SEED DISTRIBUTION . 149 



The fact that any species, such as the ordinary ragweed 

 (Ambrosia artemisicefolia), common throughout most of the 

 United States, does not promptly overrun all those portions 

 of the earth's surface suited to its growth is due to : 



(1) Lack of sufficiently thorough and rapid means of seed 

 distribution. 



(2) Multiplication of insect and plant enemies of the species 

 (often not important). 



(3) Overcrowding or competition between individuals of 

 the same species (other ragweeds) or of other species. 



138. How competition kills. The result of competition 

 among plants is sometimes to make the overcrowded individ- 

 uals dwarfish and unfruitful, or at other times to kill them 

 outright. The means by which the successful individuals 

 weaken or kill their neighbors are mainly: 



(1) Overshadowing, resulting in deficient photosynthesis 

 in the shaded plants from lack of light. 



(2) Robbing the defeated plants of water. 



(3) Robbing them of soluble salts (nitrates, phosphates, and 

 so on, from the soil). 



The deprivation of sufficient water and salts interferes 

 with the nutrition of the overcrowded plants and may soon 

 completely stop their growth. 



The extent and reality of the competition here merely out- 

 lined can be understood only by aid of careful field work. 

 Weedy ground may be found which contains as many as a 

 thousand seedlings to the square foot, and if a small area of 

 such ground is isolated and watched, the struggle for exist- 

 dnce may be followed to its end, with only one or two of the 

 thousand surviving. 1 It must be remembered that multitudes 

 of seeds get no start in life as seedlings, and so do not even 

 enter into competition, either from failure to lodge in a place 



1 See also Charles Darwin, Origin of Species, chap, iii (D. Appleton and 

 Company, New York), L. H. Bailey, Survival of the Unlike, pp. 258-261 

 (The Macmillan Company, New York), and Bergen and Davis, Principles 

 of Botany, pp. 448-450 (Ginn and Company,- Boston). 



