273 



ing the season. In general this is liable to kill prairie plants within 

 the summer, but, in a few cases, Panicum capillar e, Acnida tubercu^ 

 lata subniida, TrifoHuni rcpcns, and Salix longifolia will keep pace 

 with the incoming sand. 



Since these species which constitute the derived element of the 

 association can under no circumstances commence to grow on the 

 middle beach, and since their presence there is to be accounted for 

 solely by physical displacement of the soil upon which they were 

 growing and has absolutely no successional value, one can not say 

 that they are a real part of the association. 



The Middle-Beach Pool Associations 



In describing the middle beach (see under Development, page 270) 

 it was mentioned that in the southern part of the area its slope from 

 the lower beach was downward toward the lake-level. Just a little 

 north of the docks at Waukegan, the beach has reached the level of 

 sand which is permanently moist clear to the surface. Standing wa- 

 ter is not usually present throughout the season, and so the beach 

 pool is not permanent. This is the situation to which three groups of 

 plants give such a definite floristic character that they must be termed 

 associations, small in area and isolated though they are. In genetic 

 order these are the Juncus alpinus hisignis, the Triglochin palustris, 

 and the Carex oederi pimiila- Cyperus rivularis associations. These 

 groups of plants are not of frequent occurrence in this region. Al- 

 though they usually occur isolated from one another, they show suf- 

 ficient successional relationships to indicate that they are three asso- 

 ciations, rather than consocies of one association as Jennings (1909: 

 352) treated them. 



THE JUNCUS ALPINUS INSIGNIS ASSOCIATION 



The lowest of these associations, the Juncus alpinus insignis, has 

 been found in typical form only in exceptionally dry years, such as 

 1908 and 1910, when it occupied the dried-up bottoms of beach 

 pools. This Juncus grows in small tufts, thoroughly dominating the 

 association. With it are seldom any secondary species, and when 

 they do occur they are of very minor importance. 



LIST OF THE SPECIES OE THE JUNCUS ALPINUS INSIGNIS ASSOCIATION 



Dominant Species Secondary Species 



Juncus alpinus insignis Bidens vulgata 



