Mayfair has gained two new pieces of public art. One is of the highest quality; the other is glorified window-dressing. First, the quality. Savile Row, London's buttoned-up bastion of tailored existence, has been given a faintly surreal vibe by Verge, a gangling two-ton bronze sculpture created by the American artist, Joel Shapiro. The piece floats a few feet in front of the façade of No 23, a new building designed by the architect Eric Parry. Strung up on tensioned wires, the sculpture suggests a floating body whose languid limbs would surely represent a literally unsuitable client for the cutters and sewers at, say, Dege & Skinner, or Huntsman and Sons.

The sheer otherness of this combination of art and architecture was supported by the developers D2 Private, after their initial nervousness about the drama of the double-act (not shared, incidentally, by planners at Westminster City Council). The fusion demonstrates how two strikingly different kinds of form can bring a strangely gripping atmosphere to an urban space, 50ft above the street, that might otherwise be dead air.

Parry, a Royal Academician, has always been profoundly interested in the relationship between art and architecture and the way that, together, they can "charge" space. His Portland stone and Indian granite façade, an elegantly stitched waistcoat on the foppish 18th-century body of Savile Row, has been beautifully ruffled by Shapiro's space invader.

Two minutes' walk away, in New Bond Street, the rather trendy architectural practice DSDHA has blotted its copybook with an installation, Silver Forest 36, that allegedly transforms the Hermès flagship store into a forest. "In the 18th century," intones DSDHA director David Hills, "this location stood at the border between the city and what was once originally a forest." To "celebrate" this, 36 cast silver birch trees have been positioned on the pavement, in the window displays, and in the store itself. The visual concept "captures the imaginative spirit of Hermès, whilst paying homage to the unique qualities of the site".

Every site is unique, and most of Britain was once covered with forests. What is David Hills, an otherwise thoughtful architect, doing talking such nonsense? And why is he allowing his practice to be branded with statements about "this urban forest... shimmering like silver, evoking both the historical setting and, through their artisanal nature, the origins of Hermès, so that amid trees in winter, Mayfair is transformed".

It is not transformed. It is marketed. Which may well be something that occurred to Rebecca Cocks, creative director for Hermès's windows, who was involved in the design of DSDHA's installation. The only relationship between art, architecture and the artisanal at Hermès involves the sculptural screen by Henry Moore on top of the Time-Life building at the corner of New Bond Street and Bruton Street.

DSDHA has produced interesting architecture, and ideas; but not here. What have three dozen faux silver birch trees in New Bond Street got to do with anything other than selling undoubtedly superb leather goods and clothing? There is nothing quite like a design concept with unique qualities, is there? Particularly if those qualities will be available, after 6 January, to any private buyer who wants 36 silvery trees that have served their commercial purpose.