220 Carter . — Studies on the Chlor op lasts of Desmids. I. 
sequence a more or less lateral position, but in some individuals the 
chloroplast may be interrupted in this region, so that two chloroplasts are 
present instead of one. In the spirally twisted bands of Genicidaria and 
the parietal chloroplasts of some species of Spirotaenia the pyrenoids are 
scattered in the chloroplast near the cell-wall, but in all others they lie in 
a single row in the middle of the cell, and in Mesoiaenium there may be 
only one pyrenoid in the central position. 
Cylindrocystis and Netrium differ from all the other Saccodermae in 
the somewhat complicated form of their chloroplasts (Figs. 13 - 19 ). In both 
genera there are invariably two chloroplasts in each cell, each one consisting 
of a central axile mass containing typically one pyrenoid and a number of 
strands or plates which radiate towards the periphery. The chloroplasts of 
these two genera have been figured by de Bary (1858) and West (1902; 
1904-11, vol. i). Netrium interruptum , (Breb.) Liitkem., is unique 
amongst the Saccodermae in the possession of four axile chloroplasts in 
each cell, one above the other in a longitudinal series. 
Amongst the Placodermae there is much greater variety in the form 
of the chloroplasts, and, with the exception of some filamentous species, 
most of them have hitherto never been figured. There is almost invariably 
in the higher Desmids at least one chloroplast in each semi-cell, although in 
a few very small species, e.g. Cosmarium subtile p (West) Liitkem., C. tinc- 
tump Ralfs, C. subtilissimu m , 1 2 3 4 5 G. S. West, Cosmocladium constrictump 
Arch., and Staurastrum inconspicuump Nordst., there is only one simple 
chloroplast in each cell, typically with a single pyrenoid in the central 
position. In the larger species of the group the chloroplasts may be axile 
or parietal. When axile there may be either one chloroplast in the centre 
of each semi-cell, or else two placed side by side. In Closterium Libellida , 
Focke, var. interruptump W. and G. S. West, there are two axile chloroplasts 
in each half-cell, one on top of the other, just as in Netrium interruptum . 
When parietal there may be four or more chloroplasts in each semi-cell, and 
here there is far more variation in the actual number of chloroplasts present 
than is the case with axile ones. In species which normally have two axile 
chloroplasts in each semi-cell, more or less than two are rarely observed, and 
although in the case of species normally having one axile chloroplast in 
each semi-cell one sometimes encounters individuals which have two, this is 
quite unusual. In the case of parietal chloroplasts, however, the number of 
plates or bands present may vary from two to eight or even more. 
1 Vide W. and G. S. West (1904-11, vol. i). This species was figured under the name of 
Penhim subtile . 
2 Vide Liitkemiiller (1902), p. 356 . 
3 Vide G. S. West (1914). 
4 Vide G. S. West (1904). 
5 Vide W. and G. S. West (1904-11, vol. i). This species was figured under the name of 
Penium Libellula i var. interruptum . 
