180 GROWTH OF PLANTS 



livered to consumer pipes. As we shall see later, hydrogen sulphide, 

 although highly toxic to mammals, has a rather low order of toxicity to 

 plants. Some N2 and CO2 are present in most natural gases. Nitrogen 

 may run as high as 40 per cent and CO 2 over 1 per cent. Some natural 

 gases contain helium. All these constituents except the hydrogen sulphide 

 have relatively low toxicity to plants. If the hydrogen sulphide, when 

 present, is completely scrubbed out, the natural gases are almost inert so 

 far as plants are concerned. The most toxic constituents are the higher 

 homologs of methane, especially pentane, hexane, heptane, and octane, 

 and they are present in very low concentrations. 



Solheim and Ames ^^ found that certain natural gases of the northwestern 

 United States showed very low toxicity to plants. Tomato, potato, sun- 

 flower, castor bean, and geranium plants were uninjured by 50 per cent of 

 natural gas from the Billy Creek field in Sheridan, Wyoming, or gas from 

 the mains at Laramie after four days of exposure. Cut carnations did not 

 exhibit any symptoms of injury when they were exposed to 2 per cent of 

 the gas for four days. Fuchsia petals showed slight bro^^^ling and wilting 

 when the plants were exposed to 4 to 50 per cent of the gas for one to four 

 days. 



SchoUenberger ^* found that a natural gas composed mainly of methane 

 and ethane rendered soil highly toxic to wheat and oats. Analysis of this 

 soil showed a marked increase in soluble manganese and ammonium nitro- 

 gen, and lesser but distinct increases in sodium, potassium, and calcium. 

 Since these changes frequently occur as a result of water-logging, puddling, 

 or other conditions which favor reducing actions, SchoUenberger con- 

 cluded that the toxic efTect of the natural gas in soil was due primarily to 

 reduced oxygen pressure. 



Injury to Plants in a Greenhouse by Mercury Vapor 



Zimmerman and Crocker ''^' ^^ were asked to investigate injuries to roses 

 in a greenhouse at Attleboro, Mass. The injuries were quite different from 

 those caused by illuminating gas; also the injuries could not be attributed 

 to insects or fungal and bacterial pests, for the plants were almost free from 

 such pests and the injuries were not such as would be caused by them. The 

 only hint at a possible source of injury was the fact that the soil in some of 

 the benches had been treated with a solution of mercuric chloride, HgCl2, 

 to kill earthworms. The soil had also been fertilized with tankage. The 

 injury, however, appeared in all benches in the house, regardless of whether 

 they had been treated mth HgCU solution and tankage. Later, rather 

 extensive experimentation led the investigators to the conclusions that the 

 injury was caused by the HgCl2 solution added to the soil of some of the 

 benches, and that the organic matter of the soil reduced the chloride to 

 metallic mercury, which had sufficient vapor pressure to move through the 

 air and injure roses in untreated benches. Ratsek ^^ later reported similar 

 results from treating soils in a rose house with HgCU, but he explained the 



