FUMIGATION OF CITRUS PLANTS. oO 
injury was not especially severe; where, however, the sudden increase 
was over a wider range, as in experiments 24 to 27, of from 16° to 28° 
and accompanied by fluctuation in temperature of 5° to 10°, the 
injury was most severe. 
These data are of importance in showing that sudden and wide 
fluctuations of temperature during the gas exposure should be avoided 
where possible. Such fluctuations appear to be damaging to plants 
even where the ranges of temperatures of exposure fall below 70° F., 
which is considered within the range of optimum for field fumigation. 
GENERAL DISCUSSION OF FACTORS WHICH INFLUENCE FUMIGATION 
INJURY. 
Evidence has been presented in the foregoing experiments bearing 
on the relation of darkness, diffused light, sunshine, temperature, 
and moisture to foliage injury during ine Ameationt af citrus trees. 
Abundant additional aes have accumulated nine field and labo- 
ratory experiments to offer further corroboration of the deductions 
made from these experiments. The effect of hydrocyanic acid on 
fruit was not taken up in connection with this series of laboratory 
experiments, but a very large amount of data on this subject has been 
taken during field experimentation. 
In the fumigation of citrus trees injury to fruit and injury to foliage 
should be considered separately. The fruit has been observed to be 
severely injured without the foliage being burned in the least, while 
on the other hand trees have been noted as defoliated although the 
fruit was entirely uninjured. Several different types of injury are 
presented in the fruit and foliage of fumigated citrus trees. These 
types, though somewhat related in the case of either the fruit or the 
foliage, are sufficiently distinct to be easily detected. 
Foliage injury is properly characterized by discoloration or burning, 
and is usually accompanied by the shedding of leaves which vary in 
appearance from those completely burned to others free from deface- 
ment of tissue. The tender expanding leaves of very tender succu- 
lent stems usually show the first signs of fumigation injury, and this 
is not localized at any particular place, but sometimes occurs at the 
edge and sometimes in the body of the leaf. These affected areas 
are frequently confined to one surface of the leaf though more com- 
monly the injury is equally apparent on both surfaces. As the 
degree of injury inengases the entire tender tips are affected, this at 
first being wridenced 6 ‘wilting and finally by death. The length of 
time following fumigation before tip injury appears depends upon 
the jonclamneeo of the tip, the concentration of the gas, and such fac- 
tors as temperature and sunshine conditions surrounding the fumi- 
gation. The tender foliage of plants placed in bright sunshine imme- 
diately after treatment with a strong dosage may start wilting or dis- 
