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Family: Poaceae Barnhart

Kingdom: Plantae Rank: Family Parent: Cyperales Status: Valid

Common Names:

  • GRASS FAMILY - English, United States of America

Morphological Description

Family Recognition in the Field: Mostly herbs with 2-ranked leaves having sheathing bases, free blades, and ligules; culms round, with hollow or solid internodes; flowers small, inconspicuous, reduced to stamens and pistils, subtended by 2 scale-like bracts each, and arranged in very reduced spikes called spikelets; fruit a caryopsis; the ± similar Cyperaceae (SEDGES) have 3-ranked leaves, often 3-sided culms with solid internodes, each flower subtended by 1 scalelike bract, and fruit an achene; the ± similar Juncaceae (RUSHES) have flowers with a small 6- parted perianth and capsular fruit.

Diagnosis: Ours herbaceous or woody (in bamboos), annuals or perennials; roots fibrous; culms (= stems) usually rounded, with prominent, swollen or constricted, solid nodes, the internodes hollow or solid, and with intercalary meristem at base allowing continued elongation; tillers (= basal branches), stolons (= above ground horizontal stems or “runners”), or rhizomes (= underground horizontal stems) often present; leaves alternate, with tubular basal sheath enclosing the culm and usually split to base down one side opposite the blade and a short or elongate, usually linear, flattened or involute (= inrolled) blade, usually with a ligule (= row of hairs, a scaly membrane, or a ciliated membrane) at junction of blade and sheath on adaxial (inner, upper) side, often with intercalary meristem near the ligule; inflorescences made up of very reduced branches called spikelets, which are the basic units of the grass inflorescence and which are arranged in various ways (e.g., spikes, spicate racemes, racemes, panicles); spikelets composed of (0–)2 empty scale-like bracts (= glumes) at base (in some, glumes none or reduced to awns), 1–numerous flowers, associated scale-like bracts (each flower and its subtending (1–)2 scalelike bracts are collectively called a floret), and a short axis; florets 2-ranked, borne one above another, alternating along the usually concealed axis (= rachilla), each usually with 2 alternate, overlapping scale-like bracts (lemma—the outer or lower; palea—the inner or upper, usually smaller and more delicate, sometimes absent or not visible); flowers perfect or imperfect, without true perianth (perianth apparently represented in some species by 2(–3) minute lodicules which function by swelling and opening the floret), typically open only for a short time (usually during morning) or not at all, wind- or self-pollinated; stamens (1–)3(–6), the anthers often dangling outside the floret at anthesis to allow the pollen to be carried away but sometimes retained within the floret; pistil 1, the typically 2(–3) stigmas usually feathery, with increased surface area to catch wind-blown pollen; fruit a grain or caryopsis (= 1-seeded fruit with ovary wall adnate to the seed coat), rarely an achene.


