DEVELOPMENT OF SEED IN POLYGONACEAE 
459 
quite conceivable that the differentiated outer layer of the nucellus 
assumes this function. Further, the more rapid endosperm develop- 
ment in the micropylar region is evidently due to the fact that food 
material is brought there by way of this layer, since the broken-down 
cells of the undifferentiated nucellar tissue form a barrier to any 
passage of food substance through that channel. 
As the seed approaches maturity, a further differentiation occurs 
in the endosperm. After the "cambium" layer ceases cutting off 
starch-storing cells, it divides further by anticlinal walls; thus giving 
rise to a layer of short, thin-walled prismatic cells, which, in the micro- 
pylar region, undergo further division, developing to the exclusion of 
the starch-storing endosperm and passing abruptly into the latter. 
At the maturity of the seed, shown in figure 4, the cells of this layer, 
A, are densely filled with granular contents, but contain no starch. 
This is an aleurone layer, the "eiweisshaltige Zellen" of Harz. Except 
for this aleurone layer the remaining portion of the endosperm, EN, 
consists of large, thick walled cells, closely packed and filled with 
starch grains, which by their pressure cause the nuclei to become 
flattened and irregular in outline. 
In the mature seed, which is 2 mm. wide and 2.5 mm. long, the 
cotyledons of the embryo are flattened and slightly broader than the 
hypocotyl. The entire embryo occupies a cavity at one side of the 
seed {figs. 4, E, and 9, C), this cavity having been formed by the con- 
tinuous growth of the embryo at the expense of the endosperm. 
The cells lining this cavity are somewhat broken down and irregular 
in outline due to their depletion by the developing embryo. 
The only vestige of the original nucellus present in the mature 
seed as perisperm is a mass of broken-down tissue at the base of the 
seed {fig. 4, N') and the crushed remains of the nutritive layer. It is 
interesting to note that during the later development of the seed these 
tissues have apparently served no function, and do not represent a 
storage region, this function being entirely carried out by the endo- 
sperm with its aleurone layer. 
The thin, uniform integuments, in the development of the seed to 
maturity, have become closely compressed, so that only their crushed 
remains appear in the mature seed {fig. 4, /). 
The seed, at maturity is enclosed in a thick hard coat, consisting of 
the cutinized epidermal layer and the compressed inner layers of the 
ovary {fig. 4, 0). 
