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 sj'stem 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 

 (Marchantiaceae), the sexual organs are borne upon 

 modified branches, and in some cases peculiar non-sexual 

 reproductive bodies, gemmae, are produced in special 

 receptacles. 



The second type of differentiation is shown by the 

 foliose or leafy Hepaticse, 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 



