Early periderm development in the shoots of Ulmus alata michx.
Periderm is a protective tissue that replaces the epidermis in woody plants and consists of three layers: phellogen, phellem, and phelloderm with cells in discrete radial files. Lenticels and cork wings deviate from this arrangement. Stems of Ulmus alata were examined using scanning and transmission electron microscopy to determine the site of phellogen initiation, lenticel initiation, cork wing initiation and their development. Phellogen is initiated in the subepidermal parenchyma which is consistent with other members of Ulmus. Phellogen initiation does not appear to be associated with structures of the epidermis or underlying stem. Lenticels are initiated by irregular divisions of the lenticel phellogen and the radial expansion and lateral separation of the cells produced. Mature lenticels have alternating layers of complementary cells and closing layers. Cork wings are initiated by the radial expansion of phellem followed by increased divisions of cork wing phellogen compared to the rest of the periderm.