498 Wisconsin Academy of Sciences, Arts, and Letters. 
ence of humic acid in the water. This is brought to the lakes by 
streams filtering through immense peat beds. 
Opposite Governor’s Island the larger plants occur in a nar¬ 
row zone at a depth of about 1.6 meters. Toward the shore, the 
plants are prevented from growing by wave action on the exposed 
point, and away from the shore by the stony character of the 
bottom. 
At stations 34 and 35 a rock bottom limits the lake ward de¬ 
velopment of vegetation. Outside the bar the lake is shallow for 
a distance of more than a kilometer, with a depth of from 1 to 4.5 
meters. Here were found Ceratophyllum demersum and Potamo- 
geton RicJiardsonii in abundance, their stems reaching nearly to 
the surface. Somewhat less abundant were Najas flexilis, Vallis- 
neria spiralis, and Potamogeton pectinatus. Inside the bar the 
plants were not studied, since they belong rather to the river than 
to the lake. 
At only one point around the entire lake shore was there a break 
in the border of flowering plants, and this was opposite Maple 
Bluff (station 29), where the bottom is covered with large rocks. 
These rocks had quantities of Cladophora, an alga, clinging to 
them. 
The total number of flowering plant species found was 19. 
Chara was also found on almost all sandy bottoms at all depths 
up to 4 meters. 
The principal factor determining the depth limit of vegetation 
in Lake Mendota is probably that of light. The water of the 
lake in the month of August is not at all clear, mainly because of 
the immense quantities of plankton in suspension. 
Ceratophyllum was found at a depth of six meters. According 
to Warming (4), Spermatophyta descend at most to thirty meters. 
It may be that these are detached, since, according to Pond (5), 
six meters is given by Schimper as the maximum depth for this 
group. 
Chara and Nitella have been found at a depth of thirty meters. 
West (6) distinguishes three zones according to the intensity of 
the light received: First, the photic zone, where there is sufficient 
light for the development of higher flora: in some lakes this in¬ 
cludes depths of twelve feet or less, in others depths of as much 
as forty feet (Lake Lismore in Scotland). Second, the dysphotic 
zone, deeper than zone one. A few higher plant stragglers are 
found here. It is normally occupied by members of the lower 
