178 Principles of Plant Culture. 



an air-tight coating over them, as the resin washes 

 (295).* 



317. The Life Histories of Injurious Insects, which 

 can not here be taken up, may profitably be studied by 

 the plant grower. A standard work on economic ento- 

 mology will furnish the needed information. 



B PLANTS AS AFFECTED BY VEGETABLE PARASITES. 



318. Many of the most serious enemies of cultivated 

 plants belong to this class. As a rule, vegetable para- 

 sites contain no chlorophyll, and hence are incapable of 

 forming their own food. While most of them belong 

 to the lower orders of plants, a few species are highly 

 developed and produce true flowers and seeds. 



a By Flowering or phanerogamic (phan'-er-o- 

 ga'-mic) parasites. 



Of these, the only ones sufficiently common or inju- 

 rious to need mention are the broom rape and the 

 dodders. 



319. The Broom Rape of Hemp and Tobacco,f 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 fibre. 



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 



* The cottony cushion scale, Icerya purchasi. which was very 

 destructive to the orange in California, has been nearly suppressed 

 by the introduction of an Australian parasite, the Vedalia cardin- 



t Philipcea ramona. 



