164 Principles of Plant Culture. 
breathing pores, as kerosene (294); that dissolve their eggs 
and scales, as potash solutions; or that form an air tight 
coating over them, as the resin washes (295).* 
317. The Life Histories of Injurious Insects, which 
cannot here be taken up, may profitably be studied by the 
plant grower. A standard work on economic entomology 
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. Asa rule, vegetable parasites 
contain no chlorophyll, and hence are incapable of assimi- 
lating 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—Flowering or Phaneroganic (phan’-er-o-ga’-mic) 
parasites. 
Of these the only ones sufficiently common or injurious 
to need mention are the broom rapes, and the dodders. 
319. The Broom Rape of Hemp and Tobacco,t is the 
most injurious species of this class. The seeds germinate 
in the soil, and the young plants attach themselves to the 
roots of their host which they enfeeble by robbing it of 
nourishment. In the case of hemp, the parasite also injures 
the quality of the fibre. 
Preventives. 'The seeds of hemp or tobacco should not be 
taken from a crop infected with broom rape. Infested 
fields should be planted for several years to some crop not 
* The cottony cushion scale, Icerya purchasi, which has been very destructive 
to the orange in California, has been nearly suppressed by the introduction of an 
Australian parasite, the Vedalia cardinalis. 
+ Philipea ramona. 
