1917] BOURQUIN—ZYGNEMA 431 
some newly formed grains would always be found which howe 
indicate their origin in that way. 
Typically the pyrenoid forms a round mass in the center of the 
plastid (fig. 1). The most extreme variations in its shape are 
figured (figs. 2, 8,12, 14). In no case is there a striking similarity » 
between the contour of the pyrenoid and the starch grains, nor 
does the pyrenoid extend between the starch grains in such a way 
as to suggest that the starch grains have been cut out of the sub- 
stance of the pyrenoid. Fig. 8 is the most suggestive found. 
Although the pyrenoid is stained with different intensity in 
different parts, the whole pyrenoid stains the same color with any 
given stain. The narrow band of the substance of the pyrenoid 
connecting the halves of a dividing pyrenoid never stains deeply 
(fig. 18). In the vegetative plastid these light and dark areas 
bear no definite relation to each other or to the surrounding starch 
grains (figs. 2, 3, 8, 9, 11, 15). Moreover, these light and dark 
areas are uniformly homogeneous in structure. I believe, there- 
fore, that they do not indicate any change in the substance of the 
pyrenoid; they are simply regions of different density.” 
Grains lying radially about the pyrenoid and varying in shape 
from cuneate to rectangular might be formed by the pyrenoid by 
secretion were the pyrenoid to form them in the periphery of the 
plastid and add to them centripetally, or to form them at the 
center of the plastid and add to them continually on the inner 
edge, such additions pushing them automatically toward the 
periphery of the plastid. The first manner of formation is impos- 
sible since the pyrenoid is always confined to the center of the 
plastid and in no case was seen to approach the periphery where 
the smaller grains of starch occur. The nearest approach to such a 
situation is seen in figs. 8, 14. The second possibility cannot exist 
because the small grains are never found next to, or near, the pyre- 
noid. They were found without exception in the periphery of the 
plastid. Since there is no indication that starch grains ever split, 
they could not have been derived indirectly from the pyrenoid in 
that way. The fact that the plastid separates the pyrenoid and 
the starch grains becomes significant when added to these proofs 
that the pyrenoid does not take part in starch formation in Zygnema. 
