198 BOTANICAL GAZETTE [SEPTEMBER 
at this time there is a maximum demand by the atmosphere upon 
the water contained in the plant tissues, while at the same time 
only a minimum supply is available to replace such losses by 
transpiration. Two other periods of high evaporation are found 
to occur, one late in June and the other early in September. The 
latter is doubtless the one of greater stress, for it follows a month 
of very low rainfall. It will be seen that the maximum average 
evaporation for any week is just above 35 cc. per day, and that 
the minimum only once falls below to cc. per day. The average 
rate for the three stations upon the cottonwood dune for the 178 
days of observation is 21.1 cc. per day. 
The graphs (fig. 1) indicate that not only is the cottonwood 
dune an association with a very high rate of evaporation, but also 
that it is subject to excessive variation. This is most noticeable 
during May and June, but to a less marked extent prevails through 
the season, the fluctuations being decidedly greater than in the other 
associations (fig. 4). The mean of the readings of these three 
stations is believed to express most accurately the true measure 
of the evaporating power of the air during the growing season in 
the cottonwood dune association, and is therefore plotted and used 
in comparison with similar graphs from the other associations 
(fig. 4). 
As the dunes gradually beeoiks fixed, an association dominated 
by evergreens succeeds the cottonwood dune. This pine dune asso- 
ciation varies somewhat in composition in different localities, but 
in the area under consideration is dominated by Pinus Banksiana, 
associated with Juniperus virginiana, J. communis, and in the 
older portions containing also Pinus Strobus. In the undergrowth 
Arctostaphylos Uva-ursi is conspicuous, associated with Rhus 
canadensis, R. toxicodendron, Prunus virginiana, Celastrus scandens, 
seedlings of Quercus velutina, Smilacina stellata, Asclepias tuberosa, 
Monarda punctata, and other woody and herbaceous plants. Two 
stations were placed in this association at spots of medium density 
of growth about 1oo meters south and east of the cottonwood 
dune series, but owing to several accidents only one record is worth 
reporting. This, from station no. 4, is unbroken for 178 days, and 
is often the mean of the readings from two atmometers. 
