Plant Organs: Roots

1. Plant Organs: Roots Chapter 6

2. KEY TERMS • TAPROOT SYSTEM • A root system consisting of one prominent main root with smaller lateral roots branching from it • FIBROUS ROOT SYSTEM • A root system consisting of several adventitious roots of approximately equal size that arise from the base of the stem

3. Root Systems

4. LEARNING OBJECTIVE 1 • Describe the functions of roots • Describe two features of roots that shoots lack

5. Functions of Roots • Anchorage • Absorption • Conduction • Storage

6. Storage Roots

7. KEY TERMS • ROOT CAP • A covering of cells over the root tip that protects delicate meristematic tissue directly behind it • ROOT HAIR • An extension of an epidermal cell of a root that increases absorptive capacity of the root

8. Root Cap • Each root tip has a root cap • A protective thimblelike layer • Many cells thick • Covers delicate root apical meristem • May orient root so it grows downward

9. Root Hairs • Short-lived, unicellular extensions of epidermal cells near the growing root tip • Increase surface area of root in contact with moist soil, increasing root’s absorptive capacity

10. Structures Unique to Roots

11. Root apical meristem (area of cell division) Root cap (a) The root cap of an onion (Allium cepa) root. The root cap protects the root’s apical meristem. Fig. 6-3a, p. 114

12. Root hairs Soil air Soil water Soil particles Epidermis (b) Root hairs on a radish (Raphanus sativus) seedling. Each delicate hair is a unicellular extension of the root epidermis. Root hairs increase the surface area in contact with the soil. Fig. 6-3b, p. 114

13. LEARNING OBJECTIVE 2 • Contrast the structure of a primary eudicot root and a monocot root • Describe the functions of each tissue

14. Primary Eudicot Roots 1 • Outer protective covering • Epidermis • Ground tissues • Cortex • Pith (in certain roots) • Vascular tissues • Xylem • Phloem

15. Primary Eudicot Roots 2 • Epidermis • Protects the root • Root hairs help absorb water and dissolved minerals • Cortex • Consists of parenchyma cells • Usually stores starch

16. KEY TERMS • ENDODERMIS • Innermost layer of the cortex of the root that prevents water and dissolved materials from entering the xylem by passing between cells

17. KEY TERMS • CASPARIAN STRIP • A band of waterproof material around the radial and transverse cells of the endodermis • Ensures that water and minerals enter the xylem only by passing through the endodermal cells

18. Endodermis and Mineral Uptake

19. Endodermis Cortex Epidermis Casparian strip Endodermis Movement of water through the endodermis to the center of the root Root cross section Casparian strip Fig. 6-5, p. 118

20. KEY TERMS • PERICYCLE • A layer of cells just inside the endodermis of the root • Gives rise to lateral roots

21. Lateral Root

22. Fig. 6-7, p. 120

23. Ruptured epidermis Lateral root Cortex Pericycle Willow Fig. 6-7, p. 120

24. Primary Eudicot Roots 3 • Xylem • conducts water and dissolved minerals • Phloem • conducts dissolved sugar

25. Herbaceous Eudicot Root

26. Buttercup Epidermis Cortex Stele (a) Cross section of a buttercup (Ranunculus) root. Note that the bulk of the root is the cortex. Fig. 6-4a, p. 116

27. Cortex cells filled with amyloplasts Endodermis cell Pericycle cell Phloem cell Xylem vessel elements Intercellular space (b) A close-up of the stele of the buttercup root. Note the solid core of vascular tissues. Fig. 6-4b, p. 116

28. Comparing Monocot and Eudicot • Monocot roots often have a pithin the center of the root • In herbaceous eudicot roots, xylem and phloem form a solid mass in center of root • Monocot roots lack a vascular cambium • Do not have secondary growth

29. Monocot Root

30. Monocot Root

31. Greenbrier Epidermis Cortex Stele Fig. 6-8a, p. 121

32. Endodermis Pericycle Phloem Vascular tissues Xylem Pith Fig. 6-8b, p. 121

33. LEARNING OBJECTIVE 3 • Trace the pathway of water from the soil through the various root tissues

34. Water Movement 1 • In a primary eudicot root, water moves from soil into center of root: • Root hair → epidermis → cortex (symplast or apoplast pathway) → endodermis → pericycle → xylem of root • Water is transported upward through root xylem into stem xylem and rest of plant

35. KEY TERMS • SYMPLAST • A continuum consisting of the cytoplasm of many plant cells, connected from one cell to the next by plasmodesmata • APOPLAST • A continuum consisting of the interconnected, porous plant cell walls, along which water moves freely

36. Symplast and Apoplast

37. Movement upward Endodermis Xylem vessels Casparian strip Epidermis Cortex Symplast: interconnected cytoplasm of living cells Phloem cells Pericycle Water and dissolved nutrient minerals Plasma membrane Plasmodesma Cell wall Apoplast: interconnected cell-wall spaces Root hair Fig. 6-6, p. 119

38. LEARNING OBJECTIVE 4 • Describe several roots that are modified to perform unusual functions

39. KEY TERMS • PROP ROOT • An adventitious root that arises from the stem and provides additional support for the plant • CONTRACTILE ROOT • A specialized root, often found on bulbs or corms, that contracts and pulls the plant to a desirable depth in the soil

40. Prop Roots

41. Contractile Roots

42. Corm Contractile roots (a) Plants that produce corms or bulbs often have contractile roots that lose much of their length as root cells shorten and broaden. Fig. 6-13a, p. 124

43. Contractile roots Fig. 6-13b, p. 124

44. KEY TERMS • PNEUMATOPHORE • A specialized aerial root produced by certain trees living in swampy habitats • May facilitate gas exchange between the atmosphere and submerged roots

45. Pneumatophores

46. Other Modified Roots 1 • Buttress roots • Swollen bases or braces that hold trees upright • Aid in extensive distribution of shallow roots • Found in some tropical rainforest trees

47. Buttress Roots

48. Other Modified Roots 2 • Suckers • Aboveground stems that develop from adventitious buds on the roots • Asexual reproduction method of some roots • Certain epiphytes have roots that are modified to photosynthesize

49. Parasitic Epiphytes

50. Juniper bark Mistletoe root Juniper wood (b) Micrograph of a parasitized juniper (Juniperus) branch, showing a mistletoe root penetrating the wood (secondary xylem) of the juniper. Fig. 6-12b, p. 123