Carter —Studies on the Chloroplasts of Desmids . IV. 305 
possible to examine three specimens of the larger variety, and all these 
three differed in the arrangement and number of their pyrenoids, although 
the chloroplast in each case was of the ordinary type, being axile, with 
a pair of Jobes running into each of the three angles of the semi-cell (Figs. 
21 and 23). In practically every semi-cell there was also a single pyrenoid 
in the centre of the chloroplast (Fig. 21, lower semi-cell) ; in one, however, 
there was a group of three (Fig. 21, upper semi-cell), but that is not unusual. 
In all three specimens there were, however, in addition, other pyrenoids in 
one or more angles of the cell. Thus one specimen was quite normal in 
one semi-cell, but the other, in addition to a central group of three pyre- 
noids, had also a very small pyrenoid in one of its angles (Fig. 21, only two 
of the central group of three pyrenoids are visible). In the second specimen 
also the chloroplast was quite normal in one semi-cell, but in the other, 
besides the large central pyrenoid, there were also one or two very small 
pyrenoids in each angle of the semi-cell (Fig. 22). The third specimen also 
had one or two extra pyrenoids in each angle of the cell, but in this case 
the extra pyrenoids were of considerable size, being nearly as large as the 
central pyrenoids of the same cell (Fig. 23). 
It may be that in the larger variety, as in var. ornatum , there is 
normally a single central pyrenoid in each semi-cell, but that, in the larger 
cells of var. altum , it is impossible in most individuals for all the accumu- 
lated products of photosynthesis to be stored in a single pyrenoid, because 
a pyrenoid cannot increase in size indefinitely. Therefore the original 
central pyrenoid may in some cases divide to produce a little group of 
pyrenoids in the middle of the semi-cell, or else new pyrenoids arise in the 
comparatively large masses of chloroplast in the angles of the cell. It 
seems natural that new centres of food-storage should be formed at these 
points, because the lobes of the chloroplast in the angles are in some cases 
very massive, and photosynthesis must be going on fairly extensively, whilst 
at the same time it would be a considerable distance for all food reserves 
to be transferred to the pyrenoid in the centre of the cell. The new 
pyrenoids are very small at first, increasing in size as stores of food accumu- 
late until they are nearly as large as the older established central pyrenoid. 
In the smaller cells of var. ornatum the single pyrenoid in the middle 
of the cell is quite sufficient for the storage of all food reserves, and the 
lobes of the chloroplast in the angles are neither very large, nor are they 
removed at any great length from the central pyrenoid, so that the forma- 
tion of extra pyrenoids within them is unnecessary. 
St. furcigerum . 
In the form of St. furcigerum examined, the cell, which is triangular 
in the end view, is provided with two short hollow processes at each angle 
of the semi-cell, one above the other. The form of the chloroplast is, how- 
