THE PILOBOLUS GUN AND ITS PROJECTILE 167 



Mucor and Pilobohis are closely related genera and it may well 

 be that Pilobolus, through Pilaira, was evolved from a Mucor. It is 

 therefore not without interest to compare the sporangial walls of 

 these two genera. The sporangial wall of Mucor is wettable, non- 

 persistent in that it is diffluent in water, and more or less colourless ; 

 whereas the sporangial wall of Pilobolus is unwettable, persistent and 

 not diffluent in water, and intensely black. The unwettability, 

 persistency, and high pigmentation of the sporangial wall of 

 Pilobolus can all be regarded as special adaptations which con- 

 tribute to the success of Pilobolus as a coprophilous fungus ; for, as 

 we have seen : (1) the unwettability of the wall is a prime factor in 

 causing the sporangium to alight on any object with its gelatinous 

 side turned toward the object : (2) the persistency of the wall 

 enables the wall, when the sporangium is attached for weeks or 

 months to a flowering plant, to prevent the spores from escaping 

 from the sporangium even during rainy weather ; and (3) the 

 intensely black pigment in the wall enables the wall to absorb sun- 

 light and thus to act as a light-screen in cutting off injurious rays 

 of light from the spores which lie beneath it. 



After swallowing a sporangium attached to a blade of grass, a 

 herbivorous animal must often travel many miles before extruding 

 the faeces in which the spores have become embedded. Hence it is 

 clear that herbivorous animals are responsible for the geographical 

 distribution of Pilobolus under natural conditions. It may also be 

 remarked that, since the sporangia of Pilobolus cling so tenaciously 

 to dry grass, the commercial transportation of hay must often 

 involve the spread of Pilobolus species from one country to another. 



Often, on a cow dung-plat or a horse dung-plat in a field, one 

 may observe several hundred fruit-bodies of Pilobolus producing 

 sporangia at one and the same time. Let us suppose that the fruit- 

 bodies of Pilobolus Kleinii produced on a single dung-plat in the 

 course of several days have shot away 1000 sporangia on to the 

 surrounding grass and that, on the average, each sporangium con- 

 tains 45,000 spores. Then the total number of spores contained in 

 the sporangia on the grass will be 45,000,000. When a horse or a 

 cow comes and grazes near such a dung-plat as the one under con- 

 sideration, in a few minutes it may take hundreds of sporangia into 



