Flower:

Flowers are globe or upright bell shaped, 1/8 to 1/6 inch long and stalkless, paired in opposite leaf axils, greenish to often pinkish or even deep red in color. The base of the flower is a thickened cup surrounding the ovary call a hypanthium, the outer calyx with four short triangular lobes (sepals) folded in over the top of the floral tube. Between the sepals is a similar sized and shaped appendage that folds outward, creating a blocky or toothy appearing rim around the top of the flower. Within the floral opening are four stamens and a single central style, mostly indiscernible without a hand lens peering straight into the flower top. Petals are very minute, white to pink and withering within the same day they emerged.

Leaves and stems:

Leaves are simple and opposite, the pairs set at right angles to the ones above and below, linear to oblong, lance elliptic, ½ to 1 inch long and around ¼ inch wide, bright green, toothless with smooth surfaces, the central vein often bright red, and tapering to a point at the base. Stems are erect, often heavily branched, the branches ascending, smooth and weakly 4 angled, also often pinkish to bright red.

Fruit:

Fruit is a round capsule around 1/8 inch in diameter with four chambers.

Notes:

Toothcup enjoys a fairly broad range throughout North America down through Mexico and into Central America, even becoming invasive after introduction into Taiwan. Suggesting a more tropical origin, it should not be surprising it is typically rare throughout the northern portions of its range. In Minnesota it is a state listed Threatened species and, according to the DNR, its habitat is restricted to sandy margins of small ponds and lakes in the Anoka Sandplain, much of it already destroyed or under threat of destruction by development. While somewhat unique in form and habit, it is not without similar species with which it could be confused. Most notably is Valley Redstem (Ammannia coccinea), which is in the same family. Valley Redstem's flowers occur in tight clusters of up to 5 per leaf axil and have larger pink petals, though just as fleeting, and its leaves are more lance linear and lobed to heart-shaped at the base. Its preferred habitats are the the muddy flats and river banks of Minnesota's prairie rivers and their tributaries, most notably the Minnesota and Red rivers. Another species with marked similarities that can be found growing side by side with Rotala is Water Purslane (Ludwigia palustris), which is more spreading and creeping, frequently rooting down at the nodes. Its leaf blades are more oval then abruptly taper into a long stalk, making them more spoon-shaped, and its flowers have just the four sepal lobes flaring out with no appendages between them.