LETTUCE 221 
lettuce prefer 40 to 45 degrees at night and 10 to 15 de- 
grees higher by day. 
Most growers of Grand Rapids lettuce maintain higher 
night temperatures than they did a few years ago. For- 
merly, 45 degrees at night and 55 to 60 degrees by day 
were standard temperatures among the best gardeners. 
Now, some of the largest growers prefer 48 to 50 degrees 
at night and 55 to 60 degrees by day. A night tempera- 
ture of 45 degrees with high humidity and wet foliage is 
likely to cause much more damage than a temperature 
of 50 degrees with low humidity and dry foliage. On 
warm, sunny days during the spring, no harm will be 
caused by temperatures of 75 degrees or above, provided the 
houses are well ventilated. It is always important to have a 
definite understanding with the night fireman and watchman 
concerning the temperature that should be maintained. 
Fig. 75.—The lettuce in this large range is cultivated with a 5-pronged weeder 
attached to a long handle. 
Ventilation—Much has been said on previous pages 
about the importance of ventilation. In lettuce culture it 
is one of the means of avoiding losses from the attacks 
of fungous diseases and of producing the best, most com- 
pact heads. On very cold or stormy days the ventilators 
are not opened at all, while in mild weather it is not un- 
