1907] CHRISTMAN—MORPHOLOCY OF THE RUSTS 95 



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presented by the rusts is of primary importance. Assuming the exist- 



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ence of alternation of generations, there can be no doubt that the 



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gametophyte is to be regarded as the primitive, original generation. 

 The autoecious rusts are probably to be regarded also as more primi- 

 tive than those which are heteroecious. It is most suggestive of the 

 truth of this view that the rusts show alternation of generations at the 

 time the change from one host to another occurs. Older schemes 

 of classification of course have left all such considerations out of 

 account. 



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ScHROETER has classified the rusts on the ground of the various 

 types of spores occurring in the life-histories of the different forms. 

 To him we are indebted for the convenient separation into eu-y 

 -apsis, brachy-j hemi-, micro-, and lepto- types. Obvious difficul- 

 ' ties arise with this classification as used up to the present time. 



Assuming all primary uredospores to be in reality aecidiospores^ as 

 is the case in Phragmidium potentillae canadensis, would at once 

 throw all of the brachy- and hemi- forms into the Eiipuccinia class. 



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If we retain the term Brachy puccinia on the ground of the primary 

 uredospores having stalks^ to be consistent it would be necessary to 

 class Coleosporium with the -opsis forms, since its uredospores are 

 borne in rows as are aecidiospores. 



DeBary (10) made his first division in the group by separating 

 the aecidium-forming rusts from the so-called tremelloid forms. He 

 regards the former as being homologous in essential details with the 

 ascomycetesj and points out that the tremelloid forms may be regarded 

 as either more primitive and in process of being developed, or reduced 

 forms produced by the modification of the aecidium-bearing forms. 

 It is plain, however, that with our present knowledge no such division 

 is to be considered. As I have pointed out, the primary uredo is 

 morphologically an aecidium. Further, Dr, Olive permits me to 

 quote results, not yet published, which show that the tcleutospores of 

 Puccinia trans jormans, a micro- form, are borne upon basal cells 

 which are the outgrowth of a fusion cell produced upon a mycelium 

 having uninucleated cells. There are then telcutospores which in 

 their origin are the same as the aecidiospores and primar}' uredo- 

 spores, and this argues very strongly that the stage with uninucleated 

 cells is to be regarded as always present. 



