MYRIANGIALES 221 



black disease of hyacinths, and P. gramineum which causes the stripe 

 disease of barley; their imperfect forms belong to Cladosporium, Alter- 

 naria and Macrosporium (Fig. 145, 2), and in P. gramineum to Helmintho- 

 sporium. In a special form of P. herbarum causing a leaf spot of Sivistonia 

 australis (Corypha australis) Cavara and Mollica (1907) have demon- 

 strated that the hyphae are uninucleate. From two hyphae, two unicel- 

 lular branches are formed which, as in Penicillium "crustaceum" and 

 Sphaeroiheca Humuli, approach, embrace and copulate. Meanwhile they 

 have become surrounded by sheath hyphae and later develop ascogenous 

 hyphae in an unknown manner. As sparse as this evidence is, it may 

 be assumed that the Myriangiales have still retained the Plectascales 

 type in the form and function of sexual organs. 



Pleospora and Pyrenophora have until recently been assigned to the 

 Sphaeriales and only a special investigation of their young perithecia has 

 demonstrated their undoubted Pseudosphaeriaceous character. Besides 

 these, however, a series of genera, as Meliola and Parodiopsis in the 

 Perisporiaceae and as Parodiellina, Chevalieropsis and Botryosphaeria in 

 the Dothioraceae, show marked transitional forms which might be with 

 equal justice classified in the family in which they root, in this case the 

 Pseudosphaeriaceae, as in the group to which they lead, in this case the 

 Sphaeriales. Didymella and Leptosphaeria will be discussed here. 



Didymella has 200 species parasitic or saprophytic on the roots and dry 

 leaves of cormophytes. In structure of fructifications they develop 

 from the simpler forms similar to the Pleospora-Pyrenophora type, 

 through numerous transitions, to another extreme which extends into the 

 Sphaeriales. In the simpler forms, as in D. moravica and D. proximella, 

 the perithecia, as in Pleospora and Pyrenophora, are still entirely enclosed 

 and open only at maturity by the crumbling of the more or less sharply 

 defined tip. In this form, the interthecial plectenchyma is generally 

 still present at maturity in the form of distinctly recognizable paraphy- 

 soids. In the higher forms, as in D. Rehmii, D. cladophila and D. 

 applanata, the cause of a " brush'"' disease of raspberries, the perithecia 

 attain a more or less characteristically formed opening, an ostiole, which 

 we meet again in the majority of the Sphaeriales. 



Besides, in them the perithecial stroma is only weakly developed and 

 generally only present at maturity as a scant hyaline mass which easily 

 swells in water. While D. moravica and its relatives belong to the 

 Pseudosphaericaceae, the D. Rehmii group considered by itself would be 

 placed in the true Sphaeriales. 



This transitional character is still more marked in Leptosphaeria, 

 whose species show the characters of not less than three orders, the 

 Myriangiales, Dothideales and Sphaeriales, and hence has been divided 

 by most authors into different genera, which in practice are difficult to 

 distinguish. 



