Pits
These are valuable and well-known garden structures, utilised in their simplest form for protecting plants from the injurious effects of rain and severe frost. When heated, they are eligible for plant culture generally, for Cucumber, Melon, and Pine-growing, and for propagating. Pits are distinguished from frames by their walls being built partly beneath the ground, which consequently renders them fixtures, whereas frames are movable; generally, all the sashes are movable in both cases. Pits do not afford similar facilities for attending to the occupants as do houses, which the cultivator can enter in all weathers; yet they are indispensable where large, or even small, quantities of young plants have to be raised and grown on. For bedding plants, a single hot-water pipe is usually sufficient, in a low, narrow Pit, for expelling damp and keeping out frost, except in very severe or unfavourable weather, when coverings would be necessary. For Cucumber, Melon, or Pine Pits, a more substantial and much higher structure, and also a greater heating power, are necessary. Fig. 191 represents a useful little Pit for early forcing of Melons or Cucumbers. A narrow, sunken passage, entered from one end, affords sufficient space for attending to the plants; and if a shelter were fixed to the back wall for early forced Strawberries, these might possibly be watered from the outside, by opening the sashes a little. Special propagating Pits are best built rather low, in order that they may not be too much exposed to cold winds, and that cuttings, when inserted, may be near the light. To this end, the floor for these is also often sunk into the ground; a path passes through the centre, with a door at the end, and a heated plunging-bed is provided on one or both of the sides (see Fig. 192). If the top sashes are fixed, instead of being movable, then a structure of this sort is more correctly termed a house. Although wood is sometimes used, nothing surpasses ordinary bricks for constructing a framework on which to rest the rafters and sashes. Bricks keep out frost better than wood, and are also much more substantial. Pits are sometimes built with hollowed walls--that is, a double set of bricks is arranged so as to leave a hollow space between; the idea being that of retaining heat, which passes more readily through a solid wall than where there is a chamber midway containing air.