160 Principles of Plant Culture. 



insects. As the oil is extremely volatile, pyrethrum is 

 better adapted for use under glass or -^^itli plants other- 

 -^^ise inclosed. It is not injurious to foliage or flowers. 

 Pi-esli and pure pyrethrum powder may be diluted half 

 or more in bulk with any other light, cheap, harmless 

 powder, but the mixture should stand a day or two before 

 use, to enable the diluent to absorb the oil. The powder 

 may be used with watei- at the rate of half a pound to 

 three gallons. 



The pyrethrum jilant is comparati^'ely hardy and has 

 been successfully grown in northern United States. It 

 is said that a decoction of the unopened flowers ]30sses- 

 ses the insecticide i)rop('rties of the commercial product. 



291. Hellebore and Pyrethrum Powders should be Kept 

 in Close Vessels, since their poisonous properties are 

 volatile. In purchasing, only fi'esh samples should be 

 accepted. If fresh and pure, these powders produce a 

 tingling sensation when applied to the nostrils. 



292. Tobacco Smoke is much used for destroying 

 "lice" or "green fly" (aphidte) on plants under glass. 

 For this iDurpose, the partially-dry stems or leaves are 

 burned upon pans or bricks, or in small sheet-iron 

 stoves. Manj^ delicate flowers are, however, injured by 

 tobacco smoke. 



Stems or leaves of tobacco, strewn abundantly beneath 

 greenhouse benches, tend to prevent the multiplication 

 of aphidfe. 



Several semifluid exitracts of tobacco are sold which 

 may be evaporated in the greenhouse over an oil stove, 

 or preferably by steam under pressure. Some of these 

 are very efficient for destroying insects and do not injure 

 flowers. 



