Propagators are used to encourage the growth of seeds or cuttings. Unheated propagators usually consist of a group of seed trays under a vented lid, which helps to control temperature and humidity. Propagators may also be heated, which allows more precise control. These propagators have a built in thermostat, which can be set to the ideal temperature for the germinating seed or fragile cuttings. Propagators come in a variety of sizes, ranging from small enough for a windowsill garden to large enough for wide scale germination.