useful indication of the intensity of the different pollen 
seasons. 
The results of the pollen counts during the 4 mbrosia 
season by both methods, gravitation and rotoslide, were 
compared for station No. 9. There was aclear difference 
in the daily counts. he rotoslide appeared to be more 
efficient, especially when the pollen concentration was 
low. When the results of the rotoslide were compared 
with the average of all nine stations, the daily variations 
were similar, and there was little difference in the counts 
obtained by the two methods. Further studies with the 
other pollen seasons are necessary; but, in any case, 
these results seem to confirm the general impression that 
a single station equipped with the gravitational method 
cannot give reliable quantitative information. 
TABLE I 
POLLEN SEASONS * 
Onset Main part of the Peak(@rains/em.2) End 
season 
ACER April 21 April 24-29 April 25; (33) ** May 6 
ALNUS April 27 April 29-May 1 April 30; (6) May 5 
BETULA May 1 May 8-11 May 9; (180) May 25 
CARYA May 22 May 25-30 May 26; (15) June 10 
GRAMINEAE May 24 June 4-July 10 June 9; (24) July 30 
JUNIPERUS April 12 April 21-30 April 22; (54) May 4 
PINUS May 20 June 5-13 June 8; (133) July 3 
POPULUS April 14 April 28—May 4 April 30; (17) May 6 
QUERCUS April 30 May 11-30 May 15; (209) May 30 
AMBROSIA August 15 August 25-Sept. 4 August 28; (16) Sept. 25 
SALIX May 2 May 2-8 May 8; (22) May 20 
ULMUS April 14 April 15-25 April 22; (118) May 6 
*Average for all nine stations. 
**Numbers in parenthesis indicate pollen count at peak of season. 
Legend: Onset of season defined as the earliest date on which the average 
pollen count attained was 2 grains/cm2. End of season defined as the 
last date on which the average pollen count was 2 grains/cm2. 
[ 218 ] 
