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. III 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 
