From Seed to Seed:
Plant Science for K-8 Educators

Legumes. Humans consume both the fruits and seeds of legumes in their fresh state and also eat the seeds fresh or dried. If you eat a green bean, you are eating both the fruit (the pod) and the seed (inside the pod). Compared to cereals, legumes have a much higher protein content, often containing at least 25 percent protein.

One species of legume, Phaseolus vulgaris, accounts for many of the beans familiar to cooks and gardeners. Different types of P. vulgaris include: kidney, field, garden, haricot, pinto, great northern, and yellow-eye beans. And don't forget the green, string, snap, and wax beans that can be nibbled right in the garden. These are also types of P. vulgaris that are harvested and eaten while the fruit is still in its immature stage.

Other types of legumes (species other than P. vulgaris) are lima, scarlet runner, shell, black, and kidney bean. Pea (Pisum sativum) is also a legume, as are chick pea, cowpea, black-eyed pea, lentil, and soybean. Peanuts, of course, are a very popular legume, rich in proteins and oils (30 percent proteins, 48 percent oils, 12 percent carbohydrate).

Like the cereals, legumes are easily handled, dried, and stored. However, an even more important feature is the leguminous plants' ability to fix atmospheric nitrogen. As we have often mentioned before, many types of legumes have a certain type of soil-borne bacteria living in nodules in their roots. These bacteria are able to take in atmospheric nitrogen and convert it into a form that is useful to the plant. (Remember, nitrogen is a critical component of amino acids, and therefore proteins.) As a result, the edible parts of legumes contain significant amounts of protein. Also, legumes do not require expensive nitrogen fertilization. Furthermore, when crop residues are tilled in they add nitrogen to the soil, making it available for future crops.