1. Plant Organs: Roots
Chapter 6

2. KEY TERMS TAPROOT SYSTEM FIBROUS ROOT 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 ROOT HAIR
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. (a) The root cap of an onion (Allium cepa)
Root apical
meristem
(area of
cell division)
Root cap
Figure 6.3: Structures unique to roots.
(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. in contact with the soil.
Root hairs
Soil air
Soil water
Figure 6.3: Structures unique to roots.
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 Ground tissues
Epidermis
Ground tissues
Cortex
Pith (in certain roots)
Vascular tissues
Xylem
Phloem

15. Primary Eudicot Roots 2 Epidermis Cortex 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
Figure 6.5: The endodermis and mineral uptake.
Note the Casparian strip around the radial and transverse walls that prevents water and dissolved minerals from passing into the stele along endodermal cell walls. To reach the vascular tissues, water and dissolved minerals must pass through the plasma membranes of endodermal cells.
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 Figure 6.7: Lateral root.
A multicellular lateral root emerges from a larger root. Lateral roots originate at the pericycle.
Fig. 6-7, p. 120

23. Ruptured epidermis Lateral root Cortex Pericycle Willow
Figure 6.7: Lateral root.
A multicellular lateral root emerges from a larger root. Lateral roots originate at the pericycle.
Willow
Fig. 6-7, p. 120

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

25. Herbaceous Eudicot Root

26. (a) Cross section of a buttercup (Ranunculus) root.
Epidermis
Cortex
Figure 6.4: Structure of an herbaceous eudicot root.
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. (b) A close-up of the stele of the buttercup root.
Cortex cells
filled with
amyloplasts
Endodermis
cell
Pericycle cell
Phloem cell
Xylem vessel
elements
Figure 6.4: Structure of an herbaceous eudicot root.
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 pith in 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
Figure 6.8: Structure of a monocot root.
(a) Cross section of a greenbrier (Smilax) root. Note the extensive cortex. (b) Close-up of a portion of the center of the root, showing the vascular tissues and pith.
Stele
Fig. 6-8a, p. 121

32. Endodermis Pericycle Phloem Vascular tissues Xylem Pith
Figure 6.8: Structure of a monocot root.
(a) Cross section of a greenbrier (Smilax) root. Note the extensive cortex. (b) Close-up of a portion of the center of the root, showing the vascular tissues and pith.
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 APOPLAST
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
Figure 6.6: The symplast and apoplast.
Water and dissolved minerals that enter the root travel from one cell’s cytoplasm to another through cytoplasmic connections (the symplast) or from cell to cell along the interconnected porous cell walls (the apoplast). On reaching the endodermis, water and minerals can continue to move into the root’s center if they pass through a plasma membrane and enter an endodermal cell. The Casparian strip blocks the passage of water and minerals along the cell walls between adjoining endodermal cells.
Plasma membrane
Water and
dissolved
nutrient
minerals
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 CONTRACTILE 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. (a) Plants that produce corms or bulbs often have
Contractile
roots
Figure 6.13: 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
Figure 6.13: 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. (b) Micrograph of a parasitized juniper (Juniperus)
bark
Mistletoe
root
Juniper
wood
Figure 6.12: Specialized roots of parasitic epiphytes.
(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

51. LEARNING OBJECTIVE 5
Discuss the significance of roots to humans

52. Important Foods
Roots which store the products of photosynthesis are important sources of food for human consumption
Some roots are used as flavorings
Example: root beer flavoring (dried greenbrier roots)

53. Root Crops Predominantly taproots Some fibrous roots
carrots, beets, sugar beets, parsnips, turnips, rutabagas, radishes
Some fibrous roots
sweet potatoes, cassava

54. KEY TERMS MYCORRHIZA NODULE
A mutually beneficial association between a fungus and a root that helps the plant absorb essential minerals from the soil
NODULE
A small swelling on the root of a leguminous plant in which beneficial nitrogen-fixing bacteria (Rhizobium) live

55. Mycorrhizae

56. (a) Cross section of root showing ectomycorrhizae,
Sheath of
fungal hyphae
encircles root
Fungal hypha
between plant
cells
Figure 6.14: Mycorrhizae.
Mycorrhizae enhance plant growth by providing essential minerals to the roots.
(a) Cross section of root showing ectomycorrhizae,
fungal associations that form a sheath around the
root. The fungal hyphae penetrate the root between
cortical cells but do not enter the cells.
Fig. 6-14a, p. 124

57. (b) Cells of a root cortex showing endomycorrhizae,
Fungal hyphae
within plant
cortical cells
Figure 6.14: Mycorrhizae.
Mycorrhizae enhance plant growth by providing essential minerals to the roots.
(b) Cells of a root cortex showing endomycorrhizae,
fungal associations in which the fungal hyphae penetrate
root cells of the cortex to aid in delivering and receiving
nutrients. Endomycorrhizae colonize roots of most vascular
plant species.
Fig. 6-14b, p. 124

58. Animation: Root Systems
CLICK TO PLAY

59. Animation: Water Absorption
CLICK TO PLAY

60. Animation: Root Functioning
CLICK TO PLAY

61. Animation: Root Cross-section
CLICK TO PLAY