Taxonomy of Flowering Plants - LECTURE NOTES - Spring, 1998
Hugh D. Wilson - rm. 306, Butler Hall
20 April 1998
The Commelinidae
Family Overview - Cyperales

Poaceae (Gramineae)  - the Grass Family

Diversity: often tufted (caespitose) perennial herbs, also many annuals, rhizomatous or stoloniferous perennials and some 'woody' types (bamboo) - ca. 500 genera and 8,000 species. This family includes many important domesticates, including the world's most important cash crop - Triticum aestivum (breadwheat-Eurasia), its most important food crop - Oryza sativa (rice-Asia), the monoecious Indian Corn (Zea mays-Americas), and the most extensive crop of the local Brazos bottoms - Sorghum bicolor (sorghum, milo - Africa).  Perhaps just as important, from an economic point of view, are the suite of taxa used as forage for domesticated animals and the creation of human-dominated environments (turf grasses).  Many of these have 'escaped' cultivation and, especially in disturbed areas, established a strong position in our local flora.

Distribution:  Worldwide, present in abundance in just about all habitats and ecological zones and often forming the dominant element in open areas (prairie, savannah, etc.).  The Texas flora includes 122 genera and 523 species with 15 endemic species representing 13 genera, including one listed by the U.S. Fish and Wildlife Service as endangered, Texas Wild Rice (Zizania texana) (see also a full listing of North American taxa).

Floral structure:

 flowers can be unisexual

Significant features:  Small, reduced flowers on (often) tufted or caespitose plants with linear, sheathing leaves.  Differing from the Cyperaceae with its round (terete in cross section) stems ('culms'), hollow (not pithy or solid) at the internodes, and leaves 2-ranked or arranged in 2 rows.  Sheathing leaf bases of the are also often 'open' or loosely connected to the culm, often showing a ligule at the point where the leaf blade meets the sheath. Reproductive structures are distinct in that the inflorescences are made up of 'subinflorescences' known as spikelets.  The grass spikelet is defined by two basal 'sterile bracts' or glumes.  These subtend either a single (simple spikelet) or several (compound) florets.  The term 'floret', when applied to grasses, refers to the flower and two 'fertile' bracts that usually enclose the flower, the lemma and palea.  The perianth is reduced to vestigial structures, lodicules, positioned beneath the ovary and the fruit is single-seeded with the testa adnate to the pericarp, a caryopsis.  In contrast to most large families, the fruit is usually not needed for species identification.

Grasses have evolved in response to grazing selective pressure, probably applied initially by large ungulate populations that once inhabited the earth's prairie areas.  Their strategy has been one of accommodation, as opposed to defense, in that the grass leaf carries an intercalary meristem that allows continued growth after grazing (or mowing). This adaptation has allowed grasses to persist and diversify, relative to other possible competitors (most dicots), in the presence of strong grazing pressure.
 
 

Base structures:
 
Grass flower with vestigial perianth or lodicules
base leaf structures
compound spikelet


Wild Oats - now flowering along your local roadside:

 
 Avena fatua var. sativa - inflorescence
 Avena fatua var. sativa - spikelet

Perennial Rye Grass - another colonizing grass (diagrams):

 
 Lolium perenne - inflorescence
 Lolium perenne - spikelet
Indian Corn - imperfect flowers, multiple fruit:
 
 Zea mays - overview - Kohler's Medicinal Plants
 pistillate inflorescence
staminate inflorescence
 

See RLEM 203 (Agrostology) for more information and Internet resources
 

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