972 MINNESOTA BOTANICAL STUDIES. 



opment of Type I, the Prunus mid-strand. Generally light 

 winds, light surf, and less continuous spray favor the devel- 

 opment of Type II, the Cornus mid-strand. Light winds, light 

 surf, and frequent inundation favor the development of Type 

 III, the Salix mid-strand. 



It is interesting to notice that when strand formations of 

 this character are developed on both sides of a narrow spit as 

 at several different localities at Lake of the Woods, the side 

 exposed to the strong prevailing winds is more abundantly 

 provided with plants of Prunus pumila while it is upon the 

 less exposed side that one must look for the Cornus bushes in 

 greater number. And this difference between the two sides of 

 a sand spit will vary about with the inclination of the spit 

 towards a direction perpendicular to the prevailing winds. 



A number of conditions may modify the mid- strand forma 

 tions; the important ones are exertion of ice pressure and 

 formation of surf barriers off shore. By the former it not 

 infrequently happens that a ridge is piled up at the water's 

 edge in early spring and surf then forms mid- strand pools 

 which may be permanent for a series of years. The establish- 

 ment of such pools brings about a rearrangement of vegetation 

 which will be discussed more particularly under the head of 

 strand pools. And from this original exertion of ice pressure 

 various secondary changes in the mid strand may be initiated, 

 so that the whole aspect of the plant population is changed. 

 For example, if the mid-strand is wide, such a formation of 

 strand pools may eventually result in the appearance of dunes, 

 sand-fixing plants gaining a particularly favorable opportunity 

 for work at the borders of the pool. And if surf barriers, 

 which continue in place for a number of years, are developed, 

 the mid strand may take much the aspect of back strand, plants 

 creeping down upon it which are unable to maintain them- 

 selves through seasons of surf or inundations to which the 

 mid- strand was generally exposed before the barrier had ap- 

 peared. Such surf barriers may be in the nature of sterile 

 impermanent bars or they may become the habitation of surf 

 plants and they may eventually, as in the case of the Isle aux 

 Sables, develop into a conspicuous and permanent dune: 



The light color of the mid-strand, owing to the slight develop- 

 ment of humus, contributes to the dwarfed habit of the plants 

 growing upon it by reducing the temperature of the sub- 

 stratum. Such areas become cold soon after sundown while 

 the back strand still remains warm. By midnight the tempera 



