\Chup. XLVIII 



MOSSES AND LIVERWORTS 



651 



Fig. 300. Method of drying peat bricks cut from tfie black vertical wall in the 

 foreground. Degermoor, Southern Bavaria. 



The true mosses, or Bryales, consist of about 13,500 species classified 

 among 80 families and 655 genera. This is the largest group of the mosses. 



The liverwort plant. Two groups of liverworts are easily recognizable: 

 the thalloid and the leafy forms. The thalloid species are easily distin- 

 guished from mosses by their irregular, leaf-like, much-branched vegeta- 

 tive body (Fig. 301). The thalli (sing, thallus) may be from one to 

 several cell layers in thickness, and are sometimes dichotomously 

 branched at short intervals. The middle line of the thallus is a groove in 

 some species and has the appearance of a midrib, but no veins are 

 present. 



The leafy liverworts have flat shoots and the conspicuous leaves grow 

 in two ranks. There is a third row of smaller leaves on the under side of 

 the stem in many species. The leaves are entire, lobed, folded, or 

 dissected. 



Many of the thalloid liverworts, such as Marchantia (Fig. 302), have 

 distinctly differentiated tissues. Above is an epidermis with open pores 

 beneath which are cavities, or air chambers, containing tufts of erect 

 green filaments. Photosynthesis occurs in these upper cell layers. Below 

 are large, compactly arranged cells containing water; and extending from 

 the lower epidermis are scales and one-celled rhizoids. 



