In typical woody plants, the first outer bark is seen by the observer late during the first year or in year two, but, of course, the cell divisions to produce that first outer bark commonly start earlier, and for viewing the early stages you would need a microscope. Outer bark often begins to form long before a stem or root experiences damage to surface cells, but after internodal elongation has ceased and as the adjacent leaves become aged. Stems of tomato, garden geranium, and green beans have periderm formation in their green, relatively young lower stems. There are even some species in which periderm forms on roots of seedlings within a week after germination! Likewise, inner bark, i.e., secondary phloem, is already being produced in relatively young stems, even long before outer bark is initiated. Nevertheless, bark is most frequently regarded as the protective tissue of old stems and roots, and as characteristic of shrubs, trees, and perennial climbers.
The first phellogen of stems (tissue called the initial periderm) most commonly forms in the outermost layer of cortex. A cell wall forms parallel to the surface of the organ (i.e., periclinal), initially in a group of cells and then expanding to neighboring cells until the sheet of cells is continuous around the stem. Each cell (initial) then divides to produce derivative cells to the outside and inside. To the outside, cork cells are usually formed. The epidermis remains intact upon the cork cells until they die, while files of cork cells accumulate beneath the epidermis. For a small percentage of plants, the initial periderm of stem arises in the epidermis. These two are sometimes called the superficial periderm.
Not every species forms a continuous initial periderm around the stem circumference. In some lineages the initial periderm instead forms as longitudinal strips, alternating with green strips of original stem tissues. Some of these may therefore continue stem photosynthesis in the green portions that have no periderm.
For some species, the initial periderm arises deep in the cortex or parenchyma cells of the phloem, for which the term superficial would not be appropriate.
This initial periderm normally functions for a short time, but in apple (Pyrus), for example, the initial periderm can persist for twenty years! The first phellogen can persist if either the stem is not increasing in diameter or the initials of the phellogen are multiplying to match the increase in axis circumference (i.e., anticlinal divisions, perpendicular to the surface; occurs in oak, fir, beech, and hornbeam).
The initial periderm of a root arises deep within the axis, not near the surface. Pericycle, which is located just inside the root cortex, is the place where the first phellogen of roots arises.