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

472 ARACEAE/XANTHOSOMA
PISTIA L. WATER-LETTUCE, SHELLFLOWER, WATER-BONNET
AA monotypic genus of tropical and subtropical regions worldwide. The leaves are nearly
horizontal during the day, but move to a more vertical position at night. The leaf hairs create a
water-repellent surface. This aquatic genus seems to be an evolutionary link between the
Araceae and the Lemnaceae (DUCKWEED family). Kvacek (1995) suggested that the fossil genus
Limnobiophyllum is a fossil link between Araceae and Lemnaceae, and Stockey et al. (1997), using
a cladistic approach and material of Limnobiophyllum, concluded that Pistia plus
Limnobiophylum and Lemnaceae form a monophyletic group, and that the Lemnaceae “is a
monophyletic group imbedded within the Araceae.” The Araceae thus appears paraphyletic
when the Lemnaceae is removed. Stockey et al. (1997) suggested that if Lemnaceae is recognized
as a distinct family, Pistia should be “regarded as the basal genus in the family Lemnaceae,
rather than a genus of the Araceae.” Pistia is cultivated as an aquatic ornamental. (Greek: pistos,
water or liquid, or pistra, water trough, referring to its aquatic habitat)
REFERENCES: Kvacek 1995; Stoddard 1989; Stockey et al. 1997; Lemon & Posluszny 2000a.
Pistia stratiotes L., (soldier), WATER-LETTUCE, SHELLFLOWER, WATER-BONNET. Monoecious freefloating
herb; roots long, feathery, hanging; leaves clustered in rosettes, entire, gray-green, velvety
hairy, strongly ribbed lengthwise, cuneate to obovate-cuneate, to ca. 3–15(–20) cm long,
truncate to emarginate at apex; spathes axillary, inconspicuous, ca. 15 mm long; spadix with
staminate flowers in a whorl above, the solitary pistillate flower below; perianth absent; fruits
brownish. Streams, lakes, and ponds; San Jacinto (BRIT), Jefferson (TAES), Colorado, Comal, and
Hays (Turner et al. 2003) cos. near s margin of East TX; also apparently spreading from cultivation
in creeks in Tarrant Co. (at the Fort Worth Botanic Garden) in the Cross Timbers and Prairies
and in the Gulf Prairies and Marshes and Edwards Plateau; sporadically scattered across s 1/3
of the U.S. from NC s to FL w to CA, also NY (Kartesz 1999). The species does not tolerate salinity
or frost (Stoddard 1989), and Thompson (2000b) indicated that the populations in FL, LA,
and TX are probably the only ones that persist. The species is widespread in the tropics and
subtropics, and while there is disagreement as to whether Pistia is native to the U.S., “available
evidence suggests it is indigenous” (Thompson 2000b). Spring. This species can become a pest
in some regions by occupying areas of open water, and in TX it is considered a “harmful or potentially
harmful exotic plant”; it is illegal to release, import, sell, purchase, propagate, or possess
this species in TX (Harvey 1998). In some areas (e.g., the Nile River system in Africa) the
species is responsible for sudd formation (“sudd” is Arabic for obstruction); this term refers to
large mats of floating vegetation which impede river navigation (Bown 2000). Since its nativity
is in question and it is illegal to possess it in TX, we are denoting it symbolically as a potentially
harmful exotic. Despite being called WATER-LETTUCE, this species contains oxalates and possibly
other toxins; intense irritation of the mouth, throat, and upper digestive tract from eating
even small amounts has been reported (Morton 1982). � � m/293
XANTHOSOMA Schott YAUTIA, ELEPHANT’S-EAR, TAHITIAN-SPINACH
AA tropical American genus of 57 species of herbs with milky sap and sagittate or hastate
leaf blades; some are cultivated as ornamentals and for food. � Nearly all parts of a number of
species are injurious due to the presence of calcium oxalate crystals and possibly a toxic alkaloid;
ingestion can cause burning, swelling, and blistering of the mouth, throat, and digestive
tract. Fatalities have been reported; even tasting small amounts can cause serious reactions
(Morton 1982; Lampe & McCann 1985). (Greek: xanthos, yellow, and soma, body, alluding to the
yellow inner tissues of some species)
REFERENCES: Lemke & Schneider 1988; Loh et al. 2000; Serviss et al. 2000.
Xanthosoma sagittifolium (L.) Schott, (arrow-leaved), ELEPHANT’S-EAR, ARROW-LEAF ELEPHANT’S-
EAR, YAUTIA, TANNIA, TANIA, COCO-YAM. Large perennial herb from a tuberous corm; well-devel-