5° BOTANICAL GAZETTE [JANUARY 
rocal of the depth in inches at which the wire becomes invisible. 
As recently modified by Whipple and Jackson, turbidities are 
read directly from the scale as parts by weight of suspended 
matter in a million of water, making this a very simple observa- 
tion and worthy the attention of botanists engaged in the study 
of aquatics. November 12, 1902,a reading of 11.5 was obtained. 
No readings were taken on the second lake. 
3. Temperature. This, coupled with depth, may account for 
periodic phenomena observable particularly in the case of diatoms. 
The first lake is so shallow that stagnation never occurs. The 
bottom is a little more than a degree cooler than the surface. The 
second lake is over 35* (10.6™) deep in places, and, though no tem- 
perature readings have yet been taken, stagnation may occur, caus- 
ing at the time of the spring and fall overturning an increase in 
the diatom flora. At all events the second lake showed many 
limnetic diatoms in the spring, more than in May, leading to the 
inference that stagnation does occur. 
The chemical composition of the water, which may be the 
determining factor for the presence of blue-green algae in one 
lake and their absence from the other, has not yet been examined. 
The facts here presented show plainly that the history of 
these morainal lakes, as regards their vegetation, is by no means 
identical. Differences of depth and contour, of soil and drain- 
age, of water supply and the chances of seed dispersal, with 
other less conspicuous factors, have resulted in great differences 
in the rate of filling up and in the character of their plant life. 
The first lake stands asa representative of a large number in this 
valley that show essentially the same succession of plant societies. 
After the retreat of the ice the ground could hardly have been 
left bare for any considerable period. If this had been the case, 
the evidence would be seen in much greater erosion than bas 
actually taken place. As it is, these areas, held by vegetation, 
show the even curves of ice- and water-formed topography. We 
are to think of forest-covered hills surrounding a newly formed 
lake basin. Aquatics and amphibious plants are already growing 
about its edge, and as the humus accumulates, sedges, bushes, 
and finally trees follow. As the basin becomes filled up with 
