190 . GROWTH OF PLANTS 



is not a factor. As the National Research Council of Canada group found 

 in such concentrations, the SO2 is oxidized to less harmful sulphates before 

 enough accumulates to kill the tissue. With responses from ethylene the 

 situation is quite different. Here time of exposure is of prime importance. 

 Ethylene has low solubility in plants and consequently does not accumulate 

 with time. Moreover, it must be present until the response occurs and the 

 response ceases soon after the ethylene disappears from the surrounding air. 

 Wilted plants were more resistant than similar plants that were turgid. 

 The difference was considered to be due, at least in part, to the condition 

 of the stomata. The investigators believe that low air humidity and low 

 water content of the soil make plants more resistant by modifying the water 

 content of the plant. Plants fumigated at night showed more resistance 

 than similar plants treated during the day. Plants also gained in resistance 

 if they were placed in the dark two hours before they were fumigated in a 

 dark case. Shading while plants were being exposed to the gas was not 

 effective. 



If this work was important at the time it was done in furthering the 

 physiological point of view in SO2 injury, it has since been greatly out- 

 distanced even in this respect by the extensive and very accurate researches 

 by Hill, Thomas, and associates, and by the National Research Council of 

 Canada. Later work by Setterstrom and Zimmerman -^ confirmed the 

 earlier work of Zimmerman and Crocker and added the following facts: 

 plants are more resistant at 40° C (104° F) than at higher temperatures, 

 and also when previously grown in good rather than in poor light and when 

 exposed to full light rather than in deep shade (65 per cent or greater reduc- 

 tion of light intensity). The sulphate nutrient supply and previous treat- 

 ment \vith SO 2 did not affect the susceptibility if in the latter case sufficient 

 time was allowed for recovery from previous treatment. 



Sulphur Dioxide Content of Air at Boyce Thompson Institute. 



The SO2 content of the air at Boyce Thompson Institute ^s. 28 ^ras re- 

 corded continuously with minor interruptions for two years, November 1, 

 1936 to November 1, 1938, by use of the Thomas recorders. 



The average readings including zero readings were 0.033 ppm for the 

 first year and 0.035 ppm for the second. The maxima were 0.75 ppm for 

 the first year and 0.53 ppm for the second. Both the averages and the 

 maxima are almost identical with those at Northport, Washington 21 ■ p-^s 

 during the growing season, where claims were made for injury to crops and 

 forests. The high SO2 content of the air at Yonkers was in the winter when 

 plants outside were not gro'wdng. 



Setterstrom and Zimmerman ^s. p-i78 conclude: "Correlation of SO2 con- 

 centrations with the Avind direction indicates that the sulphur dioxide 

 comes largely from New York City (15.4 miles SSW to Times Square which 

 marks the approximate center of the metropolitan area). A study of the 

 relationships between concentrations of sulphur dioxide of the atmosphere 



