| 1 | Spikelets not squarrose. Perennials, or annuals with nuts longer than ½ mm. |
| 1' | Spikelets squarrose by the recurved mucros of the glumes. Small tufted annuals with nuts c. ½ mm long. Sp.21 |
| 2 | Involucral bracts several, foliaceous, flat. |
| 3 | Hypogynous bristles inconspicuous, shorter than the glumes, or absent. |
| 4 | Stems nodeless. Leaves basal. Sp. 1 |
| 4' | Stems noded, leafy. |
| 5 | Spikelets usually not numerous, large. Glumes 5-7 mm long. Spp. 2-3 |
| 5' | Spikelets very numerous in a decompound umbelliform inflorescence, small. Glumes much smaller. Sp. 4 |
| 5'' | Hypogynous bristles strongly elongating after anthesis, at maturity greatly exceeding the glumes, strongly curled and entangled. Sp. 5 |
| 5''' | Involucral bract non-foliaceous, single, either looking like a continuation of the stem, or glume-like, or absent. |
| 6 | Stems branched, procumbent, floating, or submerged. |
| 6' | Stems not branched, erect. |
| 7 | Hypogynous bristles absent. Spikelet several-flowered. Spp. 6-8 |
| 7' | Hypogynous bristles present. Spikelet 1(-2)-flowered. Sp. 9 |
| 8 | Inflorescence terminal, subtended by the outer caducous, often mucronate glume. Sp. 10. |
| 8' | Inflorescence pseudolateral, subtended by a persistent bract looking like a continuation of the stem. |
| 9 | Stems tall, usually stout. Bristles usually present. Nut (1-)1½-2½ mm long, smooth or transversely ridged. Spp. 11-17 |
| 9' | Stems low, setaceous. Bristles absent. Nut smooth, ¾-1½ mm long. Spp. 18-20 |
