Woody Twig Anatomy: External Features and Plant Transport

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

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

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

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

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

31. The tension-cohesion model

32. 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. 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. 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. Aphids used to study translocation in plants

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

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