Carter .—Studies on the Chloroplasts of Desmids. II. 297 
other cases the colourless gap may be quite obvious. In such individuals 
there are often apparently two chloroplasts in each semi-cell, separated from 
each other by a colourless space in the median region. On careful examina- 
tion, however, it is usually possible to locate the remains, at any rate, of the 
axile plate just beyond the nucleus connecting up the two apparently 
separate chloroplasts (Fig. 25). In the genus Micrasterias the formation 
of two separate chloroplasts in a single semi-cell by means of this extensive 
hollowing away in the apical region was not very frequently observed, but 
in Cosmarium Ralfsii, 1 which has a chloroplast exactly similar to that of 
the thick-celled species of Micrasterias , the same phenomenon also occurs, 
and here all stages in the progressive shortening of the axis in the median 
region have been observed, including the final stage in which two distinct 
chloroplasts were present. It is quite possible that this also happens 
occasionally in those species of Micrasterias having very thick cells. 
The ridges, if present, project from both surfaces of the axile plate, 
extending to the cell-wall. If the distance from the central axis to the 
cell-wall be considerable, i.e. in the thick-celled species, the ridges frequently 
fork and branch (Figs. 20-22 and 27) ; in most cases, however, they are 
simple and unbranched (Figs. 14 and 18). Their free edges are rarely 
quite straight, but either undulate (Figs. 2, 15, and 17), or, as is more 
often the case, they are much more elaborately ornamented (Figs. 13 and 
24). Sometimes their edges may be drawn out to form short thread-like 
outgrowths attached to the peripheral layer of cytoplasm lining the cell- 
wall, as in the case of the edges of the axile plate (Fig. 19). 
In many of the flat-celled species the ridges project only very slightly 
from the surface of the plate, and in some individuals may be scarcely 
perceptible. When this is the case they are usually to be found following 
the outlines of the more important incisions of the plate, just a little distance 
from .its edge (Figs. 2 and 24). 
Very often there are two very prominent ridges on both surfaces of 
the axile plate in the median region. These usually extend right from 
the nucleus at the base of the semi-cell up into the polar lobe, one on each 
side of the apical gap in the axis (Text-fig.; Figs. 9, 15, and 17). They 
usually stretch for a little way over the surface of the nucleus itself, which 
is therefore embraced by eight claw-like projections, the outgrowths of the 
four ridges on each chloroplast of the cell. In species having such 
prominent median ridges other smaller ones may or may not be present. 
If present they are sometimes irregularly scattered (Figs. 13 and 15), or 
may be in definite positions round the more important incisions as before 
(Text-fig.). 
1 This species was for some time believed to have chloroplasts in the form of parietal bands ; 
its axile form was first discovered by Dr. Liitkemuller, who figured it and published a short note on 
it in his paper, Zur Kenntnis der Destnidjaceen Bohmens, 1910. 
