Bickett—The Larger Aquatic Plants of Green Lake, 395 
ture, there are abundant records from many Wisconsin lakes, whith 
have been made available to me by Professor Juday. A brief com¬ 
parison of Green Lake and Lake Mendota serves to bring out sev¬ 
eral interesting pomts. 
• The degree of transparency of the water was measured by means 
of a white disc, 10 centimeters in diameter, which was lowered into 
the water until it disappeared from view; the depth at which this 
occurred being recorded. This depth in Green Lake varied from 
2.75 to 6.25 meters, the average being about 4.25 meters, during 
June, July and August. In Mendota, the range during the same 
months of the same year was from 1.75 to 3.8 meters, the average 
about 2.25 meters. When these figures are compared with the 
downward limits of the plants in the two lakes, it is evident that 
they confirm PearsalPs statement that light is a limiting factor in 
this respect; and, further, make it probable that it is the chief 
limiting factor. 
Pearsall also showed, by an iodine method of measuring the 
light intensity, that plants grew in as little as 2 per cent of the 
light of the surface, but not in less. According to some figures 
kindly supplied me by Dr. E. A. Birge, the light intensity in Green 
Lake is reduced to 1 per cent of that at the surface at a depth of 
about 8 meters. The same light intensity in Mendota is found at 
a depth of about 4 meters. These depths correspond approxi¬ 
mately with the limits of the plant zone in each case. Birge’s de¬ 
terminations were made in an entirely different way from those of 
Pearsall, which may partially account for the discrepancy between 
the two sets of results. 
With regard to temperature, table 14 shows the differences be¬ 
tween Green Lake and Lake Mendota. The figures are averages of 
readings taken through June, July, and August. These differ¬ 
ences in temperature evidently are not large enough to limit the 
plant zone, since plants grow in Green Lake in water 6°C. cooler 
than that at which they cease in Mendota; the temperatures at 
corresponding depths are also lower in Green Lake. It seems that 
the lower temperatures of Green Lake may partly account for the 
smaller productivity of its bottom by retarding the growth of 
plants to a slight extent. 
The effect of low temperatures and low light intensity in retard¬ 
ing growth is illustrated by the reduced stature of plants in deep 
water. Most submerged plants flower at or near the surface. This 
is especially true of such plants as Potamogeton, Vallisneria, and 
