BRYOPHYTA 



163 



In most species of Riccia the dorsal region is composed of erect rows of 

 cells separated by very narrow, vertical air chambers, but in some species 

 it is spongy, consisting of a loose network of large irregular air chambers 

 separated by one-layered partitions that extend in all directions (Fig. 

 137A). The uppermost cells form a rather ill-defined epidermis. In 

 Riccia nutans simple air pores are present, but in the other species air 

 pores are either rudimentary or wanting. Simple air pores consist of a 

 single tier of cells surrounding a small central opening, the cells being in 



Fig. 130. Section through the thallus of Marchantia polymorpha, showing epidermis with 

 an air pore that leads to an air chamber with green filaments, X200. Rhizoids are shown 

 below. 



several concentric circles (only one circle in Riccia natans). Simple air 

 pores occur on the thallus of most of the genera of Marchantiales (Fig. 

 1295). In Marchantia and a few related forms, however, the thallus 

 bears compound air pores. These are barrel-shaped, consisting of four 

 or five superimposed layers of cells and having both an upper and a lower 

 opening (Fig. 130). Conocephalum, Marchantia, and many other genera 

 have a single layer of air chambers from the floor of which special chloro- 

 phyllose filaments arise. Rehoulia and Asterella have several layers of air 

 chambers without green filaments (Fig. 1295). In many forms the limits 

 of the air chambers are plainly visible on the dorsal surface of the thallus 

 as polygonal areas, an air pore occurring in the center of each. 



In practically all the Marchantiales the lower surface of the thallus bears 

 numerous rhizoids and scales. The rhizoids are of two kinds, smooth and 

 tuberculate. The former have smooth walls, the latter peg-like thicken- 



