II MUSCINEJE— HEP ATICS— MARCH ANTIALES 23 



is fastened to the substratum by rhizoids, which are unicellular 

 and usually of two kinds, those with smooth walls and those 

 with peculiar papillate thickenings or teeth that project inward 

 (Fig. 12). The cells of the lower layers of tissue are usually 

 nearly or quite destitute of chloroplasts, which, however, occur 

 in large numbers in the so-called chlorophyll-bearing layer, just 

 below the dorsal epidermis. This chlorophyll-bearing layer 

 contains air-spaces in all forms except some species of 

 Dumortiera and Monodea, and these spaces are either simple 

 narrow canals, as in Riccia glauca, or they may be large cham^ 

 bers separated by a single layer of cells from their neighbors. 

 Such forms occur in most of the higher Marchantiaceas. 



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 mar- 

 ginal 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 typical 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 insignifi- 



