The EPA proposed a new standard on Friday for how much biofuel must be mixed into the nation's gasoline. The portion of vehicle fuel that comes from plants has increased dramatically over recent years to about 10 percent. But most of it comes from corn. Congress hoped that, by now, a billion gallons would be coming from advanced biofuels, which have much smaller greenhouse gas footprints. That hasn't happened. But the nascent cellulosic fuel industry says don't count it out. Several plants are on the verge of opening and more will be on the way.