GREENHOUSE CONSTRUCTION AND HEATING 39 
greenhouses vary from 12 to 24 inches in width. Twenty 
inches provides sufficient space under most circum- 
stances. In solid plantings of lettuce it is customary to 
omit two rows, or sometimes only one, while in cucumber 
and tomato plantations the walks are 30 inches or more. 
Special alleys (Figs. 5, 23 and 24) and roadways (Fig. 25) 
are important in very large ranges. 
The walks in some of the best houses are made of con- 
crete. These are especially desirable in heavy soils. They 
are inexpensive and simple to construct. The ground 
should be graded as level as possible before the walks are 
outlined. Use 2.x 4-inch pieces for the sides. Care must 
be exercised to get the sides straight. Tamp the soil in- 
side the 2 x 4-inch pieces (the scantling may be laid flat if 
desired) until within an inch of the top. Stretch a piece 
of poultry netting over the tamped soil, and hold it in. 
place with bent pieces of old wire stuck into the soil and 
hooked over the netting. Rather thin concrete is used, 
and the top leveled and smoothed in the usual manner. 
The poultry netting reinforcement greatly increases the 
strength of the walk, and economizes concrete. The net- 
ting should be permitted to bulge here and there over the 
soil so that the concrete will settle all around the meshes. 
Where freezing does not occur, as in the greenhouse, it 
is unnecessary to use ashes under the concrete. The 2x4 
pieces of lumber are removed after the concrete is 
properly set. 
Steam vs. hot water heating —Modern greenhouses are 
heated either by steam or hot water. Hot water is almost 
invariably preferred for small greenhouses because the 
boilers may be left for a longer period at night without 
attention. About nine-tenths of the large establishments 
are heated by steam, and the growers claim that the steam 
system costs less to install and to operate, and that it 
gives them better control of temperatures. But some of 
the owners of very large ranges of recent construction are 
