72 



EIGHTEENTH REPORT. 



genous liypbae are diploid even though the two nuclei in each cell 

 are haploid, for so lar as the functions of the cells are concerned 

 there can be little difference whether the two sets of chromosomes, 

 respectively of the male and female origin, are enclosed in one 

 common nuclear membrane or in two separate membranes. 



The sexual cycle (Fig. 9) may be represented graphically with 

 a moderate gap between the point of cell union (CU) and that of 

 nuclear union (NU), this gap representing the nuclear generations 

 during which the nuclei are in pairs and division is conjugate. The 

 main vegetable growth, however, lies between KD and CU. 



Figure 9. Sexual cycle in Pyroneina. 

 Figure 10. Sexual cycle in the Rusts. 



In the Rusts this sclieme is still further moditied. The cell 

 union occurs in the aecium, giving rise to a chain of binucleate 

 aeciosi)ores. These produce on the same or a different liost a 

 mycelium all of whose cells are binucleate. Secondary, truly 

 asexual spores, the urediniospores, nmy occur to multiply this 

 stage. Finally, however, binucleate teliospores are formed. The 

 two nuclei fuse and a diploid nucleus is formed; like that of the 

 young ascus the only one in the life history of the rusts. When 

 this nucleus di\ides it is by a reduction division to form the four 

 nuclei of the jiromycelium and so the nuclei of the uninucleate 

 sporidia. The mycelium produced by these consists of uninucleate 

 cells. It is on this mycelium that arise the hyphae which by their 

 union in the aecium begin the binucleate stage again. 



Here as in the Ascomyceteae there exists a stage with binu- 

 cleate cells and one with uninucleate cells, but the former is usually 



