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| Secondary Growth |
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| The normal process of growth that occurs in every plant body is known as primary growth. It is the result of the activity of primary meristem. The process of primary growth results in the formation of primary permanent tissues such as primary xylem, primary phloem and primary cortex. However in the dicot plants, there is a process of growth that begins after a known period of primary growth. Such a growth is known as secondary growth. It is the result of the activity of secondary meristem. It results in the formation of secondary permanent tissues such as secondary xylem, secondary phloem and secondary cortex. As a result, secondary growth brings about an increase in the girth of the plant body. |
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| In a dicot stem, secondary growth occurs both in the stele and cortex. The process occurs simultaneously but is caused by separate strips of secondary meristem. In the stele, secondary growth is initiated by vascular cambium, while in the cortex, it is initiated by cork cambium. |
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| It is the result of the activity of the vascular cambium, which occurs in between xylem, and phloem of each vascular bundle. Hence, it is also known as intra-fascicular cambium. In addition, towards the beginning of secondary growth there is a process of dedifferentiation in some of the parenchyma cells of the medullary rays, adjoining the vascular cambium. As a result, these cells now become meristematic and represent the inter-fascicular cambium. The meristematic cells in the intra-fascicular cambium and inter-fascicular cambium fuse and result in the formation of a continuous strip of meristem called cambial ring. The cambial ring at this stage has primary xylem on its inner surface and primary phloem on its outer surface. |
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| The cambial ring exhibits mitotic activity on both the sides. The mitotic activity on the inner surface results in the formation of cells, which differentiate into a set of xylem. It represents the secondary xylem. Similarly, the mitotic activity on the outer surface result in the formation of cells, which differentiate into a set of phloem. It represents the secondary phloem. Due to the formation of secondary xylem, the primary xylem becomes pushed more towards the pith and the pith gets slightly reduced. However, the secondary phloem grows and completely masks the primary phloem. Hence, it is not visible. |
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| The mitotic activity of the cambial ring is purely seasonal. It occurs only twice during every year, once in the spring and once in the autumn. Thus, every year two sets of secondary xylem and two sets of secondary phloem are formed. Each year, the mitotic division of the cambial ring usually begins in the spring season. The secondary xylem that is formed in the spring season is therefore known as springwood or early wood, while the secondary xylem formed in the autumn is known as autumn wood or late wood. The springwood is generally characterised by the presence of xylem vessels having wider lumen. This is because, spring is the ideal season for growth and the water requirement of the plant is more in the spring. The autumn wood has xylem vessels with narrow lumen, since water requirement in the winter is less. |
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| The two distinct layers of secondary xylem, the inner springwood and the outer autumn wood together represent the (or annual ring). One such annual ring is added every year due to secondary growth. Thus, it is possible to ascertain the age of a dicot tree by counting the number of annual rings. While every year two sets of secondary xylem and two sets of secondary phloem are formed, only one set is visible because the secondary phloem formed later (in the autumn) grows over and masks the secondary phloem formed earlier (in the spring). |
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| This is however applicable only to the temperate regions. The growth seasons in the tropics are different, with the growing season coinciding with the rains, in true rainforest areas where seasonality is least pronounced, many trees show continuous growth and growth rings are not seen at all. |
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| Dendrochronology: The branch of biology, which deals with the ascertaining the age of a tree by counting the number of annual rings. |
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| Thus, secondary growth in the stele results in the formation of secondary xylem and secondary phloem. Due to the addition of these tissues, there is an increase in the girth of the stele. |
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| As secondary growth continues to occur several changes take place in the secondary xylem. In a much older stem the secondary xylem (wood) shows two regions namely duramen and alburnum. |
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 Duramen, also known as heartwood, represents the centrally located, inactive portion of the secondary xylem. It appears dark in colour due to the accumulation of pigments resulting from oxidation of organic compounds like oils and tannins. It is incapable of conducting water and provides only mechanical support to the trees. It is this part of the secondary xylem that is used as the commercial wood. |
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 Alburnum, also known as sapwood, represents the peripheral, active portion of the secondary xylem. This portion is still capable of conducting water. Due to its meagre mechanical strength and lack of durability, the sapwood can not serve as commercial wood. |
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| It is the result of the activity of a secondary meristem called cork cambium, which appears between hypodermis and primary cortex. Some of the parenchyma cells in the peripheral layers of cortex undergo dedifferentiation and become meristematic. These cells now represent the cork cambium or phellogen. The cork cambium starts exhibiting mitotic activity on both the sides, just as the cambial ring in the stele. |
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| The mitotic activity on the inner surface of the cork cambium results in the formation of cells, which undergo differentiation into a living tissue, called secondary cortex or phelloderm, just above the primary cortex. The mitotic activity on the outer surface results in the formation of cells, which undergo differentiation into a dead tissue, called cork or phellem, just below the epidermis. The cork covers and masks the hypodermis. The tissue resulting from secondary growth in the cortex the cork, the cork cambium and the secondary cortex-together represent a region called periderm. |
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| The periderm along with the primary cortex represents the bark. In several dicot plants, the bark peels off regularly. |
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| Due to the formation of periderm, the epidermis is subjected to pressure and as a result it breaks at several places to form openings called lenticels. The lenticels, also known as aerating pores, enclose a group of living cells called complementary cells. Through these cells exchange of respiratory gases and to some extent transpiration take place. |
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| Thus, secondary growth in the cortex results in the formation of periderm. Due to the addition of this region there is an increase in the girth of the cortex. |
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