142 A. H. Graves, 
(1864), ten by about two thousand yw, while the microspores of Zostera 
(Rosenberg 1901, II) are three by about two thousand w when mature. 
Moreover, in Phyllospadix, grouped by Ascherson (1889) with Zostera 
in the subfamily Zostereae of the Potamogetonaceae, Dudley (1893) 
has found the pollen grains to measure about four or five by one 
thousand w. Dudley (1893, p. 412) states that “They are slightly 
flattened at the extremity and some are enlarged toward the middle.” 
During this long period of development to the mature form, 
several noteworthy internal changes occur. The tube-nucleus (PI. XI, 
figs. 92, 93 ¢w) gradually undergoes degeneration, until in the mature 
grain it often appears fragmentary or angular. The starch grains, 
so prominent in the early stages, become for the most part smaller 
and fewer, and one seems warranted in concluding that a part of 
their substance has been utilized in the formation of the grain. 
It is not until the mature condition, or when the pollen is about 
ready to be discharged, that the generative cell divides. When 
this is to take place, the latter assumes a position in which its long 
axis is more or less parallel with the long axis of the microspore. 
The two resulting male cells remain united as in Potamogeton foliosus 
(Wiegand, 1899), each surrounded by a considerable layer of cytoplasm 
(Pl. XII, fig. 94). 
Moreover, even at this stage, no wall separates the male cells from 
the cytoplasm of the pollen-grain, but there is a fine cell-plate 
formed between the two cells. As a slight variation from Murbeck’s 
figure, I tind that for the most part in Ruppia maritima a moderate con- 
striction occurs between the two male cells, in the region of the 
cell-plate. 
One feature which has not been thoroughly worked out in the 
pollen-grain of Ruppia and indeed has been much neglected in the 
study of the male gametophyte in general, is the origin and growth 
of the microspore wall. 
Murbeck (1902) has described the peculiar thickenings of the 
mature wall, the latter consisting of a single thin layer. 
Since I was fortunate enough to have a large number of sections 
of all stages of the growth of the pollen-grain, the development of 
the wall from the pollen mother-cell stage to the mature microspore 
was comparatively easy to trace. 
The wall of the pollen mother-cell, after it has separated from 
its neighbors, is very thin. After formation of the tetrads, this wall 
thickens and becomes the free or outer wall of the tetrads, while 
the interior walls of the tetrad are laid down immediately after 
the two reduction divisions, Pl. XII, fig. 88. These tetrad cells do 
