PROTOBASIDIOMYCETES 



[CH. 



216 



that Moreau found both processes (cell-fusion being considerably more 

 common than migration) in the same caeoma in Phragmidium subcor^uvn. 

 Since 1905 nuclear association by the fusion of fertile cells in pairs has 

 been observed in a number of species, and seems, according to our present 

 knowledge, to be the usual method. 



In the primary uredosorus of Triphragmidium Ulmartae and in certain 

 other species, Olive, in 1908, found arrangements of an intermediate type. 

 Here the cells of the young fructification 



form a more or less regular layer and cut 

 off sterile cells in the usual way. Other 

 hyphae then push up among them, and 

 cell fusion and nuclear association take 

 place (fig. 196). Fusion begins through 

 a narrow pore which afterwards broadens, 

 and in many cases the nucleus of the 

 younger cell migrates into the older one. 

 The process is thus intermediate between 

 those first observed by Blackman and 

 Christman respectively, and the younger 

 hypha, which does not cut off a sterile cell, 

 may be regarded as either a vegetative 

 structure or a gametangium. Olive suggests that the migrations, recorded 

 by Blackman and by Blackman and Eraser, may be merely early stages of 

 a completer cell-fusion ; the recent critical work of Welsford, however, nega- 

 tives this hypothesis, nor would the occurrence of cell-fusion be of much 

 importance once nuclear association had taken place. 



Phylogeny. The interpretation given to the processes which take place 

 in the aecidium affects the conception of other spore-forms in the Uredinales 

 and indeed of the phylogeny of the group. 



For Christman, the sporophyte arises by the conjugation of undifferen- 

 tiated or scarcely differentiated isogametes which fuse to form the basal 

 cells of the aecidium. The spermatia cannot on this interpretation be male 

 organs, and he regards them as the once-functional asexual spores of the 

 gametophyte. .The basal cells of the aecidium are homologized with those 

 of the uredo- and teleutosori, and the fact is emphasized that the basal 

 cells of the primary uredosorus and sometimes of the teleutosorus also may 

 arise by cell-fusions similar to those in the aecidium. Christman is inclined 

 therefore to regard the micro- species, in which the only spore-forms are 

 teleutospores, or teleutospores and spermatia, as the primitive rusts, and to 

 see in them a gametophytic mycelium bearing asexual spores (spermatia) 

 and undifferentiated gametes by the union of which the basal cells of the 

 teleutosorus are produced. Outgrowths of these cells bear the teleutospores 



Fig. 196. Triphragmidium Ulmariae 

 (Schum.) Link; primary uredosorus; 

 condition intermediate between migra- 

 tion and conjugation of fertile cell; 

 after Olive. 



