28 BULLETIN 907, U. S. DEPARTMENT OF AGRICULTURE. 
coloration within 2 to 3 hours after the exposure. However, like plants 
fumigated under like conditions but placed afterwards in the shade or 
darkness at a cool temperature might not show injury for at least a 
half day. As a general rule, evidence of injury to active citrus trees 
unexposed to the direct sunshine for at least several hours after the 
treatment develops within 24 hours, though the severity of injury 
may not become fully apparent for 2 or 3 days. Where plants are 
hardened or dormant at time of fumigation a much longer period is 
covered before the effects of treatment are definitely exhibited. 
Burning of the tender, fully expanded leaves in which the cuticular 
layer has not yet become fully matured requires a slightly stronger 
gas than to produce tip injury. As the expanded leaf matures greater 
resistance to the gas develops. The injury to leaves as observable 
by defaced tissue may be confined to small distinct areas, sometimes 
in the case of very tender growth not larger than the head of a pin, 
or in other, cases may include the whole of a leaf. Severely injured 
leaves drop within a few days to several weeks after treatment, but 
the shedding of foliage having little or no defacement of tissue is 
indeterminable because such foliage, especially in the case of mature 
leaves, might be and apparently is greatly affected physiologically 
even when little or no superficial evidence is presented previous to the 
actual abscission. The abscission usually occurs at the base of the 
blade (Pl. IV, B, 6) rather than the base of the petiole, but later this 
also falls. 
In any examination into the causes producing plant injury from 
fumigation with hydrocyanic-acid gas at least four distinct condi- 
tions, each of which contributes toward modifying the result, must be 
considered. These are: (1) The concentration of the gas; (2) the 
length of exposure; (3) the physiological condition of the plant; (4) 
atmospheric conditions. | 
THE CONCENTRATION OF THE GAS. 
The modifying influence of gas concentration on plant injury has 
already been briefly mentioned in this paper. It has been found in 
experimental work that under the most favorable conditions of treat- 
ment fully one-half ounce of high-grade sodium cyanid to 100 cubic 
feet of space in an air-tight fumigatorium is required to produce 
injury to normal healthy citrus plants, and dosages in excess of this 
amount were used in experiments 1 to 27. Imbrchard work fruit or 
foliage injury seldom results unless upward ot 50 per cent strength 
of the full dosage schedule recommended by this department and 
followed in commercial fumigation in California is used. As the gas 
concentration increases above the strength required to produce initial 
injury the effect on the plant becomes increasingly severe. In 
extreme cases the entire plant may be killed, but this result with 
aa ON gt =r. 8 
