SYSTEMS OF CROPPING 383 
siderable extent. The fundamental principles involved 
are practically the same as for double cropping in the 
open ground, but attention should be called to a few addi- 
tional factors, as follows: 
(1) Land values under glass are much higher than out 
of doors, and this is an argument for intensifying opera- 
tions by intercropping. 
(2) Steam sterilization destroys weed seeds and the 
soils used are usually light, so that cultivation, hoeing 
and weeding are not so necessary, and close planting, 
therefore, is less objectionable from this standpoint than 
in the open ground. 
(3) A higher standard of quality is required for green- 
house products, and companion cropping may be the 
means of lowering the quality, as when radishes and 
lettuce are crowded and shaded by larger plants. 
(4) Soil adaptation must be carefully considered. It 
is a mistake to attempt to grow two crops together 
in the greenhouse unless both are well adapted to the 
soil. 
(5) The question of temperature is the most important 
factor, and often the most serious handicap. A common 
practice is to plant cucumbers in beds of lettuce. Al- 
though the plan is successful to a greater or lesser extent, 
one or both crops usually suffer because of unsuitable 
temperatures. This condition is so serious in the opin- 
ion of some growers that they never attempt to grow 
crops together which have different temperature require- 
ments. 
(6) The manurial requirements of each crop should also 
receive consideration. This problem, however, is not 
serious, because a soil that contains sufficient manure to 
produce a good crop of lettuce will also be satisfactory 
for tomatoes and cucumbers. , 
(7) The size of each kind of plants at the time of set- 
ting should be given the closest attention. For example, 
