

1got } OVULE AND EMBRYO OF POTAMOGETON 341 | 
on all sides by a mass of tissue from five to eight cells in depth. 
This deep-seated position may bring added protection of some 
sort, or it may have no such significance. In the earliest stages 
of the nucellus no differentiation of cells can be distinguished. 
Its growth appears to be due almost entirely to divisions of 
nucellar tissue other than epidermal, since after the formation 
of the epidermis no layers were observed cut from it by peri- 
clinal walls. In fact, the occurrence of periclinal walls in the 
epidermis is exceedingly rare. 
The archesporial cell and its two daughter cells — Longitudinal 
sections of the nucellus, about the time the primordium of the 
inner integument makes its appearance, show a single hypodermal 
cell having archesporial characters. This cell is larger than the 
others and has a larger nucleus, and its protoplasm is usually 
less dense (fig. 7). When this hypodermal cell is barely distin- 
guishable from the surrounding tissue, it divides by a periclinal 
wall. The outer cell is the so-called ‘tapetal cell” (figs. 2, 3), 
from the progeny of which an extensive region of sterile tissue 
is subsequently formed. The inner cell is the primary sporog- 
enous cell or megaspore mother cell, its sporogenous character 
being plainly shown by the rapid changes which soon take place 
in both its nucleus and cytoplasm. 
The sterile tissue—-The subsequent history of the tapetal 
cell is as follows. The increase in size is followed at once by a 
periclinal division (fig. g),and this by an anticlinal wall (fg. 5). 
Doubtless an anticlinal wall follows in each of these cells, mak- 
ing a plate of four cells lying next to the epidermis. Wiegand* 
Says that the anticlinal walls may precede or follow the periclinals, 
usually preceding, but this was not verified in P. zatans. Anticlinal 
walls now follow in the second layer, thus completing a two- 
layered tapetum. In fig. 7 a three-layered tapetum is shown; 
in fig. 13 one that is five-layered. The process continues until 
in some cases eight layers lie between the embryo sac and the 
epidermal layer. As mentioned before, the embryo sac now 
lies in the very heart of the nucellus. 
* WIEGAND Kari M.: The tonnes of the embryo sac in some monocotyle- 
donous plants. Bor. Gaz. 30:25-47. pls. 6-7. 1900. 
