A device is disclosed providing an adjustable endcap (100) for use with distribution conduits in a greenhouse. The device provides a solution to seal off the end of a distribution tube while providing further means to regulate the distribution of substances such as air or gas. The device comprises a front plate (102) and a back plate (104). The front plate comprises openings (106) which may be at least partially aligned with openings (108) in the back plate. The volume of air or gas flowing past the end plate is determined by the degree of overlap of the openings, and therefore the total cross sectional area of the overlap. A adjustment groove (110) may be provides to allow direct control of the back plate, an intermediate structure (112) may be provided to control the degree of adjustment of the back plate. The intermediate structure may be a butterfly nut and corresponding bolt. The pressure of a distribution conduit may be determined by the restriction of the end cap, and adjustment of the endcap may allow adjustment of the pressure inside each distribution conduit of a greenhouse individually.
