Carter . — Studies on the Chloroplasts of Desmids. II. 301 
M. pinnatifida t . 
In this tiny species two very prominent median ridges are to be seen 
in the front view, and towards the lateral edges of the cell the plate thickens 
abruptly, at the same time splitting into two lamellae. This is best seen 
in the end view (Fig. 12). It was only possible to examine one individual, 
and this had three pyrenoids in each semi-cell, one in the polar lobe and one 
in each lateral lobe of the cell (Fig. 11). 
M. Americana. 
This species has ridges which are very large and conspicuous, but 
probably unbranched. There are two very distinct median ridges visible 
in the front view, but these usually seem to be displaced either to one side 
or the other (Fig. 13). This is due to the fact that the front faces of the 
cell are not flat, and the cell is much thicker in the middle than at the 
edges (Fig. 14). Consequently the cell always tilts over to one side when 
resting on a flat surface. The peg-like outgrowths of the cell-wall on the 
polar lobe are usually the cause of this tilting to one side of the cell, but in 
the variety examined these were not strongly developed. 
Besides the median ridges of the chloroplast there are various others 
radiating from the region of the nucleus towards the periphery. In spite 
of the thickness of the cell, the central axis, as in all the other species 
dealt with so far, retains its definite plate-like form. In Fig. 14 the axis 
is seen to be very distended with stroma starch, which has not yet, however, 
spread as far as the ridges. The pyrenoids number about twelve to fifteen 
and are scattered throughout the axis of the chloroplast. 
M. oscitans ) var. mucronata , and M. truncata . 
These two species have relatively thicker cells than any of the others 
which were examined. In both the ridges are variable in number, but they 
are always large and very much branched, quite superseding the central 
axis in their much greater development (Figs. 21 and 27). The delicacy 
of the branching of the chloroplast is largely dependent on the amount of 
stroma starch present in it. The formation of much starch causes the 
chloroplast to become very distended, and thus the elegance of the stroma 
starch-free chloroplast is lost ; cf. Figs. 20 and 21. 
The shortening of the axile plate in the median region of the cell is 
frequently considerable, and the chloroplast is in consequence often distinctly 
bilobed (Figs. 19 and 25). In such cases a transverse section near the top 
of the semi-cell is quite different from one near the nucleus. The former 
naturally cuts through the two separate lobes of the chloroplast, and two 
distinct masses are to be seen (Fig. 22), whilst towards the base of the 
semi-cell a continuous central axis is seen with various plates or branched 
Z 2 
