100 BOTANICAL GAZETTE [ FEBRUARY 
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 

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Fic. 12.—Mesophytic flood plain forest in the bottoms of the Desplaines river at 
Riverside. Elms and basswoods. Rich herbaceous vegetation, consisting largely of 
Phi 
season are largely washed away in the winter and spring, so that 
in reality the vegetation is almost exclusively annual. The first 
woody plants to get a more or less permanent foothold here are 
willows (Salix nigra and S. longifolia). 
While islands of the above type gain more soil than they 
lose, a comparison of figs. 9 and zo shows that the river erodes 
above and deposits below. As a consequence these islands 
migrate down the river, as well as grow in area year by year. 

