506 RELATION TO ENVIRONMENT. 



These zones arid areas established by Merriam, and the laws 

 governing them, have not yet been subjected to a sufficiently 

 thorough test so far as plants are concerned, and there are many 

 cases of plant distribution which they do not adequately explain. 

 Nevertheless they are in general accord with several great fac- 

 tors of plant distribution and are worthy of careful study and 

 test. Modifications it may be necessary to make, though in gen- 

 eral the fundamental principles seem well grounded. The 

 limits of these life zones and areas in the United States can be 

 seen by consulting Fig. 492. The limits and the characteris- 

 tics of the life zones and areas of North America are given in 

 Chapter LVII. 



III. Methods and Causes of Plant Migration. 



975. Advantages of plant migration. The advantages accru- 

 ing to plants through their power or tendency to migrate is that 

 individuals of a species are increased by extending their area of 

 occupation. Whether the plant concerned occupies any given 

 area to the exclusion of other species or not, an extension of the 

 area provides for an increase in individuals, and the perpetua- 

 tion of species is thus more surely safeguarded. It increases the 

 factor of safety for existence: ist. By the larger number of 

 individuals possible over a larger area. 2d. The safety of some 

 is assured in case of disaster to others in a certain region. Disas- 

 ter may come by sharp competition of other species, or by the 

 destructive effect of physical changes in the topography of cer- 

 tain areas. For example, in time of flood, areas being covered 

 by sand, gravel, or other rock debris; or by changes in the cli- 

 mate, etc. 3d. The species often gains in vigor by coming under 

 new conditions, better soil conditions, more favorable climatic 

 conditions, etc. 



976. Structural characters favoring plant migration. Many 

 of these characters are discussed more fully in the chapter on 

 "Seed Dispersal," and others in the chapter on "Stems," etc. 

 The mere enumeration of some of these characters is given here. 



