The type of banana you see in the supermarket is called a Cavendish banana. The preferred variety was originally the Gros Michel, which essentially became extinct by 1960, thanks to a fungus called Panama disease.
To whiten teeth naturally, rub the inside of a banana peel on your teeth for about two minutes every night. If you gargle with salt water, this will heighten the effect. Expect results in about two weeks. It works because of the effect of the potassium, magnesium, and manganese in the banana peel.