FUMIGATION OF CITRUS PLANTS. 30 
Temperature has a very direct relation to sunshine influence, and 
the degree of injury appears to be increased or decreased as the 
temperature of the sunshine is greater or less. Furthermore, where 
plants are subjected to sunshine immediately before treatment, the 
actual fumigation and postfumigation temperatures influence the 
degree of injury. Likewise in the case of plants exposed to sunshine 
immediately after treatment, the prefumigation and actual fumigation 
temperatures require consideration. The writer’s own experimental 
evidence shows that the optimum temperatures of fumigation are 
below 80° F., and that work conducted at higher temperatures is 
performed with increased risk. The greatest injury follows the sub- 
jection of plants to both sunshine and high temperatures both before 
and after fumigation. The effect of this combination, sunshine and 
temperature, has long influenced the time of starting orchard work in 
California. The fumigation season begins in the summer and extends 
throughout the autumn into the winter. During the warmest 
weather work is not started until the sun has set, but with the advent 
of the late autumn and cooler temperatures the first row of trees is 
sometimes covered before sundown, and in the winter period of 
dormancy entire rows are treated in the sunlight. During very hot 
weather, when the atmosphere is clear and dry, injury to the first row 
fumigated at night and the last in the morning has been of frequent 
occurrence and in the latter case has been observed to occur even 
when the tents were removed from the trees before sunrise. Full 
explanation of this situation is presented in the results of experi- 
ments 20 to 23 which show that the postfumigation sunshine influence 
may extend up to three hours after exposure, although the maximum 
of influence is confined to the first few minutes after removal from the 
gas, 
In spite of the fact that sunshine has from the first been considered 
one of the most harmful agents to plants in connection with fumiga- 
tion, the greater desirability of daylight work has led to continued 
attempts to substitute day practice for that at night. These efforts to 
revolutionize accustomed practice usually were made during the 
winter months, and frequently successfully over short periods if the 
weather was moderate and the trees well hardened. Sooner or later, 
however, this attempt to fumigate by day without modification of 
dosage or exposure was followed by severe tree damage and was 
promptly discontinued. 
It has been explained previously that cyanid injury is modified by 
the concentration of the gas and by the length of exposure. There- 
fore it is reasonable to assume that either of these factors can be so 
reduced as to render fumigation safe on the hottest sunshiny day. 
Orchard work is performed with a concentrated gas, usually as con- 
centrated as an active tree will stand safely during cool nights. In 
