592 RELATION TO ENVIRONMENT. 



pitcheri, Lathyrus maritimus, which is also a marine beach 

 plant, Euphorbia polygonifolia, (Enothera biennis, and Agro- 

 pyron dasystachyum. 2d. Shrubs. Of the shrubs the common 

 ones are the sand-cherry and a species of willow which have been 

 noted on the mid-strand of Lake of the Woods. 3d. Trees. Of 

 trees the cottonwood occurs, the Populus monolifera southward 

 and Populus balsamifera northward. 



1084. Wide distribution of beach plants. The full list of 

 even some of the most common species characteristic of the 

 different strand zones cannot be given here, but it is interesting 

 to note that while some beach plants have a very wide distribu- 

 tion, occurring throughout the length of the Atlantic coast and 

 on the shores of the Great Lakes, others have a less extensive dis- 

 tribution, so that in the tropical and subtropical regions, for 

 example, areas occupied by certain plants in the boreal and 

 austral regions are occupied by entirely different plants. Thus, 

 in the tropics, Spinifex squarrosus and Ipomcea pescaprae 

 occur on the upper beach, the latter extending as far north as 

 the beach of the island flora of Mississippi and Louisiana. 



III. Vegetation of the Dunes. 



1085. How dunes are formed. Dunes are mounds or hills 

 of sand, lying on the 'landward side of the beach. They are 

 formed by the sand which is driven from the upper beach and 

 caught in the stools or tufts of plants. They begin by the col- 

 lection of small piles of sand about tufts of certain grasses; for 

 example, marram-grass, or sea-matweed (Ammophila arenaria), 

 the northern wheat-grass (Agropyron dasystachyum), long- 

 leaved reed-grass (Calamovilfa brevipilis), and species of Andro- 

 pogon, Elymus, etc. As the dunes grow in size other plants 

 appear, finally an open formation of shrubs and trees is de- 

 veloped, and the dunes sometimes become of considerable size. 



1086. Kinds of dunes.* Dunes are described as stationary 



* Cowles has made an interesting study of the dunes on the eastern 

 shore of Lake Michigan (Ecological Relations of the Vegetation of the 

 Sand Dunes of Lake Michigan, Bot. Gaz., 27, 1899). 



