100 GRF.r.NnoiSK FL'.M Ii;A TKiX 



of wlu'thcr tlie\- were paraffined ov not, and the period of time at 

 which they were watered after fumigation. Figure 25. ddiis ex.i)eri- 

 ment shows that the injury was not due to the absorption of the 

 liv(h-oc\anic acid l)y the soil. Chemical analysis revealed the fact 

 that li\-(lr(ic\anic acid is dectimjxised 1)\- the organic matter of the 

 soil. JLxamination proved the stomata of the plants in dry soil to 

 he closed and those of plants in wet soil o])en. Soil moisture is of 

 more influence in day fumigation than in night fumigation when 

 other factors cause the closing of the stomata. Inasmuch as abund- 

 ant soil moisture favors the exudation of Avater from the water 

 pores of certain i)lants excessiAC moisture should be avoided. Stone 

 has shown that plants grown under moist conditions tend t*) produce 

 thinner cuticles, thus making them more liable to injury (2). 



Relative Humidity at the Time of Fumigation. 



After a series of preliminar\- experiments which showed that 



relati\e humidity has an intfuence. the following experiment was 



conducted as an example of the influence of relatixe humidity u]X)n 



injur\- to tomato plants. The dcjse in each case was 1 ' _- oz. of 



potassium cyanide to one thousand cubic feet of air in a small 



greenhouse. The time of exposure was one hour, at the close of 



which the house Avas aired and the plants remoxed to another house 



to prevent an\- later influence acting u])on them. lleav_\- curtains 



were drawn to shut out any possible light, although it was night. 



Av. Rel. 

 Av. Temp. Huniiditv Result. 



Set I 48.5° F. 85.5% Plants severeh- injured. 



Set II 45° 69.5% Plants not injured. 



Set III 89° 54% Plants not injured. 



Set IV 88.5° F. 62.5% Plants showed a slight 



curling of the leaves. 

 Set \' 83.5- F. 76%; Leaves of plants curled. 



The injury under the different conditions is shown in l^^igure 

 26. FrcMii this experiment it is a])parent that the actual amount of 

 moisture in the air has little inlhience but that injury is caused by 

 the influence cjf the relative humidity on the rate of e\aporatitMi. 

 In other experiments different plants were used and in general 

 similar results were obtained. Plants with thick, strongly cutinized 

 cuticle are less influenced by relati\e humidity than ])lants with 

 thin cuticles, such as tomatoes. 



The efifect of relative humiditx- is probably accomplished in 

 two ways, ( 1 ) b}- a tendenc}- of the plant to slightly open its stomata 

 under high humidity to increase transi)iration, and (2) I)}- prex'ent- 

 ing the e\a])oration of moisture from the cuticle. 



