138 VEGETABLE FORCING 
possible to maintain temperatures which are most suit- 
able for the various classes of plants at different stages of 
growth without interfering with the crops in the main 
houses. 
It is well known that the temperature for lettuce imme- 
diately after the first transplanting should average 5 to 
10 degrees higher than that for lettuce which is approach- 
ing maturity. It is impossible to provide the proper 
temperature for both lots of plants if they are in the same 
house. Again, the main houses may be filled with a 
winter crop of lettuce when it is time to start tomatoes 
or cucumbers for spring planting. While it is possible, 
with skillful management, to accomplish this, it is not a 
simple undertaking. 
The humidity of the houses can also be better regu- 
lated if there are separate nurseries for starting the plants. 
Fumigation, too, may be necessary in the plant compart- 
ment when not in the main houses, or vice versa. If 
separate houses are provided growth may be forced or re- 
tarded, as may be required to prepare the plants for the 
beds at the proper time. The advantages of separate 
plant houses are apparent. 
Fig. 45.—Lettuce plants in flats, 
