THE PILOBOLUS GUN AND ITS PROJECTILE 109 



sporangiophore growing in air, since air is a less dense medium than 

 a sporangiophore and its contents, the rays of hght after refraction 

 converge and form a bright band of hght on the side turned away 

 from the hght. Hence the back of the growing zone of the sporan- 

 giophore is hghted more intensely than the front and consequently 

 grows faster, with the result that the end of the sporangiophore is 

 turned toward the light. In a sporangiophore growing in paraffinum 

 liquidum, since paraffin (refractive index 1-47) is a denser medium 

 than a sporangiophore and its contents, the rays of light after 

 refraction diverge from one another and, therefore, do not form a 

 bright band on the side turned away from the light. Hence the 

 front of the growing zone of the sporangiophore is lighted more 

 intensely than the back and consequently grows faster, with the 

 result that the end of the sporangiophore is turned away from the 

 light. In a letter ^ to the author, Buder states that he has experi- 

 mented with Pilobolus and has found that it reacts to unilateral 

 hght in air and in paraffin in exactly the same way as Phycomyces. 

 We thus have evidence which supports the view that the heho- 

 tropism of Pilobolus is due to light-growth reactions and not primarily 

 to the direction of the incident rays of light. 



Assuming that a Pilobolus sporangiophore, when growing in 

 paraffin, turns away from the light, instead of toward the light as 

 it does when it is growing in air, the explanation of the phenomenon, 

 presumably, is as follows. The subsporangial swelhng, when 

 immersed in paraffin, owing to the divergence of the hght rays 

 after refraction, cannot form a spot of light on its wall away from 

 the source of light. Hence the side of the swelling toward the light 

 is more intensely illuminated than the side away from the light. 

 Hence the stimulus sent to the stipe from the front of the subspor- 

 angial swelhng is greater than that sent from the back. Hence, 

 therefore, the stipe grows faster in front than behind and so turns 

 the subsporangial swelling and the sporangium away from the 

 source of the light. 



The Mechanism of Heliotropic Response in Pilobolus and in the 

 Leaves of Certain Flowering Plants. — The mechanism of hehotropic 

 response in Pilobolus is not necessarily unique in the plant world ; 



1 Dated Sept. 12, 1921. 



