1. Plant Organs: Stems
Chapter 7

2. LEARNING OBJECTIVE 1
Describe three functions of stems

3. Stem Functions Support Conduct Produce new living tissues
leaves and reproductive structures
Conduct
water, dissolved minerals, carbohydrates
Produce new living tissues
at apical meristems
at lateral meristems (secondary growth)

4. Variation in Stems

5. LEARNING OBJECTIVE 2
Relate the functions of each tissue in an herbaceous stem

6. Tissues in Herbaceous Stems 1
Epidermis
protective outer layer
covered by water-conserving cuticle
Vascular tissues
Xylem conducts water and dissolved minerals
phloem conducts dissolved carbohydrates (sucrose)

7. Tissues in Herbaceous Stems 2
Storage tissues
Cortex and pith
ground tissue

8. LEARNING OBJECTIVE 3
Contrast the structures of an herbaceous eudicot stem and a monocot stem

9. Herbaceous Eudicot Stems
Have vascular bundles arranged in a circle (in cross section)
Have a distinct cortex and pith

10. Monocot Stems Have scattered vascular bundles
Have ground tissue instead of distinct cortex and pith

11. Monocot Stems

12. (a) Cross section of a corn (Zea mays)
Epidermis
Ground
tissue
Vascular
bundles
Ground
tissue
Phloem
Xylem
Air
space
Figure 7.4: Arrangement of tissues in a monocot stem.
Bundle
sheath
Corn
seedling
(a) Cross section of a corn (Zea mays)
stem, showing the scattered vascular
bundles.
(b) Close-up of a vascular bundle.
The air space is where the first xylem elements formed. The entire bundle is enclosed in a bundle sheath of sclerenchyma for additional support.
Fig. 7-4, p. 134

13. LEARNING OBJECTIVE 4
Distinguish between the structures of stems and roots

14. Differences Between Stems and Roots 1
Unlike roots, stems have nodes and internodes, leaves and buds
Unlike stems, roots have root caps and root hairs

15. KEY TERMS NODE INTERNODE
Area on a stem where one or more leaves is attached; stems have nodes, but roots do not
INTERNODE
Stem area between two successive nodes

16. KEY TERMS BUD An undeveloped shoot that contains an embryonic meristem
May be terminal (at tip of stem) or axillary (on side of stem)

17. A Woody Twig

18. Terminal bud scale scars Node
One year’s
growth
Terminal bud scale scars
Node
Internode
Axillary bud
Leaf scar
Node
Figure 7.2: The external structure of a woody twig.
The age of a woody twig is determined by counting the number of groups of terminal bud scale scars (don’t count those on side branches). How old is this twig?
Lenticels
Terminal bud
scale scars
Bundle scars
Fig. 7-2, p. 131

19. Differences Between Stems and Roots 2
Internally
herbaceous roots possess an endodermis and pericycle
stems lack a pericycle and rarely have an endodermis

20. Differences Between Stems and Roots

21. LEARNING OBJECTIVE 5
Outline the transition from primary growth to secondary growth in a woody stem
List the two lateral meristems, and describe the tissues that arise from each

22. Primary Growth: Eudicot

23. Vascular bundles Pith Cortex Epidermis Sunflower Epidermis Cortex
(a) Cross section of a sunflower (Helianthus annuus)
stem, showing the organization of tissues. The vascular
bundles are arranged in a circle.
Sunflower
Epidermis
Cortex
Phloem
fiber cap
Pith ray
Figure 7.3: Primary growth in a eudicot stem.
Phloem
Vascular
cambium
Vascular bundle
Xylem
Vessel
element
Pith
(b) Close-up of a vascular bundle. The xylem is toward the stem’s
interior, and the phloem toward the outside. Each vascular bundle
is “capped” by a batch of fibers for additional support.
Fig. 7-3, p. 132

24. KEY TERMS VASCULAR CAMBIUM
A lateral meristem that produces secondary xylem (wood) to the inside and secondary phloem (inner bark) to the outside

25. Secondary Growth Occurs in woody eudicots and conifers
Produced by vascular cambium
between primary xylem and primary phloem

26. Vascular Cambium 1 Is not initially a solid cylinder of cells
becomes continuous when production of secondary tissues begins

27. Vascular Cambium 2 Certain parenchyma cells between vascular bundles
retain ability to divide
connect to vascular cambium cells in each vascular bundle
form a complete ring of vascular cambium

