126 BOTANICAL GAZETTE [AUGUST 
are developing, one grows upward, the other downward (fig. 51). In 
the early stages the embryo sac is usually pear-shaped, the narrow 
portion marking the original position of the megaspores, while the 
expanded portion shows the region of growth (figs. 47, 51). The 
growth of the embryo sac and the enlargement of the endosperm 
after walls have been formed crowd upon the adjoining cells of the 
nucellus in such.a way as to distort and flatten them, while no 
doubt some of the nucellar tissue is also digested by the growing 
gametophyte. 
Wall formation and growth of endosperm.—The first formation 
of walls between the nuclei results in a single layer of cells sur- 
rounding the central cavity (fig. 50). The cells at this stage are 
rich in starch, the starch grains having also been present during the 
-free nuclear stages. Centripetal growth of these cells then begins 
by the radial lengthening of their walls, the walls reaching the 
center and forming a completely closed tissue before further cell 
division takes place. Several cases were found showing this feature 
in various stages, but in no case had periclinal walls formed before 
the tissue was closed (fig. 51). Cells which in a single section appear 
to be internal are merely the inner ends of cells abutting the margin 
in other sections. At the very narrow upper end the cells are very 
closely crowded together. 
JAcER’s account (6) of the formation of the endosperm in 
Taxus baccata differs in a few details from this, in that he states 
“that the cavity is filled with tissue by the inward growth of a series 
of cells formed by periclinal walls. His technique was such, 
however, that he could easily have been mistaken in his interpre- 
tation of the situation. The filling of the central cavity by growth 
and periclinal divisions seems to be the rule among gymnosperms, 
and is probably to be correlated with the size of the cavity at the 
time wall formation begins. The smallness of the cavity in Taxus 
— _may account for the method of tissue formation found here. 
a Following the complete filling of the central cavity with cell : 
oo. _tissue, periclinal walls come in, giving rise to several layers of cells 
a between the margin and the center. Anticlinal divisions also take 
- place soon and the vow of the endosperm it ins alk d rections co 
