1910] LUTMAN—CELL STRUCTURE OF CLOSTERIUM 247 
Ehrenbergii. The relative thickness of the central core as compared 
with the width of the ridges is shown in jig. 4. 
The pyrenoids of C. Ehrenbergii are imbedded in the substance 
of the chromatophore close to its surface. It is very rare, except 
near the ends of the plant, to find a pyrenoid placed far toward the 
interior of the plant. In cross section it will be seen that many are 
either placed in the ridges of the chromatophore or lie at their bases. 
Although many of them lie in the ridges, none of them are found in 
their extreme outer edges (fig. 2). 
Where strands of cytoplasm run across the central part of the 
cell body, they seem to tend to be oriented on the pyrenoids. If 
streaming of the cytoplasm occurs in these strands, as so frequently 
happens in the algae, this arrangement would put the pyrenoid in 
quick communication with all parts of the cell and would facilitate 
the movement of food products toward or away from it. It would 
seem to occupy a favorable position in its location so close to the 
surface of the chromatophore and also in its relation to other parts 
of the cell. 
In C. moniliferum there is a single row of pyrenoids down the 
central axis. In cross section (jigs. 6, 7) the pyrenoids occupy the 
center of the chromatophore. As this species is so much smaller 
than C. Ehrenbergii, there is apparently not so much necessity for 
the pyrenoids being at the surface. 
As mentioned above, the pyrenoids in C. Ehrenbergii do not lie 
in the ridges in all cases, although many of them do. In addition to 
the layer of starch which incloses each pyrenoid, there are, as in 
Hydrodictyon, numerous starch grains lying free in the cytoplasm. 
These starch grains practically all lie arranged in longitudinal series 
in the ridges (fig. r). In a longitudinal section it is rather common 
to see a number of pyrenoids lying in a ridge in a row, and scattered 
among them a number of starch grains. The grains seem to have a 
tendency to collect in these parts. These free grains undoubtedly 
had their origin around a pyrenoid, as their shape is exactly that of 
such grains, being angular at the edges and concave (fig. 8). Whether 
these grains became free by a second layer of starch forming around 
the pyrenoid and crowding out the older layer, or by the disinte- 
Sration of some of the pyrenoids themselves, or in some other way, 
