Watson on Structure and Relation of the Plastid. 339 
body, not free within the cavity, but suspended at or near the 
center by means of threadlike prolongations of the bulb 
capsule. The structure of this pyrenoid is more dense than 
that of the axial strand and seemed almost identical with 
the same bodies in Zygnema. This strand and its special- 
izations lie embedded in the mass of green chlorophyll con- 
stituting the greater part of the spiral band. Between 
adjoining chlorophyll bands delicate, slightly granular 
strands cross the interspace between two adjacent bands. 
It was impossible to determine whether these came from the 
axial portion or were prolongations of the chlorophylloid 
band substance. A well-known feature of Spirogyra is the 
great number of strands suspending the nucleus in the cell 
cavity. On careful observation it is seen that these sus- 
pensory strands are of the same material and in every way 
identical with the substance of the axial filament. 
Fig. XV shows an ending of one of these filaments, and 
while the exact continuity of the axial with the suspensory 
strands is not apparent, from the nature of the substances 
present, that is the only conclusion that seems likely. 
Cladophora.—In many respects this plant resembles Spiro- 
gyra. Here, however, the axial filament is entirely absent, 
and consequently we have a more diffuse, irregular distribu- 
tion of the chlorophyll masses. Branching processes of the 
material in a rather granular condition ramify over the 
peripheral protoplasm. Scattered about in these processes 
are the pyrenoid areas, with no attempt at systematic dis- 
tribution (Fig. Il). Along with these are smaller deeply 
staining bodies that resemble the pyrenoids. 
The same structures are present as in Spirogyra, except 
the axial filament; there being a densely staining body sus- 
pended by very delicate strands from the walls of an enclos- 
ing hyaline capsule. Many authorities describe the smaller 
of these bodies as the nuclei (each being polynucleate). 
and yet they appear to be identical in structure with the 
pyrenoids of Spirogyra. 
