244 Carter . — Shiclies on the Chloroplasts of Desmids. /. 
Although the chloroplast described above is typical for all the species 
mentioned, it is not uncommon to find odd specimens which have a slightly 
different structure. The variations usually concern the axial part of the 
chloroplast ; the parietal plates vary very little amongst individuals of the 
same species, the chief differences occurring in the lobing of the edges of 
the plate and the number and size of the outgrowths from its surface. It 
is quite a common thing, however, to find in many species specimens in 
which the central axis is much shorter than usual, leaving a space either at 
the apex or base of the semi-cell, or both (Figs. 104 and 105). The shape 
of the radiating plates is necessarily interfered with when this happens, and 
they accordingly curve outwards from the shortened axis towards the apex 
and base of the cell, so that the parietal plates are usually quite normal. In 
other cases both the central axis and the radiating plates may be missing, 
and the parietal plates are then the only remaining part of the chloro- 
plast (Fig. 102). Sometimes the parietal plates, in such cases, are not 
entirely separate and free from each other, but are variously connected by 
one or two extremely thin strands of chloroplast running across the cell 
from one plate to another (Figs. 74, 75, 96, and 103). 
Occasionally very irregular chloroplasts are found, in which the central 
axis may be perforated (Fig. 94) or displaced, together with the radiating 
plates, from its usual position. Another variation is seen in Figs. 86 
and 88. 
In typical specimens of the group, the axial part of the chloroplast, 
composed of such extremely thin plates, cannot play a very important part 
in the photosynthetic processes of the cell. This is proved to some extent 
by the usual absence of pyrenoids from the central axis in the largest 
species of the genus. The majority are always to be found in the parietal 
plates. Considering this fact, and also that parietal chloroplasts have been 
evolved in a few advanced species of other genera of Desmidiaceae, it might 
be expected that in these large species of Euastrum there would be 
a tendency for the useless part of the chloroplast in the interior of the cell 
to disappear, leaving just the parietal plates. This is possibly the reason 
why so many individuals seem to show transitional stages between the axile 
and parietal condition. 
A less important variation was observed in two isolated specimens, one 
of Eu . eras sum, and the other of Eu. oblongum . Here all the pyrenoids 
were aggregated in a compact irregular mass in the interior of the cell. 
The central axis was considerably swollen to accommodate them, and from 
it a number of short strands of chloroplast ran out towards the cell-wall, 
ending, in the first-mentioned species, in irregular parietal bands. In the 
specimen of Eu. oblongum there was a more or less complete reticulated 
layer of chloroplast lining the cell-wall. These two specimens were interest- 
ing because they appear to form a link between Groups I and II, and this 
