1	Fruiting perianth fleshy and accrescent.		1 P. chinense
+	Fruiting perianth neither fleshy nor accrescent.		(2)
2 (1)	Racemes spike-like, cylindrical to filiform, sometimes branched. Perianth segments 4-5-partite. Stamens alternating with glands. Pollen grains tri to pantoporate.		(3)
+	Racemes ovoid or subglobose, capitate. Perianth segments 4-5-lobed. Glands usually absent or obscure. Pollen grains 3-4-colpate.		(19)
3 (2)	Ochreae thick, with a herbaceous limb at the top, when young.		2 P. orientalis
+	Ochrea membranous without a herbaceous limb.		(4)
4 (3)	Ochrea usually without cilia at the mouth rarely very finely ciliate in some forms of P. lapathifolia.		(5)
+	Ochrea distinctly ciliate at the mouth.		(8)
5 (4)	Amphibious, perennials. Rhizome extensively creeping. Areal leaves usually smaller. Spikes densely conical, ovoid or shortly cylindrical.		3 P. amphibia
+	Plants terrestrial, usually annual. Leaves of one type. Spikes in axillary panicles, slender and laxly flowered.		(6)
6 (5)	Plants entirely glabrous.		5 P. glabra
+	Plants ± pubescent, scabrid, tomentose.		(7)
7 (6)	Peduncles eglandular. Leaves with white cottony tomentum on both surfaces but more below, without yellow glands. Stem without red dots.		6 P. lanata
+	Peduncles scurfily glandular. Leaves either glabrous or ± tomentose below only, usually with yellow glands. Stem usually with red dots.		4 P. lapathifolia
8 (4)	Styles 2. Nuts biconvex, rarely few nuts trigonous within the same spike.		(9)
+	Styles 3. Nuts trigonous.		(11)
9 (8)	Leaves often with a dark blotch near the middle above, without minute glands beneath. Spikes dense and compact not interrupted.		7 P. maculosa
+	Leaves without a blotch, bearing minute glands below. Spikes lax and slender, at least interrupted below.		(10)
10 (9)	Cilia much shorter than the ochreal tube. Spikes not drooping. Stamens 5. Nuts c. 1.5 mm.		11 P. tenella
+	Cilia as long as the ochreal tube. Spikes often drooping. Stamens 8. Nuts 1.5-3.0 mm.		10 P. mitis
11 (8)	Perianth and ochreolae gland-dotted.		(12)
+	Perianth and ochreolae eglandular.		(13)
12 (11)	Cilia of the ochreal tube 1-4 mm long. Spike 3-7 cm long, not drooping. Stamens 6.		8 P. hydropiper
+	Cilia of the ochreal tube 8-10 mm long. Spikes up to 15 cm long, drooping. Stamens 8.		9 P. pubescens
13 (11)	Cilia of the ochreal tube much shorter than the tube. Leaves strigosely hairy on both surfaces.		12 P. stagnina
+	Cilia of the ochreal tube either almost equalling or longer than the tube. Leaves either glabrous or if pubescent then only on the margins and midrib beneath.		(14)
14 (13)	Spikes lax and slender often interrupted, each flower distinctly visible.		(15)
+	Spikes compact, dense, not interrupted each flower not distinctly visible.		13 P. barbata
15 (14)	Leaves eglandular, sometimes whitish-dotted but not obviously glandular.		(16)
+	Leaves gland-dotted or punctate at least beneath.		(17)
16 (15)	Leaves not punctate spikes often drooping. Nuts mostly biconvex, rarely trigonous within the same spike.		10 P. mitis
+	Leaves punctate. Spikes erect. Nuts always trigonous.		16 P. punctata
17 (15)	Leaves with acute base. Inflorescence filiform up to 10 cm long.		14 P. posumbu
+	Leaves with rounded or cordate base. Inflorescence up to 4 cm long.		15 P. longiseta
18	Leaves runcinate pinnatifid; petiole auriculate.		17 P. sinuata
+	Leaves entire.		(19)
19 (18)	Plants perennial.		21 P. capitata
+	Plants annual.		(20)
20 (19)	Floral head usually with an involucral bract or leaf.		18 P. nepalensis
+	Floral head without bract.		(21)
21 (20)	Leaves sessile or nearly so. Nuts densely pitted.		20 P. humilis
+	Leaves distinctly petiolate. Nuts not pitted.		19 P. glaciale