2O SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



central position. Their shape at this time is well shown in fig. 41. It 

 is noticeable that they are thicker in their upper portion and taper con- 

 siderably below. A comparison of figs. 53-56 with fig. 41 suggests that 

 the vertical elongation of the asci has been mainly in this lower tapering 

 region, which has been extended as a stalk-like prolongation connecting 

 the upper thickened portion, in which it is to be noted lie the nuclei, 

 with the sterile cells of the ascogenous hyphae below. This tapering 

 stalk portion is never cut off by further cross-walls to form a true stalk- 

 cell. A considerable portion of it thickens up later and the nucleus 

 comes to lie much further down than in the present stage. There is 

 always, however, in the mature asci an abruptly narrowed basal portion 

 (see figs. 55, 56) where the ascus connects with the next adjacent cell of 

 the ascogenous hypha. A basal stalk-cell is, however, never cut off 

 from the lower end of the ascus. It remains always the entire cell of 

 the ascogenous hypha, as it was formed when the hyphae became septate. 



The young asci are very closely inclosed from the start by the 

 vegetative hyphse of the perithecium. At a time when the asci are 

 scarcely differentiated and the division of the ascogenous hyphse into 

 binucleated cells is scarcely complete the perithecium has the general 

 structure shown in fig. 27. No differentiation of the perithecium into 

 separate layers can be made out at this stage. There is no evidence of 

 a radial growth of hyphal branches inward from the perithecial wall. 

 The hyphae of the perithecial wall extend between the branches of the 

 ascogenous hyphaa so that the latter hardly come in contact with each 

 other, but are separated and partly inclosed by hyphal branches which, 

 from their subsequent development and differentiation, we may sup- 

 pose even at this early stage are nurse cells which give up food 

 stuffs to the ascogenous cells among which they lie and which are des- 

 tined to perform the spore-bearing function. 



Median sections show the cells of the perithecium for the most part 

 as oblong in section and apparently uninucleated. Tangential sections 

 show, however, that at this stage they are in reality flattened polygonal 

 plates and contain generally three, four, or even more nuclei. The 

 original stalk-cell of the oogonium is shown in figs. 22 and 27 forming 

 the center of support and attachment for the entire perithecium. The 

 lower cells of the ascogonium extend upward from this stalk-cell and 

 curve to one side in the base of the ascocarp. These cells are large and 

 swollen in appearance. The ascogonium as a whole is never found 

 lying in a single plane so as to appear entire in a single section. Gen- 

 erally its path is so irregular and it has become so distorted by the press- 

 ure of the perithecial cells that its successive cells bend back and forth 



