vein

A *vascular bundle , or a group of closely associated bundles, in a leaf. In a leaf vein, the xylem is almost invariably positioned adaxially and the phloem abaxially, although there is sometimes an additional layer of adaxial phloem. Veins are sometimes surrounded by bundle sheaths of collenchyma, sclerenchyma, or parenchyma, which may extend to the leaf epidermis. The pattern formed by the veins in a leaf is called the *venation.