Morphological trends in Gondwana plants

Abstract

The term Gondwana has recently been redefined to include the group of terrestrial rocks in the Indian Craton, that was initiated with a basal Permian glacigene epoch and terminated with the large hiatus at the top of the Triassic. The Gondwana Supergroup as redefined now comprises Talchir, Damuda, Panchet and Mahadeva groups and ranges in age from the earliest Permian to latest Triassic or earliest Jurassic (Venkatachala & Maheshwari, 1991).

The vegetational scenario of Gondwana shows mixture of plants belonging to quite distinct habitats. The morphological adaptations of plants that thrived at all levels on land, in continental water, upland and in environments of exceeding dryness are significant. Leaf size varies from small to large with variety of apex and base, midribless to prominent midrib, non-petiolate to petiolate, veins loosely arranged, narrow mesh type of venation to open mesh and narrow mesh type. Leaf cuticle of glossopterids also shows variations.

Most of the Gondwana woods show variation in pith and primary xylem and secondary xylem. Pith varies from homo- to hetero-cellular. Primary xylem shows variation from endarch to mesarch. The secondary xylem is pycnoxylic, homoxylous. Secondary xylem shows well-marked growth rings. There is a great variation in the pitting of secondary tracheids. Xylem rays vary from uni- to multi-seriate. The ray field-pits also show diversity.

Wide diversities are also seen in the morphology of pteridophytic megaspores and of reproductive organs of gymnosperms. The exosporium of megaspore is either smooth or variously ornamented. The mesosporium is with or without cushions. Reproductive organs are known only for the glossopterid group and are of two types.

The changing patterns in leaves, woods, megaspores and reproductive organs of Gondwana plants may provide significant data for charting of morphotrends in these organs. It can probably also be established if these morphotrends were ecologically controlled temporary and transient phase or were genetically controlled leading to evolution of new types.