The Plant Body – Tissues and Organs. Cooksonia – 408 MYA.
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Transcript of The Plant Body – Tissues and Organs. Cooksonia – 408 MYA.
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The Plant Body – Tissues and Organs
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Cooksonia – 408 MYA
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Plant Tissues
• Meristematic tissue - site of growth in plant; origin of the other tissue types: apical meristems - site of primary growth; lateral meristems - site of secondary growth
• Dermal tissue system - the outer protective covering of the plant
• Vascular tissue system - comprises the xylem and phloem - it is embedded within the ground tissue system
• Ground tissue system - the inner supportive tissues of the plant - pith
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Plant Meristems and Growth
• Primary growth is an increase in length – it occurs at apical meristems
• Secondary growth is an increase in plant diameter – it occurs at lateral meristems – in particular the vascular cambium and cork cambium
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Dermal Tissues
• Make up outermost tissue layer of plants
• In young plants, it consists of a single layer of cells – the epidermis – that may secrete cutin to make protective wax layer of cuticle
• May have hairs or trichomes
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Trichomes
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Stomata
• Scattered through the leaf epidermis are openings called stomata that allow the plant to breathe
• The opening (pore) is surrounded by two guard cells
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Periderm
• In woody plants the epidermis cracks and splits and is replaced by periderm which is formed by the cork cambium
• the periderm consists of cork cambium, cork cells and some other cells – it is the bark of mature trees -
• cork is mostly dead cells with cell walls containing much suberin
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Periderm
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Ground Tissues
• Ground tissue makes up most of the tissues in herbaceous plants• There are several ground cell types which perform a variety of
functions• Parenchyma cells – very diverse – often loosely arranged, main
location of photosynthesis and storage in leaves• Collenchyma cells – main support tissue in young plant stems –
found in leaves, stems and petals – usually with thickened corners of cell walls
• Sclerenchyma cells – can be either fibers or sclerids – fibers provide support but are dead at maturity – thick secondary cell walls; sclerids provide support as well
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Tissues in an herbaceous stem
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Mesophyll cellsare parenchyma
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Parenchyma and Collenchyma
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Sclerenchyma - Sclerids
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Yucca leaf basket – sclerenchyma fibers
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Vascular Tissues
• Vascular tissues are responsible for transporting material through the plant body
• Xylem cells move water and nutrients from roots to rest of plant
• Phloem cells move carbohydrates and other photosynthetic products from leaves to rest of plant
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Xylem
• Xylem is dead at maturity and transports water essentially through a hollow tube - angiosperms have tracheary cells are called vessels which tend to have flattened ends, angiosperms also have tracheids
• in gymnosperms the tracheary cells are called tracheids and are usually sharply tapered
• eventually the xylem becomes full of sap and is no longer used for water transport, then functions in support and forms heartwood
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Xylem –VesselsandTracheids
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Phloem
• Phloem cells are called sieve tube elements because of the sieve like plates at the end of the cells - they are alive at maturity but are crushed as the plant grows in diameter and must be continually replaced
• Some sieve cells have companion cells which govern transport of material through the sieve
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Phloem – sieve elements
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The Roots
• Roots make up most of the underground portion of the plant
• Roots anchor plant in soil
• Roots absorb water and nutrients
• Roots serve as storage organs – especially storage of starch
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Hornbeam Roots
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Carrots – Storage Root
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Carrot Flowers – Wild Carrot aka Queen Anne’s Lace
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Fibrous Tap
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Extent of Root Systems
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Size of Root Systems• The most well studied root system was for a 4
month old rye plant - its roots occupied a volume of 6 liters
• When measured the total surface area of the root system, including root hairs was 639 m2, or 130 times the surface of the shoot
• It had approximately 14 billion root hairs with an absorbing surface of 401 m2 - if laid end to end, they would extend over 10,000 km
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Size of Root Systems
• Deepest known roots – desert mesquite shrub roots down to 53.3 m
• Tamarisk and Acacia trees – roots to 30 m deep
• Herbaceous Alfalfa – roots to 6 m deep
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Root Growth
• The growth of most roots is continuous process that only stops under adverse conditions such as drought or low temperature
• During their growth through the soil, roots follow the path of least resistance and frequently follow spaces left by earlier roots which have died and decayed
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Mango Tree Root System
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More Root Growth
• The tip of the root is covered by a root cap - a mass of cells which protects the apical meristem as it pushes through the soil
• As the root pushes through the soil, cells of the root cap are sloughed off from the margins - they are replaced by new growth of cells at the center of the root cap
• The sloughed off cells and growing root tip are covered by a slimy sheath called the mucigel which lubricates the root as it passes through the soil
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Rhizosphere and Mucigel
• The mucigel provides an environment favorable for growth of beneficial bacteria
• The rhizosphere is formed of the mucigel, root hairs, sloughed off root cap cells and various microorganisms
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Mucigel Sheath
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Root Absorption
• Epidermis in young roots absorbs water and minerals and this is facilitated by root hairs - tubular extensions of the epidermal cells
• Some roots have a thin cuticle covering the epidermis; other roots have epidermal cells containing suberin - even so, the epidermal cells offer little resistence to the entrance of water and nutrients
• The innermost layer of the cortex is compact and lacks air spaces - this is the endodermis - it has Casparian strips which are bands of suberin between cells that prevent the passage of water and air - thus in endodermis, all substances must pass through cells