Growth can be referred to a permanent increase in size, length, width and changes in the shape and mass of an organism. These changes may occur over a period of time.
Most of the plants continue to grow throughout their lifetimes, and they grow through the combinations of cell growth and cell divisions. In plants, the growth is connected with the regions of meristems. They are the specialized tissues, composed of undifferentiated cells, basically, stem cells. The primary meristems of the plants are called apical meristems and are responsible for the growth of the roots and stems.
When a plant arises from seed or its vegetative parts, it necessarily grows into a plant with leaves, stems, and roots. This is called primary growth, and it develops from the apical meristem. As time passes, besides elongation of the roots and stems, the circumference of the plants starts to increase, and it is called secondary growth.
Secondary growth is a characteristic feature of dicotyledons. Most of the monocotyledons lack secondary growth.
Also refer: Anatomy of Monocot And Dicot Plants
Secondary Growth in Plants
As mentioned earlier, primary growth is the effort of the apical meristem. The lateral meristem tissues are responsible for the secondary growth of plants.
The secondary growth of plants increase in stem thickness and it is due to the activity of the lateral meristems, which are absent in herbs or herbaceous plants. There are two types of lateral tissues involved in secondary growth, namely, vascular cambium and cork cambium.
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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 centre and periphery regions. The cells in the centre, 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 or earlywood and winter or latewood. These two kinds of wood together constitute the annual ring in a tree.
In woody plants, cork cambium is the outermost lateral meristem. 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 the bark of a tree.