the sides of these furrows numerous slender roots are given off in acropetal succession. A stele of peculiar structure occupies the centre of the stem; cylindrical in the upper part (fig. 133, A), it assumes a narrow elliptical or, in species in which there are three furrows, a triangular form in the lower portion of the tuberous stem. [Sidenote: ISOETES] The stem of _I. lacustris_ represented in fig. 132, A, from which the laminae of the leaves have been removed from the summit affords an example of a species with two furrows. The drawing shows the widely gaping sides of the broad furrow with circular root-scars and a few simple and dichotomously branched roots. A short thick column of parenchymatous tissue projects from a slightly eccentric position on the base of the stem. [Illustration: FIG. 133. _Isoetes lacustris_. A. Transverse section of stem: _cr_, cortex; _x_, _x²_ xylem; _c_, cambium; _a_, thin-walled tissue; _lt_, leaf-traces; _b_, dead tissue. B, C, D. Portions of A enlarged. E. Longitudinal radial section of sporophyll-base: _v_, velum; _l_, ligule; _bb_; vascular bundle; _m_, megaspores; _t_, sterile tissue. F. Longitudinal section through the base of a root. G. Transverse section of root. H. Transverse section of sporophyll, showing sporangium with trabeculae, _t_; leaf-trace, (_lt_), and two groups of secretory cells. I. A group of secretory cells enlarged.] The primary vascular cylinder[140] consists of numerous spiral, annular or reticulate tracheids (fig. 133, A, _x_) which are either isodiametric or longer in a horizontal than in a vertical direction, associated with parenchyma. Lower in the stem crushed and disorganised xylem elements are scattered through a still living trabecular network of parenchymatous tissue. From the axial cylinder numerous leaf-traces (fig. 133, A, _lt_) radiate outwards, at first in a horizontal direction and then gradually ascending towards the leaves. The vascular cylinder is of the type known as cauline; that is, some of the xylem is distinct in origin from that which consists solely of the lower ends of leaf-traces. As in _Lycopodium_ the development of the metaxylem is centripetal.