ARTHUR: PROBLEMS IN THE Stupy OF PLANT Rusts 3 
Farlow, Thaxter and Halsted upon the connection between cedar 
rusts and the aecidia on Pomaceae, from which most of the pres- 
ent knowledge of American Gymunosporangia has been derived, a 
genus of rusts especially North American in the number and diver- 
sity of its species. And it was during this period that Plowright 
in England cultivated the rusts and ably presented the claims ot 
heteroecism. In Germany Rostrup, Schroter, and Winter made 
cultures, and the last two, more than all others of this period, per- 
mitted the doctrine to influence their systematic treatment of the 
order. Each began, and carried well forward, the fungous flora ot 
a large region: the Kryptogamen Flora von Schlesien and the 
Kryptogamen Flora von Deutschland, Oesterreich und der Schweiz, 
in which the Uredineae were elaborated in a manner yet unap- 
proached. These two works, with Burrill’s Parasitic Fungi of 
Illinois, Part I., which was issued during the same period, and 
inspired by Winter’s work, are the only systematic handbooks of 
the order, founded upon a critical study of specimens, that are 
available to the American collector even to this day, although they 
were published from fifteen to twenty years ago. Since that time 
the number of known species has more than doubled, and the 
volume of information about them has quadrupled. 
We may pause a moment here to recall what Sachs, writing 
near the beginning of the era of culture studies, said in his History 
of Botany about parasitic fungi, of which the Uredineae are the 
chief representatives. He characterized them as ‘‘ the most attrac- 
tive objects in the whole field of mycology,” and said that ‘“ here 
were difficulties in abundance, here were the darkest enigmas with 
which botany has ever had to deal.’ Among the difficulties he 
had in mind were “to discover what properly belonged to one 
cycle of development” and to find where in this cyle the sexual 
fusion occurs. 
Regarding sexuality of the rusts there is still dense ignorance, 
but regarding the exact cycle of development great advances have 
been made. DeBary supplied the key to this garden of knowl- 
edge, and during two or three decades some twenty botanists es- 
sayed the tedious but interesting task of growing one or more 
species of Uredineae and tracing the succession of forms througha 
life-cycle, some sixty or seventy heteroecious species being thus 
