98 GRKKNTKirSl': FLMKiATlON 



Moisture Upon the Leaves of the Plants. 



Moisture upon the lea\es al)Sorbs hych^ocyanic acid, thus forni- 

 iui^' (h'lips of prussic acid on the leaves. These increase in strength 

 during fumigation, giving the hydrocyanic acid an excellent oppor- 

 tunity to pass from the drop through the cuticle to the plant tissues. 

 The ability to enter the citticle depending upon its cutinization, 

 workers tising ditterent plants have arrived at different conclusions 

 as to the injury resulting from wet leaves at the time of fumigation. 

 Johnston states, "Lettttce, cucuml)ers. and cantelotipes are very 

 easily injured if the plants are damp, even with small amounts 

 of gas" (6). A\'oglum (7), working with citrus plants, reports, 

 'VhTom all this exi)erience. not a single authentic instance has been 

 seen in which burning was directly attributable to absorption of 

 gas bv the moisture on the fruit or leaves."' In a series of experi- 

 ments carried out where the leaves of the plant were wet, the 

 results showed an injury to the foliage of lettuce, tomatoes, begonia, 

 tradescantia, cineraria, geranium, and similar plants with thin cttti- 

 cles while citrus, aspidistra palms, and ivy, were not injured. 



In the fumigation of plants with thin, slightly cutinized cuticles, 

 the leaves should be perfectly dry. but in dealing with plants with 

 thick. strc)ngly cutinized cuticles the leaves may be wet or dr}- 

 withotit causing injury. Moisture on the leaves may be present, 

 due to several causes, watering the plants just preceding ftimiga- 

 tion. dripping" from snow (»r rain on the roof, and exudation of 

 water from the water pores of the plant. A peculiar case of injury 

 due to moisture on the leaves is shown in Figure 24-1 of eucalyp- 

 ttis leaves, burned bv hvdroc\anic acid dissolved in nectar which 

 had dropped down ui)on them from the flowers of a col)ea growing 

 overhead. 



Soil Moisture. 



In daylight fumigation soil nioisttire fa^■ors injury to the plants. 

 A typical experiment was one in which si.x sets of tomato ]ilants 

 were used. Set 1 dr}-. not watered for 24 hours, nearly wilting: 

 .Set II wet. abundant water in the pots; .Set III dry, as in Set I. but 

 the soil Coated with paraftine ; Set I\' wet and coated with. i)araffine ; 

 -Set \ dry, l)ut watered immediateh' after fumigation to bring an}- 

 hydrocyanic acid in the loose soil in contact with the organic matter 

 and thus aid its decomposition ; Set \ I, both wet and dry soil coated 

 with paratfine btit not fumigated. The result showed the plants in 

 wet Soil were more severely injured than those in dry soil resfardless 



