454 Plant Physiology 



serting a beak directly into the tissues, for example, plant 

 lice and squash bugs. 1 



278. Destruction of weeds by poisons. - - For years it 

 has been more or less customary to employ salt for the 

 destruction of weeds in the lawn or garden, or to suppress 

 all plants growing in walks and playgrounds. It is only 

 within recent years, however, that any special study has 

 been bestowed upon the use of toxic solutions in the form 

 of sprays as one of the recognized methods of weed control 

 in lawns and cultivated fields. It is not a method which 

 may be expected to replace the usual practices of clean 

 cultivation, rotation, or pasturing, nor is it one which 

 should lead the grower awa}^ from a close study of the root- 

 ing and reproductive habits of weeds. 



1 The poisons or insecticides commonly employed for biting insects 

 are such as Paris green, arsenate of lead, arsenite of soda, arsenite of lime, 

 London purple, and hellebore. Of these, Paris green is by far the most im- 

 portant. The use of this substance attracted general attention between 

 1860 and 1870, when the Colorado potato beetle became an important 

 factor in potato production. Shortly afterwards, the same mixture was 

 employed throughout the South against the so-called army-worm of cot- 

 ton, and it has since been used to give protection against an endless num- 

 ber of biting insects. 



In employing the usual means of control against sucking insects, such 

 substances as kerosene emulsion, miscible oils, whale oil soap, and lime- 

 sulphur wash may be used, as well as methods of fumigation. The first 

 three substances mentioned may be employed, with care, upon the foliage 

 and growing parts. Miscible oils, carbolic acid, and relatively strong 

 kerosene may be used only when the plant is in a dormant condition. 

 Fumigation with tobacco smoke is common. The highly toxic vapor of 

 hydrocyanic acid, prepared from potassium cyanide and sulfuric acid, 

 has also been employed in the fumigation of trees under tents and with 

 nursery stock in a dormant condition. It may also be used in the green- 

 house with care, but special instructions are needed in any particular case. 



The effective use of chemical agents as protective measures against 

 fungous diseases, dates from the discovery of Bordeaux mixture by Millar- 

 det in France, 1883. Since that time, there has been organized through- 

 out the United States and in foreign countries extensive methods of con- 

 trolling these diseases. 



