10 ARTHUR: PROBLEMS IN THE StTubDY oF PLANT Rusts 
Schroter in 1887 recognized four willow rusts and three poplar 
rusts. The characters for these were drawn chiefly from the 
uredospores, although the latter author was guided somewhat by 
the limited cultures of Rostrup and Nielsen. 
The matter presents a wholly different aspect when we turn to 
the conclusions reached by Klebahn. From an external knowl- 
edge of the species we are presented with an intimate knowledge, 
and from conjecture we pass to reasonable certainty. It is some- 
what startling, nevertheless, to find that the original single willow 
rust has become fourteen, and the one poplar rust has become 
seven. One of the willow rusts, quite unsuspectedly, proves to be 
autoecious, having its aecidium on willow leaves along with the 
uredo and teleutostage; all the other species are heteroecious, 
with their aecidia on Larix, Euonymus, Ribes, Orchis, Allium and 
Galanthus. The poplar rusts are all heteroecious, with their 
aecidia on Larix, Pinus, Allium, Mercurialis, Chelidonium and 
Corydalis. Most of these species can be told from one another 
by morphological characters quite as readily and as certainly as 
can the species of Viola, Crataegus and Antennaria in the present 
congested condition of these genera; but some of them are diffi- 
cult to distinguish in this way. For instance, the five species 
which occur on Populus tremula can scarcely be told apart unless 
the aecidial host is known. 
As an incidental piece of information it may be stated in this 
connection that all the willow and poplar rusts investigated by 
Klebahn are European. Although similar rusts are common in 
this country, and often sufficiently abundant to be destructive to 
their hosts, yet none of them has so far been identified with Euro- 
pean species ; and, moreover, the most common forms may be as- 
sumed, with some degree of certainty, to be distinctly American. 
But to return to the question regarding species that are mor- 
phologically similar, but ecologically dissimilar; it appears to be 
possible for all five of these Melampsorae on Populus tremula to 
grow side by side on the same individual leaf, in which case the 
uredospores would be practically indistinguishable by any direct 
morphological or cultural test, the teleutospores could only be 
told apart by ascertaining what species of host they could be made 
to infect, while the aecidia would be known by the particular host 
