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 321% inches 
by 13% inches by 1534 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 
