204 



PROTOBASIDIOMYCETES 



[CH. 



But the fact that these sori are developed on the same mycelium as the 

 spermogonia, the fact that in their "fertile cells nuclear association takes 

 place and the fact that in the formation of the fertile cell a sterile cell is cut 

 off, all suggest that the true homology is with the aecidium. 



The mycelium formed by the germination of the aecidiospore grows with 

 renewed energy. It consists of binucleate cells giving rise to uredospores. 

 These are borne in groups or uredosori (fig. 179) which may be surrounded 



Fig 



179. a. Phragviidium Rubi Pers.; uredosorus, x6oo; after Sappin-Tiouffy ; /'. Phragmidium 

 violaceum 'VimX..; uredosorus, X480; after Blackman. 



by paraphyses, or in certain genera {Pucciniastrum, Uredinopsis) by a pseudo- 

 peridium. In the young sorus a regular layer of somewhat rectangular basal 

 cells is formed, from which the uredospore mother-cells arise. In Coleo- 

 sporium, in Chrysomyxa, and in the secondary caeomata of Phragmidium 

 subcorticiuin, they are produced in vertical rows like the typical aecidiospore 

 mother-cells and divide to form uredospores and intercalary cells, but, 

 in the large majority of cases, they appear as a succession of buds from 

 different parts of the basal cell. Each bud elongates, its nuclei undergo 

 conjugate division, a stalk is cut off which grows in length but remains 

 narrow, while the uredospore enlarges considerably, its contents acquire an 

 orange or yellow colour, and its wall is variously roughened in most species 

 by minute projections on the surface. Two or more germ-pores are usually 

 present and the uredospore, like the cells which give rise to it, is invariably 

 binucleate ; it produces a binucleate mycelium on which teleutospores or 

 further crops of uredospores are formed. 



Certain s-pecie.s{Puccinia vexans, etc.), occurnng under very dry conditions. 



