The Morphology of Ruppia Maritima. 163 
In the root system, although Ruppia has usually only one root 
at each node, Zannichellia and Althenia have two, and the Pota- 
mogetons several, even Potamogeton pectinatus developing four 
or more (Irmisch, 1858). In Zostera and Phyllospadix a cluster of 
roots occurs at the nodes. The coleorrhiza has been reported in 
Potamogeton, Zannichellia, and Cymodocea, and probably occurs 
also in Althenia and Zostera. In the root anatomy, Potamogeton 
pectinatus, Zannichellia and Althenia correspond closely with Rup- 
pia, as is the case in the other vegetative organs, Zannichellia 
showing the four distinct meristematic regions at the growing-point 
as in Ruppia. 
Passing to the reproductive organs, where, owing to our lack of 
information, the comparison is most unsatisfactory, we have seen 
that the inflorescence is spadix-like, resembling Potamogeton and 
Zostera, and that the flowers themselves are apparently reduced 
from some form lke Potamogeton. 
The archesporium of the microsporangium probably arises in 
Zannichellia in much the same way as in Ruppia, i. e., originating 
simultaneously from a large group of cells. Only one species of 
Potamogeton has been investigated on this point, and here the 
archesporium is traceable to a single hypodermal cell. Like Ruppia, 
Zostera also derives tapetum from the outer sporogenous cells, but 
Potamogeton foliosus differentiates it from the wall. 
Parietal tissue exhibits a much greater development in the mega- 
sporangium of Potamogeton than in Ruppia, and in this respect 
Zannichellia probably more closely resembles Ruppia. This con- 
sideration again points to Potamogeton as the more primitive form. 
In Potamogeton natans (p. 138) an arrangement of the potential 
megaspores has been found such as occurs in Ruppia. 
The pollen-grains of Zannichellia and Potamogeton are more or 
less globular, those of Ruppia somewhat elongated, while those of 
Zostera and Phyllospadix have attained extreme length so that 
they might be termed filamentous. These differences depend pre- 
sumably on the different modes of pollination prevailing among 
these genera. 
More than any other member of the Potamogetonacee so far in- 
vestigated, the embryo of Zostera has points in common with Ruppia 
in the manner of its development, and, as is the case with the 
mature embryo of Zannichellia, resembles it in its mature form. 
The primary root of Zostera, however, functions for a time during 
germination. 
It will be seen from these considerations that, on the whole, 
