PLANTS AND ANIMAL PARASITES 159 



is said that a decoction of the unopened flowers possesses 

 the insecticide properties of the commercial product. 



291. Keeping these powders. — Hellebore and pyre- 

 thrum powders should be kept in close vessels, since their 

 poisonous properties are volatile. In purchasing, only 

 fresh 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 " (aphids) on plants under glass. 

 For this purpose, the partially dry stems or leaves are 

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

 There are also powdered forms of tobacco on the market 

 which can be used in much the same way. Many delicate 

 flowers are, however, injured by tobacco smoke. 



Stems or leaves of tobacco, strewn abundantly be- 

 neath greenhouse benches, tend to prevent the multipli- 

 cation of aphids. 



Several semifluid extracts 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. 



293. Tobacco solutions. — A strong decoction of to- 

 bacco is often used for destroying aphids on plants in 

 rooms where tobacco smoke would be objectionable. 

 The plants are immersed in, or washed with, the decoction. 

 It is often effectually used on young plants of cabbage, 

 cauliflower and turnip, to prevent their destruction by 

 the flea beetle (Phyllotreta vittata). Florists commonly 

 use strong extracts of tobacco, sold under different 

 proprietary names, as tobacine, nicotine and the like, 

 for destroying plant-lice. They are much diluted and 



