Periderm

 Periderm

The stems and roots of woody plants are covered by bark consisting of a functional secondary phloem and rhytidome, a complex tissue Comprised of successive layers of the periderm. The outer bark of woody dicots consists primarily of rhytidome. It is a protective layer that prevents the entry of insects and microorganisms and protects the inner living tissues from temperature extremes. It also inhibits the loss, of water through evaporation but also performs gaseous exchange through specialized regions in the periderm called lenticels.

Periderm develops in response to continued secondary growth. This is because, as secondary tissues are formed from vascular cambium, they exert continuous pressure on the phloem, cortical and epidermal cells are pushed outwards, deformed and finally epidermis gets ruptured. At this stage, the epidermis is replaced a group of secondary protective tissue called the periderm. Periderm consists of three types of tissues.

(a) Phellogen (b) Phellem (c) Phelloderm.

Structure & functions of Periderm

Phellem (cork) and phelloderm (secondary cortex) are both derived from a single-layered secondary lateral meristem called the phellogen (cork cambium)

(a) Cells of phellogen are tubular, radially thin, elongated cells and originate beneath the epidermis, however, in some plants, it may be formed deep within the cortex or even in the outer secondary phloem. The phellogen may become a complete cylinder or it may be initiated in segments that ultimately connect to form a cylinder.

Phellogen initials behave like cambial cells and divide tangentially producing new tissues both towards the inner and outer sides. The cells on the inner side form the secondary cortex and those on the outer side form phellem or cork.

Phellogen initials arise by dedifferentiation (reversal to meristematic state) of mature parenchyma cells followed by periclinal cell divisions to form phellem or phelloderm cell in an alternate manner, one of the cell remains phellogen initial (similar to the division of cambial initials)

The phellogen produces larger quantities of phellem cells than phelloderm. In many plants of temperate regions, the phelloderm may consist of only one to several layers of cells but in some tropical plants, thick layers of phelloderm are produced. Both phellem and phelloderm cells occur in well-defined radial files. (b) Phellem (Cork): Phellem or cork is formed to the exterior of phellogen and is a compact tissue consisting of cells that are similar in shape to phellogen but are non-living at maturity and have no intercellular spaces except in lenticels. Their secondary walls are heavily suberized and thus water impermeable.

Two distinct types of phellem cells are recognized on the basis of morphology and cell wall structure, thick-walled and thin-walled.

A single type may be characteristic of a species or the o types, thick and thin-walled cells may occur in alternate layers. For example n species of Picea, Tsuga Abies, and Betula.

(i) Thick-walled phellem: These types of phellem cells are radially narrower than the thin-walled cells and have a three-layered wall-an outer primary wall that is mostly lignified, a middle secondary wall layer that is heavily suberized and an inner layer sometimes called tertiary wall, impregnated with waxes.

The Phellem cells are not completely impermeable to water due to suberization but the impregnation of the inner tertiary wall with waxes is responsible for the cell becoming fully impermeable and the death of cell protoplast.

The thick-walled phellem cells contain dark-staining resins and tannins.

(ii) Thin-walled phellem: These phellem cells have a much greater radial dimension than thick-walled cells001 and have a thinner secondary wall but lack the inner tertiary!" wall. The phellem cells are used in making bottle corks. At maturity, phellem cells in some species are known to have gas-filled lumina with suberized, wax-impregnated walls, contributing to their impermeability. Some specialized phellem cells lack, suberized walls, they are called phelloid cells and sometimes alternate with layers of suberized cells.

(c) Phelloderm (Secondary cortex): It is formed on the inside of phellogen and, consists of cells with living protoplasts like cortical parenchyma cells. Phelloderm cells in the first formed periderm are photosynthetic and contain. chloroplasts and starch grains. Later, on maturity, some phelloderm cells may differentiate into sclereids

Function of Periderm

The main function of the periderm is the protection of the underlying tissues from drying out, affected by the maintenance of corky and suberized layers. Cork layers, when of considerable thickness, may also afford a certain amount of protection, against mechanical injury to the tissues beneath. In addition to these functions, the periderm layers serve for the protection of various specialized structures or plant parts. Thus, in fruits and tubers, a periderm layer frequently takes the place of a heavily cutinized epidermis as in potato, tropical fruits such as sapodilla (Achras sapota) and Calocarpum mammosum, in which the surface cork layers are well developed and give the fruit a gray-brown, somewhat rough appearance. Cork layers also occur on the dorsal side of bud scales of many woody plants, and as wings and ridges on niany dry fruits. They also occur on leaves of some species of Ficus.

Wound Cork: One of the special functions of the periderm is the protection of wounds by the production of wound cork. Dead tissue is usually cut off from healthy tissue by a suberized layer formed from pre-existing cells that become chemically changed. This change may be followed by the formation of a phellogen layer in the layers of uninjured living parenchymatous tissues adjacent to the wound. This layer forms Phellem and phelloderm in the normal way, thus sealing the wound. Such a layer not only prevents water loss from the wound but also protects the healthy tissues against infection by fungi and bacteria because cork is particularly resistant to the action of microorganisms.

Wound cork may occur in any part of the plant though the readiness with which it is formed varies with the species, the organ or tissues concerned, and the enrivonmental conditions. In general, wound cork forms more readily in woody plants and dicotyledons than in herbaceous plants and monocotyledons. It develops only occasionally in leaves and rarely in sclerenchymatous tissue.

Reference:

• Bsc 2nd year botany 3 dots.
• Shutterstockphotos.