432 BOTANICAL GAZETTE [NOVEMBER 
Everything bears out the following theory concerning the origin 
and growth of the starch grains in the form. The plastid gives rise 
to minute starch grains in its periphery, either between the larger 
grains (figs. 15, 16) or entirely beyond them (figs. 8, 11). Their 
growth seems to be regulated by their position in relation to the 
larger grains and to be a mechanical matter. Figs. 5, 6, 13 show 
how the shapes of the grains seem to fit naturally into the place 
occupied. If the small grains lie between larger grains, the plastid 
adds to them in such a manner that they become cuneate in shape. 
The enlargement on the inner face continues to be more rapid 
lengthwise of the grain than laterally as it grows down between 
the grains toward the center of the plastid, so that when it attains 
the length of the large grains it is still cuneate. A comparison of 
grains ¢ in figs. 3, 8, 14, 16 shows a complete gradation in the 
lengths of the grains from the shortest to the longest, so that a 
perfect series could be arranged were the grains removed from their 
respective plastids. 
After lengthening the grain in the manner described, the plastid 
begins to add most rapidly to the narrow base laterally, so that the 
grain becomes trapezoidal and then rectangular in shape. In rare 
instances this basal broadening may continue until the grain is 
once more trapezoidal (fig. 2). Grains / in figs. 2, 10, 16 show 
every stage in the change of shape just mentioned. If the small 
grains are formed above larger grains, they grow rectangular or 
oval, pushing the grain below toward the pyrenoid. This is the 
unusual rather than the normal method of growth, however. 
Summary 
The chromatophore of Zygnema is a plastid containing imbedded 
in its substance a pyrenoid which lies near the middle, and starch 
grains which usually lie radially about the pyrenoid. 
The pyrenoid cannot take part in starch formation because it 
is always confined to the center of the plastid and is separated 
from the starch by the plastid, and because the small young 
grains of starch are always found in the periphery of the 
plastid. The plastid therefore must form these minute starch 
grains. 
