| 1 | Perennials. |
| 1' | Annuals. |
| 2 | Glumes glabrous or minutely hairy. Spikelets not in globose clusters, at most 6 mm long. |
| 2' | Glumes long-hairy. Spikelets in a dense, globose, terminal cluster, 1-2 smaller lateral clusters whether or not added. Spikelets large, (6-)8-9 mm long. Spp. 20-21 |
| 3 | Hypogynous disk well developed. |
| 3' | Hypogynous disk much reduced, obsolete. Nut-bearing (or all) spikelets bisexual. Nut trigonous, with 3 basal depressions. Spp. 18-19 |
| 4 | Nut-bearing spikelets bisexual (not always in S. cyathophora) Nut usually trigonous. Glumes shortly hairy. Spp. 1-5 |
| 4' | Nut-bearing spikelets usually unisexual, the ♂ part reduced to a sterile glume or sometimes to 1-2 flowers. Nut terete or obscurely trigonous. Glumes glabrous or minutely ciliolate. Spp. 6-17. |
| 5 | Inflorescence linear, spiciform, unbranched, without leafy bracts. Sp. 22. |
| 5' | Inflorescence otherwise, with leafy bracts. |
| 6 | Nut bearing spikelets with at least 3 glumes. Spp. 23-31 |
| 6' | Nut-bearing spikelets with 2 glumes. |
| 7 | Ripe nuts falling out of the glumes which are persistent on the rachilla. Sp. 32. |
| 7' | Ripe nuts closely enveloped by the glumes and falling with them. Spp. 33-34 |
