12 ARTHUR: PROBLEMS IN THE STupy oF PLANT Rusts 
adopted by students of bacteria and yeasts ; they have their place. 
In fact, I do not deem it necessary to bring up for discussion the 
hackneyed subject of what constitutes a species. Even were I 
polemically disposed, it would be hazardous ; but I have in mind 
only to call attention to the hyphenated nomenclature so effectively 
used by Klebahn, Fischer, and others, to indicate dichotomous 
parasitism. For instance, what has been considered Melampsora 
populina occurring upon Populus nigra is found to be separable by 
cultures into two forms, one having its aecidium on Larix and the 
other on Aliiwm ; these two forms are named by Klebahn J/e/am- 
psora Larici-populina and M. Allit-populina, although they may not 
be morphologically distinguishable. 
Such names are descriptive, and a great help in following the 
diversified results of cultures. Whatever their status in strictly 
systematic work, they would appear to have their use and be per- 
missible for cultural studies. But when the conclusions of the cul- 
turists are transferred to systematic botany the provisional nomen- 
clature must necessarily be subjected to the rigid laws of priority ; 
moreover, questions regarding the propriety of accepting the cul- 
tural forms as species must also be settled to accord with general 
usage. In making such changes it is doubtful if the systematist 
could safely rank as species forms that have not a reasonable 
morphological distinctness in addition to their host connections. 
The morphologically distinguishable species, like JJelampsora 
populina, may be separated into varieties showing selective para- 
sitism, like the varieties Larici-populina and Allu-populina. If some 
such method is not adopted, we may well be appalled at the prob- 
able condition of systematic uredinology a few years hence. 
At present there are only about a dozen culturists active. In 
North America alone a hundred species of grass and sedge rusts 
are known, which of course does not include all the heteroecious 
forms. Of these less than twenty have been traced to an aecidium, 
and none have had more than a preliminary investigation. In 
what proportion these hundred species are likely to exhibit selec- 
tive parasitism can not of course be told, but it is likely to be large. 
What is the future collector to do, if when he turns to his manual 
he finds that on the host he has in hand there are recorded from 
two toa dozen species, none of which can be told except by a 
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