V. MOSSES (MUSCI): SPHAGNALES—ANDREMALES i73 



The central portion usually remains but little altered and con- 

 stitutes a sort of pith composed of thin-walled colourless par- 

 enchyma, which merges into the outer prosenchymatous tissue 

 of the central region. The cells of the latter are very thicl: 

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

 with a brown or reddish pigment. In their earlier stages, ac- 

 cording to Schimper ((i), p. 36), the prosenchyma cells have 

 regularly arranged and characteristic pitted markings on their 

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

 come largely obliterated. Cross-sections of these prosenchyma 

 cells show very distinct striation of the wall (Fig. 90, G), 

 which become less evident as they approach the thinner-walled 

 parenchyma of the central part of the stem. No trace of a cen- 

 tral cylinder of conducting tissue, such as is found in most of 

 the Mosses, can be found in Sphagnum, 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. 89, C), but 

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

 layers, which at this stage are very conspicuous (Fig. 89, B). 

 Later there may be a further division, so that the cortex of the 

 main axes frequently is four-layered. While the cells of the 

 young cortex are small, and the tissue compact, later there is 

 an enormous increase in the size of the cells, which finally lose 

 their protoplasmic contents and resemble closely the hyaline 

 cells of the leaves. Like the latter, the cortical cells are per- 

 fectly colourless, and usually have similar circular perforations 

 in their walls. The resemblance is still more marked in S. 

 cymbifolium, where there are spiral thickened bands, quite like 

 those of the hyaline leaf cells. On the smaller branches the 

 cortical cells (Schimper (i), p. 39), have been found to be of 

 two kinds — the ordinary form and curious retort-shaped cells 

 with smooth walls and single terminal pore. 



The Branches 



Leitgeb ( i ) has studied carefully the branching of Sphag- 

 num, which corresponds closely to the other Mosses investi- 

 gated. The branch arises from the lower of the two cells into 



