2 2 THE PLANT SOCIETIES OF 



in figs. 2 and j" it will be seen that most of the trees stand approxi- 

 mately vertical. But the activity of the erosive forces, slow as it may 

 be, is nevertheless revealed by occasional leaning, or even falling, trees. 

 From the above it is easy to understand that cycles of vegetation often 

 pass much more rapidly than cycles of erosion, but never more slowly. 

 During one erosion cycle the mesophytic forest develops at least twice 

 — once on the ravine slopes, and then finally on the gentler slopes that 

 betoken approach toward base level. 



C. The flood plain. — We may now follow the successive stages in 

 the development of the flood plain vegetation. While the ravine is 

 still young, as '\Tifig. 2, there is no permanent stream, but merely tor- 

 rents, which remain but a short time. As the ravine deepens. Widens, 

 and lengthens, thus approaching the underground water level and 

 increasing the drainage area, the water remains for a longer and longer 

 time after each rainfall. As the ravine conditions thus become m9re 

 and more hydrophytic, the original flora, perhaps of shade mesophytes 

 (as Impatiens), becomes replaced by amphibious shade plants, such as 

 the common buttercup (Ranunculus septentrionalis), Plantago cordata, 

 various mosses, etc. Together with these forms algae of short vegeta- 

 tive period may be found in the wet seasons. When the ravine at last 

 is sufficiently developed to have a permanent stream, a definite hydro- 

 phytic flora appears, consisting largely of algae (<?. g., Batrachos- 

 permum), aquatic mosses, and seed plants with finely dissected leaves 

 and strong holdfast roots (such as Myriophyllum), though these latter 

 plants are more characteristic of ponds. In the early phases of a 

 stream the currents are rapid and the vegetation (apart from lower 

 forms) is sparse, by reason of the difficulty which plants have in secur- 

 ing and retaining a foothold on the stream bed. This difficulty is due 

 to the rapid erosion and consequent instability of the substratum, as 

 well as to the direct destructive action of the currents. Fig. y shows 

 ohe of these young streams whose flora is sparse. 



Springs and spring brooks may be classed with ravine streams, but 

 differ from them in the relative absence of erosion phenomena. This 

 type of stream is uncommon in the Chicago area, though there are a 

 few spring brooks near Chesterton. The water supply is much more 

 constant than in ravine streams, and the shade of the ravines is often 

 lacking. Besides the aquatics, there may be mentioned a characteristic 

 brookside flora, including such plants as Symplocarpus fxtidus (Skunk 

 cabbage), Asclepias incarnata (Swarap milkweed), Chelone glabra. Poly- 



