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 (rhizotds) 
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 
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 (s, 3, the bounding walls) contain- 
ing cells with chloroplasts (chl) and pierced by a chimney-like air-pore (sp).— 
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 
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