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Plant tissues
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Meristematic tissue Meristematic tissue is undifferentiated tissue.
Meristematic tissue contains actively dividing cells that result in formation of other tissue types (e.g. vascular, dermal or ground tissue). Apical meristematic tissue is found in buds and growing tips of plants. Lateral meristematic tissue make the plant grow thicker. Examples of lateral meristematic tissue include the cambium or 'bark' found in trees. Lateral meristems occur in woody trees and plants.
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Meristematic tissue continued…
Structural adaptation Function Cells are small, spherical or polygonal in shape. This allows for close packing of a large number of cells. Vacuoles are very small or completely absent. Vacuoles provide rigidity to cells thus preventing rapid division. Large amount of cytoplasm and a large nucleus. The lack of organelles is a feature of an undifferentiated cell. Large nuclear material contains the DNA necessary for division and differentiation.
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Permanent tissue The meristematic tissues give rise to cells that perform a specific function. Once cells develop to perform this particular function, they lose their ability to divide. The process of developing a particular structure suited to a specific function is known as cellular differentiation. We will examine two types of permanent tissue: Simple permanent tissues Epidermis Parenchyma Collenchyma Sclerenchyma Complex permanent tissues xylem vessels (made up of tracheids and vessels) phloem vessels (made up of sieve tubes and companion cells)
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Structure Function Layer of cells covering surface of entire plant. Acts as a barrier to fungi and other microorganisms and pathogens. Layer is thin and transparent. Allow for light to pass through, allowing for photosynthesis in the tissues below. Epidermal tissues have abundant trichomes which are tiny hairs projecting from surface of epidermis. Leaf trichomes trap water in the area above the stomata and prevent water loss. Epidermis tissue The epidermis is a single layer of cells that covers plants' leaves, flowers, roots and stems. It is the outermost cell layer of the plant body and plays a protective role in the plant. The function of key structural features are listed in the table.
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Epidermis tissue continued...
Root hairs are elongations of epidermal cells in the root. Root hairs maximise the surface area over which absorption of water from the soil can occur. Epidermal tissues in leaves are covered with a waxy cuticle. The waxy outer layer on the epidermis prevents water loss from leaves. Epidermal tissues contain guard cells containing chloroplasts. Guard cells control the opening and closing of the pores known as stomata thus controlling water loss in plants. Some plant epidermal cells can secrete poisonous or bad-tasting substances. The bitter taste of the substances deter browsing and grazing by animals.
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Epidermis continued: guard cells and stomata
Guard cells are bean shaped specialised epidermal cells found mainly on the lower surface of leaves which are responsible for regulating the size of the stoma opening. Together, the stoma and the guard cells are referred to as stomata. The stomata in the epidermis allow oxygen, carbon dioxide and water vapour to enter and leave the leaf. The guard cells also contain chloroplasts for photosynthesis.
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Parenchyma tissue Structure Function Thin-walled cells. Thin walls allow for close packing and rapid diffusion between cells. Intercellular spaces are present between cells. Intercellular spaces allow diffusion of gases to occur. Parenchyma cells have large central vacuoles. This allows the cells to store and regulate ions, waste products and water. Also function in providing support. Specialised parenchyma cells known as chlorenchyma found in plant leaves contain chloroplasts. This allows them to perform a photosynthetic function and responsible for storage of starch. Some parenchyma cells retain the ability to divide. Allows replacement of damaged cells. Parenchyma tissue forms the majority of stems and roots as well as soft fruit like tomatoes and grapes. It is the most common type of ground tissue.
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Collenchyma tissue Collenchyma is a simple, permanent tissue typically found in the shoots and leaves of plants. Collenchyma cells are thin-walled but the corners of the cell wall are thickened with cellulose. This tissue gives strength, particularly in growing shoots and leaves due to the thickened corners. The cells are tightly packed and have fewer inter-cellular spaces.
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Collenchyma tissue continued...
Structure Function Cells are spherical, oval or polygonal in shape with no intercellular spaces. This allows for close packing to provide structural support. Corners of cell wall are thickened, with cellulose and pectin deposits. Provides mechanical strength. Cells are thin-walled on most sides. Provides flexibility, allowing plant to bend in the wind.
