Diesel fuel powers engines in which pressure rather than a spark causes combustion. It's big in Europe and, thanks to a new generation of quieter, cleaner diesel engines, might see dramatic growth in the United States and elsewhere. So a renewable source of this fuel would have a big potential market. Biodiesel's story is similar to that of ethanol: Today's version is made from vegetable sources like soy and palm oil. But that's a dead end, for a variety of reasons. First, these sources don't generate the yields per acre necessary to scale up and lower costs. Their trajectory, to use a term that's becoming popular in clean-tech investing, is insufficiently steep. Second, there are consistency problems when utilizing fuels from different feedstocks, which results in biodiesel with varying properties, quality, and consistency. One 2007 survey of biodiesel samples found that half of them failed to meet basic standards. Third, palm oil comes from the tropics, so ramping up production means cutting down rain forests that are far more valuable than a marginal bit of auto fuel. But the biggest problem is that soy and palm oil, like corn, are foods, and their diversion to fuel raises prices. Palm oil especially is a staple of Asia's poorest families, and its rising price is already causing real hardship.

But for every problem presented by palm- and soy-derived diesel, oil from jatropha, a hardy, drought-resistant perennial, appears to offer a solution. Jatropha grows on marginal land, so it doesn't compete with food crops. Not only isn't it food but it's inedible and is frequently used as a living fence to keep cattle in and sand out. It can survive for 50 years, and its seeds yield oil equal to 30 percent or more of their weight. Lots of big players are betting on its commercialization: In 2007, British Petroleum and British biodiesel producer Dl Oils formed a joint venture to plant nearly 3 million acres of jatropha in Africa, with the goal of meeting 18 percent of Europe's biodiesel needs by 2011. And California-based SE-Energy Technology is building the largest U.S. biodiesel plant, using primarily Mexican jatropha, in Virginia. In early 2008, millions of acres of jatropha were either growing or being planted around the world.