82 



SECOND GROUP. — AIUSCINEAE. 



beneath its summit every year after the ripening of the fruit and grows beside it, so that 

 the stem gets a false bifurcation every year : as the plant dies away slowly from 

 below upwards these innovations are in time separated from it and become inde- 

 pendent plants. But some branches of each tuft turn downwards, become long and 

 slender and sharp-pointed, and lie along the stem forming a closely applied envelope ; 

 others again turn outwards. The leaves, which are sessile on stem and branch 

 with a broad insertion and a divergence of two-fifths, are ligulate or pointed above, 

 and with the exception of the first on the young stem are composed of two kinds 

 of cells with a regular arrangement. The leaf consists necessarily at first of homo- 

 geneous tissue, but as it developes the cells of the nerveless lamina are differentiated 

 into large broad cells of somewhat elongate rhombic form, and into narrow tubular 

 cells which run between them and bound them, and form a network among them- 



FlG. 136. Transverse section of a young stem of Sphrtgrnivi 

 cymbi/olium ; x inner cells with colourless walls, r outer layer of 

 cells, becoming- narrower and thicker-walled towards the out- 

 side ; e, e peripheral layers, / holes through which the cells of the 

 peripheral layers communicate with each other. Mag. 900 times. 



Portion of a stem beneath the : 

 . but still closed sporogonia. mai^r 



antheridial branches, /' leaves of tht 

 After Schiniper. 



selves ; they have the appearance of being squeezed in between the others. The 

 large cells lose their contents and therefore appear to be colourless, while their walls 

 show narrow spiral bands with irregular and distant coils and large pits with a thickened 

 margin ; the absorption of the enclosed membrane converts these pits into holes, which 

 are usually circular in form. The narrow tubular cells between them retain their contents 

 and produce chlorophyll-corpuscles, and are therefore the nutrient tissue of the leaf, 

 though the whole surface they expose is smaller than that of the colourless tissue (Fig. 

 138). The a.\es are composed of three layers of tissue; the innermost is an axile 

 cylinder of thin-walled colourless parenchymatous cells ; this is surrounded by a layer 

 of thick-walled pitted prosenchymatous cells with firm, perhaps lignified, wails of a 

 brown colour; lastly, the cortical tissue consists of from one to four layers of very broad 

 thin-walled empty cells, which in Sphagnum cymbifolhim have spiral threads and 



