SUMMARY. 75 



more species of one host genus, while others range over two or 

 more genera and often on (Ufferent tribes of the same family. AVhat 

 appear to be the same forms macroscopically and microscopically are 

 often physiologically different, and may consist of a large number of 

 strains or varieties conveniently called biologic forms. This fact 

 accounts, to a large extent, for the differences in results obtained by 

 American and European investigators working on what are appar- 

 ently the same species. 



In an attempt to break down the barriers between biologic forms 

 the writers have been able to transfer rusts in the uredo stage as fol- 

 lows: Stem rust of wheat (Puccinia graminis tritici) from wheat to 

 wheat, rye, and barley, but not to oats; from wheat to barley and 

 then to wheat and rye; and from wheat to barley successively three 

 times and then to oats. Stem rust of barley (P. graminis hordei) 

 from barley to barley, oats, rye, and wheat; from barley to wheat 

 and then to barley, wheat, oats, and rye; and from barley to rye, to 

 barley, and then to wheat, oats, and rye. Stem rust of rye {P. graminis 

 secalis) from rye to rye and barley; from rye to barley and then to 

 barley, oats, and rye; and from rye successively to barley, to barley, 

 and to rye. Stem rust of oats (P. graminis avenae) froin oats to oats 

 and barley, but not to wheat or rye. Leaf rust of wheat (P. ruhigo- 

 vera tritici) from wheat to wheat, rye, and barley. Leaf rust of barley 

 (P. simplex) from barley to barley only. Leaf rust of rye (P. ruhigo- 

 vera secalis) from rye to rye only. Leaf rust of oats (P. coronata) 

 from oats to oats and barley, but not to wheat or rye. 



There is a measurable difference in size between the uredospores of 

 the stem rust on wheat and the stem rust on barley. In continuous 

 culture experiments of wheat stem rust on barley and barley stem 

 rust on wheat, the uredospore of the wheat stem rust approached the 

 uredospore of the barley stem rust in size and the barley stem rust 

 approached the wheat stem rust in size. 



The following points in regard to biologic forms of rusts of cereals 

 may be emphasized: (1) The stem rusts on wheat, barley, rye, and 

 oats are undoubtedly biologic forms of the same species, Puccinia 

 graminis Pers.; (2) these forms are not entirely confined to their 

 hosts, but vary in range in part according to the host plants they have 

 been recently inhabiting; (3) the leaf rusts on wheat and rye are more 

 highly specialized than the corresponding stem rusts; (4) the stem 

 rust on barley has orcUnaril}^ the widest, while the leaf rusts on barley 

 and rye have the most restricted range; (5) under favorable conditions 

 all the stem rusts can be carried successfully to the four cereals; (6) 

 when rusts are transferred to uncongenial hosts, if pustules are pro- 

 duced they are small and weak; (7) two biologic forms may inhabit 

 the same cereals without being identical; (8) by gradual variation 



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