THE RESIDUAL CAVITY 21 
may filla fourth or more of the residual cavity before finally becom- 
ing indistinguishable from the remnants of protoplasm around it. 
Immediately after the organization of this vegetative nucleus, 
numerous leucoplasts appear in the residual cavity. They are 
: found chiefly clustered closely around the large nucleus, which 
they sometimes completely envelop with their own mass and that 
of the starch they form. The basal nucellar tissue is being rapidly 
broken down at this time, and the starch stored in the inner 
integument is being withdrawn. It seems to be the business of 
this vegetative nucleus and the accompanying leucoplasts to 
elaborate the food secured from surrounding tissues for absorption 
by the growing endosperm, or in the case of over supply, to re- 
organize it into stable starch form. Even after the disorganization 
of the nucleus the leucoplasts seem active. They are the last 
organized bodies to disappear from the residual cavity, and may be 
observed singly or in globular masses of the size of a normal endo- 
sperm nucleus after the large nucleus of the cavity has completely 
broken down. 
A point of special interest to the writer is the fact that in ovules 
having no fecundated egg, the primary endosperm nucleus behaves 
much as the nucleus just described. The leucoplasts congregate 
about it and often almost fill the embryo-sac with masses of starch 
grains before the general decline of the tissues begins. 
The activity of the vegetative nucleus is accompanied by a 
marked increase in the size of the residual cavity. It not only 
occupies the place of the disintegrated antipodal cells and basal 
nucellar tissue, but it crushes the inner integument from which 
the food material has been removed. Finally by the pressure of 
its increasing liquid contents it forces the endosperm toward the 
micropyle, and spreads the base of the ovule, giving it the char- 
acteristic form of the seed (PLATE 2, FIG. 31). 
THE SEED COATS 
In order to understand fully the steps in the development of 
the seed coats, a statement of some nutritive processes preceding 
fertilization is necessary. During the tetrad divisions and the 
maturing of the megaspores, the cells of the nucellus become filled 
with starch. During the growth of the embryo-sac all this starch 
