Results 
In this report, we present the principal pollen seasons 
as revealed by the gravitational sampler for the nine sta- 
tions. The results of the rotoslide and comparison of the 
two methods will be the subject of another report. 
Identification of pollen derived from different species 
within a genus was not attempted. In the case of Betula- 
type, pollen grains so identified may have included other 
members of the Betulaceae and the closely similar pollen 
from Myrica. Likewise, discrimination of pollen from 
the different genera of the Gramineae was not attempted. 
The seasonal patterns accorded with expectations. At 
the beginning of the tree season in April, Ulmus is the 
more important with an average of 118 grains per square 
em. during the peak of the season. Other early trees are 
Juniperus and Populus, with Acer following later in 
April. May is dominated by Betu/a-type, with an aver- 
age of 180 grains per square cm. during the peak, and 
Quercus inthe second part of May, with an average peak 
of 209 grains. Carya and Pinus appear laterin May. The 
Gramineae season starts at the end of May and continues 
to the end of July. Asis shown in the table and figures, 
the trees produced greater concentrations of pollen than 
do the grasses and Ambrosia, which pollinates from the 
middle of August to the end of September. 
It was interesting to observe the wide variations in the 
daily pollen counts from station to station. There are 
many local factors (such as location of the sampler, local 
flora and wind direction) which presumably account for 
these variations. For this reason, reports of daily pollen 
counts based on one station equipped with a gravitational 
sampler only are of little value from a quantitative stand- 
point. As indicated below, however, the average of 
several stations scattered over a wide area can give a 
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