VEGETATION OF THE STRAND. S9 l 



ground for 30^ or more, the trailing wild bean (Strophostyles 

 helvola). These trailing morning-glories occur on the beach in 

 tropical regions of South America. For protection against 

 strong insolation (excessive heat) the blade of the leaves is brought 

 into a vertical position. The trailing wild bean, on the other 

 hand, extends as far north as Quebec and west to South Dakota, 

 Nebraska, and Texas in sandy soil. The sea-rocket and salt- 

 wort also extend northward along the middle beach of the Atlan- 

 tic coast of the United States. 



1083. The upper beach is the third parallel zone and corre- 

 sponds probably to the back strand. This is beyond the reach 

 of the waves, and perhaps in some cases represents a former por- 

 tion of the beach acted upon by the surf, though it is principally 

 developed by the activity of the wind. The limits of all of these 

 zones are constantly changing, due to the action of the waves on 

 the water side and of the wind on the landward side. Chemical 

 and physical conditions of the soil are also different, there is a 

 higher percentage of humus, shows darker color, evaporation 

 and radiation are less, the soil water is nearer the surface, and 

 the soil retains more heat. This encourages different biological 

 conditions which are shown in the different character of the 

 plants. Though sand is still the principal constituent of the soil, 

 the characteristic plants are those which have more of a thermo- 

 phytic or nitrophytic character. The conditions of plant life are 

 less severe than on the middle beach. Still the vegetation is 

 similar to that of sand formations, and the nitrogenous material 

 is not so abundant as to encourage true humus plants like Coral- 

 lorhiza or Pyrola. The vegetation of the upper beach may also 

 be divided into several formation groups according to certain 

 soil characters, and these again are still further subdivided 

 according to areas which encourage certain dominant types of 

 plants, whether herbaceous, shrubby, or arboreal, and these fur- 

 ther into the individual formations. According to Cowles the 

 most characteristic species of the upper beach on Lake Michi- 

 gan are: ist. Grasses and other herbs. Artemisia caudata and 

 A. canadensis, while these are often associated with Cnicus 



