MUSCI AND HEPATICAE 365 



HEPATICAE OR LIVERWORTS. 



The Life-Cycle of Liverworts is on the same plan as that of the 

 Mosses, the gametophyte being the predominant generation. In the 

 simpler types it is thalloid, and may be forked. Pellia, which is 

 common on moist clay banks, is constructed like a large Fern pro- 

 thallus, but more fleshy. Most Liverworts, however, bear appendages. 

 Thus the thalloid Riccia has scales upon the ventral (lower) surface 

 of its fleshy thallus. Its upper surface is deeply penetrated 

 by narrow air-canals, each bounded by four rows of chlorophyll- 

 containing cells, of which the outermost may be enlarged. The result 

 is a ventilated photo-synthetic structure. In the series of the 

 Marchantiales this ventilated construction is further developed, so as 



FIG. 309. 



Vertical section through part of the thallus of Targionia, showing the cavities opening 

 by pores on the upper surface, and containing filaments of chlorophyll-cells. ( x 75.) 



to render the thallus very efficient for photo-synthesis on land. For 

 instance, in Targionia (Fig. 309), the flattened thallus, bearing ventral 

 scales below, and fixed by rhizoids in the soil, is differentiated into a 

 massive lower region chiefly for storage (though it is also penetrated 

 by a mycorhizic fungus through the root-hairs), and an upper photo- 

 synthetic region marked by large, overarched air-chambers. Each 

 of these communicates with the outer air by a large pore, which 

 is more or less under control. From the floor of the chamber 

 arise active green cells, grouped in simple, or branching filaments. 

 Such developments, with varying detail, are characteristic of the 

 Marchantiales. The analogy with Angiospermic leaves is obvious, 

 but the origin of the structure is here quite different, being chiefly due 

 to surface-involution. 



A distinct line of vegetative advance is shown by the Junger- 

 manniales, in which successive steps may be found from the thalloid 

 state, through various forms of marginal lobes, to a full leafy develop- 

 ment. In the truly leafy Liverworts there is a ventral row of leaves, 

 and a row of lateral leaves on either side. These leaves are more or less 



