Il] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 327 



a single point, overshadow the crustaceous types, holding water and 

 fragments more effectively than their predecessors, and preparing 

 the way for the moss stage whose components usually start entering 

 as soon as soil particles accumulate in crevices and depressions. 

 Such hardy Mosses are able to withstand prolonged desiccation and 

 sometimes pioneer on uncolonized rock surfaces, making the Lichens 

 unnecessary and ' proseral '. The Mosses usually enter as spores, 

 and, among their closely aggregated axes and often densely matted 

 rhizoids, young soil accumulates rapidlv. Sometimes in the larger 

 cushions this accumulation becomes quite thick (Fig. 92), forming 



Fig. 92. — Telescoped ^tayo ot xerosere in Norwegian Lapland. Gnarled speci- 

 mens of the dominant Scots Pine ha\e managed to grow in crevices of the rock 

 whose surface is often still colonized by crustaceous Lichens, although in other 

 places cushions of Mosses or ground-shrubs have accumulated some soil. 



fine nests for the colonization and establishment of herbs — especially 

 annuals of xerophvtic tendency. These contribute further to the 

 humus accumulation and soil-building, and are typically followed by 

 more exacting biennials and herbaceous perennials, which in their 

 turn replace members of former stages while accelerating the further 

 processes of succession. Often more important than colonization 

 of the open rock surfaces is extension from crevices, at which Mosses 

 and ground-shrubs are particularly adept. In time taller woody 

 plants enter, constituting a less enduring stage that tends to overtop 

 and oust the herbs but meanwhile to improve the soil and often 



