The common bean, Phaseolus vulgaris, indigenous to the Americas, is an herbaceous annual plant domesticated independently in ancient Mesoamerica and the Andes, and now grown worldwide for its edible bean, popular both dry and as a green bean. The leaf is occasionally used as a leaf vegetable, and the straw is used for fodder. The common bean is a dicot.
The common bean is a highly variable species. Bush varieties form erect bushes 20-60 cm tall, while pole or running varieties form vines 2-3 m long. All varieties bear alternate, green or purple leaves, divided into three oval, smooth-edged leaflets, each 6-15 cm long and 3-11 cm wide. The white, pink, or purple flowers are about 1 cm long, and give way to pods 8-20 cm long, 1-1.5 cm wide, green, yellow, black or purple in color, each containing 4-6 beans. The beans are smooth, plump, kidney-shaped, upto 1.5 cm long, range widely in color, and are often mottled in two or more colors.
As the common bean is a dicot, it germinates as such:
* The primary root emerges through the seed coats while the seed is still buried in the soil.
* The hypocotyl emerges from the seed coats and pushes its way up through the soil. It is bent in a hairpin shape — the hypocotyl arch — as it grows up. The two cotyledons protect the epicotyl structures — the plumule — from mechanical damage.
* Once the hypocotyl arch emerges from the soil, it straightens out. This response is triggered by light (phototropism). Both red light, absorbed by phytochrome and blue light, absorbed by cryptochrome can do the job.
* The cotyledons spread apart, exposing the
* epicotyl, consisting of two primary leaves and the apical meristem.
* In many dicots, the cotyledons not only supply their food stores to the developing plant but also turn green and make more food by photosynthesis until they drop off.
Toxicity
Before they are eaten, the raw bean seeds should be boiled for at least ten minutes to degrade a toxic compound - the lectin phytohaemagglutinin - found in the bean which would otherwise cause severe gastric upset. This compound is present in many varieties (and in some other species of bean), but is especially concentrated in red kidney beans. Although in the case of dry beans the ten minutes required to degrade the toxin is much shorter than the hours required to fully cook the beans themselves, outbreaks of poisoning have been associated with the use of slow cookers whose low cooking temperatures may be unable to degrade the toxin.
Green beans
Green common beans are also called string beans, stringless beans (depending on whether the pod has a tough, fibrous "string" running along its length), or snap beans. Compared to the dry beans, they provide less starch and protein, and more vitamin A, vitamin C, and calcium.
The green beans are often steamed, stir-fried, or baked in casseroles.