Inflorescence a pseudolateral sessile head of 1–several (occasionally numerous) spikelets with numerous flowers. Involucral bracts 0–2 exceeding inflorescence, usually lowest erect and leaf-like, often falling early. Glumes spirally arranged, mostly folded laterally when immature but spreading almost flat at maturity, rarely distichously arranged and then spikelet is flattened (I. levynsiana). Hypogynous bristles absent. Stamens 1–3. Style 2- or 3-fid, not thickened at base, deciduous.
Nut trigonous or lenticular or rarely subglobose, smooth or trabeculate or verruculose but not rugose.
| | Key to the species | |
| 1 | Style 2-fid | 2 |
| Style 3-fid | 6 |
| 2 | Culms weak, floating or creeping, usually long and leafy (at least in water); lowest involucral bract glume-like or with a short thickened appendage, usually shorter than inflorescence; spikelet always 1 | 3 |
| Culms usually erect, to 45 cm high; involucral bract solitary, leaf-like, usually as long as or exceeding the inflorescence, to as long as the culm; spikelets 1–15
Back to 1 | 5 |
| 3 | Spikelets ovate to broad-elliptic in outline, only slightly flattened, c. 15-flowered, 5–8 mm long | Isolepis crassiuscula |
| Spikelets more or less elliptic in outline, slender, strongly flattened, 5–10(–15)-flowered, 3–5 mm long
Back to 2 | 4 |
| 4 | Glumes 2.5–3.3 mm long, usually dark red-brown to blackish; stamens 3; anthers 1.5–2.7 mm long; nut obovoid with angles slightly ribbed | Isolepis producta |
| Glumes 1.7–2.8 mm long, occasionally with red-brown patches; stamens 2(–3); anthers 0.5–1.0 mm long; nut broad-obovoid to ellipsoid, angles not ribbed
Back to 3 | Isolepis fluitans |
| 5 | Spikelets usually 3–15 per inflorescence, often proliferating; stamens 1 or rarely 2; nut 0.7–1 mm long | Isolepis inundata |
| Spikelet 1, not proliferating; stamens 2 or rarely 3; nut 1.4–1.8 mm long
Back to 2 | Isolepis fluitans |
| 6 | Plant with a long-creeping, thick, woody rhizome to 10 mm diam. (formerly I. nodosa) | Ficinia nodosa |
| Plants tufted or with a filiform rhizome
Back to 1 | 7 |
| 7 | Nut longitudinally striate and transversely trabeculate | Isolepis hookeriana |
| Nut surface smooth or minutely reticulate, punctulate or verruculose
Back to 6 | 8 |
| 8 | Slender perennial with numerous spikelets clustered in a more or less dense head, usually proliferating; culms 1–3 mm diam | Isolepis prolifera |
| Small perennials or annuals with 1–15 spikelets per inflorescence, sometimes proliferating; culms mostly filiform, usually less than 1 mm diam
Back to 7 | 9 |
| 9 | Glumes arranged distichously (in two rows) so spikelet is laterally flat | Isolepis levynsiana |
| Glumes arranged spirally so spikelet is clearly 3-dimensional, more or less terete
Back to 8 | 10 |
| 10 | Spikelets squarrose; glumes with long mucro about as long as the body of the glume; nut about a third as long as glume body | Isolepis hystrix |
| Spikelets not squarrose; glumes not long-mucronate, occasionally short-mucronulate; nut mostly at least half as long as glume (except in Isolepis congrua and I. victoriensis)
Back to 9 | 11 |
| 11 | Stamens 3 | 12 |
| Stamens 1 or 2
Back to 10 | 20 |
| 12 | Nut less than 0.7 mm long | 13 |
| Nut 0.7–1.5 mm long
Back to 11 | 14 |
| 13 | Nut dark brown to black; glumes 1–1.5 mm long, with sides irregularly but conspicuously 2–5-nerved | Isolepis platycarpa |
| Nut straw-coloured to dark red-brown; glumes 0.8–1.2 mm long, with sides nerveless or rarely with 1 nerve at edge of midrib
Back to 12 | Isolepis australiensis |
| 14 | Plant with leaves to twice as long as culms and involucral bracts same length as culms | Isolepis montivaga |
| Plants with leaves and involucral bracts shorter than culms
Back to 12 | 15 |
| 15 | Glumes obtuse | 16 |
| Glumes acute
Back to 14 | 18 |
| 16 | Nut with angles not well-defined, dark brown to black, 0.5–0.8 mm long; tufted annual or ?short-lived perennial | Isolepis platycarpa |
| Nut with angles well-defined, pale yellow to dark grey-brown, 0.7–1.1 mm long; perennials, mostly shortly rhizomatous
Back to 15 | 17 |
| 17 | Nut yellow-brown to dark grey-brown, half to two-thirds as long as glume; glumes mostly straw-coloured to orange-brown or red-brown | Isolepis cernua |
| Nut pale yellow to pale red-brown, about seven-eighths as long as glume; glumes straw-coloured tinged dark red-brown to mostly dark red-brown
