288 ECOLOGY [CH. 



Phragmites followed by Scirpus lacustris ; next, a belt of plants 

 with floating leaves, among which Nymphaea lutea is the domi- 

 nant species, and, still farther from the shore, a zone of plants 

 with leafy shoots reaching to the water surface, or nearly, con- 

 sisting mainly of Potamogetons. To this succeeds a region in 

 which the upper layers of the water are free from vegetation, 

 while the grappling iron brings to light various plants which 

 grow on the bottom, such as Ceratophyllum, Naias, Chara and 

 Nitella. Fig. 165 shows, in the form of a section, the essentially 

 similar zones of vegetation in the White Moss Loch in Perth- 

 shire 1 . 



; !G/U.IUM PALUSTRE 

 ' COMAWH 



i CARPC FlUFORMISi POT/IMOCET/JN HETEROPHV1U/S 

 | ^EWANTHE|5 MyRIOPHYIjUJM 



I PitlflCHOJDES P PEIfFOUATUS 

 ! I ' 'VITF 



FIG. 165. Section nearly N. and S. across White Moss Loch, Perthshire, showing 

 relations of plants to water environment. [Matthews, J. R. (1914).] 



One of the chief reasons determining this zonation seems to 

 be that plants with floating leaves can only flourish if guarded 

 from the wind. For this reason they generally do not occur at 

 a great distance from the shore, except in very sheltered basins, 

 and often obtain the necessary protection by growing among 

 reeds. It has been pointed out that in the larger English Broads, 

 the "floating-leaf association" is almost coterminous with the 

 "open reed-swamp 2 ," while in Lake St Clair (Michigan) pre- 

 cisely the same thing occurs, the plants with large floating leaves 

 all belonging to the " Phragmitetum 3 ." In the case of the White 



1 For recent views on ecological classification of aquatics, see 

 Pearsall, W. H. (1917-1918) and (1918). 



2 Pallis, M. in Tansley, A. G. (191 1). 3 Pieters, A. J. (1894). 



