32 



THE PLANT SOCIETIES OF 



nant of one of these oxbows; on the farther side are old and dying 

 willows, trees that look back to the well-drained river margin. On 

 either side of the pond are seen clumps of the button bush {Cepha- 

 latithus occidentalis), one of the most characteristic plants of undrained 



swamps. Thus the willows are antecedent and the button bush subse- 



Fig. 17. — A dead oxbow lake in the flood plain of Thorn creek. A willow still remains at 

 the right, while the shrubs (Cephalanthus) have closed in upon the lake. 



quent to the formation of the cut-off. Fig. ly shows a portion of the 

 same, in which the willows, and even the pond itself, have gone, and 

 onl}' the marginal button bush is left, though in this case the margin 

 occupies the center of the original pond. Near Starved Rock an 

 extinct oxbow lake on the flood plain of the Illinois river contains an 

 extensive patch of Sphagnum and Osmunda, among the most character- 

 istic plants of undrained swamps. There are many undrained swamps, 

 some with tamaracks, in the Calumet valley. The future of these 

 swamps is like that of other swamps, and will be described in the next 

 section. Fig. i8 shows a morainic island in the Thorn creek flood 

 plain; the stream has meandered, but has thus far left this detached 

 fragment of the morainic mainland with a large part of its original 

 flora. 



