The size of the beds is governed by convenience in making the beds and handling the crop. The beds on the side tiers, therefore, are often three to three and one-half feet in width, affording a convenient reaching distance from the alley. The beds of the center tier have access from the alley on either side and are usually seven feet in width. The width of the alley varies according to the mind of the owner, from two to three or three and one-half feet. The narrow alley economizes space in the structure of a house; the wide alley, while slightly increasing the cost of the structure, makes it much more convenient in handling the material, and in moving about the house. The beds are constructed of one-inch boards. Various kinds of lumber are used, the hemlock spruce, the oak, Georgia pine, and so on. The beds are supported on framework constructed of upright scantling and cross stringers upon which the bottom boards are laid. These occur at intervals of three to four feet. The board on the side of each bed is 10 to 12 inches in width. The bottom bed, of course, is made on the ground. The upper beds in the tier are situated so that the distance is about three feet from the bottom of one bed to the bottom of the next above. Figs. 228 to 231 show the general structure of the beds. =Heating.=--One portion of the house is set apart for the boiler room, where a small hot water heater is located. The position of the heater in one of these houses is shown in Fig. 227. In other cases, where the plant is quite a large one, a small separate or connecting boiler apartment is often constructed. In other cases, where the house is connected with or adjoining a system of greenhouses devoted to hothouse vegetables, the water pipes may run from the general boiler house which supplies the heat for all the houses. The water pipes in the mushroom houses are sometimes run beneath the boards or the walk in the alley, or in other cases are run just beneath the roof of the building.