FUMIGATION OF CITKUS PLANTS. 33 



whose growth has not kept pace with the rest of an orchard, due 

 possibly to inferior subsoil, to hardpan, gravel, etc., are not uncom- 

 mon. Trees under such adverse conditions have sometimes been 

 noted to be more adversely affected by fumigation than healthier 

 trees. 



The general conclusion to be drawn from this discussion is that 

 plants best resist cyanid gas if in a hardened or dormant condition 

 at the time of fumigation. Hardening is brought about either by 

 cold weather or a dry soil. From the standpoint of the action of 

 cold, plants are most matured or dormant during the winter season 

 and at this time least injury from fumigation is to be expected. 

 Since citrus is mostly grown in countries that practice irrigation, the 

 dryness of the soil can be regulated by regulating irrigation. There- 

 fore, as a general rule, fumigation should precede the run of water 

 rather than follow, as is frequently the practice at the present time. 



ATMOSPHERIC AND LIGHT CONDITIONS. 



DARKNESS AND DIFFUSED LIGHT. 



Experimental evidence presented in this paper has shown that 

 diffused light before, during, or after fumigation in no way modifies 

 the degree of injury to citrus trees. Since the active stomata of 

 citrus plants open during the daytime and for the most part remain 

 closed at night it is evident that the condition of the stomata does 

 not noticeably alter the degree of injury from fumigation. Such a 

 conclusion is not fully in accord with the statement of Clayton that 

 "the stomata seem to be the most important single factor in deter- 

 mining the amount of injury resulting from hydrocyanic acid * * * . 

 Injury closely paralleled the stomatal movement, increasing as the 

 size of stomatal aperture increased." 



These differences in results are readily explained in the light of 

 the work of Stone (18) and Moore (11). The former states that the 

 condition of the stomata does not appear to have anything to do with 

 susceptibility to burning from fumigation but the injury is due rather 

 to the development of the cuticle and texture of the tissue in general; 

 that tender immature tissue is least resistant to fumigation injury. 

 Moore has shown that hydrocyanic acid enters plants to a greater or 

 less extent through the cuticle and that those with thin cuticles are 

 far more severely injured than those with thick, strongly cutinized 

 cuticles. Geranium, Tradescantia, and tomatoes, the plants with 

 which Clayton worked, have very thin cuticles and were injured with 

 a concentration of gas at the rate of approximately ^ ounce of potas- 

 sium cyanid to 100 cubic feet. The smallest dosage used in the 

 writer's experiments with the thicker and more heavily cutinized 

 citrus plants was 1 ounce of potassium cyanid to 100 cubic feet. 



