506 Wisconsin Academy of Sciences, Arts, and Letters. 
gram, was used. This proved, on calibration with known weights, 
to be sufficiently accurate. Weights were read to the nearest 5 
grams. The difficulty in obtaining the wet weight lies in the im¬ 
possibility of separating the surface water adhering to submerged 
plants from the contained water. The method followed was to 
allow the plants to drain for a short time, and then to shake off 
as much of the surface water as possible. Another error in the 
weighings is due to the fact that the plants on removal from the 
water become matted together so that they sometimes retain a con¬ 
siderable quantity of sand or mud. For these reasons it is prob¬ 
able that the wet weights listed in the tables represent values 
slightly above the true weights. 
The case of the Potamogetons is peculiar. These plants are 
heavily incrusted with a deposit of calcium carbonate (2, p. 171). 
It is especially noticeable on the broad-leaved forms. An effort 
was made to conserve as much as possible of this material but it 
was impossible to pluck the plants without disturbing a certain 
quantity of it. On shaking the plants, it is dislodged in the form 
of fine flakes of considerable size, which, of course, become im¬ 
mediately distributed through the water. On drying, it takes the 
form of a fine powder, which is difficult to conserve. 
Dry Weight. After being weighed wet, the samples were spread 
out on sheets of paper until air-dry. They were left in this condi¬ 
tion until the end of the collecting period, so that all had reached 
about the same degree of dryness. A number of samples of various 
species were then selected and dried in ovens at various tempera¬ 
tures until a constant weight was reached. It was determined 
that after 48 hours at 60° C. further loss of weight was so small— 
a small fraction of 1 per cent.—that for the purposes of these de¬ 
terminations the samples could be considered as dry. Accordingly, 
all the samples collected were dried at this temperature for 48 
hours, and weighed. Weights were read to the nearest 0.1 
gram. It was found necessary to weigh each individual sample 
because of the great variation in the ability of different samples 
even of the same species to retain water in an air-dry condition, 
making it impossible to obtain the air-dry weights and then to 
deduct a percentage calculated by oven-drying a small number of 
samples. This variation among the samples depends probably to 
a large extent upon the different types of bottom from which the 
samples come. 
