ORDER UREDINALES (tHE RUSTS) 397 



solved and a nucleus passes from one cell into the other. From this cell 

 arise dicaryon hyphae which occupy that portion of the sorus while the 

 monocaryon hyphae degenerate. These dicaryon hyphae give rise to bi- 

 nucleate teliospores on slender binucleate stalks. The nuclei unite in the 

 teliospore and the pedicel breaks, so that the teliospores are distributed by 

 air currents. The zygote nucleus passes into the apical promycelium where 

 the two normal divisions of meiosis occur. Usually but one septum is 

 formed dividing the promycelium into a basal uninucleate cell which soon 

 degenerates, and a large terminal cell with three nuclei. This sends out a 

 slender germ tube into which the nuclei enter and infect the host. After 

 the epidermis is penetrated the nuclei divide further and septa are formed, 

 dividing the resultant mycelium into uninucleate cells. In this species the 

 terminal cell of the promycelium takes the place of a sporidium in infect- 

 ing the host while the distribution of the teliospore by air currents makes 

 distribution by means of sporidia unnecessary. Apparently the spermo- 

 gonia and sperm cells do not function. 



In other microcyclic forms there are other ways in which the life cycle 

 is completed. Miss Allen (1933b) found that in Puccinia malvacearum the 

 teliospores arise from dicaryon mycelium. The two nuclei unite and then 

 divide in the promycelium in the usual way. The nucleus then undergoes 

 division in the sporidium but Miss Ashworth (1931) finds that it gives 

 rise to a monocaryon mycelium. According to her investigations certain 

 cells in the telial sorus show nuclear migrations producing the dicaryon 

 phase. Miss Allen (1935) reported that occasional conidia are formed at 

 the tips of hyphae emerging from the stomata and suggested that possibly 

 these may have a part in the diploidization of the mycelium that forms 

 the telial sorus. However, in the main this appears to result from the 

 intermingling and fusion of hyphae from mycelia produced from two 

 separate but adjacent sporidial infections. In Puccinia prostii Duby, ac- 

 cording to I. M. Lamb (1934), no receptive hyphae occur in the spermo- 

 gonia which are produced. The mycelium within the host (Tulipa sp.) 

 remains monocaryotic. The teliospores arise from the lateral fusion of two 

 adjacent hyphal tips or from the passage of a nucleus through a small 

 opening in the septum from a basally placed cell to the cell above. Al- 

 though normal teliospores are produced, apparently infection rarely if 

 ever occurs by means of sporidia, the mycelium being carried over from 

 year to year through the bulbs. 



In most of the short-cycled rusts studied in which normal spermogonia 

 are produced (spermogonia and telia, or spermogonia and aecia whose 

 spores function as teliospores) it has been shown that the mycelium is of 

 monocaryon type until the telium or aecium is formed, when dicaryon 

 cells appear. It is probable that this dicaryon phase arises in the same way 

 as is described above for macrocvclic rusts. 



