Podostemaceae-the strange family of aquatic angiosperms

Abstract

Podostemaceae is a unique family of aquatic angiosperms. The vegetative body of the podostemads is represented by a thallus resembling that of an alga, lichen or a bryophyte, and lacking the conventional demarcation into stem, leaf and root. The podostemads invariably inhabit gushing mountain streams or falls and generally grow attached to rocks. They lack aerenchyma. Flower buds are initiated while submerged, but flowering and fruit development occurs when the water level subsides and the plants become exposed. Taxonomists believe that podostemads have affinities with Crassulaceae, Saxifragaceae and Hydrostachydaceae. Podostemaceae are termed an embryological family owing to the lack of antipodals, triple fusion, endosperm, the prevalence of single fertilization, and the presence of a pseudo-embryo sac. The wide range of phenotypic plasticity, morphological nature of the thallus as understood by developmental studies using in vitro cultures, and embryological features of the family are discussed. The origin, adaptation and affinities with closely related families have also been examined.