LETHAL GASES 201 



exposure. Animals endure 33 ppm for 500 hours without injury. Sulphur 

 dioxide kills the leaf parenchyma of the medium-aged leaves, thus cutting 

 doA\Ti assimilation, but there is no reduction of assimilation or growth if 

 no tissue is killed, that is, there is no "invisible injury." Darkness and 

 partial wilting increase the resistance of plants to SO 2, partly at least by 

 closing the stomates. Many continuous records have been made of SO2 

 content of the air about industrial and population centers. At Boyce 

 Thompson Institute the average and highest aimual concentration of SO 2 

 in the air, largely blowing in from New York City, is about the same as 

 that in the upper Columbia Valley at Northport, Washington, during the 

 growing season, where injury from the Trail Smelter is claimed. The highest 

 concentration of SO 2 in the air about cities is during the A\inter when most 

 coal is being burned. Finally, the SO2 given off by industries and cities 

 aids soil fertility by replenishing sulphur deficiency of the soU. 



Hydrogen sulphide differs from SO 2 in several ways as to its effect upon 

 plants; it requires a much higher concentration to injure plants, 40 to 

 400 ppm ; it kills the young leaves and stems rather than spotting middle- 

 aged leaves, and its toxicity is not so greatly reduced by darkening and by 

 •wilting the plants. 



Chlorine acts much like SO2 on plants and spots them in even lower con- 

 centrations. Chlorinated water has relatively low toxicity for land plants 

 when used either for syringing or watering them. Chlorinated water 

 bearing 1 .0 to 1 .5 ppm of chlorine will kill fish, and water plants are a little 

 less sensitive. City water supplies may at times contain enough chlorine 

 to injure fish and water plants. 



By use of the continuous air-flow method, a study was made of relative 

 sensitiveness of plant and animal pathogens, sclerotia, seeds, green plants, 

 and houseflies, rats, and mice to the five gases, CI2, HCN, H2S, NH3, 

 and SO2. Chlorine and SO2 showed high toxicity to pathogens and other 

 gases low toxicity. Sclerotia and seeds were little injured by any of the 

 gases. Green leaves were very sensitive to these gases, and the gases show^ed 

 the followmg order of toxicity : Cl2>S02>NH3>HCN >H2S. Green stems 

 were more resistant than leaves, with no significant difference in the degree 

 ' of toxicity of the five gases. Animals were readily killed by HCN and H2S 

 and the order of toxicity for animals was HCN>H2S>Cl2>S02>NH3. 



Literature Cited 



1. Alway, F. J., "A nutrient element slighted in agricultural research," J. Am. Soc. 



Agron., 32 : 913-921 (1940). 



2. Barton, L. V., "Toxicity of ammonia, chlorine, hydrogen cyanide, hydrogen sulphide, 



and sulphur dioxide gases. IV. Seeds," C. B. T. I., 11 : 357-363 (1940). 



3. Boussingault, "Sur Taction deletere que la vapeur cmanant du mercure exerce sur 



les plantes," Compt. Rend. Acad. Sci. [Paris], 64 : 924-929 (1867). 



4. Bredeman, G., and H. Radeloff, "Ueber Schadigung von Pflanzen durch Ammoniak- 



gase und ihren Nachweis," Zeitschr. Pflanzenkrankh. u. Pflanzenschntz, 42 : 457- 

 465 (1932). 



