Sagittaria platyphylla (Engelm.) J.G. Sm.

Arrowhead

© SRTurner

Family - Alismataceae

Habit - Perennial emergent aquatic forb, monoecious, with rhizomes and corms.

Stems - Not present.

	Vegetative plants.
© SRTurner

Leaves - Basal, simple, long-petiolate, 20-45 cm long, the petioles rounded to somewhat flattened or trigonal in cross section, the blades lanceolate to ovate, entire, glabrous. Submerged leaves uncommonly produced, linear.

	Leaf adaxial.
© SRTurner

	Leaf abaxial.
© SRTurner

Inflorescences - Unbranched racemes with whorled flowers, ascending to erect. Bracts at nodes of the inflorescence 3-7 mm long, fused in the basal half, the free portions triangular, obtuse. Lower 1-4 whorls of flowers pistillate, with reflexed, thickened pedicels 14-30 mm long.

	Bracts and sepals.
© DETenaglia

Flowers - Actinomorphic, unisexual. Sepals 3, green. Petals 3, white, dropping soon after flowers open. Stamens and pistils numerous. Pistils separate, in a dense, headlike cluster on an expanded receptacle. Style 1 per pistil, the stigma decurrent. Ovaries superior, with 1 ovule.

	Staminate flowers.
© SRTurner

	Staminate flowers.
© DETenaglia

	Pistillate flowers.
© SRTurner

Fruits - Fruiting pedicels stout, usually reflexed. Achenes obovate, 1.4-2.0 mm long, the beak 0.3-0.6 mm long, slender, curved.

	Fruits.
© SRTurner

Flowering - May - August.

Habitat - Mud flats, sloughs, wet ditches.

Origin - Native to the U.S.

Lookalikes - Other members of the genus, especially S. graminea.

Other info. - This is one of the rarer species of arrowhead in Missouri, where it is mostly restricted to a few counties in the Bootheel region. Its scarcity in the state makes it a species of conservation concern, with a state ranking of S1 (critically imperiled). The bulk of its U.S. distribution is within four states: Arkansas, Louisiana, Texas, and Oklahoma, with additional small disjunct populations elsewhere in the Southeast.

Differentiation of this species from the closely related S. graminea is difficult and problematic. In both of these species, the leaves lack the basal lobes and sagittate shape associated with the "arrowhead" common name. In S. platyphylla the fruit cluster stalks tend to deflex (droop), whereas in S. graminea they are usually spreading. The shape of the inflorescence bracts has been used to distinguish between the two species, but these are easily torn or distorted and may thus be misleading. More study is needed to establish reliable differentiating characters, and whether the two species are truly distinct.

Photographs taken just east of Otter Slough Conservation Area, Stoddard County, MO, 7-31-2015 and 6-26-2019 (SRTurner).