484 RELATION TO ENVIRONMENT. 



water even where rainfall is abundant. The same may be said 

 of the sand dunes farther back from the shore. The plants 

 of these areas are then usually xerophytes. Some of the plants 

 accustomed to growing in such localities are American sea-rocket, 

 seaside spurge, bugseed, sea-blite, sea-purslane, the sand- 

 cherry, dwarf willow, marram-grass, certain species of beard- 

 grass, etc. 3d. Rocky shores or areas. Here lichens and mosses 

 first grow, later to be followed by herbs, grasses, shrubs, and 

 trees, as decayed plant remains accumulate in the rock crevices. 

 4th. Shores of ponds, or swamp moors. Here the vegetation 

 often takes on a zonal arrangement if the ground gradually 

 slopes to the shore and out into the pond. In Fig. 493 is shown 



Fig. 493. 



Macrophytes in the upper zone of the photic region. Ascophyllum and Fucus 

 t low tide, Hunter's Island, New York City. (Photograph by M. A. Howe.) 



zonal distribution of plants. The different kinds of plants are 

 drawn into these zones by the varying amount of ground water 

 in the soil, or the varying depth of the water on the margin of 

 the pond as one proceeds from the land towards the deeper 

 water. On the border lines or tension lines between the different 

 zones the plants are struggling to occupy here ground which is 

 suitable for each adjacent individual formation. Other edaphic 

 societies are those of marl ponds, alkaline areas, oases in deserts, 



