Generations , and General Cytology of the Uredineae . 339 
age one can observe the transition from the uninucleate to the binucleate 
condition. This failure led to a close examination of the fertile cells at 
about the point of transition, and it was observed that in a number of cases 
the fertile cell was occupied by two nuclei of different size and structure 
(Figs. 62-65). One was usually a larger nucleus with the characters given 
above for the original nucleus of the cell, the other, usually a smaller, 
denser nucleus containing no nucleolus or only a small one, and having, 
in fact, more the characters of a nucleus of an ordinary cell of the 
mycelium. 
The differences in size and structure of these two nuclei was hardly 
compatible with the view that they were sister-nuclei, and when this fact 
was further considered in conjunction with the absence of all stages of the 
necessary division, there seemed no escape from the view that the smaller, 
denser nucleus must have had some other origin. This view was amply 
confirmed by the discovery of a number of cases in which a nucleus zvas 
actually found passing into the fertile cell from one of the smaller cells 
of the mycelium at its base (Figs. 66-70) 1 . The migrating nucleus is 
reduced to a narrow thread during the process, the actual aperture through 
which it passes being very small ; neither before nor after its passage could 
a pit in the wall be observed 2 . 
In many cases there is to be found lying below the fertile cell a cell 
which obviously belongs to the same cell-row ; it may be called a basal cell 
though it is in no way specially differentiated. When this basal cell is 
present it is often from it that the migrating nucleus comes ; and it is 
interesting to note that the nucleus seems to pass more often into one of the 
neighbouring fertile cells (Figs. 66, 67, and 71), with which it is in contact 
at the sides owing to the irregularity in length of these cells, rather than 
into the fertile cell immediately above it (Figs. 68 and 69). In the former 
case the relationship between the two nuclei which meet in the fertile 
cell may be considerably distant, while in the latter case the two nuclei 
are separated in origin only by one division, that which cuts off the 
sterile cell. 
In some cases in which the fertile cell contained two dissimilar nuclei, 
the smaller, denser one lay in the upper part of the cell, and at the same 
time the sterile cell was without a nucleus (Fig. 65). It seemed then 
possible that the nucleus of the sterile cell might have migrated into the 
fertile cell below ; but no direct evidence could be obtained for this view, 
and it is rendered improbable by the fact that a similar state of affairs was 
1 More than twenty-four cases were observed in which the fertile cell contained two dissimilar 
nuclei and more than fifteen cases in which a nucleus was in a stage of actual migration. 
2 The migration of nuclei in the tissues of Phanerogams which have been placed under abnormal 
conditions has been described by several observers (cf. Koernicke, Ber. d. Deutsch. Bot. Ges. xxi, 
1904, p. 100). 
