606 RELATION TO ENVIRONMENT. 



moves above and beyond the wave action of the salt water. Rains change 

 the salt moisture from salt to fresh and the vegetation changes from halo- 

 phytes to psammophytes, the latter being grasses adapted to sandy soil 

 with fresh-water infiltration. By wind action sand is carried farther from 

 the shore, and dunes of varying size and age are formed parallel with the 

 shore. The younger ones near the shore have the open formations, while 

 farther inland the older ones begin to take on a closed formation with trees 

 and shrubs more abundant, a sign that the climax type is being reached. 

 MacMillan in 1898 has used the same method with excellent results in 

 his study of the distribution of plants on the shores of the Lake of the 

 Woods in northern Minnesota. Later Cowles (1898) has applied the 

 same principle in the study of the Physiographic Ecology of Chicago, and 

 in his studies of the sand-dunes of Lake Michigan. Whitford (1898), 

 Genetic Development of the Forests of Northern Michigan; Kearney, Vege- 

 tation of the Dismal Swamp; Ganong (1903), The Vegetation of the Bay 

 of Fundy Salt and Diked Marshes; Fink (1903), Some Talus Cladonia 

 Formations, and others. 



II. Vegetation of Swamps and Moors. 



1098. Mud swamp or reed societies. The flora of some soil 

 shores is very closely related to that of the mud swamp. The 

 ground is rich in nitrogenous plant-food from well-decayed vege- 

 tation matter mixed with the soil. The characteristic plants are 

 bullrushes, reed-grasses, cattail-flag, sparganium, sedges, arrow- 

 leaf, and often pondweeds and pond-lilies. These plants grow 

 in rather shallow water of varying depth, and thus where the 

 ground is sloping often show the zonal arrangement with very 

 distinct formations (as Typha formation). The reeds and bull- 

 rushes usually stand out with the greater part of the plant above 

 the water, and while they are by some called hygrophytes, or 

 hydrophytes, the aerial parts are of such a structure as will retard 

 transpiration. This would seem necessary since so large a part 

 of the plant is exposed to sun and wind, while the roots are im- 

 mersed in water and absorption is thus lessened. Where they 

 stand in this relation to water they are often called semiaquatic 

 plants. The water-lilies, pondweeds, arrow-leaf, etc., are true 

 hydrophytes. With the accumulation of vegetation year after 

 year the water becomes shallower in the mud-swamp, until the 



