Nuclear Divisions in the Rusts. 
34i 
the teleutospore sorus, although he is quite at a loss to explain how the 
transition has come about. 
Vegetative Nuclear Division. 
As indicated in the review of the present status of our knowledge 
concerning the nuclear division in the Rusts, the subject greatly needs 
further investigation. Triphragmium ulmariae proved to be an especially 
favourable form, since the large nuclei show in their division stages many 
details not hitherto described. The caeoma-like aecidium of the thistle 
Rust as well as some other forms were also found to possess comparatively 
large nuclei, but the mitotic figures were not so clear in these species. 
Fig. 1 shows one of the exceptionally large nuclei characteristic of the 
spermogonial hyphae of Triphragmium . A comparison of Fig. 1 with 
Figs. 12, 36, and 37, which are drawn to the same scale, will serve to 
emphasize the difference in size between the nuclei of the spermogonium 
and those of the germ cells of the young sporophyte. In Fig. 1 will be 
seen a small nucleole, a lightly-staining chromatin network, and, in particular, 
two small, deeply-staining bodies on the nuclear membrane, each surrounded 
by a small aggregation of a less deeply-staining archoplasm-like substance. 
These two centrosomes, as I shall term them, now lie close together, and 
each has attached to it a well-defined chromatic filament. I am inclined 
to think that this figure represents an early prophase condition, in which 
the centres have just undergone division ; but there is another possible view, 
which will be explained in some detail later, viz. that the two centres have 
been carried over as a double centre from the last previous division. 
Fig. 3 is much easier to interpret. The two distinct centres shown in 
this figure have obviously migrated apart, until they now lie at quite a dis- 
tance from each other, and they are joined together by a long, slender 
strand. Judging from careful focussing, the strand apparently lies upper- 
most, on the surface of the nucleus, which still contains some nuclear sap 
and is still enclosed in a nuclear membrane. In this figure it is clear that 
the deeply-staining chromatin, particularly in the basal portion, is attached 
partly to the centres, and, at least apparently, partly to the narrow strand 
itself. As will perhaps be more clearly seen in figures of conjugate 
division, to be explained later, this strand connecting the diverging centres 
undoubtedly corresponds to the ‘ Centralspindel ’ of Hermann (’ 91 ). 
In the next figure, 3, which is somewhat more highly magnified, the 
central spindle now extends to the extreme ends of the elongated nuclear 
figure. Apparently the nuclear membrane has not yet completely broken 
down, as some sap still seems to be present in the lower part of the nucleus. 
The nuclear content in the upper half of the figure, in the spermatial bud, 
is obscure. It appears to be composed mainly of a thready substance. 
The chromatin obviously lies at one side of the spindle figure, and apparently 
