Selaginella erythropus

Selaginella erythropus is a spikemoss from the wet lowland forests of Costa Rica and tropical South America, where it grows along riverbanks and in thickets beneath dense forest. Each upright red stem branches into a flat, fern-like frond that is dark green on top and ruby red underneath. It is a lycophyte — one of the oldest surviving lineages of vascular plants — rather than a true moss, and in a closed terrarium it usually stays 5–15 cm tall, spreading slowly from underground rhizomes. It does best in shade to bright indirect light with constantly moist substrate and humidity of 70% or more; the fronds crisp quickly if the substrate dries out, and strong light can bleach their colour.