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Chapter 23

Chapter 23. Plant Structure and Function. Section 23-1. Specialized Tissues in Plants. Structure of a Seed Plant. 3 organs -> roots, stems, leaves Linked by tissues that provide support, protection, nutrient production and transport. Structure of a Seed Plant. Roots

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Chapter 23

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  1. Chapter 23 Plant Structure and Function

  2. Section 23-1 Specialized Tissues in Plants

  3. Structure of a Seed Plant • 3 organs -> roots, stems, leaves • Linked by tissues that provide support, protection, nutrient production and transport

  4. Structure of a Seed Plant • Roots • Anchor plants, prevent erosion, absorb nutrients/water and transport them, store food, hold plants upright • Stems • Produce leaves/reproductive structures, contain transport systems • Leaves • Photosynthesis, have adjustable pores to reduce water loss and help gas exchange

  5. Plant Tissue Systems • 3 main tissue systems -> dermal, vascular, and ground • Dermal Tissue - protective, outer covering • Single cell layer in young plants called epidermis, the outer surface often covered with a waxy cuticle • In older plants usually many layers, sometimes covered with bark • Some epidermal cells have trichomes, which protect leaves and give them a fuzzy appearance • In roots, incudes root hairs to absorb water

  6. Plant Tissue Systems • Vascular Tissues – support plant bodies, transport water and nutrients • Xylem – transports water • Phloem – transports products of photosynthesis

  7. Xylem • Cells called tracheids • As they mature, they die and leave their cell walls which contain lignin (gives wood strength) • Cells have connecting openings for water to pass • Pits allow water to diffuse into ground tissue

  8. Xylem • Angiosperms have second xylem tissue called vessel elements – wider than tracheids, arranged end to end • Mature and die, cell walls develop slits at each end for water to move freely

  9. Phloem • Alive at maturity • Main cells called sieve tube elements, arranged end to end forming sieve tubes • Small holes at ends so nutrients can move from cell to cell • Lose nuclei and most organelles as they mature

  10. Phloem • Companion cells surround sieve tube elements - keep nuclei/organelles

  11. Plant Tissue Systems • Ground tissue – produces/stores sugars, contributes to physical support • Parenchyma cells – thin walls, large central vacuole surrounded by thin layer of cytoplasm – chloroplasts in leaves • Collenchyma cells – thicker walls, flexible, provide support • Sclerenchyma cells - thickest walls, rigid, makes up seed coat

  12. Parenchyma Collenchyma Sclerenchyma

  13. Plant Growth and Meristems • Meristems – regions of unspecialized cells in which mitosis produces new cells ready for differentiation • Apical meristems found in places of rapid division – tips of stems and roots

  14. Plant Growth and Meristems • At first, cells produced in apical meristems are all thin, unspecialized • Gradually mature and differentiate to form each tissue system • Meristems also create highly specialized cells of cones and flowers • Patterns of gene expression changes the stem’s apical meristem

  15. Section 23-2 Roots

  16. Root Structure and Growth • As soon as a seed sprouts, its first root brings in water/nutrients from soil • Cells divide rapidly, pushing root tips into soil, providing raw materials for developing stems and leaves

  17. Root Structure and Growth • Taproot Systems • Primary root grows long and thick (taproot) giving rise to smaller branches • Can store sugars and starches • Fibrous Root Systems • Begin with one primary root, which is replaced by many equally sized branches that grow separately from the base of the stem • Help prevent soil erosion

  18. Dandelion (taproot) Grass (fibrous root)

  19. Anatomy of a Root • Epidermis made of dermal tissue – protection and absorption • Surface covered in root hairs – penetrate between soil particles and increase surface area • Cortex composed of ground tissue • Water/minerals move from epidermis • Stores products of photosynthesis and starches

  20. Anatomy of a Root • Endodermis – layer of ground tissue enclosing vascular cylinder – moves water and minerals to center of root • Vascular cylinder in the center composed of xylem and phloem • Dicot roots have central column of xylem

  21. Anatomy of a Root • Apical meristems near root tip allow roots to increase in length • Root cap protects meristem, secretes slippery substance to ease progress through soil • Cells at tip scraped away and replaced continually

