212 AfOSSES AND FERNS chap. 



gonium, and which alone is provided with stomata, sometimes 

 becomes very large, and in the genus Splachnuni ^ especially 

 forms a largely -developed expanded body, which must be 

 looked upon as a specially -developed assimilating apparatus 

 (Fig. 1 06, A). 



Undoubtedly the Polytrichaceae represent the highest stage 

 of development among the Musci. This is true both in regard 

 to the gametophore and the sporogonium. The former reaches 

 in some species, i.e. P. comniune, a length of 20 centimetres 

 and sometimes more. The stem is usually . angular and the 

 closely-set leaves thick and rigid. The numerous rhizoids are 

 often closely twisted together and form cable-like strands. The 

 structure of the leaves is very characteristic, and differs very 

 much from that of the simpler type found in Funaria. In 

 PolytricJmm (Fig. 107, B) the midrib is very broad, and only 

 at the extreme margins of the leaf is the lamina one-layered. 

 Seen in cross - section " the leaf appears somewhat crescent- 

 shaped. The cells of the margin of the section, and also a line 

 of cells running through the central part, are comparatively thin- 

 walled, and the latter are empty water-conducting cells similar 

 to the tracheae in the vascular bundles of the higher plants. 

 Next to these cells on the lower side of the leaf are a number 

 of similar but somewhat smaller cells containing starch, and the 

 rest of the section is made up principally of very thick-walled 

 sclerenchyma. The outer cells contain more or less chloro- 

 phyll, but the principal assimilative tissue consists of a series 

 of vertical cell plates or laminae parallel to the long axis of the 

 leaf, and seen in cross-section appear as short vertical rows of 

 four or five cells. These cells contain numerous chloroplasts, 

 and the laminae cover the whole upper surface of the leaf 

 except the extreme margin and the sheathing part of the base, 

 where they are wanting. 



The structure of the stem of P. commune is thus described 

 by Goebel.^ " A transverse section of the stem shows the 

 following structure. In the centre is a cylinder of broad thick- 

 walled cells, with here and there those whose walls have remained 

 thin. The thickened walls show a yellowish colour. Surround- 

 ing this cylinder is a ring of several layers of thin-walled narrow 

 cells, which is bounded on the outside by from one to three 

 layers of cells with thin, mostly very dark brown walls. These 

 1 Vaizy (3). ^ Strasburger (10). ^ Goebel (8), p. 369. 



