252 
THIRTEENTH REPORT. 
hence a break water was constructed far enough from the pontoons 
to allow a boat to pass between. The other two sides were closed in by 
a log boom, whose purpose was to keep inquisitive visitors at a proper 
distance. 
Native plants for the most part were placed in the tanks, the water 
depth being regulated to suit the natural conditions of the various 
species used. Reference to the accompanying diagram will indicate the 
position of the various plant groups. Two tanks, as indicated, con- 
tained neither soil nor plants and were used to obtain the figures for 
the evaporation from the free water surface. The points were so placed 
that the surface of the water in the tanks was l%ths inches from the 
rim. Every leaf outline was traced on paper and every leaf stalk was 
measured at the beginning of the experiment, after which the pontoons 
were filled with rocks to bring the rims of the tanks to within 2 inches 
of the surface of the lake. As leaves died from time to time during the 
progress of the experiment, a record was made of the area of the leaf 
and time of its removal. At the close of the experiment the leaves 
and stems were again measured so that a curve of growth may be 
made for the period of time consumed, from which the area of surface 
exposed to the air at any time and for any species may be taken. 
Measurements of the evaporation were begun August 8th and were 
continued with some interruptions until August 30th. Readings were 
made twice daily when weather conditions permitted, once in the early 
morning and again in late afternoon. A record was each time made 
of the volume of water added to bring the water level to the normal 
point. At the same time the temperature of the water within the tank 
was taken, both at the surface and just above the soil. The tempera- 
ture of the water of the lake at the station was likewise ascertained 
both at the surface and at a depth of 30 inches. A whirling psychro- 
meter was also used to ascertain the wet and dry bulb temperatures, 
from which the relative humidity has been derived. 
Some of the data acquired at this station have been worked up. For 
tli e sake of ready reference and comparison these figures have been 
plotted graphically, charts I -VI I. Chart I shows the evaporation which 
took place from the two control tanks, the average being represented 
by the heavy line. (Fractions of centimeters have been dropped to 
facilitate the plotting of the lines.) It will be noticed that the rate of 
evaporation from the free water surface was greater during the day 
than at night. A glimpse at charts II-VII shows this to a more marked 
extent. Allowing for any considerable error which might occur, these 
charts show conclusively that unsubmerged water plants transpire large 
quantities of water, and that this takes place principally during the day- 
time. The one exception in the plants used is the Water Lily. In 
this case the presence of the plants retards evaporation. It must be 
understood, however, that these figures refer only to plant stands in 
the open. They do not explain the conditions in cases of large areas 
of dense growths. It should be remarked, also, that in those cases where 
a large leaf surface was presented, as in the cases of those tanks contain- 
ing Pickerel -weed, Arrow-head, and Cattail the results charted are 
