Nuclear Divisions in the Rusts . 
353 
appear at all in T riphragmium ; on the contrary, cell division takes place 
by means of the usual process of constriction. 
Blackman’s idea that we have in the Rusts three distinct kinds of 
nuclear division — the vegetative division, in the spermogonium and in the 
vegetative hyphae, which he regards as of the nature of amitosis ; and 
the two promycelial divisions, which he thinks are reduced forms of 
indirect division — is apparently based on the study of unfavourable 
material. I have not yet compared the nuclear division in the promycelium 
of T riphragmium with the vegetative divisions of this form ; but since the 
latter are found to conform in the main features with the mitotic phenomena 
as described for other organisms, I think it more than likely that the 
promycelial and vegetative divisions in the Rusts will be found to differ 
from each other only in those characteristics which distinguish the hetero- 
typic and homoeotypic divisions from the vegetative. 
In T riphragmium the fibrous structures seen in the telophases, which 
represent the chromosomes, are seen to number invariably eight (Figs. 6 a 
and io). Poirault and Raciborski’s (’95) conception as to the single longi- 
tudinally-split chromosome of the Rust nuclei, as well as Sappin-T rouffy ’s 
(’96), and Maire’s (’00) conclusion that the two chromatin masses so 
commonly observed represent two chromosomes, results without doubt from 
the study of poorly-differentiated mitotic figures. The well-differentiated 
nuclear figures obtained in the vegetative cells of T riphragmium, as well 
as the occasional favourable cases in some other forms, furnish convincing 
evidence of the correctness of Holden and Harper’s view that the more 
or less solid dumbbell-shaped chromatin masses, characteristic of the 
vegetative nuclei of so many Rusts, result from poor fixation and poor 
differentiation. Blackman (’04) also regards the two chromatin masses in 
each nucleus as resulting from poor differentiation. He did not succeed, 
however, in making out distinct chromosomes ; nor did he see more than 
a ‘simple spindle ’ in the achromatic structures. 
Blackman offers without further comment the pregnant suggestion that 
the two chromatin masses first noted by Sappin-T rouffy ‘ probably represent 
the chromatin derived respectively from the two nuclei which fuse in the 
teleutospore ’ (’04, p. 346). The double centres seen in Figs. 6, 6 a, and 10, 
to each of which four chromosomes are seen to be attached, result, in my 
opinion, from this tendency for the chromatin contents of each daughter- 
nucleus to segregate into two more or less distinct masses. But to the 
views expressed in certain recent important investigations on related 
problems bearing on the cytological aspects of heredity in the higher 
plants (Allen, ’05 ; Strasburger-Allen-Miyake-Overton, ’05) Blackman’s 
theory would present a striking contrast. The double character of the 
nuclei in the case of higher plants, resulting from the close proximity in the 
same nucleus of the chromatin derived from the two parents, clearly persists 
