182 GROWTH OF PLANTS 



plate atomized with HgCl2 solution, no mercury was deposited on gold 

 foil by such air. If in addition the bell jar contained a potted plant, 

 mercury was deposited on the gold foil. The plant evidently gave off some 

 material that reduced the chloride, or HgCU vapor reached the plant and 

 was reduced to the metal. 



Addition of HgCl2 solution to tankage injvired untreated plants enclosed 

 with it much more than did HgCU solution added to sand or powdered 

 charcoal. Mercurous chloride dust, which has much lower vapor pressure 

 than HgCU, is not toxic to a growing plant with which it is enclosed. If 

 HgCl2 is added to soil and the soil enclosed with a plant, the plant is in- 

 jured. Zimmerman and Crocker found that various inorganic mercury 

 compounds as well as several organic mercurial fungicides, Dubay, Nu- 

 Green, Semesan, and Uspulun, when added to fertile soils, gave off emana- 

 tions that injured plants in untreated soils in the same enclosure. Finally, 

 Daines ^ says that in soils where mercurials are effective as fungicides, 

 mercury compounds are reduced by the soil to metallic mercury, which 

 migrates in the soil as mercury vapors, and that any factor that prevents 

 the conversion of the mercury salt to metallic mercury destroys the 

 fungicidal effects of the mercurial. 



Walker, et al.^^ give a similar explanation for the effectiveness of HgCU 

 solution in controlling club root of cabbage when applied at the time of 

 transplanting. This is the type of evidence we have that fertile soils reduce 

 mercury compounds to metallic mercury, and that it is vapors of the 

 metallic mercury that travel through the air and injure the plants in un- 

 treated soil in the same enclosure. Regardless of the correctness of this 

 interpretation of the mechanics by which additions of mercury compounds 

 to the soil in greenhouses injure plants in untreated soils in the same house, 

 there is no doubt that mercury compounds, organic and inorganic, must 

 be used with caution in greenhouse soils, because of the release of mercurial 

 vapors in the air. In the use of mercurials in outside planting the danger 

 of injury is through the roots. The air will be kept relatively free from 

 dangerous vapors by diffusion and wind currents. 



That mercury vapor in the air is injurious to both plants and animals 

 has long been known. The only new claim here introduced is that adding 

 mercury compounds, even those with very low vapor pressures and solu- 

 bilities, to a greenhouse soil releases mercurial vapors into the air in suffi- 

 cient concentrations to injure plants throughout the greenhouse, including 

 those in untreated soils. 



Zimmerman and Crocker ^2- p-^'^^ state: "In 1797 there was made known 

 in a letter addressed to Van Mons by Lauwerenburgh that four Dutch 

 chemists, Deiman, Paats, Van-Troostwyck, and Lauwerenburgh,^'' had 

 discovered the deleterious effects of metallic mercury vapors on plants. 

 The results of 15 experiments mentioned in the letter showed that where 

 beans, mints, or spiraea we're enclosed in bell jars with metallic mercury 

 the leaves became spotted after 24 hours, and if left exposed to the vapors 



