266 Carter .—Studies on the Chtoroplasts of Desmids . III. 
C. cucumis ) a species characterized by parietal chloroplasts. Such classifi- 
cations based on the structure of the chloroplast were very unwise, 
particularly since at that time the form of the chloroplasts was known only 
in comparatively few species, and these had usually only been investigated 
in the living condition. Thus it is not surprising that several species were 
wrongly placed. C. Ralfsii and C. elegantissimum , two species believed by 
Lundell to possess parietal choroplasts and placed by him in Pleurotaeniopsis , j 
have since been found to be provided with axile ones, whilst in two other 
species hitherto believed to have axile chloroplasts, C. Brebissonii and 
C. contr over sum , the chloroplasts are now known to be parietal. So that in 
any case such a division was quite unjustifiable whilst information was so 
scanty concerning the chloroplasts of the genus. 
Again, it has been pointed out by W. and G. S. West ( 1904 - 11 ), that 
this classification according to the form of the chloroplasts is an unnatural 
one, resulting in the separation of closely related species. Thus it some- j 
times happens that chloroplasts of very different forms are found in quite 
closely related species, e. g. C. cucumis and C. subcucumis , C. binum , and 
C speciosum . 
Parietal chloroplasts seem to have been developed in general in the 
largest species of the genus, although there are a number of very large 
species which still retain the axile form. But in these there is a marked 
tendency to the formation of parietal films of chloroplast. These parietal 
films were observed in several species (Figs. 21, 37, 56, 69, and 7 1), but they 1 
are frequently very delicate and are difficult to distinguish except after 
staining. The extent of the parietal layer depends on the profuseness of 
the chlorophyll. If the chloroplast is fairly massive, then the film will be 
fairly continuous, whilst if it is scanty there may be just a slight extension 
of the ridges over the surface of the walk 
Thus it appears that in these large cells the chloroplasts tend to con- 
centrate as much as possible towards the periphery, where they can be of 
greater help in the process of photosynthesis ; but, whilst in some species 
the chloroplasts have become entirely parietal, others are content to retain i 
the axile form, but mantle the surface of the cell-wall with photosynthetic 
material, so as to make the best use of their substance. The distinction 
between parietal and axile chloroplasts is therefore not so clear as was 
formerly believed to be the case, since in certain axile chloroplasts there 
may be parietal layers as well. That a sharp line cannot be drawn 
between parietal and axile chloroplasts is also illustrated by C. Brebissonii . 
Here the chloroplasts are very variable, and whilst in some individuals they 
may be distinctly parietal, in others they penetrate into all parts of the cell ; 
cf. Figs. 77-81. 
LUtkemiiller ( 1893 ) drew attention to a peculiar character of the 
parietal chloroplasts of some species of Cosmarium — that the parietal bands | 
