As we know that the bark is formed as a result of secondary growth of cork cambium in the dicot stem.it includes :-
(1) Cork (Phellem)
(2) Cork cambium (Phellogen)
(3) Phelloderm
(phellem,phellogen &Phelloderm constitutes periderm).
(4) Cortex
(5) Phloem

Bark Layers

Bark is the outermost protective layer of a tree trunk, and it consists of several different layers that work together to provide structural support, protection, and transportation of nutrients. These layers vary in composition and function. Let's explore the various kinds of cell layers that constitute the bark:

1. Epidermis:
- The epidermis is the outermost layer of the bark.
- It consists of a single layer of cells that protect the tree from external factors such as pathogens, ultraviolet radiation, and desiccation.
- The epidermis also helps in preventing water loss by secreting a waxy substance called cuticle.

2. Cork:
- The cork layer lies beneath the epidermis and is composed of dead cells.
- These cells are packed tightly together and contain a substance called suberin, which makes them impermeable to water and gases.
- The cork layer provides insulation, protection against physical damage, and helps prevent the loss of water.

3. Phellogen:
- Also known as the cork cambium, the phellogen is a layer of meristematic cells.
- It lies between the cork and the secondary phloem.
- The phellogen produces cells both inwardly and outwardly.
- The inward cells differentiate into cork cells, while the outward cells differentiate into secondary phloem.

4. Secondary Phloem:
- The secondary phloem is the innermost layer of the bark.
- It is responsible for the transportation of nutrients, primarily sugars, from the leaves to other parts of the tree.
- This layer consists of sieve tube elements, companion cells, fibers, and parenchyma cells.
- The secondary phloem also provides structural support to the tree trunk.

5. Vascular Cambium:
- The vascular cambium is a meristematic layer located between the secondary phloem and the secondary xylem.
- It is responsible for the production of secondary xylem (wood) on the inner side and secondary phloem on the outer side.
- The vascular cambium plays a crucial role in the growth of the tree by increasing the girth.

6. Secondary Xylem:
- The secondary xylem, also known as wood, is the layer found beneath the vascular cambium.
- It consists of xylem vessels, fibers, and parenchyma cells.
- The primary function of the secondary xylem is to transport water and minerals from the roots to the leaves and provide mechanical support to the tree trunk.

In conclusion, the bark of a tree is a complex structure composed of multiple layers, each with its specific functions. From the outermost epidermis to the innermost secondary xylem, these layers work together to protect the tree, transport nutrients, and provide structural support.