Poaceae Pollen

The grass family (Poaceae) is the fifth largest plant family with 780 genera and 12,000 species. Grasses are the dominant vegetation in prairies and savannas but can also be found wherever plants can grow, under a wide variety of environmental conditions from arctic marshes to tropical swamps. Approximately 25% of the world’s vegetation belongs to the grass family. The Poaceae includes cereal crops, prairie and pasture grasses, bamboos, and lawn grasses. There are 331 species of grass in Oklahoma.

Flowers in the grass family occur on an inflorescence, usually a spike or panicle. Individual flowers are small, inconspicuous, and incomplete, with sepals and petals lacking completely or replaced by small scale-like structures called lodicules. In the majority of grasses each flower normally has three stamens, and one carpel with a single ovule in the ovary but two styles and stigmas. The stigmas are enlarged and feathery, and the mature stamens with long hanging filaments; these features facilitate wind pollination. 

Pollen grains are monoporate in all grasses making genus identification impossible in the analysis of air samples by microscopy. As a result, they are only identified as Poaceae pollen. Grass pollen grains are often 20-25 µm in diameter; however, there is a large range in size among the thousands of species in the family and some species have very large pollen grains. For example, corn pollen ranges from 80 to 120 µm in diameter. Poaceae pollen is seen in our Tulsa air samples from late March through October. The peak concentrations occur in late May. Although some native grasses release pollen in spring, other grasses, including lawn and ornamental grasses flower throughout the summer and into fall.