Rickett—The Larger Aquatic Plants of Lake Mendota. 507 
A few samples were lost before their dry weights were ob¬ 
tained; the latter were calculated in these cases on the basis of 
the average percentage of water obtained from all the other 
samples of the species in question. 
The method of recording the weights, wet and dry, is illustrated 
in table 2. 
Calculations 
General Averages. The first step in the utilization of the data 
obtained was to summate the weights, wet and dry, of each spe¬ 
cies, and to compute from these figures the average percentage of 
water for each species. For this purpose only those samples were 
used for which both wet and dry weights had been obtained. The 
dry weights of those samples that were lost in drying were cal¬ 
culated from the average percentage of water for the species in 
question. The totals, thus corrected, for all species, were then 
summated, and to the total were added the weights of a few sta¬ 
tions in which it was impossible to separate the various species. 
From this final total was determined the average percentage of 
water for all species, and, by a comparison with the total area 
from which collections were made, an average weight per square 
meter. The total area of the plant zone being known (see be¬ 
low), it was possible to calculate on this basis the total quantity 
of plants in the lake. These results are shown in table 3. 
Zones. Each station was divided into zones according to 
depth, as follows: zone 1, 0 to 1 m.; zone 2, 1 to 3 m.; zone 3, 
3 to 7 m. This division was based on an examination of the 
samples gathered from various depths. It was evident, for in¬ 
stance, that in water 1 meter or less in depth the prevailing 
species were Potamogeton pectinatus, P. Richardsonii, and Vallis - 
neria spiralis; while in water deeper than 1 m., P. amplifolius 
predominated, with large masses of Ceratophyllum and Myrio- 
phyllum in places. In water deeper than 3 m., vegetation was 
for the most part scanty, and was characterized by the absence 
of many species found in shallower water. Of course the line 
of division between the zones is arbitrary, but the samples were 
gathered well within the zone limits. 
Weight per Square Meter of Stations. From the actual weights, 
wet and dry, of each sample, was calculated the weight per square 
meter of each species for that particular spot. The samples within 
