The Morphology of Riippia Maritinia. 143 



not separate : the protoplasts forming the future pollen grains may 

 be said to simply occupy the four quadrant-like compartments in 

 the spherical structure comprising the tetrad. The walls at this 

 stage are of noteworthy appearance by reason of their considerable 

 thickness — being thicker than the wall of the pollen-grain at any 

 future time — and also from their transparency, refracting the light 

 in such a way that they appear gelatinous. 



The tetrad walls appear now to expand, leaving the pollen proto- 

 plasts naked within them. Such a condition is shown in PL XII, 

 tig. 97. Finally the walls entirely dissolve. 



Judging from Murbeck's account, these conditions escaped his 

 notice, for he speaks of the membrane of the tetrads as showing 

 " noch keine Spur von Oberflachenstruktur " (1902, p. 8j. 



The process is, however, essentially the same as that described 

 by Strasburger (1882, pp. 87 ff. ; 1889, pp. 36 ff.) for the pollen- 

 grain of Malva crispa and other plants. Only one marked difference 

 occurs, namely, that in Malva crispa, &c. the old pollen mother- 

 cell wall is apparently cast off and does not take part in the wall 

 formation of the tetrads. 



From now on a wall develops around the young pollen-grain, 

 but, as in the cases of Naias and Zannichellia, remains a single 

 thin membrane (PI. XII, fig. 94). 



Very early (PI. XII, fig. 97) the local thickenings begin to be 

 formed on the outer surface of the wall. These in this early stage 

 are difficult to see clearly on account of their transparence. They 

 seemed, however, to be the result of depositions on the wall and 

 not do develop from radial bands in its interior. Ultimately these 

 thickenings appear in the shape of low ridges arranged to form 

 irregular polygons, at whose intersection are short spines, with a 

 slight knob at the end (PI. XII, fig. 96, a and b). At the ends of 

 the grain and at its expanded center, these spines become shorter, 

 or are absent altogether (PI. XII, fig. 94). It is probable that at 

 these localities the pollen tube commences its formation. 



Pollination 



Ruppia is one of the few of the higher plants to which, in 

 respect of the process of pollination, the term hydrophilous may be 

 applied. In other words, water is the transporting agent for the 

 pollen, instead of the commoner methods by insects or the wind. 



