Callitriche chathamensis

Callitriche chathamensis has stems with axillary scales and leaf bases that are fused. Its lingulate leaves are slightly wider at the apex, measuring 1.0–1.4 mm in length and 8.0–9.7 mm in width. The expanded submerged or floating leaves are ovate-elliptic, 2.2–4.1 mm long and 0.8–2.5 mm wide, with complex venation. The petiole is 1.6–4.5 mm long. The species is monoecious, with solitary flowers alternating between female and male along the stem. Styles are erect, 1.1–2.3 mm long, and the filaments are 4.5–6.8 mm long with quadrilocular anthers. The fruit is subsessile, pale brown when mature, 0.6–0.9 mm long and 0.7–1.1 mm wide. It can be distinguished from other Callitriche species in the region by its pale, unwinged fruit that is wider than high, the absence of bracts, and the presence of both male and female flowers on the same plant.