322 



THE LEMNA-RICCIA ASSOCIATION 



An alternating association with the one just described is the 

 Lemna-Riccia association. It shows a tendency to inhabit the 

 longer, narrower pools, where there is less chance of the wind dis- 

 turbing the water. The plants differ from those of the Ranunculus 

 association in that they are free-floating. They mass together, how- 

 ever, in great mats which cover the surface of the water with vege- 

 tation. Lcnina seems to prefer the more open water, while Riccia 

 shows a tendency to remain nearer the border association of Nyin- 

 phaea or Typha. The Leiiina and the Riccia are, however, so inter- 

 mingled with one another that they have essentially the same ecologi- 

 cal conditions to meet, and so are parts of the same association. 

 This association can only exist as such in quiet water, for in streams 

 the plants are washed away. On this account it is more conspicuous 

 in the small pools, although careful search usually revealed its plants, 

 especially the Lemna, among the grasses or sedges that form the 

 bordering amphibious vegetation of the rivers. Numerous small 

 animal forms are associated with these plants, but no other species 

 of plants have been observed wnth it in this region. 



THE MENYANTHES-SAGITTARIA ASSOCIATION 



In fairly wide and shallow (2-4 dm.) sloughs the Casfalia- 

 Nyiiiphaea association occupies the central part, where there is a lit- 

 tle running water, especially during the spring floods. Bordering it 

 on either side is the expanse of the Menyanthes-Sagittaria associa- 

 tion, which reaches to the sedges. As it occurs in a few of the 

 situations it is a typical bog, like those so much more common 

 farther north. The bottom is very level and somewhat peaty. The 

 plants of this association have their root systems entirely submerged, 

 while the leaves and the flowers are usually above the surface of the 

 water. The vegetation is very dense, as shown in the center of Fig- 

 ure 2, Plate LIII. 



Arrowleaf (Sagittaria latifolia) is always one of the dominant 

 species in the bogs that- occur in this region. It occurs along streams 

 of running water as well, and associated with it are many of the 

 same secondary species that accompany it in the typical bog situation. 

 This association is boreal in distribution. Here, near its southern 

 limit, as shown in Transeau's map of the distribution of bog plants 

 (1903:406), it is not typically developed. The species that is most 

 abundant in this association in this region, Sagittaria latifolia, is not 

 listed by Transeau as a bog plant because it is not characteristically 



