THE PILOBOLUS GUN AND ITS PROJECTILE 107 



According to the modern conception of heliotropism, for which 

 Blaauw ^ is chiefly responsible, an organ, such as the sporangiophore 

 of Phy corny ces nitens or the coleoptile of Avena sativa, turns toward 

 or turns away from a source of Hght because its sides are at first un- 

 equally lighted and because, as a result of this unequal illumination, 

 one of the sides grows more rapidly than the other until the illumina- 

 tion of the two sides has become equalised. The turning of an organ 

 until it points toward a source of light is therefore in itself a secondary 

 phenomenon which results from unequal light-growth reactions. 



The explanation of the heliotropism of Pilobolus which I have 

 already given is in reality but an extension of Blaauw's general 

 theory of hehotropism to the special case of a fungus provided with 

 an ocellus. In Phycomyces nitens, so thoroughly investigated by 

 Blaauw, the sporangiophore is a simple cylinder and therefore 

 relatively undifferentiated, and its upper part, when unequally 

 lighted, is the part which reacts by unequal growth. In Pilobolus, 

 on the other hand, the sporangiophore is differentiated into a sub- 

 sporangial swelhng and a stipe, and there is a division of labour 

 between these two structures of such a kind that, when the sporan- 

 giophore is bending hehotropically, the subsporangial swelhng 

 receives the rays of hght and becomes unequally illuminated but 

 does not alter in form, while the top of the stipe, which hes below 

 the swelhng and may or may not be illuminated, reacts to the 

 unequal lighting of the subsporangial swelhng by unequal growth. 

 In the sporangiophore of Phycomyces nitens one and the same part 

 both receives the light and responds to it, whereas in that of Pilobolus 

 one part receives the light but another part responds to it. 



That the decisive condition for hehotropic response in Phycomyces 

 nitens is not the direction of the rays of light but unequal illumination 

 is shown by the results of a beautiful experiment made by Buder.^ 



1 A. H. Blaauw, " Licht und Wachstum," I and II in Zeitschrift fiir Botanik, 

 Jahrg. VI, 1914, and VII, 1915 ; III (" Die Erklarungdes Phototropismus") in part 

 XV of Mededeelingen van de Landbouwhoogeschool, Wageningen, 1918. 



2 J. Buder, " Die Inversion des Phototropismus bei Phycomyces," Ber. d. D. 

 Bot. Gesellschaft, Bd. XXXVI, 1918, pp. 104-105. Some other ingenious experi- 

 ments made by Buder demonstrate that it is not the direction of the light but 

 imequal lighting that is responsible for the heliotropism of the coleoptile of Avena. 

 Ibid., Bd. XXXVIU, 1920, pp. 14-19. 



