Chapter 4
Lecture Outline
Stems
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Outline

External Form of a Woody Twig

Origin and Development of Stems

Tissue Patterns in Stems
• Herbaceous Dicotyledonous Stems
• Woody Dicotyledonous Stems
• Monocotyledonous Stems

Specialized Stems

Wood and Its Uses
External Form of A Woody Twig

Cells produced by stem meristem become
shoot system with branches and leaves.

Woody twig consists of an axis with attached
leaves.
• Node - Area of stem where leaves are attached
–
Alternate or spiral
–
Opposite - Attached in pairs
–
Whorled - In groups of three or more
• Internode - Stem region between nodes
• Leaf has a flattened blade and is usually attached
to the twig by petiole.
External Form of A Woody Twig

Axil - Angle between
petiole and stem
• Axillary Bud located in axil.
– Will become branches or
flowers in flowering plants
– Bud scales protect buds.

Terminal Bud at twig tip
• Growth makes twig longer.
• Number of groups of bud
scale scars tells age of twig.

Stipules - Paired, often
leaflike appendages at
base of a leaf
External Form of A Woody Twig

Deciduous trees and
shrubs (lose all leaves
annually) - After
leaves fall, have
dormant axillary buds
with leaf scars below
• Bundle scars mark food
and water conducting
tissue within leaf scars.
Origin and Development of Stems

Apical meristem at
stem tip
• Contributes to increase
in stem length
• Dormant before growing
season begins
• Protected by bud scales
and by leaf primordia
–
Leaf primordia - Tiny
embryonic leaves that
develop into mature leaves
Longitudinal section
through stem tip
Origin and Development of Stems

Apical meristem cells
form 3 primary
meristems.
• Protoderm - Gives rise
to epidermis
• Procambium - Produces
primary xylem and
phloem
• Ground Meristem -
Produces pith and
cortex, both composed
of parenchyma cells
Longitudinal section
through stem tip
Origin and Development of Stems

Leaf primordia and bud
primordia develop into
mature leaves and buds.
• Traces branch off from
cylinder of xylem and
phloem, and enter leaf or
bud.
–
Trace = strand of xylem
and phloem
–
Each trace leaves a gap filled with parenchyma in the
cylinder of vascular tissue, forming leaf gap or bud
gap.
Origin and Development of Stems

Narrow band of cells between the primary
xylem and primary phloem may become
vascular cambium.
• Cells produced by the vascular cambium become
components of secondary xylem toward center
and secondary phloem toward surface.
Origin and Development of Stems

In many plants cork cambium (phellogen)
produces cork cells with suberin and
phelloderm cells.
• Function to reduce water loss and to protect stem
against injury
• Lenticels - Parenchyma cells in cork for
exchange of gases
Tissue Patterns in Stems
Steles

Stele - Central cylinder of primary xylem,
primary phloem, and pith (if present)
• Protostele - Solid core, phloem surrounds xylem
–
In primitive seed plants, whisk ferns, club mosses and
ferns
• Siphonosteles - Tubular with pith in center
–
Common in ferns
• Eusteles - Discrete vascular bundles
–
In flowering plants and conifers
Tissue Patterns in Stems

Cotyledons - Seed leaves attached to
embryonic stems
• Store food needed by young seedling

Dicotyledons (Dicots) - Flowering plants that
develop from seeds having two cotyledons

Monocotyledons (Monocots) - Flowering
plants that develop from seeds with a single
cotyledon
Tissue Patterns in Stems
Herbaceous Dicotyledonous Stems

Annuals - Plants that die after going from
seed to maturity within one growing season
• Usually green, herbaceous plants
• Most monocots are annuals, but many dicots are
also annuals.
• Tissues largely primary
Tissue Patterns in Stems
Herbaceous Dicotyledonous Stems

Herbaceous dicots have discrete vascular
bundles arranged in a cylinder.

