11 MUSCINE.^— HEPATIC^— MARCHANTIACEM 29 



Marchantiaceae. Their walls are undulating, and projecting 

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

 berances. The root-hairs 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 ■a.wA 

 allied forms four well-marked tissue zones can be readily 

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

 layers of colourless rather loose parenchyma, from which the 

 root-hairs arise, and to which the ventral lamelke 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 character- 

 istic 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. Jiirta, some of 

 the surface cells grow out into stout thick -walled pointed 

 hairs. 



TJie Sexual Organs 



In Riccia the sexual organs are formed in acropetal suc- 

 cession from the younger segments of the initial cells, and 

 continue to form for a long time, so that all stages may be met 

 with upon the same thallus. While both antheridia and 

 archegonia may be found together, in the two species R. glauca 

 and R. hirta, mainly studied by myself, I found that as a rule 

 several of one sort or the other would be formed in succession, 

 and that not infrequently antheridia were quite wanting from 

 plants that had borne numerous archegonia. Both archegonia 

 and antheridia arise from single superficial cells of the younger 

 dorsal segments of the initial cells. In their earliest stages 

 they are much alike, the mother cell of the antheridium being, 

 however, usually somewhat larger than that of the archegonium. 

 The cell enlarges and projects as a papilla above the surface, 

 when it is divided by a transverse wall into an outer cell and 