Notes: This is a huge cosmopolitan family with 9,500–12,000 species in 668 (Mabberley 1997) to as many as 785 genera (Watson & Dallwitz 1992). The family follows only Asteraceae, Orchidaceae, and Fabaceae in number of species. Species range from tiny annuals 2–3 cm tallto huge bamboos 40 m (ca. 130 feet) tall. On a worldwide basis, the Poaceae is ecologically the most dominant vascular plant family, occurring over vast areas of prairie, plain, steepe, and pampas. It is estimated that 20–24% (Judd et al. 1999) of the world’s vegetational cover is made up of grasses. The family is also economically the most important, containing all the cereal crops including Avena sativa (OATS), Hordeum vulgare (BARLEY), Oryza sativa (RICE), Secale cereale, (RYE), Sorghum bicolor (SORGHUM), Triticum aestivum (WHEAT), and Zea mays (CORN), as well as Saccharum officinarum (SUGARCANE). Cereals are among the oldest types of plants cultivated by humans, with some being brought into cultivation at least 10,000 years ago (Judd et al. 1999); most major civilizations have been based on the food value of the triploid endosperm of grasses. More than 70% of farmland worldwide is devoted to cereals, which provide humans with more than 50% of all calories (Heiser 1990). Just three plants, WHEAT, RICE, and CORN, provide ca. 45–50% of total human caloric intake (Chrispeels & Sadava 1977; Mabberley 1997). Most grasses are extremely well-adapted to fire, grazing, trampling, and mowing. This is due in part to the presence of intercalary meristems (located in the culms just above the nodes and in the leaves near the ligule, thus allowing growth from the base even if terminal parts are damaged), the large amount of below ground biomass, and the tendency to branch (“tiller”) or produce stolons or rhizomes near or below ground level. The Poaceae, which includes some species with C₃ and some with C₄ photosynthesis, is one of only two monocot families (the other is the Cyperaceae) with the typical C₄ photosynthetic pathway (Soros & Bruhl 2000). C₄ plants are better able to capture CO₂, and so are able to get the CO₂ they need for photosynthesis while keeping their stomata open less (and thus reducing water loss) in comparison to C₃ plants. The resulting increase in water use efficiency is an advantage in arid environments. Grasses are mostly wind-pollinated and shed large amounts of pollen, a major source of allergic reactions (e.g., hay fever) in humans. The family is well known for polyploidy, with at least 80% of the species thought to be of polyploid ancestry (e.g, Triticum aestivum, WHEAT, is a hexaploid—6 sets of chromosomes). Recent phylogenetic analyses (e.g., Hahn et al. 1995; Kellogg & Linder 1995; Linder & Kellogg 1995; Chase et al. 2000; Kellogg 2000b; Soltis et al. 2000; Grass Phylogeny Working Group 2001) indicate that Poaceae is a monophyletic group most closely related to Joinvilleaceae (a very small family of forest-margin plants of se Asia and the s Pacific) and more distantly related to other members of a “graminoid” clade including a number of small e Asia and s hemisphere families, including Anarthriaceae, Centrolepidaceae, Ecdeiocoleaceae, Flagellariaceae, and Restionaceae. More distant still are other members of the Poales, including Bromeliaceae, Cyperaceae, Juncaceae, Mayacaceae, Typhaceae, and Xyridaceae (Chase et al. 2000; Kellogg 2000b). The most recent large-scale work on the family (Grass Phylogeny Working Group 2001) divides the family into 12 subfamilies, 9 of which occur in East TX. The Poaceae is a particularly important part of the East TX flora; the 410 species present represent slightly more than 12% of the total species known for the region and make the Poaceae the largest family in East TX (the Asteraceae is a close second). Turner et al. (2003) mapped a number of additional exotic species for East TX, based on specimens erroneously entered into the Digital Flora of Texas Herbarium Specimen Browser (2002). These specimens were either cultivated or associated with agricultural research facilities: Blepharoneuron tricholepis (Torr.) Nash, Eleusine tristachya (Lam.) Lam., Eragrostis spicata Vasey, Hyparrhenia hirta (Nees) Stapf, Melinis repens (Willd.) Zizka, Pappophorum vaginatum Buckley, Paspalum unispicatum (Scribn. & Merr.) Nash, Pennisetum nervosum (Nees) Trin., Pennisetum setosum (Sw.) Rich., Phalaris aquatica L., Pleuraphis (Hilaria) jamesii Torr., Pleuraphis (Hilaria) mutica Buckley, Urochloa brizantha (C. Hochstetter ex A. Rich.) R. Webster, Urochloa mosambicensis (Hack.) Dandy, and Vetiveria zizanioides (L.) Nash ex Small. We do not consider these species to be members of the East TX flora. A number of additional species of ornamental grasses are cultivated and long persist in East TX, including Pennisetum species and a variety of bamboos. Dissection is usually necessary to see the parts of the grass spikelet, and because of the small size of the structures involved (e.g., lemmas, paleas), a dissecting scope or at minimum a hand lens is often necessary for definitive identification. In the keys, measurements of glumes, lemmas, or spikelets do not include awns (= hair-like or bristle-like appendages) if these are present. (subclass Commelinidae—Cronquist; order Poales—APG II) (Gramineae Juss.)