In vitro studies on developmental morphology of Indian Podostemaceae

The early developmental biology of seven species of Indian podostemads based on in vitro growth of plants from seeds is discussed. Unpublished observations on seed germination, destiny of the radicular pole of the embryo, origin of the ‘primary axis’, location of the site of initiation of the thallus, and variations in seedling morphology in three thalloid members of the subfamily Podostemoideae (Zeylanidium lichenoides (S. Kurz) Engler, Hydrobryopsis sessilis (Willis) Engl., Willisia selaginoides (Bedd.) Warming ex Willis) are communicated. Additional observations are added to previously published accounts of the seedling biology of the thalloid Cladopus hookerianus (Tul.) C. Cusset and Polypleurum stylosum (Wight) Hall (Podostemoideae) and one thalloid Tristichoideae (Dalzellia ceylanica (Gardn.) Wight). Special attention is focused on the sequence of developmental events from seed to flowering in Indotristicha ramosissima (Wight) Van Royen (Tristichoideae), the only Indian species that seems to correspond to the Classical Root Shoot (CRS) model. In I. ramosissima exogenous root primordia arise from the flattened radicle, of which only three develop into main roots with asymmetric root caps. Although the mature seed lacks a plumule, a ‘primary axis’ of limited growth is formed at the apex. The plant body, with its numerous long branches, develops from the main roots. Flowering has been stimulated by subjecting vegetative plants to water and nutrient stress. Comparisons are made of early seedling developmental patterns among the seven investigated species. Special attention is focused on determining at what stage in ontogeny the root-shoot versus thalloid growth forms diverge. A model of seedling development is presented that depicts interspecific variability among species examined. The homology of the thallus in Podostemaceae remains unclear.