Leaves usually small in proportion to the size of the whole plant, arranged in whorls at the nodes. Sporangia borne on specially modified sporophylls or sporangiophores, which are aggregated to form a definite strobilus or spore-bearing cone. EQUISETACEAE. (Recent Species.) The leaves are in whorls, coherent in the form of a sheath, and traversed by longitudinal veins which do not fork or anastomose. The stem is divided into comparatively long internodes separated by the leaf-bearing nodes, and the branches arise in the leaf-axils at the nodes. The fertile leaves or sporophylls differ from the sterile leaves, and usually occur in definite aggregations or strobili containing spores of one kind (_isosporous_). In the single living genus _Equisetum_, the outer coat of the mature spore forms two hygroscopically sensitive filamentous structures or _elaters_. On the germination of the spore the gametophyte is developed in the form of a small lobed prothallium 1–2 cm. in length. In most cases there are distinct male and female prothallia. The genus _Equisetum_ L., the common Horse-tail, is the sole living representative of this Family. It occurs as a common native plant in Britain, and has a wide geographical distribution. Species of _Equisetum_ are abundant in the temperate zones of both hemispheres, and occur in arctic as well as tropical latitudes. Wallace[484] speaks of Horse-tails, “very like our own species,” growing at a height of 5000 feet on the Pangerango mountain in Java. In favourable situations the large British Horse-tail, _Equisetum maximum_ Lam. (= _E. Telmateia_ Erhb.), occasionally reaches a height of about six feet, and growing in thick clusters forms miniature forests of trees with slender erect stems and regular circles of long and thin branches. A tropical species, _Equisetum giganteum_ Linn.[2] living in the marshes of Mexico and Cuba[485], and extending southward to Buenos Ayres and Chili, reaches a height of twenty to forty feet, but the stem always remains slender, and does not exceed an inch in diameter. Groves of such tall slender plants on the eastern slopes of the Andes[486] suggest to the palaeobotanist an enfeebled forest-growth recalling the arborescent Calamites of a Palaeozoic vegetation. The twenty-five existing species of _Equisetum_ are remnants of various generic types of former epochs, and possess a special interest from the point of view of the geological history of plants. A brief description of the main characters of the recent genus will enable the student to appreciate the points of difference and agreement between the extinct and present representatives of the Equisetales.