1 9 14] FROMME—AECIDIUM CUP 17 



cytoplasm of the fusion cells is more dense in the upper and central 

 part and is often vacuolate in the base. For this reason it is often 

 impossible to make out the two-legged character of an old fusion 

 cell. Further, the fusion of the cells is often complete, and this 

 also serves to make the detection of fusion cells more difficult in the 

 cup aecidia than in the caeomas, where the fusion is usually between 

 the tips of the gametes, and the bases remain distinct. After the 

 elongation of the fusion cell, the nuclei divide conjugately in the 

 upper central part. In the early stages of division the two spindles 

 stand out sharply (fig. 10). They lie parallel to each other, with 

 their poles in the long axis of the fusion cell. The process of mitosis 

 was not studied in detail, but it seems evident, from the examina- 

 tion of a number of stages in both gametophytic and sporophytic 

 cells, that the essential features are as described by Olive (22). 

 The spindle figures in the early stages are very small. No central 

 bodies could be differentiated at the poles and radiations were only 

 rarely seen. Fig. 11 shows a late anaphase with the dumb-bell- 

 shaped appearance that has been figured by many investigators of 

 nuclear division in the rusts. 



With the completion of the conjugate nuclear division, the 

 fusion cell divides and the apical one-third, containing two daughter 

 nuclei from separate spindles, is cut off (figs. 8, 14). This cell, the 

 so-called aecidiospore mother cell, or, as it should more properly be 

 termed, the aecidiospore initial cell, redivides soon afterward, pro- 

 ducing the aecidiospore and the small intercalary cell. The basal 

 cell meanwhile elongates and repeats the process of nuclear and 

 cell division. Fig. 8 shows a distinctly two-legged basal cell of 

 P. Claytoniata. Both the basal cell and the aecidiospore initial cell 

 are four-nucleated as the result of conjugate division. The subse- 

 quent divisions of the basal cell and aecidiospore initial cell produce 

 a catenulate series of alternating aecidiospores and intercalary cells. 



The terms "fusion cell" and " basal cell" are not synonymous 

 as I have used them. The fusion cell is the immediate product of 

 the sexual cell fusion and may or may not function as a basal cell. 

 It becomes a basal cell with the production of aecidiospores. The 

 basal cell is a conidiophore ("basidium" of the earlier writers) from 

 which the spores are abstricted. The term "basal cell" may with 



