The reusable Black End Plugs are the third part of the Knox Cellars  'System' tube assembly. Systems include 1) water resistant Cardboard Tube (reusable), 2 ) White Paper Liner and 3) reusable Black End Plugs that all together make it so easy to harvest cocoons. Plugs are sold as bags of 50 in case some get lost or you would like to assemble the Systems or they can be used to cap one side of the 48 or 96 Wood Nesting Blocks