247 



this species prevails lies in the southern third of the lake and 

 alon^ its sides for a distance of several hundred feet from 

 shore, and again at the northern end for a distance of three 

 quarters of a mile from the outlet. During low-water years 

 scattered clumps of CeyatophyUum and Potamogefon were found 

 as far north as the middle of the lake. Thus, at all seasons 

 about half of the lake — often two thirds of it — is open water 

 with scarcely a trace of fixed vegetation. 



The seasonal changes in the vegetation of this lake are very 

 marked. In midwinter, during the ice blockade, which contin- 

 ues much longer upon the lakes than upon the river, the vege- 

 tation is not much in evidence. At low stages dead rushes 

 rise above the ice in a few places, but give little hint of the 

 great mass of broken and more or less comminuted vegetable 

 debris which covers the bottom in those portions with vegeta- 

 tion of a semi-littoral character. This debris is of great extent, 

 and in the absence of current and buoyancy is not carried away, 

 but remains to enrich the waters and the unstable ooze upon 

 which it lies. Most of the vegetation in this belt is dormant at 

 this period, little trace of green appearing on the half-buried 

 root-stalks and rhizomes of the perennial species belonging to 

 this zone. In the deeper water, on the other hand, a consider- 

 able quantity of Cenitophyllum, with some E'/oc/ea, remains upon 

 the bottom throughout the winter, keeping its foliage beneath 

 the ice. This is an important factor in preserving the equilib- 

 rium in the gaseous contents of the water, and thus in the main- 

 tenance of the winter plankton. 



With the rise in temperature in spring the vegetation starts 

 into growth which the spring floods to a large degree conceal. 

 This growth and the decline in river levels combine to make 

 its appearance at the surface or its emergence above it some- 

 what sudden. The greatest growth takes place during the 

 months of May and June, and is in large part attained by the 

 close of the latter month. The changes subsequent to this pe- 

 riod which are incident to growth are but slight, and have but 

 little effect upon the "waterscape." 



