Lycopodiella diffusa (R.Br.) B.Øllg.

Horizontal stems dichotomously branching, subterranean, 0.5–2.0 mm diameter, bearing roots at intervals; leaves widely spaced, spreading, narrowly ovate, adnate. Aerial stems prostrate or curving upwards at branch apices, dichotomously branching 1–5 times, glabrous; longest fertile aerial branching system 25–170 mm long, 20–250 mm wide. Sterile leaves monomorphic, spirally arranged, similar throughout, spreading or incurved, linear to narrowly ovate, apices acuminate, margins entire or minutely ciliate at apex, bases adnate, 2.5–6.0 mm long, 0.5–1.1 mm wide, dull green or tinged red-brown or purple-brown. Sporophylls appressed when young, spreading at maturity, ovate to broadly ovate, apices acuminate, margins ciliate, bases adnate, 2.2–3.5 mm long, 1.4–2 mm wide, aggregated into strobili. Strobili solitary, lateral or terminal on aerial stems, erect, sessile or on short leafy lateral branches up to 7 mm long, yellow-brown when young, chocolate-brown at maturity, 4–24 mm long, 1.5–6 mm wide.

Lycopodiella diffusa is distinguished from all other species of Lycopodiaceae in New Zealand, except L. lateralis, by its strobili usually being borne laterally on the aerial stems, and characteristically chocolate-brown at maturity. The aerial stems grow prostrate with only the ends of the branches upturned, branching dichotomously up to five times. In L. lateralis the aerial stems are held erect or scramble upwards, and are generally much longer than in L. diffusa (50–600 mm, cf. 25–170 mm tall). The leaves of L. diffusa are generally incurved, and slightly shorter and wider than in L. lateralis (2.5–6.0 mm long, 0.5–1.1 mm wide; cf. 3.5–9.0 mm long, 0.3–0.9 mm wide).

Lycopodiella diffusa forms prostrate mats on pākihi or sandy soils, in mossy turf, on boggy ground, at the edge of tarns and on granite pavement, often with Juncus and Empodisma spp., or under Gleichenia, low mānuka, kānuka, epacrid or subalpine scrub.

Burnard et al. (2016) detected several haplotypes in Lycopodiella diffusa and L. lateralis, but they did not correspond to the two different morphologies. Three samples with L. lateralis morphology shared the haplotype found in L. diffusa. Whether this is evidence of hybridisation between the two taxa or a result of ancestral polymorphism is unclear. Their status needs further investigation.