160 Bliss . — A Contribution to the Life-history of Viola. 
network of this nucleus is extremely delicate, and only slight condensations 
of chromatin are visible at the periphery and in the meshes of the net. The 
endosperm nucleus is very easily recognized, when present, by the large size 
of its nucleolus. This nucleolus is always characterized by a conspicuous 
vacuole which very nearly fills it. 
Pollination and Development of the Pollen-tube. 
Before the pollen-grain is shed the contents of the microspore divide 
into a larger and a smaller cell. The smaller or generative cell becomes 
lenticular and lies free in the cytoplasm of the tube cell (Fig. 18). 
Chamberlain (1) and Wylie ( 9 ) figure similar generative cells for Salix and 
Elodea. The pollen-grains lodge on the inner surface of the stigmatic 
cavity where germination takes place. Often the upper portion of the 
stylar canal is completely filled with these pollen-tubes, although I could 
trace comparatively few into the ovary. There is no evidence of branching 
in the pollen-tube as it passes down the stylar canal, and no penetration of 
the stylar tissue. The passage of the pollen-tube through the micropyle 
and between the tapetal cells was clearly observed many times. 
The pollen-tubes stain densely at this period, compared with the egg, 
and in most cases their exact contents could not be determined. From 
careful observations, I am led to believe that the generative nucleus divides 
immediately after the entrance of the pollen-tube into the embryo-sac. In 
cases where the sperm nuclei could be distinguished lying in the densely 
staining mass emitted from the pollen-tube, they were small, very nearly 
equal in size, and spherical in outline, giving no suggestion of the spiral 
form described by so many authors. The nucleus proper stains lightly, but 
it contains a large, spherical, deeply staining nucleolus. 
In Viola , the embryo-sac has reached its mature size previous to the 
entrance of the pollen-tube (Fig. 15). At this time it is broadest at its 
middle diameter, narrowing somewhat at the micropylar end, but tapering 
more conspicuously at the antipodal extremity ; the egg apparatus occupies 
nearly three-fourths the width and about one-third the length of the sac, 
and the endosperm nucleus has moved away from the egg, and occupies 
a central position in the embryo-sac. Following the entrance of the pollen- 
tube into the embryo-sac, one or both of the synergids disintegrate, the 
nucleus of the egg increases in size, and, in some instances, the growth of 
the antipodal cells is very conspicuous (Fig. 16). In such cases the size 
and shape of the antipodals recall the antipodal egg described by 
Chamberlain ( 12 ) and by Opperman ( 11 ) in Aster , but in no case was there 
any evidence of fertilization of these cells. Never more than two enlarged 
cells were found at the antipodal end of the embryo-sac, thus there was 
never found any structure suggesting an egg apparatus. 
