Various plans have been devised to secure the needed conditions as to light, moisture, heat, and transpiration. In all of them nearly, glass is the covering used, as it is cheap and best combines the essentials needed. Its fault is that it not only admits light and lessens transpiration and evaporation, but it admits the sun's rays that do not return. In other words, the radiant heat of the sun passes through glass and accumulates to such extent that a hot-bed or other structure of limited space under glass may burn and scorch every plant in two hours if the ventilation is neglected. Yet glass is the best covering known, and with a little timely care it answers the purpose well.
Fig. 27.—Simple propagating bed.
The bottom heat is given in various ways. One of the simplest forms of combining the confined air and bottom heat is shown at Fig. 27. The box is about two feet deep with glass over the top. The water over the lamp is in a sheet-iron tray supported as shown. The sand shown is held up by placing in a shallow wooden box. With a little practice and experience cuttings can be rooted in a small way in this frame with about as much certainty as in a greenhouse cutting-bench.
The principle is the same in the private house, conservatory or the great commercial greenhouse. But in these cases the heating of the houses in winter and the needed bottom heat of the cutting-bench are given by steam- or hot-water pipes.