Oct. IS, i92t Aecial Stage of the Orange Leaf rust of Wheat 169 



of some one species denote that the rust is not to be found in the habitat 

 of such a resistant species, for susceptibility or resistance is not dependent 

 upon the presence or absence of the rust but may develop with the species 

 in any region. It is regarded as significant, however, that of the species 

 of Thalictrum tested the most susceptible are exotic. This fact, taken 

 with the foreign origin of wheat itself, is confirmative of the foreign 

 origin of the rust. 



The native habitats of two of these species of Thalictrum are known 

 with some degree of certainty. Thalictrum flavurn is found throughout 

 Europe, western Asia, and Asia Minor. T. Delavayi is given by the Index 

 Kewensis ^ as occurring in western China, probably indicating a distri- 

 bution in the little-known mountainous regions of Tibet and Chinese 

 Turkestan. These two species, taken together, would therefore indicate 

 as the most probable original distribution a region in which the two 

 Thalictrum species may border or overlap, such as that of southwestern 

 Asia. Such an origin would indicate a like origin for wheat itself, 

 which, we believe, would agree with the latest theories advanced as to 

 the original home of wheat. 



Concerning the occurrence and distribution of the aecia of Puccinia 

 triticina but little can be said with the data at hand. It is also probable 

 that the aecial stage occurs, and probably assumes greater importance, 

 in other regions than it may in either Europe or North America, where the 

 rust is known to overwinter in its uredinial stage. Thus in such countries 

 as India, where Butler has shown there is no oversummering of the rust, 

 the Thalictrum species of the foothills may be of importance in starting 

 the rust the next season. The question of the role which the aecia of the 

 leafrust of wheat plays in its life history and distribution must, however, 

 be left for future research to solve. Whether native species of Thalictrum 

 serve as aecial hosts in North America and, if so, whether they serve as 

 important factors in the development of the leafrust of wheat and 

 whether there is more than one race of the leafrust, as indicated by the 

 results obtained from the Great Plains area, or whether these results 

 were due to other causes, such as climatic or seasonal effects weakening 

 the vitality of the teliospores, are all questions on which further investi- 

 gation is planned. Other species of Thalictrum from foreign botanic 

 gardens also v/ill be studied in regard to their susceptibility to the orange 



leafrust of wheat. 



SUMMARY 



(i) The aecial stage of Puccinia triticina has been produced in green- 

 house cultures upon several species of Thalictrum. 



(2) The various species of Thalictrum show varying degrees of sus- 

 ceptibility to the rust. Thalictrum occidentale was apparently immune. 

 Upon T. dasycarpum and T. polygamutn an occasional devlopment of 



> INDEX KEWENSIS PLANTARUM PHANEROGAMARUM. SUPPI.EMENTUM PRIMUM . . . CONFECERUNT 

 THBOPHIlrUS DURAND ET B. DAYDON JACKSON. p. 42^. BniXCllis I9OI-06. 



