CHAPTER XVIII 

 DOTHIDEALES 



It is still more impossible to give a discussion of natural groups cor- 

 responding in any way to natural relationships in the Dothideales than 

 it was in the Sphaeriales. Originally the classification was based upon 

 the fact that the ascigerous locules arose without perithecial wall directly 

 inside darker, harder stromata. Thus they were regarded in part as 

 Xylariaceae without perithecial walls. When it was subsequently 

 recognized that in the highest Scolecosporeae of the Hypocreales, forms 

 without perithecial walls gradually arose from forms with solitary peri- 

 thecia lacking stromata, it was necessary to reorganize the Dothideales. 

 As a matter of fact, in the last decade numerous genera and several 

 families have been removed to the Myriangiales, not to the Sphaeriales 

 or even to the Hypocreales; thus the Dothioraceae are mainly composed 

 of genera removed from the Dothideales. 



Even at present this process is incomplete and the classification is 

 only provisional. At present one can best describe them as Myriangiales 

 with polyascous loculi, thus expressing the Pseudosphaeriaceous character 

 of the majority of their forms as, in the higher Myriangiales, the stroma 

 consists of parallel compact hyphae rising vertically from the host. In 

 the centrally attached forms they first spread out flabelliformly until 

 they have attained the entire basal area of the fructification, and only 

 then rise. In this compact mass, the loculi are formed by resorption. 

 Since there is no perithecial wall, there is no ostiole; the top of a loculus 

 is always formed by part of the stroma which may or may not be thick- 

 ened and may or may not have a crumbling papilla, rounded bluntly or 

 drawn out into a neck. 



The main families of the Dothideales, the Dothideaceae and Phyl- 

 lachoraceae, differ in the topographic position of the mature stromata; 

 in the former, these lie on the surface but in the latter they are at first 

 covered by host tissue. 



Dothideaceae. — This family falls into three tribes: the Dothideae, 

 the Leveillelleae and the Coccoideae. The Dothideae are the central 

 group. Their ascus stromata arise subepidermally and are freed by the 

 rupture of the epidermis. In Systremma Ulmi (Dothidella Uhni), para- 

 sitic on elm leaves (Killian, 1920), the ascospores infect the young leaves 

 during the spring rains and grow there between epidermis and cuticle 

 to a flat, plectenchymatic crust whose elongate, palisade-like, terminal 



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