Fig. 16. Fig. 15.Fig. 17.
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.
Fig. 18.
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.
Fig. 19. Fig. 20. Fig. 21.
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.
Fig. 20. Spore of Equisetum, surrounded by two filaments with club-shaped extremities. The filaments are represented as coiled round the spore.
Fig. 21. Spore of Equisetum, with the filaments (elaters) expanded.
Among vascular Acrogens is included the natural order Marsileaceæ or Rhizocarpeæ, the Pepperworts (Fig. 22). The order consists of aquatic plants, with creeping stems, bearing leaves, which are either linear, or divided into three or more wedge-shaped portions not unlike clover. The fructification is at the base of the leaf-stalks, and consists of sacs (sporocarps) containing spores of two kinds, microspores and macrospores. The order contains Marsilea, Pilularia, Azolla, and Salvinia.