Cambium

Cambium tissue is present in plants. The cambium layer is a tissue layer that helps in plant growth. The cambium cells help in the secondary growth of the plant by providing an undifferentiated mass of cells. The cambial activity is seen between the areas of the xylem and phloem. Secondary tissues arise from the parallel rows of the cells that are made from the cambium in plants. The cambium is seen in dicotyledon stems. It is absent in the monocot plants. As the cambium is present between the vascular bundles, therefore, it is also known as intrafascicular cambium. The main cambium function is to provide secondary growth to the plants. This meristematic tissue is present between the permanent tissues. We will learn more about what is cambium in plants, what is the function of cambium, the structure of cambium. 

Classification of Vascular Bundles

In the above paragraph, we learned a bit about what cambium is. Here we will understand how vascular bundles are classified. The cambium is the basis of this classification. These vascular bundles are classified on the presence and absence of cambium. 

• Open Vascular Bundles: In these types of vascular bundles, cambium tissue is present. It is present in between xylem and phloem elements. The bundles are said to be open. These types of bundles due to the presence of cambium have the ability to form secondary xylem and phloem or we can say that they are capable of secondary growth. As they are open for secondary growth therefore they are named open vascular bundles. They are present in dicots. 

• Closed Vascular Bundle: The cambium tissue is absent in these types of vascular bundles. The xylem and phloem tissues are joined together and no cambium is present in them. No secondary growth is seen in these types of vascular bundles. Due to this, they are known as closed vascular bundles as they are closed for secondary growth. 

Secondary Growth

We got a rough idea about what cambium is and now we will study its role in plant growth. The growth of the root and stem is known as primary growth. This growth takes place with the help of apical meristems. The lateral meristems are responsible for the secondary growth of the plant. They help to increase the girth of the root and stem. The lateral meristems are the Intrafascicular cambium, Interfascicular cambium and the cork cambium. These lateral meristems are responsible for the secondary growth of the plant. 

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 function also starts and 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.

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Cork Cambium

We will study the cork cambium function. The cork cambium is the main tissue that is responsible for the formation of wood. The girth of the stem increases due to increased activity of the vascular cambium. As the girth keeps increasing the outer cortical layers start 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.

Cambium - at a Glance

• The plants which have only primary growth have limited size and longevity.

• The diameter of the dicotyledons and perennial gymnosperms is caused by secondary growth thereby supporting the growth size and height of the plants.

• On differentiation of procambium into primary vascular tissue, that is, xylem and phloem, an active meristematic region is present in between the xylem and the phloem. This region is known as the cambium which is responsible for adding fresh tissues when required.

• Ray parenchyma cells present between vascular cambium give rise to the interfascicular cambium.

• The complete cylinder of vascular cambium is formed by the joining of the fascicular cambium and the interfascicular cambium.

• Cambium contains longitudinal divisions which enable the stem to increase in girth only.

• Phellem and phelloderm arise from the phellogen that differentiates near the surface of the stem.

• The cells are made impervious to gas and liquids because of suberin present in the wall of the cork cells.

• Gas exchange is facilitated by the lenticels present in the bark.

• Cork cambium originates in deeper tissues like cortex, epidermis and phloem.

• The secondary tissues of the dicotyledons are arranged in the form of concentric circles.

• Larger vessel elements are present in the early wood whereas the late woods contain smaller vessels and predominant tracheids.

• Annual rings represent the one-year growth of the xylem.

• The histological data of the annual rings can give an insight into the age of the tree and the ecological aspects of the tree as well.

• The wood which is young is living and located on the periphery and is commonly referred to as sap wood.

• The wood which has become old cannot perform any function and gets accumulated in the centre forming a dead core known as heartwood.

• The recently formed one or two rings participate in the ascent of sap.

• Some monocots show true secondary growth, that is, through cambium that produces secondary vascular bundles and parenchyma, for example, Agave.

• In certain dicotyledonous stems, the variants of cambium contribute to unusual secondary growth.

• The variants of cambium may arise due to conditions like when cambium is in an abnormal position but has normal activities when there is the formation of accessory cambium or formation of the interxylary phloem because of the abnormal activity and the position of the cambium.

• The variants of cambial are observed in the roots of Ipomoea batatas and Beta vulgaris.