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 (PI. IV, B, h) 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. In orchard work fruit or 

 foliage injury seldom results unless upward of 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 



