CHAPTER XXXVIII. 

 MIGRATION AND DISTRIBUTION OF PLANTS. 



612. The migration of plants is the movement of plants over 

 the earth, not only into new territory, but also movement from 

 place to place within the territory already occupied by a given 

 species. The word distribution is sometimes used in place of 

 the word migration, and is also used to indicate the area already 

 occupied by a species. 



613. One very important principle in plant migration is 

 that of the pressures, as they are called, which are behind many 

 of the plant movements. Plants produce great quantities of 

 seeds or other reproductive bodies, the larger number of which 

 fail to make new plants. These seeds are scattered yearly from 

 their centers of production so that whenever a chance comes for 

 growth the seed is there to start it. In the area occupied by the 

 species in question some of them grow to new plants to take the 

 place of those which perish. Other seeds are carried beyond 

 this area into new territory. If this territory is suitable for the 

 growth of the species in question it may gain a foothold and 

 thus extend the " range " of the species. If the seeds are carried 

 into a climate unfavorable for this species, they die. This great 

 fertility of species therefore constitutes a pressure which is ever 

 forcing the species onward and outward, in many cases into 

 territory where it cannot possibly obtain a footing. Great 

 changes in climate also produce pressures which result in the 

 migration of plants. Some of the principles of plant migration 

 may be considered under the following heads: First, methods 

 and causes of plant migration; second, barriers to plant 

 migration. 



