218 THE STUDY OF PLANT COMMUNITIES • Chapter IX 



FlG. 106 (A). Xerarch succession as illustrated by vegetational develop- 

 ment on granitic rock in the Piedmont of the southeastern states. Early stage 

 (upper) of mat formation initiated by the pioneer moss (Grimmia laevigata) 

 upon which a lichen (Cladonia leporina) is well established. As mat thickens 

 (lower), herbs come in, with eventual Andropogon spp. dominance. 1 



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on the one hand, by light penetration of the water and, on the 

 other, by a zone of floating-leaved species. These latter (water 

 lilies, etc.) exclude submerged species by shading but cannot move 

 into the zone of submerged forms until the bottom is built up or 

 the water level falls. In still shallower water, emergent species pre- 

 dominate. These have their roots and rhizomes in the mud and 

 extend upward into the air (rushes, reeds, cattails, sedges). The 



