Cabomba caroliniana A. Gray

Carolina Water Shield, Fanwort

Cabomba caroliniana plant

Family - Cabombaceae

Habit - Perennial, emergent aquatic forb.

Cabomba_caroliniana_plant2Plant.

Stem - Mostly submerged, inconspicuously hairy when young.

Cabomba_caroliniana_stemStem.

Leaves - Strongly dimorphic, with emergent leaves only present at flowering. Submerged leaves opposite, petiolate, to 4 cm long and broad, fan-shaped, palmately divided into 5-7 parts, each of these further divided into narrow, threadlike segments. Emergent leaves alternate, long-petiolate, peltate, with blades to 2 cm long, entire, roughly oblong but often shallowly notched.

Cabomba_caroliniana_leaves2Submerged leaves.

Cabomba_caroliniana_leaves

Cabomba_caroliniana_leaves3Emergent leaves.

Flowers - Stalked, held just above the water, the stalk bending to become submerged as the fruits develop. Sepals 3, 5-10 mm long, petaloid, elliptic to obovate, white. Petals 3, 5-10 mm long, elliptic to obovate, sagittate toward the base and thus appearing short-stalked, white with the basal auricles each with a yellow nectary. Stamens 6. Pistils 2-4, the stigma capitate.

Cabomba_caroliniana_flower2Flower.

Cabomba_caroliniana_flower

Cabomba_caroliniana_flower3

Fruits - Achenelike, 4-7 mm long, narrowly ovoid to somewhat peanut-shaped, tapered to the persistent style. Seeds 1.5-3.0 mm long, oblong-ovate in outline, the surface finely pebbled, with 4 longitudinal rows of dense tubercles visible when moistened, greenish brown to brown.

Flowering - May - September.

Habitat - Submerged aquatic in ponds, sloughs, swamps, ditches.

Origin - Native to the U.S.

Lookalikes - Ceratophyllum spp., Ranunculus aquatilis

Other info. - This attractive but easily overlooked aquatic species can produce large numbers of small white flowers at the surface of still waters, while most of the plant remains submerged. The plant is easily identified by its flowers and the dimorphic leaves. The sepals and petals are similar, collectively forming a six parted perianth. The plant is not common in Missouri, with most collections coming from the Bootheel area. Its U.S. distribution is mainly southeastern. The plant's foliage could be confused with species of Ceratophyllum or Ranunculus aquatilis, which also have dissected, fan-shaped leaves. However, the leaf arrangement on these latter species is alternate.

C. caroliniana provides good cover for fish and other aquatic invertebrates, and is commonly grown as an ornamental for aquarium use. This use has led to its accidental escape and introduction at some sites beyond its natural range. The flowers are pollinated by flies, and show an interesting staged behavior. Individual flowers are only open for two days. On the first day, stigmas are receptive within flowers which are open in mid-morning. In the late afternoon, the flowers close and the stalks bend downward, submerging the flower. On the second day, the flowers are once again held erect, and at that time shed pollen, with the stigmas no longer being receptive. The flowers then re-submerge and the fruits develop underwater.

Missouri populations of this species are assignable to var. caroliniana.

Photographs taken at Duck Creek Conservation Area, 8-26-2015 (SRTurner).



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