1. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition Solomon Berg Martin Chapter 33 Stems and Plant Transport

2. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 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

3. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport External structure of a woody twig in its winter condition

6. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Herbaceous stems possess Epidermis Vascular tissue Either –Ground tissue or –Cortex and pith

7. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Epidermis, cont. Storage functions carried out by –Cortex –Pith –Ground tissue

9. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Cross section of a Helianthus annuus stem

11. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Closeup of two vascular bundles

12. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Cross section of a Zea mays stem

13. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Closeup of a vascular bundle

14. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Lateral meristems Vascular cambium produces –Secondary xylem (wood) –Secondary phloem (inner bark) Cork cambium produces periderm –Cork parenchyma –Cork cells

15. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Secondary growth occurs in Some flowering plants (woody dicots) All cone-bearing gymnosperms

17. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Develop- ment of secondary xylem and secondary phloem

19. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Onset of secondary growth

21. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Beginning of division of vascular cambium

22. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport A young woody stem

23. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport

24. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Pathway of water movement Water and dissolved nutrient minerals move from soil into –Epidermis –Cortex, etc.

25. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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

26. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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

27. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Water potential, cont. Water moves from an area of higher (less negative) water potential to an area of lower (more negative) water potential

28. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport The tension-cohesion model explains the rise of water and dissolved nutrient minerals in xylem Transpiration causes tension at top of plant

29. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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

30. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Transpiration, cont. Column of water pulled up through plant remains unbroken due to properties of water –Cohesive –Adhesive

31. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport The tension- cohesion model

32. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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

33. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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)

34. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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

35. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Aphids used to study translocation in plants

36. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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

37. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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

38. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport Pressure-flow hypothesis, cont. Sugar therefore accumulates in sieve tube element This causes movement of water into sieve tubes by osmosis

39. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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

40. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport 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

41. Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh EditionCHAPTER 33 Stems and Plant Transport The pressure-flow hypothesis (diagram divided in two)