MOSSES, 



327 



tongue-shaped or apiculate, and, with the exception of the first on the young stem, are 

 composed of two kinds of cells arranged regularly. The young leaf necessarily consists 

 of homogeneous tissue ; but as the development progresses the cells of the veinless 

 lamina become differentiated into large broad cells about the shape of a long lozenge, 

 and into narrow tubular cells, running in the midst of the former, bounding them, and 

 united with one another into a network ; they are, as it were, squeezed in among the 

 larger ones. The larger cells lose the whole of their contents, and hence appear 

 colourless ; their walls show irregular narrow spiral bands with the turns some distance 

 apart, as well as large dots, each of which has a thickened edge, while the part of the 

 cell-wall which closes the dot is absorbed. Large holes usually circular are thus formed 

 in the cell-wall of the colourless cells. The intermediate tubular narrow cells retain 

 their contents, form grains of chlorophyll, and thus constitute the tissue of the leaf, 



FIG. zyj.-Sphagnum acnti/olium ; part of the stem below the apex ; a, a tlie male branches, b leaves of the primary 

 stein ; ch perichcCtial branch with old still closed sporogonia (after Schimper X5-6). 



the entire area of which is, however, smaller than that of the colourless tissue (Fig, 

 240). The stems consist of three layers of tissue, the innermost of which is an axial 

 cylinder of thin-walled colourless cells elongated in a parenchymatous manner; it is 

 enveloped by a layer of thick-walled, dotted, firm (lignified?) prosenchymatous cells, 

 with their walls coloured brown. The epidermal tissue of the stem, finally, con- 

 sists of from I to 4 layers of very broad thin-walled empty cells, which, m S, cjm- 

 bifolium, possess spiral threads and round holes similar to those of the leaves (r/ 

 Fig. 70, p. 82). These colourless cells, both those of the leaves and of the epidermal 

 layer of the stem and of the branches, serve as a capillary apparatus for the plant, 

 through which the water of the bogs in which it grows is raised up and carried to 

 the upper parts; hence it results that the Sphagna which always grow erect are 



