168 Journal of Agricultural Research Voi. xxii. no. 3 



collective species to be designated, according to the limitations of 

 the species concept held and the system of nomenclatm-e used, as 

 P. Agropyri B. and E. (j, v. 9, p. 304), P. Clematidis (DC.) Lagerh., or 

 Dicaeoma Clematidis (DC.) Arth. (j, p. 333-337). 



The close biological specialization of Puccinia triticina to wheat is of 

 considerable significance with respect to the bearing it has upon the 

 possible origin of this rust and of wheat itself. Since wheat is an intro- 

 duced plant, it is logical to assume that a rust showing such close biolog- 

 ical specialization to it is also introduced and of foreign origin. 



It is generally recognized among students of the rusts that a high de- 

 gree of host specialization must have been acquired in certain groups of 

 species at a very early stage in the evolutionary history of this group of 

 fungi. It is also recognized that the host is the most important factor 

 in the evolution of highly specialized pai-asitic fungi. As the higher 

 plants have gradually developed during geological times, their rust 

 parasites have developed with them. It therefore appears reasonable to 

 assume that Puccinia triticina, which shows such a high degree of spe- 

 cialization to wheat at the present time, had its origin as a distinct strain 

 comparatively early in the development of the group of grasses from 

 which our cultivated wheats have originated. The original distribution 

 of the rust presumably would coincide with the distribution of the 

 ancestral wheats. 



A study of the relative susceptibility of various species of Thalictrum 

 to infection by this rust is of interest in this connection. The four most 

 susceptible species of Thalictrum encountered in this investigation are 

 all of foreign origin. The most susceptible of our native North American 

 species, Thalictrum dioicum, does not compare in susceptibility with 

 these four foreign species— T. flavum, T. Delavayi, T. sp. 55, and T. sp. 

 98 — but is comparable to the resistant foreign species such as T. minus. 

 That these foreign susceptible species of Thalictrum are also to be con- 

 sidered as indicating a foreign origin of the rust would appear to follow if 

 the nature of aecial infection is considered. Heteroecious rusts in most 

 cases infect their aecial hosts only for a comparatively short period of 

 tune while the teliospores are germinating in the spring. The infection 

 produced, not being able to propagate itself upon such hosts, causes little 

 or no damage, and they are in most cases soon able to outgrow it. On 

 this account it is hardly to be expected that a natural selection of resist- 

 ant strains of aecial hosts takes place in nature comparable to that which 

 occurs where the host is killed or prevented from producing seed. Should 

 this occur in heteroecious rusts which are not able to survive adverse 

 conditions in winter or summer by means of urediniospores, such a selec- 

 tion would be fatal to the rust itself. For this reason the susceptibility 

 of the aecial hosts of P. triticina may be taken as indicative of its origin. 

 It is true that susceptibility of a host species does not necessarily indi- 

 cate that such a species was a native host of the rust nor does resistance 



