Our knowledge of the anatomy of _Selaginella_, thanks chiefly to the researches of Harvey-Gibson[116], is much more complete than in the case of _Lycopodium_. The stems, which may be either trailing or erect, are usually dorsiventral, and it is noteworthy that different shoots of the same plant or even the same axis in different regions may exhibit considerable variation in the structure and arrangement of the vascular tissue. In the well-known species, _Selaginella Martensii_, the stem, which is partly trailing, partly ascending, possesses a single ribbon-shaped stele composed of scalariform tracheids with two marginal protoxylems formed by the fusion of the leaf-traces of the dorsal and ventral leaves respectively. As in _Lycopodium_ the metaxylem tracheae are as a rule scalariform, but reticulate xylem elements are by no means unknown. The tracheal band, surrounded by parenchymatous elements, is enclosed by phloem with external protophloem elements. The characteristic features of the stele are shown in the diagrammatic drawing of a section of another species—_S. Willdenowii_—represented in fig. 131, A. [Illustration: FIG. 131. A. _Selaginella Willdenowii._ Transverse section of stem: _a_, outer cortex; _p_, phloem; _t_, trabeculae. B. _S. spinosa_, stem: _px_, protoxylem. C. _S. laevigata_ var. _Lyallii_, section of stele: _t_, ridge of xylem cylinder; _e_, endodermis. D. _S. rupestris_, seedlings with cotyledons (_c_) protruding beyond the sporophylls (_b_). E. Transverse section of _Selaginella_ leaf-base: _l_, ligule; _lt_, leaf-trace. F. Portion of G. enlarged. G. _S. grandis._ Longitudinal section of strobilus: _bb_, sporophyll-trace; _l_, ligule. (A, B, C, E, F, after Harvey-Gibson; D, after Miss Lyon.)]