992 MINNESOTA BOTANICAL STUDIES. 



more closely aggregated, and hence gives shade, raising the 

 temperature of the soil at night and assisting in the harboring 

 of soil water. Thus the surface becomes moist enough, espe- 

 cially near the base of the largest trees of the area, which act 

 as drains for the rain drops, for the establishment of such 

 plants as Vagnera and Onagra The soil is less oxidized, not 

 being exposed to such frequent shiftings by the wind, and the 

 humus- content rises. Directly or indirectly, all of these condi- 

 tions co-operate for the development of a more stable sub- 

 stratum, and one upon which a greater variety of plants can 

 dispose themselves. Therefore, in number of species, the 

 dune summits quite surpass the dune slopes or even the dune 

 hollows. These latter, unless they become sinks for rain- 

 water, do not show any characters particularly different from 

 those of the dune slopes. 



Dune pools. While the dune pool resembles the pools of the 

 back strand previously described in being formed from rain 

 water drained in from the slopes which surround it, it is rather 

 more nearly related to the pools of the mid strand in its scanty 

 vegetation and in its failure, in almost every case, to be sur- 

 rounded by zones of plants developed upon moist or mixed 

 humus, such as the Vleckia and Scutellaria formations described 

 for the pools on the back strand of Oak point. The dune pools, 

 too, from the shifting of the sand that continually takes place are 

 never deep and hence their vegetation is limited by this condi- 

 tion. In Plate LXXV a characteristic dime pool is shown on 

 Isle aux Sables surrounded by Juniperus sabina, Hudsonia 

 tomentosa, Primus pumila and Artemisia- caudata. In the back 

 ground is seen a low dune summit with Celtis trees and one or 

 two taller plants of Quercus macrocarpa. The vegetation of 

 this dune pool consisted exclusively of Eleocharis and Juncus — 

 a very common grouping in such shallow sink holes on these 

 dunes. In one deeper hole a plant of Castalia, doubtless sown 

 by birds, was found growing but such a plant is exceptional in 

 these pools, and even Sagittaria so abundant in the region is 

 more often absent from the pools of the dunes. 



The shifting of the sand is probably the principal occasion 

 for such limited pool formations, because the pools when 

 formed under natural conditions of the drainage are quickly 

 blown full of sand and can not maintain the depth necessary 

 for the establishment of a more varied aquatic population. Thus 

 the exposure to wind of the dune as a whole indirectly limits 

 the flora of the dune pool. 



