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Annual or perennial herbs, stem erect, prostrate, procumbent or scandent, sometimes with recurved prickles. Leaves petiolate, mostly lanceolate, often ovate, elliptic, hastate or sagittate. Ocrea tubular, often with foliaceous limb. Inflorescence usually spiciform branched raceme. Tepals 4-5. Stamens 5-8. Carpels 2 or 3, syncarpous. Nut biconvex, biconcave or trigonous, included in persistent perianth.

Key to the species

Serial Key Reference
1a. Stems and leaves with recurved prickles 2
1b. Stems and leaves without prickles 3
2a. Stamens 6-8; lower surface of lamina copper-coloured; peduncles not dichotomously branched; tepals reddish P. strigosa
2b. Stamens not more than 5; both the surface of lamina green; peduncles dichotomously branched; tepals not reddish P. praetermissa
3a. Tepals usually 4 4
3b. Tepals usually 5 6
4a. Plants aquatic or semi-aquatic; stem and leaves covered with white hairs P. lanata
4b. Plants terrestrial or marshy; stem and leaves not covered with white hairs 5
5a. Creeping herbs; stamens 5 P. assamica
5b. Erect herbs; stamens 6 P. lapathifolia
6a. Ochrea with a spreading herbaceous limb at apex 7
6b. Ochrea without spreading herbaceous limb at apex 8
7a. Leaves ovate; plants heterostylous; peduncles densely hairy P. orientalis
7b. Leaves elliptic-lanceolate; plants not heterostylous P. limbata
8a. Carpels 2 9
8b. Carpels 3 13
9a. Stamens 7 10
9b. Stamens not more than 6 11
10a. Aquatic herbs; petiole and mid-rib densely tomentose; ochrea strigose-tomentose, margin ciliate P. lapathifolia
10b. Terrestrial herbs; petiole and mid-rib glabrous; ochrea glabrous, margin eciliate P. glabra
11a. Stamens 6 P. hydropiper
11b. Stamens 5 12
12a. Flowers white; leaves petiolate; raceme dichotomously branched P. dichotoma
12b. Flowers usually red; leaves subsessile; raceme not dichotomously branched P. minor
13a. Flowers pink or crimson-red 14
13b. Flowers white 15
14a. Whole plant covered with glandular hairs; perianth crimson-red, eglandular; inflorescence unbranched raceme P. viscosa
14b. Plant glabrous to pubescent; perianth bright pink, glandular; inflorescence branched raceme P. flaccida
15a. Cilia of the ochreal tube much shorter than the tube; leaves strigosely hairy on both surfaces P. stagnina
15b. Cilia of the ochreal tube either almost equaling or longer than than the tube; leaves either glabrous or if pubescent only on the margin and mid-rib beneath 16
16a. Leaf margin eciliate P. eciliata
16b. Leaf margin ciliate 17
17a. Flaccid herbs; spikes laxly flowered; leaves elliptic-lanceolate P. posumbu
17b. Robust herbs; spikes densely flowered; leaves linear-lanceolate 18
18a. Leaves base sub-cordate; petioles and mid-rib usually glabrous; anthers white P. serrulata
18b. Leaves base acute to obtuse; petioles and mid-rib strigosely ciliate; anthers pink P. barbata