JUNGLE FORMULA: Construction of what is said to be the world's largest greenhouse is due to start this spring in a 15ha former china clay pit at St Austell, Cornwall. A contracting joint venture of the two McAlpines, Sir Robert and Alfred, will erect a series of interlinked 60m high 'biomes' to house fully mature rain forest plants. The 9m diameter biomes have a lightweight steel frame roof supporting clear foil instead of glass. When complete in mid-2000, the £74M Eden project, 50% funded by a Millennium Commission grant, will offer the public a 10,000 plant conservatory and provide extensive research facilities. Architect is Nicholas Grimshaw & Partners with engineering by Ove Arup & Partners and Anthony Hunt Associates.