296 CLASS ASCOMYCETEAE 



a somewhat primitive order from which have arisen the Sphaeriales. The 

 asci become exposed by the weathering away of the outer part of the 

 stroma or of its apical portion. Rarely the central apical tissues dissolve 

 away to form an ostiole. In the forms with a small perithecium-like 

 stroma the tissues between the ascus locules break or dissolve away leav- 

 ing shreds that have been taken for paraphyses, so that the numerous asci 

 appear to stand in a true perithecial cavity. Careful investigation has 

 shown that many species formerly assigned to the genera Pleospora and 

 Leptosphaeria (Family Pleosporaceae, Order Sphaeriales) have the fore- 

 going structure, hence must be transferred to this order. Just how far 

 this may apply to the many remaining species of these and other genera 

 can be determined only by careful study of the development of the young 

 ascocarps. To say, as does Gaumann, that these represent a transition 

 from one order to the other may represent the truth but the author pre- 

 fers to reserve judgment until further light is thrown on the subject by 

 ontogenetic investigations throughout the various genera of the Sphae- 

 riales. It is possible that these are transitional forms but that evolution 

 has progressed in the contrary direction, from the Sphaeriales to the 

 Pseudosphaeriales. Sexual reproduction has not been investigated in 

 enough undoubted members of the order to enable the information gained 

 to be used in classification. Conidia are produced in a number of families. 



Theissen and Sydow recognize several families. The most important 

 are the following: 



Family Pseudosphaeriaceae. The stromata resemble separate peri- 

 thecia of the Sphaeriales and open at the apex either by crumbling of the 

 tissues or by their dissolution to produce an ostiole. The asci are clustered 

 at the base, separated by paraphysis-like hyphae which are attached at 

 the top as well as bottom. Or it may be that the interior tissues and part 

 or all of the interlocular tissues may dissolve, leaving a cluster of asci at 

 the base of a hollow perithecium-like structure so that with mature asco- 

 carps it can not be determined easily whether the fungus belongs to the 

 Pseudosphaeriales or to the Sphaeriales. Among the fungi assigned here 

 with more or less confidence are some species of Pyrenophora, of which P. 

 teres (Died.) Drechsl. (P. irichostoma) is the perfect stage of Helnimtho- 

 sporium teres Sacc, the cause of net blotch of barley. In this genus the 

 ascocarps are hairy and the phragmosporous or muriform ascospores 

 colored. Whether Dothiora should be included here or in a separate family 

 is doubtful. In its general structure it resembles somewhat the foregoing 

 but the perithecium-like stromata are broader and flatter and the muri- 

 form ascospores hyaline. (Fig. 96.) 



Family Mycosphaerellaceae. In the Mycosphaerellaceae the 

 genera Guignardia and Mycosphaerella deserve attention. G. hidwellii 

 (Ellis) V. & R. is the cause of the very destructive black rot of the grape. 



