34 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



of the plankton — the more or less passively floating or drifting plant 

 and animal population of seas and lakes — and as such are of vital 

 importance as the ultimate source of food of many fishes and other 

 sea and freshwater animals. They are virtually world-wide in dis- 

 tribution, being found, for example, on and in damp soil and upon 

 as well as under the sea-ice even about the North Pole. When 

 they decompose or are digested by animals, their siliceous valves 

 usually do not decay but sink in considerable quantities to the 

 bottom of the body of water, often forming extensive deposits of 

 ' diatomaceous earth '. This is widely used for scouring, filtering, 

 insulation, and other purposes. 



DiNOPHYCEAE : These are the Peridinians or Dinoflagellates — 

 usually motile, microscopic unicellular organisms of yellowish or 

 brownish colour and sometimes of marked luminescence. A few 

 are naked but the vast majority have cellulose walls, which are 

 often composed of several sculptured plates. Fig. 7 shows three 

 flagellated, motile examples ; but even those types which are non- 

 motile and filamentous reproduce by motile spores (zoospores) that 

 have the form of typical Dinophyceae. These 

 have a particularly characteristic feature — two 

 grooves at right-angles, one of which encircles 



ABC 



Fig. 7.— Dinoflagellates (Dinophyceae). A, Gyumodimum, a type without plates 



{/. about 1560); B, Goniaulax, a type armoured with plates ( • about 1200); 



C, Ceratium, with plates and form-resistance ( •, about 300). 



