keys to the vascular plants of east texas - Botanical Research ...

796 POACEAE
to 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 (see Appendix
4). 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.