Artists ranging from Constable to Pollock have stressed the part played in their work by nature, by the chance slurring or dripping of paint, by chemical reactions. Now, in our age of environmental concern, artists are trying to invent ways of working in some kind of collaborative accord with natural forces. Stephen Turner pushes this inclination further out than most by working with woodland muck or letting the trees, as it were, paint theirvery own self-portrait. Cherry tree leaves and berries are simmered for six hours to achieve the precise quality of what Turner calls "a golden brownessence". Canvases are spread beneath trees to receive the various stainingof the changing weather.