209 



entirely adjusted to the increase of water level following the 

 establishment of the Chicago Drainage Canal. 



The Platanus occidentalis association is represented fairly well 

 in the bog area by a number of seedlings in the thickets and along 

 the little creeks, and by a few young trees between the Cephalan- 

 thus and the Ulmus-Acer association. Platanus persists quite 

 readily as a relic after the Ulmus-Acer association obtains domi- 

 nance. Both the Platanus and the Ulmus-Acer associations 

 occupy the drier portions of the bog area and there they readily 

 obtain dominance over the thickets. 



The Ulmus-Acer association is represented by several of the 

 species of trees which characterize it. The proncipal ones 

 involved are Acer saccharinum, with many, well-developed, 

 medium-sized trees, 2-3 dm. in diameter, furnishing an abun- 

 dance of seedlings; Ulmus americana; Ulmus racemosa, with a few 

 small trees, i. 0-1.5 dm. in diameter and several young trees; 

 Fraxinus nigra, with a few fair-sized trees and several small ones; 

 Fraxinus americana, with several fair-sized and many small trees; 

 Betula nigra, with a few medium-sized and several small trees; 

 Quercus platanoides , with a few small trees; Tilia americana, with 

 a few small trees and one large one; and Platanus occidentalis, 

 with a few large relics. Of these the Acer, Betula, Ulmus and 

 Tilia incline towards the higher and consequently drier ground, 

 often forming oases in the bog. In such places the shade is very 

 dense and the undergrowth is entirely absent. There is usually 

 considerable undergrowth elsewhere, although but little of it is: 

 characteristic of the Ulmus-Acer association. This undergrowth 

 is a curious melange of several species from different associations 

 and formations. In point of numbers the thicket elements are 

 probably best represented with numerous plants of Cornus 

 amomum, Rosa Carolina, Cephalanthus occidentalis, Salix discolor 

 and Salix longifolia. Several young trees are present, notably 

 Juglans nigra, Gleditsia triacanthos, Celtis occidentalis, Diospyros. 

 virginiana and Betula nigra, all of which are characteristic trees in 

 the mesophytic forests of central Illinois. The herbaceous flora 

 includes such a typically northern bog plant as Spathyema foetida. 

 mixed in with typical swamp plants, as Asclepias incarnata, Spar- 



