8 ARTHUR: PROBLEMS IN THE StTupy oF PLanr Rusts 
investigated. During the last decade of this era, however, more 
botanists have worked at solving these riddles, and some of them 
have made the work continuous from season to season, taking 
great pains to explore suggestive by-paths. The results have not 
been confined to a knowledge of life-cycles of individual species, 
but have brought to view some unexpected facts of far-reaching 
significance, regarding the limits of species and their biological be- 
havior. Doubtless many who listen to me to-day also heard the 
vice-presidential address of Professor Farlow before the botanical . 
section of the American Association for the Advancement of 
Science in 1898, in which he traced with great clearness and with 
many side-lights, the modification and broadening of our concep- 
tion of species through recent cultural work on rusts. It would 
be superfluous to retrace the ground, even were I capable of equal- 
ling the admirable presentation, but some phases of the subject, 
which have since become prominent, may well be mentioned here. 
One of the most eminent investigators in this line of research, to 
whom science and also the world at large owes much for his work 
on grain rusts, is Dr. Eriksson, of Stockholm. The great unfold- 
ing and clarification of a subject of such vast economic importance 
justly merited the gold medal and other honors, which have been be- 
stowed. That what had previously been rated three species of rusts, 
Puccinia graminis, P. rubigo-vera and P. coronata, have been found 
under the search-light to be seven or eight species or possibly ten 
or twelve, with numerous specialized forms or so-called biological 
species, is some indication of the thoroughness and extent of the 
labor. Practically it has been of the greatest value to know that 
the rust of grains belonging apparently to one species cannot 
usually be spread by the uredospores from rye to wheat, rye to 
oats, oats to barley, or in general from one kind of grain to another. 
But mapping out the possible distribution of each species of grain 
rust still requires a vast amount of work, as not only the culti- 
vated grains are involved, but many species of wild grasses. 
In passing, attention may be called to a remarkable and seem- 
ingly anomalous property belonging to the principal grain rusts, 
that of attacking hosts standing in two or more tribes of the 
Gramineae. Of the hundreds of other species of rusts inhabiting 
grasses it is probably safe to assert that not one spreads to hosts 
