These excellent quality 4 and 7 inch Ultra Clear Humidity Domes now have built in light tracks that accommodate the 18 inch Super Sprouter? T5 HO Grow Light Fixture. The light tracks are directional to allow the light to run the length of the domes or the width of the domes, allowing two trays to run side by side. Vertical sides allow the use of all plant sites in a propagation tray. Two vents in the top of the dome can be opened or closed to control humidity. These are the most clear and transparent domes on the market, allowing for maximum light penetration and the best viewing of your plant starts.