Generations , and General Cytology of the Uredineae . 341 
The development of the aecidium is centrifugal, and when it has 
reached a certain size the peripheral cells, instead of forming ordinary 
sterile and fertile cells, grow out into uninucleate paraphyses : these may, 
however, in some cases may be absent. 
The process of nuclear migration, by means of which the fertile cell 
is stimulated to rapid growth and continued division, is clearly a process 
of fertilization , the exact relation of which will be discussed later 1 * . 
Gymnosporangium clavariaeforme. 
The first beginning of the aecidium takes place deep down in the 
hypertrophied tissue of the leaf, & c., after the majority of the spermogonia 
have faded. The aecidium in this genus is of the typical form, with a 
definite pseudoperidium. Unfortunately the very early stages, which are 
much more difficult to obtain than in Phragmidium , were not observed, 
so that the presence or absence of sterile cells and the exact behaviour 
of the nuclei could not be investigated. It was clear, however, that the 
transition from the condition of single to that of paired nuclei took place, 
as described by Sapin-Troufify, in connexion with the aecidiospore-bearing 
cells (fertile cells) ; for while in the whole mycelium, and even in the 
dense layer of fungal cells surrounding the actual aecidium, the nuclei 
were single, yet the fertile cells and their products showed clearly two 
nuclei in the paired (conjugate) condition. 
It is evident that a comparative study of aecidium-development in the 
Uredineae generally, with special relation to the existence of sterile cells 
and the origin of the binucleate condition, is much to be desired. The only 
other forms of which we have any information are Endophyllum Sempervivi , 
De Bary, and Puccinia Bunii , DC., in which Maire (29, 30) is of opinion 
that the two nuclei of the fertile cell are daughter-nuclei arising by division 
of the original single nucleus. The migration of nuclei, however, is very 
easily overlooked unless special attention is drawn to such a point ; it is 
interesting to note that Maire does not appear to have observed in 
Puccinia Bunii the actual division, for he merely states, ‘ A little later, it 
can be seen that the terminal cells [fertile cells] contain each two nuclei 5 
(30, p. 39 ). It is of course possible that in the aecidium of Phragmidium 
with its simple structure we have a more primitive condition, and that 
in some other forms the association of nuclei of two different cells may 
have been replaced by association of daughter-nuclei of the same cell ; but 
judgement must be suspended in the matter. 
1 Whether any protoplasm passes over with the migrating nucleus could not, of course, be 
determined. 
B b % 
