508 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



marked ones in the numbers of species of Green, Brown, and Red 

 Algae present, and in the general luxuriance of the vegetation. At 

 the lower concentrations in middle and high latitudes some of the 

 persisting Brown Algae, particularly, often take on characteristic 

 dwarf forms, and the plankton becomes poor in species and more 

 and more limited to freshwater or brackishwater types. There may 

 also be superimposed layers of waters of different salinities, and 

 supporting different Algae — especially when currents of different 

 origin meet but do not mix. 



By far the most abundant solid in solution in sea- water is sodium 

 chloride (common or table salt), which on the average forms nearly 

 78 per cent, of the total salts present, contributing over 27 grams 

 per litre. It is followed in the scale of abundance by magnesium 

 chloride (nearly 1 1 per cent.) and magnesium sulphate (nearly 5 

 per cent.), though salts of calcium and potassium are also fairly 

 plentiful. Sea-water is a ' buffered ' solution, exhibiting resistance 

 to changes in its degree of basicity (the so-called ' reaction ' or 

 ' pH level ', due to the concentration of free hydrogen-ions present). 

 Thus plentiful carbon dioxide is normally available for photosynthesis 

 without disturbance of the buffered state, and the prevailing slight 

 alkalinity enables living organisms to extract calcium carbonate, etc. 

 Especially is this easy in warm seas — hence their numerous large 

 calcareous shells, coral-reefs, and so forth. Containing, as it does, 

 all of the chemical elements essential to the growth and maintenance 

 of protoplasm, sea-water is in general a very appropriate environment 

 for living cells — provided they are adapted to its concentration of 

 salts. 



Different Algae vary enormously in their tolerance to variations 

 in the salt-content of the water — from the narrowly stenohaline 

 species requiring the salinity to remain within a narrow range (these 

 are represented by most oceanic forms), to the broadly euryhaline 

 ones that grow in puddles high up on the shore. Here they are 

 bathed in sea-water when the tide comes in or waves reach them, 

 the salinity being often further increased through evaporation ; but 

 after heavy rainfall they may find themselves in almost entirely fresh 

 water. The turgor adjustments involved within the cells in endur- 

 ing such changes are not fully understood, although it is known that 

 certain Diatoms living in the mouths of streams where the salinity 

 varies rapidly are able to take in or let out salt very quickly according 

 to its concentration in the water. 



Owing to the intimacy of aquatic organisms with the medium in 



