332 UREDINEAE. 



host-species. I have previously shown, 1 with regard to the 

 mistletoe (Viscum album), that the different forms on P/////X, 

 Abie*, and various broad-leaved trees, which some authors regard 

 as distinct species, might equally well be regarded as forms of 

 one species differing slightly on account of their different 

 substrata. Magnus 2 designates as " habitat-races " these forms 

 of heteroecious Uredineae whose aecidial generation has become 

 adapted in some varying degree to each of their respective 

 species of host-plant. Thus the various forms of Accidium 

 convallariae, on its different host-plants, he regards as forms of one 

 and the same fungus, the Puccinia of which occurs on Phalm'is 

 arundinacea. 



The manner in which such adaptations originate is indicated 

 by my experiments with Gymnosporanyinm. Thus G. clavariae- 

 formc can infect leaves of Crataegus and produce aecidia 

 without failure ; whereas the same infection carried out on 

 Sorbus and Ci/donia results in incomplete development of aecidia 

 (see Table, p. 885). In this way there might easily be pro- 

 duced one form which infected Crataec/us, and another confined 

 to Cydonia. The same thing occurs with the various Pcridermia 

 of pine-needles ; these, according to the investigations of 

 Klebahn, are caused by one or other species of Coledsporium 

 from very different species of host-plant. 3 



The best examples of all, however, are presented by the 

 cereal-rusts, as demonstrated by Eriksson. This investigator 

 believes that the forms distinguished by him as " specialized 

 forms " (by Kostrup as " biological species or varieties ") are 

 of common origin. In course of time these have taken on 

 different biological characteristics in adapting themselves to 

 the varied nature of their substrata, their various host-plants, 

 so that in many cases they can no longer suit themselves 

 to the host-plant of the original parental form. In fact, species 

 were found with aecidia of similar shape when occurring on 

 the same host-plant, yet completely specialized from the aecidia 

 on another host. They thus present a stage intermediate to 

 that of the " habitat-races " just mentioned. 



1 v. Tubeuf, Hot an. Cfutrn/li/ti/f, xu, 1SS9, p. 312. 

 - Hl n-li/lfi, 1S!)4, p. 77, ami 1895. 



: 'Klebahn's views on this subject, along with further investigations on other 

 fungi, will be found in Zeitschrift f. Pftanzenierankheiten, 1895, p. 153. 



