1 64 MOSSES AND FERNS chap. 



(Fig. 78, D). The chlorophyll cells are sometimes so crowded 

 and overarched by the hyaline ones that they are scarcely 

 perceptible, and of course in such leaves the green colour is 

 very faint. Cross-sections of the leaves show a characteristic 

 beaded appearance, the large swollen hyaline cells regularly 

 alternating with the small wedge-shaped sections of the green 

 cells (Fig. 78, E). Russow^ has shown that the leaves of the 

 sporogonial branch retain more or less their primitive character, 

 and the division into the two sorts of cells of the normal leaves 

 is much less marked. He connects this with the necessity for 

 greater assimilative activity in these leaves for the support of 

 the growing sporogonium. From his account too it seems that 

 the stem leaves lose their activity very early. 



The degree of development of the thickenings upon the 

 walls of the hyaline cells varies in different species, and in 

 different parts of the leaf It is, according to Russow," best 

 developed in the upper half of the leaf, where these thickenings 

 have the form of thin ridges projecting far into the cell cavity. 



The development of the central tissue of the stem varies. 

 The central portion usually remains but little altered and 

 constitutes a sort of pith composed of thin-walled colourless 

 parenchyma, which merges into the outer prosenchymatous 

 tissue of the central region. The cells of the latter are very 

 thick walled, and elongated, and their walls are usually deeply 

 stained with a brown or reddish pigment. In their earlier 

 stages, according to Schimper,^ the prosenchyma cells have 

 regularly arranged and characteristic pitted markings on their 

 walls, but as they grow older and the walls thicken, these 

 become largely obliterated. Cross-sections of these prosen- 

 chyma cells show very distinct striation of the wall (Fig. 78, 

 G), which becomes less evident as they approach the thinner- 

 walled parenchyma of the central part of the stem. No trace 

 of a central cylinder of conducting tissue, such as is found in 

 most of the Mosses, can be found in Sphagmun, and this is 

 correlated with the absence of a midrib in the leaves. 



The cortex at first forms a layer but one cell thick, but is 

 from the first clearly separated from the axial stem tissue. In 

 the smallest branches it remains one-layered (Fig. jy , C), but 

 in the larger ones it early divides by tangential walls into two 

 layers, which at this stage are very conspicuous (Fig. yy, B). 



^ Russow (4). ^ Russow, I.e. p. 8. ^ Schimper (i), jd. 36. 



