FUMIGATION OF CITRUS PLANTS. 37 



tain experiments already presented in this paper have shown that 

 plants in a prefnmigation and fumigation environment of darkness (in- 

 dicating closed stomata) were severely injured by placement in sun- 

 shine immediately after the exposure, while check plants placed in 

 the shade at an equal temperature were little affected. In these cases 

 the increased damage to the sun-exposed plant was brought about in 

 spite of the fact that the stomata were apparently closed during the 

 fumigation. Furthermore, data have been collected during daylight 

 work, both in the morning and afternoon, showing that trees some- 

 what protected from the direct sun were little affected by a strength 

 of gas that severely injured trees in the direct sunshine treated at the 

 same time. On the other hand, the increased injury to plants in a 

 prefumigation condition of sunshine, as previously explained, and 

 observation that greater injury is usually apparent during morning 

 orchard fumigation than during that performed late in the afternoon 

 at an equal temperature, might indicate possible stomatal influence 

 when viewed in the light of Lloyd's (9) conclusions that the morning 

 sun may hasten stomatal opening, that this opening is at its maximum 

 toward midday, and that closure occurs during the afternoon. 



Stone, Moore, and others conclude that a strong concentration of 

 gas tends to close the stomata. This closure of the stomata from 

 fumigation would reduce the rapidity of the escape of gas which 

 remained in the intercellular spaces after treatment and might 

 thereby modify the degree of injury, especially in plants subjected to 

 such adverse conditions as postfumigation sunshine. 



The condition of the soil apparently influences cyanid injury from 

 sunshine, as shown in the case of a 10-acre citrus orchard fumigated 

 during a clear hot day hi November, 1919. This orchard was so 

 irrigated that the soil nearest the head of the furrows was thoroughly 

 wet to a normal depth, whereas the soil at the lower end of the furrows 

 was for the most part wet only for a few inches at the surface, or 

 sometimes not at all. The trees reflected this lack of required moisture 

 in their general less healthy appearance. The tents were strung in 

 the direction of the irrigation furrows.^ Severe injury resulted from 

 the fumigation, amounting almost to complete defoliation on the 

 sunward side of a large part of the trees. This injury was confined 

 almost exclusively to the trees on the dry soil, those on the moist 

 soil being very little affected. The explanation is that the trees which 

 had long suffered from lack of moisture were in a weakened condition 

 at the time of fumigation, whereas the others were not. 



TEMPERATURE. 



It has been clearly shown by the experimental evidence presented 

 in this paper that temperature exerts one of the most important 

 modifying influences on injury from fumigation. Furthermore, not 



