Lecture 14

Periderm

I.  Introduction

A. Periderm = secondary tissue derived from the cork cambium (phellogen) and the cork cambium itself; replaces the epidermis in woody stems and roots (Liquidambar twig). It is usually a continuous ring (not always), all the way around the stem, root, or fruit. Can identify periderm by radial files of cells.

B.  Can also be found in some leaves, winter bud scales, herbaceous dicots, some monocots.

C.  Produced in parts that abscise or upon wounding

D.  Bark is not the same thing as periderm. Bark is all the tissues outside the vascular cambium, i.e. secondary phloem, any primary tissues that are still there, periderm, and dead tissue outside the periderm. Composed of an inner living part and an outer dead part. Cut trunk of Robinia.

II. Structure of Periderm

A. Components
1. Phellogen = cork cambium. Meristem that produces the periderm. Relatively simple, rectangular, radially flattened cells.  Produce derivatives in radial files. Figure 12.1A of Ipomoea (sweet potato). Pelargonium stem, with interpretation by Mauseth.

2. Phellem = cork layer. Tissue that differentiates to the outside of cork cambium. Either regular or irregular in shape (Figure 12.2) and cactus (Leptocereus) phellem. Lack intercellular spaces. Nonliving but may have solid or liquid contents.  In these photos, tannins were deposited into the phellem cell vacuoles of Quercus and Clusia.
a. Walls are suberized.  Suberin and wax forms a lamellar layer on top of primary wall (that may or may not be lignified). 
b. Walls of Quercus suber (cork oak) phellem are thin, cells filled with air, thus lightweight and insulating.  Form a barrier that is impervious to water and resistent to oil. 
c. May also have phelloid cells – non-suberized, may become sclereids (Rhododendron, Figure 12.2D).  Layering of cork and sclerified cells occurs in seasonal increments. Similar phenomenon in this cactus (Oreocereus celsianus) root.

3. Phelloderm = living parenchyma that differentiates to the inside of the cork cambium. Usually 1-4 cell layers thick, if present at all. First produced cork cambium often makes phelloderm.  It is important if stems remain green as it may be photosynthetic.

B.  Polyderm.  In roots and rhizomes of families such as Hypericaceae, Myrtaceae, Onagraceae, and Rosaceae.  Layers of suberized cells one cell layer deep alternating with non-suberized cells several layers deep (Figure 12.3 in Fragaria, strawberry).  Only the outermost layer is dead.  The non-suberized cells are used for storage.

C.  Rhytidome
1.  The outermost bark. Layers of tissue isolated by the periderm and layers of the non growing periderm. Diagram in Figure 12.4.
2. When periderm arises in succcessively deeper tissue, accumulation of old periderms and alternate tissues (cortex, 1˚ phloem, 2˚ phloem).  All of these tissues are no longer functional – just a barrier.
3.  Shrubs exfoliate older bark, then accumulate rhytidome.
4.  In conifers, two kinds of periderm that make rhytidome. Initial cells are brown (exophylactic) and later reddish purple (necrophylactic); these cells act to protect living cells from affects associated with dead cells.

III.  Development of Periderm

A. Where does it form? Mostly in the outside cortex of a stem. Often directly under the epidermis but sometimes a bit deeper. The phellogen can derive from various different living, potentially meristematic cells, e.g. epidermis, subepidermal parenchyma or collenchyma, pericycle (roots), phloem and phloem rays.  In cacti, such as this Acanthocereus, cork cambium only arises from the epidermis, not from deeper layers of the cortex or secondary phloem.

B. The first cork cambium often forms at the same time or after the vascular cambium (first year). Later (sequent) periderms are initiated deeper, either the same year or many years later, if they appear at all. Affected by environmental conditions.

C.  Example of shallow periderm initiation. Pelargonium (geranium).  Subepidermal cells undergo periclinal divisions, producing phellogen and phelloderm inside of it (Figure 12.5). In ivy (Hedera helix), the cork cambium forms just below the epidermis.

D. Example of deeper periderm initiation. Berberis, Ribes, Vitis (Figure 12.6). Periderm arises deep in primary phloem. As it grows, it causes the death and collapse of the cortex cells.

