iqiq] CLAYTON— fumigation 495 



Concerning the conditions under which plants are most resistant, 

 the following statements are possible : 



The resistance of the tomato to cyanide is increased by the 

 presence of water on the leaf surface, but with some species wet 

 leaves increase injury. Temperature should be moderately low. 

 Light intensity should be low during the day preceding the fumiga- 

 tion. The plants should go into the fumigation with the stomata 

 closed. A large amount of reducing sugars in the plant is correlated 

 with maximum resistance. Checking growth by reducing water 

 supply, during the period preceding the fumigation, increased 

 resistance. 



The action of cyanide on entering the plant is a very interesting 

 question, and all evidence favors the conclusion that cyanide 

 combines readily with the substances there. In certain species of 

 plants cyanides in the form of glucosides occur, and on these 

 naturally occurring cyanides a great deal of work has been done. 

 The investigators (12, 13) are unanimous in considering that this 

 cyanide is never in an uncombined state. Granting then that the 

 hydrocyanic acid, on entering the plant, unites quickly with plant 

 compounds, there may still be variations in the type of union. 

 Thus possibly there may be adsorption under certain conditions and 

 under other conditions chemical combination. 



Methods for the determination of cyanide in plants have been 

 devised (2, 11), but tKey are rather laborious. One method is based 

 on the fact that a cyanide in a picric acid-sodium carbonate solu-' 

 tion gives a red color. The depth of the color, as compared with 

 a standard range, is the basis of estimation. The coloration is due 

 to the strong reducing action of the cyanide, but unfortunately the 

 cyanide is not the only substance present in the plant which gives 

 the reaction. For accurate quantitative work it is necessary to 

 isolate the cyanide compounds. This was not done at this time; 

 instead, estimations were made on unit weights of leaf tissue. The 

 leaves were put in the picric acid-sodium carbonate solution, and 

 observations were made after 24 hours by means of a colorimeter. 

 The phrase ''reducing substance content" is used because, as 

 already stated, the cyanides are not the only things reacting. The 

 variation in values secured, during the course of a series of 



