LETHAL GASES 189 



Experiments at Boyce Thompson Institute. Beginning in 1930, Zimmer- 

 man and Crocker ^*'' *^ made a study of the effect of SO2 on more than 

 30 species of plants representing more than a dozen families and including 

 farm, garden, ornamental, and wild species. The experiments were carried 

 on under a considerable range of conditions as to SO 2 concentration, dura- 

 tion of fumigation, light, moisture, and other factors. These experiments 

 were made before automatic self-recording fumigating apparatus was 

 available. The concentration of the SO2 was adjusted by flow meters 

 and the concentration of SO2 checked in the fumigating chamber by periodic 

 analyses of air samples. 



Figure 72. Buckwheat leaves from plants treated for six hours with various con- 

 centrations of sulphur dioxide gas. Left to right: (1) control, (2) 1.0 ppm, (3) 0.7 ppm, 

 (4) 0.5 ppm. 



The experiments led to the following conclusions. There is a great range 

 in the sensitiveness of different species of plants: buckwheat proved the 

 most sensitive; it showed killing of leaf tissue in 0.46 ppm A\dth seven hours' 

 fumigation. Some species of orchids were very resistant, withstanding 

 60 ppm for several hours without injury. The cereals and various weeds 

 were sensitive and carnations, gardenias, and rhododendrons were rather 

 resistant. The leaves were injured, but not the stems and buds. In dicoty- 

 ledons middle-age leaves were more sensitive than young or old leaves; 

 interveinal parenchyma was more injured than the veins, and small veins 

 more than large ones. Fig. 72 shows the leaf injury on buckwheat by 1.0, 

 0.7, and 0.5 ppm with six hours' exposure. The plates (some colored) in 

 the Trail Smelter book show the type of injury caused in leaves by SO 2, 

 and the Report of the Selby Smelter Commission contains colored plates of 

 leaf injury by SO2. The Trail report speaks of marginal as well as veinal 

 killing of leaf tissue in dicotyledons. 



Both duration of exposure and concentration of the SO2 were important 

 in determining the extent of injury. Sulphur dioxide is very soluble in 

 plant tissues and accumulates with time. In subtoxic concentrations time 



