V] SPHAERIALES 165 



extended as a thin black layer over a considerable area and ending irregularly ; 

 sometimes, as in species of Valsa, forming black cushions erumpent through 

 the bark of the host. In a few cases the stroma surrounds only the upper 

 part and not the base of the perithecium, and we have thus a transition from 

 the rudimentary stromata of some of the earlier families. 



The peridium is black and leathery, the asci usually long stalked, the 

 spores uni- or multicellular, and hyaline or dark-coloured. 



Conidia are frequently present, borne on free conidiophores or produced 

 within pycnidia. 



The genus Valsa includes some four hundred species and Diaporthe a 

 rather larger number. The majority are saprophytic on wood and other re- 

 sistant parts of plants. 



Diatrypaceae 



In the Diatrypaceae the stroma is developed under the bark of the host, 

 and forms either a cushion or a thin, flat layer which later becomes exposed. 

 Conidia of various kinds are produced, but the conidial and perithecial 

 stromata are often distinct and whereas the latter are of the usual dark 

 colour and carbonaceous consistency the former are frequently light-coloured 

 and fleshy. This separation and the usually unicellular, small, hyaline, 

 curved ascospores are the principal characters of the family. 



The genus Calosphaeria is exceptional in lacking a perithecial stroma; 

 its perithecia are free and it could appropriately be placed in one of the 

 groups near the Pleosporaceae but that a conidial stroma is present and 

 closely resembles that of the Diatrypaceae; the ascospore.s, moreover, are of 

 the characteristic curved form, so that Calosphaeria may, it appears, more 

 fitly be regarded as a reduced member of the group. The species of Calo- 

 sphaeria, like the other Diatrypaceae, occur chiefly on dead wood but 

 C. princeps infects the living branches of cherry, plum and peach. 



In Diatrype the most characteristic stroma is a black corky tissue of 

 indefinite extent in which the perithecia are completely immersed ; the ascus 

 contains eight spores in contrast to the numerous spores of certain species of 

 Calosphaeria and of Diatrypella, a genus further distinguished by the cushion- 

 shaped stroma. 



Xylariaceae 



The Xylariaceae occur chiefly on wood; they represent the highest 

 development of the Sphaeriales and are characterized by the free superficial 

 stroma which is only very rarely, as in Hypoxylon, partly sunk in the sub- 

 stratum, and shows every variety of form from a spreading crust on the 

 surface of the host, as in the genus Numimilaria, some species of which 



