1901 | PHYSIOGRAPHIC ECOLOGY OF CHICAGO 97 
as the common buttercup (Ranunculus septentrionalis\, Plantago 
cordata, various mosses, etc. Together with these forms algae of 
short vegetative period may be found in the wet seasons. When 
the ravine at last is sufficiently developed to have a permanent 
stream, a definite hydrophytic flora appears, consisting largely 
of algae (¢. g., Batrachospermum), aquatic mosses, and seed 


1G. 9.—Young island in the Illinois river at Starved reek (close view of island 
in foreground of fg. 8), seen from above, and showing the destructive action of tke 
iver, 
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 sub- 
stratum, as well as to the direct destructive action of the currents 
/ig. 7 shows one of these young streams, whose flora is sparse 
