208 DISEASES OF PLANTS 



ANIMAL PARASITES 



Worms. — The commonest worms to cause disease in 

 plants are thread worms or nematodes. These attack 

 the roots or other parts of their characteristic host plants, 

 e.g., potatoes, flower bulbs, etc., entering the adjoining 

 tissues and being covered by enlargements often attain- 

 ing considerable size. In these abnormal growths, which 

 are sometimes called galls, these animals reach maturity 

 at the expense of the host plant which feeds and shelters 

 them. These worms not only rob the host of food, but 

 may also poison it to a greater or less extent by their 

 excretions. 



Fia. 53. — I^af Gall on California White Oak. 

 Kellogg, Atncrican Insects. 



Flies of various sorts, particularly those called gall 

 flies, are known to cause the formation of leaf- and stem- 

 galls, the structure and composition of which, as well as 

 their appearance, are often very striking. Thus on a 

 certain species of oak the large balls which form 



