327 



The following floating plants are frequently present : 

 Riccia fiiiitans Leiiina minor 



THE SCIRPUS VALIDUS ASSOCIATION 



In still shallower water than the preceding association occurs the 

 Scirpiis validus association. It is characterized by the bulrush, Scir- 

 pus validus, and the very closely related species, S. hcterochaetus and 

 S. occidentalis. The former species grows in water which varies in 

 depth from one to ten decimeters. In this area, and in general in the 

 lake region in northeastern Illinois and southeastern Wisconsin, the 

 Scirpns validus association grows in deeper water only when there 

 are no other associations of emersed plants between it and the open 

 water. This association is one of the commonest aquatic pioneers, 

 and will grow either in still water or in a moderate current. Al- 

 though this association agrees with the Phraginites-Typha association 

 and the Scirpns anicricanus association in having the roots in satur- 

 ated soil and the tops of the plants in the air, they can hardly be 

 grouped into a single association, as Jennings (1909:354) has 

 pointed out, because of the definite arrangement they always exhibit 

 w4th respect to one another. This differentiation is most evident in 

 the relations of the plants to the varying depths of water. The Phrag- 

 mites-Typha association grows in deeper water than the Scirpus vali- 

 dus, while Scirpus americanus grows in shallower water and will 

 persist out of the water as a relic. Where Typha or PJiraginites have 

 been found surrounded by Scirpns validiis, or vice versa, investiga- 

 tion has always shown a dift'erence in level. Scirpns validus grows 

 in soil which contains rather more humus than that in which Typha 

 grows. Both associations have few secondary species, which for the 

 most part are unimportant. Taken as a whole, the secondary species 

 constitute less than 3 per cent, of the association, the remaining 97 

 per cent, being the dominant species. Neither Phragmites nor Typha 

 have more than a very slight ability to persist among Scirpns validus 

 as relics, but the Scirpus itself is, to a limited extent, capable of being 

 an invader in the Phragmites-Typha, and to a greater extent pos- 

 sesses the power of growing as a relic in the Scirpus americanus as- 

 sociation. The color tone of the Scripus validus during the grow- 

 ing season is dark green, which very decidedly separates it from the 

 light green of the Scirpns americanus. The two latter associations 

 are shown in Figure i, Plate LIV, where Scirpns americanus occupies 

 the left half and the lower part of the right half of the figure, while 

 Scirpus validus, appearing darker in color, is in the upper part of the 

 risfht half of the fig-ure. 



