Where this method of cultivation is employed, as the main issue, houses are constructed especially for the purpose. In general the houses are of two kinds. Those which are largely above the ground, and those where a greater or lesser pit is excavated so that the larger part of the house is below ground. Between these extremes all gradations exist. Probably it is easier to maintain an equable temperature when the house is largely below ground. Where it is largely above ground, however, the equability of the temperature can be controlled to a certain extent by the structure of the house. In some cases a wall air space is maintained around the sides and also over the roof of the building. And in some cases even a double air space of a foot or 18 inches each is maintained over the roof. In some cases, instead of an air space, the space is filled with sawdust, single on the sides of the house, and also a 12 01-18-inch space over the roof. The sides of the house are often banked with earth, or the walls are built of stone or brick.

Figure 226

Single mushroom house (Wm. Swayne, Kennett Square, Pa.), "curing " shed at left. This house is heated in connection with other hothouses.

All of these houses, no matter what the type of construction, require ventilation. This is provided for by protected openings or exits through the roof. In some cases the ventilators are along the side of the roof, when there would be two rows of ventilators upon the single gable roof. In other cases a row of ventilators is placed at the peak, when a single row answers. These ventilators are provided with shut-offs, so that the ventilation can be controlled at will. The size of the house varies, of course, according to the extent of the operations which the grower has in mind.

The usual type of house is long and rather narrow, varying from 50 to 150 feet long by 18 to 21 or 24 feet wide. In some cases the single house is constructed upon these proportions, as shown by Fig. 226, with a gable roof. If it is desired to double the capacity of a house, two such houses are built parallel, the intercepting wall supporting the adjacent roof of the two houses, as shown in Fig. 227. A still further increase in the capacity of the house is often effected by increasing the number of these houses side by side. This results in a series of 8 or 10 houses forming one consolidated block of houses, each with its independent ridge roof and system of ventilation. The separating walls between the several houses of such a block are probably maintained for the purpose of better controlling the temperature conditions and ventilation in various houses. If desired, cummunication from one house to another can be had by doors.

Figure 227

Double mushroom house (L. S. Bigony's Mushroom Plant). Packing room at left, "curing" shed at right, next to this is boiler room.