165] FLORA OF COLUMBIA AND VICINITY 23 



Cardamine Pennsylvanica Conobea multifida 



Potentilla paradoxa Ilysanthes gratioloides 



(Missouri river) Gerardia Besseyana 



Centunculus minimus Valerianella radiata 



(rare) Eclipta alba 



Gratiola Virginiana 



3. Marginales littorales. Plants of the littoral sub-zone. 

 The littoral vegetation proper is but scantily developed in 

 the region. There are, however, two kinds of soil along streams, 

 less often about ponds, which support a sparse littoral xerophy- 

 tic growth, but consisting in the main of dwarfed and depauper- 

 ate limose species. 



a. Littorales xanthioides. The first kind of soil con- 

 sists of the shingle and pebbles of old creekbeds, or of 

 creeks dry the greater part of the year. Most of this soil 

 is absolutely barren of vegetation. A few plants, mainly 

 species of Polygonum, Bidens, Heleocharis, and Xanthium, 

 come up among the stones. 



b. Littorales moUuginoides. The other soil consists of 

 the shore-sands. These occur at the head of the Hinkson 

 reservoir, and in certain flats of Grindstone creek and other 

 streams. Here there is a thin covering of small creeping 

 plants, which may be called the carpet-weed association. 

 Mollugo verticillata. Euphorbia mactilata and E. nutans are 

 the chief species. 



c. Littorales salicoides. The thicker sandbars are cov- 

 ered with willows, mainly SalLv longifolia and S. cordata 

 which assume a peculiar recumbent form. 



d. Littorales equisetoides. The only other well-marked 

 plant-group is that of the littoral horsetail association, 

 consisting of Equisetum arvense and E. hiemale, which 

 often prick up through the sand in almost pure communi- 

 ties. 



