598 TEXTBOOK OF BOTANY 



A marsh is dominated by cattails, grasses, sedges, and rushes, and 

 may be found in either temperate or tropical regions. A swamp is 

 usually regarded as an area where the water never covers the soil deeply 

 but is never far below the surface, and where the vegetation is domi- 

 nated by shrubs and trees, such as buttonbush, low willows, alders, 

 and swamp trees. 



Bogs differ from marshes and swamps in that mosses form an im- 

 portant part of the vegetation; they and the other aquatic plants become 

 enmeshed in a floating substrate which rises and falls with the water 

 level and upon which sedges, grasses, low and tall shrubs, and even 

 trees may subsequently grow. Many northern lakes have an open area 

 of water surrounded by floating marginal mats of herbs, bog mosses, and 

 shrubs. As peat accumulates below the mat the bog may become more 

 or less solid and support a forest of conifers, such as black spruce, tama- 

 rack, and arbor vitae. 



Bogs may be either acid or alkaline. If alkaline, the water is gen- 

 erally clear, and shrubby cinquefoil occurs among the shrubs; if acid, 

 the water may be brown in color, and sphagnum moss is usually 

 present in the substrate. Bog water is usually lower in dissolved oxygen, 

 free carbon dioxide, and the salts of potassium and nitrogen than that 

 of the more open type of lake. Likewise, the plant and animal popula- 

 tion in bog lakes is comparativelv low, and there are fewer species. 

 In the Great Lakes region and northward, a peat substrate may accumu- 

 late on valley bottoms from bog plants that grew there. 



The term "bog" is frequentlv applied in the Southern States to wet, 

 acid sand flats on which manv characteristic bog plants grow, as well as 

 to areas having a peat and muck substrate. Some typical bog plants 

 associated with the grasses, sedges, and mosses of acid bogs are cran- 

 berry, blueberry, snowberry, leatherleaf, pitcher plant, sundew, and 

 dwarf birch. 



The Salt-water Environment 



The oceans, bays, gulfs, and a few inland bodies of salt water are also 

 habitats of aquatic plants and animals. These marine habitats differ 

 from the fresh- water environment principally in being saline. The salin- 

 ity is due largely to chlorides and sulfates with smaller percentages of 

 carbonates. Few species of plants can grow both in the sea and in fresh 

 water. The salinitx' is essential to the marine species and toxic to the 

 fresh- water species. 



