385 



merit, 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 hetercecisin 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 Blackmail, 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. Enough 

 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 gametophytie generation has uninucleated 

 mycelium, and gives rise to two kinds of spores, basidiospores and pycnio- 

 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 (seciospore), and usually a 

 repeating form, in addition to the teliospore. All spores of this genera- 

 tion are binucleated. In the garnetophytie 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 secidiuin, cseonia, or primary uredo, they are 

 essentially of the same physiological nature, whatever form they may take. 

 Any such sorus may be called an aecium, and the spores seciospores, 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—230031 



