15] VEGETATIONAL TYPES OF FRESH WATERS 503 



the sedimentary ooze. This last usually includes contributions 

 from the plankton and pleuston which may accordingly be considered 

 part of the sere, though their non-essential nature, provided inorganic 

 sediments are sufficient to build up the bed, makes them rather of 

 proseral significance. Similarly proseral in nature are many of the 

 benthic communities of the profundal and lower infralittoral, though 

 those of the shallower, upper littoral zones commonly form part 

 of the autogenic main sere. These may include the ' Characetum ', 

 which often occupies the floors of bodies of water or shoals chiefly 

 from 8 to 12 metres deep, and is characterized by the curious Green 

 Algae known as Stoneworts (species of the genera Chara and Nitella), 

 by Bushy-pondweeds or Naiads {Najas spp.), and by various aquatic 

 Mosses such as species of Drepanocladus and Fontinalis. These 

 plants are widely important not only in retaining silt and depositing 

 humus, but also in ' binding ' the surface on which they grow. 



The first of what appear to be the normally essential portions of 

 the hydrosere is the ' submerged aquatics ' stage. Although this 

 may to some extent be represented by members of the Characetum, 

 more familiar in the zone expressing this stage in the north-temperate 

 regions are such plants as many of the Pondweeds {Potamogeton 

 spp.), Tape-grasses [Vallisneria spp.). Water-milfoils {Myriophyllum 

 spp.), Horn-worts {Ceratophylhim spp.), and Water-weeds {Elodea 

 spp.). Most of these are normally rooted, but various lower plants 

 such as the often associated larger Algae and Mosses lack true roots 

 although they may grow attached to the substratum by means of hold- 

 fasts, rhizoids, etc. Some even of the higher plants are normally 

 unattached, floatmg freely as more or less dense carpets or * beds ' 

 near the bottom of quiet waters whence they may extend into the 

 general body of the pond or lake. These submerged plants tend 

 to cover slopes down to a depth of about 6 metres, where decreasing 

 light-intensity becomes a limiting factor, though some Mosses may 

 persist more deeply. Spring-moss {Fontinalis sp.) being said to form 

 carpets down to depths of about 20 metres in some clear alpine 

 lakes. Stoneworts (Charales), looking like small vascular plants, 

 may grow even deeper down, and a single Moss has been found 

 growing at a depth of 60 metres in Lake Geneva. Fig. 167 showed 

 the relationship of this stage to other early ones of the hydrosere. 



These relatively bulky submerged plants collect silt and organic 

 matter produced by other living forms and add this material, 

 together with the results of their own decomposition, to the bed 

 which accordingly becomes built up more quickly. In time it is 



