Koons's Made in Heaven is perhaps the most thoroughgoing example here of the marriage of art and mass culture. This set of sculptures, photographs and paintings of Koons and his porn-star ex-wife, La Cicciolina, in various states of copulation have the cheap glamour of Hollywood film posters and the high-finish production qualities of upmarket gewgaws and knick-knacks. Their closest relative is graphic art: they share both that genre's instant impact and its lack of texture, its essential soullessness.  

The artist as celebrity, the publicity value of shock and a detailed knowledge of the market are the most obvious lessons learned and mastered by Hirst and his peers. There is, however, one item that isn't included in the show that sums up how confused the conflation of art, mass culture and commerce has become. In 2002, Tracey Emin's cat went missing and the distressed artist put up posters near her Spitalfields home asking for help in finding the lost pet. The fliers themselves immediately went walkabout, removed by astute individuals who promptly sold them on. This reductio ad absurdum was made all the more absurd by poor Tracey, a woman who insists that everything she touches is art, wailing that the posters were not "a conceptual piece of work" and they had "nothing to do with her art". What a tangled web it is that Andy Warhol wove.

A less pernicious side to Pop Art is on display at the Hayward Gallery with its retrospective of the work of Ed Ruscha — the first major survey here of one of the movement's founding fathers. Although Ruscha has a reputation as something of a safe artist, his cinemascopic images of suburbia and petrol stations, landscapes and floating words, created over the course of half a century, nevertheless amount to a highly distinctive record of American life. 

An early interest in cartooning and a spell working at an advertising agency mean that his work is marked by its clarity but he also believes that, "Art has to be something that makes you scratch your head." There is indeed a deeply enigmatic, and very un-Pop Art, strain to his work. 

The emptiness of his American scenes recalls Edward Hopper's versions of alienation, while his word pictures — "Annie", "Oof" or "It's only vanishing cream" — are visualisations of sounds that seem ripped from billboards with their meaning lost in the act of removal. What fascinates Ruscha are "things we have looked at but not examined". 

Ruscha is also notable in that he has never lost either the urge to experiment or his feeling for beauty: ideas that separate him from the more knowing and meretricious artists at Tate Modern.