28 MOSSES AND FERNS CHAP. 



ing increase in the number of the marginal cells. The central 

 cells of the marginal group now begin to grow more vigorously 

 than the others and to project as a sort of lobe (Fig. 3, C, L), 

 and this lobe divides the initial cells into two groups lying 

 on either side of it. As soon as this is accomplished each 

 new group of initial cells continues to grow in the same manner 

 as the original group, and two new growing points are estab- 

 lished, each of which develops a separate branch. The growth 

 of the middle lobe is limited, and it remains sunk in the fork 

 between the two new branches. 



The thallus is attached to the substratum by rhizoids of 

 two kinds. The first are smooth-walled elongated cells, with 

 colourless contents, the others much like those of the higher 

 Marchantiaceae. Their walls are undulating, and projecting 

 inward are numerous more or less developed spike-like protu- 

 berances. The rhizoids arise from large superficial cells of 

 the ventral part of the midrib. They are readily distinguished 

 from the adjacent cells by their much denser contents, even 

 before they have begun to project. 



The arrangement of the tissues of the fully-developed 

 thallus is best seen in vertical cross-sections. In R. glauca and 

 allied forms four well-marked tissue zones can be readily 

 recognized in such a section. The lowest consists of a few 

 layers of colourless rather loose parenchyma, from which the 

 rhizoids arise, and to which the ventral lamellae are attached. 

 Above this a more compact, but not very clearly limited region, 

 the midrib. The elongated form of the midrib cells, which 

 contain abundant starch but no chlorophyll, is, of course, not 

 evident in cross-section. Radiating from the midrib are 

 closely-set rows of chlorophyll-bearing cells with the charac- 

 teristic narrow air-spaces between. The median furrow is very 

 conspicuous in such a section, and extends for about half the 

 depth of the thallus. Terminating each row of green cells is 

 the enlarged colourless epidermal cells, often extended into a 

 beak-like appendage. In some species, e.g., R. trichocarpa, 

 some of the surface cells grow out into stout thick-walled 

 pointed hairs. 



The Sexual Organs 



In Riccia the sexual organs are formed in acropetal suc- 

 cession from the younger segments of the initial cells, and 



