REED: SPECIALIZATION OF PARASITIC FUNGI 349 



Whatever term has been appHed, the underlying conception has 

 been the same, namely, that these races, strains, forms, etc., of distinct 

 morphological species of fungous parasites differ, not in discernible 

 structural features, but in their physiological behavior, as indicated 

 by their ability to infect some hosts and not others. They differ in 

 their ability to establish the parasitic relation with particular hosts and 

 thus secure the necessary food for their normal development. The 

 phenomenon is distinctly physiological and is doubtless quite com- 

 parable to the well-known behavior of saprophytic fungi on different 

 chemical substrata. Various saprophytes, structually similar, vary 

 in their ability to utilize different chemicals as sources of food, de- 

 pendent on their capacity to secrete the necessary enzymes. While 

 the strains of parasites may differ essentially in their ability to secure 

 food from a particular host, we must keep in mind the possibility of a 

 more complicated series of relations in which toxin and antitoxin 

 production are involved. 



Many investigators of the phenomenon of host specialization have 

 made a large number of species on the basis of the results of their 

 inoculation tests. This is especially the case in the rusts where 

 Klebahn, Eriksson, Schneider, Fischer and others have raised many 

 forms to specific rank, although no distinct structural differences can 

 be observed. It may be noted that the races of Puccinia dispersa, 

 P. sessilis, P. Rihesii-Caricis , P. extensicola, Coleosporium Campamilae, 

 Melampsora populina, M. Tremtdae and others, referred to below, are 

 regarded as good species by some students. 



Fischer (45), in connection with the rusts, accepts as species the 

 following: 



1 . All rusts which are structurally distinct. 



2. All rusts which have a different life-cycle; for example, forms 



which are distinguished by the presence or absence of certain 

 spore-forms. 



3. All forms which differ in their choice of hosts, in so far as the 



hosts belong to different genera. In heteroecious rusts species 



are recognized when the hosts of one generation, aecidial or 



uredo and teleuto, belong to two different genera. 



Fischer unites under one species as formae speciales or specialized 



races all rusts which differ only physiologically and whose hosts are 



species of a single genus. Whether a ^^articular rust is a physiological 



species or a specialized race is thus determined by the range of its 



hosts. 



It is doubtless true that many rusts, and other parasitic fungi 

 as well, which can be distinguished only by the hosts upon which they 

 grow, are just as distinct forms as others which are characterized by 



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