246 Journal of Agricultural Research voi. xv, No. 4 



Barley, which Freeman and Johnson (9) found to increase the range 

 of parasitism of biologic forms has not been found to do this in the 

 writers' experience. Attempts to induce bridging by means of this 

 form have been made continuously for almost four years, and during 

 all of that time no evidence whatever has been obtained that this host 

 is able to change biologic forms. It is true that barley, as well as a 

 number of the grasses, serves as a meeting point for a number of biologic 

 forms and theoretically it seems as though it ought to change their 

 parasitic capabilities. It also appears as though, if all of the common 

 biologic forms of P. graminis could be kept on barley for a long period 

 of time, they ought eventually to become practically uniform. 



Although it is possible that rusts may change and new biologic forms 

 may develop, it seems more probable that the change is either a very 

 gradual one, extending over long periods of time, or that they change 

 by mutation. No evidence of mutation, however, was obtained in the 

 present investigation. The difference may be one of evolution as com- 

 pared with experimentally induced change. For practical purposes, 

 however, it seems perfectly safe to say that no certain and marked 

 changes in biologic forms need be expected as a result of growing on 

 bridging hosts; nor does it seem probable that biologic forms are able 

 to gradually adapt themselves to semicongenial hosts by constant 

 association with those hosts. The writers unsuccessfully tried to get 

 evidence of such adaptation. Hybridization may possibly account for 

 some unexplained phenomena and deserves investigation. 



It still seems probable that rusts may change as a result of selecting 

 strains from a given biologic form. While there is no positive evidence 

 for this, it seems reasonable that biologic forms may be somewhat 

 analogous to pure lines in genetics and that some forms may possibly be 

 composite from which it is possible to isolate the component pure lines. " 

 This last supposition is theoretical only, and is prompted by a study of 

 the avenae, phleipratensis, and agrostis forms. These are similar para- 

 sitically and the avenae form is variable morphologically, containing 

 spores which after being isolated could be interpreted as being agrostis 

 spores and others which might be determined as being phleipratensis 

 spores. It is possible, although the experiments do not support the 

 idea strongly, that a number of pure lines might be isolated from the 

 avenae strain if extensive attempts were made. 



Recently evidence has accumulated which seems to show that some of 

 the apparent bridging obtained by previous investigators may have 

 been due to the fact that several very closely related biologic forms may 

 have been used in the experiments. The discovery of the differential 

 hosts for these biologic forms is largely a matter of accident and the 

 writers are of the opinion that all existing forms of P. graminis have not 

 yet been discovered. The fact that several distinct biologic forms which 

 attack various wheats are now known, is especially suggestive. A few 

 varieties of wheat are differential hosts for these forms. Other closely 

 related forms may exist and unless the investigator is lucky in stumbling 



