L88 PLANT RELATIONS. 



forms associated with sedges and grasses may vary widely 

 in different societies (Fig. 1G4). 



In almost all swamp-moors there is a lower stratum of 

 vegetation than that formed by the sedges. This lower 

 stratum is made of certain swamp mosses, which grow in 

 very dense masses. Towards the north, where the tempera- 

 ture conditions are not so favorable for the sedge stratum, 

 it may be lacking almost entirely, and only the lower moss 

 stratum left. In these cases the swamp-moor becomes 

 little more than a great bed of moss, and it is in such con- 

 ditions that peat may be formed. 



139. Swamp-thickets. — Swamp-thickets are very closely 

 associated with swamp-moors, and are doubtless derived 

 from them. If a swamp-moor, with its sedge stratum and 

 moss stratum, be invaded by shrubs or low trees, it becomes 

 a swamp-thicket. It will be noticed that these shrubs and 

 trees are of very uniform type, being mainly willows, alders, 

 birches, etc. Such willow and alder thickets are very com- 

 mon in high latitudes. 



II. Xerophytic hydrophytes. 

 A. Fresh-water societies. 



140. Sphagnum-moors. — The sphagnum-moor is a very 

 peculiar type of swamp society. It is so named because the 

 common bog or peat moss, known as sphagnum, gives a 

 peculiar stamp to the whole area. Sphagnums are large, 

 pale mosses, whose lower parts may die, and whose upper 

 parts continue to live and put out new branches, so that a 

 dense turf is formed. In walking over such a bog the moss 

 turf seems springy, and sometimes trembles so as to sug- 

 gest the name "quaking bog." These are the great peat- 

 forming bogs. It is interesting to know what conditions 

 keep the swamp-moor plants out of the sphagnum-moor. 

 The plants of the sphagnum-moor seem to be entirely dif- 

 ferent from those of the swamp-moor, although the amount 



