378 BOTANICAL GAZETTE [NOVEMBER 
The innermost megaspore is functional and develops rapidly into 
the embryo sac at the expense of the other megaspores and the tapetal 
cells. This enlarged cell (fig. 3) now touches the epidermis at the 
micropyle and passes rapidly through the two-, four-, and eight- 
nucleate stages. The mature embryo sac is very small and straight, 
and the enlargement is principally in the direction of the long axis 
of the ovule. Starch is usually very abundant and persists through- 
out the two- and four-nucleate stages, and in Brasenia purpurea 
and Cabomba piauhiensis usually throughout the eight-nucleate stage. 
It gradually disappears, first from the micropylar end of the sac and 
finally from the antipodal end (fig. 6). It probably undergoes modi- 
fication to form the first food for the development of the embryo 
and endosperm. 
Occasionally two embryo sacs were produced, but one was always 
absorbed by the other (fig. 7). This was more frequent in Cabomba 
piauhiensis than in any other species studied. Fertilization occurs 
almost immediately upon the completion of the eight-nucleate stage of 
the sac. The same very pronounced sclerification of the inner part of 
the epidermal cells (jig. 4), previously observed by me (7) for Cas- 
talia odorata, was observed in the tropical species of Castalia studied, 
but not in the other genera, The actual penetration of this epi- 
dermal wall by the pollen tube was observed in only a few cases. In 
fertilization the pollen tube (fig. 4) enlarged and stained so deeply 
that it was impossible to observe what fusion of nuclei did occur. 
At the time of fertilization the polar nuclei are very large, usually 
rather indistinct, and unite at the micropylar end of the sac just 
below the egg apparatus. At the same time the antipodals, which 
are very inconspicuous, undergo degeneration (fig. 4). The syner- 
gids may persist a short time after fertilization (figs. 5,6, 7), but usually 
disappear very quickly. They are most persistent in Cabomba 
piauhiensis. 
The primary endosperm nucleus, which is now very large and ced 
spicuous, moves to the antidopal end of the sac (jigs. 5, 6), where it 
divides (figs. 7, 8, 20), and a very delicate wall is formed across the 
sac between the two daughter nuclei. This wall can be observed 
without great difficulty in Nymphaea advena (jig. 8) and in Castalia 
am pla (fig. 20), but is very difficult to see in the other species studied. 
