Stems have several functions including supporting leaves, flowers, and fruits; transporting water and nutrients between roots and other plant parts; and storing food. They originate from the epicotyl region of seed embryos. Herbaceous stems are soft and green while woody stems are tough with secondary growth. Stems have internal specialized tissues like xylem and phloem that conduct water and nutrients respectively. Some stems are modified for storage, protection, or reproduction.
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.
4. Stems support a display of
leaves.
Stems orient the leaves
toward the light with minimal
overlap among the leaves.
17. 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.
21. Typical Stem Cross Section (Dicot Stem)
Epidermis Helianthus annuus-
sun flower annual
Cortex
A ring of vascular bundles
Pith
22. 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?
23. 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?
Vascular Bundle:
to center outside
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 to center
25. 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
26. The vascular cambium makes 2° tissues:
Vitis vinifera - grape
2° xyl
2°
ph em
loe
m
28. Three years of Secondary Growth
Tilia - basswood
Secondary
c ambium
Phloem
Secondary
Xylem
29. A cork cambium differentiates and produces a periderm.
Epidermis
cutin
suberin
Cork Cells
Cork Cambium
Phelloderm
30. 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!
31. Bark =
epidermis +
periderm +
cortex +
phloem +
vascular
cambium
Wood =
secondary
xylem only!
Pith =
a small
percentage of
tree diameter
at maturity
33. The trees pictured below have long lost their
epidermis on the woody portion of the stem
Sequoia sempervirens - giant sequoia
34. The study of the growth rings in wood: Dendrochronology
35. 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
36. Stems
Stems—the axes of plants—consist of nodes (where
leaves and axillary buds are produced) separated by
internodes.
37. 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. 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
40. 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.
41. Rhizomes
• Rhizomes - horizontal
stems that grow
below the ground with
adventitious roots
• Eg. irises, ferns, and
grasses.
48. 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
49. Stolons
• Stolons or runners - Bermuda grass (Cynodon dactylon).
horizontal stem that
grow above the
ground with long
internodes
Eg. Bermuda grass
(Cynodon dactylon)
Spider plant (Chlorophytum)
Fern (Nephrolepis)
51. Wild Radish – Rosette & Bolt
A FLOWERING ANNUAL
YEAR ONE YEAR ONE
52. Common Mullen – Rosette & Bolt
A FLOWERING BIENNIAL
YEAR ONE
YEAR TWO
53. 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.
54. 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
55. 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
