WEEDS AND POISONOUS PLANTS 



341 



cause a serious irritation of the skin ; and some species of 

 lady's-slipper are reported to have an effect upon some 

 persons very like that of poison ivy. 



357. Hay Fever. — Just as some sub- 

 stances produced by plants cause irritation 

 of the skin, others give rise to serious diffi- 

 culties because they are poisonous to the 

 mucous membranes of the nasal passages, 

 eyes, and throat. The irritated condition 

 that gives rise to " hay fever " and " rose 

 fever " is due to the poisonous action of 

 the pollen of certain plants. The plant 

 that seems to cause most trouble in this 

 way is the common ragweed which grows 

 abundantly along roadsides and on waste 

 land. Some cases of hay fever are apparently caused by 

 the pollen of certain grasses. 



Fig. 192. 

 ragweed. 

 Weed. 