Back to 16 | Isolepis subtilissima |
| 18 | Glumes with sides strongly 4–6-nerved, shining; tufted annual | Ficinia marginata |
| Glumes with sides irregularly and finely 2–4-nerved, dull; usually rhizomatous perennials
Back to 15 | 19 |
| 19 | Nut unequally trigonous, remaining on bare axis after glumes and involucral bracts fall | Isolepis aucklandica |
| Nut equally trigonous, not remaining on bare axis
Back to 18 | Isolepis habra |
| 20 | Nut 0.4–0.6 mm long | 21 |
| Nut 0.7–1.5 mm long
Back to 11 | 25 |
| 21 | Glumes with 3 or 4 nerves spread over each side | 22 |
| Glumes with sides markedly hyaline, with 1 nerve or none on each side close to midrib
Back to 20 | 23 |
| 22 | Glumes 1.2–2 mm long; spikelets 2–7 per inflorescence, 3.5–6 mm long; involucral bract 1–6 cm long (often as long as culm); nut straw-coloured | Isolepis gaudichaudiana |
| Glumes 0.5–1 mm long; spikelets 1–3 per inflorescence, 1.5–3 mm long; involucral bract less than 1 cm long; nut dark brown to black
Back to 21 | Isolepis sepulcralis |
| 23 | Hyaline sides of glumes 0.4–0.6 mm wide, with the cell outlines clearly visible at 15–20 times magnification (cells usually c. 0.1 mm long, 0.05 mm wide); glumes 1.5–2 mm long (rarely as short as 1.3 mm); nut broad-ellipsoid to broad-obovoid, with the sides more or less concave, the angles acute, prominent; leaf blades often 4–9 cm long | Isolepis congrua |
| Hyaline sides of glumes 0.2–0.3(–0.4) mm wide, with inconspicuous cell outlines; glumes 0.8–1.6 mm long; nut obovoid to broad-obovoid, with the sides convex or flat, the angles obtuse; leaf blades usually less than 3 cm long
Back to 21 | 24 |
| 24 | Glumes usually hyaline tinged dark red-brown, 0.8–1.2 mm long, nerveless or rarely with 1 nerve close to the midrib on each side; nut half to two-thirds as long as glume, pale brown to dark red-brown | Isolepis australiensis |
| Glumes usually hyaline, rarely with red-brown patches, 1.1–1.6 mm long, with 1 nerve at the midrib margin on each side; nut about a third as long as glume, white with underlying red-brown to blackish at maturity
Back to 23 | Isolepis victoriensis |
| 25 | Plant with leaves to twice as long as culms and involucral bracts 1–6 cm long, mostly as long as culms | 26 |
| Plants with leaves and involucral bracts shorter than the culms (bracts to 2 cm long)
Back to 20 | 27 |
| 26 | Nut 0.6–0.8 mm long, more or less equally trigonous, with angles well-defined, straw-coloured, minutely punctulate, glistening; spikelets 2–7 per inflorescence | Isolepis gaudichaudiana |
| Nut 0.9–1.3 mm long, unequally trigonous, with angles not well-defined, straw-coloured to grey-brown, smooth with a minutely reticulate pattern under the surface, shining; spikelets 1–3 per inflorescence
Back to 25 | Isolepis montivaga |
| 27 | Glumes with the broad midrib produced in a relatively stout excurved or straight mucro | 28 |
| Glumes mucronulate or with the midrib not produced at all
Back to 25 | 29 |
| 28 | Hyaline sides of glumes 0.2–0.3(–0.4) mm wide, with inconspicuous cell outlines, 1–3-nerved; nut unequally trigonous | Isolepis stellata |
| Hyaline sides of glumes 0.4–0.6 mm wide, with the cell outlines clearly visible at 15–20 times magnification, nerveless or with 1 nerve close to midrib; nut equally trigonous
Back to 27 | Isolepis congrua |
| 29 | Glumes straw-coloured to orange-brown or red-brown; nut yellow-brown to dark grey-brown; inflorescence not proliferating | Isolepis cernua |
| Glumes straw-coloured with dark red-brown patches to uniformly dark red-brown; nut pale yellow to pale red-brown; inflorescence often proliferating
Back to 27 | 30 |
| 30 | Spikelets usually 3–15 per inflorescence; nut with third angle not well-defined; inflorescence often proliferating | Isolepis inundata |
| Spikelets 1–3 per inflorescence; nut with all three angles well-defined; inflorescence occasionally proliferating
Back to 29 | 31 |
| 31 | Nut unequally trigonous, 0.7–1 mm long, 0.5–0.7 mm diam., about seven eighths as long as glume; glumes 0.8–1.3 mm long; culms to 10 cm high | Isolepis subtilissima |
| Nut equally trigonous, 0.8–1.4 mm long, 0.6–0.8 mm diam., about three-quarters as long as glume; glumes 1.3–1.8 mm long; culms 7–30 cm high
Back to 30 | Isolepis habra |
