976 MINNESOTA BOTANICAL STUDIES 



The conditions which determine the character of the back 

 strand group of vegetation are more complex than those which 

 determine the mid strand character, but for clearness they may 

 be considered under three heads. 1. Conditions which orig- 

 inate in the back strand itself. 2. Conditions which arise lake- 

 ward. 3. Conditions which arise landward. Of the first group 

 of conditions, those that originate in the back strand itself, a 

 division into chemical, physical and biological, may be made 

 with value, and these may be reviewed in order. The most 

 important chemical difference between back strand and mid- 

 strand lies in the greater percentage of nitrogenous material 

 which is mingled with the sand. This serves to make back 

 strand more nutritive, as a substratum, than the areas nearer 

 the water's edge. Hence plants of more nitrophytic habit can 

 become established upon it, as, for example, Vagnera and Smi- 

 lax. In typical back strand I do not find that this increment of 

 soil nitrates and nitrites becomes so considerable that true 

 humus plants may gain a foot-hold, and therefore CoraUorhiza 

 or Pyrola are absent from this formation, but the freedom 

 from surf activity tending to wash out organic substances 

 formed in the interstices of the sand and the less rapid drain- 

 age off of soil water derived from rains, both contribute to 

 a degree of nitrogen-content that is favorable to special spe- 

 cies and groups of species. The larger leaved and more ro- 

 bust habit of the back strand vegetation by diminishing wind 

 activity has also an effect favorable to the retaining of dead 

 leaves upon the surface, which, in course of their decay, add to 

 the nitrogen- content. 



Of physical differences the rise in soil temperature, owing 

 to the changes in its texture and color, are doubtless of con- 

 siderable importance, while the decrease in illumination, owing 

 to the shade- affording bodies which are developed, and the re- 

 ducing of the general reflection -value of mounds and hummocks 

 must not be ignored. Thus the retention of moisture is pro- 

 moted, and this is still further accentuated by the withdrawal 

 of the whole area from surf- activity, and the substitution of 

 wind-activity, which tends to develop a more irregular surface 

 with consequent greater tendency to accumulate soil-water. As 

 a whole, the back strand is of higher temperature, less illu- 

 mination and greater superficial saturation than the mid- strand, 

 and this in a broad way is true, notwithstanding the periodic 

 inundations of the mid-strand. At a considerable depth below 

 the surface, however, the saturation of the mid strand is greater 

 than that of the back strand. 



