1913.] PUBLIC DOCUMENT — No. 31. 09 



In tlie preceding table it is shown that different percentages 

 of soil moisture exert a marked influence on the development 

 of the plants, and their susceptibility to burning from fumiga- 

 tion. It will be noticed (see Summary Table) that the average 

 height of the plants was greater and the leaves larger in the 

 pots containing the largest percentage of water. (Cf. Figs. 1 

 and 2.) Also the diameter of the stems and length of leaf 

 petioles and internodes was greater in the plants growing in a 

 higher percentage of soil moisture, although in the case of the 

 plants growing in pots containing 70 per cent, of water there 

 was too much moisture for the best development. 



In Experiment I. the water was supplied largely to the sur- 

 face of the soil, and in II. and III., below the surface. The 

 plants growing in soil containing over 50 per cent, of moisture 

 burned most severely, while those in soil containing less were 

 not affected nearly so much, showing the extent of injury to be 

 proportionate to the amount of soil moisture. Examination of 

 some of the leaves under a microscope showed much variation 

 in the number of stomata. The upper leaf surface of a cu- 

 cumber plant grown under normal moisture conditions had 27 

 stomata to the square millimeter, while that of a plant grown 

 in soil containing 10 per cent, of moisture had only 11. 



Those who have used cyanide to any extent in greenhouses 

 are well aware that it has a rather narrow range of safety, and 

 that burning will occur at one time and not at another when 

 the conditions are apparently identical. Most fumigating is 

 done after dark, and it is usually customary to first wet down 

 the greenhouse, on the idea that if the house is well filled with 

 moisture, fumigation is more effective, especially when tobacco 

 is used ; and by fumigating after dark less injury is supposed to 

 result from burning of the foliage. This latter notion is based 

 on the assumption that the leaf stomata close in darkness, and 

 therefore less gas is absorbed by the foliage, and burning is not 

 likely to occur. 



There are numerous factors associated with the burning of 

 foliage by fumigation which are not well understood. The 

 effect of sunlight and atmospheric moisture, etc., on the burning 

 of tomatoes and cucumbers has been studied by Dr. H. T. Fer- 



