14] VEGETATIONAL TYPES OF TROPICAL LANDS 463 



Particularly characteristic are the peaty moor-forests originating 

 in oHgotrophic waters, which are widespread in the rain-forest region 

 of southeastern Asia, where they are evergreen and dominated by 

 dicotyledonous trees. These last may be as much as 30 metres 

 high on the edge of such areas where the climax has been reached, 

 but often diminish gradually towards the centre where the vegeta- 

 tion typically consists of earlier stages, including dwarf forest and 

 even pools of water. Kneed and other aerating roots may help to 

 make the surface of the ground an ' impenetrable ' jungle even in 

 the mature forest. The dominants are often species peculiar to this 

 type of vegetation ; they are relatively few in number and show a 

 strong tendency to be gregarious, with a single species sometimes 

 forming an almost pure community. Often associated are Palms 

 and Screw-pines, with abundant epiphytes and herbaceous swamp- 

 plants among the furnishings. 



In eutrophic waters the early stages clearly consist of free-floating 

 ' sudd ' and communities of submerged aquatics, followed, when the 

 water becomes sufficiently shallow owing to silting, by rooted 

 floating-leaf vegetation consisting of Water-lilies, etc. This prepares 

 the habitat for emergent aquatics that soon constitute the reed- 

 swamp stage, which in turn is succeeded by scrub or low forest. 

 Though it may contain more trees per unit area, the (edaphic) 

 climax canopy tends to be more open than in rain forest, so that 

 light-loving species are commonly included in the undergrowth. 

 Moreover, the total number of species per unit area of this climax 

 tends to be smaller than in the rain forest, but greater than in the 

 serai stages. 



It is of interest to note that not only do tropical hydroseres run 

 much the same course as temperate ones, the recognizable stages 

 often having a closely comparable physiognomy, but many of the 

 genera involved are the same in both these main climatic zones, 

 and not a few of the species are closely related or, in some instances, 

 identical. This is especially the case in the early stages in eutrophic 

 waters, when, as in other extreme habitats, only a few co-dominants 

 or even a single dominant may prevail. Thus in Panama the 

 Common Reed [Phragmites communis agg.) and/or Narrow-leafed 

 Cattail {Typha angustifolia) may largely dominate the reed-swamp 

 stage, as may identical or similar species over much of temperate 

 North America and western Eurasia. Characteristic inhabitants of 

 rocks in rushing water are representatives of the peculiar family 

 Podostemaceae. 



