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Chapter 33

Chapter 33. Stems and Plant Transport. External features of a woody twig Buds (undeveloped embryonic shoots) Terminal bud at tip of stem Axillary buds (lateral buds) in leaf axils Dormant bud covered and protected by bud scales which leave bud scale scars.

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Chapter 33

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  1. Chapter 33 Stems and Plant Transport

  2. External features of a woody twig • Buds (undeveloped embryonic shoots) • Terminal bud at tip of stem • Axillary buds (lateral buds) in leaf axils • Dormant bud covered and protected by bud scales which leave bud scale scars

  3. External features of a woody twig, cont. • Node is area on a stem where leaf is attached • Internode is region between two successive nodes • Leaf scar remains when leaf is detached from stem

  4. External features of a woody twig, cont. • Bundle scars are areas within a leaf scar where vascular tissue extended from stem to leaf • Lenticels are sites of loosely-arranged cells allowing oxygen to diffuse into interior of woody stem

  5. External structure of a woody twig in its winter condition

  6. Herbaceous stems possess • Epidermis • Vascular tissue • Either • Ground tissue or • Cortex and pith

  7. Epidermis • Protective layer covered by a water-conserving cuticle • Stomata permit gas exchange • Xylem conducts water and dissolved nutrient minerals • Phloem conducts dissolved sugar

  8. Epidermis, cont. • Storage functions carried out by • Cortex • Pith • Ground tissue

  9. All herbaceous stems have same basic tissues, but arrangement thereof varies • Herbaceous dicot stems have circular arrangement of vascular bundles and distinct cortex and pith • Monocot stems have vascular bundles scattered in ground tissue

  10. Cross section of a Helianthus annuus stem

  11. Closeup of two vascular bundles

  12. Cross section of a Zea mays stem

  13. Closeup of a vascular bundle

  14. Lateral meristems • Vascular cambium produces • Secondary xylem (wood) • Secondary phloem (inner bark) • Cork cambium produces periderm • Cork parenchyma • Cork cells

  15. Periderm, cont. • Cork parenchyma functions primarily for storage in a woody stem • Cork cells are the functional replacement for epidermis in a woody stem

  16. Secondary growth occurs in • Some flowering plants (woody dicots) • All cone-bearing gymnosperms

  17. Transition from primary growth to secondary growth in a woody stem • Vascular cambium, which develops between primary xylem and primary phloem divides in two directions, forming • Secondary xylem (to the inside) • Secondary phloem (to the outside)

  18. Develop-ment of secondary xylem and secondary phloem

  19. Transition from primary growth to secondary growth in a woody stem, cont. • As secondary growth proceeds, in the original vascular bundles, two elements become separated • Primary xylem • Primary phloem

  20. Onset of secondary growth

  21. Beginning of division of vascular cambium

  22. A young woody stem

  23. Pathway of water movement • Water and dissolved nutrient minerals move from soil into • Epidermis • Cortex, etc.

  24. Pathway of water movement, cont. • Once in root xylem, water and dissolved minerals move upward from • Root xylem to stem xylem • Stem xylem to leaf xylem • Most water entering leaf exits leaf veins and passes into atmosphere

  25. Water potential is a measure of the free energy of water • Pure water has a water potential of • 0 megapascals • Water with dissolved solutes has • Negative water potential

  26. Water potential, cont. • Water moves from an area of higher (less negative) water potential to an area of lower (more negative) water potential

  27. The tension-cohesion model explains the rise of water and dissolved nutrient minerals in xylem • Transpiration causes tension at top of plant

  28. Transpiration, cont. • Tension at top of plant results from water potential gradient ranging • From slightly negative water potentials in soil and roots • To very negative water potentials in atmosphere

  29. Transpiration, cont. • Column of water pulled up through plant remains unbroken due to properties of water • Cohesive • Adhesive

  30. The tension-cohesion model

  31. Root pressure • Caused by movement of water into roots from soil as a result of active absorption of nutrient mineral ions from soil • Helps explain rise of water in smaller plants (especially when soil is wet) • Pushes water up through xylem

  32. Pathway of sugar translocation • Dissolved sugar is translocated up or down in phloem • From a source (area of excess sugar, usually a leaf) • To a sink (area of storage or of sugar use)

  33. Pathway of sugar translocation, cont. • Area of storage or of sugar use • Roots • Apical meristems (fruits and seeds) • Sucrose is predominant sugar translocated in phloem

  34. Aphids used to study translocation in plants

  35. Pressure-flow hypothesis explains the movement of materials in phloem • Companion cells actively load sugar into sieve tubes at source • ATP required for this process

  36. Pressure-flow hypothesis, cont. • ATP supplies energy to pump protons out of sieve tube elements • Proton gradient drives uptake of sugar by cotransport of protons back into sieve tube elements

  37. Pressure-flow hypothesis, cont. • Sugar therefore accumulates in sieve tube element • This causes movement of water into sieve tubes by osmosis

  38. Pressure-flow hypothesis, cont. • Companion cells unload sugar from sieve tubes at sink • Actively (requiring ATP) • Passively (not requiring ATP) • As a result, water leaves sieve tubes by osmosis

  39. Pressure-flow hypothesis, cont. • Unloading of sugar causes decrease in turgor pressure inside sieve tubes • Flow of materials between source and sink is driven by turgar pressure gradient produced by • Water entering phloem at source • Water leaving phloem at sink

  40. The pressure-flow hypothesis(diagram divided in two)

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