Unlikely as it may seem, this "vertical garden" on a highway overpass in Aix en Provence, France, is the work of a serious scientist. Patrick Blanc is a biologist at the prestigious, government-funded Centre National de Recherche Scientifique, and regards his walls as something of a sideline -- even though they now adorn buildings around the world from Madrid to Bangkok. Blanc's plantings specialize in striking juxtapositions of texture and color: bold tropical elephant ears jammed next to dense clusters of hostas of every conceivable shade; bright spikes of creeping bentgrass peeping out from a drapery of ferns. People may say that it's overused these days, but there are times when "green" is the only word that will do.