Size, variability, and identification of grass pollen

Studies of grass pollen identification were initiated as an aid to the study of pollen dispersal in its relation to the production and maintenance of pure seed stocks of cross-pollinated grasses. Studies of pollen dissemination previously have been of major interest chiefly to the medical profession in its consideration of pollen allergies. Data collected and methods used in pollen identification during two years of these studies are summarized. The size and variability of pollen of 40 grasses growing on the Nebraska Agricultural Experiment Station Farm were studied in 1944 and 1945. Composite samples of each grass pollen were collected and stored in glycerine-alcohol solution. Measurements of pollen diameters were made from permanently mounted slides. Studies of pollen dissemination of different grasses showed that bromegrass, timothy, Kentucky bluegrass, Russian wildrye, orchard grass, common rye, and field corn were prolific producers of pollen. Other grasses were intermediate or produced pollen in much lesser quantities. Shape and natural color are apparently of little value in the identification of pollen of different grasses. The 40 grasses are classified into five major groups on pollen size, determined by measurement of diameters expressed in mean values with their standard errors. Sizes of pollen range in mean values from 24.5 microns for sand lovegrass to 99.5 microns for a variety of popcorn, with 30 of the 40 grasses having pollen grains between 30 and 50 microns in diameter. Highly significant differences may be shown between the sizes of pollen of many of the grasses. However, the difficulties of identifying grass pollen within the several group classes on the basis of size alone are apparent. Results of studying pollen from the same grass stand in two different years indicate that pollen size may vary as a result of environmental conditions. Although 17 of the 31 grasses studied in both years produced pollen with no significant variation between the two years, 5 grasses showed significant differences at the 5% level and 8 showed significant differences at the 1% level. Seasonal environment has a pronounced effect on the pollen size of at least some grasses. Knowledge of the season and time of day of pollen shedding of the many grasses growing in a region is important in addition to measurement in the identification of grass pollen.