VI. 



THE BRYALES 



221 



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, e. g., P. commune, 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. 



FIG. 120. Dawsonia superba. A, upper part of female plant bearing a sporogonium, 

 Xi; B, a leaf, slightly enlarged; C, section of leaf, X about 70; D, part of the 

 same more highly magnified; E, two views of the capsule, 



In the Polytrichacese (Fig. 121) the midrib of the leaf is 

 very broad and only at the extreme margin of the leaf is the 

 lamina developed at all. A cross-section of the leaf shows that 

 the midrib is greatly thickened in the centre, and gradually 

 merges into the rudimentary lamina. In Dawsonia (Fig. 120) , 

 the leaf is almost flat, in Polytrichum (Fig. 121), usually 

 more or less incurved at the margin. 



The outer, or dorsal, surface of the leaf is covered with a 

 well marked epidermis, whose outer cell-walls are strongly 



