3° 



THE PLANT SOCIETIES OF 



(Blue grass) and Agrostis alba vulgaris (Red top), there are often other 

 plants in abundance, e. g., Thalictrum purpurascens (Meadow rue), Fra- 

 garia Virginia?ia (Strawberry), and Anemone Pennsylvanica. The eco- 

 logical meaning of the meadow is not clear. Probably mowing or 



grazing is responsible for the failure of a mesophytic forest to develop. 



Fig. 15.— Terrace in the flood plain of tlie Des Plaines river at Glendon Parle, showing how 

 a mesophytic flood plain may become xerophytic. The opposite bank shows deposition and 

 flood plain enlargement {fi^, ii). 



Extensive thorn (Crataegus) thickets sometimes occur in these meadows, 

 and probably betoken the beginning of a mesophytic forest. E.\ten- 

 sive and apparently natural meadows are found in the Calumet valley. 

 As we have seen, the climax type of vegetation on the flood plain 

 is the mesophytic forest; but here, as well as on the river bluffs, the 

 climax may be but temporary. Retrogression is almost sure to come 

 in connection with terrace formation. While it is true that deposition 

 is the main feature of flood plains, it is also true that erosion has not 

 ceased; the downward cutting of the river once more causes vertical 

 banks, though this time in its own flood plain. This action is seen in 

 fig. 14, which shows the beginning of the new erosive phase, and its 

 indication in the falling elm. There has doubtless been lateral erosion 

 here also, since elms are not usually marginal trees. Mg. /j shows 



