Periderm is the corky outer layer of a plant stem formed in secondary thickening or as a response to injury or infection. It is a cylindrical tissue that covers the surfaces of stems and roots of perennial plants during early secondary growth; therefore it is not found in monocots and is confined to those gymnosperms and eudicots that show secondary growth.
In this article, Periderm, it’s structure and development will be discussed briefly.
Due to continued formation of secondary tissues, in the older stem and roots, however, the epidermis gets stretched and ultimately tends to rupture and followed by the death of
epidermal cells and outer tissues and a new protective layer is developed called periderm.
Structure of periderm
Periderm consists of three parts:
- A meristem known as phellogen or cork cambium.
- The layer of cells cut off by phellogen on the outer side constitutes phellem or cork cell.
- The layer of cells cut off by the phellogen towards the inner side constitutes phelloderm.
These three parts are described as follows:
Phellogen is a type of lateral meristem. It contrast to the vascular cambium, the phellogen is relatively simple in structure.
- Phellogen arises in epidermis, hypodermis, cortex and phloem tissue.
- Phellogen is composed of one type of cells. Cells are rectangular in cross section and radially flattened.
- Protoplasm contain tannins, chloroplast.
- Intercellular spaces are lacking except in lenticels.
- Due to the activity of phellogen plant axis increase in thickness.
- Phellogen divide by periclinal division and produced phellem or cork cells on
the outerside and phelloderm on the inner side.
Place of origin of phellogen
- In most stems the first phellogen arises in the subepidermal layer. In a few plants the phellogen arises in the epidermal cells (Nerium, Pyrus). Sometimes only a part of the phellogen is developed from epidermis while the other part arise in subepidermal cells (Pyrus). In some stem, the second and third cortical layer initiates the development of periderm (Robinia, Aristolochia). In still other plants the phellogen arises near the vascular region or directly in the phloem (Punica, Vitis).
- At the time of beginning of the development of a phellogen in epidermal cells, the protoplast lose central vacuoles and the cytoplasm increases in amount and become more richly granular. As soon as this initial layer develops, it divides tangentially and to a lesser extent radially, in the similar way as division takes place in cambium.
- Generally several to many times as many cells are cut off towards the outside (phellem) as towards the inside (phelloderm). Phelloderm cells are few or absent. Rarely phelloderm is greater in amount than phellem.
Origin and development of Phellogen Diagrams are given below:
2. Phellem (Cork cells)
The cells that constitute phellem are commonly known as cork cells. They are like the phellogen from which they are derived.
- Cells are dead at mature stage.
- There are no intercellular spaces in cork cells.
- Suberin is present in cell wall. In some cells suberin are absent called phelloid cells.
- Wall of the cork cell may be colorless or colored substance is present in the cell lumen.
- As seen in tangential section, cork cells are polygonal and uniform in shape, often radially thin as seen in cross section of stem.
- The cells of commercial cork (Quercus suber) are radially elongated as seen in transverse section.
- In the periderm of Betula and Prunus, the cork cells are elongated tangentially as seen in cross section.
The phellogen cuts off the phelloderm cells towards inner side which are living cells with
- The cells are loosely arranged and pitted.
- In most plants, they resemble cortical cells in wall structure and contents.
- Their shape is similar to that phellogen cells.
- They may be distinguished from cortical cells by their arrangement in radial series resulting from their origin from the tangentially dividing phellogen.
- In some species, they act as photosynthetic tissue and aid in starch storage.
- They are pitted like other parenchyma cells.
- Occasionally the sclereids and other such specialized cells occur in phelloderm.
- The term secondary cortex is sometimes applied to phelloderm which does not seen to be appropriate.
The development of the periderm layers in the cork Oaks (Quercus suber) is of special interest. The ability of the plant to produce phellogen in deeper layers, when the superficial periderm is removed, is utilized in the production of commercial cork (Q. Suber). At the age of about twenty years, when the tree is about 40 cm in circumference, this outer layer, known as virgin cork is removed by stripping to the phellogen.
1. Class Lecture of Parveen Rashid, PhD
Professor, Department of Botany, University of Dhaka.