38 BULLETIN 907, U. S. DEPARTMENT OF AGRICULTURE. 
only must the temperature of treatment be considered, but also the 
temperature surrounding plants after exposure and to a much less 
extent that before exposure. It has been shown that high tempera- 
tures are More injurious than low, and in the case of each of the three 
fumigation environments, the prefumigation, actual fumigation, and 
postfumigation, the maximum optimum fell below 80° F. Exactly 
how much this maximum for any particular environment fell below 
80° F. depended on the temperature of the other two; when any two 
- were low the maximum optimum for the third approximated 80°; 
when they were high, however, the maximum for the third was a few 
degrees less than 80°. In one case it did not exceed 75°. These 
conclusions differ very little from the writer’s experience in orchard 
fumigation’ in southern California, for which 70° is held as the maxi- 
mum when a heavy dosage is used. This same maximum is recom- 
mended by Sasscer and Borden (1/6) for greenhouse plants. An 
interesting relation apparently exists between the maximum opti- 
mum temperature for fumigation and the activity of plants, for Mac- 
Dougal (10) states that temperature is one of the most widely inter- 
locking factors concerned in the activity of protoplasm, and that the 
temperature of greatest activity in seed plants varies from 80° to 
100° F. The experimental work presented in this paper shows 
increasing fumigation injury as the temperature of 80° is approached 
or exceeded, which corresponds with the degree at which greatest 
protoplasmic activity commences. 
It is possible to conduct fumigation at temperatures of 80° F. or 
above without serious injury provided the prefumigation and post- 
fumigation conditions are ideal. A high postfumigation temperature 
increases the probability of damage, and especially is this true if the 
fumigation temperature is also above the optimum. The greatest . 
damage follows when all three temperatures surrounding the treat- 
ment are high. A prefumigation temperature in shade and darkness, 
even up to 100° F., appears to alter the results very little unless the 
fumigation or postfumigation temperature is also high, in which case 
the high prefumigation temperatures are more injurious than the low. 
The temperature of 55° F. was the minimum at which experimental 
work was conducted. The little injury evidenced at this temperature 
showed it to be within the range of the optimum. In field work it 
has been stated by the writer (9) that operations are safe as low as 
38° F., although fumigation below this point is not advocated. 
In experiments 24 to 27 it was shown that a sudden increase in 
temperature immediately preceding or during the first few minutes 
of exposure produces very severe injury, especially if followed by 
sharp fluctuations of temperature. This factor offers a partial 
explanation for the severe injury in sunshine fumigation on the sun- 
ward side of the tree, especially toward the top, and also presents a 
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