[485.] Horsetails, Equisetaceæ, is the name of a family which consists only (among now-living plants) of Equisetum, the botanical name of Horsetail and Scouring Rush. They have hollow stems, with partitions at the nodes; the leaves consist only of a whorl of scales at each node, these coalescent into a sheath: from the axils of these leaf-scales, in many species, branches grow out, which are similar to the stem but on a much smaller scale, close-jointed, and with the tips of the leaves more apparent. At the apex of the stem appears the fructification, as it is called for lack of a better term, in the form of a short spike or head. This consists of a good number of stalked shields, bearing on their inner or under face several wedge-shaped spore-cases. The spore-cases when they ripen open down the inner side and discharge a great number of green spores of a size large enough to be well seen by a hand-glass. The spores are aided in their discharge and dissemination by four club-shaped threads attached to one part of them. These are hygrometric: when moist they are rolled up over the spore; when dry they straighten, and exhibit lively movements, closing over the spore when breathed upon, and unrolling promptly a moment after as they dry. (See Fig. [493-498].)
Fig. 493. Upper part of a stem of a Horsetail, Equisetum sylvaticum. 494. Part of the head or spike of spore-cases, with some of the latter taken off. 495. View (more enlarged) of under side of the shield-shaped body, bearing a circle of spore-cases. 496. One of the latter detached and more magnified. 497. A spore with the attached arms moistened. 498. Same when dry, the arms extended.
Fig. 499. A Tree-Fern, Dicksonia arborescens, with a young one near its base. In front a common herbaceous Fern (Polypodium vulgare) with its creeping stem or rootstock.
Fig. 500. A section of the trunk of a Tree-Fern.