326 



INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



and consequently may long remain uninvaded unless it be by pro- 

 seral colonies of Bacteria, Blue-green Algae, etc. In time, however, 

 the extreme exposure and general lack of water and nutrients are 

 usually overcome by crustaceous Lichens or other hardy cryptogams 

 as the first essential stage. They spread over the surface, helping 

 the weathering forces of nature by corrosivelv or otherwise ' eating 

 into ' the rock and adding plant material to form something of a 

 nidus (nest) for ecesis of foliose Lichens, etc. These, attached at 



Fig. 90. — Diagram illustrating stages of hydrosere with deposition of peat. The 



usual stages are discernible on the gently-sloping shore, being, from left to right, 



submerged benthic, floating-leaf, reed-swamp, sedge-meadow (fen), hydrophytic 



scrub, hygroph\tic forest, and final!}- climax forest. 



Fig. 91. — Sedge-meadow stage of hydrosere colonized by some hygrophytic shrubs 



and, on right, trees, near Prout's Neck, Maine. An extensive reed-swamp is 



visible in the middle-distance, surrounding the lake shown in Fig. 89, 



