142 A. H. Graves, 



(1864j, ten by about two thousand //, while the microspores of Zostera 

 (Rosenberg 1901, II) are three by about two thousand fi 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 //. 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. XII, 

 figs. 92, 93 tn) 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 

 (PI. 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 find 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, PI. XII, fig. 88. These tetrad cells do 



