332 THE VALUE OF GREEN PLANTS TO MAN 



divided in jive parts. Every boy and girl should know how poison 

 ivy looks in order to avoid it. Injury from poison ivy may often 

 be prevented by a prompt washing with soap and water. 



Tobacco, although strictly a poisonous plant because of the 

 nicotine it contains, is, nevertheless, one of this country's largest 

 crops. Nearly 1,500,000,000 pounds were raised in 1924, having 

 a value of about $300,000,000. Atropine and belladonna, both 

 deadly poisons used as drugs, are obtained from plants related 

 to the tobacco. 



Numerous other poisonous common plants are found, one of 

 which deserves special notice because of its presence in vacant 

 city lots. The Jimson weed {Datura) is a bushy plant, from two 

 to five feet high, bearing large leaves. It has white or purplish 

 flowers, and later bears a four-valved seed pod containing several 

 hundred seeds. These plants contain a powerful poison, and 

 people are sometimes made seriously ill by eating the roots or other 

 parts by mistake. 



Weeds and their Control. — From the economic standpoint 

 the green plants which do the greatest damage are weeds. Those 

 plants which provide best for their young are usually the most 

 successful in life's race. Plants which combine with the ability 

 to scatter many seeds over a wide territory the additional char- 

 acteristics of rapid growth, resistance to extreme cold or heat and 

 to attacks of enemies, inedibility, and peculiar adaptations to 

 cross-poUination or to self-pollination, are usually spoken of as 

 weeds. They flourish in the sterile soil of the roadside and in 

 the fertile soil of the garden. By means of rapid growth they 

 kiU other plants of slower growth by usurping their territory. 

 Slow-growing plants are thus actually exterminated. Besides 

 depriving other plants of soil salts and water, weeds do more 

 harm. Some are poisonous, as the loco-weed, hemlock, and laurels. 

 Cattle eating them may become poisoned. Other weeds, as the 

 wild onion or garlic, may be eaten by cows, and the milk produced 

 will be ruined in flavor. Some weeds are hosts to injurious para- 

 sitic insects or fungi ; witness the Hessian fly, which lives in some 

 wild grasses, and the wheat rust, which lives in the barberiy. The 

 pollen of the ragweed and of other weeds undoubtedly causes some 

 people to have '' hay fever." Many of our common weeds have 



