FUMIGATION OF CITRUS PLANTS. a0 
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 (48) 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 atethe rate of approximately 34, 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. 
