988 MINNESOTA BOTANICAL STUDIES. 



II. Pools of the mid- strand. These are formed by the 

 waters of the lake thrown back in the heavy surf, or produced in 

 the mid-strand area by the construction of bars off shore. Such 

 pools may remain for several years, as long, indeed, as the 

 bars which bound them are permitted to exist. By the slope 

 of the beach such pools also receive accessions of water from 

 the rains and drainage channels are formed which bring water 

 to them from portions of the strand some distance away. 

 Water falling upon the lakeward slope of the back strand is 

 also brought into such pools. They are therefore formed by 

 surf and by rains. Owing to their real impermanence they do 

 not usually become tenanted by many species nor by those of 

 most robust growth. A list of the main plants in a mid- 

 strand pool at Oak point is appended. 



Mid-strand pool formatiou at Oak point. 



Sagittaria graminea. Eleocharis tenuis. 



latifolia. J uncus articulatus. 



rigida. Lenina naiuor. 



Eleocharis palustris. 



These plants were distributed about the shallow pool, and 

 towards the center the water was open. None of them were of 

 great age, as was determined by an examination of the rhiz- 

 omes, and the scanty development of species was clearly con- 

 nected with the shallowness of the water and the short duration 

 of the pool, as a feature in the topography. 



III. Pools of the back strand. These being for the most 

 part Outside the region of surf influence are rather rain-water 

 sinkholes in the back strand, than surf-fed or surf-formed 

 pools, like the two first mentioned. They are dependent upon 

 the contour of the back strand for their depth and size, and are 

 commonly permanent features of the topography. Conse- 

 quently they are populated by a greater variety of species and 

 by plants of more robust growth. A list of varieties found in 

 a back Ftrand pool at Oak point is furnished. 



Back strand pool formation at Oak poiut. 



Typha latifolia. Scirpus cyperinus. 



Potaniogeton foliosus. lacustris. 



Potainogeton heterophyllus. Lemna minor. 



perfoliate var. richsrdsonii. Juncus articulatus. 



Sagittaria graminea. Iris versicolor. 



latifolia. Koripa palustris. 



rigida. Spiraea salicifolia. 



Eleocharis palustris. Utrlcularia vulgaris. 

 tenuis. 



Such pools of the back strand are commonly surrounded by 

 plants of hydrophytic aptitudes, such as Cyperus, Betula, 1m- 



