n] 



ASCOMYCETES 



Fig. 8. Humaria rutilans (Fr.) Sacc.; a. ascogenous 

 cell containing two nuclei cut off from the uninu- 

 cleate terminal cell and stalk ; b. fusion in the ascus, 

 the nuclei are just passing into synapsis; both 

 x 1875. 



cytoplasm forming the primordium of a spore. In 1879 Schmitz observed 

 nuclei in the vegetative cells of several Ascomycetes, and in 1893 Gjurasin in 

 Pesiza vesiculosa recognized that the divisions in the ascus are karyokinetic. 

 The Fusion in the Ascus. In 1894, Dangeard showed in Pesiza vesi- 

 culosa and other forms with a well-developed fructification, that the ascus at 

 its first inception is binucleate and that the two nuclei subsequently unite 

 to form the definitive nucleus of de Bary. He at first believed that the ascus 

 was produced in these cases, as in 

 Eremascus, by the fusion of two 

 independent filaments, but he 

 was soon able to ascertain that 

 the young ascus arises, not by the 

 union of two separate hyphae, but 

 from the penultimate cell of a 

 single recurved filament. The 

 apex of this filament receives two 

 nuclei which undergo a simulta- 

 neous karyokinesis, so that four 

 are formed. One of these lies at 

 the tip of the hypha, and one 

 passes back into the lower part, while the other two lie in the curved 

 portion, and become separated from their sister nuclei by transverse walls 

 (fig. 8). The terminal cell of the hypha thus contains a single nucleus 

 and the penultimate cell is regularly binucleate ; it grows out laterally 

 to form the ascus, and its two nuclei fuse soon after they come together. 



The fusion in the ascus was the first nuclear fusion observed among 

 Ascomycetes and its discoverer, Dangeard, accepting it as a sexual process, 

 regarded the ascus as an egg, and the sexual apparatus described by de 

 Bary and his pupils as at most vestigial. It is probable that this view was 

 influenced by his first opinion that the ascus arose from the fusion of two 

 separate filaments, but it was not modified by his subsequent discovery of 

 the true process. 



Fertilization. In the following year (1895) both de Bary's observations 

 and those of Dangeard were confirmed by Harper, working on the common 

 mildew Sphaerotheca Humuli^. Harper saw the development of a uninucleate 

 oogonium and a uninucleate antheridium. He observed the passage of the 

 male nucleus into the oogonium and the fusion there of the sexual nuclei. 

 After fertilization the oogonium underwent septation, giving rise to a row 

 of cells of which the penultimate contained two nuclei, and, after these had 

 fused, developed into the ascus (fig. 9). He thus demonstrated that in this 

 fungus there is a normal fusion of male and female nuclei, followed by a 



i Sphaerotheca Humuli (DC.) Burr.= Sphaerotheca Castagiiei Lev. 



