THE PILOBOLUS GUN AND ITS PROJECTILE 131 



photically sensitive and directs the growth of the stipe towards 

 the Hght. 



The stipe is positively heHotropic so long as it is growing in 

 length ; but, as soon as it ceases to elongate and begins to develop 

 a sporangium at its free end, it ceases to respond to unilateral 

 illumination. Even when it is exposed to the direct rays of the 

 sinking sun, so that its axis is at right angles to the rays, it shows no 

 heliotropic reaction whatever. The inability of the stipe to make 

 heliotropic curvatures coincides roughly with the hours of darkness 

 and lasts from about 7 o'clock in the evening until early next morning, 

 by which time the subsporangial swelling has been developed. 



What advantage to Pilobolus, if any, is its periodical develop- 

 ment which results in the production of diurnal crops of fruit-bodies 

 and in the discharge of the sporangia between about 10 a.m. and 

 early afternoon ? The answer to this question is not far to seek. 

 Pilobolus uses light to direct its guns toward open spaces, with the 

 result that its projectiles are shot away from dung-plats in pastures 

 on to the surrounding herbage where they remain until they are 

 swallowed by herbivorous animals. If the guns were to be developed 

 and discharged during the night, light could not be used to lay them 

 and the sporangia would not be scattered nearly as effectively as 

 they actually are ; but the periodicity in the development of a single 

 fruit-body is such that the diminishing afternoon light can be 

 employed for directing the growth of the naked sti])e and the strong 

 late-morning hght for the precise orientation of the mature gun just 

 before it shoots away its sporangium. We may conclude, therefore, 

 that the periodicity in the development of the fruit-body of a 

 Pilobolus indirectly favours the dispersion of its sporangia and thus 

 assists the species in maintaining its place in nature. 



The Subsporangial Swelling and the Discharge of the Pilobolus 

 Gun. — The first special function of the subsporangial swelling — that 

 of acting as an ocellus for perceiving the direction of the strongest 

 incident rays of light and for assisting in the heliotropic laying of 

 the Pilobolus gun — has been fully treated of in previous pages. 

 The second special function of the subsporangial swelhng — that of 

 acting as part of a squirting mechanism for the discharge of the 

 sporangium — will be discussed in what follows. 



