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Field Obsebvations on Rusts fob the Genebal Botanist. 



By J. C. Arthur. 



(Abstract). 

 The polymorphic character of many species of rusts, together with 

 the discontinuous growth between the forms of most of such species, and 

 the further fact that some species live upon unlike hosts at different stages 

 of their life cycle, make the study of the rusts unusually attractive for 

 those who enjoy a varied problem. A very important part of the field 

 observation consists in later visits to the spot where a rust has been found 

 in order to ascertain if it is followed by another form of the same species 

 either on the same host or on a host of some other kind. In either case, 

 but especially the latter, a suitable specimen of the rust may be taken to 

 a locality where it does not occur and placed beside a healthy plant of 

 the kind observed. It is then watched to see if the rust infects the healthy 

 plant. If it does, the correctness of the inference from the first observation, 

 that the two forms of rust found to succeed each other in the same locality 

 belonged to the same species, is established. But if the healthy plant does 

 not become infected, either the two forms found in the original locality 

 belong to distinct species and only incidentally happen to follow one 

 another, or else the sowing of the rust in the new locality was not well 

 done. In either case further trials and observations are required. There 

 are many variations to the inferential and deductive reasoning required 

 to solve these problems, and to come to a conclusion repeated observations 

 may be required extending over many months or even years. 



