PLANT SUCCESSION 



217 



duplicated regardless of the region. It is, therefore, possible to pre- 

 sent a general description of such successions, which can be ap- 

 plied almost anywhere and which will illustrate what we have just 

 discussed. 



FIG. 105. Hydrarch succession illustrated by swamp vegetation. The zone 

 of cattails occupies the partially flooded, muddy margins. When soil builds 

 up or drainage improves, bog shrubs (buttonbush, alder, willow) appear as 

 in the middle background. On wet, but drained soil a swamp forest of mixed 

 hardwoods develops as in background.-Ptoo by H. L. Blomquist. 



Hydrarch succession progresses in response to better moisture 

 conditions in combination with improved aeration. Initiated in a 

 lake, pond, or stream margin where water movement is not too 

 great, the pioneer vascular plants are submerged aquatics with 

 thin, dissected, or linear leaves. Their depth of growth is limited, 



