DEVELOPMENT OF SEED IN POLYGONACEAE 
457 
about the developing embryo. At about the time of the first indication 
of cotyledons in the embryo, cellular formation appears in this region, 
giving rise to a thin layer of cells around the embryo and also extending 
across the embryo sac below it. As further cell formation proceeds 
in the endosperm, a marked differentiation becomes apparent. At the 
stage shown in figure 2, the cell formation has proceeded until a por- 
tion of the endosperm, EN, about 6 cells thick extends across below 
the embryo. The endosperm above and at the sides of the embryo 
consists of a layer one or two cells thick, while below the thickest 
region, the endosperm again becomes gradually thinner and thinner, 
until at the base, for about one-fourth its length it no longer shows cell 
division but consists of a layer of nucleated cytoplasm, enclosing the 
large central sap cavity. This cytoplasmic layer merges into a cylin- 
drical haustorium-like mass of dense protoplasm, iJ, at the base of 
the embryo sac, which apparently has no special function, as it under- 
goes very little change in the later development of the seed. 
This marked differentiation of the endosperm into a cellular and 
non-cellular region, presented a condition similar to that described by 
Hofmeister (8, p. 185) and Strasburger (19, p. loi) in which the 
first division of the primary endosperm nucleus gives rise to a two 
chambered embryo sac, in only one of which the endosperm is de- 
veloped. 
A secondary differentiation in the cellular portion of the endosperm 
becomes evident very soon after the stage just described; the outer- 
most layer assuming the appearance and function of a "cambium" 
layer which cuts off cells only from the inner side {fig. 2a, CA). 
Chamberlain (4, p. 344), in the developing endosperm of Dioon edule, 
and Stevens (18, p. 61), in Fagopyrum esculentum, have found similar 
conditions. These "cambium" cells divide rapidly, forcing the newly 
formed cells toward the center and downward as shown in figure 2a. 
The size of the sap cavity is thus diminished through the active growth 
of these cells. Very soon after the stage shown in figure 2, the coty- 
ledons appear in the embryo, the sap cavity continuing to become 
smaller, until it becomes completely obliterated. As the embryo 
continues in development at the expense of the endosperm, it comes 
to lie in one angle of the seed with the faces of the cotyledons parallel 
with the flat surfaces of the seed as shown in figure 3. At this stage 
the endosperm cells differ considerably in appearance. The "cam- 
bium" cells and those adjacent are prismatic and densely filled with 
