DEVELOPMENT OF THE PERITHECIUM. I9 



possibility for special arrangements of the nuclei as to their distribution 

 in asci and stalk-cells. Fig. 290 shows an ascogenous hypha in which 

 the end cell is enlarged and is apparently destined to form an ascus, but 

 contains only a single nucleus. I am inclined to think this is due to 

 the fact that the cell in question is cut in two and that a portion of it 

 should appear in the next section. In this particular case, however, I 

 have been unable to find the second nucleus, and it is possible that we 

 have here a case of a young ascus just cut oif and containing only one 

 nucleus. If this is the fact it is certainly to be regarded as a rare excep- 

 tion to the rule that the young ascus contains two nuclei when first 

 formed. The cell below this in the same hypha contains two nuclei 

 and is pushing out laterally and upward to form an ascus in the normal 

 fashion. The third cell is plainly, from its lack of content, to remain 

 sterile and contains a single, much smaller nucleus. It is evident that 

 there is in Phyllactinia, as also in Erysiphe, no such regular process of 

 forming an ascus from the penultimate cell of a curved lateral branch 

 of an ascogenous hypha as one finds in Ascobolus, Pustularia, Pyro- 

 nema, and some other Discomycetes. 



The yoving ascus regularly contains two nuclei, and since, as noted, 

 the ascogenous hyphse are multinucleated before they are cut up into 

 cells and their cells, which are to form asci, regularly contain two nuclei 

 from the time they are formed, there is no reason for supposing that 

 the pairs of nuclei are daughter nuclei of the same mother nucleus ; but 

 there is no such apparent method of preventing the inclusion of two 

 sister nuclei in the same ascus as one finds in the Discomycetes men- 

 tioned above. Certain cells of the ascogenous hyphse which remain 

 sterile contain only one nucleus, and this may be the reason for their 

 failure to develop further. Still other cells of the ascogenous hyphse 

 containing two nuclei are apparently prevented from developing by 

 overcrowding, so that one can not conclude positively that the number 

 of nuclei which it contains determines necessarily the fate of the cell 

 of an ascogenous hypha. On the analogy with the Discomycetes it 

 seems fair to assume, in the absence of any evidence to the contrary, 

 that the two nuclei in the young ascus are not sister nuclei, though 

 there is again no reason for assuming in Phyllactinia that their relation- 

 ship is a very distant one. The asci now elongate rapidly in a vertical 

 direction, while the sterile cells of the ascogenous hyphae with which 

 they are connected below undergo no further development. As a result 

 these sterile cells and the old ascogonium come to lie further from the 

 center of the perithecium and are more noticeably left behind, as it were, 

 in the basal portion. The asci, on the contrary, continue to occupy a 



