a structural arrangement is provided which is particularly useful in connection with solar greenhouses . the structural member is suitable for use in a parallel arrangement of the same particularly for the purpose of providing controlled shading . the structural member is provided as a hollow bar of elongated form provided with at least one longitudinally extending track channel having a relatively narrow longitudinally extended slot - mouth . the bar may be provided with two such channels and these channels are substantially circular in cross - section with the mouths defining reentrant angles therewith . in a solar greenhouse , members of the above type support a glazing with a curved - eave section , the bars being curved to shape and the channels therein accommodating the bulbous lateral edges of shades which are retained therein . the shades are transferred from one roller in the direction of a second roller . the rollers may be provided with internal motors which rotate the same although the arrangement disclosed is susceptible of being operated manually as well and additionally in conjuction with one or more spring loaded rollers . the arragement is also suitable for selectively extending shields against wind and other such elements including rain and snow by the raising up of walls constituted by transparent sheets of plastic .