"Predisposition" and "Immunity" in^ Plants. 309 



right in England, Klebahn in Germany, and others, have led to 

 the recognition of a state of affairs even more complex than this, 

 however, for they have disclosed the facts that not only are there 

 several species of rust on our cereals capable of distinction by 

 visible characters, and, therefore, morphological species, but that 

 each .of these actual species may behave differently towards 

 different species of cereals or other grasses, thus giving rise to 

 -physiological races or forms, each with its own distinctive biolo- 

 gical characters, but indistinguishable by means of the microscope. 



Eriksson especially has done so much, and such able work 

 in connection with these points, that the reader may be referred 

 to his papers for further details. Here it will suffice to point out 

 that in his latest summary of his work on this species question 1 , 

 he accepts no less than thirteen " species " of Puccinia on the 

 cereals, in place of the three hitherto mentioned, breaking up 

 P. graminis into two, P. rubigo-vera into eight, and P. coronata 

 into three distinct species, in addition to more than twenty 

 specialised varieties or races. With these matters we are not 

 now concerned: I mention them here merely to show that P. 

 graminis of the text-books is not a single or simple species. 



Of these " species " I selected some time ago one which forms 

 the characteristic Brown Rust of the Bromes, for an exhaustive 

 investigation. I may remark that at the time this work was 

 begun, eighteen months ago, the species was known as Puccinia 

 dispersa (Erikss.), a form or variety of the old P. rubigo-vera of 

 De Candolle 2 , but during the progress of the investigation two 

 discoveries have been made, both of importance in other con- 

 nections, but not bearing on my work, which is concerned only 

 with the behaviour of the Uredo stage. It has been shown (1) by 

 Eriksson 3 that this Brome-rust does not form its secidium on 

 Anchusa, as does the typical P. dispersa (the old P. rubigo- 

 vera), and (2) by Midler 4 that the secidium-form is developed 

 not on Anchusa but on Pidmonaria and Symphytum. 



The plan of my work was to test (1) how far and in what 

 manner this particular Uredine is specialised to the Bromes, and 

 the nature of this specialisation ; (2) what are the conditions of 

 infection and the relations between host and parasite during in- 

 fection and incubation. 



In September last 5 1 gave a brief account of the principal results 

 of a long series of infections, the details of which have ovdy been 

 withheld because I was anxious to solve some questions depending 



1 Ann. des Sc. Nat. 1901, vol. xiv. p. 101. 



- Eriksson, Die Getreideroste, 18. 



:1 Eriksson, Ann. des Sc. Nat. 1901, vol. xiv. p. 101. 



4 Bot. Centr. Beihefte, B. x. 1901, p. 182. 



5 See British Association lieport, Glasgow, 1901, and Ann. of Bot. vol. xv. 1901, 

 p. 560. 



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