294 



THE BRYOPHYTES 



instead of running rapidly off in floods. The lichens and mosses 

 are among the first plants to appear on barren soil or exposed 

 rocks and cliffs, and are also the plant pioneers that push 

 their way up mountains and into the arctic regions where no 

 other vegetation can live. 



294. The life history of a moss. The life history of the 

 common mosses is more complex than that of a liverwort. The 

 moss spore does not develop directly into the leafy moss plant. It 



prim 



FIG. 262. The protonema of a common moss (Funaria) 



prim, primary shoot; br, branches from primary shoot; pi, young moss plant 



or bud. After Sachs 



produces a preliminary filamentous growth, called the protonema 

 (meaning preliminary thread), which sometimes forms an exten- 

 sive network over the ground, resembling at first sight such 

 terrestrial algae as certain species of Vauclieria. The protonemal 

 filaments (Fig. 262), however, consist of cells placed end to end 

 (they are never coenocytic) ; they have generally oblique cross 

 walls and contain numerous disk-shaped chloroplasts. There 

 are no algae known which the protonema resembles in detail, 

 and yet this phase in the life history suggests what may have 

 been the life habits of ancestors of the mosses. Certain cells of 



