342 



GENERAL SCIENCE 



Some few plants are poisonous. Probably one of the 

 best known of these is the poison ivy, a three-leaved 

 climbing plant which attaches itself to walls, trees, and 

 fences by means of small roots growing from the stem. 

 Berries from wild plants should not, be eaten until their 

 identity is determined, since some of them are poisonous. 

 Plant Diseases. Many parasitic fungi live on useful 

 plants which are cultivated. They attack trees, grains, 



fruits, and vege- 

 tables to such an 

 extent that the 

 damage done by 

 them annually 

 amounts to mil- 

 lions of dollars, 

 while probably as 

 much more is 

 spent in combating 

 them. The most 

 common parasitic 

 plants are rusts, 

 molds, smuts, and 

 blight-producing 

 fungi. 



Wheat Rust. - 

 For many years 

 wheat rust , has 

 been one of the 

 most destructive 

 and most dreaded of plant diseases because it destroys 

 a plant upon which so large a part of the civilized world 

 is dependent for food (Figure 304) . It has long been sus- 

 pected but only recently determined beyond a doubt, that 



U. S. Dept. of Agriculture. 

 FIG. 304. Wheat Heads and Straw Showing Rust. 



