1907] CHRYSLER—POT AMOGETONACEAE ' 18^ 



argued by Schenck (27) that we have here a stage in the assumption 

 of aquatic life by the genus. The leaves of all the other members of 

 the family are submersed. 



(7) The inflorescence is a spike or spadix, an admittedly primitive 

 form. The other genera show a specialized spadix, such as Zostera, 

 or a reduced inflorescence, as in Zannichellia (and Naias). 



(8) The spadix in Potamogeton is raised above the surface of the 

 water so that pollination is anemophilous. The nearest approach 

 to this is in Ruppia, where the pollen floats on the surface of the 

 water, while in the other genera the inflorescence is submersed and 

 the pollen does not reach the surface. In connection with this habit 

 the pollen of Zostera and Cymodocea is filamentous. Evidently these 

 cases of hydrophilous pollination are adaptations. 



(9) The floral parts are in whorls of four, which is a characteristic 

 dicotyledonous number, although the whorls do not alternate in the 

 simplest w^ay. There is only slight coalescence of parts. 



(10) The flowers are perfect^ while in ZannicheUia and Naias they 

 are monoecious or dioecious. 



As to the relationships among the genera, the tendency of recent 

 classifications. is indicated by the fact that the old family Naiadaceae 

 is now split into four families, and one of these (the Potamogctonaceae) 

 into five not very closely related sections. It appears that the leading 

 principle at work in the evolution of the families has been that of 

 simplification or reduction, both in the reproductive and the vegetative 

 organs. Some features of specialization and complexity have also 

 appeared, such as the filamentous poflen and flattened spadix of 

 Zostera. This genus in spite of its peculiarities seems to stand more 

 closely related to Potamogeton than to the reduced genera Ruppia, 

 Zannichellia, and Naias, and its va:>cular system at least may be 

 derived from that of Potamogeton, as has already been shown. The 

 vascular system of Ruppia may represent a reduced condition deri\'cd 

 from that of Zostera or Potamogeton, while Zannichellia and Naias 

 are still more reduced both as regards the vascular system as a whole 

 and the elements of which it is composed. Naias represents the 



extremo of rrdnrtinn fr^nnH in tV»p nrrrinn fnr tfinrp 1*; a Sintrle leaf 



N 



fl' 



