"The dead-horse arum of Corsica looks and smells like the south end of a horse that died going north," says Roger Seymour. He's actually talking about a plant, and a more prosaic soul might add that it belongs to the same family as calla lilies and jack-in-the-pulpits. Seymour is a zoologist, and the plants he studies show an animalistic feature: They can generate body heat. Most plants, including calla lilies and jack-in-the-pulpits, simply assume the ambient temperature because their metabolic reactions hum along so gently that they don't give off bursts of heat. The dead-horse arum, however, belongs to the group of several-hundred plant species scattered among some 10 families that can rev up their own furnaces. That heat can launch strong odors, like those of a dumpster in August. In winter, warm flowers can melt snow.