l8 UNIVERSITY OF MISSOURI STUDIES [l6o 



b. Fluitantes potamogetonoides. The pondweed as- 

 sociation of ribbon-leaved aquatics finds its best development in 

 slow-flowing streams, the leaves floating in the direction of the 

 current. In ponds and the quiet waters of lakes this association 

 is found mingled with the next. There are but two species, the 

 first of which is restricted to a few old ponds : 



Potamogeton fluitans P. hybridus 



3. Aquatiles demersae. There is but one association 

 of submersed aquatics ; Demersae ceratophylloides, the 

 hornwort association. There are five species, all of which 

 are local, except the first: 



Potamogeton Hillii Elodea minor 



P. pusillus Ceratophyllum demersum 



Naias flexilis (More's lake) 



B. Marginales. Planis of the marginal zone. 



Marginal plants occupy the shores and shallow waters of 

 ponds, lakes and streams, or are central in the wettest places of 

 swales and swamps. While in the main the marginal zone is 

 well defined, it merges into all the other hydrophytic zones. In 

 low water the true aquatics are sometimes forced to assume a 

 terrestrial existence. Even Lemnaceae, at the drying up of 

 ponds and puddles, root feebly in the soft earth. The marginal 

 zone is divisible into three subzones: i. The amphibious; 2. 

 The limose; 3. The littoral. 



1. Marginales amphibiae. Plants of the amphibious 

 subzone. 



The amphibious vegetation is fitted both to an aquatic and 

 a terrestrial life. This class is therefore intermediate between 

 the true aquatics and all other hydrophytic plants. The close 

 relationship to the former is shown by the development of true 

 floating leaves, as in Alisma, or by heterophylly as in Cicuta, 



