982 MINNESOTA BOTANICAL STUDIES. 



In concluding this discussion of influences tending to modi- 

 fy the back strand population, it may be said that the suc- 

 cessive zones of beach formation — front strand, mid- strand and 

 back strand, show an interesting progression in complexity. 

 The first named is by far the simplest, the last by far the most 

 complex. In the case of front strand, impact of surf places a 

 rigid limit on the types of plants that can develop. For the 

 mid- strand the spray, wind, surf and occasional inundations 

 place a limit, but not so definite a one as for the front strand. 

 For the back strand a limit is placed by the texture and chem- 

 ical nature of the soil and by the mid-strand border. Otherwise 

 it is free to be tenanted by whatever back country plants may 

 attempt to push in. Thus back strand, of the three zones, is 

 the most complex in its plant population. Since more, and 

 more complex, modifying conditions affect it than affect the 

 mid-strand, or the front strand, it is correspondingly a more 

 diversified area and a larger number of types of back strand 

 need to be examined. The ecological reasons for such a gen- 

 eralization have already been given; it remains to note in order 

 the principal types of back strand formations that have been 

 studied at Lake of the Woods. 



I. Gramineous back strand may appear in two principal 

 forms — as meadow-like slopes with strong development of 

 Agrostis and Alopecurus or as dune-like slopes with Elymus 

 and Calamagrostis as the characteristic plants, with often a 

 considerable mixture of Hordeum and even of Agropyron. The 

 representative of the latter genus — A. tenerurn — according to 

 my observations, less commonly exists upon back strand than 

 do the others. The first variety of the formation indicates a 

 greater percentage of nitrogenous matter mixed with the 

 sandy substratum than does the second. Therefore under the 

 general conditions that form gramineous back strand, the one 

 variety or the other may be regarded as an indication of the 

 humus-content and the differences between two areas of grassy 

 back strand, in this respect, are controlled by the conditions 

 regulating different percentages of nitrogenous material — for 

 example, slow drainage, low elevation giving less sweep to 

 the wind, shade from neighboring objects or contour of surface 

 making accumulation of organic debris more probable. 

 Gramineous back strand often develops just lakeward of a 

 dense tree growth classified as belonging to the back country, 

 but in some cases at Lake of the "Woods the grass covered 

 slopes are bare and open. In such cases the ridge is low, 



