Marshweeds
Marshweeds are annual or perennial herbs. Plants of the genus vary in form, from erect to prostrate, and with branching or unbranched stems. Submerged leaves are whorled; aerial leaves are whorled or oppositely arranged. The leaves are lance-shaped or pinnate, and the blades have smooth or serrated edges. The sepals are arranged in a tubular calyx, and the corolla is tubular or funnel-shaped. The corolla has a lower lip with three lobes and an upper lip that is unlobed or double-lobed. There are about 40 species in the genus. It is distributed in tropical and subtropical regions of Africa, Asia, Australia, and the Pacific Islands.