LIVERWORTS AND MOSSES. 



47 



ing the whole of the outer region occupied by a distinct mass 

 of mechanical tissue for stiffening the stem, and, near the 

 center, a strand known as " conducting tissue," which may 

 act as a line of transfer for water or food. 



Fig. 42.-/4, gametophyte of Polytrichutn commune^ with rhizoids below. B, gameto- 

 phyte of Hylocomium sphndens, bearing three sporophytes near top. Natural size. 

 — ^After Kemer. 



57. The leaves are also more highly developed than in 

 liverworts. They are always sessile and are arranged in two 

 (rarely), three, or more vertical ranks along the stem, and 

 consist usually of a single sheet of green cells, the blade (figs. 

 43, 44), and a central rib running from base to apex (fre- 

 quently wanting), which is composed of elongated conduct- 

 ing and strengthening cells (figs. 43, 44). In some the 



