338 Watson on Structure and Relation of the Plastid. 



Observations. 



Zygnema. — Each cell of Zygnema possesses two pyrenoids 

 suspended within the lumen of the cell by means of strands 

 of a clear substance. Each pyrenoid consists of a very 

 dense, deeply staining nodule; while around it is a less 

 densely staining area of a granular character. 



In Fig. V this is clearly shown, and in some cases the 

 granular character extends a distance into the suspensory 

 strands. Midway between the two pyrenoids is seen a clear 

 refractive body, the nucleus, not staining as deeply as the 

 pyrenoids, and embedded, as it were, in the substance of a 

 broad band of cytoplasm joining the two pyrenoids. This 

 body is constant in all cells in the normal resting stage and 

 always in this position. It is evidently associated closely 

 with the active areas on either side of it. 



Fig. Ill shows the condition after conjugation is com- 

 pleted. The four pyrenoids are distinctly shown suspended 

 by filaments from the wall of the aggregation capsule. 



Fig. IV shows division of a pyrenoid. The two dense 

 bodies formed by fission of the parent pyrenoid gradually 

 separate, while between them is a clear band in the center of 

 which will later appear the nucleus which for the time is not 

 observable. In comparative structure, relative stainability 

 and close association the pyrenoids and nucleus of Zygnema 

 suggest a close relationship. 



Spirogyra. — The most available type chosen was S. nitida 

 and the minute structure of the chlorophyll bands was first 

 studied. Running in the center of the band, as shown in 

 Fig. XV, is a continuous clear strand about 5 microns in 

 width at its narrow portions. At intervals in this apparently 

 clear spiral strand appear dilatations into sac-like structures. 

 The sac-like portions are budded off from each other by con- 

 strictions, then by growth of the intermediate portion, due 

 to the constriction, the two sacs become separated. 



Within the lumen of each sac rests the so-called pyrenoid 



