ZOtfATION 475 



birch patches. A grass-land formation may consist of patches of 

 timothy and others of redtop, and so on. Such minor groups 

 are often called associations. 



In some cases it may be possible to show that the association 

 is based on the mutual relations to each other of the plants 

 which compose it, while the formation as a whole depends on 

 the characteristics of the station in which it exists, i.e. on soil, 

 climate, and so on.. 1 



453. Zonation. The most striking occurrence of plant forma- 

 tions is in localities where sharply contrasted conditions of life 

 exist side by side. It is often possible, within the radius of a 

 few hundred feet, to travel from the floating aquatic vegetation 

 of the deeper waters of a pond, through the rooted aquatic forms 

 of the shallower water, to the sub-aquatic species of the wet 

 shore, then past the sand-loving plants of the sand dunes farther 

 back from the water, and finally into the wood or meadow vege- 

 tation of ordinary soil. Such a series of zones is shown in 

 Fig. 366 and in Plate X. 



Similar diagrams may be made to illustrate the distribution 

 of plants about a salt spring or pool, along the seashore or the 

 margin of a salt marsh, on the top and sides of an isolated hill 

 with a dry, ledgy, or sandy summit, or even about an old unused 

 gravel pit or a railroad embankment. Less clearly defined but 

 very interesting and extensive zones may be studied with rela- 

 tion to submerged aquatics, particularly among marine plants, as 

 shown in Fig. 202. 



Among the most striking and symmetrical instances of zona- 

 tion are those to be found about the salt marshes of some of 

 the deserts of the far West. The waters of these marshes are 

 too salt to support vegetation, but encircling their borders may 

 sometimes be found as many as six broad concentric bands of 

 abundant vegetation. 



1 Some authors use the term association as an equivalent for the term 

 formation as here employed. Consocies is sometimes used with the same 

 meaning as is here given to" association. 



