282 ELEMENTARY STUDIES IN BOTANY 



and its plants have been called the pioneers of land vegeta- 

 tion ; for their growth and the entangled detritus make the 

 water more and more shallow, until finally the reed plants 

 are compelled to migrate into deeper water. In this way 

 small lakes and ponds may become converted first into 

 ordinary swamps, and finally into wet meadows. Instances 

 of nearly reclaimed ponds may be found, where bulrushes, 

 cat-tails, and reed grasses still occupy certain wet spots, 

 but are shut off from further migration. 



In many regions, especially in our northern states, there 

 is a peculiar kind of swamp association, characterized by the 

 abundant growth of the bog moss or peat moss, and de- 

 veloped in undrained swamps. Growing out of the springy 

 moss turf there are numerous peculiar plants, such as heaths 

 and orchids, and the curious carnivorous plants ( 127, 

 p. 218). Often trees encroach upon peat bogs and a swamp 

 forest is the result. The chief types in this case are the coni- 

 fers, and on this bog-moss foundation there occur larches, 

 certain hemlocks and pines, junipers, etc. The larch or 

 tamarack is a very common swamp tree of the northern 

 regions, usually occurring in small patches ; while the larger 

 swamp forests are composed of dense growths of hemlocks, 

 pines, etc. (Fig. 234). 



The two illustrations just given are from hydrophytic asso- 

 ciations, but the swamp forest is approaching the mesophytic 

 conditions. 



Plant associations inhabiting dry, sandy ground are very 

 common, but perhaps the most extreme type of sand as- 

 sociations is that which inhabits dunes, and may be called 

 dune associations. On certain borders of the Great Lakes 

 and of sea coasts, beyond the beach, the dunes occur. They 

 are billows of sand formed by the prevailing winds, and in 

 many cases they are continually changing their form and are 

 frequently moving landward (Fig. 235) . In the case of these 

 moving dunes a peculiar type of vegetation is demanded, 



