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

1116 RUPPIACEAE/RUPPIA
waters (Hellquist 1980), possibly explaining its distribution only on the extreme w margin of
East TX where soils are alkaline. Guo and Cook (1989) found that at least some pollination can
occur underwater in this species.
RUPPIACEAE Horan. ex Hutch.
DITCH-GRASS FAMILY
AA very small (ca. 10 species in a single genus—Haynes 2000c), nearly cosmopolitan family of
submersed, glabrous, aquatic herbs of brackish or saline conditions or of fresh waters with high
calcium or sulfur ion concentrations. The reduced flowers (perianth absent) and morphological
variation have made it difficult to delimit species within the group (e.g., some workers have recognized
only a single variable species). Ruppiaceae are unusual in lacking vessel elements
(Mabberley 1997). The family is similar to and has sometimes been included in the
Potamogetonaceae (e.g., Haynes 1978; Jacobs & Brock 1982; Dahlgren et al. 1985; Thorne 1993b),
and it has been suggested that the Ruppiaceae evolved from the Potamogetonaceae by reduction
(Heywood 1993). Molecular studies have suggested a closer relationship to the marine families
Cymodoceaceae and Posidoniaceae (Les et al. 1993; Les & Haynes 1995; Haynes et al. 1998b),
and we are following the recent treatment by Haynes (2000c) in segregating Ruppia into its
own family. (subclass Alismatidae—Cronquist; order Alismatales—APG II)
FAMILY RECOGNITION IN THE FIELD: submersed aquatic herbs of brackish, saline, or mineral waters,
with thread-like leaves 0.5 mm or less wide, flowers without perianth, and a terminal spike
that in fruit has an elongate, usually coiled peduncle.
REFERENCES: Haynes 1978; 2000c; Rico-Gray 1991; Haynes et al. 1998b.
RUPPIA L. DITCH-GRASS, WIDGEON-GRASS, RUPPIA
AOnly two species occur in North America, with both known from TX (one known only to
the s of East TX). The two are very similar and have been treated as variants of a single species
by some authorities (e.g., Fernald & Wiegand 1914). However, we are following Thorne (1993b)
and Haynes (2000c) in recognizing them at the specific level. Pollination takes place on the surface
of the water. After release, the boomerang-shaped pollen grains aggregate by adhering end
to end in chains or in snowflake-like groups and are rafted to the stigma, which is held at the
water surface due to the elongated peduncle and buoyancy provided by stomata and intercellular
spaces in the spongy tissue of the inflorescence (Haynes 1978; Dahlgren et al. 1985; Cox &
Knox 1989; Lacroix & Kemp 1997). This type of hydrophily (= water-mediated pollination) occurring
at the water surface is known as epihydrophily (in contrast to hypohydrophily or underwater
pollination as seen in Najas—Hydrocharitaceae) (Philbrick 1993). Some workers (e.g.,
Cox 1988) further divide epihydrophily into a category in which pollen is transported just
above the surface of the water (e.g., Hydrilla, Vallisneria) and a category in which pollen is transported
directly on the surface of the water (e.g., Elodea, Ruppia). Cox and Knox (1989) have indicated
that a variety of plants with pollen transported directly on the water surface (a two dimensional
system) show striking convergent evolution that can be defined as a “surface-pollination
syndrome”—floating pollen grains that join together in long floating rafts or “search vehicles”
whose large size greatly increases the probability of encountering a stigma, as well as surface
borne stigmas that are either elongate or that create a slight depression in the water surface (thus
maximizing the probability of being encountered). Ruppia species are considered to be among
the most valuable submersed aquatics as sources of food for waterfowl and other wildlife. They
provide food and cover for fish, all parts of the plants are eaten by many species of waterfowl, and
marsh and shorebirds eat the fruits and foliage (Correll & Correll 1972; Haynes 1978; Godfrey &
Wooten 1979). (Named for Heinrich Bernhard Ruppius, 1688–1719, German botanist)
REFERENCES: Fernald & Wiegand 1914; Gamerro 1968; Richardson 1980; Jacobs & Brock 1982;
Lacroix & Kemp 1997; Morgan & Holmes 2004.