182 Cook: EmBryoLocy or RuyYTIDOPHYLLUM 
embryo now increases in size and the differentiation into dermato- 
gen, periblem and plerome occurs very early (figures 20-23). The 
cell a nearest the embryo (figure 78), to which we will hereafter 
refer as the basal cell, now divides by a cross-wall into two cells, 
6 and 6’ ( figure 20). Cell 3’ now divides by longitudinal division 
into four cells, which form the root-tip (figures 27, 22, 23a, 236). 
Cell 6 now divides by both transverse and longitudinal divisions 
( figures 22, 23) into four cells of which the upper layer 4’ (figures 
23a, 236) forms the dermatogen of the root-tip. 
At about the time of the formation of the dermatogen of the 
root-tip the two cotyledons appear (figure 23 a), one of which is 
usually slightly larger than the other. Examination of a large 
number of embryos slightly older than figure 23 indicated that 
these cotyledons have an independent origin. The endosperm has 
entirely disappeared by this time and the nucellus is very much 
reduced ( figure 16). The embryo now enlarges rapidly, gradually 
utilizing the last of the nucellar tissue, and when fully mature is 
surrounded only by the integuments. 
Abnormal embryos. — Four abnormal embryos were observed. 
Two of these embryos ( figure 24) were about the same age as 
those in figure 23 but had elongated without producing cotyledons 
and without differentiation of tissues into plerome and periblem. 
The other two abnormal embryos each had three equally well de- 
veloped cotyledons. They were the same age as in Jigure 25, and 
were cut in cross sections, 
SUMMARY 
1. The archesporium is a single sub-epidermal cell. 
2. This usually becomes the functional megaspore without pre- 
vious subdivisions. 
3. There is nothing unusual in the formation of the embryo-sac. 
4. The formation of the embryo is typical of the dicotyledons 
as illustrated by Capsella Bursa-pastoris, except for variations in 
the formation of the dermatogen and in the basal (hypophysis) 
region. 
5. The endosperm is of the cellular type and is formed in the 
usual manner but disintegrates very early. 
6. Both endosperm and nucellus are utilized to feed the grow- 
ing embryo, eventually leaving the embryo surrounded only by 
the integuments, 
