Family Hydatellaceae

Hydatellaceae Hamann

~ Centrolepidaceae (formerly, now far removed)

Habit and leaf form. Aquatic or semi-aquatic herbs (submerged or partly emergent). Glabrous annual; with a basal aggregation of leaves (plants only 2–5 cm high, the stems very short). Hydrophytic to helophytic (in fresh water); rooted. Leaves submerged and emergent. Leaves small to medium-sized (5–40 mm long); alternate; spiral; sub- terete, or flat (filiform, thin); sessile; more or less non-sheathing; simple; epulvinate. Lamina entire; filiform- linear; one-veined. Leaves eligulate; exstipulate; with a persistent basal meristem, and basipetal development, or without a persistent basal meristem (? - it would be interesting to know, given recent suggestions that the family is not even monocotyledonous, let alone related to Poaceae).

General anatomy. Plants without silica bodies.

Leaf anatomy. Stomata present; anomocytic.

Lamina internally centric. The mesophyll not containing mucilage cells; without calcium oxalate crystals. Vessels present (Wagner 1977); end-walls scalariform.

Stem anatomy. Secondary thickening absent. Xylem with vessels. Vessel end-walls scalariform.

Root anatomy. Root xylem with vessels; vessel end-walls scalariform.

Reproductive type, pollination. Fertile flowers functionally male, or functionally female. Plants monoecious (the heads mixed in Trithuria, mostly unisexual in Hydatella), or dioecious (sometimes, in Hydatella). Floral nectaries absent (nectaries lacking). Pollination autogamous or by water.

Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in heads; more or less in ‘spikelets’. Inflorescences usually more or less scapiflorous; terminal; with involucral bracts (the involucral bracts 2–4(-6), translucent); pseudanthial. The involucres non-accrescent. Flowers ebracteate; ebracteolate; minute to small; 1 merous. Hypogynous disk absent.

Perianth absent.

Androecium 1 (i.e. the male flower consisting of a single stamen); exclusively of fertile stamens. Stamens 1. Anthers basifixed (the filament stout); non-versatile; dehiscing via longitudinal slits; bilocular; tetrasporangiate. Microsporogenesis successive. Pollen grains aperturate; 1 aperturate; sulcate. Interapertural interstitium columellate.

Gynoecium ostensibly 1 carpelled. The pistil 1 celled. Gynoecium monomerous, or syncarpous (perhaps pseudomonomerous, depending on interpretation); of one carpel, or eu-syncarpous (?); superior. Carpel non-stylate, or stylate (? - the ‘styles’ alternatively interpretable as stigmatic papillae); apically stigmatic; 1 ovuled. Placentation apical. Ovary 1 locular; shortly stipitate. Gynoecium non-stylate (if a tuft of filamentous structures represents stigmas), or stylate (if they are stylar). Styles 3–10 (if considered stylar!); free; apical. Stigmas 3–10 (i.e., if the filamentous structures be are so interpreted). Placentation apical. Ovules in the single cavity 1; pendulous; non-arillate; anatropous; bitegmic; crassinucellate to pseudocrassinucellate. Endosperm formation cellular (sic).

Fruit non-fleshy. The fruiting carpel (if considered monomeric) dehiscent (sometimes, via 3 slits, in Trithuria), or indehiscent; a follicle, or an achene (Hydatella). Fruit if considered syncarpous dehiscent, or indehiscent; a capsule, or achene-like (? - small and dry). Capsules when dehiscent, three valvular (i.e., in Trithuria). Fruit 1 seeded. Seeds ‘almost’ non-endospermic. Perisperm present (starchy). Seeds minute. Seeds with starch. Embryo rudimentary at the time of seed release. Testa operculate; probably without phytomelan (?).

Geography, cytology. Western Australia, Tasmania, New Zealand.

Taxonomy. Subclass Dicotyledonae, or Monocotyledonae (? - see comments); if dicotyledonous, presumably Tenuinucelli. Dahlgren’s Superorder perhaps Nymphaeiflorae (?); Dahlgren et al. Superorder Commeliniflorae (i.e., as assigned Monocot by Dahlgren, Clifford and Yeoh); Hydatellales. APG 3 peripheral angiosperms; Superorder Nymphaeanae; Order Nymphaeales.

Species 8. Genera 2; Hydatella, Trithuria.

Long regarded as a Monocot perhaps related to Poaceae, nucleic acid-sequencing data now (2007) suggest that this family is closer to the Nymphaeaceae.

Illustrations.
• Hydatella, Trithuria.