40 BULLETIN 907, U. S. DEPARTMENT OF AGRICULTURE. 
protoplasm passes into a state of immobility. Since the maximum 
daily temperature of the Tulare citrus belt during the summer fre- 
quently exceeds 100° F., the mean maximum for the hottest months 
seldom falling more than 1° to 3° below this temperature, a reason for 
reduced activity is presented.? This condition is doubtless promoted 
by the usual practice of withholding wrigation until after fumigation. 
In the more equable, damper climate of the coastal region, where the 
temperature very seldom attains a maximum of 100° F., but rather 
approaches the optimum for protoplasmic activity, dormancy during 
the summer is less noticeable and the physiologically active plants 
are more subject to fumigation injury. 
MOISTURE. 
Moisture, even when present in excessive amounts, appears to 
have no influence on injury to citrus trees either before, during, or 
after the treatment, under conditions of shade or darkness, and this 
conclusion agrees with the work of Gossard (7), Morrill (13), and 
others. .U ae conditions of exposure to hot sunshine before fumi- 
gation the application of cool water appears to reduce the degree of 
injury slightly. These results on the relation of moisture to the 
fumigation of citrus trees do not necessarily apply to tender green- 
house plants, for moisture on such plants with thin cuticles has been 
shown by various authors (11, 78) to produce increased injury. 
Clayton (1), however, in a recent paper states that some species are 
made more susceptible to injury by wetting the leaves while other 
species are not visibly affected. He places the tomato in the latter 
class although Moore found that wetting tomatoes as well as various 
other plants with thin cuticle increased their susceptibility to injury. 
The influence of soil moisture has been referred to in previous dis- 
cussions and may be passed with the statement that soil moisture in 
sufficient quantity to make plants physiologically very active and 
tender renders plants more susceptible to fumigation injury than 
where present only in such quantities as place the plants in a re- 
sistant or hardened condition. 
Hydrocyanic-acid gas has a great affinity for water, and under con- 
ditions of excessive moisture sufficient gas might be absorbed to so 
materially reduce the concentration that less injury would be pro- 
duced than otherwise, and, furthermore, its insecticidal value would 
be lessened. Where fumigation is conducted in gas-tight con- 
tainers this condition can not be ignored, and the necessity of atten- 
tion thereto has been clearly shown by Penny (1/4), and Sasscer and 
Borden (16). in orchard work under cloth tents the absorption 
of gas by moisture is offset by the greater gas-holding POP, 
erties of the moist canvas. 
2 Taken from published records of the U. S. Weather Bureau, which are lower than orchard sunshine 
temperatures. 