E.  Roots: periderm arises in the pericycle or near the surface [we will cover this later]

F. Sequential periderms
1. Forms each year and in successively deeper layers beneath the first (burried by their own meristematic activity). 
2.  Originate from parenchyma cells of secondary phloem and ray cells, which on the outside is no longer functional. So, for many trees, secondary phloem band remains the same thickness.  In Pyrus, the sequent periderm has enclosed some of the secondary phloem.
3.  The first phellogen is uniform around the circumference of the stem – or – it may form in isolated areas that later become contiguous by lateral spread and meristematic activity. Most cell divisions are periclinal but anticlinal divisions are involved in the phellogen keeping pace with the stem’s increase in circumference.
4.  Sequent periderms appear as discontinuous but overlapping layers (Figure 12.4, 12.7B for Quercus).  They may also appear continuous around the circumference (Figure 127C for Lonicera tatarica).

G.  Monocots (palms and others) have storied cork.  This is where parenchyma cells in successively deeper layers divide periclinally – products are suberized.  Appears storied in cross section (Figure 12.9 of Cordyline, Laxmanniaceae).

H. Wound periderm.  Formed as a cytological response to damage; closes the wound.
1. First the cut surface is sealed by scar (cicatrice) tissue. Tissue includes dead (necrosed) cells on surface and living, suberized and lignified cells below called the closing layer. (Figure 12.1B of Ipomoea, sweet potato). 
2. Wound phellogen arises beneath the closing layer. With phellogen cell divisions, cork is produced and this pushes the dead scar tissue outward.
3. Monocots and dicots differ in their wound resonses. When the cork is removed from cork oak (Quercus suber), living cells below the periderm respond by producing new periderm.

IV. Outer Aspect of Bark

A.  Periderm determines how the outer bark appears.  Smooth bark has a thin periderm with little cork as compared with thick, furrowed bark. Some examples of campus plants with characteristic bark:
 B.  Wings. Some woody plants produce periderm in localized areas on the branches. Euonymus alatus has periderm activity only in localized areas.  Others such as Ulmus alata has symmeterical longitudinal splitting of cork followed by uneven expansion.

C.  Rhytidome can be of two types:
1. scale bark =  curved (shell-shaped) sequent periderms develop in localized overlapping strata.  Bark peels as scales. Examples: Quercus alba, Cephalanthus (Figure 12.8), Pinus, Pyrus, Tilia.
2.  ring bark = concentric rings of rhytidome, formed deeper and deeper. Examples: Vitis, Clematis, Lonicera
3.  If fibers are enclosed in the rhytidome, gives a characteristic aspect (e.g. Tilia, Fraxinus, Sambucus).  If fibers are absent, bark breaks away in scales or shells (Pinus, Acer). Fibers or phelloids contrasting with phelloderm cells cause exfoliation along the weak points (Figure 12.2D Rhododendron).
4. Paper on rhytidome formation in Crytomeria japonica.

V.  Lenticels

A. What are they?  Openings in periderm of stems and roots, and because of intercellular spaces (in contrast to phellem), they are used for gas exchange. Lenticel in ivy (Hedera helix).

B. Appear as horizontally or vertically elongated mass of cells, dots or lines, of various sizes, either scattered or in rows. Lenticels are evident on younger stems and are usually found in fissures thicker, older bark.  Examples:
 C.  How they form (Figure 12.10 for Persea A, B and Fagus C). First appear under stomata where phellogen is more active. Phellogen is bent downward, and in this “cup” is produced loose cells called filling (complementary) tissue.  The outer layers of the lenticel are pushed and the whole structure ruptures the surface.

D. Two types of lenticels
1.  With loose, filling tissue but no closing layers (Liriodendron, Magnolia, Malus, Persea, Populus, Pyrus, Salix). Esau (1977) recognizes variations on this type where the plants that have compacted filling tissue at the end of the season (Aristolochia, Hedera, Sambucus, Fraxinus, Quercus,Tilia).
2. With closing layers = layers of more compact, thinner walled cells that hold complementary cells in place. (Figure 12.10C, Betula, Fagus, Prunus, Robinia).
Last updated: 14-Oct-22 / dln