36 BULLETIN 907, U. S. DEPARTMENT OF AGRICULTURE. 



the light of the experiments presented in this pa,per it is to be expected 

 that the use of such dosages during warm sunshine would cause severe 

 injury. Experiment 3 shows that by reducing the strength of the gas 

 the influence of the sunshine is correspondingly reduced. One 

 orchardist known to the writer has practiced daylight fumigation on 

 his small lemon orchard during the growing season for several years, 

 accomplishing his purpose through reduction of both dosage and 

 exposure. He was observed on one occasion to fumigate lemon trees 

 safely at a temperature of 84° to 86° F. by using a dosage calculated 

 as less than one-half the usual dosage, with an exposure of 30 minutes. 

 The effect on the scale was not noted. 



Since liquid hydrocyanic acid has come to be used in fumigation, 

 daylight practice is no longer considered a dangerous experiment. 

 During the winter months outfits operate throughout the daytime in 

 bright sunshine, in many cases with complete safety, and under 

 conditions which in the past with pot or machine generated gas were 

 wont to produce severe injury. Outside of possible differences in 

 physical properties of the gas due to the method of generation and 

 application, the one most plausible reason for the increased safety of 

 daylight operation is the difference in diffusion throughout the tree. 

 In pot-generated gas the greatest concentration is toward the tree 

 top, whereas with liquid hydrocyanic acid in warm weather the greatest 

 concentration is toward the bottom of the tree (21 ) . The writer has 

 determined by a series of experiments that the temperature of the 

 tented tree rapidly rises on the sunward side after covering and that 

 the greatest increase is toward the top of the tent. In pot-generated 

 gas the maximum gas concentration, maximum heat, and most 

 sudden change of temperature are exerted at the same place, the top 

 of the tree, whereas in trees fumigated with liquid hydrocyanic acid 

 the greatest concentration of gas at the bottom of the tree is at the 

 coolest part of the sunward side of the tree, while at the top or point of 

 maximum temperature the gas is most dilute. A seeming explana- 

 tion is presented in the comparative appearance of damaged trees 

 under these two methods of gas application. Sunshine-injured trees 

 from pot-generated gas show the greatest damage on the sunward 

 side toward the top; in the case of trees treated with liquid hydro- 

 cyanic acid the damage toward the bottom is greatly increased over 

 that as compared with pot-fumigated trees while that toward the top is 

 lessened. 



It was stated in a previous paragraph that the open or closed 

 condition of the stomata does not appear to affect the degree of injury 

 to citrus plants in darkness or diffused light before, during, and after 

 fumigation. In the case of plants subjected to sunshine either im- 

 mediately before or immediately after exposure to cyanid gas, equally 

 conclusive data bearing on this subject have not been developed. Cer- 



