A mycelium growth bed for optimal production of pure mycelium or a pure mycelium composite with controlled or predictable properties, the bed comprising a tray, a conveying platform, a permeable membrane, a substrate, a porous material and a lid. The permeable membrane is positioned on the conveying platform within the tray. The substrate is positioned on the permeable membrane and the porous material is positioned on top of the substrate. The system provides a configuration having a substrate weight to surrounding space volume ratio between 0.5 and 5.0 g/cc, an air volume (surrounding space) to substrate volume between 0.01 and 1.0, and an air volume (surrounding space) to substrate area is between 0.5 and 5 cc/cm, wherein the CO2 concentration is held above 3%, the relative humidity is held above 40% and the O2 concentration is held below 20% in steady state conditions to produce leather-like mycelium without fruiting bodies.