28. Dividing Vascular Cambium

29. Second division of vascular cambium forms a phloem cell.1X 2X 3X 4X 2P 1P 1X 2X 3X 2P 1P 1X 2X 2P 1P
Time
Secondary xylem
Secondary phloem
Figure 7.6: Radial view of a dividing vascular cambium cell.
To study this figure, start at the bottom and move up. Note that the vascular cambium (blue cell) divides in two directions, forming secondary xylem (X) to the inside and secondary phloem (P) to the outside. These cells, which are numbered in the order in which they are produced, differentiate to form the mature cell types associated with xylem and phloem. 1X 2X 1P
Second division of vascular
cambium forms a phloem cell. 1X 1P
Division of vascular cambium forms
two cells, one xylem cell and one
vascular cambium cell. 1X
Vascular cambium cell when
secondary growth begins.
Vascular cambium cell
Fig. 7-6, p. 136

30. KEY TERMS
CORK CAMBIUM
A lateral meristem that produces cork parenchyma to the inside and cork cells to the outside
Cork cambium and the tissues it produces make up the outer bark of a woody plant

31. Cork Cambium Arises near the stem’s surface
Is either a continuous cylinder of dividing cells or a series of overlapping arcs of meristematic cells that form from parenchyma cells in successively deeper layers of the cortex and, eventually, secondary phloem

32. Development: Woody Eudicot

33. (a) At the onset of secondary growth, vascular cambium arises in the
Primary
xylem
Epidermis
Cortex
Primary
phloem
Vascular cambium
Pith
(a) At the onset of secondary growth, vascular cambium arises in the
parenchyma between the vascular bundles (that is, in the pith rays), forming
a cylinder of meristematic tissue (blue circle in cross section).
Figure 7.5: Development of secondary growth in a eudicot stem.
Vascular cambium and the tissues it produces are shown; cork cambium is not depicted. (The figures change in scale from part a to part c because of space limitations; pith and primary xylem are actually the same size in all three diagrams, but the change in scale makes those tissues appear to shrink.)
Fig. 7-5a, p. 135

34. and secondary phloem on the outside.
Remnant
of cortex
Remnant
of epidermis
Remnant
of primary
phloem
Secondary phloem
(inner bark)
Secondary xylem
(wood)
Periderm
(outer bark)
Remnant of
primary xylem
Remnant
of pith
Vascular
cambium
Figure 7.5: Development of secondary growth in a eudicot stem.
Vascular cambium and the tissues it produces are shown; cork cambium is not depicted. (The figures change in scale from part a to part c because of space limitations; pith and primary xylem are actually the same size in all three diagrams, but the change in scale makes those tissues appear to shrink.)
(b) Vascular cambium begins to divide, forming secondary xylem on the inside
and secondary phloem on the outside.
Fig. 7-5b, p. 135

35. (outer bark; remnants of primary phloem, cortex,
Periderm
(outer bark; remnants of
primary phloem, cortex,
and epidermis are gradually
crushed or torn apart and
slough off)
Secondary xylem
(wood)
Secondary phloem
(inner bark)
Figure 7.5: Development of secondary growth in a eudicot stem.
Vascular cambium and the tissues it produces are shown; cork cambium is not depicted. (The figures change in scale from part a to part c because of space limitations; pith and primary xylem are actually the same size in all three diagrams, but the change in scale makes those tissues appear to shrink.)
Remnant of
primary xylem
Remnant of
pith
Vascular
cambium
(c) A young woody stem. Vascular cambium produces more secondary xylem
than secondary phloem.
Fig. 7-5c, p. 135

36. 3-Year-Old Stem

37. Secondary xylem (wood)
Pith
Primary xylem
Annual ring of
secondary xylem
Secondary xylem (wood)
(a) LM of cross section of basswood (Tilia americana) stem. Note the
location of the vascular cambium between the secondary xylem (wood) and secondary phloem (inner bark).
Vascular cambium
Secondary phloem
Periderm and remnants of primary
phloem, cortex, and epidermis
Expanded phloem ray
Xylem ray
Remains of
epidermis
Cork
Cork
cambium
Expanded
phloem ray
Figure 7.7: Three-year-old stem in cross section.
It may be easier to study the parts labeled on the sketch and then locate them on the micrograph.
(b) Sketch of a pie-shaped segment of the cross section. The primary phloem is not labeled because it is crushed beyond
recognition.
Cortex
Secondary phloem
Vascular cambium
Secondary xylem (third year)
Xylem rays
Secondary xylem (first year)
Primary xylem
Pith
Fig. 7-7, p. 137

38. Onset of Secondary Growth

39. Cortex Phloem fiber cap Primary phloem Secondary phloem
Vascular cambium
Secondary xylem
Primary xylem
Figure 7.8: Onset of secondary growth.
This cross section through part of a magnolia (Magnolia) stem shows a vascular bundle that was split apart by secondary growth. Compare this vascular bundle with the one in Figure 7-3b, which has primary growth only.
Cross section
of twig
Vascular
cambium
Pith
Fig. 7-8, p. 138

