214 



PROTOBASIDIOMYCETES 



[CH. 



fertile cells of Phragmidium violaceum was shown by Blackman and subse- 

 quently by Welsford to be derived from one of the smaller cells at the base 

 of the fertile layer. It is thus a vegetative nucleus; it enters the fertile cell 

 by migrating through the wall, becoming much drawn out and laterally com- 

 pressed. It leaves a pore which maybe identified after its passage (fig. 192). 



Fig. 19:. Phragmidium violaceum Went.; caeoma; a. migration of nucleus 

 from vegetative cell of one hypha to fertile cell of another, x 1040; b. and 

 c. binucleate cells showing the pore through which the second nucleus has 

 passed, x 1010; after Welsford. 



After enterint 



Fig. iq.v Uro'iiyces Poae 

 Kaben.; nuclear migra- 

 tionsin young aecidiuni 

 X950; after Blackman 

 and Fraser. 



the fertile cell the second nucleus is at first smaller and 

 denser than that originally present, but soon becomes 

 similar to it in size and consistency. The fertile cell 

 now elongates and in doing so pushes through and 

 destroys the sterile cell above. The associated nuclei 

 divide simultaneously, a transverse wall is formed 

 between the pairs of daughter nuclei cutting off the 

 aecidiospore mother-cell, and the process is several 

 times repeated so that a row of cells is formed. Each 

 of these divides again to separate a small, binucleate 

 intercalary cell below from the binucleate aecidiospore 

 above. 



A similar type of development initiated by the 

 migration of a vegetative nucleus into the fertile cell, 

 was observed by Blackman and Fraser in the aecidia 

 of Puccinia Poarum and Uromyces Poae (fig. 193). But 

 in neither of these was the sterile cell satisfactorily 

 identified. 



In such cases it seems reasonably clear that the 

 entrance of the second nucleus is not a primitive 

 process but a form of reduced fertilization where a 



