15] VEGETATIONAL TYPES OF FRESH WATERS 479 



of the Entomostracan genera Daphnia and Cyclops, range from the 

 polar regions to the tropics, so suggesting again that at least those 

 conditions which are critical for them are relatively uniform over 

 remarkably wide areas. 



Plankton 



Planktonic organisms are those which float freely on or in a body 

 of water ; they may be roughly divided into animal and plant types, 

 constituting, respectively, zooplankton and phytoplankton. The main 

 categories of freshwater plankton are (/) the ' limnoplankton ' of 

 lakes and ponds ; {ii) the ' potamoplankton ' of slow streams and 

 rivers ; and (m) the ' cryoplankton ' of lasting snow, neve, and ice. 

 The cryoplankton is so distinct in habitat and form that it seems best 

 treated separately in the next section. In addition we may distin- 

 guish in lakes and ponds {iv) the ' tychoplankton ' of forms transported 

 from the littoral or from affluents by currents. 



Planktonic organisms must be able to remain suspended in water, 

 and this they do either by having the power of active locomotion, 

 particularly by flagella, or, more often, in the case of phytoplankton, 

 by having suitable * form-resistance '. This latter is commonly 

 expressed in projections from the surface of the usually minute body 

 and, in addition, for success requires a specific gravity near that of 

 the surrounding medium. Thin, light, and flat or curved cefl-walls, 

 such as are often found in planktonic Diatoms, help considerably in 

 this connection ; so do gelatinous sheaths possessing almost the 

 same density as water, and light food-reserves of oil, or, of course, 

 still lighter bubbles of gas. Very important is smallness of size, 

 for we all know that large bodies sink more rapidly than small ones 

 of the same material. Also significant is the specific surface area, 

 which is the ratio of the total surface area to the volume of the body ; 

 for the larger the ratio, the greater will be the friction caused during 

 sinking, and consequently the slower this last will be. Hence the 

 frequent provision of spines, horns, ridges, and the like on the out- 

 side of planktonic organisms, such as are illustrated in Figs. 5 (note 

 especially the forms of Desmids), 6, and 7. Moreover, owing to the 

 greater viscosity of water as well as protoplasm at low temperatures, 

 the ability of planktonic organisms to float tends to be greater during 

 winter than summer. All such ' flotation-adaptations ' are, however, 

 unable to prevent fairlv rapid sinking of most except the smallest 

 among the non-motile planktonic organisms if the water is entirely 