40. Variation in Bark

41. Lenticels

42. Lenticel Cork cells Cork cambium and cork parenchyma (phelloderm)
Figure 7.10: Structure of a lenticel.
This cross section through the periderm of a calico flower (Aristolochia elegans) stem shows a lenticel.
Cork
cells
Cork
cambium
and cork parenchyma
(phelloderm)
Calico flower
Fig. 7-10, p. 141

43. Heartwood and Sapwood

44. Heartwood Sapwood Fig. 7-11, p. 141
Figure 7.11: Heartwood and sapwood.
This cross section through a butternut (Juglans cinerea) trunk reveals the darker heartwood in the center of the tree and the lighter sapwood. The sapwood is the functioning xylem that conducts water and dissolved minerals.
Fig. 7-11, p. 141

45. Tree-Ring Dating

46. Long, slender core of wood extracted by a boring tool Vascular cambium
Pith
Annual rings
Bark
Tilia
(basswood)
Long, slender core of wood
extracted by a boring tool
Vascular
cambium
Outer
bark
Annual rings
Pith
Tree-ring dating. Scientists develop a master chronology by using progressively older pieces of wood from the same geographic area. They can then accurately determine the age of a wood sample by using a computer to match its rings to the master chronology.
Sample from a living tree
1950
1940
1932
Outermost ring is the year when the tree was cut.
Sample from a dead tree
in the same forest
1940
1932
1931
1926
Matching and overlapping older and older wood sections extends dates back in time
Sample from an old building in the same area as the forest
1931
1926
1920
1918
p. 143

47. Annual Rings

48. Secondary phloem Vascular cambium Late summerwood Annual ring of xylem
Figure 7.12: Annual ring.
This portion of a basswood (Tilia americana) stem, in cross section, shows one annual ring, or growth increment. Note the differences in size between the vessel elements of springwood and late summerwood.
Springwood
Cross section
of 3-year-old
Tilia stem
Late summerwood
of preceding year
Fig. 7-12, p. 144

49. Sections

50. (b) Tangential section
Annual
rings
(a) Cross section
Ray
(c) Radial section
(b) Tangential section
Figure 7.13: A block of wood showing (a) cross, (b) tangential, and (c) radial sections.
Both tangential and radial sections are longitudinal sections. A tangential section is cut in a plane that does not pass through the center of the stem but instead passes at a right angle to a radius. A radial section is cut in a plane that passes through the center along a radius of the stem. Rays and annual rings are distinctive for each section.
Rays
Annual
rings
Annual
rings
Rays
Fig. 7-13, p. 144

51. KEY TERMS DEFORESTATION
The temporary or permanent clearance of large expanses of forests for agriculture or other uses

52. Rain Forest Distribution

53. KEY TERMS
VINE
A plant with a long, thin, often climbing stem

54. LEARNING OBJECTIVE 6
Contrast the various stems that are specialized for asexual reproduction

55. KEY TERMS
RHIZOME
A horizontal underground stem that often serves as a storage organ and a means of sexual reproduction
Example: iris

56. A Rhizome

57. Rhizome Adventitious roots Fig. 7-14a, p. 147
Figure 7.14: Modified stems.
Fig. 7-14a, p. 147

58. KEY TERMS
TUBER
The thickened end of a rhizome that is fleshy and enlarged for food storage
Example: white potato

59. A Tuber

60. Rhizome
Figure 7.14: Modified stems.
Tuber
Roots
Fig. 7-14b, p. 147

61. KEY TERMS
BULB
A rounded, fleshy underground bud that consists of a short stem with fleshy leaves
Example: onion

62. A Bulb

63. Bulb Fleshy leaves Stem Adventitious roots Fig. 7-14c, p. 147
Figure 7.14: Modified stems.
Stem
Adventitious
roots
Fig. 7-14c, p. 147

64. KEY TERMS
CORM
A short, thickened underground stem specialized for food storage and asexual reproduction
Example: crocus

65. A Corm

66. Axillary bud Leaf scars Corm (modified stem) Old corm (last year’s)
Figure 7.14: Modified stems.
Old corm
(last year’s)
Adventitious
roots
Fig. 7-14d, p. 147

67. KEY TERMS
STOLON
An aerial horizontal stem with long internodes; often forms buds that develop into separate plants
Example: strawberry

68. Stolons

69. New Scale leaf shoot (at node) Stolon (runner) Adventitious roots
Figure 7.14: Modified stems.
Stolon
(runner)
Adventitious
roots
Fig. 7-14e, p. 147

70. Animation: Layers in a Woody Stem
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71. Animation: Growth in a Walnut Twig
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72. Animation: Annual Rings
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