliotropic reaction of the sporangiophore of 

 Pilobolus, at least in its final stages, is not due to the action of the 

 light which falls on the motor region or any other part of the stipe 

 but to the action of the light which falls on the subsporangial 

 swelling and, in particular, to the action of the light which forms an 

 asymmetrically situated light-spot on the protoplasm lining the 

 cell-wall of the swelling. 



In Pilobolus Kleinii the protoplasm in the lower part of the 

 subsporangial sweUing and at the top of the stipe, as shown in Figs. 46 

 and 47 by shading, is strongly pigmented with carotin ; and, as 

 we have seen, at the top of the stipe just above the stipe's motor 

 region, the protoplasm is usually heaped up so as to form a strongly 

 biconcave, very red, centrally perforated septum. Direct observa- 

 tions with the microscope and a study of the course of the rays of 

 light represented in Fig. 46 make it clear that the protoplasmic 

 septum, owing to its peculiar shape and to its distance from the top 

 of the subsporangial swelling, is admirably adapted for receiving 

 the rays of light as they begin to diverge from one another after 

 coming to a focus in the cell-sap when the sporangiophore is in a 

 position of complete or almost complete physiological equilibrium. 

 From the point of view of general structure and function, the layer 

 of protoplasm lining the lower part of the subsporangial swelling 

 and forming the incomplete septum at the top of the stipe — in its 

 concave shape, its strong pigmentation, its position in respect to 

 the subsporangial lens, and its mode of functioning by sending a 

 stimulus to the motor region of the stipe — is comparable with the 



