30 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



SPECIAL NUCLEAR PHENOMENA. 



Phyllactinia is especially favorable for . the examination of the 

 structure of the nuclei and their fusion and division in the ascogenous 

 hyphas and asci. The large number of asci formed in each ascocarp 

 makes it possible to readily obtain large numbers of the nuclei in all 

 stages of their development. The young perithecia of Phyllactinia 

 also, in most of their stages, seem especially favorable for fixation, so 

 that the nuclear figures in the sexual cells, the ascogenous hyphse, and 

 young asci are differentiated with exceptional distinctness and clearness 

 of detail. As a result, certain structures which could be made out 

 only at particular stages and without full details in Erysiphe can be 

 found at every stage of nuclear development in Phyllactinia. I have 

 described (38) for Erysiphe a peculiar and characteristic attachment of 

 the chromatin of the nucleus to the central body, giving the nucleus a 

 characteristic polar rather than a radial structure. This condition is 

 very conspicuous in the young daughter nuclei in the ascus of Erysiphe. 

 In Phyllactinia the nuclei throughout the entire plant, in both the 

 mycelium and ascocarps, show this characteristic relationship of the 

 chromatin and the central body; and in the ascogenous hyphae and* 

 young asci a very definite type of orientation of the chromatin threads 

 as a cone or partial aster extending from the central body into the cavity 

 of the nucleus is conspicuous. This persists through the fusion stages 

 in the ascus as well as through division and the resting-stages, and the 

 central body is thus shown to be a permanent structure through the 

 whole life history of the mildews. 



I shall continue, as in former papers, to call this structure a central 

 body rather than to use the term sentrosphere or centrosome. I do this 

 not to indicate that I consider this body in the mildew as different from 

 apparently similar structures found in the karyokinetic figures of other 

 plants, but merely because I prefer the more general descriptive term 

 rather than a more technical one, since the bodies in question are still so 

 variously described by the different authors who have worked on them. 



It is a conspicuous and important fact that the nuclei and cells of 

 the mildews undergo extreme variation in size in the course of the 

 development of the fungus, and in general it is plain that the nuclei are 

 larger in the larger cells. Before proceeding with the description of 

 the organization of the nucleus I shall briefly summarize the facts as 

 to this variation, since, as we shall see later, these facts may have an 

 important bearing on the interpretation of the nuclear fusion in the ascus. 

 The nucleus of the oogonium is considerably larger (fig. 7) than the 



