168 Mottier.—Nuclear and Cell Division 
though smaller than in the ordinary epidermal or superficial 
cells of the thallus. This is because of their rapid multi¬ 
plication during the growth of the tetraspore mother-cells. 
In the medullary or central cells of the thallus only a few 
smaller chloroplasts are present. They may be elongated, 
elliptical, bean-shaped, or often globular. They do not as 
a rule lie in the cavities of the alveoli, but are applied to 
their walls or to the threads of the cytoplasmic framework. 
Even after the process of fixing, imbedding, and staining, 
they still retain their brown colour, which, however, is paler 
than in the living cell. 
The Nucleus. 
As in Stypocaidon , there is only one nucleus in each cell. 
In one case an apical cell was observed in which two nuclei 
in the resting condition lay very close together. In this 
instance nuclear division had been completed before a cell- 
plate had put in an appearance. Whether this is purely an 
exceptional case, or whether in the apical cell, cell-division 
is later in point of time than in other vegetative cells could 
not be decided. It is highly probable that this is exceptional, 
for in other vegetative cells cell-division was well under way 
before the daughter nuclei had passed into the complete 
resting stage. When the tetraspore mother-cell has increased 
considerably in size, the nucleus also becomes proportionately 
enlarged. Indeed, as pointed out by Swingle (’ 97 ) for Stypo - 
caulon , the size of the nucleus seems to be in some degree 
proportional to the size of the cell. The nucleus contains one 
very large nucleolus, invariably much vacuolated, and a fine 
linin-reticulum, upon which are distributed fine granules 
varying somewhat in size. Many of these granules, but not 
all, constitute the chromatin. With the exception of the 
larger ones, which are more densely stained, they present 
the same greyish colour as the linin. 
The amount of chromatin in the primary nucleus of the 
tetraspore mother-cell is certainly less in proportion to the 
size of the nucleus than in the vegetative cells. In fact 
