DIV. 



BRYOPHYTA 



491 



ment of their small leaves, which are rarely arranged in two rows. 



In Mosses which have prostrate stems the leaves, although arranged 



spirally, frequently assume a somewhat outspread position, and all 



face one way, so that in such cases 



a distinction between an upper and V* a 



a lower side is manifested, but in a 



manner different from that of the 



Liverworts. 



The STEM OF THE Moss PLANT is formed 

 of cells which become gradually smaller 

 and thicker- walled towards the periphery. 

 In the stems of many genera (e.g. Poly- 

 trichum, Mnium, ig. 96 and p. 82) there 

 is found a central, axial strand consisting 

 of elongated, conducting cells with narrow 

 lumina. These strands are not as highly 

 differentiated as the vascular bundles of 

 Pteridophytes. They have neither vessels 



FIG. 453. Andreaea petrophila. ps, Pseudopodiura ; 

 Spf, foot ; k, capsule ; c, calyptra, ( x 12.) 



FIG. 454. Polytrichum commune, rh, Rhi- 

 zoids ; s, seta ; c, calyptra ; ap, apophysis ; 

 d, operculum. (Xat. size.) 



nor sieve-tubes, but serve for the conduction of water and organic substances. 

 They are wanting in the Sphagnaceae or Bog Mosses which grow in swampy 

 places. The stems of the Sphagnaceae show a peculiar development of the outer 

 cortical layers. The cells in these layers are devoid of protoplasm, and are 

 in communication with each other and the atmosphere by means of large, open 

 pores ; to secure rigidity, they are also provided with spirally- thickened walls. 



