15] VEGETATIONAL TYPES OF FRESH WATERS 505 



The floating-leaf plants tend to be still more prolific than preced- 

 ing stages and to build up the bed still more rapidly by their col- 

 lection of silt, etc., and addition of their own material on death. 

 Consequently in time there can be colonization by swamp plants 

 and emergent hydrophytes of which at least half the body is aerial. 

 These rapidly predominate over the floating-leaf plants, many of 

 which are soon crowded out, and so the reed-swamp stage is attained. 

 Whereas the water is commonly ^-3 metres deep in the floating- 

 leaf stage, in the reed-swamp stage it is usually less than i metre 

 deep. Here the main dominants are of such types as the Common 

 Reed [Phragmites communis agg.). Bulrush {Scirpus lacustris). Reed- 

 maces or Cattails {Typha spp.), Water Horsetail {Equisetum fluviatile), 

 and various Sedges [Carex spp.), or Papyrus {Cyperus papyrus) in 

 tropical rivers, one or other of which often forms a practically pure 

 stand. Even within this zone there may be some differentiation, 

 the Bulrushes, for example, occupying the deeper water. 



Subsequent events in the hydrosere are outlined and illustrated 

 in Chapter XI ; but it should here be remarked that many marsh- 

 plants {helophytes, recognizable within the more general category 

 of ' hygrophytes ', or plants of moist habitats) and water-plants 

 {hydrophytes) are much alike in their morphological and anatomical 

 characteristics. There are, however, some marked differences, as 

 the following characterizations will indicate. In underwater parts 

 the cuticle and other features curtailing transpiration are reduced, 

 stomata being abolished but aerenchyma (^^e below) much developed. 

 Moreover, the vascular tissue even in stems is often arranged centrally 

 when they develop under water, there being no secondary thickening. 

 Such a structure gives tensile strength but allows flexibility, as water 

 affords suflicient support for rigidity to be unnecessary — even 

 though this same water may exert a dangerous pull. In the stems 

 of real marsh-plants, however, the supporting and conducting 

 elements are usually arranged peripherally, as these stems have to 

 stand more or less erect after the manner of those of land plants 

 and meanwhile must have sufficient conducting elements for rapid 

 transpiration. On floating leaves, stomata are usually confined to 

 the upper surface, while submerged leaves are often finely dissected 

 to facilitate the exchange of materials. The same characteristic may, 

 incidentally, save them from being torn by currents. 



As it is more difficult to obtain oxygen in water than in air, and 

 still more diflicult in waterlogged mud, most aquatic and marsh 

 plants have systems of large air-canals or air-filled cells, constituting 



