I9to] LUTMAN—CELL STRUCTURE OF CLOSTERIUM 249 
them, and some of the very largest may be without starch or have 
only a very thin layer. It is apparent that it is the physiological con- 
dition of the plant concerned and not the size of the pyrenoids that 
determines this difference. This is in harmony with the observed 
fact that in any one Closterium the starch layers around its pyrenoids 
will all tend to have the same thickness. 
In fixed material, at least, the starch grains are not in direct con- 
tact with the pyrenoid. Between the body of the pyrenoid itself 
and the starch around it there is ‘a layer of substance which either 
does not stain or stains the same color as the cytoplasm. It seems 
natural, of course, that there must continue to be some organic con- 
nection between the pyrenoid and the starch grains surrounding it; 
otherwise the starch would be moved away from its pyrenoid by the 
streaming of the cytoplasm, which undoubtedly occurs here in the 
chromatophore even if not so rapidly as it does in the cytoplasm next 
the cell wall. Whatever the connection may be, it must be main- 
tained through this unstained layer, which seems from its staining 
reaction to be neither pyrenoid nor starch. 
There is great variation in the appearance of the pyrenoid itself 
in different specimens. In one specimen the pyrenoids may all 
appear angular, while in the specimen lying next to it they are full 
and globular. This difference may be due to differences in the way 
in which the fixing solutions happen to affect the particular Closterium 
containing the pyrenoids, but I am inclined to think that it is rather 
due to some condition in the pyrenoid itself. The angular form was 
especially noticeable where the pyrenoids were very large and with 
little starch. The fixation of the general cell structure with the 
F lemming weak, half strength, was good in these cases, and many 
details in the structure of the pyrenoid could be made out. It is 
an interesting fact that a single cell presents in general the same 
type of pyrenoid throughout its whole extent. 
Very few of the pyrenoids are homogeneous, although they are 
usually described as being so; practically all of them show parts that 
are denser. One of the more common forms which this dense region 
takes is that of an irregular star-shaped portion toward the center 
of the pyrenoid (fig. 15). In this case the central part may be very 
irregular, with rays extending out from it. The denser portion has 
