A pericycle composed of one or two layers of chlorophyll-containing cells encircles the whole stele which is suspended in a lacuna by trabeculae (fig. 131, A, B, _t_) connecting the pericycle with the inner edge of the broad cortex. The trabeculae consist in part of endodermal cells characterised by cuticular bands. The cortex is usually differentiated into three fairly distinct regions. Mechanical tissue of thick-walled fibres constitutes the outer region (_a_); the middle cortex consists of thinner-walled parenchyma, the elements of which become smaller and rather more compactly arranged in the inner zone. The middle cortex is frequently characterised by the presence of spaces and by the hyphal or trabecular structure of the tissue, a feature which, as Bower[117] pointed out, is common to many recent and fossil members of the Lycopodiales. In some cases, _e.g._ _S. erythropus_, from tropical America, the cortex of the creeping stem consists entirely of thick-walled cells. _Selaginella grandis_ (fig. 130) has “a short decumbent stem rooted at close intervals[118],” from which thick erect aerial shoots rise to a height of one foot or more. In the apical region these erect axes give off repeatedly forked foliage shoots on which the spiral phyllotaxis of the homophyllous axis is gradually replaced by four rows of two kinds of leaves (fig. 130, 2). The anatomy of this species agrees with that of _S. Martensii_. The trailing or semi-erect and homophyllous shoots of _Selaginella spinosa_[119] present a distinct type of vascular anatomy. The upper part of the ascending stem has an axial strand of xylem with seven peripheral groups of spiral protoxylem tracheae (fig. 131, B); in the trailing portion of the shoot the protoxylem elements occur as one central group in the solid rod of metaxylem through which the leaf-traces pass on their way to the axial protoxylem. This type is important as affording an exception, in the endarch structure of the xylem, to the usual exarch plan of the stelar tissues. This species is the only one in which any indication of the production of secondary xylem elements has so far been recorded. Bruchmann[120] has shown that, in the small tuberous swelling which occurs at the base of the young shoot (hypocotyl), a meristematic zone is formed round the axial vascular strand and by its activity a few secondary tracheids are added to the primary xylem. With this exception _Selaginella_ appears to have lost the power of secondary thickening, the possession of which constitutes so striking a feature of the Palaeozoic Lycopods. Another type is represented by _S. inaequalifolia_, an Indian species, the shoots of which may have either a single stele or as many as five, each in its separate lacuna. The homophyllous _S. laevigata_ var. _Lyallii_ Spr., a Madagascan species, affords a further illustration of the variation in plan of the vascular tissues within the genus. There is a considerable difference in structure between the erect