Abstract: A modular construction system for forming a greenhouse structure includes a plurality of glazing panels and a plurality of support members for connecting and supporting the glazing panels. In the preferred embodiment the glazing panels and support members are connected utilizing interlock brackets connected to and extending outwardly from the edges of the glazing panels, to form a positive interlock between glazing panels and support members. A bypass sliding ventilation window assembly is provided, and a bypass sliding door assembly, or alternatively a hinged door assembly, is also provided. At least one glazing panel or sliding ventilation window panel preferably includes a self-sealing pass-through aperture for an electric cord or the like.