CHAP, xxxvn FORMATIONS OF AQUATIC PLANTS 155 



disturbed sand is utterly devoid of plants, as is the case with vast tracts 

 of the North Sea bed ; Heligoland lies like an oasis in a desert whose 

 sandy surface is ceaselessly set in motion by breakers or by tides, and 

 is therefore absolutely unfitted for the germination of algal spores. 



On the shore, hydrophytic formations often very gradually pass over 

 into the marsh-plant formations : this is especially true of communities 

 living on loose soil. Marsh-plants are also zonally arranged. 1 



Subjoined is a general synopsis of the oecological class constituted 

 of hydrophytes, and grouped into formations, which will be treated 

 in the succeeding chapters. 



I. FREE-FLOATING OR FREE -SWIMMING (PLANKTON AND 



PLEUSTON) 



1. Plankton-formation. 



2. Cryophyte-formation. 



3. Hydrocharid-formation (Pleuston). 



II. FIXED (BENTHOS) 



A. To rocks or stones. 



1. Lithophilous spermophytic formation. 



2. Lithophilous algal formation (Nereid). 



a. Halophilous nereid-formation. 



b. Limnophilous nereid-formation. 



B. To loose soil. 



1. Microphyte-formation. 



2. Enhalid-formation. 



3. Limnaea- formation. 



CHAPTER XXXVIII. PLANKTON-FORMATION 



THE term ' plankton ' was introduced in 1887 by Hensen, 2 to denote 

 bodies, dead or living, plants or animals, that float passively in water 

 and are conveyed about by wind or current. Here we are concerned 

 only with phytoplankton, which always consists of minute plants (micro- 

 phytes) ; some of these are autophytes, which can manufacture organic 

 substances from inorganic material, while a far smaller number are 

 bacteria and fungi living on the autophytes or on their products. 



FLORA 



Phytoplankton-organisms are all minute ; they are solitary or colonial, 

 and unicellular or multicellular. They belong to widely different syste- 

 matic groups of low organization, to wit : 



i. Cyanophyceae 3 may occur in masses and colour the water bluish- 

 green, sap-green, grey-green, or red. In the sea there occur species of 



1 Warming, 1906. " Hensen, 1887. * See Wille, 1904. 



