294 COMPARATIVE MORPHOLOGY OF FUNGI 



Hypocreales and Sphaeriales and which tends to pass from endoparasitism 

 to ectoparasitism and to the asterinoid habit, i.e., to become free from the 

 host and transfer the fructifications from the interior of the leaf to its 

 surface. Thus we will observe how in many Leveillelleae the intramatrical 

 hypostroma gradually degenerates; its cross section diminishes in propor- 

 tion to that of the ascus stroma and the hypostroma finally shrinks to a 

 spatially limited foot from which the fertile hyphae rise in a single column 

 (Fig. 192) . The forms which belong to this new type are united in the sub- 

 family Coccoideae. Under these conditions, a natural line between the 

 Leveillelleae and Coccoideae cannot be drawn, but this line rests upon 

 a more or less arbitrary relationship of breadth between fertile ectostroma 

 and sterile entostroma. No representative of either of these tribes has 

 been fully investigated. 



Phyllachoraceae. — While the ascus stromata of the Dothideaceae 

 are independent of the place of their formation, and at maturity are 

 always free, in this family they are always covered by the host tissues; 

 in the tribe Trabutieae they lie between cuticle and epidermis, in the 

 Scirrhieae between epidermis and palisade layer, and in the Phylac- 

 choreae in the mesophyll. The artificiality of this division is shown by 

 the fact that the ascus stroma of Phyllachora graminis (Phyllachoreae) 

 is formed directly under the epidermis and later penetrates deeper. 



Morphologically the Phyllachoraceae may be derived from Dothi- 

 diaceae which, on account of their exclusively intramatrical life, have 

 undergone all sorts of modifications. Thus instead of the tuberous 

 or pulvinate stromata of the true Dothideae, there appears a more flat- 

 tened stroma. Further in most genera the cuticle or epidermis is pene- 

 trated in all directions by stromatal hyphae bound inseparably with 

 the stroma and incorporated with it. The cuticle in the Trabutieae, 

 or the epidermis in the remaining genera, thereby becomes an organic 

 component of the stromatal cover layer. This is called the clypeus. 

 Beginnings of such clypeal formation are already present in the Pseudo- 

 sphaeriaceae (in the Didymella Rehmii group), in the Sphaeriales, in the 

 Mycosphaerellaceae and in the Clypeosphaeriaceae, not otherwise dis- 

 cussed in this book. However, they never attain the typical structures 

 of the Phyllachoraceae and can hardly be confused with these as the 

 stroma forming them in the Sphaeriales contains true perithecia. 



How the limits of the Phyllachoraceae, as contrasted with the 

 Dothideaceae and simpler Sphaeriales, may best be drawn will appear 

 only with extensive ontogenetic investigation. As characteristic of the 

 position, a single example will be cited: the common Phyllachora graminis, 

 which causes elongate black wales on grasses, has no stroma (in spite of 

 the specific character of the order), but true solitary perithecia which are, 

 however, held together by a common clypeus. Futhermore, it possesses 

 true paraphyses which grow into the cavity of the perithecia (C. R. 



