Montana Field Guide

Plants thallose, simple, with few advancements (rhizoids and spores) beyond algae (Bland 1971). Thallus 4-10 mm in length and 4-9 mm in across (Doyle 2006), brilliant green, composed of shallowly-forked heart-shaped lobes marked with a purple central line, forming rosettes, free-floating on water (Nicholson 1966, Schofield 2002). Air chambers on dorsal surface form hexagonal pattern perceptible to unaided eye; a pore in the center of each may appear as a dot (hand lens helpful). Numerous long, violet-colored scales, spread throughout on the ventral surface, protrude downward and provide equilibrium in the water (Nicholson 1966, Schofield 1985, 2002). Plants marooned on soil are greyish to yellow-green, possess a central groove on upper surface, and maintain violet or hyaline scales below (often hidden among the abundant rhizoids) (Doyle 2006).

Riccia cavernosa, which may also be found on soil at the water’s edge, resembles the terrestrial form of Ricciocarpos natans in size. However, the former’s thallus is a very light green. The thallus of R. natans is a dark green and has a purple line bisecting the lobes (Schofield 2002).

Quiet or sheltered and slow-moving waters such as ponds, ditches, backwaters, and lake coves (Nicholson 1966, Schofield 2002), often among aquatic vascular plants (e.g. Azolla, Lemna, Wolffia) where R. natans is difficult to notice (Doyle 2006). Following its deposition with the retreat of water, R. natans is adapted to grow on muddy shores (Schofield 2002).

Broken-off thallus lobe pieces that embed sporangia may provide primary means of reproduction following decomposition of the thallus. Sporophytes seldom produced (Schofield 2002).

Grows well in waters polluted with organic compounds. Large groups of these plants may drift on the water’s surface (Schofield 1985).