Question

Which of the following options correctly shor sequence of different tissues of the periderm staticc\\} periphery? (a) Phellogen \(\rightarrow\) Phellem \(\rightarrow\) Phelloderm (b) Phellem \(\rightarrow\) Phelloderm \(\rightarrow\) Phellogen (c) Phellem \(\rightarrow\) Phellogen \(\rightarrow\) Phelloderm (d) Phelloderm \(\rightarrow\) Phellogen \(\rightarrow\) Phellem

Short answer

(a) Phellogen \rightarrow Phellem \rightarrow Phelloderm

Step-by-step solution

Understanding Periderm Tissues

The periderm is part of the secondary protective covering of plants, replacing the epidermis in parts of plants that undergo secondary growth. It consists of three layers: phellogen, phellem, and phelloderm.

Define the Order of Tissues

The phellogen (cork cambium) is the first layer and is responsible for the production of the periderm. The phellem (cork) is the second layer produced by the phellogen outwardly. The phelloderm is the third layer produced by the phellogen inwardly.

Match the Order to the Options

Knowing that the correct sequence starts with phellogen followed by phellem and ending with phelloderm, match this sequence with the provided options.

Key concepts

Phellogen

Often termed cork cambium, phellogen is a form of meristematic tissue which plays a critical role in the secondary growth of plants. This cellular layer acts as a substitute for the epidermis in mature plant stems and roots that have ceased to expand in length and have started to thicken.

Phellogen is composed of cells that are capable of dividing and is found in the outer cortex layer. As it divides, it forms two distinct types of cells: those that will become part of the phellem, or cork layers, to the outside; and those that will develop into phelloderm, or secondary cortex, towards the inside. This dynamic layer is the origin point from which the protective periderm tissue is generated.

Phellem

The phellem, more commonly known as cork, is the outermost layer of the periderm. It is produced by the phellogen as it undergoes radial cell division. Cork is a protective barrier that shields the plant from environmental hazards and prevents desiccation, contributed by its waterproof and suberized cell walls.

The phellem layer is made up of dead cells at maturity, which are compactly packed without intercellular spaces, creating a durable and impervious layer. These dead cork cells are filled with air, adding to the insulation and protective properties of the layer. Furthermore, it is the cork we encounter daily, being utilized for various practical applications like wine stoppers and bulletin boards.

Phelloderm

Parallel to the development of phellem, the phellogen also generates phelloderm on its inward-facing side. The phelloderm is sometimes referred to as the secondary cortex and is comprised of living parenchyma cells. Contrasting to the protective role of the phellem, the main function of phelloderm is to store food and nutrients.

It forms a thin layer compared to the other two constituents of the periderm and contains chloroplasts in some species, contributing to photosynthesis in the underlying tissues. Moreover, it helps in the transport of nutrients and repair of injured plant areas. The balance between the generation of phellem and phelloderm by the phellogen is crucial for healthy plant secondary growth.

Secondary Growth in Plants

Secondary growth refers to the process by which plants increase in girth, primarily in their roots and stems, this process is critical for the plant's ability to grow taller and to support itself. This growth is predominantly governed by the activity of vascular cambium (producing secondary xylem and phloem) and cork cambium or phellogen (producing periderm).

As plants undergo secondary growth, they develop stronger and more resilient structures to withstand various environmental stresses. This includes the formation of wood — a result of secondary xylem accumulation — and the necessary protective layers including the bark, of which the periderm is a part. The harmonious coordination of these growth processes is essential for the plant's overall health, longevity, and adaptation to its surroundings.