THE PILOBOLUS GUN AND ITS PROJECTIT.E 165 



the sporangium which enables the drop of cell-sap accompanying 

 the discharged sporangium to turn the sporangium round at the 

 moment of landing so that the gelatinous ring comes into contact 

 with the surface of the object struck. The unwettability of the 

 sporangium-wall, in promoting the fixation of the sporangium, 

 indirectly promotes the dispersion of the spores. 



After a sporangium has become attached to the leaf or stem of 

 a grass or other flowering plant, many weeks or even months must 

 often go by before it is swallowed by a herbivorous animal. During 

 this time it may be exposed for a great many hours to brilliant 

 sunshine and yet (there is every reason to suppose) the spores retain 

 their vitality unimpaired. The spores of Pilobolus have colourless 

 spore-walls and therefore, like the colourless spores of Schizophyllum 

 commune and Daedalea unicolor,^ they might be killed by prolonged 

 exposure to the sun's direct rays ; but they are protected from any 

 possibly injurious rays of light by the sporangium-wall which is so 

 intensely black that it must absorb practically all the light which it 

 does not reflect or diffuse.^ 



1 These Researches, Vol. I, 1909, pp. 24-2G. 



2 Coprinus, Panaeolus, Anellaria, Sordaria, and certain other genera of copro- 

 philous fungi have black spores. These spores, like the black sporangia of Pilobolus, 

 settle on and become firmly fixed to the stems and leaves of grasses and other 

 flowering plants in pastures, and they must often wait through long periods of 

 sunny weather before they are swallowed by herbivorous animals (c/. these Researches, 

 Vol. Ill, pp. 229-230). While exposed to sunlight, these spores are protected 

 from possibly injurious rays of light by their own light-screens, namely, their rather 

 thick black walls. In Pilobolus where the spores in a sporangium are protected 

 by a common light-screen, namely, the black sporangial wall, and the spore-walls 

 therefore cannot function as light-screens, the spore-walls are colourless. The 

 presence of a blackish pigment in the wall of the sporangium and the absence of 

 such a pigment from the walls of the spores are just what might be expected if, as 

 I think probable, the sporangial wall does actually protect the spores from injurious 

 rays of light. 



While in Coprinus, Panaeolus, etc., as we may suppose, the pigment in the 

 spore-wall enables the wall to act as a light-screen and is therefore functionally 

 useful, in other equally coprophilous fungi, e.g. Aleuria vesiculosa, Lachnea stercorea, 

 and Huinaria granuJata, the spore-walls are colourless. It is possible that, in these 

 species, owing to the fact that the spore-walls are colourless and therefore cannot 

 function as a light-screen, the spores, when exposed to sunlight, are more readily 

 killed than the black spores of Coprinus, etc. An experimental test of this suggestion 

 is desirable. 



The spores of Hypoxylon, Xylaria, Rosellinia, Bulgaria, and certain other 

 fungi which are not normally coprophilous but live on dead wood or other plant 



