2 ARTHUR: PROBLEMS IN THE Srupy oF PLANT Rusts 
In the selection of a subject for the present address it has 
seemed fitting that the central thought of the society should find 
a measure of expression, and therefore I have brought together 
some of the suggestions, entanglements and problems that have 
arisen during a series of investigations into the life histories of a 
number of the plant rusts. I do this with the more alacrity as it 
enables me to bring before you, and possibly to introduce to some 
of you for the first time, a remarkable class of plants of great 
scientific and economic importance. 
The plant rusts, or Uredineae, are microscopic fungi, strictly 
parasitic in their nature, inhabiting all classes of flowering plants 
and ferns. The mycelial body of the fungus consists of colorless 
hyphae which ramify through the tissues of a larger or smaller 
part of the host. Sometimes they permeate every part of the 
plant above ground, but more usually are confined to certain 
areas, often very small and circumscribed. It is at all times diffi- 
cult to distinguish this vegetative part of the fungus from the 
cellular structure of the host, unless special differentiating methods 
are employed. 
On the other hand the reproductive parts are comparatively 
prominent, being yellow, red, brown, black or some characteristic 
tint, and with few exceptions are formed at or near the surface of 
the host, and of sufficient size to permit recognition under a hand 
lens or often by the unaided eye. These fruiting bodies are gen- 
erally dehiscent, exposing the spores as a waxy or powdery layer. 
When produced abundantly, and of an orange or brownish color, 
the supporting plants have the appearance of being dusted with 
iron rust, which has suggested the now common name of plant 
rusts. The most generally recognized examples are the rusts of 
wheat, oats and other grains, which cause a loss to the people of 
this country of many millions of dollars annually, often equalling 
that caused by the Hessian fly, chinch bug and all other insect and 
fungous enemies combined. 
The fruiting parts and especially the spores are the only por- 
tion of the rust fungus seen or recognized by the general observer, 
the botanical collector, or the average student. Not only are the 
spores readily seen, but as a rule they are easily removed and 
examined under the microscope, needing no preparation except 
