THE GREAT GROUPS OF BRYOPHYTES 
309 
203. Marchantia forms. In this line the simple thallus 
gradually becomes changed into a very complex one. The 
thallus retains its simple 
outlines, but becomes thick 
and differentiated in tissues 
(groups of similar cells). 
The line may be distin- 
guished, therefore, as one 
in which the differentia- 
tion of the tissues of the 
gametophyte is emphasized 
(Figs. 280-282). In Mar- 
chantia proper the thallus 
becomes very complex, and 
it may be taken as an illus- 
tration. 
The thallus is so thick 
that there are very distinct 
green dorsal and colorless 
ventral regions (Fig. 283). The latter puts out numerous 
rhizoids and scales from the single layer of epidermal cells. 
Above the ventral epidermis are several layers of colorless 
FIG. 280. A very small species of Riccia, 
one of the Marchantia forms : A, a 
group of thallus bodies slightly en- 
larged ; B, section of a thallus, show- 
ing rhizoids and two sporogonia im- 
bedded and communicating with the 
outside by tubular passages in the 
thallus. After STKASBUBGEB. 
FIG. 281. Rictiocarpus, a Marchantia form, showing numerous rhizoids from ventral 
surface, the dichotomous branching, and the position of the sporogonia on the 
dorsal surface along the " midribs." GOLDBERGER. 
