ance: though small in temperate regions, in the tropics it attains a size of fifteen or sixteen feet. The Horse-tail is composed of a series of hollow tubes joined end to end, the articulations being separable, and these tubes are marked externally by furrows running longitudinally. In place of leaves, the Equisetum exhibits green-colored branchlets; it has also rhizomes, or underground stems, sometimes extending to the depth of many feet. The spores are contained in a spike-shaped or conical cap, terminating the stem of the plant; the spores produce a cellular structure, the Prothallus, from which the new Equisetum will be developed: this kind of reproduction is seen in the Ferns, of which we will presently speak. There is found in all parts of the Horse-tail such a large amount of silex that the plant becomes important in a commercial point of view, it being much used for polishing.
FILICES
Ferns are not only interesting to botanists on account of their structure and reproduction, but also equally attractive to the laity, their graceful stems and exquisite leaves furnishing specimens for the greenhouse and ornaments for the parlor. These beautiful plants are abundantly found in damp, shady places, though a damp soil and moist climate seem more necessary than shade for their luxuriant growth. If an oblique section (Fig. 123) of the stem of a Fern be magnified, the most important features observed are the vessels or ducts running down the middle of the stem, which have in them some woody tissue. So characteristic is the presence of vessels in the higher flowering plants, that Ferns, from having these organs, are often associated with them. At certain seasons there are seen, generally on the under surface of the leaf of a Fern, small bodies usually supported on stems, known as Sori. (Fig. 124, a.)
