Dec. 23. 1918 Parasitism of Cronartium ribicola 639 



Alb. and Schwein. on Peonia officinalis. It is apparently a common 



occurrence. Just how the sporidia reach the pine is not known, but it 



may be assumed that they are usually air borne. Infection of the pine 



host follows their germination under favorable circumstances on young 



pine shoots. 



CYTOLOGY 



PREVIOUS INVESTIGATION 



Our knowledge of the cytology of the genus Cronartium is very frag- 

 mentary; in fact, no reference has been found to the nuclear phenomena 

 accompanying the formation of any bark-inhabiting aecium of this group. 

 Poirault and Raciborski {44) figure silhouettes of the nuclear division, 

 as they interpreted it in the formation of the seciospores of the form known 

 to them as Peridermium pini acicolum} Sappin-Trouffy (57) gives a fair 

 diagram of this acicolous type of aecium and the nuclear division at its 

 base. His diagram of the telial column of Cronartium fiaccidum indicates 

 the phenomena accompanying the production of teliospores, though the 

 figures are too minute to be more than suggestions of the actual condi- 

 tions. He regards the processes as closely similar to the nuclear phe- 

 nomena in other rusts, in which he is quite correct. Aside from the work 

 of these authors, nothing beyond incidental mention of the cytology of 

 the genus Cronartium has come to the writer's attention. 



It is hardly necessary to give an extended resume of the work of pre- 

 vious investigators on the cytology of the rusts, on account of the excel- 

 lent reviews which have been published by Blackman (2), Christman 

 {4, 5, 6), Fromme {14, 15), Maire {32), Guilliennond {16), and Ramsbot- 

 tom {46). Moreau {36, 37, jp) revives the older view that there are but 

 two chromosomes in the rust nucleus, which hardly seems tenable, and 

 establishes the presence of centrosomes in the resting nuclei {36). This 

 subject will receive further consideration later. Fromme's (75) recent 

 paper on the morphology and cytology of the aecium clears up the matter 

 of the formation of the deeper seated sori and establishes the similarity 

 of spore formation in the caeoma and aecium. The life history of a com- 

 plete rust is divided into two stages : the gametophyte, with uninucleate 

 cells, and the sporophyte, with binucleate cells. The gametophyte be- 

 gins with the reduction division, or its equivalent, in the promycelium 

 and continues up to the fusion of the gamete cells at the base of the aecium, 

 which initiates the dikaryon. The sporophyte begins with the inception 

 of the dikaryon and continues up to the reduction division, or its equiv- 

 alent, in the promycelium. The association of the nuclei in the basal 

 cells of the secium is regarded as the equivalent of a true fertilization, 

 and the fusion of the two nuclei in the young teliospore as the completion 

 of the process necessary for a mixing of the chromatin elements previous 

 to reduction {31). The nuclear divisions are true mitotic divisions, accom- 



' Vuillemin {.62) also retx>rts some of the phenomena accompanying seciospore formation in Pert</«rmtUffi 

 pini. 



