186 GROWTH OF PLANTS 



fumes by the use of Cottrell precipitators. This prevents other toxic 

 substances from reaching the atmosphere about the smelters and gives 

 valuable by-products. 



Research by Hill, Thomas, and associates. The advance in methods and 

 instrumentation was started by Wells of the Selby Smelter Commission and 

 later further developed and perfected by scientists of the Department of 

 Agricultural Research, American Smelting and Refining Company of Salt 

 Lake City, Utah, early by O'Gara and later by Hill and Thomas and 

 associates. The outstanding advance in apparatus was the Thomas 

 autometer for continuously analyzing and recording low concentrations of 

 SO2 in the atmosphere. This has later been adapted for measuring and 

 recording low concentrations of many other gases and vapors in the atmos- 

 phere. The automatic recording fumigating apparatus used for SO 2 and 

 other gases at the Institute is illustrated in Fig. 71,2'^ and a full description 

 of the apparatus and method of its operation was made by Setterstrom.^^ 

 Thomas and associates ^^ have recently improved this apparatus for long- 

 time large-scale study of the effect of SO 2 fumigation on the nutrition and 

 physiology of plants. The new apparatus gives control of the soil conditions 

 as well as the atmosphere about the plants. 



Besides doing so much to develop suitable apparatus, the group at the 

 American Smelting and Refining Company has contributed much accurate 

 knowledge on the physiological effect of SO2 in the air on plants. Space will 

 permit the mention of only two of the later contributions. They studied 

 the effect of SO2 fumigations on the rate of photosynthesis and respira- 

 tion 3" as measured by the CO2 absorbed or released by alfalfa. Heavy 

 fumigations, 0.7 to 1.26 ppm, of durations too short to kill any tissue of the 

 leaf reduced the rate of photosynthesis during the exposure; but imme- 

 diately after exposure the photosynthesis rose to normal or greater than 

 normal, so the net effect of such fumigations was practically zero. Light 

 fumigations for long periods, 0.24 ppm for 3 days, 0.19 ppm for 11 days, 

 0.14 ppm for 39 days, either showed no effect or suggested stimulation. 

 Even when heavy fumigations were long enough to cause extensive killing 

 of leaf tissue, much of the photosynthetic power was restored by the 

 development of new leaves within 10 or 15 days. The workers conclude 

 that fumigations that do not produce visible injury (spotting of leaves) do 

 not produce "invisible injury" (reduction of photosynthesis and yield). 

 Thomas and co-workers, "^ in a study of sulphur nutrition of alfalfa in the 

 improved culture chambers, found that sulphur-deficient plots were im- 

 proved in yield by light SO2 fumigations. We shall later discuss the sig- 

 nificance to soil fertility of release of SO 2 into the atmosphere by mdustries. 



American Smelting and Refining Company has faced realistically the 

 problem of possible injury to vegetation by the fumes from its smelters, 

 and has built and manned a fine laboratory to work out in detail the effect 

 of SO2 upon plants. They have contributed materially to improved instru- 

 mentation in plant and industrial science and to fundamental knowledge 



