210 that when projected in an outward direction normal to the interior member 24 is subtended by the exterior member 26 likewise the exterior ventilation apertures 204 may be characterized as defining an area 212 that when projected in an inward direction normal to the exterior member 26 is subtended by the interior member 24 in embodiments that include the axial offset 208 the interior and exterior ventilation apertures 202 and 204 may be dimensioned so that the projections of areas 210 and 212 onto the exterior and interior members 26 and 24 respectively do not overlap the interior and exterior apertures 202 and 204 may each comprise a continuous slot fig1 or intermittent apertures fig1 the interior and exterior ventilation apertures 202 and 204 may be formed by removing material along a line or lines that cut into the internal and external walls 24 and 26 respectively for the co extruded structure 46 that comprises members defining waved shapes 50 e g fig2 the removal of material may be to a depth 206 that is less than the amplitude of the waved shapes 50 fig1 to form apertures that are non continuous the material removal operation may be performed numerous ways including with a sharp edge or with a laser in operation the solar radiation load imposed on the spectrally selective grow tube 20 b may cause the air within the intermittent gap 48 to be at an elevated temperature relative to the ambient air 214 and or the interior air 216 within the interior member 24 buoyancy forces may then cause a u201c chimney effect u201d on the air within the.