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 caroliniana, Spirodela polyrhiza, 
Ceratophyllum demersun, Utricularia vulgaris. 
Lemna minor, 
With them are often found plants of Sagzttarza 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, Azo//a 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 flexilts, Potamogeton pusillus, 
Naias guadalupensis, Potamogeton zosteraefolius. 
