RELATION BETWEEN EVAPORATION AND PLANT SUCCESSION 1 73 
entire range of evaporation conditions present in the region. Thuja 
seedHngs commence development on the open beach, but are de- 
stroyed by ice action. On the low fringing dune, where the evapora- 
tion was 618 cc. from atmometer No. 27, Thuja seedlings were more 
frequent. 
At Bryant's bog, where conditions were intermediate between the 
sand dune and a normal cedar bog, atmometers No. 39 and No. 40 
gave 590 and 555 cc. respectively for the season of 1916. As pre- 
viously noted, this bog has been repeatedly devastated by fire and 
there are no Thuja seed trees in the immediate vicinity. The absence 
of Thuja, therefore, can not be attributed to the conditions of eva- 
poration. 
In certain of the East Point bogs, conditions pre-eminently suitable 
for the development of Thuja prevail. Although Thuja seedlings are 
found under a wide range of evaporation conditions, there are places 
in the bog where it is too dark for them to grow. Darkness is here 
attended by low evaporation. With an increase in light, evaporation 
is increased. Since a certain amount of light is necessary for the 
development of the Thuja seedling, low evaporation is not in itself a 
sufficient reason for the absence of Thuja seedlings. Darkness results 
from the dense canopy formed by the trees, but even in the darkest 
places Thuja seeds may be found. The evaporation from such a spot 
where no Thuja seedlings were present was 155 cc. for the season of 
1916. In a small opening nearby, where Thuja seedlings were actively 
growing, the evaporation was 225 cc. The increased rate of evapora- 
tion in itself could hardly be held responsible for the presence of seed- 
lings in one case and not in the other. The development of seedlings 
in openings tends to restore a dense canopy and thus to lower the 
evaporation from the chamaephytic layer. When a clearing of con- 
siderable size is made, the evaporation is increased to a much greater 
extent, as in the case of atmometer No. 3, in Reese's clearing with an 
evaporation of 587 cc. Many Thuja seedlings were present. 
Reese's bog occupies a low rolling site at the head of Burt Lake. 
A road and several trails improve its circulation. A comparison of 
atmometers No. 33 and No. 35 brings out the effect of circulation. 
Atmometer No. 33 on the ground beneath a canopy of Thuja so dense 
as to prevent ground vegetation, gave 435 cc, a higher rate than 345 cc. 
from No. 35 in the crown of a small Thuja seedling in an opening 
nearby. In the latter case, the development of edge conditions in the 
