Joni Mitchell, Janis Joplin, David Bowie or Jane Birkin are all points of reference for many designers this season. In other words, the seventies are back this summer. With the spirit of the '70s in the ascendent, expect Bohemian designs: long, print-patterned dresses, romantic brocaded and embroidered whites, rock-star metallic golds and denim everywhere. Turning the clock back ...