448 COMPETITION AND INVASION 



seeds and occupies as much as four square feet of earth. The 

 offspring of an individual of this species, therefore, if all the seeds 

 grew to mature plants, would cover nearly 2.3 acres. It may 

 interest the student to calculate in what generation the descend- 

 ants of one plant would cover the entire area of his state. 



424. Statistics of overcrowding. Charles Darwin seems to 

 have been one of the earliest observers, if not the very first, to 

 collect exact statistics in regard to the severity of competition 

 among plants. He found that out of 20 species which occurred 

 on a plot of turf three by four feet in area nine species died 

 from overcrowding by the others. On a piece of dug and cleared 

 ground he found that 60 weed seedlings to the square foot 

 sprang up and 49 of them were destroyed, chiefly by slugs 

 and insects. 1 



In a rich and weedy bit of land Professor L. H. Bailey 

 found in an area of twenty by twenty square inches ten spe- 

 cies of weeds. Reduced to the number per square foot, there 

 were: July 10, 30 plants; August 13, 31 plants ; September 25, 

 25 plants. Several of these were large weeds, such as the 

 redroot (Amarantus retroftexus) and the ragweed (Ambrosia 

 artemisicefolia) . 



On June 23 of the next year there were on the same plot 

 (which had remained undisturbed) eleven species, numbering 

 108 plants to the square foot, and now the dominant plants 

 were red clovers. Most of the other plants were puny and 

 suffering from lack of light under the shade of the clovers. 2 



If one selects a plot hi which seedlings are just starting, the 

 number of individuals to the square foot will often be found to 

 be much greater than those above given. Under a full-grown 

 tree of the wild black cherry the writer has found on June 9 

 portions of the ground containing hardly any other seed plants 

 except cherry seedlings at the rate of 104 to the square foot. 

 Not one of all the thousands which had begun to grow could 



1 Origin of Species, Chapter in. 



2 The Survival of the Unlike, pp. 258-261. 



