324 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



form families (of a single species) or colonies (of two or more species) ; 

 (5) competition (virtually the struggle for existence) among the 

 colonists, particularly for space, light, water, and nutrients ; (6) 

 invasion by other plants, usually from adjacent areas ; (7) reaction, 

 which essentially comprises the changes wrought in habitat 

 conditions by the plants themselves {e.g. in soil formation) ; (8) 

 coaction, the influence of organisms upon each other ; (9) stabiliza- 

 tion, which of course is only relative, change being inevitable in all 

 living organisms and aggregates thereof; and (10) attainment of a 

 climax, by which time competition has generally become so intense 

 that further invasion is problematical unless the community is 

 drastically disturbed. 



The complete sere just indicated is a primary sere (prisere), be- 

 ginning on a bare substrate without organic material. The chief 

 types of primary seres are those initiated (i) in fresh water (hydro- 

 seres), from which may be distinquished ' haloseres ' beginning in 

 saline water ; (2) on damp aerial surfaces such as alluvial mud 

 (mesoseres) ; and (3) on dry materials (xeroseres), of which out- 

 standing examples are those starting on bare rock (lithoseres) and 

 on dry sand (psammoseres). Secondary seres (subseres) are merely 

 partial, beginning after the succession has been stopped, and thus 

 not going back to a purely inorganic substratum unaffected by plants. 

 They are distinguished as ' hydrarch ', ' mesarch ', or ' xerarch ', 

 according to whether their initiation is under damp, median, or 

 dry conditions, respectively. 



The broad tendency of succession is from simplicity to complexity 

 of organization, and from dominance by lower to higher life-forms 

 which make more and more exacting demands on the habitat. Yet 

 sometimes we see ' retrogression ' to dominance by a lower life-form, 

 for example when the habitat undergoes a change to less favourable 

 water conditions. An incidental change in normal successions is 

 from open to closed conditions, involving also an increase in the 

 intensity of competition and marked alteration of local climatic and 

 edaphic factors such as atmospheric humidity, wind, and the humous 

 content of the soil. Such ' reactions ' are reciprocal, the plants 

 affecting the habitat, which in turn affects the plants ; indeed, many 

 of the higher life-forms only enter when the ground has been suitably 

 ' prepared ' for them by the dominants of earlier stages, which in 

 turn they ruthlessly overshadow and frequently oust. 



We will now outline examples of the main tvpes of priseres as 

 postulated for temperate forested regions ; although the tendencies 



