502 INTRODUCTION TO PLANT GEOGRAPHY [cHAP. 



The determining factor appears to be the acidic reaction, coupled 

 with the low salt-content and the abundance of humic colloids 

 in solution. 



Saline waters occurring inland vary from faintly brackish to 

 several times the average salinity of the ocean, some being completely 

 saturated with salts. The plants of very salty inland waters, such 

 as are found practically throughout the ice-free land of the world 

 but especially in arid regions, are largely Green Algae, though 

 Diatoms and Cyanophyceae may also occur, with, in addition, higher 

 plants around the shallow margins. These higher plants are usually 

 sea-shore species, so that the communities often resemble some of 

 those already described. Dunaliella salina is probably the com- 

 monest of the Green Algae inhabiting brine. It and a species of 

 Stephanoptera can even form light-green areas on solid salt crusts 

 where the water has a concentration of 33 per cent, of dissolved solids. 

 Such waters in salt ponds practically anywhere in the world may be 

 coloured reddish by the Dunaliella and have their bottoms covered 

 with a carpet of Cyanophyceae such as Microcoleus chthonoplastes. 

 Quite large species of Enteromorpha are known among the Green 

 Algae from salt springs and brine lakes, as well as from fresh waters 

 and brackish lagoons. Here fluctuating salinities often lead to the 

 production of peculiar specimens that are difficult to identify : 

 indeed the characters of several supposed ' species ' can be exhibited 

 by different parts of a single thallus! In many brackish waters, 

 such as those near the sea where incoming streams bring fresh water, 

 a wide range of Algae and other plants of both fresh and salt waters 

 often flourish : they are normally species tolerant of varying salinity 

 but, being of diflFerent degrees of halophytism, still tend to give 

 populations that vary with the habitat. Thus under only slightly 

 brackish conditions the flora is predominantly a freshwater one, 

 whereas with a close approach to full oceanic salinity it is mainly 

 marine. 



Hydroseres 



Apart from the bog succession dealt with above, there are the 

 normal seres occurring in open fresh water — especially around the 

 shallow margins of eutrophic lakes and ponds. Here the diff^erent 

 stages characteristically form more or less well-marked zones. Such 

 hydroseres can have their inception in deep waters that gradually 

 become shallower with the deposition of various materials comprising 



