Igor | PHYSIOGRAPHIC ECOLOGY OF CHICAGO 103 
Gradually the growing flood plain becomes dry enough to 
permit the germination and development of a true mesophytic 
flora. The trees named 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 (Zilia Americana), the walnut and 


Fic. 15.—Terrace in the flood plain of the Desplaines river at Glendon park 
showing how a mesophytic flood plain may become xerophytic. The opposite bank 
shows deposition and flood plain enlargement (fg. 7/). 
butternut (Juglans nigra and J. cinerea), the pig-nut (Carya por- 
cima). In this rich flood plain forest there are many lianas 
climbing over the trees, e. g., greenbrier (Slax hispida), grape 
(Vitis spp.) , Virginia creeper (Ampelopsis quinguefolia),and poison 
ivy (Rhus T. oxicodendron). 
The undergrowth in these river woods is very dense and 
luxuriant, the alluvial character of the soil making it very fertile. 
Among the shrubs are the thorns (various species of Crataegus), 
the gooseberry (Ribes Cynosbati), and many others. ate Bee: 
baceous vegetation is dominantly vernal, the shade being too 
dense for a typical estival flora. Prominent among the spring 
