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AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
between some forms may be only two or three microns, but these differences 
easily can be recognized by quantitative methods; and they are as constant 
as are the differences between many recognized species of fungi. There is 
a consistent average difference of ten microns between the length of the 
urediniospores of the tritici form and that of urediniospores of the agrostis 
form. But even if there were no morphologic differences, these biologic 
forms are distinct and constant pathogenically, and we must recognize 
their existence. 
In a recent paper Brierley expresses views similar to those expressed in 
this paper and makes the concrete suggestion that Lotsy’s terminology, 
proposed for the phanerogams, be modified to meet mycological needs as 
follows. He suggested that the term linneon replace the species in the 
Linnaean sense—the description being based on morphological grounds 
only; jordanon would be based on morphological characters which were 
demonstrated to be transmissible; and species would be established only on 
the basis of morphologic and physiologic reaction under standardized 
conditions. The term modification would be used to designate non-trans- 
missible effects of external conditions. Whether or not this terminology is 
adopted, the principles involved are worthy of serious consideration. 
The physiological concept already has been added to a certain extent 
to the morphologic concept of species. We raise no particular objection 
to basing the determination of a species of the Uredinales partly on life 
history, and all pathologists use physiologic characters in establishing species 
of phytopathogenic bacteria. If everything which the pathologist must 
know is classified, it will be necessary to add more and more of the physio¬ 
logic concept. The simple fact is that as scientists we ought to want to 
classify plants on the basis of those characters which are really characteristic, 
whether they be morphologic or pathologic, and as practical pathologists 
we must do so. We shall no doubt encounter difficulties, but, as technic 
becomes more standardized and refined, it will become possible to recognize 
still less obvious differences in species of pathogenic fungi than we now do. 
If the criticism be made that the proposed recognition of physiologic 
characters in classification would be drawing too fine distinctions, all that 
can be said is that the real distinctions were drawn by Nature; and, if we 
are dealing with pathogenic fungi in a practical way, we must recognize these 
distinctions; and, if we are seeking the ultimate truth regarding fungi, 
surely we ought to accept it in plant behavior as well as in plant structure. 
University of Minnesota 
