462 GENERAL MORPHOLOGY OF PLANTS 



I. METHODS AND CAUSES OF PLANT MIGRATION. 



629. Advantages of plant migration. The advantages 

 accruing 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 perpetu- 

 ation of species is thus more surely safeguarded. It increases 

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

 of individuals possible over a larger area. Second. The safety of 

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

 Disaster may come by sharp competition of other species, or by 

 the destructive effect of physical changes in the topography cf 

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

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

 etc. Third. The species often gains in vigor by coming under 

 new conditions, better soil conditions, more favorable climate 

 conditions, etc. 



630. 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. 



1. Seeds. First. The buoyancy of seeds produced by the 

 development of so-called " wings " on the elm, maple, etc., the 

 pappus of the composites and other hairy or wooly outgrowths 

 form part of the seed or fruit which enable it to catch the wind. 

 Small size and lightness of many seeds also give them a certain 

 amount of buoyancy. Second. The development of structures for 

 grasping hold upon other objects; for example, barbs and hooks 

 on seeds or fruits for clinging to animals. Third. The use of 

 seeds as food by animals. Fourth. Seeds which are capable of 

 floating on the water. 



2. Fruit. There are many fruits which are used for food by 

 animals and the seeds of which often pass through the body 



