200 GROWTH OF PLANTS 



must be considered when one organism is compared with another; flies, for 

 instance, resist 1000 ppm of HCN more than twice as long as mice or rats, 

 but mth 63 ppm mice and rats endure HCN five to seven times as long as 

 flies. At 16 ppm neither rats nor mice showed any deaths or even signs of 

 injury during 960 minutes, while 50 per cent of the flies were killed in 

 48 minutes. For SO2 we have already mentioned that guinea pigs resisted 

 higher concentrations better than mice, but that mice were much more 

 resistant to low concentrations. It is certain that flies could be killed in the 

 presence of mammals by use of HCN in perfectly regulated low concentra- 

 tions for long periods. It is regrettable that several other insects were not 

 included in these studies to see how generally very low regulated concentra- 

 tions of HCN proved fatal to insects. 



Those that are interested mil want to read the original articles, especially 

 for details on symptoms and internal pathological changes caused in ani- 

 mals and on types of injuries produced in green plants by the several gases. 



Summary 



As we saw in the previous chapter, ethylene is the constituent of artificial 

 illuminating gas that injures plants in greenhouses when this gas seeps 

 through the soil and into the houses. Hydrocyanic acid is the most deadly 

 constituent of artificial illuminating gas to plants growing outdoors near 

 leaking gas pipes. ISIost natural gases have very low toxicity to plants 

 because they contain no ethylene or other unsaturated hydrocarbons, and 

 no HCN or other highly toxic gases. Some natural gases contain H2S 

 which might injure plants if the gas were not thoroughly scrubbed. 



Mercuric chloride, calomel, or organic mercury fungicides must be used 

 with caution on soils in greenhouses or other enclosed spaces because the 

 soils reduce these compounds to metallic mercury which has sufficient vapor 

 pressure, especially at higher growing temperatures, to injure plants 

 throughout the enclosed space. Use of mercury in respirometers or to seal 

 apparatus may vitiate experiments in plant physiology either by injuring 

 or stimulating the plants. The health of laboratory workers is endangered 

 by exposure of large surfaces of mercury to the air, as in the case when 

 pellets of mercury are allowed to lie on the floor or laboratory tables. 



Injuries from SO 2 from smelters have led to the development of accurate 

 apparatus for recording the SO2 concentration in the air and for fumigating 

 plants and animals in continuous-flow chambers with the SO2 regulated 

 accurately and continuously recorded. The Department of Agricultural 

 Research of the American Smelting and Refining Company did much to 

 develop this apparatus and contributed greatly to knowledge of the physi- 

 ological effect of SO2 on plants. The National Research Council of Canada 

 in connection with Trail Smelter injury added much to our understanding 

 of the effect of SO 2 on vegetation. Boyce Thompson Institute added to the 

 knowledge of the physiological effect of SO2 on plants and animals. The 

 most delicate plants are injured by 0.46 ppm of SO2 with seven hours' 



