24 MOSSES AND FERNS chap. 



as in Riccia glauca, or they may be large chambers separated 

 by a single layer of cells from their neighbours. Such forms 

 occur In most of the higher Marchantiaceae. 



The growth of the thallus is due to the division of a small 

 group of cells occupying the bottom of the heart-shaped indent- 

 ation in the forward part of the thallus. Sections parallel to 

 the surface, cutting through this group, show a row of marginal 

 cells that appear very much alike, and it is impossible always 

 to tell certainly whether or not there is a single definite initial 

 cell. Such a single initial is unquestionably present in the 

 earlier stages, and it is quite possible that it may persist, but 

 owing to its small size and its close resemblance to the adjoin- 

 ing cells, this cannot be positively asserted. In vertical sections 

 the initial cell (or cells) appears nearly triangular, with the 

 free outer wall somewhat convex. From this cell two sets of 

 segments are cut off, the dorsal segments giving rise to the 

 green tissue, and the lower segments producing the ventral 

 lamellae and colourless lower layers of cells of the thallus. 



The plants multiply asexually either by the older parts of 

 the thallus dying away and leaving the growing points isolated, 

 or lateral branches, which are often produced in great numbers 

 from the lower surface of the midrib, become detached and each 

 branch forms a separate plant. The well-known gemmae of 

 Marchantia and Lunularia are the most striking examples of 

 special asexual reproductive bodies. 



The sexual organs are always derived from the dorsal 

 segments of the apical cell, either of the ordinary branches or 

 of special shoots. The archegonium is of the regular form, and 

 the antheridium always shows a series of transverse divisions 

 before any longitudinal walls are formed in it. 



While the gametophyte may reach a very considerable 

 degree of specialisation, the sporophyte is relatively insignificant 

 even in the higher forms, and has the foot and stalk poorly 

 developed. While the Marchantiaceae grow for the most part 

 in moist situations, and some of them, e.g. Marchantia polymoi'pJia, 

 are very quickly killed by drying, some species, e.g. Riccia hirta^ 

 a common Californian species, grows by preference in exposed 

 rocky places exposed to the full force of the sun. This latter 

 species as well as several others of the same region, e.g. 

 Fivibriaria Califomica, Targionia ]iypop]iylla, do not die at the 

 end of the rainy season, but become completely dried up, in 



