THE PILOBOLUS GUN AND ITS PROJECTILE 163 



and other plants growing on many square feet in the neighbourhood 

 of the dung-plat. 



The discharge of the Pilobolus guns brings about the dispersion 

 of the sporangia but not of the spores ; for each sporangium, on its 

 flight through the air, carries all its spores with it and, after it has 

 become fixed to the leaf or the stem of a flowering plant (Fig. 80), 

 its tough black indissoluble sporangial wall prevents the spores from 

 escaping from its interior. When a sporangium has once become 

 attached to a flowering plant, neither wind nor rain can dislodge it 

 or open it, so that clearly wind and rain can have nothing to do with 

 the dispersion of the spores. The dispersion of the spores can be 

 effected only through the agency of herbivorous animals. 



The greater the horizontal range of the Pilobolus gun, the greater 

 the number of square feet of herbage surrounding dung-plats which 

 will become sprinkled with sporangia, and the greater the chance 

 of the sporangia being swallowed by horses, cows, and other 

 herbivora. Hence the great violence of discharge of the sporangia 

 is a factor which, in the end, favours the dispersion of the spores 

 and the persistence of Pilobolus species. 



The heliotropic response of the fruit-bodies of Pilobolus results in 

 each sporangium being shot toward the source of the strongest inci- 

 dent rays of light and, therefore, 171 the direction in which there are 

 fewest obstacles to its flight through the air. In laying the Pilobolus gun, 

 light is used in the most efficient manner as a directive agent. The 

 discharge of the sporangia in the direction of least resistance to 

 their flight favours the scattering of the sporangia on surrounding 

 herbage and, therefore, ultimately increases the chance of the 

 sporangia being swallowed by herbivorous animals. 



As a rule, only the spores of sjJorangia which are swallowed by a 

 grazing animal find their way into a dung-plat. Hence the import- 

 ance of the sporangia becoming firmly fixed to the flowering plants 

 which they happen to strike and being thereby prevented from 

 falling to the earth where they would be wasted. Hence therefore, 

 also, the advantage of the discharged sporangium being provided 

 with a strongly adhesive gelatinous ring, and of the sporangium 

 always landing with the ring toward the surface of the flowering 

 plant. It is the resistance to being wetted offered by the wall of 



