The Morphology of Ruppia Maritima. 135 
MErGASPORANGIUM 
The usual method of development of the megasporangium cor- 
responds in every particular to that of Ruppia rostellata, as des- 
cribed by Murbeck (1902, pp. 10, 11), so that it would be use- 
less to duplicate his careful description here. Stated briefly, the 
process is as follows: At astage in the growth of the pistil represented 
in Pl. X, fig. 62, or when it is about 0.25 mm. in length, a hypo- 
dermal cell in the young nucellus, in the region marked x, becomes 
considerably larger than its neighbors, with a larger nucleus, and 
more densely staining contents. This then divides by a periclinal 
wall, forming an outer, primary parietal cell, and an inner, megaspore 
mother-cell. The primary parietal cell now divides twice succes- 
sively by anticlinal walls, at right angles to each other, forming a 
plate of four cells, or, through a third anticlinal division, sometimes 
six cells. During this time the two integuments successively make 
their appearance. Meanwhile, the megaspore mother-cell enlarges, 
and with preparations for the first reduction division the history 
of the female gametophyte begins. 
In Zannichellia, Campbell (1897, pp. 45, 46) finds two parietal 
layers formed at first, but these later divide into several layers. 
A much greater development of parietal tissue has been observed 
by Wiegand (1900, pp. 31, 32) in Potamogeton folhosus and by Holferty 
(1901, p. 341) in Potamogeton natans. In the latter case it is def- 
initely stated that sometimes eight layers le between the embryo 
sac and the epidermis. In other respects the development of the 
megasporangium in both of these genera is essentially the same 
as in Ruppia. In Lemna, according to Caldwell (1899, pp. 56, 
57), there are not more than two parietal layers formed, and the 
other details of the megasporangial development are practically 
the same as in Ruppia. 
Coulter and Chamberlain (1903, p. 65) state that the suppression 
of the parietal tissue among the monocotyledons ‘is usually asso- 
ciated also with the greater or less development of this tissue,” a 
point which is illustrated here in the Potamogetonaceae by the 
condition in Potamogeton. “The strongest argument,” to quote 
these writers further, “that suppression of the parietal tissue of the 
megasporangium is a strong tendency among Angiosperms, is that 
this condition is universal among the Sympetalae so far as investig- 
ated.” 
1 Rarely two layers of parietal cells are formed (Fig. 78). 
