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E. LUCY BRAUN 



nature of the matrix, facilitate the entrance of rain water. The 

 fine-grained parts of the masses serve as reservoirs to retain and 

 gradually feed out the water after the surface portions become 

 dry. Part of the rock, too, is buried deep in the soil of the hill- 

 side where there is always a constant supply of moisture. 



The amount of water available to the vegetation of these 

 rocks depends directly on the steepness of slope of the surface 



Fig. 1. Conglomerate rock outcrop on hillside; patches of moss and Sedum 

 are prominent, with shrubs on the ledges. 



and on the texture of the material, and indirectly on the direc- 

 tion of slope. 



The influence of steepness of slope is seen in the distribution 

 of plant communities (fig. 4). We find the most xerophytic 

 communities on the vertical faces and edges of shelves, the 

 xero-mesophytic communities on gently sloping and irregular 

 parts of the rock, and the most mesophytic communities in deep 

 and sheltered angles where both surface water and water drain- 



