105 



— e. g., the species above marked with an asterisk — find a home 

 in similar situations along the Great Lakes and westward. 



There is still another group of plants, which are neither halo- 

 phytic nor littoral but which, while frequent or common in the 

 vicinity of the coast, rarely extend inland more than a few miles. 

 Prominent among these are: 



Woodwardia areolata Opuntia vulgaris 



Lycopodium inundatum Bigelovii Hottonia inflata 

 Lilium superhum Rhexia virginica 



Iris prismatica Gerardia purpurea 



Habenaria ciliaris Eupatorium hyssopifolium 



Quercus stellata Artemisia caudata 



The cause of the coastward affinity in such plants is not wholly 

 clear, but it should be pointed out that practically without 

 exception the species concerned attain their optimum develop- 

 ment farther south, while a large percentage of them are restricted 

 to the Atlantic coastal plain, the Mississippi basin, and the region 

 around the Great Lakes. Hardly less remarkable from a dis- 

 tributional standpoint is a second group of plants, also mainly 

 southern in range, which are largely confined to the coast and to 

 the valleys of the larger rivers. Representative of this class are: 



Panicum virgatum Solidago odora 



Dioscoraea villosa Aster novi-belgii 



Cassia Chamaecrista Helianthus giganteus 



Onosmodium virginianum Bidens laevis 



Sicyos angulatus Bidens discoidea 



It seems probable that the limited distribution of these last two 

 groups may in some way be associated with atmospheric condi- 

 tions. There is an additional element in the river flora, however, 

 whose presence is unquestionably due to edaphic rather than 

 climatic factors. As illustrations, the cottonwood (Populus 

 deltoides) and the silver maple (Acer saccharinum) may be 

 selected. Although scattered locally throughout the state, it is 

 only on the flood plains of the larger rivers that these two trees 

 find conditions congenial to their optimum development. Here, 



