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

674 HYDROCHARITACEAE/LIMNOBIUM
purchase, propagate, or possess this species in the state (Harvey 1998). It is also considered noxious
in AZ, CA, OR, and SC (Kartesz) and is a U.S. federal noxious weed (Kartesz 1999; USDA
Natural Resources Conservation Service 2002). The special hibernating or overwintering organs
(turions), which are short specialized shoots, can be of two types: axillary (cylindrical or
conical in shape) or subterranean (� boat-shaped) (Pieterse 1983). Hydrilla can easily be confused
with Egeria densa (see key to genera). � I
LIMNOBIUM Rich. FROG’S-BIT, AMERICAN FROG’S-BIT, SPONGEPLANT
ADepending on taxonomic authority, a genus of 2 species (Haynes 2000b; Haynes & Holm-
Nielsen 2001) or 1 very variable species composed of 2 subspecies (Lowden 1992). The genus
occurs from the e U.S. s to Argentina. Pollination is accomplished by pollen movement through
the air (nectar and scent are lacking). Pollen released from anthers in the male flowers falls onto
a pollen-collecting platform composed of the spreading sepals and petal bases; from there the
pollen can be blown or fall onto the female flowers which are often positioned below the male
flowers (Cook & Urmi-König 1983; Cook 1998a). (Greek: limnobios, living in pools)
REFERENCES: Wilder 1974a; Díaz-Miranda et al. 1981; Hunziker 1981; Catling & Dore 1982; Cook
& Urmi-König 1983; Lowden 1992; Les & Capers 1999.
Limnobium spongia (Bosc) Rich. ex Steud., (a sponge), COMMON FROG’S-BIT, AMERICAN FROG’S-BIT,
AMERICAN SPONGEPLANT. Free-floating or rooting, stoloniferous, perennial, aquatic herb, sometimes
forming mats; roots pendent, conspicuous; plants of two types: plantlets developing from
seeds or at the ends of runners produce floating rosettes of nearly reniform leaves with a characteristic
obvious central disk of purplish spongy tissue beneath; from these rosettes develop
more robust, flowering and fruiting plants with a short axis and erect to ascending, long-petioled
leaves (Godfrey & Wooten 1979); leaves mostly emersed or floating; leaf blades ovate to
suborbicular or reniform, 3–7(–10) cm long, 3–7(–8) cm wide, firm-textured, entire or nearly so,
basally cordate to rounded, apically obtuse or rounded, with 5–8 principal veins; petioles 1–15
(–40) cm long; flowers unisexual (plants monoecious), held above water on pedicels from
spathes that are sessile or on scapes shorter than the petioles; spathes composed of 1 or 2 free
bracts, usually with 1 or 2 flowers open at a time; pedicels 3–12 cm long; sepals 3(–4), greenish
purple; petals 3(–4), greenish white to yellowish, (4–)7.4–13.6(–14) mm long; stamens (3–)9–12
(–18), with filaments fused to form a column from which anthers diverge at different levels;
anthers ca. 2–4 mm long; staminodes 0–5, linear; styles usually 6–9, filiform, deeply 2-parted;
ovary inferior; fruit a berry, subglobose to globose, ca. 6–15 mm long, on a stout recurved
peduncle, developing below the water surface; seeds numerous, with erect, spike-like hairs.
Shallow, mostly stagnant, quiet water; Harrison (Caddo Lake), Liberty (BRIT), Jefferson, Marion,
Panola, Polk, Shelby, Wood (ASTC), and Leon (TAMU) cos. in the Pineywoods and e Post Oak
Savannah; also n Gulf Prairies and Marshes; e U.S. from MD s to FL w to MO and TX, also NY.
Jun–Oct. [Hydrocharis cordifolia Nutt., Hydrocharis spongia Bosc] This species is sometimes cultivated
as an ornamental. The root hairs are used in the laboratory (under high magnification)
to demonstrate cytoplasmic streaming (Mabberley 1997). Where common, the fruits of this species
can be an important food for ducks (Les & Capers 1999).
NAJAS L. NAIAD, WATER-NYMPH, BUSHY-PONDWEED
Submersed, monoecious or dioecious, aquatic herbs; leaves subopposite, opposite, or crowded
and appearing whorled but inserted at barely different levels, linear or linear-lanceolate, 3 mm
wide or less, with sheathing bases; flowers axillary, few, without a perianth; staminate flowers
with 1 stamen; pistillate flowers sessile, 1-carpellate; stigmas 2–4; fruit a 1-seeded nutlet enclosed
in a loose coat.
AA cosmopolitan genus of 40 species (Haynes 2000d). The stems have reduced xylem which
lacks vessels. Some are serious weeds in rice fields, but make good green fertilizer. Najas species