102 (;Ri-:ENHorsK fim hiaiiox 



W hen till' rc-lati\ e humidity is low and e\ aporatidii is rapid, the 

 h\(h"oc_\anic acid woidd be evaporated as fast as it dissolved in 

 the moisture of the cuticle, and no opportunity would he given it 

 to diffuse from the cuticle into the tissues of the leaf. Plants with 

 thin cuticle are intluenced more b}' the relatix e humidity than those 

 with a thick, waxy cuticle. 



Relative Humidity After Fumigation. 



The iiitluence of relative Inmiiditv is not confined to the time of 

 fumigation. A ntimber of tomato plants fumigated side by side 

 were divided into two lots at the close of the fumigation, one of 

 which was placed under humid conditions, 99^^, 86° F., the other 

 drv conditions, 15%, 86° F. for a period of 12 hours. The plants 

 under a high relatiAe humidit}' were more severely injured, as 

 shown bv Figtire 27. The hydrocyanic acid is apparently dissolved 

 in the cuticle of the leaf, but is ra^jidly e\a])orated under the low 

 relati\e htimidit\' ])ef(^re it has an op]:)orttinity to reach the cells 

 below, while it is retained under the high relative humidity and 

 passes from the cuticle to the cells below, producing its typical 

 injur\-. [Mants with thick, waxy cuticles are not so easily aft'ected 

 bv a high relative humidity. Citrus plants e\en fumigated with 

 drops of water on their leaves were placed under wet and dry condi- 

 tions after fumigation without injury resulting. 



TEMPERATURE. 



Influence on the Gas in the House. 



An ob\ious influence of high temperature is to increase the 

 diff'usibility of the hydrocyanic acid i)resent in the air during fumi- 

 gation. Inasmuch as the uneven distribution of the gas in the 

 greenhouse at the beginning of the fumigation is a source of danger 

 to the plants, a higher temperature which hastens dift'usion is bene- 

 ficial. ( )n the other hand, a warm house, when the outside temper- 

 ature is low, facilitates the escape of the vapor through the crevices 

 of the glass, thus rapidly reducing the concentration of hydrocyanic 

 acid. When the outside temperature is so low that ice fills the 

 crevices of the glass the leakage of vapor from the house is checked. 

 .Such a house is extremely tight and in fumigation under such con- 

 ditions it is achisable to reduce the dose. 



Influence of Temperature on the Penetration of the Gas. 



One would expect that an increase in temperature causing an 

 increase in the rate of dift'usion through the stomata of the plants 

 would tend to increase injury to the plants. An increase in the 



