VEGETATION OF 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), Juncus bufonius (toad- 
rush), and Bartsia viscosa (yellow Bartsia), very common 
in south-western England, but rare or unknown else- 
where. 
The associations of plants growing in water-laden soil 
may be separated into two 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. 
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. 
A characteristic feature of the reed-swamp is that the 
lower parts of the plants are submerged, the upper parts 
standing straight out of the water, whilst many floating 
aquatics find a sheltered home amongst the stems. The 
dominant plants are monocotyledons and_horsetails, 
plants of a social or cespitose habit, with Jong, erect. 
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