"I ASCOMYCETES 53 



In the forms here grouped together as Plectomycetes, we have three or 

 four varieties of sexual apparatus, but the richly septate types are never 

 reached and detached spermatia are not known. The antheridium is a uni- 

 nucleate or coenocytic cell borne at the end of a stalk; the most elaborate 

 archicarp is that of Eurotium with a unicellular trichogyne and an oogonium 

 which becomes septate after the fertilization stage. In Gymnoascus there is 

 no trichogyne and the compact sheath of the Aspergillaceae is represented 

 by an open weft of hyphae. 



In the Erysiphaceae a trichogyne is not developed and the oogonium 

 becomes septate after fertilization, but only one cell gives rise to ascogenous 

 hyphae or (in Sphaerotheca) becomes the single ascus. 



In the Endomycetaceae two enlarged cells unite, and the single ascus is 

 the immediate product of their union. The fusing cells arise on the same 

 mycelium as the conidia, and in Endomyces they are of different size; each 

 is uninucleate and after the union of their nuclei the fusion nucleus divides 

 to form the nuclei of the ascospores which may be four or eight in number; 

 in view of what is known in other asci it may be inferred that in the course 

 of these divisions meiotic reduction takes place. 



Should this inference prove correct, and there seems no other stage of 

 the life-history at which meiosis is likely to occur, we have here a diploid 

 phase of the briefest possible duration, meiosis immediately succeeding 

 fertilization. This condition may be either primitive or reduced ; ascogenous 

 hyphae have not yet been interpolated or have already disappeared. The 

 only indication in favour of the latter hypothesis is the occurrence in certain 

 species of three nuclear divisions in the ascus and eight ascospores; there 

 are, however, other cases, such as the oogonium of the Fucaceae, where a 

 third division regularly follows meiosis even when all the nuclei formed are 

 not to be utilized, and in consequence the significance of the third mitosis 

 cannot be pressed. If the Endomycetaceae be regarded as primitive, or 

 rather as a simple offshoot from a primitive common ancestor, the ascogenous 

 hyphae must be regarded as an interpolated phase in the life-history, and 

 this interpolation seems to have entailed (i) the septation of the cell in which 

 fertilization takes place, and (2) the formation of one or more asci from one 

 or more of its subdivisions. 



These changes have been established in certain simple forms through 

 which the rest may have been derived : 



( i ) Erysiphaceae ; the vegetative cells are generally uninucleate ; the oogo- 

 nium after fertilization divides to form a row of cells, and from one of these 

 ascogenous hyphae grow out. The single ascus of Sphaerotheca and Podosphacra 

 is presumably the result of reduction, since there seems no explanation of 

 the development of a sporophyte unless the number of spores resulting from 

 a single sexual act is thereby increased. As simple forms go, the sheath is 



