Carter. —Studies on the Chlor op lasts of Desmids. /. 225 
reported unusually large numbers of pyrenoids in Hyalotheca neglecta , 
Racib., and Cosmarium sphagnicola , W. and G. S. West. 
The observations of these investigators are supported byDucellier ( 1917 ), 
who noted some irregularities in the number of the pyrenoids in various 
species of the genus Cosmarium, and gave some evidence that the number 
of pyrenoids in this genus is not always one or two, as was formerly 
stated. The present investigation also supports this, not only with regard to 
the genus Cosmarium , but also to species of other genera which have 
chloroplasts containing typically one pyrenoid. The single pyrenoid may, 
under favourable conditions, give rise to a group of two or more pyrenoids, 
and so long as the group of pyrenoids occupies the same relative position as 
the original one, the specimen can be regarded as being quite ordinary ; 
cf. Fig. 59. Of course one occasionally meets with individuals which have 
an abnormal number of pyrenoids in unusual positions. Thus specimens of 
Euastrum pectinalum , Breb., Eu. bidentatum , Nag., Cosmarium subtumidum, 
Nordst., &c., have been encountered having two chloroplasts in each semi- 
cell, each containing one pyrenoid, whereas normally there should be only 
one chloroplast, containing one pyrenoid ; cf. Figs. 51 and 54, 66 and 70. 
If the two pyrenoids had simply occupied the same position as if there had 
been only one, the specimens would have been regarded as normal, but 
since each pyrenoid was contained in a distinct and separate chloroplast, 
they were regarded as being unusual — and were omitted as such. Different 
anomalies were met with in other species. 
Ducellier also raises the question whether the number of pyrenoids 
contained in any cell belonging to the genus Cosmarium is not dependent 
to some extent on the size of the individual. This does not seem to be the 
case, either in Cosmarium or in any other genus of Desmids. Ducellier 
does not show the chloroplast in any of his figures, and it would appear 
that this is a factor which he has entirely neglected. The actual mass of 
chlorophyll-bearing material contained in the cell undoubtedly does have 
a direct effect on the amount of food-reserves stored as pyrenoids, but the 
amount of chloroplast present in individuals of one species varies consider- 
ably, irrespective of their size. Thus in some large cells the chloroplast 
may be particularly scanty, whilst in smaller individuals of the same species 
it may be quite massive ; in such cases the large cell probably contains less 
reserve food than the smaller one. Again, the conditions which control the 
division of the pyrenoid are very obscure, and cannot be determined. For 
in the same collection one individual may contain a few large pyrenoids, 
whilst another of the same species may contain many smaller ones. 
Furthermore, the conditions governing pyrenoid division may vary in 
different parts of the same semi-cell. For example, in one specimen of 
Xanthidium Brebissonii containing two axile chloroplasts in a semi-cell, 
one chloroplast was provided with a single extremely large pyrenoid, whilst 
