DESCRIPTIVE MANUAL 399 



with seeds buried at different depths to be brought under condi- 

 tions favorable for germination at intervals for several years. Ma- 

 ture plants should be mowed and burned before plowing. The 

 seed appears as an impurity in clover, millet and the heavier grass 

 seeds, and the plant is doubtless most frequently introduced by 

 this means. As the seeds may be carried a long distance by the 

 wind, the plants must be cleared out of the fence rows, waste land 

 and roadsides." 



Clark and Fletcher recommend as follows: "The seed is 

 short-lived and if the plant is kept closely cut and prevented 

 from seeding in waste places for two or three years it will soon 

 disappear from cultivated areas. Clean waste lands and seed 

 to permanent vigorous grasses. Ordinary methods of cultiva- 

 tion will suppress it in the fields." 



Wild Lettuce (Lactuca canadensis L.). 



Description. — Stem leafy, glabrous or nearly so, glaucous; lower 

 leaves sinuate, pinnatifid, upper entire; heads numerous, in a 

 rather long, open panicle ; flowers yellow. 



Distribution. — Nova Scotia to Ontario, Ohio and westward; fre- 

 quently found in Iowa. 



Extermination. — This perennial is not difficult to destroy by 

 cultivation. 



