HYDROPHYTE ASSOCIATIONS. 



185 



killed, but that aeration of the root system occurs through 

 the knees remains to be proved. 



Another habit of swamp plants is called turf -building, 

 which means that new individuals arise from older ones, 

 and so a dense mat of roots and rootstocks is formed. Very 

 prominent among these turf-building swamp plants are 



Fig. 162. Two leaves of a water buttercup, showing the difference in the forms of 

 submerged and aerial leaves on the same plant, the former being much more 

 finelv divided. — After Stkasbubgee. 



the sedges. Some of the prominent swamp societies may be 

 enumerated as follows : 



137. Reed swamps. — The reed-swamp plants are tall wand- 

 like forms, which grow in rather deep, still water (see Fig. 

 163). Prominent as types are the cat-tail flag, bulrushes, 

 and reed grasses. Such an assemblage of forms usually 

 characterizes the shallow margins of small lakes and ponds. 

 In such places the different plants are apt to be arranged 

 according to depth, the bulrushes standing in the deepest 

 water, and behind them the reed grasses, and then the 



