There are two types of growth that take place in plants. They are primary growth and secondary growth. The primary growth is related to the growth of apical meristems whereas the secondary growth is related to the growth of roots and stems in the lateral meristems. Secondary growth is shown by the dicotyledon plants. Due to a lack of vascular tissue, secondary growth does not occur in monocots. The wood plant tissue is formed by the secondary growth in dicot stems. This wood plant tissue is responsible for providing protection to the plant. The lenticels present in the wood material helps in the exchange of gases. We will learn about how this growth takes place and about the wood yielding plants that are useful for us. The wood obtained from the plants is also used in making paper. It also finds its uses in making furniture and shelter.
As we studied above, the secondary growth in plants is responsible for the formation of wood. The vascular bundles get arranged in a ring-like manner. They are arranged around the central pith and are conjoint and open. As they possess cambium therefore they are called open tissues. It is known as intrafascicular cambium. The cells start the process of dedifferentiation and in this way, the cambium rings are formed. These cambium rings that are formed by the cambium start dividing. From the observations, it is seen that the cambium is more active on the inner side as compared to the outer side.
The cork cambium is the main tissue that is responsible for the formation of wood. This will give us a brief idea about wood information. The girth of the stem increases due to increased activity of the vascular cambium. As the girth keeps increasing the outer cortical layers starts rupturing. So, cork cambium produces new layers that replace the damaged or ruptured layers. Cork cambium is also called phellogen. The cork cambium has another name that is stellar cambium. Phellogen is thick and has two layers. The outer one forms the cork and the inner one forms the secondary cortex. The cells of cork are compactly arranged and in the beginning, they have thin cellulose cell walls. When they mature the living part is replaced by the non-living part which is the formation of wood material. The cell walls of the cork become thick by the deposition of suberin. This chemical makes the cork or wood material impervious to water by getting deposited in the cell walls. The phelloderm is the secondary cortex. It is called so because it develops at the time of secondary growth. It is made up of thin-walled parenchymatous cells. They have cellulose cell walls and are living in nature. The periderm is the collective name given to phellogen, phellem and phelloderm. They are the protective layers of the cell. They grow when the epidermis layer is ruptured and also when the outer cortical layers are ruptured. When secondary growth in the vascular cambium takes place then only the secondary growth of the cork cambium happens. As the growth of cork cambium is continuous, the layers peripheral to phellogen are damaged and they need continuous replacement.
The wood of the tree is mainly present in its bark. The bark is made up of all the tissues that are present outside the vascular cambium. The layers that make up the bark are the periderm, primary cortex, pericycle, primary and secondary phloem. These all tissues are present outside the vascular cambium. The bark formed in the early season is known as soft bark and the bark that is formed later in the season is called hard bark. When the entire ring of the cork cambium makes a bark, it is called ring bark. In this, a complete cylinder of bark is formed.
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Figure: Wood made from the bark of the tree
They are the small openings that are formed in the bark. They are formed by a small portion of the periderm. They are produced by the activity of the phellogen. As we read above, the phellogen is the meristematic tissue that is formed during the secondary growth of plants. They are lens-shaped openings. As they are openings, they help in the exchange of gases. The exchange of gases takes place between the internal tissue of the stem and the outer atmosphere. They help in exchanging gases from the woody areas of the plant. Water is also lost from them in the form of vapours. As they help in the exchange of gases, they are also known as breathing pores.
The wood that can be used for carpentry purposes is known as timber. It is of two types: heartwood and softwood. Some common timber yielding trees are:
There are two other types of wood that are known as heartwood and sapwood. Heartwood is a modified and non-functional secondary xylem. Due to the deposition of organic compounds, the heartwood becomes resistant to microbial growths. It also becomes hard and durable in nature. It has dead elements and the cell walls are highly lignified. No water is conducted from this heartwood. It helps in providing mechanical support to the plant. The sapwood is present in the periphery region of the secondary xylem. It is actively involved in conducting water and minerals. Due to the passage of time and the addition of layers in the cork cambium, more and more sapwoods are changed to heartwoods.
1. How is the Cork Cambium Originated?
Answer: It is a meristematic tissue that arises from the cells of the pericycle. The cells of the pericycle get divided and it results in the formation of cork cambium. This cork cambium further gives rise to the periderm. The activity of the cork cambium is similar in both the dicot root and dicot stem. Cork cells are produced on the outer side and secondary cortex on the inner side. The cork cells have the presence of suberin in their cell wall. These cells become dead due to more and more deposition of suberin. The activity of cork cambium builds pressure in the layers that are peripheral to the phellogen.
2. What is the Role of the Pericycle in the Secondary Growth of the Dicot Roots?
Answer: The cells of the pericycle lie opposite to that of the protoxylem. The cells of the protoxylem divide and give rise to the vascular cambium. Also, it gives rise to parenchymatous cells that are present below the patches of phloem. They help information of a complete and continuous cambial ring. Lateral roots are also developed by the pericycle.