It is important to know not only the airborne pollen-producing plants in the general area under investigation but also their positions with respect to distance from the pollen sampler. If this is known and daily records of wind direction are kept, these data are of value in understanding the catch with respect to the sources. Some pollen may travel long distances and in large quantities (Tyldesley 1973), so knowledge of occurrence over regions beyond the local area is desirable. Range maps for many species are available.
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