LEGUMES AND NUTS
Legumes are next in importance to cereals as sources of human food. They contain more protein material than any other vegetable product, and so are nearer to animal flesh in food value. Carbohydrates and fats are also present. The proteins occur as small granules in the same cells with the starch grains. The high protein content is correlated with the presence, on the roots of many legumes, of tubercles that, contain nitrogenfixing bacteria. These bacteria are able to utilize free atmospheric nitrogen and convert it into nitrates, thus augmenting the supply of nitrogenous material available for the plants.
The legumes or pulses all belong to the great family Leguminosae, which is characterized by having a special kind of fruit, a legurne, which is a pod that opens along two sutures when the seeds axe ripe. Nearly 11,000 species of legumes are known, and many are of importance as industrial, medicinal, or food plants. They have been cultivated and used for food for centuries all over the world. The seeds (Fig. 168) are of greatest importance; as in the case of the other dry seeds, the low water content and impervious seed coats enhance their value for storage purposes and increase their longevity. Legumes are easily grown, mature rapidly, and are highly nutritious. They are not only rich in proteins but also in minerals and vitamin B. They are an absolute necessity in countries where little meat is eaten. Before the advent of the potato, they constituted a great part of the food of the poorer classes in Europe. Legumes have a high energy content and are particularly well adapted for use in cold weather or where physical exertion is involved. The immature fruits also serve as a food.
Owing to the fact that not only the seeds but all other parts of the plant are rich in protein, legumes are very valuable as field and forage crops. When plowed under they are an excellent fertilizer and greatly increase the nitrogenous content of the soil.
The common pea (Pisum sativum) is a native of Southern Europe and has been cultivated since before the beginning of the Christian era. Peas were well known to the Greeks and Romans. Although an old