[Sidenote: Fig. 11, _a_. Bifurcating (forked or dichotomous) trunk (caudex) of a Tree-fern (_Alsophila Perrottetiana_), showing the scars (cicatrices) left by the fallen fronds. These scars exhibit the arrangement of the vascular bundles. Fig. 11, _b_. Rhizome of _Lastrea Filix-mas_ (male fern), showing scars of the leaves, _c_, with markings of the vascular bundles.] [Sidenote: Fig. 12. Transverse section of the stem (caudex) of a Tree-fern (_Cyathea_), showing the arrangement of the cellular and vascular tissue. The cellular tissue of the centre, _m_; that of the circumference, _p_; vascular cylinder, _f v_, consisting of dark-coloured pleurenchyma or ligneous tubes, _f_, and paler vessels, _v_, chiefly scalariform and closed spiral, and pierced by the meshes for the leaf-bundles at _m_; the outer cortical portion connected with the bases of the leaves, _e_.] [Illustration: Fig. 13-17.] [Sidenote: Fig. 13. Scalariform vessels taken from a Tree-fern. They are marked with bars like the steps of a ladder, hence their name. The membrane occasionally disappears, so that the walls are made up of fibres only at some parts. Fig. 14. Sporangia of a Fern, supported on stalks, _p_, each of which ends in an elastic cellular ring, _s_, partially surrounding the spore-case, and opening it when mature. Fig. 15. _Lycopodium clavatum_, a common Club-moss. The leafy branch, _l_, ends in a stalk bearing two spikes of fructification, _f_. Fig. 16. A kidney-shaped 2-valved case, containing small spores (microspores) of Lycopodium. Fig. 17. Two-valved case, containing large spores (macrospores) of Selaginella.] [Illustration: Fig. 18.] [Sidenote: Fig. 18. Fructification of _Equisetum maximum_, Great Water Horse-tail, showing the stalk surrounded by membranous sheaths, _s s_, which are fringed by numerous processes called teeth. The fructification, _f_, at the extremity, is in the form of a cone bearing polygonal scales, under which are spore-cases containing spores with filaments.] Another important order of vascular Acrogens is the Equisetaceæ or Horse-tails (Fig. 18). These are Cryptogams, having rhizomes, bearing hollow, striated branches, which secrete in their epidermis a considerable amount of silex. These branches are jointed and have membranous sheaths at the articulations, which are whorls of leaves reduced to a very rudimentary condition. The fructification consists of cone-like bodies (Fig. 18, _f_) bearing peltate polygonal scales, under which are spore-cases (Fig. 19), enclosing spores with four hygrometric club-shaped filaments called elaters (Figs. 20 and 21). At the present day some of these plants in tropical regions have stems of 15 or 16 feet high. [Illustration: Fig. 19-21.] [Sidenote: Fig. 19. Polygonal scale, _s_, of a species of Horse-tail (_Equisetum_), bearing membranous sacs, _t_, which open on their inner surface to discharge spores.