The bean family pulls its own fertiliser out of thin air
Peas, peanuts, soybeans, chickpeas, liquorice and tamarind all belong to one family of nearly 20,000 species. Many of them house bacteria in root nodules that grab nitrogen gas from the air and turn it into plant food, a partnership that helped make legumes a human staple for millennia.
Fabaceae, also called Leguminosae, is the third-largest land plant family after orchids and daisies, with about 765 genera. Five genera alone, led by Astragalus with more than 3,000 species and Acacia with over 1,000, account for about a quarter of it. No family is more common in the wet and dry tropical forests of Africa and the Americas, and it ranges from giant trees such as Koompassia excelsa to climbing vines and small herbs. The name comes from the Latin faba, bean.
Two features give legumes away: compound leaves with stipules at their base, and the pod, a dry fruit that usually splits along two seams. Pea-type flowers have a distinctive build, with a large upper banner petal, two side wings and two lower petals joined into a boat-shaped keel. Mimosa-type flowers instead gather into fluffy balls where long coloured stamens do the showing off. Leaflets often have hinge-like cushions that let them fold, and some acacias shelter guard ants inside hollow stipules.
Inside the root nodules, rhizobia bacteria convert atmospheric nitrogen into ammonia or nitrate for the host, receiving shelter in return. The family also carries unique chemistry: pterocarpans occur nowhere else, and forisome proteins in its sap-conducting tubes work without ATP. Many seeds contain antinutrients that need processing before eating. Not all legumes are welcome; gorse, broom, black locust and kudzu are pests in parts of the world.
The wider order Fabales traces back 94 to 89 million years. The earliest fossils definitely assignable to the family appear about 65 million years ago, and most modern lineages were present by the middle to late Eocene. Some splits are recent: the tree genus Inga, with roughly 350 species, seems to have arisen within the last 2 million years. Legumes may have first evolved in dry lands along the ancient Tethys seaway, though Africa or the Americas remain possible birthplaces.
Source: Fabaceae
