385 
ment, if the extent of the life cycle and characters other than those per- 
taining to the teliospore were called into account. 
The third epoch in the study of plant rusts (the second one being 
ushered in by DeBary’s demonstration of hetercecism and the first epoch 
preceding that time), may be considered to have started with the study 
of the nucleus and its behavior. This was begun by the work of Sappin- 
Trouffy and of Poirault and Raciborski some fifteen years ago, and ably 
continued by Blackman, Christman, Holden and Harper, Olive and others. 
The nuclear history in the rusts is still in a very incomplete state, and 
part of what has been gone over needs further substantiation. Hnough 
has been demonstrated, however, to modify profoundly our ideas of the 
Significance of the different spore forms, the relation of the spore struc- 
tures, and the possibility of sexuality. 
While it may be interesting to review the present knowledge of nu- 
clear changes in the rusts and show the bearing on taxonomy, it will 
suffice for the present purpose to bring up briefly a few points. It has 
been rather clearly shown that the rusts. possess well marked antithetic 
alternation of generations. The gametophytic generation has uninucleated 
mycelium, and gives rise to two kinds of spores, basidiospores and pyenio- 
spores, both uninucleated, and these are the only truly asexual spores 
formed in the life cycle. The sporophytic generation begins shortly after 
the pycnia mature, being inaugurated by a sexual fusion of cells. This 
act introduces the binucleated condition. In many species of rusts only one 
spore form (teliospore) is produced in the sporophytic generation. In 
other species there is an initial spore form (:eciospore), and usually a 
repeating form, in addition to the teliospore. All spores of this genera- 
tion are binucleated. In the gametophytic generation all species behave 
essentially the same. It is in what follows during the sporophytic genera- 
tion that the great diversity of the rusts is shown. 
If the first binucleated spores arising after sexual cell fusion are 
teliospores, no other spore forms in this generation are produced, and the 
life cycle is a brief one. But if the first binucleated spores are formed in 
what has been called an ecidium, coma, or primary uredo, they are 
essentially of the same physiological nature, whatever form they may take. 
Any such sorus may be called an :ecium, and the spores zeciospores, this 
being an extension in the previous application of the terms to cover the 
primary uredo. Possibly new terms would be less liable to introduce am- 
[| 25—23003 ] 
