144 A. H. Gf'aves, 



According to my own observations, which agree in the main 

 with those of Roze (1894), the process of poUination takes place 

 about as follows. Soon after the extension of the peduncle above 

 the surface of the water, the anther sacs split open by a longitudinal 

 cleft, and the pollen, shed in large yellowish masses, may be seen 

 floating on the surface of the water. 



Immediately subsequent to the shedding of the pollen, the rhachis, 

 which up to this time has been erect, commences to incline toward 

 the surface of the water. It becomes more and more horizontal 

 until eventually, after two or three hours, it comes to lie on the 

 surface of the water. 



In this position, the stigmas are of course so situated that the 

 floating pollen grains, with which, in the height of the flowering 

 season, the water is fairly well covered, have easy access to them. 

 The final step occurs when the currents of water, always moving 

 in one direction or another in a tidal ditch — or sometimes set in 

 motion by gusts of wind — bring the pollen grains into contact with 

 the stigmas. 



Wylie (1904), has described a similar mode of poUination in 

 Elodea. He attributes the floating of the pollen-grains to the air 

 imprisoned between the spines of the pollen-grain and the surface 

 of the water, this being sufficient to keep the grain afloat. Pol- 

 lination on the surface of the water also takes place in Vallisneria, 

 as is well known. In Zostera, however (Strasburger, 1908, p. 258), 

 pollination is performed below the surface of the water. 



Fertilization 



No investigator has yet been able to demonstrate the process of 

 fertilization in Ruppia. Murbeck (1902, p. 15) has, indeed, found 

 the pollen tubes in the ovary, but has been unable to distinguish 

 either of the sperm-nuclei in the embryo-sac. For various reasons, 

 he concludes that the act of fertilization takes place very rapidly. 

 If this is true, it may account for the fact that in my own prep- 

 arations I also have been unable to find any unmistakable evidence 

 of the sperm-nuclei. 



As is shown in PI. X, figs. 62-66, a definite stylar canal exists 

 from the stigma to the cavity of the ovary, but Murbeck (1902, 1. c.) 

 claims that the pollen tubes, however, penetrate through the cellular 

 structure of the style to the cavit}^ of the ovary. 



