Poaceae – The Grass Family
By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
Alabama Allergy & Asthma Center Collection Station – Birmingham, Alabama
Grasses begin releasing pollen in this part of the United States during the early spring (March), and grass pollen reaches a peak in grains/mm3 of air in our pollen collections during the late spring/early summer (May and June). All grass pollen looks very similar. The grains are spheroidal to suboblate (slightly flattened at the poles) and have a single circular pore. The monoporate pollen grain of grass typically has an annulus (a distinct thickening of the outer and inner walls of the pollen grain), but some lack an annulus. The annulus may be covered by an operculum comprised of both wall layers, or it may be non-operculate. The exine (outer wall of the pollen grain) may be areolate (a pattern of block-like areas like cracked, dried mud) to scabrate (roughened, as if covered with scabs). Grass pollen varies in size from approximately 20 µm up to 90 µm. For practical purposes, all grass pollen looks very similar when stained with basic fuchsin and viewed under 400X magnification. (See photographs below of grass pollen from two grass species.) Hence, our pollen counts lump all grass pollen into one category, the family Poaceae.
Economically, Poaceae is the most important family of plants. All cereal grains such as wheat, rice, corn, barley, and oats are in this family. In addition, sugarcane, sorghum and bamboo are members of the Poaceae (formerly called Gramineae) as well as pasture grasses that serve as a food source for grazing animals. This family has some 700 – 800 genera, and 10,000 – 12,000 species. These numbers make it one of the top five families of flowering plants with reference to the number of species present in the family. Grasses are worldwide in their distribution and the family is the most abundant of the Earth’s flora. Grasslands cover some 20% of the Earth’s surface. In addition, grasses are present in abundance in nearly all habitats and ecological zones. Grasses are the dominant vegetation in the biome called prairie or savannah. In short, Earth would not be recognizable to us without grasses, and we would certainly have a challenging time feeding ourselves without some of the species in this plant family.
Most grasses are herbaceous (some are woody such as Arundinaria, Phyllostachys, Bambusa, (bamboos). The stems of grasses have distinct nodes and internodes; the nodes are solid and the internodes are usually hollow. The grass blades (leaves) are flat, have parallel venation, and are arranged alternately and in 2-ranks (on opposite sides of the stem). Some grasses are annuals while others are rhizomatous or stoloniferous perennial herbs.
The basic inflorescence of grasses is a spikelet (see illustration below), and spikelets are themselves arranged in spikes, racemes, or panicles, (see illustration below). Spikelets are composed of one or more flowers that are arranged along an axis. The flowers may be perfect or imperfect with a very reduced perianth (petals and sepals), that is represented by structures called lodicules (see illustrations and photo below). The lodicules serve to help force the lemma (the lowermost of two chaff-like bracts enclosing the grass floret) and palea (the uppermost chaff-like bract enclosing the grass floret) apart during anthesis, which facilitates exsertion of the anthers and stigmas. There are typically three stamens present and two stigmas. The stigmas are feathery and often densely pubescent, which aids in trapping pollen present in the air. The ovary is one-ovulate and the fruit produced is called a grain or caryopsis.
Grass flower and inflorescence
Typical inflorescences of grasses
Timothy grass pollen – Muhlenbergia glomerate
Canary grass pollen – Phalaris arundinacea