GRF.EXIIOL'SE I'TMIGATION 107 



CONCLUSION. 



From the results reported the fcjllowing- procedure shouhl Ije 

 followed in the fumigation of a greenhouse. Plants should Udt l)e 

 watered for 24 hours preceding" fumigation to insure that the house 

 is dry and the relative humidity low. The day temperature preced- 

 ing fumigation should he low, gradually rising to 68°-70° F. at the 

 time the charge is put in. This insures the temperature of the soil 

 being lower than the air of the house, and the relative humidity 

 being low prevents exudation of water from the water pores of the 

 leaves. After the fumigation is started, the temperature is permit- 

 ted slowly to rise to 75° F. The rising temperature causes a de- 

 crease in the relative humidity and favors eva])oration from the 

 leaf surfaces. If a strong dose is given for a short time the temper- 

 ature during fumigation should be lower and care taken in airing 

 out the house that moistiu'e containing hydrocyanic acid is not 

 deposited upon the plants. After ventilating the house the temper- 

 ature should rise to 70° to 7S° to remove any hydrocyanic acid on 

 or in the cuticles of the leaves. Plants with thin cuticles such as 

 lettuce, cucumbers, tomatoes, etc.. are more liable to injury from 

 strong doses for short periods than from small doses for longer 

 periods. Plants with thick, waxy cuticles can withstand \ ery 

 strong doses for short periods, granting that the fumigations are 

 carried out during the night when the stomata are closed. 



Doses of from 34 o^- to 1-2 oz. of potassium cyanide i)er 1,000 

 cu. ft. may be used overnight if the plants have thin cuticles. As 

 most of the hydrocyanic acid has disappeared by morning the house 

 may be ventilated without danger to the plants. 



Following these principles a number of greenhouses have been 

 successfully fumigated. In one case a greenhouse contained roses 

 and chrysanthemums in full bloom, smilax, ferns, and other green- 

 house plants. One-fourth ounce was used and the following morn- 

 ing the white fly and even the aphids in the heart of the chrysan- 

 themum flowers were dead without the slightest injury to any of 

 the ]ilants. 



SUMMARY. 



Hydrocyanic acid may enter a plant either through the stomata 

 or directly through the cuticle. 



The amount of hydrocyanic acid which will enter the cuticle of 

 the plant depends upon the thickness and the degree to which it has 

 been cutinized. 



