Bliss . — A Contribution to the Life-history of Viola . 161 
The sperm nuclei, small at first and spherical in outline, increase rapidly 
in size. One passes down and fuses with the endosperm nucleus. This 
fusion is clearly shown in PI. XIX, Fig. 19, in which the endosperm nucleus 
is easily recognized by the characteristic vacuole of its nucleolus. The sperm 
nucleus is very large, nearly spherical in outline, and differs from the 
endosperm nucleus very conspicuously in appearance, in that masses of 
chromatin are held in the rather close network of the nucleus, and it 
contains a small densely staining nucleolus. The second sperm enters the 
egg at its apex. Fusion may take place in the central or in the basal 
portion of the egg. In both cases the two nuclei are surrounded by a mass 
of densely staining cytoplasm. Before fusion of the sperm and the egg 
nucleus is completed, and in some cases before the two nuclei are in contact, 
the so-called endosperm nucleus divides from one to several times (Figs. 21, 
22). The young endosperm surrounds the egg and outlines a cavity beneath 
it which extends to the base of the embryo-sac. 
This cavity persists in the seed of both the chasmogamic and the 
cleistogamic flowers when the embryo is fully formed and the seed is nearly 
mature. The formation of endosperm is very rapid at the last, and in the 
mature seed we find a solid mass of endosperm surrounding the tiny green 
embryo at the micropylar end of the seed. 
The first division of the egg is transverse, dividing it into two cells 
nearly equal in size (Fig. 24). There is no suggestion of a suspensor. The 
oldest embryo figured (Fig. 27) shows a spherical mass of actively dividing 
cells at the apex of the embryo-sac ; the endosperm is limited to a parietal 
layer lining the embryo-sac, and forming a sheath of cells which completely 
encloses the embryo, and separates it from the cavity beneath. In mature 
seeds of the chasmogamic flowers collected and studied in the spring of 1909, 
the embryo was an eighth of an inch in length, bright green in colour, and 
clearly differentiated into radicle, caulicle, and cotyledons. 
Summary. 
The primary archesporial cell arises from a hypodermal cell of the 
nucellus. 
A tapetal cell is cut off, which by vertical and transverse divisions forms 
a cap three or four cells in depth, directly beneath the micropyle. 
The definitive archesporial cell divides into an axial row of four cells. 
The innermost of the four cells is generally the functional megaspore, but 
cases were observed where the second or third cell of the axial row was 
enlarging to form the embryo-sac. 
The polar nuclei fuse in contact with the egg, generally before the 
entrance of the pollen-tube. 
The synergids are pear-shaped with a large vacuole in the basal portion, 
and the nucleus lies just above the vacuole. 
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