Oct. IS, 1921 Aecial Stage of the Orange Leaf rust of Wheat 167 



tion to their hosts is very pronounced. This appears to hold true for the 

 aecial as well as the grass hosts. P. Cockerelliana and P. alternans go to 

 Thalictrum dioicum, and P. Elymi to T. minus, as aecial hosts, neither of 

 which is a favorable host for P. triticina. The rust of T. dasycarpum, 

 used by Fraser {14) in his cultures, is on a host which was not infected 

 by P. triticina. P. horealis and P. ohliterata on T. alpinum offer no com- 

 parison, as P. triticina was not sown on that host. P. persistens, as 

 cultured by Fischer on T. minus and T. aquilegifolium, is upon species of 

 Thalictrum unfavorable for P. triticina, while P. persistens as originally 

 cultured by Plowright upon T. flavum is on the most congenial host for 

 the leafrust of wheat. It is very probable that Plowright and Fischer 

 were working with two distinct biologic strains. Although T. flavum 

 appears to be a favorable host for both P. persistens and P. triticina, and 

 these two rusts are very similar in their morphology, the inability of the 

 latter to infect Agropyron repens shows that it is biologically distinct 

 from the former. A study is being made of the relationship of P. triticina 

 to grass hosts other than wheat. From the data now at hand, it would 

 appear that, in addition to the grasses listed in Table IV, species of 

 Bromus, Festuca, Agrostis, Poa, and Anthoxanthum are immune from 

 the leafrust of wheat. These results indicate that, as far as its telial 

 host is concerned, P. triticina also is biologically distinct from other grass 

 rusts having aecia on Thalictrum. 



A similar situation exists in the relationship of Puccinia triticina to 

 rusts having aecia upon species of other genera of the Ranunculaceae. 

 Slight morphological differences, such as urediniospore size, wall color, 

 and pore number, exist among the different races producing aecia upon 

 species of such genera as Actaea, Anemone, Clematis, Delphinium, etc. 

 A similar, or perhaps greater, biologic specialization is also to be found 

 among these races. The importance of these morphological characters 

 and biological differences which occur among the members of this group 

 can not be fully determined at present on account of our comparatively 

 limited knowledge of but few races. Any final interpretation must 

 await further study of a greater number of such races. On the basis of 

 our present knowledge, the disposition of P. triticina must depend largely 

 upon the species concept held. In Europe there is a tendency among 

 certain students of the rusts to consider as species those rusts showing 

 distinct biologic specialization regardless of the absence of morphological 

 difference. In this country, on the other hand, the general tendency is 

 to include in a single species all closely related forms having but little 

 difference in their morphology. Forms limited to a definite host, or 

 hosts, are considered as races of such species. On the former basis, P. 

 triticina would be considered a distinct species comparable to P. Elymi, 

 P. Agropyri, P. persistens, etc., while with the latter concept it would be 

 united with all or part of these, each being considered a race of a 



