1	Plants annual.		(10)
+	Plants perennial.		(2)
2 (1)	Ochreae membranous or somewhat hyaline, conspicuously nerved throughout or more than half of their length.		(3)
+	Ochreae hyaline, not conspicuously nerved or nerved at the base only.		(6)
3 (2)	Nerves of the ochreae thickened and exserted like 2 aristae.		1 P. biaristatum
+	Nerves of the ochreae though strong but not thickened and aristate, rarely exserted.		(4)
4 (3)	Ochreae longer or almost equalling the internodes, covering the internodes, with 12 strong nerves. Nuts 2.5-3.5 mm long.		4 P. maritimum
+	Ochreae shorter than the internodes, not covering the internodes, with 2 nerves only. Nuts 1.5-2 mm long.		(5)
5 (4)	Ochreae shorter than the internodes, not covering the internodes, with 2 nerves only. Nuts 1.5-2 mm long.		2 P. recumbens
+	Plants prostrate. Leaf blades linear, gland-dotted. Nuts dull and striate.		3 P. roylei
6 (2)	Leaves sessile, linear lanceolate.		(7)
+	Leaves petiolate, elliptic, oblong to oblong-spathulate, ovate.		5 P. cognatum
7 (6)	Plants erect. Rhizome distinctly many branched. Nuts ± symmetrical, regular, often trigonous, acute to pointed at the tip (leaves Thymus-like).		6 P. thymifolium
+	Plants prostrate to subprostrate. Rhizome unbranched. Nuts asymmetrical, distinctly curved, not pointed at the tip (leaves not Thymus-like).		(8)
8 (7)	Flowering branches numerous, filiform, ± ascending, with few leaves. Leaves 0.5 mm broad. Ochreae ± 3 mm long.		7 P. sarobiense
+	Flowering branches few, thick, with many leaves. Leaves 1-2 (-3) mm broad. Ochreae 5-15 mm long.		(9)
9 (8)	Leaves up to 10 (-12) mm long, ochrea always equalling or exceeding the leaves especially on young branches. Plants usually glabrous.		8 P. paronychioides
+	Leaves 10-25 mm long. Ochrea always shorter than the leaves. Plants papillose-puberulous.		9 P. afghanicum
10 (1)	Plants very small, stem dichotomously branched, up to 5 cm long. Perianth hyaline. Ochrea almost covering the flowers.		20 P. molliaeforme
+	Plants large. Stem >5 cm long not dichotomously branched. Perianth petaloid, white, green, roseate or red. Flowers not covered by ochreae.		(11)
11 (10)	Flower bearing branches either without leaves or bracteal leaves much smaller thus the floral branches conspicuously differentiated from the leafy vegetative branches.		(12)
+	Flowers bearing branches with normal leaves and the flowers ± disposed over the entire plant almost from the base.		(13)
12 (11)	Inflorescence compact. Floral branches almost naked. Tepals parted almost half the length. Nuts 1.5-2.0 mm long.		10 P. argyrocoleum
+	Inflorescence lax. Floral branches not completely naked. Tepals parted almost up to the base. Nuts 2-3 mm long.		11 P. patulum
13 (11)	Internodes ± covered by the ochreae and closely imbricated leaves.		(14)
+	Internodes not covered by the ochreae and closely imbricated leaves.		(16)
14 (13)	Plants mostly glabrous. Perianth subcampanulate, tube shorter than the lobes usually not fused with nuts. Nuts not shining.		(15)
+	Plants mostly puberulous. Perianth tubular, tube longer than the lobes, usually fused with nuts. Nuts shining.		19 P. rottboellioides
15 (14)	Perianth scarcely 1 mm long, exceeded by the mature nuts. Leaves membranous, oval.		17 P. cashmeriense
+	Perianth c. 2 mm long, equalling to mature achenes. Leaves coriaceous, linear to ovate.		18 P. polycnemoides
16 (13)	Plants heterophyllous. Cauline leaves larger, ramal leaves smaller.		(17)
+	Plants not heterophyllous. Cauline and ramal leaves ± equal size.		(18)
17 (16)	Nuts smooth. Stem and branches not striate.		14 P. olivascens
+	Nuts rugolose. Stem and branches mostly striate.		12 P. aviculare
18 (16)	Plants erect, ochrea distinctly nerved from the base to the apex.		11 P. patulum
+	Plants prostrate to procumbent, ochrea either not nerved or faintly nerved.		(19)
19 (18)	Ochrea deeply bifid. Nuts 2-2.5 mm long.		13 P. arenastarum
+	Ochrea undivided, mostly lacerate or toothed only at the apex. Nuts 1-1.25 (1.5) mm long.		(20)
20 (19)	Flowers distinctly pedicellate, exserted from the ochreae.All tepals almost equal in size.		15 P. effusum
+	Flowers sessile to subpedicellate, mostly sunken among the ochreae, rarely exserted. Outer tepals slightly longer.		16 P. plebejum