36 BULLETIN 907, U. S. DEPARTMENT OF AGRICULTURE. 
the light of the experiments presented in this paper 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 lquid 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- 
