28 Transactions British Mycological Society. 



considered distinct are really identical, and it is unquestionable 

 that there should also be a considerable elimination of species 

 in other genera. In parasitic genera of this kind it is only by 

 cross inoculations that proper host relationships can be dis- 

 covered, so that it is urgent here for the systematist either to 

 become a pathologist for the time being or to work in close 

 co-operation with one. Species diagnosis must at present be 

 primarily based upon structural characters, and however useful 

 physiological characters may be for the separation of distinct 

 strains, physiological differences alone cannot yet be taken as 

 adequate criteria for species differentiation. In many patho- 

 genic fungi there is exhibited the phenomenon of specialised 

 parasitism, i.e. the existence of a number of forms differing in 

 host relationships but either only slightly or not at all in 

 structure, within the limits of a single species as ordinarily 

 understood. This phenomenon is particularly well shown in Puc- 

 cinia graminis which comprises many forms of distinct physio- 

 logical relationship such as the variety Tritici confined almost 

 entirely to wheat, and the variety Secalis upon rye, couch grass, 

 and a few other grasses. Prof. Stakman of America has recently 

 told me that some of these physiological varieties in America 

 show slight structural differences, for example, in the dimen- 

 sions of the uredospores and aecidiospores. If these structural 

 differences were more considerable, there would be a strong 

 claim for converting these specialised races into distinct species. 

 In systematic work there will always exist, both the "lumper" 

 and the "splitter," for in the main it is a matter of opinion as 

 to what degree of difference constitutes specific differentiation. 

 To the plant pathologist it does not much matter whether the 

 species have been diagnosed by a "lumper" or a "splitter" 

 as long as the basis of distinction is clearly set forth in language 

 that he can understand. I must confess a preference for the 

 methods of the "lumper" for, as stated in another place*, 

 mycological systematy will become altogether too unwieldy if 

 fungus species are set within too narrow limits. 



Another difficulty encountered by the plant pathologist is 

 that even the generic limitations hitherto imposed by systema- 

 tists not infrequently break down in practice. In the Fungi 

 Imperfecti in particular this difficulty is often met with, and 

 it was pointed out a short time ago in dealing with certain 

 tomato diseases* how the generic distinctions between Phoma, 

 Phyllosticta, Ascochyta and Diplodina broke down. There cannot 

 be any doubt but that many genera in this group are entirely 

 artificial, and one looks forward to the time when some systema- 



* Brooks, F. T. and Searle, G. O., An investigation of some tomato diseases. 

 Trans. Brit. Myc. Soc. vii, p. 191 (1921). » 



