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TEXTBOOK OF BOTANY 



Fig. 265. Fifteen diflFerent galls on hickory leaves caused by as many different 

 insects. Drawings by B. W. Wells. 



They eat their way into the roots of many plants, feeding upon the juices, 

 and cause the overgrowth of the infested organs (Fig. 264B). They 

 infest the roots of tomato, tobacco, cucumber, lettuce, peony, straw- 

 berry, cotton, and many other cultivated and wild plants. The so-called 

 nematode disease of wheat affects the flowering parts, transforming 

 the grains into galls. Stems and leaves of rye, clover, alfalfa, begonia, 

 and many bulbous plants become distorted, swollen, and variously 

 colored because of infestation by leaf and stem nematodes. An ex- 

 cessive development of fibrous roots of sugar beet is also caused by 

 nematodes. The chief methods of control are soil disinfestation and 

 crop rotation. 



Virus-diseases of plants. Among the diseases recognized in relativelv 

 recent times as being caused by viruses are tobacco mosaic, curlv top 

 of sugar beet, peach yellows, yellow dwarfing of onions, tomato streak, 

 leaf roll of potatoes, virus gall of sugar cane, and witches' broom of 

 sandalwood tree. Viruses also affect animals and cause such diseases 

 as infantile paralysis, smallpox, influenza, common colds, measles, and 

 rabies. There are many viruses; each has characteristic effects on the host 

 plant (Fig. 266). Some viiTises are limited to a single host, whereas 

 others occur in a wide variety of unrelated plants. Viruses have been 

 detected in more than a thousand species of plants growing in all 



