Wheeler :' FLORA OF SOUTHEASTERN MINNESOTA. 355 



For consideration, the water plants of the river valley may be 

 classified into four main groups : plankton, attached submerged 

 aquatic, attached aquatic plants with natant leaves, and adaptive 

 shore plants. 



PLANKTON. 



The plants forming this group are those which are not at- 

 tached to any soil substratum, and so are rarely found in any of 

 the swift-flowing currents, but rather on the surface of protected 

 lakes and ponds and near the high banks of sloughs, where 

 they are protected from rapid currents of wind and water. The 

 plants comprising this group are : 



Azolla carolintana, Sptrodcla -polyrhiza, 



Ceratophyllum demersum, Utricularia vulgaris. 



Lemna minor , 



With them are often found plants of Sagittaria and Pota- 

 mogeton which have been dislocated from their original position 

 on the soil. They seem to grow nearly as well and bloom 

 nearly as profusely as when attached. In this condition they 

 form part of the plankton, but as they are originally attached 

 and ordinarily remain so, I have not included them in the list 

 of plankton types. 



One of the most beautiful and interesting plants of this group 

 is the small heterosporous fern, Azolla caroliniana. In the 

 early part of the summer it is green or but slightly red in color 

 and only scattered plants or very small patches can be found. 

 In the later summer and autumn it covers large areas of water 

 with a deep red pure growth or mixed with the duckweeds. In 

 restricted areas it often grows so rapidly late in the summer that 

 it is pushed up from the surface of the water and forms ridges 

 and bunches above the water-level. 



ATTACHED SUBMERGED AQUATIC PLANTS. 



The floor of some of the very shallow ponds and sloughs is 

 covered with a growth of bassweeds and pondweeds that are en- 

 tirely immersed. This group contains but few species of the 

 higher plants. The species collected are : 



Naias flex His ) Potamogeton pusillus, 



Naias g-uadalupensis, Potamogeton zosteraefolius. 



