Compared with fields like genetics and neuroscience and cosmology, botany comes up a bit short in the charisma department. But when scientists announced last week that they had figured out how plants grow, one had to take note, not only because of the cleverness required to crack a puzzle that dates to 1885, but because of what it says about controversy and certainty in science -- and about the evolution debate. In 1885, scientists discovered a plant-growth hormone and called it auxin. Ever since, its mechanism of action had been a black box, with scientists divided into warring camps about...