106 VEGETABLE FORCING 



No general rule can be given relating to the frequency 

 of fumigations. This will depend on the crops under 

 cultivation and the prevalence of aphids. It is important 

 to start with plants apparently free from lice. If this is 

 done, once a week may be sufficient. When there are 

 evidences of serious trouble, it will be best to fumigate 

 lightly on three successive nights. This is regarded as 

 more effective than one strong treatment, which may 

 injure the plants. If the three treatments are successful, 

 no further attention may be needed for a week or ten 

 days. 



The danger of injury to the crops will depend upon the 

 plants that are under cultivation; cucumbers are more 

 easily affected than tomatoes, but tomatoes are more 

 susceptible to injury than lettuce. If the plants have 

 been grown too rapidly and the tissues are soft and 

 tender, injury is likely to occur. High temperatures are 

 largely responsible for injuries from tobacco fumigation. 

 Gourley made the following interesting experiment : 



"A small test was run on the effect of smoke on lettuce 

 in the following manner: A rectangular box 32^ inches 

 by 13^4 inches by 15^4 inches (inside dimensions) with a 

 capacity of 753.6 cubic inches was placed over four let- 

 tuce plants of a size ready to be marketed. The tempera- 

 ture within the box before starting the smudge was 54 

 degrees. A dense smudge was created in one end of the 

 box with dried tobacco leaves. When the box was raised 

 after an exposure of 15 minutes the temperature was 115 

 degrees, and the plants covered with a viscid, brownish 

 precipitation of nicotine compounds which was intensely 

 bitter and sickening to the taste. The leaves were 

 mostly limp and brown. 



"Again the box was placed over four fresh plants of 

 the same size as the former ; the temperature was stand- 

 ing at 60 degrees within the box. Two sections of stove 

 pipe were secured ; the lower one had a false bottom of 



