6 
BULLETIN" 183, U. S. DEPARTMENT OF AGRICULTURE. 
green color. The chlorophyll-bearing cells themselves become some- 
what thickened and remain plainly discernible as a layer, even at 
maturity. The inner integument does not disappear as does the 
outer one. The cell walls undergo considerable modification and the 
layer becomes crushed, so that it exists in the ripened fruit only as 
a dull brown line. It is in the investing membrane of the nucellus, 
however, that the greatest functional transformation occurs. The 
cell walls of this delicate tissue become thickened, and the layer 
quickly assumes, either independently or in conjunction with the 
inner integument, the functions of a selective semipermeable mem- 
brane. Brenchley states that even before the glumes are grown fast 
to the fruit it is necessary to prick through this membrane to bring 
about the entrance of killing fluids. The absorption of the nucellar 
tissue begins almost immediately after fertilization has taken place. 
It proceeds rapidly and is almost total, a small body persisting near 
the funicle and fragments of collapsed cell walls in chance locations 
elsewhere. 
Of the tissues which envelop the seed, the semipermeable membrane 
is the only one performing more than mechanical service. On the 
other hand, all the growth within the embryo sac is of physiological 
purpose in one or more ways. There are three important structures — 
the aleurone layer, the embryo, and the starch endosperm (PI. II, 
fig. 1) . The three are associated in development and are intimately 
related in later germination and growth. Preliminary to fertili- 
zation, the nucleus of the embryo sac divides and redivides until 
eight nuclei are present. The egg cell and two companion cells locate 
near the micropyle. Two others unite to form a large nucleus near 
the center, while the remaining three, the antipodal cells, pass to the 
end opposite the egg cell. 
Following fertilization, the egg cell begins the initial divisions of the 
growth of the embryo, but its development is less rapid than that of 
the large central nucleus, which is the source of the endosperm. The 
earliest and most vigorous cell division of the endosperm-forming 
tissue occurs near the distal end of the grain upon the ventral side and 
adjacent to the antipodal cells which, according to Johannsen, 1 have 
by this time increased to the number of 30 or thereabouts. After the 
first layer of endosperm cells has been formed, the growth continues 
radially toward the center of the grain. The mature stage is always 
found on the ventral side and particularly in the flanks of the 
furrow. It is only after the embryo sac is entirely filled with cells 
that the outer layer of the endosperm begins to differentiate. Al- 
though at this time starch grains are infrequent throughout the 
endosperm, its external layer begins to exhibit the proteolytic 
i Johannsen, W. Om Fr0hviden og dens Udvikling hos Byg. Meddelelser, Carlsberg Laboratories 
Bd. 2, Hefte 3, p. 103-133, 3 pi., 1884. (French ed., p. 60-77.) 
