296 THE SOILS AND CROPS OF THE FARM. 



sections of our own country. It is afiPected by both 

 soil and climate. The Canada thistle and the ox-eyed- 

 daisy are not especially troublesome in Ohio, Indiana 

 and Illinois (because they usually do not produce 

 seed) but to keep an eastern farm free of them re- 

 quires eternal vigilance. 



Prolificacy. — The difficulty of eradicating weeds 

 is due to at least four causes; viz.: their prolificacy, 

 the vitality of the seed, their means of dissemination 

 and the adaptability of the plant. 



The ordinary burdock burr will contain fifty seeds. 

 Each seed may grow into a plant, producing thirty- 

 five to forty thousand seeds. A common thistle-head 

 may contain three hundred seeds. Each seed may 

 produce a plant with fifty thousand seeds. The com- 

 mon tumble-weed (AmarantiLS albus) growing in some 

 fence corner of a twenty acre field may have seed 

 enough to supply one seed to every square foot of 

 land in the field. 



Below are given a few common weeds, with the 

 number of seeds produced on a single plant of not 

 unusual size. 



Number of seeds on 

 a plant. 



Native— Annual- 

 Ragweed 4,000 



Foreign — Annual — 



Purslane 400,000 to 2,000,000 



Jamestown weed 100,000 



Pigweeds 150,000 to 800,000 



Fox-tails 15,000 to 45,000 



Velvet leaf 8,000 



Chess 4,000 



Cockle 3,000 



JFoRBiGN— Biennial- 

 Common thistle 10,000 to 65,000 



Burdock , , 3B,000 to 50,000 



