372 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



with sand and plant remains, and gradually the vegetation extends 

 to form a complete covering of the shingle, so that on old shingle 

 beaches away from the direct influence of the sea, inland types of 

 vegetation commonly develop, and in time scrub or even forest. 



Except that, usually, only inland plants are involved and the early 

 stages tend to form long downward stabilized strips separated by 

 dynamic tracts, much the same sequence takes place on many talus 

 screes and heaps of detritus. These, when reasonably stable and 

 including a fair amount of comminuted ' soil ', can, with the help 

 of their surface crevices, run the whole gamut of the xerosere with 

 relative speed and ease right up to a forest climax. Otherwise the 

 lithosere is usually very slow in progressing, especially in its early 

 stages. Although these stages may be represented by highly 

 characteristic cryptogamic or herbaceous communities, these norm- 

 ally occupy only very limited areas of rock, such as occur naturally 

 in temperate regions chiefly on clifi^s and rocky outcrops. 



The hydrosere also affords characteristic serai communities such 

 as floating-leaf, reed-swamp, sedge-meadow, and damp scrub ones 

 which were outlined in the last chapter and may severally cover 

 considerable areas. Although they vary widely in different instances 

 and especially in different regions within the temperate and allied 

 belts, these communities are mostly too familiar and obvious to 

 require description in any detail here. Dominants on both sides 

 of the North Atlantic include Water-lilies {N ymphaea spp.) in the 

 floating-leaf stage. Cattails or Reedmaces {Typha spp.) in reed- 

 swamp, various Sedges {Carex spp.) in the sedge-meadow, and 

 Alders {Alnus spp.) in the damp scrub. Examples of most of these 

 features are described and illustrated elsewhere in this chapter or 

 the immediately preceding one (stages of hydroseres being shown 

 especially in Eigs. 89, 90, 91, and 102). 



Besides the various natural or semi-natural scrublands mentioned 

 above, there are the often characteristic scrubs and thickets belonging 

 to subseres that follow cutting or burning of forests in temperate 

 and allied regions. These unstable types may occupy considerable 

 areas, though usually not without more or less recent human dis- 

 turbance or intervention. Even the later and taller stages of these 

 secondary growths are frequently dominated by plants — such as 

 ' weedy ' Birches or Poplars — that do not occur in the climax 

 forest. 



There remain to be mentioned among tvpes naturally occupying 

 substantial areas in temperate and allied regions the various marshes. 



