980 MINNESOTA BOTANICAL STUDIES. 



of a uniformly lower area towards which whatever drainage 

 currents there may be will flow. At Sandy beach, although 

 the back strand is a relatively low ridge, yet as one stands up- 

 on it he sees landward an apparently interminable low swamp 

 with spruce and larch and intervening reed-bogs and sphag- 

 num bogs. Nowhere near does a continuous rise take place 

 from the back strand to the general country-level. Conse- 

 quently at the Northwest Angle the back strand forms a levee 

 like ridge, and frequently this ridge is less than fifty yards in 

 diameter. On the one side is the lake and on the other swamps 

 and low lands extending for miles. Such a condition does not 

 favor the constant diversion of moisture to the back strand by 

 higher land behind, and under such conditions the back strand 

 does not form strand pools so abundantly as on such an area as 

 Oak point. Here higher land lies behind the strand, except 

 at the end of the point, and thus a drainage current sets to- 

 wards the edge of the lake and in the back strand this accu- 

 mulates in the strand- pools. Again, the greater breadth of 

 the back strand at Oak point as compared with Sandy beach, 

 permits the wind to execute more of an irregular dune-like con- 

 tour of the surface and this contributes to the formation of pools. 



The character of the back country has further effect in pro- 

 ducing physical conditions tending to modify the back strand 

 by its alteration of the direction, intensity or humidity of 

 atmospheric currents and by the nature of substances washed 

 down from it in drainage- water — if such exists A dank, 

 illimitable swamp affects the back strand atmosphere differ- 

 ently by its proximity from a succession of pine-clad hills or 

 ridges of rock covered by crevice plants and mosses and 

 lichens. The first named condition actually prevails at Sandy 

 beach while the second is observed at Oak point. 



In general, modifications of the substratum and of the atmos- 

 pheric conditions, as well, may originate in the region behind 

 the back strand and the temperature, illumination, humidity of 

 the affected region may be correspondingly changed. 



Of biological conditions originating landward and tending to 

 modify the back strand, a great deal might be written, for the 

 group is a complex one. In the first place it must be observed 

 that another minor tension line runs at the rear of the back 

 strand similar to the one which runs along its outer edge next 

 to the mid-strand. And just as in the former case, reciprocal 

 influences are set in motion between the adjacent formations so 

 that one falls back or advances while the other moves in the 



