1. STEMS Origin Functions External Anatomy Internal Anatomy
Specialized Stems
Physiology 1

2. Origin
First stem of a plant develops from part of a seed embryo called epicotyl, w/c is a continuation of the hypocotyl

3. The Plant Body: Stems FUNCTIONS OF STEMS
Produces & support appendages of plant (leaves, flowers, fruits)
transport water and solutes between roots and leaves.
Stems in some plants are photosynthetic.
Produce & store materials necessary for life (e.g., water, starch, sugar).
In some plants, stems have become adapted for specialized functions. 3

4. Stems support a display of leaves.
Stems orient the leaves toward the light with minimal overlap among the leaves. 4

5. The stem supports a display of flowers5

6. The stem does photosynthesis… and stores water. Opuntia-prickly pear6

7. Two Types of Aerial Stems
Herbaceous Stems
Woody Stems
Soft & green
Little growth in diameter
Tissues chiefly primary
Chiefly annual
Covered by epidermis
Buds mostly naked
Tough & not green
Considerable growth in diameter
Tissues chiefly secondary
Chiefly perennial
Covered by corky bark
Buds chiefly covered by scales

8. Herbaceous Stem woody stem

9. EXTERNAL ANATOMY 9

10. 10

11. STEM APICAL MERISTEM 11

12. 12

13. 13

14. 14

15. PRIMARY & SECONDARY GROWTH15

16. Apical Dominance
Apical dominance refers to the suppression of growth by hormones produced in the apical meristem. 
Lateral branch growth are inhibited near the shoot apex, but less so farther from the tip.
Apical dominance is disrupted in some plants by removing the shoot tip, causing the plant to become bushy. 16

17. INTERNAL STEM ANATOMY 17

18. Monocotyledonous & Dicotyledonous Flowering Plants18

19. Monocot Stem – cross section19

20. Typical Stem Cross Section (Dicot Stem)
Helianthus annuus-
sun flower annual
Epidermis
Cortex
A ring of vascular bundles
Pith 20

21. - window, reduce water loss
Epidermis
- window, reduce water loss
Cortex Collenchyma
- extensible support
Cortex Parenchyma
- photosynthesis, etc.
Fibers- rigid support
Functional Phloem
- conduct sugars etc. away from leaf to rest of plant
Vascular Cambium
- adds 2° xylem and 2° phloem
Xylem
conduct water and minerals up from soil
Pith
water storage, defense? 21

22. VIP Stem: Provide both name and function labels:
Epidermis: reduce evaporation, gas exchange
Cortex: photosynthesis, collenchyma support
Vascular Bundles: conduction
Pith: water storage? defense? disintegrate?
outside
to center
Vascular Bundle:
outside
to center
Phloem Fibers: support
Functional Phloem:
conduct CH2O away from leaf
Vascular Cambium:
add 2° Xylem and 2° Phloem
Xylem:
conduct minerals up from soil 22

23. Vitis vinifera - grape 23

24. Remnants of the procambium: Intrafasicular cambium
Notice how the vascular cambia of adjacent vascular bundles line up side by side.
Notice that cambium tissue differentiates between the bundles, connecting the cambia together.
Remnants of the procambium:
Intrafasicular cambium
Interfasicular cambium
Vitis vinifera - grape 24

25. The vascular cambium makes 2° tissues: Vitis vinifera - grape
2° phloem
2° xylem 25

26. Basswood – 1 & 2 years old 26

27. Three years of Secondary Growth
Tilia - basswood
Secondary
Phloem
cambium
Secondary
Xylem 27

28. A cork cambium differentiates and produces a periderm.
Epidermis
cutin
suberin
Cork Cells
Cork Cambium
Phelloderm 28

29. Over time, the epidermis dies.
The cork cells build up to for a thick layer for the bark of a tree. We use this to make stoppers for wine bottles and so on.
When suberin is fully developed, the cortex cells will eventually be in the dark. So these chloroplasts will lose their function! 29

30. epidermis + periderm + cortex + phloem + vascular cambium
Bark =
epidermis + periderm + cortex + phloem + vascular cambium
Wood =
secondary xylem only!
Pith =
a small percentage of tree diameter at maturity 30

31. Anatomy of a Woody Stem 31

32. The trees pictured below have long lost their epidermis on the woody portion of the stem
Sequoia sempervirens - giant sequoia 32

33. The study of the growth rings in wood: Dendrochronology33

34. Each year the cambium produces a layer of secondary xylem and a layer of secondary phloem.
This photo shows secondary xylem from parts of three years in Pinus strobus (white pine).
spring of the next year
winter of that year
fall of that year
mid-summer of one year 34

35. Modified & Specialized Stems35

36. Stolons
Stolons or runners - horizontal stem that grow above the ground with long internodes
Eg. Bermuda grass
(Cynodon dactylon)
Spider plant (Chlorophytum)
Fern (Nephrolepis)
Bermuda grass (Cynodon dactylon). 36

37. Rhizomes
Rhizomes - horizontal stems that grow below the ground with adventitious roots
Eg. irises, ferns, and grasses. 37

38. Food Storage Stems
Prickly Pear Cactus
Bamboo Shoots
Kohlrabi 38

39. Food Storage Stems - Sugarcane39

40. Tubers
Tubers – swollen regions of stems that store food for subsequent growth
The "eyes" of a potato (irish potatoes Solanum tuberosum) are the nodes of a starch-ladened stem 40

41. Rosette
Rosette - stem with short internodes and leaves attached at nodes 41

42. Wild Radish – Rosette & Bolt
A FLOWERING ANNUAL
YEAR ONE
YEAR ONE 42

43. Common Mullen – Rosette & Bolt
A FLOWERING BIENNIAL
YEAR ONE
YEAR TWO 43

44. Bulbs
Bulbs - large buds with a small stem at the lower end surrounded by numerous fleshy leaves that store nutrients; adventitious roots at base
Eg. onion, tulip, hyacinth, daffodil and lily 44

45. Corms
Corms - resemble bulbs but composed entirely of stem tissue surrounded by a few papery scale like leaves, food storage organs with adventitious roots at the base of corms
Eg. crocus and gladiolus. 45

46. Cladophylls
Cladophylls or cladodes - leaf-like stems modified for photosynthesis
Eg. butcher's broom, asparagus, orchids (eg. Epidendrum) 46

47. Succulent Stems
Succulent stems - stout fleshy stems that are modified for water and food storage
Eg. cacti 47

49. Thorns- for protection from grazing animals
Bougainvillea
Honey locust (modified stem)
Black Locust (modified leaf stipules) 49

50. Tendrils – for support Eg. Squash, ampalaya, cadena-de-amor
Grape Tendrils 50

52. PHYSIOLOGY OF STEMS Conduction of Materials by Xylem
1) Root pressure – powered by transpiration of water from the leaves
2) Transpiration pull and water cohesion – water is pulled up from the roots due to adhesion of water to the xylem walls & tension generated by the water-potential gradient bet. leaves & xylem
Other contributing factors:
3) Atmospheric pressure
4) Action of Living cells
5) Imbibition in cell walls of xylem
6) Capillary attraction

53. PHYSIOLOGY OF STEMS Conduction of Materials by Phloem
- nutrient-rich fluid in the phloem moves from areas of high solute concentration & water pressure to areas of low solute concentration & water pressure
Hypotheses of phloem function are:
1) cytoplasmic streaming
2) movement through interface
3) pressure flow or mass flow

54. Transpiration-Cohesion Hypothesis for Water Movement54

55. Sugar Loading of Phloem and Bulk Flow55

56. Sugar Loading of Phloem and Bulk Flow56