Citation: L., Sp. Pl. 234 (1753).

Derivation: Greek hydro, water; kotyle, a dish or plate; alluding to the shape of the leaves slightly depressed in the centre.

Synonymy: Not Applicable

Common name: Pennyworts.

Description:
Perennial herbs with prostrate or ascending stems, rooting at least at the lower nodes, or with underground stolons, or small annuals with erect or ascending branched stems; leaves petiolate, with scarious stipules; blades palmately veined, simple, rhomboid, orbicular, cordate or peltate, entire, palmately lobed or dissected, with crenate, dentate or serrate margins, or digitately compound with 3-5 entire, crenate, dentate, serrate or lobed leaflets.

Flowers-small, bisexual or unisexual (plants sometimes dioecious or polygamous), in pedunculate (rarely, sessile) simple umbels or small few- to many-flowered heads or short spikes, with or without bracts, sometimes in more or less irregularly compound inflorescences of paniculately arranged sessile umbels or placed in whorls on simple or branched umbellately arranged interrupted spikes; sepals minute or absent; petals entire, valvate in bud, usually acute, white, cream-coloured, greenish, often pink to purplish tinged, their apex not inflexed; disk flat with a raised margin.

Fruit laterally compressed; endocarp woody; carpophore absent or, in most annual species, undivided and persistent; mericarps more or less distinctly 5-ribbed, smooth or, especially in the annual species, variously sculptured; vittae usually absent.

Distribution:  More than 130 species, mainly of the tropics and southern temperate zone; about 55 species in Australia, mostly endemic, some also in New Zealand and New Guinea, 2 introduced and naturalised.

Biology: No text

Taxonomic notes: Annual species are known only from Australia; they show many features which otherwise do not occur in Hydrocotyle. Their exclusion from Hydrocotyle may be justified.

Key to Species:

1. Leaf blades peltate, more or less orbicular, glabrous; flowers whorled in interrupted spikes
2. Inflorescence an umbel with many rays along which the pedicellate flowers are arranged in whorls; fruit cordate at the base	H. bonariensis 1.
2. Inflorescence a simple or few-branched interrupted spike of whorls of almost sessile flowers; fruit broadly cuneate at the base	H. verticillata 18.
1. Leaf blades cordate to truncate at the base, simple or digitately divided, glabrous or hairy; flowers in simple umbels or small heads
3. Perennials with prostrate or ascending stems or stolons, rooting at the base; carpophore absent
4. Leaf blades divided to the base, with 3-5 leaflets
5. Leaf blades divided into 5 (rarely 4) leaflets; lateral leaflets entire or 3-dentate at the apex	H. muscosa 11.
5. Leaf blades divided into 3 leaflets; lateral leaflets usually 2- lobed; lobes dentate	H. tripartita 17.
4. Leaf blades simple, sometimes deeply lobed
6. Leaf blades glabrous or almost so, crenate to almost entire; petioles glabrous; fruits flat
7. Mericarps with a dorsal wing	H. pterocarpa 14.
7. Mericarps with a sharp dorsal keel but not winged	H. plebeya 13.
6. Leaf blades pubescent, with crenate-dentate lobes; petioleshairy; fruits thick; mericarps with convex sides, dorsally not sharply keeled
8. Pedicels 2-4.5 mm long at anthesis; flowers numerous in almost globular umbels	H. laxiflora 9.
8. Pedicels usually less than 1.5 mm long; flowers in dense globular heads
9. Heads many-flowered; fruits c. 1.5 mm long; styles long; stamens obsolete or rudimentary; petals usually falling at the beginning of anthesis, sometimes connate	H. laxiflora 9.
9. Heads with 5-15 flowers, all bisexual; fruits 1-1.5 mm long; styles often short; petals spreading at anthesis	H. hirta 8.
3. Annuals with erect or ascending stems, not rooting at nodes; carpophore mostly persistent
10. Fruits quite flat, light-brown; intermediate ribs inconspicuous; flowers 2 (rarely 1)-4 in one umbel, distinctly pedicellate	H. diantha 6.
10. Fruit rather thick, with protruding intermediate ribs; flowers usually more per umbel, sometimes almost sessile
11. Fruit with a rim of long spreading white fringes or hairs near its apex	H. comocarpa 4.
11. Fruit glabrous, without a fringed rim at the apex
12. Mericarps with 3 or 4 thick connecting-ridges between the dorsal and intermediate ribs, almost as high as the longitudinal ribs, with 1 pit between each of the horizontally connecting-ridges and between the intermediate and lateral ribs	H. rugulosa 15.
12. Mericarps smooth or finely pitted or tuberculate between the dorsal and intermediate ribs, without thick connecting-ridges of equal height between them
13. Mericarps smooth or faintly wrinkled between the dorsal and intermediate ribs
14. Umbels usually shortly pedunculate, with 6-15 flowers; leaves lobed to about the middle; teeth of lobes hair-pointed (minutely setose); mericarps dark purplish-brown to almost black, with a flat depression between the intermediate and lateral ribs; plants usually grey-green and often purplish tinged	H. callicarpa 2.
14. Umbels mostly sessile, leaf-opposed, with 3-6 flowers; leaves often almost 3-partite with lateral segments deeply 2-lobed; teeth of lobes not setose; mericarps light reddish-brown, with a deep white-rimmed pit between the intermediate and lateral ribs; plants usually bright- to yellowish-green	H. foveolata 7.
13. Mericarps with tubercles or many pits between the dorsal and intermediate ribs
15. Mericarps with many deep pits impressed into the thickened more or less even surface between the dorsal and intermediate ribs
16. Mericarps pitted between the intermediate and lateral ribs; dorsal and intermediate ribs irregularly and bluntly swollen	H. medicaginoides 10.
16. Mericarps not pitted between the intermediate and lateral ribs; dorsal and intermediate ribs narrowly keeled	H. capillaris 3.
15. Mericarps with tubercles between the dorsal and intermediate ribs
17. Tubercles between the ribs of mericarps very prominent and thick, almost globular, usually protruding sidewards from the dorsal and intermediate ribs, close to each other and almost interlocking; flowers almost sessile; mericarps 1.5-2 mm long	H. pilifera var. glabrata 12.
17. Tubercles small, distant from each other, in a single row between the dorsal and intermediate ribs
18. Peduncles stout; pedicels very short, thick; leaves rather thick; petioles often with long spreading hairs just below the blade; mericarps c. 1.5 mm long; central lobes of .upper leaves usually cuneate-narrow-obovate	H. crassiuscula 5.
18. Stem, branches, peduncles and pedicels very slender; leaves papery thin; petioles glabrous; mericarps c. 1 mm long; central lobes of upper leaves oblong with almost parallel sides	H. trachycarpa 16.

Author: Not yet available