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 threadHke 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 Zygiiema. 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. II). 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 hyahne 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. 



