Department of Botany, University of Karachi, Karachi.
Annual or perennial, monoecious or dioecious, mostly submerged aquatic herbs in fresh or saline waters. Stem corm-like or slender, stoloniferous or not. Leaves exstipulate, sessile or petiolate, radical or cauline and spirally whorled, opposite or alternate, submerged, rarely partly emergent or floating, variously shaped; veins ± parallel, straight or curved. Flowers solitary or clustered, enclosed at first by a bifid sessile or long stalked pair of opposite united and spathe-like or free bracts, bisexual or unisexual, when unisexual then males usually more than 1, females solitary, actinomorphic or occasionally ± zygomorphic, mostly 3-merous. Sepals 3, free, often green, valvate. Petals 3 or absent, membranous, showy or minute and inconspicuous, imbricate, or con¬volute. Stamens (1-) 2 to numerous, uni or multiseriate, inner ones rarely staminodial, filaments ± slender free or united in pairs, rarely absent; anthers dithecous, dehiscing by parallel longitudinal slits; 1 or more pistillodes usually present in male flowers. Ovary inferior, composed of 2-15 fused carpels, linear, lancecolate or ovate, generally protruding into a filiform beak or rostrum at the apex, unilocular with parietal placentation and numerous orthotropous to anatropous, erect to pendulous, bitegmic ovules; styles as many as carpels, entire or 2-lobed. Fruit ripening under water, linear-lanceolate, ovate to globose, dry or pulpy, many-seeded, dehiscing irregularly. Seeds fusiform, elliptic, ovate or globose, glabrous, warty or spinose, exalbuminous; embryo upright.
A family of c. 17 genera and 76 species, distributed in marine, saline or fresh waters in tropical, subtropical and temperate regions. Represented in Pakistan by 6 genera and 6 species.
There are several extraordinary pollination mechanisms throughout the family, as in Hydrilla and Vallisneria (Cook, C.D.K. in Symoens, J.J. et al. (ed.) Studies in Aquatic Vascular Plants pp. 1-15,1982).
Acknowledgements: We are grateful to Prof. Dr. C.D.K. Cook (Botanischer Garten and Institut fur Systematische Botanik der Universitat Zurich, Switzerland) and Mr. I.C. Hedge (Edinburgh) for going through the manuscript and offering suggestions for its improvement. The financial assistance received from the United States Department of Agriculture under P.L. 480 with the coordination of the Pakistan Agriculture Research Council, Islamabad, is thankfully acknowledged.