8 INTRODUCTION [ch. 



under the ring itself and none in the area enclosed by it. The ring is depen- 

 dent upon the growth of the fungus which spreads outwards in all directions 

 from the centre, the mycelium dying off as the food materials in the soil are 

 exhausted; in the transition region, where the fungal hyphae themselves are 

 disintegrating, the soil is in high condition, it contains organic residues recently 

 formed and capable of rapid change and the grass is .especially luxuriant; 

 the ring accordingly is just inside the region of maximum fungal activity. 



A certain number of fungi belonging to theTuberales, Elaphomycetaceae, 

 Terfeziaceae and Hymenogasteraceae are completely subterranean or 

 hypogeal in their development. They produce closed fructifications pro- 

 tected by a stout wall of interwoven hyphae. As the spores approach 

 maturity the fructifications develop a strong scent, varying much in character 

 and from the human standpoint either pleasant or disagreeable, which serves 

 to attract animals and especially rodents. The fructification is eaten and 

 the spores pass uninjured through the alimentary canal, and are thus dis- 

 tributed. The truffles {Tuber spp.) are the best known of these forms. 



Coprophilous Fungi. Fungi feeding on organic remains in the soil often 

 benefit by the presence of natural manures and incidentally help to break 

 up these substances so that they become available for the higher plants. 



From such fungi it is no great transition to the extensive coprophilous 

 flora of which the habitat is the dung of various animals and especially of 

 herbivorous species. In addition to the rich nitrogenous food supply which 

 these fungi obtain, the presence of cellulose in the straw and other vegetable 

 debris in the dung is an important factor in their nutrition. This is well 

 shown by the fact that many coprophilous species fail to fruit in artificial 

 culture of dung decoction and agar, unless they are provided with cellulose. 

 Cotton wool or pieces of filter paper laid on the substratum admirably serve 

 this purpose; the latter are soon broken into small flocculent scraps. In 

 nature Zygomycetes, Ascomycetes and Basidiomycetes succeed each other 

 in fairly regular order and, speaking generally, show very similar adaptations 

 to their habitat. In many Ascomycetes (Ascobolaceae, Sordariaceae) the 

 spores are surrounded by mucilage and form together a projectile which 

 owing to its weight can be shot to a much greater distance than would be 

 possible for single spores. The sudden ejection of the spore mass seems to 

 depend on the absorption of water by the mucilaginous contents of the 

 ascus. After ejection the mass dries up and becomes firmly attached to the 

 substratum on which it has fallen. In the same way the spores in the 

 sporangium of Pilobolus are surrounded by a gelatinous envelope which 

 swells in the presence of water and bursts the sporangium wall, so that the 

 whole sporangium is shot off as a single mass and adheres by means of the 

 gelatinous layer to the body against which it strikes. 



The grass surrounding the dung receives an ample supply of spores and 



