159] FLORA OF COLUMBIA AND VICINITY 17 



fluvial; D. The palustrous. These zones tend to blend into one 

 another, but they stand for natural plant groupings. 



A. Aquatiles. Plants of the aquatic zone. 

 Aquatic plants inhabit ponds, lakes, and flowing waters, but 

 inasmuch as the lakes, being artificial, have produced no lacus- 

 trine aquatic types, and as the streams have no aquatic higher 

 plants, this portion of the flora is restricted to plants dwelling in 

 stagnant waters. They belong to three chief types according as 

 the plants swim freely on the surface, or being fixed by roots 

 float upon the water, or are wholly submerged except at 

 anthesis. 



1. Aquatiles natantes. There is but one association 

 of free swimming plants, Natantes lemnoides, the duckweed 

 association of minute thalloid plants, either with or without root- 

 lets, and seldom flowering and fruiting in this latitude. They 

 cover thickly the surfaces of all stagnant ponds and pools, and 

 the backwaters of streams. There are but four species: 



Spirodela polyrrhiza L. minor 



Lemna Valdiviana Wolffia papulifera 



2. Aquatiles fluitantes. There are two well marked 

 associations of fixed floating aquatics : 



a. Fluitantes nymphaeoides, the water-lily associa- 

 tion. This attains its best development in lakes, the large float- 

 ing leaves being admirably adapted to withstand the shock of 

 waves. The water-lily association occurs very rarely. Nym- 

 phaea odorata grew till a few years ago in a pond on the Uni- 

 versity Farm. Niiphar advena was listed from Boone county by 

 Tracy in 1885. Nelumbo lutea occurs in a pond near Roche- 

 port. This plant, interesting because of its intricate root-system 

 and its putting forth both erect and floating leaves, is in some 

 respects intermediate between aquatic and amphibious plants. 



2 



