i66 MOSSES AND FERNS chap. 



their hygroscopic movements, play an important part in scat- 

 tering the spores, and physiologically take the place of the 

 elaters of the Hepaticse. 



Some Mosses live but a few months, and after ripening 

 their spores, die. This is the case with Funaria hygrometrica, 

 at least in California. Other Mosses are perennial, and some 

 species of peat or tufa-forming Mosses seem to have an un- 

 limited growth, the lower portions dying and the apices grow^ 

 ing on until layers of peat or tufa of great thickness result, 

 covered over with the still living plants whose apices are the 

 direct continuation of the stems which form the basis of the 

 mass. 



With the exception of a very few forms all the Mosses are 

 readily referable to three orders. The first two, the Sphagnales 

 and the Andreaeales, are represented each by a single genus, and 

 are in several respects the types that come nearest the Liver- 

 worts. All the other Mosses, except perhaps Archidium and 

 Buxhaumia, conform to a very well-marked type of develop- 

 ment, and may be referred to a common order, the Bryales. 

 The Phascacese or cleistocarpous Mosses are sometimes sep- 

 arated from the higher Bryales as a distinct order, but a study 

 of their development shows that they belong to the same series, 

 and only differ in the degree of development from the more 

 specialized stegocarpous forms. 



Order I. — Sphagnales 



The Sphagnales, or Peat-Mosses, are represented by the 

 single genus Sphagnum. They are Mosses of large size, 

 which, as is well known, often cover large tracts of swampy 

 land and about the borders of lakes, forming the familiar peat- 

 bogs of northern countries. Owing to the empty cells in the 

 leaves and outer layers of the stem, they suck up water like a 

 sponge, and the plants when growing are completely saturated 

 with water. The colour is usually pale green, but varies much 

 in depth of colour, and in many species is red or yellow. When 

 dry the colour is much duller, largely owing to the opacity of 

 the dry, empty cells which conceal to a great extent the colour 

 of the underlying tissues. They branch extensively, and, ac- 

 cording to Schimper, a branch is always formed corresponding 

 to every fourth leaf ; but Leitgeb has shown that although this 



