Carter. — Studies on the Chloroplasts of Desmids. IV. 315 
In Cosmarium Brebissonii , Menegh., the complicated chloroplasts press 
into the young semi-cell as a comparatively shapeless mass, the pyrenoids 
thronging in at the same time (Fig. 57). In the figure the nucleus is seen 
pressed by the chloroplast to the right of the isthmus. 
It was noticed that in the axile chloroplasts of many species there is 
a distinct tendency for the peripheral parts of the chromatophore to enter 
the young semi -cell very much more rapidly than the central part, as if there 
were an attempt on the part of the organism to cover the cell-wall with 
photosynthetic material as quickly as possible, no matter what happened 
in the interior of the cell. This is very pronounced in many species of 
Staurastrum , in which the angles of the young semi-cell may be quite filled 
by the chromatophore whilst yet the axial part of it has scarcely entered 
(Figs. 49 and 58). It is quite possible that very often the division of the 
chloroplast at the isthmus occurs before the axial part has entered the 
young semi-cell at all. Thus several distinct chloroplasts would be produced 
in the new half-cell instead of a single central one. In the next generation 
the entire cell would probably be provided with such chloroplasts in half 
the individuals. 
This phenomenon has already been mentioned as being probably 
responsible for the frequent occurrence of wholly parietal chloroplasts in 
St. grande (Bulnh.), which normally possesses a single axile one, and also 
for the frequent shortening of the axis of the chloroplast in the median 
region in Micrasterias truncata , (Corda) Breb., M. oscitans , var. mucronata , 
(Dixon) Wille, and Cosmarium Ralfsii , Breb., and also for the production, 
in the extreme case, of two distinct chloroplasts in a semi-cell in the three 
latter species. 
Further, in other species, one sometimes encounters odd specimens in 
which the chloroplasts, which normally should be axile, are more or less 
parietal. Such have been observed in Tetmemorus Brebissonii, (Menegh.) 
Ralfs, T. granulatus , (Breb.) Ralfs, and others. The same explanation 
probably applies to these. 
Again, it has been noted earlier that in the larger species of Euastrum 
a considerable proportion of individuals are found in which the axis of the 
chloroplast is either shortened, or else, together with the radiating plates, is 
altogether absent. 1 In extreme cases there may be several entirely parietal 
chloroplasts in a semi-cell. Unfortunately dividing specimens of the species 
concerned were not frequently met with in stained material, and so it was 
impossible to ascertain the real cause of the discrepancy, but it is not 
improbable that the abnormalities arose in young semi-cells during cell- 
division by the mantling of the cell-wall by the parietal plates long before 
the central axis and radiating plates had entered. 
Finally, it is also suggested that in those species which normally possess 
1 Carter, N. : Studies on the Chloroplasts of Desmids. 1 . Ann. Bot., vol. xxxiii, 1919. 
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