ARTHUR: PROBLEMS IN THE StTupy OF PLANT Rusts. 13 
cultural test? Would it not be better that he should find one) 
species recorded for his host, having from two to a dozen varieties 
separable by cultures? Moreover, would not this accord better 
with usage in other fields of plant life, and at the same time give 
a more just conception of relationship ? 
Difficulties in this line are not confined to the heteroecious 
rusts. The autoecious species are likely to prove quite as per- 
plexing and intricate when studied with equal thoroughness. The 
American species of rust on Euphorbia, called Uromyces Euphorbiae, 
has been somewhat examined culturally, and four cultural species 
have been found respectively inhabiting Auphorbia maculata, E. 
nutans, £, dentata and E£. marginata. Both aecidiospores and 
uredospores have been used for the cultures, and in no case has it 
been possible to transfer the rust of one of these hosts to another ; 
morphological characters by which to distinguish them, however, 
have not been found. Fancy the systematic situation for the 
Euphorbia rusts, when the other hosts are examined, not only of 
the eastern United States for which Britton’s Manual gives thirty- 
six species, but of the western regions, and especially of Mexico 
and other warm countries where Euphorbias attain a greater devel- 
opment, many being woody and even tree-like, and where rusts 
on them are especially abundant. 
But I ought not to weary you with more of this perplexing 
topic. Let the culturists and the systematists fight out their bat- 
tles. It is difficult to foresee whether the outcome will be the 
establishment of order or the release of pandoric confusion. 
Probably no part of the many-sided subject of the rusts is 
superior in interest to the fact of their complete parasitism. They 
not only vegetate inside the host without destroying it, but even 
at the fruiting points the tissues are rarely killed. On the con- 
trary they often exert a stimulating effect upon the cells of the 
host, so that the tissues remain green, especially about aecidia, 
when other parts of the organ are yellow and dying. Another 
phase of the stimulating effect is seen in hypertrophied develop- 
ment. So intimate is the association of parasite and host that as 
a rule the vigor of the parasite is directly proportional to the vigor 
of the host. Every culturist soon learns, that to have success in 
his work he must employ strong, rapidly growing plants. Even 
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