1902] SURVEY OF THE HURON RIVER VALLEY 129 



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depth. The soil of the central basin is almost pure loess-like 

 clay mixed with a little fine vegetable detritus which settles 

 from the water of the lake ; this area, so far as numerous dredg- 

 ings show, is entirely barren of vegetable life of any kind. 



The water of the lake enters chiefly by underground channels 

 and from springs at the bases of the gravel hills surrounding the 

 lake. During the greater part of the year the supply is just 

 about equal to the loss by evaporation, and except during 

 seasons of maximum precipitation there is very little water 

 passing into the outlet. The water dissolves and holds in solu- 

 tion so much organic substance from the decaying vegetation in 

 the lake that a very perceptible taste is imparted to it. ' The 

 fine detritus held in suspension together with the dissolved sub- 

 stance renders the water brownish, making it impossible to dis- 

 tinguish objects more than six feet below the surface. The, 

 opacity of the water is an important factor in determining the 

 distribution of plants. Obviously there are no perceptible cur- 

 rents in so small a lake. 



2. The plAxN'T societies.— The deep central portion of the 

 lake does not, so far as could be ascertained bj dredging, sup- 

 port any plant life; this cannot be due to the soil, for in other 

 lakes the same soil is covered with Chara and Potamogeton, but 

 is undoubtedly due to the feeble intensity of light at depths 

 greater than twenty feet. 



The plants at the lake are grouped in five fairly well defined 



-concentric zones, occupying all the suitable lake bottom less 



than twenty feet under water, and all the land surrounding the 



lake which feels the influence of the presence of the water. 



Beginning with the innermost, they are: 



^ C'^) ^ zone of Pota?nogeton, which extends in water from 



eighteen feet to six feet in depth completely around the lake 

 just shoreward of the central depression, and forms a zone 

 averaging thirty feet in width. This zone is composed almost 

 exclusively of Potamogeton sosteraefolhis Schum., which grows 

 very luxuriantly, forming a dense tangled mat. The only other 

 plant found in this zone is P. hicois L. 



