of an Alga or Fungus. In its Protonema stage the Moss is only a cellular plant, a Thallophyte. Later, out of the Protonema is developed the true Moss, with its stem, leaves, and reproductive organs.
The Mosses have probably descended, through forms like the Hypoterygiae, from the Jungermanniae.
FILICALES.
The so-called Horse-tails of ditches, etc., our common Ferns, the aquatic plants known as Pillwort and Club-moss, are generally considered by botanists as representing four different orders of _the class Filicales. While Ferns, etc. are as highly organized as Mosses, in having stems and leaves, the vascularity of their stem exhibits a considerable advance as compared with the same structure in Mosses. The Fern and Horse-tail, though differing in appearance, are usually associated, since their reproduction is the same. The Pillwort and Club-moss, agreeing in their reproduction, differ, however, from that observed in the Horse-tail and Fern: hence their frequent union. The Filicales of the present day play an inferior part as compared with those of past time. Tropical climates even do not give us an idea of what the class once was, as regards their size, variety, and importance in the economy of nature. They are sometimes called Acrogens, or summit-growers. We will examine now a little more closely the living Filicales, leaving for the chapter on Geology the account of those forms that have died out.
EQUISETACEAE.
The Horse-tail, or Equisetum (Fig. 121), is a very common plant, abounding in ditches, woods, marshes, etc., and is readily distinguished by its very characteristic appear-