  22. Root Functions • Uptake of plant nutrients • Soil contains sand, silt, clay, air, bits of decaying animal/plant tissue in varying amounts • Plants need inorganic nutrients like nitrogen, phosphorus, potassium, magnesium, calcium • Trace elements also important, but excessive amounts can be toxic

  23. Root Functions • Active transport of dissolved nutrients • Active transport proteins in root hairs, other epidermal cells • Bring in mineral ions from soil • Water movement by osmosis • Mineral ions accumulate in root, water “follows”

  24. Root Functions • Movement into vascular cylinder • Move through cortex • Cylinder enclosed by endodermis – cells meet and cell walls from waterproof zone called Casparian strip • Casparian strip forces water/minerals to move through cell membrane rather than between cells – filter and control water • Ensures one-way flow

  25. Root Functions • Root pressure • Minerals pumped into vascular cylinder, water follows by osmosis creating pressure • Water has to go up - root pressure forces water through vascular cylinder into the xylem • Up and up!

  26. Section 23-3 Stems

  27. Stem Function • Produce leaves, branches and flowers • Hold leaves up to sun • Transport substances • Xylem and phloem form continuous tubes from roots to stems to leaves • In many plants they function in storage and aid in photosynthesis

  28. Anatomy of a Stem • Surrounded by layer of epidermal cells with thick cell walls and a waxy protective coating

  29. Anatomy of a Stem • Growing stems have nodes where leaves are attached • Buds contain apical meristems to produce new stems and leaves • Larger plants have woody stems to support leaves and flowers

  30. Monocot Stems • Vascular bundles (clusters of xylem and phloem) scattered throughout ground tissue composed mainly of parenchyma cells

  31. Dicot Stems • Vascular bundles arranged in a ring pattern • Parenchyma cells inside ring called pith, outside form cortex • Complexity increases as stem increases in diameter

  32. Primary Growth • Occurs in all seed plants – apical meristems increase plant length

  33. Secondary Growth • Larger plants require older parts of stem to increase in thickness • Common in dicots and gymnosperms

  34. Secondary Growth • Takes place in meristems called vascular cambium (produced vascular tissues, increase thickness of stem) and cork cambium (outer covering)

  35. Growth from Vascular Cambium • Thin layer of cells between xylem and phloem • Xylem pushed in, phloem pushed out • Increases diameter of stem each year

  36. Wood Formation • Layers of secondary xylem produces by vascular cambium • Older xylem near center no longer carries water – heartwood (dark) • Surrounded by sapwood – active in fluid transport (light)

  37. Tree Rings • In spring, vascular cambium produces light colored rings of xylem (early wood) • Cells grow less as season continues, have thicker cells walls, darker in color (late wood) • A ring = a year of growth • Thick rings mean favorable weather

  38. Formation of Bark • Everything outside the vascular cambium in a mature stem (phloem, cork cambium, cork) • Expansion leads to oldest tissue splitting • Cork cambium surrounds cortex producing a thick layer of cork to prevent water loss • Outer layers may flake off as stem thickens

  39. Section 23-4 Leaves

  40. Anatomy of a Leaf • Blade – thin, flat part of leaf – maximum light absorption • Blade attached to stem by petiole • Outer covering of dermal tissue • Top and bottom covered by epidermis, tough irregular cells with thick outer walls • Covered by waxy cuticle – waterproof, prevents water loss

  41. Anatomy of a Leaf • Vascular tissues bundles into veins that run from stem through leaf • Palisade mesophyll beneath upper epidermis – closely packed cells that absorb sunlight • Spongy mesophyll contains air spaces connected to stomata – small opening in epidermis allowing for gas exchange

  42. Transpiration • Mesophyll cell walls moist for easy diffusion • Water can evaporate from these surfaces by transpiration • May be replaced by water from xylem • Cools leaves on hot days, but can threaten survival

  43. Gas Exchange • Exchange gases between air spaces in spongy mesophyll and exterior by opening stomata

  44. Homeostasis • If stomata were always open, too much water would be lost to transpiration • Open just enough to allow photosynthesis • Guard cells control opening and closing of stomata, regulating movement of gases

  45. Homeostasis • When water is abundant, increaser in water pressure in guard cells opens stoma by curving • When water is scarce, water pressure in guard cells drops and stoma closes • Stomata usually open during day, closed at night • Can be closed in bright sunlight or hot/dry conditions • Respond to environment

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