Have you ever pulled a bean plant out of the ground and examined its roots? If so, you may be familiar with one of the defining characteristics of the Fabaceae, and are privy to one of the secrets of life on Earth. Nearly all species in this clan, commonly known as the legume family, have the ingenious ability to create their own nitrogen fertilizer through a symbiotic relationship with bacteria that live on their roots. These bacteria, called rhizobia, cause the tiny whitish-pink nodules that you find attached to the roots of a legume when you pull it out of the ground and are largely responsible for making nitrogen available to the entire plant kingdom — otherwise it would only exist in the air, which plants can't use.
Nitrogen is the nutrient responsible for lush green growth in all plants. Because legumes can take nitrogen from the air and convert it into a soluble from the plants can use, they are often found growing in infertile, nitrogen-poor soils — places like road cuts, eroded hillsides and anywhere that the topsoil has been removed. Thus the Fabaceae are considered a 'pioneer' plant family. They go into places where few other plants would survive, 'fixing' nitrogen (that's the technical term) and making enough to share with other plants in the process. As the soil improves, other plant families move in and the fast-growing, but short-lived legumes move on.
All peas and beans are in the Fabaceae family along with a host of other plants that you would never imagine were related (though their elongated seed pods should give a clue). Besides the legumes we like to eat, there are many others that livestock enjoy, such as clover, alfalfa and vetch. There are also stunning ornamentals, ranging from the tropical bauhinias to the cool-climate lupines. There are common trees like mimosa, locust, mesquite and acacia, along with kudzu, a weedy vine in the Southeast known for completely covering and killing mature trees. And there are also more obscure and exotic leguminous cousins, like indigo (source of the dye used for blue jeans), licorice (used to flavor candy) and carob, a healthy chocolate substitute.