188 PLANT RELATIONS. 



the forms associated with sedges and grassea may vary 

 widely in different associations (Fig. 164). 



In almost all swamps there is a lower stratum of vege- 

 tation 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 temperature con- 

 ditions 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 becomes little more than a 

 great bed of moss, and it is in such conditions that peat 

 may be formed. 



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

 associated with swamps, and are doubtless derived from 

 them. If a swamp, 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 associations. 



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

 peculiar type of swamp association. 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 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 suggest 

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

 ing bogs. It is interesting to know what conditions keep 

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

 of the sphagnum-moor seem to be entirely different from 



