462 INTRODUCTION TO PLANT GEOGRAPHY [cHAP. 



rain forest, the swamp-forest may consist of several tiers and may 

 be plentifully supplied with lianes, that in some instances are 

 described as having a short stem which reaches up only to the surface 

 of the water in the rainy season, and from which arise dispropor- 

 tionately long, slender shoots.^ The numbers of terrestrial herbs 

 depend upon such features as the depth and duration of flooding : 

 often they are few, being chiefly members of the Sedge family 

 (Cyperaceae). However, epiphytic Orchids and Ferns can be plenti- 

 ful, as can Mosses and Liverworts. 



Whereas the swamp vegetation just described is evidently hydro- 

 seral, exemplifying stages in the succession, from open water to 

 forest, that appears to take place in the same general manner as in 

 cooler regions, there is one peculiarity in the tropics, where humus 

 does not normally accumulate to any great degree. This is the fact 

 that the majority of tropical swamp soils are not peaty, containing 

 as they do little if any more humus than soils of normal drainage. 

 However, where the water is poor in dissolved mineral matter 

 (oligotrophic) some peat formation can occur, leading to the develop- 

 ment of ' moor-forests ', which are commonly called the tropical 

 equivalent of the ' highmoors ' of cool regions. Corresponding to 

 this on one hand and, on the other, to the normal (non-peaty) 

 swamp soil with a relatively eutrophic water-supply, there thus 

 appear, in the tropics, to be two types of hydrosere leading to 

 different types of climax forest which in both cases are edaphic 

 rather than climatic. For in eutrophic waters the raising of the soil 

 level is due mainly to the accumulation of inorganic sediments, 

 while in oligotrophic waters such raising is chiefly the result of 

 accumulation of plant remains. In both cases the soil level rises 

 scarcely if at all above the height of the highest water-level once 

 this has been reached, as conditions for further substantial accumula- 

 tion then cease to exist in the tropics where organic breakdown is 

 rapid. Consequentlv the hvdrosere appears to end with the forma- 

 tion of ground in which the water-table is near the surface during 

 at least part of the year. Such ground bears forest more or less 

 like the climatic climax in structure, but different in floristic composi- 

 tion owing to the local water conditions. It is thus an edaphic 

 climax and, like the hydrosere which engenders it, exists in two 

 forms according to whether the soil consists largely of deposited 

 silt or peat. 



^ A similar phenomenon may be observed in son\e areas of Amazonian rain 

 forest that are subject to periodical inundation (R. E. Schultes voce). 



