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| Tissue Systems in the Plant Body |
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| A section taken through any region of a typical plant body shows the presence of three tissue systems, namely |
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 Epidermal tissue system |
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 Ground tissue system and |
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 Vascular tissue system |
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| It is a derivative of protoderm. It is represented by the epidermis and the associated structures, which are mainly involved in providing protection to the plant body. The epidermis is the outermost covering of the plant body. It is commonly described as the skin of the plant body. It is usually represented by a single layer of compactly arranged, barrel-shaped parenchyma cells. In the stem and the leaf, the epidermal cells are thick-walled and are meant for protection. In the roots, the epidermal cells are thin-walled, since they are mainly involved in the absorption of water and mineral salts. Hence, the epidermis of root is described as epiblema. In the leaf, there are two epidermal layers, one on each surface of the leaf. |
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 The epidermis usually forms projections known as epidermal hairs. In the root, the epidermal hairs are unicellular and are called as root hairs. The root hairs penetrate between the soil particles to absorb water. In the stem and the leaf, the epidermal hairs are multicellular and are known as trichomes. They are involved in the secretion of some terpenoid compounds. |
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 The epidermis of the stem and the leaf is usually surrounded by a thin, covering called cuticle. It is formed by a waxy substance called cutin. It is meant for preventing excessive evaporation of water. Cuticle is absent in the root epidermis. |
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 The epidermis of the leaf and herbaceous stem (green stem) contains numerous minute openings called stomata. Each stoma is surrounded by a pair of modified epidermal cells called the guard cells. The stomata take part in vital physiological functions such as transpiration, respiration and photosynthesis. Stomata are absent in the epidermis of the root and woody stem. |
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| It is a derivative of ground meristem. It is represented by the different regions in the plant body that are mainly involved in storage. In the cylindrical parts of the plant body such as root and stem, the ground tissue becomes differentiated into various regions. In a typical condition, following regions can be recognised in the ground tissue. |
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 Hypodermis is a region that is found immediately below the epidermis in the stem. It is made up of a few layers of either collenchyma cells (as in dicot stem) or sclerenchyma cells (as in monocot stem). Hypodermis is meant for providing additional protection and mechanical support. |
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 Cortex is a major component of the ground tissue represented by many layers of loosely arranged parenchyma cells. Intercellular spaces are prominent. Cortex represents the major storage region in the plant body. |
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 Endodermis is the inner most layer of cortex represented by a single layer of compactly arranged barrel-shaped parenchyma cells. In the stem, it is meant for storage. In the root it plays an important role in the movement of absorbed water. Some of the cells in the root endodermis contain secondary thickenings called casparian thickenings, formed by a waxy substance called suberin. |
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 Pericycle is a region that lies immediately below the endodermis. In the stem, it is represented by a few layers of sclerenchyma cells while in the root, it is represented by a single layer of parenchyma cells. |
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 Medullary rays/Conjunctive tissue are represented by |
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| parenchyma cells that are found in between the vascular tissues. In the stem, the parenchyma cells are meant for storage of food and represent the medullary rays while in the root, they store water and represent the conjunctive tissue. |
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 Pith is the innermost part of the stem or root representing the axis. It is formed by loosely arranged parenchyma cells with prominent intercellular spaces. It is another major storage organ in the plant body. The pericycle, the medullary rays or conjunctive tissue, and the pith along with the vascular tissues, together represent a region, which is known as stele or central cylinder. |
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| In the leaf, the ground tissue system is known as mesophyll. It is formed by several layers of chlorenchyma cells found between the two epidermal layers. The mesophyll is mainly involved in photosynthesis. |
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| The vascular tissue system is a derivative of procambium. It is represented by the conducting tissues-xylem and phloem. These vascular tissues occur in the form of strands extending from one end of the plant body to another. These strands are known as vascular bundles or fascicles. In the leaf, the vascular bundles are known as veins. |
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| The vascular bundles may sometimes enclose a piece of meristematic tissue called cambium, which brings about secondary growth in the bundles. When cambium is present, the vascular bundle is described as open and when the cambium is absent, the vascular bundle is described as closed. |
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| A vascular bundle may contain either only xylem or only phloem or both. Accordingly, based on the relative position of xylem and phloem the bundles can be distinguished into three types. |
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| Conjoint Bundle |
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| In conjoint bundle xylem and phloem occur in the same bundle along the same radius. |
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| The conjoint bundle is of two types: |
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 Collateral where, the vascular bundle consists of only one set of xylem and one set of phloem. The collateral bundle can be either open or closed. |
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 Bicollateral where, the vascular bundle has one set of xylem, two sets of cambium and two sets of phloem. The bicollateral bundle is always open. |
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| The conjoint bundles are characteristic features of the leaf and the stem. |
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| Radial Bundle |
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| In radial bundle, xylem and phloem occur in separate bundles, alternately. Radial bundles are characteristic features of the root. |
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| Concentric Bundle |
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| In concentric bundle, xylem and phloem occur in the same bundle surrounding each other. When the bundle has xylem in the centre surrounded by phloem, the concentric bundle is described as amphicribal (or hadrocentric). e.g., ferns. When phloem is in the centre surrounded by xylem, the concentric bundle is described as amphivasal (or leptocentric). |
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| Example: |
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| Dracena, Yucca. The concentric bundles are characteristic features of some monocot plants. |
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| A Typical Vascular Bundle |
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| In the xylem, the protoxylem and metaxylem vessels exhibit characteristic orientation with reference to the central axis. On this basis, two types of arrangement can be recognised in the xylem. |
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 Centripetal arrangement in which, the younger metaxylem vessels are found towards the pith while the older protoxylem vessels are found away from the pith. In this condition, xylem is described as exarch. It is a characteristic feature of the root. |
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 Centrifugal arrangement in which, the older protoxylem vessels are found towards the pith and the younger metaxylem vessels are found away from the pith. In this condition, xylem is described as endarch. It is a characteristic feature of leaves and stem. |
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