Growth can be referred to as an increase in length and width i.e., change in size and mass. These changes may occur over a period of time. When a plant arises from a seed or its vegetative parts, it necessarily grows leaves, stems, and roots. This is called primary growth, which develops from apical meristem. As the time passes, besides elongation of the roots and stems, the girth of the plants starts to increase. This is called secondary growth. Secondary growth is a characteristic feature of dicotyledons. Most of the monocots lack secondary growth.
As mentioned earlier, primary growth is the effort of the apical meristem. The lateral meristem tissues are responsible for the secondary growth of plants. There are two types of lateral tissues involved in secondary growth, namely, vascular cambium and cork cambium.
In the primary stage, a layer of meristematic plant tissues is sandwiched between vascular tissues- primary xylem and phloem. This layer is not continuous but appears as patches. On maturity, these patches develop and separate the vascular tissues. This tissue is called vascular cambium. They form the cambial ring in plants.
Formation of the cambial ring can be explained by recalling the anatomy of dicot stems. In a dicotyledonous stem, the primary xylem and primary phloem are separated by cambium cells called intrafascicular cambium. During secondary growth, the medullary rays adjacent to the intrafascicular cambium develop into meristematic tissue and are called interfascicular cambium. Both intrafascicular and interfascicular cambiums form a continuous ring called cambial ring.
An active cambial ring initiates differentiation of new cells; numerous cells are formed towards the center and periphery regions. The cell in the center, mature into secondary xylem while the periphery cells mature into the secondary phloem. Depending on the activity of the cambial ring, two types of woods are formed- spring/early wood and winter/latewood. These two together constitute the annual ring in a tree.
Cork cambium, also called phellogen, is another meristematic tissue developed in the cortex region. Due to the cambial ring activity, the outer layers such as cortex cells and epidermis get crushed. This is the time when the cork cambium develops as a new protective layer. Cork cambium starts to differentiate cells and form outer cork (phellem) and inner secondary cortex (phelloderm). Phellogen, phellem, and phelloderm, all together make up the periderm. Non-technically, the secondary phloem and periderm are collectively called as the bark of a tree.
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