586 A TEXTBOOK OF BOTANY [Cn. XIV 



THE HEATH FORMATIONS. These include the BOG FORMA- 

 TIONS, both flat bogs, with a vegetation much like swamps, 

 and raised bogs, with a very sparse heath vegetation. North- 

 ward they merge to MUSKEAG and ultimately to Arctic 

 TUNDRA. THE HEATH FORMATION occurs on thin organic 

 soil, and consists chiefly of Heather, and other dwarf Erica- 

 ceous plants. Moor formations are the same. 



THE SALINE FORMATIONS. These include the SALT MARSH 

 FORMATION, MANGROVE SWAMP FORMATION, the open COAST 

 FORMATIONS, and those of interior saline basins. 



THE ROCK FORMATIONS, those of cliffs and stony plains. 



SAND AND GRAVEL FORMATIONS, those of sand dunes and 

 exposed beaches. 



FORMATIONS OF WASTE AND CULTIVATED GROUND are also 

 sometimes recognized. 



THE AQUATIC FORMATIONS. These include SWAMP, MARSH, 

 and WATERSIDE FORMATIONS, with their characteristic asso- 

 ciations. In the water occur two, the BENTHOS (Formation), 

 including the vegetation attached to the bottom, and the 

 PLANKTON (Formation), the free-swimming and drifting 

 forms, whether fresh water or marine. 



The chief value of this conception of plant formations and 

 associations consists in the power it gives to understand 

 causatively the vegetation of a country, and also to describe 

 it vividly. ' From a study first of the flora, and then of the 

 vegetation groups, with illustration by photography, dia- 

 grammatic maps and transects, it is possible to prepare the 

 most expressive descriptions of the plants of any country, 

 and moreover in a way to make comparison possible with the 

 plants of other countries. Few such studies have yet been 

 made, and herein lies not the least of the opportunities for 

 scientific service open to the interested student, especially 

 to one so fortunate as to possess the instinct of the field 

 naturalist. 



