Dec. 23, 1918 Parasitism of Cronartium ribicola 645 



aeciospore initials gives rise to a row of alternating aeciospores and intercalary 

 cells (PI. 58, Y), as has been stated in the description of aeciospore forma- 

 tion elsewhere in this paper. The nuclei of the aeciospores soon become 

 completely reorganized (PI. 59, AA) and typical of the dikaryon in all its 

 stages, but the nuclei of the intercalary cells slowly degenerate and dis- 

 appear. A centrosome, the center for the polarization phenomena pre- 

 viously mentioned, can be differentiated by proper staining on the men- 

 brane of each nucleus. 



NUCLEAR PHENOMENA IN UREDINIUM 



The mycelium in leaves of species of Ribes has already been described. 

 The nuclei at this stage of the dikaryon are only slightly less in diameter 

 than that of the hyphae, and they are therefore somewhat separated — 

 not side by side as in the elements of the ^ciospore chain (Pi. 59, C). 

 They do approach each other in the larger cells at the base of theuredinium 

 and undergo conjugate division by a process apparently identical (PI. 59, 

 E-I) to that described for the basal cell and aecispore initial of the aecium. 

 The same holds true for the division in the urediniospore initial (PI. 59, 

 J-M, N, b). The wall formed between the two pairs of daughter nuclei 

 divides the initial into an upper, larger cell, the young urediniospore, and 

 a lower, smaller cell, the young stalk cell, which rapidly elongates. The 

 nuclei in the spore become organized similarly to those of the aeciospore 

 (PI. 58, AA), while the stalk nuclei slowly degenerate (PI. 59, N). The 

 second and subsequent conjugate divisions in the basal cell (PI. 59, O) are 

 similar to the primary division, but the nuclei often are not so definitely 

 placed side by side, an irregularity probably due to the tendency of the 

 cytoplasm flowing into the second urediniospore initial to pull one of the 

 nuclei along with it. As a rule, both in the basal cell of the aecial chain 

 and in the basal cell of the uredinium at the time of the primary division, 

 the nuclei, though they may commence to divide when located at different 

 levels in the cell, become arranged side by side at metaphase. In the 

 second division of the urediniospore basal cell this orientation may not 

 take place, so that at telephase one of the daughter nuclei may be well 

 up in the new urediniospore initial and its companion just passing into it; 

 but on reorganization after the initial is cut off from the basal cell, the 

 two nuclei take a position side by side in the typical manner. Their 

 division and the subsequent cutting off of the stalk cell by a wall give rise 

 to a second urediniospore. This process may be repeated several times, 

 for the basal cell seems to retain its powers of division until the sorus 

 dries up. 



NUCLEAR PHENOMENA IN TELIUM 



The telia are borne on the same mycelium that gives rise to uredinia. 

 By an exactly similar process of conjugate division (PI. 59, S-W) a cell 

 is cut off from the basal cell of one of the telial unit columns; but this 



