24 THE PLANT SOCIETJES, OF 



and more destructive than constructive at that point, there are to be 

 seen small flood plain areas with their typical tree inhabitants. 



There is no place where flood plaip development can be better 

 studied than on growing islands in relatively rapid and yet essentially 

 depositing streams, such as the Illinois river at Starved Rock. Fig. 8 



Fig. 9. — Young island ^.n the Illinois river at Starved Rock (close view of island in foreground 

 oifig. S), seen from above, and showing the destructive action of the river. 



gives a general view of the Illinois islands and flood plain. In Jigs, 

 g and lO the lower island (foreground of fig. 8) is seen close at hand. 

 Any obstacle, such as a partially submerged tree trunk, serves to check 

 the river current and cause a deposition of sand or silt, and before 

 long a sand bar originates. As in the case of a sand dune, the bar 

 itself becomes an obstacle to the currents, and hence continually grows 

 larger. 



The first vegetation, as on the lake beach, consists largely of annuals, 

 especially the giant ragweed {Ambrosia trifida); rushes and sedges, 

 some annual and some not, are also present, but are less conspicuous. 

 The perennials that manage to survive one season are largely washed 



