VIII] 



UREDINALES 



20 1 



cells below them (Fromme '14), or each may receive a second nucleus by 

 migration from a neighbouring vegetative cell (fig. 173). In each case they 

 now constitute the basal 

 cells of the rows of spores 

 and they proceed at once to 

 cutoffaecidiospore mother- 

 cells, each of which in turn 

 divides to separate a small 

 intercalary cell below from 

 the aecidiospore above. 



Exceptionally binucle- 

 ate cells may be observed 

 before the fertile layer is 

 d i fferen t iated . In Puccinia 

 Poarnm nuclear migrations 

 sometimes take place be- 

 tween the vegetative cells 

 at the base of the very 

 young aecidium. These 

 cells may grow up, either 

 at once or after division, to 

 form fertile cells. 



The aecidiospores, then, 

 are the products of a sexual 

 process by means of which 

 two nuclei become associated within the 

 limits of a single protoplasmic mass, form- 

 ing the dikaryon or synkaryon of Maire. 

 The nuclei thus brought together do not 

 fuse, but undergo simultaneous division 

 (fig. 174), so that a daughter nucleus from 

 each passes into every new cell. Conjugate 

 division is continued when the aecidiospore 

 germinates and a mycelium of binucleate 

 cells is produced. The sporophyte of the 

 rusts is thus normally inaugurated in the 

 fertile cells of the aecidium. 



It is not unusual to find spores and 

 vegetative cells which contain three or 

 more nuclei ; in these, as in the binucleate 

 cells, and, indeed, in multinucleate cells of 

 many different groups of plants, conjugate 



Fig. 172. Phragmidium speaosum Fr. ; 



cells; b. fusion of two fertile cells; after Christman. 



Fig. 173. Phragmidium violaceum Wint.; migration of 

 second nucleus into fertile cell of caeoma, x 950 ; after 

 Blackman. 



Fig. 1 74. Melampsora Rostrupi Wagn. ; 

 paired fertile cells, x uoo; after 

 Blackman and Fraser. 



