Il] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 325 



are generally similar, the outcome is apt to be different in polar and 

 tropical regions, seres in the former being often much mixed and 

 disturbed, and, in the latter, commonly retarded by lack of humus 

 accumulation. These items are explained in the appropriate 

 chapters. 



In temperate regions the typical hydrosere, after various non- 

 essential (proseral) stages of plankton, etc., starts in bodies of fresh 

 water whose beds where suitable are colonized by attached or other 

 benthic {i.e. bottom) aquatic vascular plants and Mosses besides Algae 

 as deep down as light conditions allow. These plants often form 

 dense mats that collect silt and humus, there being frequently 

 insufficient oxygen for rapid decay. The bed is thus built up 

 gradually until, at a depth of some i to 3 metres, it can be invaded 

 by floating-leaf types such as Water-lilies [Nymphaea spp.) or certain 

 Pondweeds {Potamogeton spp.), which tend to shade out the sub- 

 merged plants. The long stalks of these floating-leaf plants trap 

 silt and their coarse bodies after death become deposited as, ulti- 

 mately, humus — so that the bed is built up with relative rapidity until 

 the water is shallow enough for swamp plants to enter the community. 

 These typically form a reed-swamp whose dominants are only partly 

 submerged, building up the beds quickly and ousting the previous 

 types. As the level continues to rise owing to the deposition of 

 humus, etc., fully terrestrial invaders enter to characterize the sedge- 

 meadow stage, the reed-swamp plants disappearing in due course 

 as conditions are rendered unsuitable for them. With further rising 

 in level of the soil surface and relative depression of the water-table, 

 shrubs and ultimately trees enter and in time give rise to a hygro- 

 phytic woodland. The Alders, Poplars, Willows, etc., which com- 

 monly constitute this, will, in their turn, shade out the lower types 

 and prepare for the climax forest which requires drier and more 

 favourable soil conditions. Fig. 90 gives a diagrammatic repre- 

 sentation of the stages of a hydrosere in section, a typical example, 

 extending from the floating-leaf stage, having been shown in Fig. 89, ^ 

 while Fig. 91 continues this to the sedge-meadow and early timbered 

 stages. A more detailed account of the early stages of some hydro- 

 seres is given in Chapter XV. 



As a characteristic xerosere we will take a lithosere initiated on 

 bare rock. Such surfaces are apt to be extremely difficult to colonize 



1 This was of eutrophic (' good foods ') type, an example of the oligotropic 

 (' few foods ', geologically young) type, which may be expected in time to develop 

 into a eutrophic lake, being shown in Fig. 88. 



