Evolution of a cormophytic plant body in lower vascular plants

Abstract

Possession of a cormophytic plant body and stelar system distinguishes vascular plants from thallophytic ancestors. Based on morphology and development of sporophyte and gametophyte of pteridophytes it is argued that current interpretations of morphology and evolution of these features in terms of Axial theory and Stelar theory are untenable in these primitive vascular plants. The leaf, and not the axis/stem, is the primary organ and the first to evolve. The leaf along with its associated root constitutes a Phyllorhize which is the basic unit of construction of the plant body. Shoot apical meristem functions only in initiating leaves and branches and what appears as stem is a product of conjoined leaf bases, the shoot meristem contributing little to its construction. Vasculature is developed only in tissues derived from leaf and root meristems; no vasculature is developed in tissue derived from the shoot meristem. Stelar cylinder consists of secondarily interconnected basal regions of leaf vasculatures and sometimes also vasculature of leaf-associated roots. The pattern of growth (different in taxa having erect rhizome and plagiotropic rhizome) determines the nature of the stelar cylinder. Evolution apparently followed the same course in gametophytic and sporophytic generations, and evolution of pteridophyte gametophyte indicates that the primitive form was an amorphous cushion shaped thallus devoid of meristem and vasculature. It is shown how a phyllorhize unit evolved from such a plant body. Morphological, anatomical and ontogenetic evidences are presented in support of the contention that the pteridophyte plant body is formed of conjoined leaf bases and its stele is the product of leaf base vasculatures interconnected in a regular pattern. Also, it is shown that stelar evolution did not follow the sequence suggested by Stelar theory.