N. I. Vavilov 63 



Secondly, as has already been remarked by Prof. Biffen (2, pp. 428 — 

 429), two facts speak against the great role of " bridging species " : first, 

 the fact that immune varieties may be grown dozens of years in close 

 proximity to susceptible ones, severely attacked by fungi, and still 

 remain quite resistant ; the second fact is the possibility of obtaining 

 immune races by crossing. 



Finally, it may well be conceded that the exactitude of fungal 

 reactions must be studied before using them for genetic purposes, 

 as with reactives in chemistry. With fungi of cereals, this preliminary 

 work is happily already more advanced than it is with any other group 

 of fungi'. 



In conclusion, we need only remark that the degree of sensitiveness 

 of fungal reaction, for instance, with cereals up to the present time 

 is not exceeded by that of the so-called "serum" methods, applied to 

 plants. It is hardly necessary to add that fungal reaction technically 

 is much simpler than the " serum " method in its application for the 

 recognition of individuals. 



JIarc/i, 1914. 



Note. While this paper was being printed there appeared in the Zeitschrift fur 

 Pflanzenziichtimg, April 1914, Bd. Ii, Heft 2, a very interesting paper by Dr Zade 

 entitled : " Serologische Studien an Leguminosen and Gramineen." In his investi- 

 gation with cereals, using the " serum " reactions as a chemical test for the genetics 

 of these jilants, Dr Zade, as we understand his tables, conies to quite the same 

 conclusions concerning the genetic relationshij) of oats and wheats, to which we 

 came using fungal tests for the same purpose. 



So, the tables of experiments on dift'ereut species of Avena show that the 

 A. fatiM gave almost the same reaction as A. sativa. 



A. strigosa,yi\Ac\i according to our investigation is genetically more distant from 

 A. sativa than A. fatua, gave in the experiments of Dr Zade, when he used the weak 

 " serum " solution of A. sativa, a much weaker reaction than A. fatua. 



specialization of this fungus in different countries, suggest the suspicion that it is possible, 

 in the same biologic form, more than one race of fungus may exist, and these races differ 

 more or less in their specialization. And, perhaps, in some instances the same phe- 

 nomenon of "bridging species" is the result of unconscious selection of different races 

 of fungi by the aid of different hosts. Certainly this question can be solved only by 

 means of pure cultures of fungi. 



' For the purpose of greater exactness, the conditions of the use of fungal reactions 

 must be borne in mind too, because, like chemical reactions, they change under differing 

 conditions ; although in general the role of external conditions (climate, soil, manure, 

 etc.) in changing immunity is very often too exaggerated in mycological literature (16, 

 pp. 99—102). 



