134 DISCOMYCETES [CH. 



some it branches in a stellate manner, in others the ascocarp is raised and 

 laterally compressed so as to resemble a miniature mussel or oyster shell 

 standing on its hinge and with the opening uppermost. In this case, or 

 when the ascocarp is superficial, it is rigid and carbonaceous in consistency, 

 when developed beneath the epidermis of the host it is membranous. 



The ascospores are coloured or hyaline and are frequently septate ; they 

 may be very long and narrow and may be surrounded by a gelatinous 

 membrane. 



In a few cases pycnidia are known, producing oblong, unicellular, hyaline 

 conidia. 



The majority of species are saprophytic on old wood, bark, or dry leaves. 

 The mycelium is intercellular, and is sometimes parasitic on living plants 

 though the apothecia reach maturity only on parts that have been killed. 



The details of cytology and development are not known, nor do these 

 minute species, growing often on a hard substratum, seem very promising 

 objects of study. 



The subdivisions of the Hysteriales, of which Hysteriaceae and Hypo- 

 dermataceae are the chief, depend upon the consistency of the sheath, on 

 the form of the ascocarp, and on whether it is superficial or immersed. 



Lophodermium Pinastri (Hypodermataceae) produces pine-blight or 

 needle-cast in the seedling of Pinus sylvestris and other conifers, causing 

 them to drop their leaves. The mycelium ramifies in the leaf and gives rise 

 first to pycnidia and later, usually after the leaf has fallen, to ascocarps. 

 These are black and oblong, opening by a narrow slit. The spores are filiform 

 and continuous. The disease does very considerable damage to young plants, 

 often causing death. It attacks mature trees also and, though these are not 

 themselves seriously injured, they act as centres of infection, particularly in 

 the neighbourhood of seedbeds and nurseries. 



In- the form of their fructification the Hysteriales are intermediate between 

 the Discofflycetes on the one hand, and the Pyrenomycetes on the other, 

 and have been variously included under either of these headings. Their 

 black, coriaceous ascocarps, opening by a narrow slit, differ from those of 

 certain Phacidiales chiefly in the less exposure of the disc. 



They approach the Sphaeriales in the frequent occurrence of coloured, 

 septate spores, as well as in the consistency and often in the form of the 

 ascocarp, which is distinguished from a true perithecium chiefly by its 

 elongated opening, and by the absence of periphyses\ Possibly a study of 

 their minute anatomy may lead to more definite knowledge of their 

 relationships. 



' For definition, see p. 140. 



