286 INTRODUCTION TO CEYPTOGAMIC BOTANY. 



294. We come now to a very important division, both on 

 account of the value of many of the species, and of the sin- 

 gularity of their habit and structure.* With the exception of 

 JEndogone and Sjohcerosoma, they all grow more or less com- 

 pletely beneath the surface of the soU, and Endogone is, in fact, 

 a condensed Mucor and Sphcerosoma little different from a 

 spherical Helvella. In consequence of their subterranean 

 mode of growth the hymeniuni is internal, even in those 

 species which are most nearly allied to others with exposed 

 hymenia, which grow in the open air. Some are, in fact, little 

 more than closed Pezizm, as Genea and Hydnocystis. Indeed, 

 there is a small group of Pezizm, which grow in sand or on loose 

 earth, as P. sepulta, affunis, and arenicola, in which the cups are 

 more or less buried. These species are scarcely distinguishable 

 from Hydnocystis. In some the cavity is quite simple ; but in 

 others, from the protrusion and depression of parts of the 

 walls, it becomes irregular. In the more compact and solid 

 species, either the mass is traversed by flexuous cavities, the walls 

 of which constitute a more or less distinct hymenium, or the 

 cavities are quite obliterated, and the mass presents a marbled 

 section, the lighter parts of which consist of a kind of stroma, 

 while the darker are speckled with sporangia immersed in veins, 

 which arise from two confluent hymenia placed front to front. 

 In the species which are more nearly allied to Pezizce, the asci 

 are often cylindrical, and the sporidia of moderate dimensions, 

 though often curiously sculptured ; but in the real truffles the asci 

 are represented by large pyriform sacs, and the sporidia are large, 

 with a reticulated and sometimes spinulose episporium (Fig. 

 65, d). In some cases, as in the red truflle, the episporium is 

 of considerable thickness, and the cells very large and loose. 

 They afford beautiful objects for the microscope, but they often 



* Those who wish for full information respecting these plants must 

 consult the splendid work of Tulasne, which leaves scarcely anything 

 for future observers, whether as regards morphology, history, or phy- 

 siology. The British truflSes have been most successfully investigated 

 by Messrs. Broome and Thwaites, to whose researches a long list both 

 of genera and species is due. Their work was so well performed that 

 no new British species has occurred very lately. 



