164 A. H. Graves, The Morphology of Ruppia Maritima. 
Ruppia is closer to Potamogeton, Zannichellia, and Althenia than to 
any others of the Potamogetonaceae, especially with regard to the 
vegetative organs; and yet, in its embryo, it most strongly resem- 
bles Zostera. Similarities in the vascular structure of the vegetative 
organs are looked upon generally as comparatively fixed indications 
of relationship; so that in this case the evidence points to Ruppia 
as being derived from some form similar to the present submerged 
Potamogetons, with Zannichellia and Althenia serving as examples 
of still further reduction. 

