sem-3

1. Prepared by:
Pooja Khanpara
APIP, Jamnagar
STEMS

2. STEMS
• Origin
• Functions
• External Anatomy
• Internal Anatomy
• Specialized
Stems

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

4. 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.

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

6. The stem supports a display of flowers

7. The stem does photosynthesis…
and stores water.
Opuntia-prickly pear

8. This stem does
photosynthesis, stores
water, but also produces a
defense chemical:
mescaline…a hallucinogen.
Lophophora williamsii -
peyote

9. 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

10. Herbaceous Stem woody stem

11. EXTERNAL ANATOMY

13. STEM APICAL MERISTEM

17. PRIMARY & SECONDARY GROWTH

18. 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(thick).

19. INTERNAL STEM
ANATOMY

20. Monocotyledonous &
Dicotyledonous Flowering Plants

21. Monocot Stem – cross section

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

23. 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?

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

25. Vitis vinifera - grape

26. Vitis vinifera - grape
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

27. Vitis vinifera - grape
The vascular cambium makes 2° tissues:
2° phloem2° xylem

28. Basswood – 1 & 2 years old

29. Three years of Secondary Growth
Tilia - basswood
Secondary
Xylem
Secondary
Phloemcambium

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

31. 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!

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

33. Anatomy of a Woody Stem

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

35. The study of the growth rings in wood: Dendrochronology

36. 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).
mid-summer of one year
fall of that year
winter of that year
spring of the next year

37. Stems
Stems—the axes of plants—consist of nodes (where
leaves and axillary buds are produced) separated by
internodes.

38. Node - region of the stem where the leaf and bud are
borne.
Internode - the part of the stem between two adjacent
nodes.
Herbaceous - not woody; dying down at the end of the
growing season.
Woody - hard in texture, containing secondary xylem, and
persisting more than one growing season.
Acaulescent - having an inconspicuous stem.
Caulescent - having a distinct stem.

39. Modified & Specialized Stems

40. 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

41. 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.

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

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

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

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

47. Food Storage Stems
Prickly Pear
Cactus
Bamboo Shoots Kohlrabi

48. Food Storage Stems - Sugarcane

49. 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

50. 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).

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

52. Wild Radish – Rosette & BoltWild Radish – Rosette & Bolt
YEAR ONEYEAR ONE YEAR ONEYEAR ONE
A FLOWERING ANNUALA FLOWERING ANNUAL

53. Common Mullen – Rosette & BoltCommon Mullen – Rosette & Bolt
YEAR ONEYEAR ONE
YEAR TWOYEAR TWO
A FLOWERING BIENNIALA FLOWERING BIENNIAL

54. Buds
Buds are short embryonic stems. In
angiosperms they are found at the nodes,
in the leaf axil (the angle formed by the
stem and the petiole of the leaf).
Axillary bud - a bud located in the leaf
axil.
Terminal bud - a bud at the apex of a
stem.

55. 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

56. 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

57. Transpiration-Cohesion Hypothesis
for Water Movement

58. Sugar Loading of Phloem and Bulk Flow

59. Sugar Loading of Phloem and Bulk Flow