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AMERICAN JOURNAL OF BOTANY 
[Vol. io f 
Among pathogenic fungi similar chemical differences apparently exist; there 
appear to be distinct forms of Sclerotinia sp., causing brown rot of stone 
fruits in the United States, which consistently produce strikingly different 
types of growth on various synthetic media. The differences between 
biologic forms of Erysiphe, of Puccinia graminis, of Puccinia coronata, and 
other pathogenic fungi, no doubt also are chemical, although the exact 
nature of these differences has not yet been ascertained. It is significant, 
however, that the reaction of several biologic forms of P. graminis 11 tritici” 
to hydrogen-ion concentration differs perceptibly. Whether the physiologic 
differences between biologic forms can be detected on artificial culture media 
or only by the action of the forms on host plants, it would seem that the 
nature of the difference is essentially the same—that is, physico-chemical. 
( Why were biologic forms not called species when first discovered? 
Probably because the morphological concept of species had become so 
firmly fixed that it was considered too heterodox to use physiologic differences 
as a sole basis for classification. Furthermore, morphologic differences 
were considered more permanent than physiologic differences. But, even 
as early as 1898, Farlow read the following to the Botanical Section of the 
American Association: 
When therefore the botanist denies that physiological species are properly species, he 
is practically admitting that his own definition, the perennial succession of like individuals, 
is used by him in a special sense, and he does not seem to be aware that species as he limits 
them are artificial and not natural. The belief that species should be based on morpho¬ 
logical rather than physiological characters rests on the assumption that the former are 
more likely to be inherited and thus show the temporary attempts of the organism to 
adapt itself to the environment. It is perhaps a question whether the grounds for this 
belief are as valid as has been supposed. We readily see the morphological characters 
which have been inherited, but it is usually only by accident or experiment that we recognize 
the physiological or pathological qualities. 
Biologic forms long were considered to be unstable. Ward, Salmon, 
Freeman, Freeman and Johnson, Pole-Evans, and Johnson all obtained 
evidence which led them to conclude that the parasitic capabilities of biologic 
forms of Puccinia dispersa , Erysiphe graminis , Puccinia graminis , and 
Puccinia phleipratensis easily could be changed by bridging hosts and by 
other influences. The results of these investigations indicated that biologic 
forms readily acquire the ability to parasitize normally immune hosts, 
provided they are grown first on some closely, or sometimes even distantly 
related, susceptible species of host plant. Thus P. graminis tritici is 
incapable of attacking oats, but can grow on barley. On barley, according 
to Freeman and Johnson, the rust acquires the ability to attack oats slightly, 
presumably on account of some chemical change in the biologic form. If 
biologic forms could be changed so easily, the objection to using their 
physiologic characters in classification certainly would be valid. But do 
they change easily? 
The so-called plasticity of biologic forms of Puccinia graminis has been 