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Sclerenchyma tissue Sclerenchyma is a simple, permanent tissue.
It is the supporting tissue in plants, making the plants hard and stiff. Two types of sclerenchyma cells exist: fibres and sclereids. Sclerenchyma fibres are long and narrow and have thick lignified cell walls. They provide mechanical strength to the plant and allow for the conduction of water. Sclereids are specialised sclerenchyma cells with with thickened, highly lignified walls with pits running through the walls. They support the soft tissues of pears and guavas and are found in the shells of some nuts.
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Sclerenchyma tissue continued…
Structure Function Cells are dead and have lignified secondary cell walls. This provides mechanical strength and structural support. The lignin provides a 'wire-like' strength to prevent from tearing too easily. Sclereids have strong walls which fill nearly the entire volume of the cell. Provide the hardness of fruits like pears. These structures are used to protect other cells.
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We will now examine the complex permanent tissues
We will now examine the complex permanent tissues. Remember the difference between simple and complex permanent tissues is that simple permanent tissues are made up of cells of the same type whereas complex permanent tissues are made up of more than one cell type that combine to perform a particular function.
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Xylem tissue Xylem has the dual function of supporting the plant and transporting water and dissolved mineral salts from the roots to the stems and leaves. It is made up of vessels, tracheids, fibres and parenchyma cells. The vessels and tracheids are non-living at maturity and are hollow to allow the transport of water. Both vessels and tracheids have lignin in their secondary walls, which provides additional strength and support. Xylem vessels are composed of a long chain of straight, elongated, tough, dead cells known as vessel elements. The vessel elements are long and hollow and they make a long tube because the cells are arranged end to end, and the point of contact between two cells is dissolved away. The role of xylem vessels is to transport water from roots to leaves. Xylem vessels often have patterns of thickening in their secondary walls. Secondary wall thickening can be in the form of spirals, rings or pits. Tracheids have thick secondary cell walls and are tapered at the ends. The thick walls of the tracheids provide support and tracheids do not have end openings like the vessels. The tracheids' ends overlap with one another, with pairs of pits present which allow water to pass through horizontally from cell to cell.
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Xylem tissue continued...
Structure Function Long cells Form effective conducting tubes for water and minerals Dead cells: no cytoplasm No obstruction to water transport Thick, lignified walls Support the plant and are strong enough to resist the suction force of transpiration pull, so they don’t collapse Pits in cell walls Allow lateral water transport to neighbouring cells
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Tracheids have tapered ends
Improved flexibility of the stem in wind Vessels elements have open ends Water is transported directly to the next cell No intercellular spaces Added support for the stem Living parenchyma cells in between xylem Form vascular rays for water transport to the cortex of the stem Patterns of secondary wall thickening Improve flexibility of the stem in wind and allow the stem to stretch as it lengthens
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Phloem tissue Phloem tissue is the living tissue responsible for transporting organic nutrients produced during photosynthesis (mainly as the carbohydrate sucrose) to all parts of the plant where these are required. The phloem tissue is made up of the following major types of cells: sieve elements: these are conducting cells which transport sucrose. parenchyma cells: which store food for transport in phloem. companion cells: are associated with parenchyma cells and control the activities of sieve tube elements, since the latter have no nuclei. Companion cells are responsible for providing energy to the sieve elements to allow for the transport of sucrose. fibres: unspecialised cells and supportive cells.
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Phloem tissue continued...
Structure Function Companion cells: Contain large number of ribosomes and mitochondria. Due to absence of organelles or nucleus in sieve tube, companion cells perform cellular functions of sieve tube. Has many plasmodesmata (intercellular connections) in the wall attached to the sieve tube. Allows transfer of sucrose-containing sap over a large area. Sieve tubes Sieve tube elements are long conducting cells with cellulose cell walls. Form good conducting tubes over long distances. Allows for transfer over a large area. They are living cells with no nucleus or organelles such as vacuoles or ribosomes. Allows for more space to transport sap. It is also why sieve elements need companion cells to carry out all cellular functions.
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