A cylindrical shield prevents impaction of pollen on the 
sample surface while the slides are not rotating. When 
the slides are in motion the shield is pulled down so they 
are exposed to normal air flow. When rotation ceases, 
the shield is pulled up to shelter them from the wind’’(2). 
The slides from the gravitational samplers were stained 
and mounted with methy! green in jelly. Such slides can 
be stored for a long time without deterioration. The roto- 
sampler slides were placed in special holders and stained 
with methyl green. There is no satisfactory method 
for preserving the rotosampler slides after reading, a dis- 
advantage when there are difficulties in identification. 
Identification was carried out in the Paleobotanical 
Laboratory of the Botanical Museum, Harvard Univer- 
sity, directed by Professor Elso S. Barghoorn. We had 
previously obtained experience in pollen identification 
from two year’s study of the flora of the Sabana de Bogota 
in Colombia, South America. During a four-month period 
before initiating the survey, we became familiar with the 
flora of Boston by studying the pollen collections of the 
laboratory and our own collection prepared from herbar- 
ium specimens. During the season, we had the coopera- 
tion of Mr. Richard Eaton, Phanerogamic Curator of 
the New England Botanical Club Herbarium, who gave 
us helpful information as well as specimens of the dif- 
ferent native plants in flower. 
Zeitz microscopes were used with the 10X objective. 
On occasion, it was necessary to use 40% or oil immer- 
sion. Each of us took four stations, and the remaining 
one was rotated every two months. One square cm. of 
each slide was studied, and the pollen grains were iden- 
tified and counted. When a particular grain was difficult 
to identify, the place was marked with a microlocater 
for further study. About 95% of the pollen grains were 
identified. 
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