FRAGILE STATE.

A horticultural greenhouse in the grounds of Prague Castle refers to historical precedent and is a shining addition to the city's buildings.

Prague Castle has had an orangery since the middle of the fifteenth century. Built on the south side of the royal gardens, at the edge of the moat dividing them from the citadel, it was protected from the extremes of climate by a stone wall. Orange trees, insulated with straw and wooden beams, managed to survive the winters (when temperatures can sink to -20 degrees Celsius) and their nurture continued until the First World War. Then, deprived of care, the trees died and the Orangery fell into disrepair.

After the Second World War, there was renewed interest in the idea of a horticultural greenhouse. A simple structure with a glass roof was built to house plants and trees but this slowly deteriorated and by 1996 this too was derelict.

Developed with Matthew Wells of Techniker, Jiricna's winning design was of a delicate cylindrical vault of glass and stainless steel. In its poetic expression of engineering virtuosity, the building is a clear relation of the series of magical glass and steel staircases for which the practice is famous. As a response to the historic site and context, to the requirements of modern hothouses and to climate, it is both sophisticated and thoughtful. Undoubtedly it will help to change perceptions of architecture in the Czech Republic.

The new Orangery by Eva Jiricna Architects is a consequence of having an asthmatic and dramatist in power. After he became President of the Czech Republic in 1989, Vaclav Havel was in the habit of retreating to the greenhouse, elevated above the polluted city, for relief from his asthma and to write plays. He conceived the idea of an international competition to build a new and permanent glasshouse on the same site, which would at the same time forge a historical link and be a shining example of twentieth-century architecture.

Initially, the business of constructing so refined a building presented apparently insuperable problems until a Czech subsidiary of Seele, the German company, agreed to manufacture the components and act as contractor.

Some alteration to the site proved necessary. The sixteenth-century wall at the back was retained, but the front wall which had no foundations and was of relatively recent construction, was demolished. Jiricna's structure, conforming to the plan of the original building, could not spring directly from the rear wall for this too had no foundations, but from a tetrahedral space-frame girder running above it along its length. Angled legs prevent the arch from flattening and the structure is further braced by four cross frames which also accommodate cross walls. Sliding joints capable of 30mm movement accommodate extreme changes of temperature.

The cylindrical form of the toughened laminated glass shell gives an efficient volume-to-surface ratio and cuts down heat loss. It is suspended from an exoskeletal structure, a lightweight diagonal steel mesh of stainless-steel tubes, 48mm in diameter. For strength and stability the tubes are short (the site's difficult access anyway inhibited use of large building components), and the mesh made up of welded crosses connected by mechanical fixed nodes with four solid ends. Tubes are clamped top and bottom to the node ends by plates designed to be held by a single countersunk screw. Glass panels hung beneath the diagonal grid are point fixed by spider castings and silicon sealed.

The vault, enclosed at either end by glazed shutter doors, is stiffened internally by the glazed cross walls dividing three climatic zones: one for decorative plants and trees, one for young plants and one for seedlings.

Essential heating is concealed in the floor slab, under stone paving, in the brick retaining wall, and behind roller blinds which, silvered on the outside, provide shade in summer and insulation in winter. Pipes suspended from the structure spray the plants with a fine vapour. Horizontal louvres at the base of front and back walls, and butterfly vents within glass panels at the apex, provide cross ventilation.

Jiricna's empathetic response to her clients' wishes, her concern for the interior of buildings and the way they function, is extraordinary. A concomitant of this are the demands she makes on structure, insisting on extracting the most from the most minimal. Such exacting habits have resulted in a collection of truly original structures, to which the Orangery is a remarkable addition. Here, structural efficiency has produced clear volumes and expanses of delicate filigreed shell without sacrificing functional efficiency. In its arboreal surroundings, the building is a knock-out; and visitors wandering along the new path from Klarov to Powder bridge can experience the delight of seeing how this modern building sits so lightly on its historic site.