492 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



proportion of the biomass of most bodies of fresh, brackish, or inland 

 saline waters. It is chiefly away from the shallowest waters, in 

 which coarse vascular plants usually predominate provided the bed is 

 suitable for their rooting, that these smaller benthic and allied forms 

 are in real evidence, though among the marginal vascular plants there 

 are usually to be found numerous bottom-attached, epiphytic, and 

 unattached Algae or other cryptogams. Indeed a definite gradation 

 can often be traced in the ' microflora ' of smaller forms as one passes 

 from marginal to outer reed-swamps and from the latter to stony 

 or muddy inorganic beds. For example, there are progressive 

 changes in the algal flora associated with modifications in the bottom 

 as it becomes less and less organic in nature, passing from an eutrophic 

 to an oligotrophic condition. This is regardless of the central lake- 

 basin usually being covered with sedimentary ' ooze ' consisting of 

 mixed organic and inorganic matter {see pp. 496-7). 



The benthic and allied organisms exist about the interface between 

 free water and the usually heterogeneous bottom or its covering, and 

 consequently their relationships are apt to be more complicated than 

 those of plankton. This is furthermore the case because different 

 lake-basins are formed in various ways and are overlain by different 

 materials. Also the basins are variously altered after being filled 

 with water — the shores by wave-action, the bed by deposits of 

 sedimentary ooze — so that, with time, there is normally a pro- 

 gressive decrease in depth. Frequently the form indicated dia- 

 grammatically in Fig. 167 develops, in which, on the outermost 

 shore, wave action has created a small cliff and an erosional terrace. 

 To this there is adjoined a usually much wider depositional terrace 

 that consists of sediments, the surface of which is controlled by 

 wave action. This latter, sublittoral terrace may extend 100 metres 

 or more out into the lake before descending more steeply as the in- 

 fralittoral slope, which, in turn, changes into the central plain of 

 descending or level deep-water sediments. The water above this 

 central plain is the main region of pure plankton, for the shore 

 terrace, the infralittoral slope, and to some extent often the central 

 plain itself, are all inhabited by benthos. 



It is convenient here to distinguish as littoral that portion of the 

 profile inhabited by photosynthesizing plants, and to subdivide it 

 into the following three zones : (a) The enlittoral, which is character- 

 ized by fluctuating water-levels and hence conditions, being more- 

 over a zone in which wave action on shore-lines may have a consider- 

 able effect, {b) The usually much more extensive sublittoral, which 



