Carter. — Studies on the Chloroplasts of Desmids. III. 275 
the middle of the semi-cell, from which a number of branching ridges radiate 
towards the periphery (Fig. 59). The branching and spreading out of the 
ridges near the cell-wall result in the formation of broad peripheral masses of 
chloroplast, which in the living condition doubtless give the impression ol 
parietal bands (Figs. 59 and 61). The ultimate branches of the ridges end 
against the cell-wall with smooth or fringed edges. The pyrenoids vary 
considerably in size and number (6-14), and are embedded in the thicker 
parts of the chloroplast, either where the ridges arise from the axile plate, or 
nearer the periphery, at the points of branching of the ridges themselves 
(Fig. 61). 
There is a strong tendency in this species towards the hollowing away 
of the axile plate of the chloroplast in the apical region of the semi-cell, 
exactly similar to that observed in the thicker-celled species of Mzcrasterias. 1 
The chlorophyll-free portion is often quite small, but in some individuals it 
extends right as far as the nucleus, and consequently in such cases there are 
two chloroplasts in each semi-cell instead of one ; cf. Figs. 59, 60, and 58. 
The general structure of the chloroplast, and its variations in the median 
region of the cell, together with the arrangement and variation in number 
and size of the pyrenoids, are all very suggestive of the thicker-celled 
species of Micrasterias ; in fact it is scarcely possible to distinguish any 
differences at all between the chloroplasts of this species and M. oscitans in 
transverse section. Moreover, its chloroplast is quite different from that of 
any of the other species of Cosmarium examined, with the possible exception 
of the small species C. ornatum. There are some similarities between the 
chloroplasts of the latter species and those of C. caelatum and C. speciosum ; 
cf. Figs. 33, 10, and 28 ; but in the irregular form of its ridges and scattered 
pyrenoids it agrees more nearly with C. Ralfsii . In C. ornattim there are 
almost invariably two or three pyrenoids, and they never give one the 
impression that they might have been formed by the division of an original 
central one. Thus we may assume that in C. ornatum , as in C. Ralfsii , 
there is not merely one point of pyrenoid formation. 
The seventeen species dealt with so far possess normally one chloro- 
plast in each semi-cell. The thirteen species which will now be described 
have two chloroplasts transversely disposed in each semi-cell. In each case 
the chloroplast has a funnel-shaped axis arising on one side of the nucleus, 
with which it is in close connexion, and this axis stretches towards the corre- 
sponding lateral region of the semi-cell (Fig. 47). It increases in thickness as 
it passes upwards, and finally ends about half-way between the apex of the 
semi-cell and the nucleus in a swollen head which contains the pyrenoid or 
group of pyrenoids (Fig. 47). Two such axes arise, one on each side of the 
nucleus, and they lie symmetrically, one on each side of the median line. 
1 Vide Carter, N. : Studies on the Chloroplasts of Desmids. II. Ann. Bot., vol. xxxiii, 1919. 
