FUMIGATION OF CITRUS PLANTS. 6 



hydrocyanic acid fumigation, although Stone (18) concluded from his 

 investigations that susceptibility to injury is due more to the condi- 

 tion of the tissue, whether thin and tender or resistant, than to the 

 open or closed condition of the stomata. 



It has not been uncommon in California to hear practical fumi- 

 gators state that plant injury during fumigation is due to impurities 

 in the cyanid or sulphuric acid. One writer (23) has attributed 

 injury to sulphuric-acid fumes given off during the gas generation 

 and has discussed in great detail how such action is brought about. 

 The work of Schroeder, Moore, and others disproves any conclusion 

 that does not mention the cyanid gas itself as the cause of plant 

 injury, and experiments by the writer (19) furnish further data in 

 disproving these theories with reference to sulphuric acid. Further- 

 more, the recent wide use of high-purity liquid hydrocyanic acid, a 

 material free of sulphuric acid, has been attended by the customary 

 fruit and foliage injury. 



DETAILS OF EXPERIMENTS. 



Boxed seedling orange trees, pruned to several branches and from 

 1 to 2 feet in height, were grown beneath a canvas shelter which 

 afforded protection from the midday sun. The foliage was dense 

 and for the most part consisted of heavily cutinized leaves, except 

 for the tender growth toward the top. 



The gas-tight fumigatorium (PI. I, A) in which the experiments 

 were performed contained 100 cubic feet of space and was equipped 

 with two large glass windows, which permitted the regulation of light 

 conditions. Treatment in the shade signifies that both windows were 

 fully exposed to diffused light. The temperature of the fumigato- 

 rium for any one experiment was uniform during the exposure, unless 

 otherwise noted. All records were made in the Fahrenheit scale. 

 Only high-grade commercial cyanids of 96 to 99 per cent purity were 

 used. Potassium cyanid was used according to the 1-1-3 formula, 

 or sodium cyanid according to the 1—1^—2 formula (19). The foliage 

 in all cases was dry, unless otherwise noted. Check plants were used 

 in all experiments and failure to refer to them signifies that the checks 

 were in no way injured. The dosages in these experiments approxi- 

 mate those employed in orchard treatment in California. All plants 

 in any one experiment were fumigated at the same time. Immedi- 

 ately after treatment they were removed from the fumigatorium "and 

 placed in different environments of temperature and light. Final 

 notes on results were taken five to seven days after treatment. Shade 

 signifies protection from the sun by a canvas shelter or a wooden 

 building. Darkness means total exclusion of light. Moisture does 

 not include atmospheric humidity. Injury as included in this paper 

 should be interpreted as meaning damage to or death of tissues, so 



