The Morphology of Ruppia Maritima. 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 PI. XII, 
fie. 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. 8). 
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 (Pl. XII, tig. 94). 
Very early (Pl. 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 (Pl. XII, fig. 96, @ and 6). At the ends of 
the grain and at its expanded center, these spines become shorter, 
or are absent altogether (Pl. 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. 
