V] SPHAERIALES 153 



on which they produce stromata usually below the epidermis and finally 

 exposed by its rupture. 



The stfoma is externally black and hard, built up of hyphae closely 

 interwoven to form a pseudoparenchyma; internally it is of much looser 

 consistency, and is often white or brownish in colour. The perithecia are 

 without definite walls, so that the asci develop in mere cavities in the stroma, 

 which however have the globose form of ordinary perithecia, and are 

 bordered by cells rather smaller and narrower than those of the surrounding 

 mycelium. In some cases, where the inner tissue of the stroma is very 

 loosely interwoven, the perithecium is, however, definitely delimited. 



In Dothidea the stromata form black projecting cushions, which in 

 D. virgidtorum occur on the living, as well as the dead stems and branches 

 of the birch. 



In Plowrightia the very similar stromata run together in masses. 

 P. niorbosa is a serious disease attacking species of Prunus, especially the 

 cherry and plum. The mycelium penetrates the living branches which 

 become swollen and deformed and on which stromata and finally perithecia 

 are produced. 



SPHAERIALES 



The Sphaeriales are distinguished by the dark colour and membranous, 

 corky or carbonaceous texture of their perithecia, and of their stromata when 

 present. They number already considerably over six thousand species, and 

 new species are constantly being brought to light, so that there is no doubt 

 that a study of the tropical forms, at present very incompletely known, 

 will greatly increase their number. Not only the number of species, 

 but the number also of individuals is very considerable; the majority are 

 saprophytes, and serve a useful purpose in bringing about the first stages of 

 decay in such resistant materials as wood and straw. They greatly outnumber 

 the Hypocreales and Dothideales, and it is from their black or brown colour 

 and often charred appearance that the name Pyrenomycetes is derived. 



The origin of the group has been proposed through Chaetoinium, which 

 is sometimes without an ostiole, from the Erysiphales, or, in view of the 

 structure of the sexual organs, from an Eiirotium-\\ke. form among the 

 Plectascales. 



Unfortunately their small size and resistant texture as well as the nature 

 of their habitat make many of the simpler species unfavourable subjects of 

 study, and our knowledge of their development is at present very fragmentary. 

 Sordaria and some others can be grown on artificial media and satisfactory 

 results may be anticipated from a further application of this method. Some 

 of the larger forms with a well-developed stroma can readily be handled but 

 in none of these has normal sexuality yet been observed. 



The perithecia in the simplest forms are borne singly, free or partially 



