1919] CLAYTON— FUMIGATION 



499 



appreciation of the necessity for delicate regulation of amounts of 

 cyanide. Every separate greenhouse varies in its ability to retain 

 the gas, and it is not alone the amount of cyanide which is put in 

 that counts, but rather the amount which is retained. Thus 

 definite recommendations cannot be given more than it is possible 

 to give a universal fertilizer formula. Second, it is the usual thing 

 to start the fumigation during the latter part of the afternoon when 

 the stomata are still open. This is sure to induce excessive injury. 

 It is safe to start fumigation 2 . 5-3 hours after sundown on a bright 

 day, or at sunset on a very dark day.. 



Summary 



1. Different concentrations of hydrocyanic acid gas gave effects 

 ranging from stimulative to depressive. The maximurn of bene- 

 ficial results was secured from concentrations deadly to insect life, 

 but just a little below the point of first injury to the plant. 



2. External factors having important action on the resistance 

 are as follows : (a) wetting the leaves had a beneficial effect on the 

 tomato; {b) reduced temperature and low light intensity during 

 the day preceding fumigation increased resistance. 



3. Injury closely paralleled the stomatal movement, increasing 

 as the size of stomatal aperture increased. 



4. A higher or lower water supply in the soil affected resistance, 

 through hastening or retarding the growth rate. Rapid growing 

 plants were susceptible to injury, while slow growing plants were 

 more resistant. 



5. High reducing sugar content seemed to be correlated with 

 maximum resistance. 



I wish to acknowledge the assistance given by Dr. William 

 Crocker and Dr. Sophia H. Eckerson, under whose direction 

 this work was conducted. 



University of Wisconsin 

 Madison, Wis. 



