26 



THE PLANT HOCIBTIBS OF 



The outermost fringe of land at ordinary low water is often almost as 

 barren of vegetation as are the islands, but the soil is fine, and hence 

 makes a mud flat instead of a sand bar. Immediately after the spring 

 freshets have gone, an alga vegetation is frequently found on these 

 flats, consisting especially of Botrydium and Vaucheria. Later in the 



Fig. 11. — Flood plain of the Des Plaines river at Glendon Park, showing encroachment on 

 the river. Willows in the foreground, cottonwoods farther back. 



season, annuals, or even scattered perennials, may occur here, though 

 the winter and spring floods uproot or bury most of this vegetation. 

 The Ambrosia and willow vegetation soon appears, as described above. 

 The river maple (Acer dasycarpun^ usually appears with or soon after 

 the willows. After the willows the cottonwood [Populus monilifera) 

 and the ash {JFraxinus Americana) soon come in. Fig. ii shows an 

 advancing flood plain of this type; willows are seen on the margin 

 and cottonwoods farther baclc. 



Gradually the growing flood plain becomes dry enough to permit 

 the germination and development of a true mesophytic flora. The 

 trees nained above, especially the willows, are largely replaced by others 

 that seem better adapted to the changed conditions; among these are 

 the elms {Ulmus Americana and U. fulva), the basswood (Tilia Amer- 



