Origin, structure, and interpretation of the thallus in Hydrobryopsis sessilis (Podostemaceae)

The Indian Podostemoideae are characterized by a dorsiventral thalloid plant body, the nature of which is controversial. Hydrobryopsis sessilis, selected for this investigation, is an endemic and highly reduced member. The mature embryo lacks a plumule and a radicle. Seed germination and seedling histology have shown that a diminutive SAM-like meristem is formed endogenously at the junction of the cotyledons. This meristem gives rise to a determinate primary axis apically as well as an indeterminate thallus primordium laterally. The radicular pole does not show activity, and a root is not formed. Based on histological studies, the thallus is interpreted as a flattened stem as it has a tunica-corpus-like organization at its apex. The origin, structure, and nature of the secondary vegetative shoots and thallus lateral branches have been described and discussed. Interestingly, lysigeny precedes the inception of endogenous secondary vegetative shoots. The primary axis and secondary vegetative shoots are similar, being stemless, determinate, and endogenous in origin.