348 Aquatic Societies 



ing of the root stocks; it also prevents deep freezing 

 after the fires have run. Plants that are capable of 

 renewing their vegetative shoots from parts below the 

 level of the burning, are the ones that year after year, 

 maintain their place in the sun. 



Ill 



Bog Societies. Bogs belong to moist climates and to 

 places where water is held continuously in an amount 

 sufficient to greatly retard the complete decay of plant 

 remains. Acids accumulate, especially, humous acids. 

 The soil becomes poor in nutriment, especially in 

 available nitrogen. Plants can absorb little water, 

 at least at low temperature; and the typical bog 

 situation is therefore said to be " physiologically dry." 

 With such conditions there go some striking differences 

 in flora and fauna. The plants are "oxylophytes" like 

 sphagnum and cranberry, i. e., plants that can grow in 

 more or less acid media, and that have many of the 

 superficial characteristics of desert plants; such as 

 vestiture of hairs or scales or coatings of wax, thickened 

 cuticle, leaves so formed or so closed together as to 

 limit or retard transpiration. The kinds of plants are 

 fewer; the individuals crowd prodigiously. They are 

 eaten by animals less than in any other situation. 

 Their remains, partly decomposed, are added to the soil 

 in the form of deposits of peat. The animal population 

 is correspondingly reduced and scanty. 



Sphagnum. The most characteristic single organism 

 in such a situation is the bog-moss, Sphagnum (fig. 206; 

 see also fig. 59 on p. 147). This grows in cushion-like 

 masses of soft erect unbranched stems, that are in- 

 dividually too weak and flaccid to stand alone, but that 

 collectively make up the largest part of the bog cover. 

 The masses are loose and easily penetrated by the roots 



