LIVERWORTS 167 



The plant body is flat and compact, lying prostrate upon 

 its substratum, and is often a thallus; that is, it shows no 

 distinction of stem and leaves, the whole body appearing 

 leaf-like (Fig. 160). The upper surface of the body is 

 freely exposed to the light, but the lower surface is against 

 the substratum and puts out hair-like processes (rhizoids) 

 for anchorage. If the body is thin, all the cells contain 

 chloroplasts; but if the body is so thick that the light 

 cannot penetrate it, the under layers of cells are not 

 green. 



92. Marchantia. Marchantia is one of the most com- 

 mon and conspicuous liverworts. The body is a thick 



chl 



FIG. 161. Marchantia, cross-section of thallus: showing lower epidermis (from 

 which, in other parts of the thallus, rhizoids are developed), two layers of 

 colorless cells (p), and one large air-chamber (, a, the bounding walls) contain- 

 ing cells with chloroplasts (chl) and pierced by a chimney -like air-pore (p). 

 After GOEBEL. 



thallus that forks repeatedly, giving the appearance of 

 notches of greater or less depth (general habit as in Fig. 

 160). The central axis of the thallus, or of a branch, ends 

 in the terminal notch, in the bottom of which, therefore, 

 is the growing tip. The upper surface of the Marchantia 

 body is blocked off into small rhombic areas, in the center 

 of each one of which is a minute opening (Fig. 162). 



A section through this body shows its general structure 



(Fig. 161). Beginning with the lower side, there is seen 



first the layer of cells forming the epidermis, from which 



the rhizoids and certain other appendages arise; above this 



12 



