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PLANT PHYSIOLOGY AND ECOLOGY 



nuded area is reached, and hence a larger number fall within it. 

 A second reason lies in the fact that migrants from more distant 

 formations must pass through or over adjacent ones. In this 

 case the number of disseminules becomes smaller and smaller 

 as the distance increases, and the few invaders from a distance 

 reach the denuded area only to find it already occupied. Owing 

 to the radiate nature of migration, only a part of the fruits 

 of a plant or a group can be carried ordinarily in any one direc- 

 tion. The bulk of these fall near the parent, the number de- 



FiG. 108. A polydemic plant, Frasera speciosa, found in several of the 



mountain formations. 



creasing rapidly as the distance increases, until at a few hundred 

 meters or a few kilometers even the last fruits disappear. 



286. Endemism. A species is said to be endemic when it 

 occurs in a single region or country, and polydemic when it is 

 found in two or more regions. These terms refer merely to posi- 

 tion or distribution, and are distinct from indigenous, originating 

 in the place where found, and exotic, native in a place other than 

 that in which the species is found. A species is best said to be 



