FUMIGATION OF CITRUS PLANTS. 



21 



under shade temperatures of 79° and 90° F. immediately after the 

 exposure, although plants exposed to the sunshine (76° F.) at the 

 same time had almost all foliage, both tender and resistant, destroyed. 

 Plants held in the shade for 30 minutes and one hour, respectively, 

 after treatment and then subjected to sunshine at temperatures of 79° 

 and 80° F. were about equally affected and this amounted to haying 

 a large part of the old resistant foliage destroyed. The injury, how- 

 ever, was noticeably less than where the plants had been exposed 

 immediately after the treatment. Plants withheld for a period of 

 two hours before placement in the direct sunshine had the tender 

 growth destroyed and a few old leaves slightly affected, while plants 

 held three hours before placement in the sunshine were no more 



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X 



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Y 











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2 

 HOURS 



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Fig. 1. — Graph shovring relation of plant injury to exposure to sunshine at different periods after fumiga- 

 tion. 



injured than those placed in the shade, only the tenderest growth 

 being burned in either case. 



The results of this experiment are fully corroborated by both 

 experiments 22 and 23, and in part by experiment 21. In number 

 21, however, although the results agree with those in the other three 

 experiments in showing that sunshine increases the toxic action of 

 hydrocyanic-acid gas to the plant for a period of fully one hour after 

 exposure, it differs somewhat in that the full effect of the sun is shown 

 for one hour after winch its influence quickly disappears. 



The evidence presented in these four experiments shows that 

 sunshine affects the degree of injury to fumigated citrus trees usually 

 for a period of at least two hours after treatment where dosages 

 equivalent to those used in these experiments are given to growing 

 plants; that the greatest injury follows exposure to sunshine imme- 



