486 
overflowed district, and are to a varying degree traversed by 
waters from the bottom-lands above and adjacent to them, and 
the purely local factors of their environment, such as vegeta- 
tion, thus become less potent. Again, it is not until the latter 
part of this period that the vegetation attains the development 
which continues throughout the remainder of the summer. 
The relative barrenness (in plankton) of the vegetation-rich 
waters is thus least striking when the vegetation is least in 
evidence. 
During the period from August to November inclusive the 
ratios are very much higher, rising to 1 to 16 or 20. This is the 
low-water period, when the vegetation in the vegetation-rich 
lakes isat its maximum development both in quantity and in 
the relative volume of the lake occupied by it. It is also at 
such times that these several bodies of water are more distinct 
units of environment, with their local factors no longer merged 
by flood conditions. The relative barrenness of the vegetation- 
rich waters is thus greatest when the vegetation is at its maxi- 
mum development and is most emphasized as a factor in the 
environment. The conclusion from this comparison of the mean 
production of plankton in vegetation-rich and in vegetation-poor 
waters in our locality is thus inevitable that vegetation (in the 
usual sense of the word) is inimical to the development of an 
abundant plankton. It may also be said that the contrast 
would be considerably heightened if it were possible to elimi- 
nate from all the collections on which this comparison is based 
the adventitious organisms—such as small insect larve, mol- 
lusks, oligochetes, Hydra, etc., which form a considerable vol- 
ume of many of our catches in the vegetation-rich waters. 
On the other hand, it must be maintained that the vegeta- 
tion is only one of the factors concerned in the phenomenon pre- 
sented by this contrast. It is quite probable that other fac- 
tors, especially the current, tributary waters, and the chemic- 
al constituents of the water, affect the problemin hand. Dur- 
ing high water both Thompson’s Lake and the Dogfish-Quiver 
region are traversed by a considerable current from the bottom- 
