MOSSES AND LIVERWORTS 107 
Starting from this simple type, the development of 
the gametophyte has proceeded in several directions, 
two of which are specially noteworthy. In the first 
place, while the gametophyte has retained its primitive 
thallose form, there has been a very considerable 
amount of differentiation in the tissues, which are 
divided into a dorsal region, mainly occupied by an 
elaborate system of assimilating tissues, and a ventral 
mass of colorless cells. The assimilative apparatus in 
the most highly specialized forms consists of a series of 
large chambers into which the chlorophyll-bearing cells 
project, which communicate with the outside atmosphere - 
by means of curious pores which may be compared func- 
tionally at least with the stomata of the higher plants 
(Fig. 27, B). The rhizoids are also peculiarly modi- 
fied, and scales are developed from the ventral surface 
of the thallus. In the higher members of this group 
(Marchantiacee), the sexual organs are borne upon 
modified branches, and in some cases peculiar non-sexual 
reproductive bodies, gemmz, are produced in special 
receptacles. 
The second type of differentiation is shown by the 
foliose or leafy Hepatic, the “scale-mosses.” These 
comprise much the greater part of the existing liver- 
worts, and are distinguished from the lower forms by 
having a distinct axis with definite leaves or assimila- 
tive organs (Fig. 27, E). Both stem and leaves are of 
the simplest possible structure, all the cells being alike, 
and the leaves are composed of but a single layer of 
cells, but these simple leaves form very efficient assimi- 
lating organs. The scale-mosses are much the com- 
monest of liverworts, and their adaptation to various 
