134 BOTANICAL GAZETTE [august 



line in such places is quite well marked and permanent {fig- 3)^ 

 A few rods south of the lake is the site of the *' dead lake*' 

 mentioned above, a description of which may, as suggested, throw 

 some light upon the life history of the present lake. At present 

 the only indication of the former pond is an elliptical depression 

 surrounded by representatives of the Gramineae and Compositae 

 zone, but in the spring of the year the depression is filled with 

 water to a depth of about two feet, in w^hich several species of 

 Polygonum grow vigorously. Before July the water has all dis- 

 appeared from this basin and for the remainder of the year it is 

 dry- In the center of the depression there is a group of sedges 

 and ferns which is surrounded by a wide belt of willows ; outside 

 of these again is a zone of grass and terrestrial plants. The 

 whole state of affairs suggests that this has been the site of a 

 pond which has been steadily encroached upon by the zones of 

 vegetation in the manner previously described, until now the 

 pond is practically obliterated. The sedges have exterminated 

 the water plants and now the willows have all but exterminated 

 the sedges. 



This pond, originating from the primitive lake, must have been 

 essentially the same in character as the lake under considera- 

 tion ; it seems reasonable therefore to conclude that the pond 

 represents an advanced stage yet to be realized in the life history 

 of the lake. 



IV. ECOLOGICAL FACTORS. 



The plant societies afford unmistakable evidence of the influ- 

 ence of the glacial epochs. The flora of glacial lakes may be 

 compared to boreal islands composed of plants forced southward 

 by the advance of the ice sheet. The sphagnums and sedges 

 flourish best in those conditions which best reproduce boreal 

 environment, and so long as any plant society remains which is 

 distinct!}' lacustral it will undoubtedly show traces of its boreal 

 origin. 



Agencies now at work may be considered unde^r four groups 

 of factors, viz.: hydrodynamic, edaphic, atmospheric, and biotic. 



^Whitford, H. N., Box. Gaz. 31 : 291. 1901. 



