CHAPTER XIII 



PLANTS AS AFFECTED BY VEGETABLE 

 PARASITES AND BY WEEDS 



MOST of the vegetable parasites are less conspicuous or 

 evident in themselves than are the quadrupeds, birds 

 and insects, but their work is often peculiarly devastating 

 and usually insidious. 



318. Types of vegetable parasites. Many of the 

 most serious enemies of cultivated plants belong to this 

 class. As a rule, vegetable parasites contain no chloro- 

 phyll, and hence are incapable of forming their own food. 

 While most of them belong to the lower orders of plants, 

 a few species are of the higher types and produce true 

 flowers and seeds. Of parasites of the higher orders, seed 

 plants, the only ones sufficiently common or injurious to 

 need mention are the broom rape and the dodders. 



319. The broom rape of hemp and tobacco is the 

 most injurious species of this class. The seeds ger- 

 minate in the soil, and the young plants attach them- 

 selves to the roots of their host, which they enfeeble by 

 robbing them of nourishment. In the case of hemp, 

 the parasite also injures the quality of the fiber. 



Preventives. The seed of hemp or tobacco should not 

 be taken from a crop infested with broom rape. In- 

 fested fields should be planted for several years to some 



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