The Morphology of Ruppia Mantima. 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 like 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 Potamogetonacese 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, 



