Comparative morphology of Selaginella rhizophore

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A morphogenetic investigation was made of the rhizophore of three large-sized tropical Selaginella species. The rhizophores of Selaginella delicatula, S.caudata, S. plana arise exogenously at the points of branching of the main stems. In S.caudata they are initiated at the junction of the second youngest branching. The rhizophore apical meristem has a tetrahedral apical cell and is capless. The rhizophores are usually three or four times dichotomously branched in S.delicatula and S.plana and four or five times in S.caudata. In S.delicatula, dichotomous branching of the rhizophore involves formation of two new apical cells subsequent to loss of an original apical cell. A pair of roots is formed endogenously from inner cells below the dermal layer at the apex of ultimate rhizophore branches. The finding that the rhizophore is an autonomously branched, leafless, and capless axis leads us to argue that Selaginella rhizophores, like lepidodendrid rhizomorphs, are fundamental axial organs that coordinate with the stem and root.The plants of Psilotaceae are the most simply organized among living vascular plants and consist of irregularly branched subterranean "rhizome" and simple or dichotomously branched, leafy aerial shoots. They lack roots throughout the life cycle. The subterranean "rhizome" of Psilotaceae is an enigmatic organ. While the "rhizome" has uncapped apical meristem, it differs distinctly from a genuine stem by lacking appendicular leaves(and also roots), and bearing only rhizoids. The "rhizome" is considered to be one of the fundamental organs of vascular plants. It is compared with non-stem axial organs such as the Selaginella rhizophore, the lepidodendrid (including lsoetes) rhizomorphs, and the subterranean axes of some early fossil vascular plants of subtly differentiated organization.