278 MINNESOTA BOTANICAL STUDIES 



over the greater portion of all of the lake centers of the region 

 that such factors as relative depth of water, alkalinity, light, and 

 clearness of the water, influence to some degree the distribution of 

 the various species in the different lakes. The position of the aqua- 

 tics within any given lake appears to be determined largely by chance 

 dissemination. ^Movements of the water under the force of the 

 wind may be the distributive factor in some cases, especially in the 

 larger lakes that are more exposed to wind action than the slender 

 finger lakes that lie between sheltering ranges of hills. Seldom 

 does one see a species with floating leaves, such as Potamogeton 

 natans, in the larger (though also shallow) lakes where wave action 

 is most pronounced. The effect of light, though probably of 

 primary importance in the vertical distribution of aquatic plants, 

 is not clearly evident here. Alkalinity and turbidity, in extreme 

 cases, are phases of the lake biology that are most pronounced. 

 Until field methods for the determination of light values in water 

 are far more perfect than at present, the relation of turbidity and of 

 pigmented waters to the distribution of hydrophilous life must 

 remain, as largely at present, great unknowns. 



In many of the less alkaline or turbid lakes of the sandhills the 

 above species often reach such density of growth in the form of 

 extensive submerged beds that boating becomes difficult or impos- 

 sible during late summer after the various species of the association 

 have passed their maximum vegetative period. As one looks down 

 into the clear water of the richer lakes the appearance of the vege- 

 tation is that of a tangled aquatic garden of low stature in which 

 Myriophyllum spicatum and the Potamogetons are most commonly 

 dominant. It is a rather notable fact in these dense aggregations of 

 plants that the various species seldom become mixed to a marked 

 degree. The tendency is toward the production of alternating 

 pure colonies. Thus in rowang about one passes over such dense 

 relatively pure communities of Myriophyllum, Potamogeton richard- 

 sonii, or P. heterophyllus. In the more open water one sees similar, 

 but much more open aggregations of P. zosteraefoUus, P. natans, 

 and P. pectinatus. Potamogeton foliosus is most abundant in the 

 alkahne waters where it often fills broad areas of shallow water with 

 a dense grassy tangle to the exclusion of most all other plants. 

 This is the one of the species of this association that appears to 

 have solved the problem of alkali resistance to a remarkable degree, 

 and it is seen in particular abundance in the hills wherever there are 



