IO SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



stages of development of the sexual apparatus. It is not easy to bring 

 out by shading the various bendings of the cells, as they can be fol- 

 lowed by focusing up and down through the thickness of the section, 

 and at the same time to show the contents of every part of each cell as it 

 appears in median optical section. For the most part I have contented 

 myself here, as in my earlier figures of Erysiphe, with fulfilling the 

 second and, of course, the more important of the two requirements. 

 Hence the figures, with the partial exception of 2 and 3, are to be 

 regarded as representing projections of the outlines of the cells as they 

 are followed by focusing up and down through the section, with the 

 content of each cell as shown in median optical section. As a further 

 result, the relative length of the cells in each section and in successive 

 figures is not in every case correctly indicated. In a majority of cases 

 the long axis of the oogonium tends to be parallel to the surface of the 

 leaf or to rise slightly at its outer end, as shown in figs. 2, 3, 4, 10, while, 

 as noted above, the antheridial branch is more nearly vertical to the 

 leaf surface. Many exceptions to these relations are, however, to be 

 found, as shown in the other figures of these stages. 



Fig. i shows a stage when the antheridial branch is not yet cut off 

 from the hyphal cell from which it arose, and it contains a single nucleus. 

 This nucleus divides, and a cell-wall is formed between the daughter 

 nuclei, so that one of them becomes the nucleus of the antheridial 

 branch and the other remains in the hyphal cell below (fig. 3). This 

 cross-wall is put in quite constantly at the narrowed region where the 

 antheridial branch is partially encircled by the oogonium (figs. 3, 10). 

 The nucleus of the antheridial branch now divides, and one of the 

 daughter nuclei migrates into the tip of the branch (fig. 4). The tip is 

 cut off by a cross-wall and becomes the antheridial cell (figs. 5, 6, 7). 



At this stage no trace of the enveloping perithecial hyphse can be 

 found. The whole apparatus consists only of the two gametophores 

 and the gametes which have developed almost simultaneously and are 

 thus sharply distinguished in the time of their development from the 

 protective enveloping hyphal branches which come later. In Phyllac- 

 tinia, as in Sphaerotheca and Erysiphe, the claim that the antheridial 

 branch is to be interpreted as merely the first of the protective hyphse 

 is negatived by its appearing almost simultaneously with the oogonium, 

 while the protective hyphse arise much later and not singly, but in series 

 around the base of the sexual branches. 



Fertilization occurs, as in Sphaerotheca and Erysiphe, by the break- 

 ing down of the walls separating the protoplasts of the oogonium and 

 antheridium and the fusion of the two protoplasts, and ultimately their 



