The stems extend underground into structures called stolons. The ends of the stolons may enlarge greatly to form a few to more than 20 tubers, of variable shape and size, usually ranging in weight up to 300 grams (10 ounces) but occasionally to more than 1.5 kg (3.3 pounds). The skin varies in colour from brownish white to deep purple; the starchy flesh normally ranges in colour from white to yellow, but it too may be purple. The tubers bear spirally arranged buds (eyes) in the axils of aborted leaves, of which scars remain. The buds sprout to form clones of the parent plant, allowing growers to vegetatively propagate desired characteristics. Indeed, vegetative reproduction is always used commercially, though the resulting decrease in genetic diversity has made the popular varieties more vulnerable to pests and diseases.