l8 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



the perithecial cells and the young ascogenous hyphse. The questions 

 here involved are of interest as bearing on the relationship of the mil- 

 dews with such forms as Ascobolus and Pyronema, but have no direct 

 bearing on our interpretation of the ascogonium or the entire ascocarp. 



In view of Blackman's and Christman's remarkable discoveries in 

 the rusts, noted further below, I have devoted special attention to the 

 question as to whether in Phyllactinia, either in the ascogonium or the 

 ascogenovis hyphse, binucleated cells may not be present. My results 

 have been entirely negative. While occasionally a binucleated cell is 

 found (fig. 2ia) in the ascogonium, it is even here uncertain whether 

 a subsequent cell division may not separate these nuclei. The great 

 majority of the cells are uninucleated. As just noted, the ascogenous 

 hyphse in Phyllactinia, in whole or in large part, arise from the penulti- 

 mate cell of the ascogonium, and this cell regularly contains several 

 nuclei (figs. 19, 20, 21, 25a). The cells that are to become asci contain 

 two nuclei (figs. 28, 29). The remaining cells of both the ascogonium 

 and the ascogenous hyphse after cell division is complete are almost 

 without exception uninucleated (fig. 19-29). This comes out very 

 strikingly in the case of the old cells of the ascogonium and ascogenous 

 hyphse, which are largely filled with a watery cell-sap (figs. 2$b, 29). 



The enveloping hyphas of the perithecium continue to grow, and 

 thus the protective envelope is enlarged. The ascogonium reaches 

 its full development rather early in forming the row of cells above 

 described, and then ceases to enlarge and remains lying in the basal 

 portion of the growing perithecium (fig. 27). With the enlargement 

 of the perithecium the ascogenous hyph^ push up vertically and away 

 from the ascogonium. It is qjiite plain from fig. 27, as also from a 

 study of the earlier stages, that the ascogenous hyphse develop consid- 

 erably before they become septate. Later, after cell division is com- 

 plete, the asci are formed, either as lateral outgrowths from intercalary 

 cells or from the terminal cell of an ascogenous hypha. In fig. 2gb we 

 find the terminal cell developing as an ascus and also evidence of the 

 repeated pushing out of the subterminal cells, though it is possible that 

 this appearance is due to the formation of the entire branch before cell 

 division occurred. In this figure the second cell appears empty of con- 

 tents, but this is due to the fact that only a small portion of its base 

 appeared in the section drawn, the remaining portion extending into 

 the next section. In this case the two cells which are to produce asci are 

 each binucleated, while the stalk-cells below are uninucleated. Quite 

 possibly the cell division does not occur in the ascogenous hyphse until 

 the stage when the young asci are differentiated, and this gives full 



