40 



NATURAL HISTORY 



Estimate ok Seeds Produced by a Single LARtiB WE>;n 



Dandelion . 

 Cockle-bur . 

 Oxeye daisy 

 Prickly lettuce 

 Beggar's ticks 

 Ragweed 



1,700 



9,700 



9,750 



10.000 



10,500 



23,000 



Crabgrass . 

 Russian thistle 

 Pigweed 

 Purslane (large) 

 Tumble mustard 

 Lamb's-quarters 



89,600 



150,000 



305,000 



1,250,000 



1,500,000 



1,600,000 



Some fruits, like those of violets and the witch-hazel, explode, send- 

 ing their seeds to a distance. Even gravity may sometimes be re- 

 sponsible for spreading plants by means of soil-slides, while animals 

 in such accidentally disturbed soil may be carried considerable dis- 

 tances to a new situation. 



Birds inadvertently scatter fruits and seeds by first swallowing 

 and then depositing them elsewhere with their droppings. As a 

 result cherry bushes and poison-ivy vines may often be seen growing 

 along fences where birds have roosted. 



Adaptability to New Conditions 



The fact that some organisms do not invariably adapt themselves 

 to new localities which they have invaded is a great deterrent to 

 their permanent spread. Successful invaders that gain a new foothold 

 as pioneers, and retain it as settlers, are conspicuous enough to be 

 discovered and remembered, but unsuccessful ones, reaching the 

 Promised Land but unable to establish themselves there, escape atten- 

 tion. Indian corn, for example, seems unable to reproduce and main- 

 tain itself if allowed to run wild. The yellow-fever mosquito has a 

 certain dead-line, north of which it cannot successfully continue to live. 



Just as in economic life, so in communities of plants and animals, 

 undesirable individuals frequently appear, bumming their way into 

 places where they are not wanted. Weecis are notorious plant- 

 hoboes that are pre-eminently successful on their own part, but are 

 unwanted by man, and reckoned as outlaws with a bad reputation, 

 because they rob other plants which man favors, of food, moisture, and 

 sunlight. Having great natural vitality, they are successful because 

 they usually grow even in unfavorable conditions which would kill 

 competing plants, and produce enormous numbers of seed. Their 

 persistence and varied means of seed dispersal are easily realized by 

 anyone who has tried to pick "beggar's ticks," and "sticktights," 

 and burrs from his clothes after a ramble in the autumn woods. 



