4 ;2 BOTANY OF THE LIVING PLANT 



distance. The spores thus shed germinate to form protonema, as seen 

 above (Fig. 357, p. 463). 



The Mosses show by their ubiquitous spread that they are a success- 

 ful type of Land Vegetation, though restricted by dependence on 

 water not only for their fertilisation, but also for their physiological 

 activity. Their capacity for retaining their vitality under drought, 

 and their subsequent recovery often saves them. But the feature which 

 leads most directly to their success is their profuse vegetative propaga- 

 tion by protonema and by gemmae. Not only do the latter secure 

 their spread, but the former provide also for their persistence in the 

 soil whenever conditions at the surface are unfavourable. The 

 prevalence of Mosses in all relatively humid climates is thus easily 

 explained. 



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 flat thallus, 

 of form similar to that of a Fern-prothallus, with rhizoids on the 

 lower surface, but no other appendages. Most Liverworts, how- 

 ever, bear appendages. Thus the thalloid Riccia has scales upon 



Fig. 368. 



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.) 



the ventral (lower) surface of its fleshy thallus. Moreover, 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 photosynthetic structure. 

 In the series of the Marchantiales this ventilated construction is 

 further developed, so as to render the thallus very efficient for photo- 

 synthesis on land. For instance, in Targionia (Fig. 368), the flattened 

 thallus, bearing ventral scales below, and fixed by rhizoids in the 

 soil, is differentiated into a massive lower region chiefly for storage 



