59 RELATION TO ENVIRONMENT. 



conditions are severe because evaporation of water and radia- 

 tion of heat are rapid. The vegetation is, therefore, sparse, 

 scattered, and xerophytic. Characteristic sand plants along 

 Lake of the Woods, like certain grasses (Agropyron tenerum, 

 Elymus canadensis) and other sand herbs (Artemisia caudata 

 and canadensis), are among the first to gain a foothold, and 

 species in these genera are found in similar strand formations 

 on the Great Lakes and on the Atlantic and Gulf coasts. These 

 assist in the formation of humus and in holding it, and soon 

 other plants appear. The formations vary greatly according 

 to variations of soil and exposure. Thus one with considerable 

 wind exposure has as a dominant plant, a low shrub with small 

 leaves, Prunus pumila, the sand-cherry (Prunus formation), with 

 subordinate species of a similar habit, as well as sand grasses 

 and herbs. Another formation with less wind exposure and a 

 less per cent of humus has a taller shrub with broader leaves as 

 the dominant plant and is a Cornus formation. In still another 

 where inundation is frequent and the plants would be more 

 liable to be torn out, certain dwarf willows which are low, small- 

 leaved and deep-rooted are dominant, and this is the Salix for- 

 mation, and so on. While all these formations .are adapted to 

 severe conditions, each one presents peculiar types of austere 

 conditions and is populated with dominant plants which pre- 

 sent special adaptations to specific conditions. 



On the east shore of Lake Michigan the dominant plant is a 

 crucifer, American sea-rocket (Cakile edentula), while two others 

 are common, bug-seed (Corispermum hyssopifolium) and sea- 

 side spurge (Euphorbia polygonif olia) . The Cakile and Euphor- 

 bia are characteristic also of the beach on the Atlantic coast, 

 while on the Gulf coast or island shores of Mississippi and Louisi- 

 ana low succulent plants with radiate habit of growth occur. 

 There is here a seaward zone of sea-blite (Dondia linearis), sea- 

 rocket (Cakile fusiformis), saltwort (Salsola kali), sea-purslane 

 (Sesuvium maritimum), large burr-grass (Cenchrus macrocepha- 

 lus); and a landward zone of the same with tropical morning- 

 glories (Ipomoea pes-caprae and I. acetosaefolia) trailing on the 



