VEGETATION OP THE MARSH AND BOG 241 



in the wetter parts is low for the greater part of the year, 

 and only becomes warmed by the summer sun. The 

 vegetative season is consequently short, and annuals 

 have no chance of completing their life-cycle before the 

 renewal of the adverse period. In the drier parts com- 

 petition with perennials prevents the annual from be- 

 coming firmly established. The most common marsh- 

 annuals are : Ranunculus sceleratus (celery-leaved butter- 

 cup), Impatiens fulva (orange balsam), Peplis Portula 

 (water- purslane), Bidens cernua and B. tripartita (bur- 

 marigolds), Pedicularis palustris (marsh-lousewort), Poly- 

 gonum Hydropiper (water-pepper), and J uncus bufonius. 

 The associations of plants growing in water-laden soil 

 may be separated into three groups, according to the 

 character of the water : 



I. Associations in Fresh, Sweet Water. 



1. Reed-Swamp: the lower part of the plant is 



submerged in often deep water. 



2. Woodland or Bush-Swamp: associations of 



woody plants in marshy soil. 



3. True Marsh: water may reach the surface, 



but lower part of the plants not submerged. 



II. Associations in Sour, Acid Water. 



4. Bog. 



III. Associations in Salt Water. 



5. Salt-Marsh (see p. 278). 



1. Reed-Swamp. 



The reed-swamp is found along the margin of rivers, 

 canals, lakes, etc. It reaches its greatest development, 

 as in the case of aquatic vegetations, in the lowlands 

 where the water is rich in mineral salts. It is either 

 absent from the shores of highland lochs, or is very 

 scanty. The reed-swamp* closely approaches the associa- 

 tions of aquatic plants in many characters, andis sometimes 

 regarded as a constituent of the aquatic formation. The 

 lower parts of the plants are always submerged, and 

 many floating aquatics occur amongst the stems. The 

 dominant plants are monocotyledons and horsetails, 

 plants of a social or csepitose habit, with long, erect, 



16 



