256 BOTANICAL GAZETTE [APRIL 
sperm cells fully differentiated before pollination. As the tube 
passes through the micropyle it is considerably constricted, but 
when it reaches the apex of the embryo sac it increases appre- 
ciably in diameter. The tube takes exactly the same course as 
in Alisma, passing down on one side near the wall of the sac, 
and encountering the nucleus of one of the synergids, which 
disappears at this time and is never seen again (figs. 27, 25,26, 29). 
The other synergid, with its nucleus, persists for a long time, and 
can still be seen above the vesicular suspensor cell of rather 
large embryos (jigs.65,69). The pollen tube after entering the 
embryo sac stains very dark, and it is often difficult to distin- 
guish the two sperm nuclei as they are traveling down the tube. 
_In lightly stained material, however, they can be seen very 
readily. As the lower one approaches the tip of the tube it is 
preceded by two centrospheres, which can be seen always in 
well stained sections because of their position and the light 
colored cytoplasm with which they are usually surrounded 
(figs. 22, 23, 24, 26,28). When the sperm nucleus breaks out 
of the tube it makes a decided perforation, the appearance being 
as though the tip had been softened and the nucleus had broken 
forcibly out of it. In some cases the edges of the perforation 
are rather smooth, while in other cases they are somewhat 
ragged (figs. 29, 30, 37, 72). A stream of cytoplasm escapes 
from the tube after the lower sperm nucleus (fig. 30), but the 
upper sperm nucleus never leaves the tube (fig.32), which is 
also the case in Alisma. After the rupture of the pollen tube, 
the cytoplasm between the sexual cells usually contains numerous 
granules, which may have escaped from the tube, or they may be 
: fragments of the disintegrated tip of the tube (figs. 25, 27)- 
This often makes it difficult to identify the centrospheres at this 
stage, it being very easy to lose sight of them entirely although 
they may be present in the section. 
_ In the meantime changes have been taking place in the 
oosphere. Its nucleus i is no longer symmetrical in outline, but, 
_ just as in Alisma, it is drawn out into a considerable bulge on 
the side toward the 5p rm “nucleus: gs. 21, 2h: Cae This 