Vascular cambium arises between primary
xylem and primary phloem.
• Adds secondary xylem and secondary phloem
Tissue Patterns in Stems

Woody Dicotyledonous Stems
Wood = Secondary xylem

Differences in wood:
• Vascular cambium and cork cambium active all year:
–
–
Ungrained, uniform wood produced
Some tropical trees
• If wood produced seasonally:
–
–
–
In spring: Relatively large vessel elements of secondary
xylem produced - Spring wood.
After spring wood: Fewer, smaller vessel elements in
proportion to tracheids and fibers - Summer wood.
In conifers, vessels and fibers absent
o Tracheids in spring larger than later in season
Tissue Patterns in Stems
Woody Dicotyledonous Stems

One year’s growth of xylem = annual ring
• Vascular cambium produces more secondary
xylem than phloem.
–
Bulk of tree trunk consists of annual rings of wood.
• Indicates age of a tree
• Indicates climate during tree’s lifetime

Vascular Rays consist of parenchyma cells
that function in lateral conduction of
nutrients and water.
• Xylem Ray - Part of ray within xylem
• Phloem Ray - Part of ray through phloem
Tissue Patterns in Stems
Woody Dicotyledonous Stems
Cross section of
young stem
with secondary
growth
Tissue Patterns in Stems
Woody Dicotyledonous Stems
3-D view of
dicot wood
Tissue Patterns in Stems
Woody Dicotyledonous Stems

Tyloses - Protrusions of adjacent parenchyma
cells into conducting cells of xylem
• Prevent conduction of
water
• Resins, gums, and
tannins accumulate, and
darken wood, forming
heartwood.
–
Heartwood - Older, darker
wood in center
–
Sapwood - Lighter, stillfunctioning xylem closest
to cambium
Tissue Patterns in Stems
Woody Dicotyledonous Stems

Softwood - Wood of conifers
• No fibers or vessel elements

Hardwood - Wood of dicot trees

Resin canals - Tubelike canals scattered
throughout xylem and other tissues
• Lined with specialized
cells that secrete resin
• Common in conifers
• In some tropical
flowering plants
–
Frankincense and myrrh
Resin
canals
in pine
Tissue Patterns in Stems
Woody Dicotyledonous Stems

Bark - Tissues outside the vascular cambium,
including secondary phloem
• Mature bark may consist of alternating layers of
crushed phloem and cork.
Cross
section of
young
stem with
secondary
growth

Laticifers - Ducts found mostly in phloem that
have latex-secreting cells
•
Rubber, chicle (chewing gum), morphine
Tissue Patterns in Stems
Monocotyledonous Stems

Monocots stems have neither a vascular
cambium nor a cork cambium.
• Produce no secondary vascular tissues or cork
• Primary xylem and phloem in discrete vascular
bundles scattered throughout the stem
–
–
Vascular bundles
oriented with xylem
closer to center of
stem and phloem
closer to surface.
Parenchyma (ground
tissue) surrounds
vascular bundles.
Cross
section
of
monocot
stem
Tissue Patterns in Stems
Monocotyledonous Stems

In a typical monocot vascular bundle:
• Two large vessels with several small vessels
• First formed xylem cells
stretch and collapse.
–
Leave irregularly shaped
air space
• Phloem consists of
sieve tubes and
companion cells.
• Vascular bundle
surrounded by sheath
of sclerenchyma cells.
Monocot vascular bundle
Specialized Stems

Rhizomes - Horizontal stems
that grow below-ground and
have long to short internodes
• Irises, some grasses, ferns

Runners - Horizontal stems
that grow above ground
and have long internodes.
•

Strawberry
Stolons - Produced beneath the
surface of the ground and tend
to grow in different directions.
• Potato
Specialized Stems

Tubers - Swollen, fleshy,
underground stem
• Store food
• Potatoes - Eyes of potato are
nodes

Bulbs - Large buds
surrounded by numerous
fleshy leaves, with a small
stem at lower end
• Store food
• Onions, lilies, hyacinths, tulips
Specialized Stems

Corms - Resemble bulbs,
but composed almost
entirely of stem tissue,
with papery leaves
• Store food
• Crocus and gladiolus

Cladophylls Flattened, leaf-life
stems
• Greenbriars, some
orchids, prickly
pear cactus
Prickly pear cactus
Wood and Its Uses

In a living tree, 50% of the wood weight
comes from water content.
• Dry part of wood composed of about 60-75%
cellulose and about 15-25% lignin.

Density - Weight per unit volume

Durability - Ability to withstand decay
• Tannins and oils repel decay organisms.
Wood and Its Uses
Types of Sawing

Radially cut
(quartersawed) boards
show annual rings in
side view.

Tangentially cut (plainsawed or slab cut)
boards are cut
perpendicular to rays.
• Show annual rings as
irregular bands of light
and dark streaks
Wood and Its Uses

Knots - Bases of lost branches covered by
new annual rings produced by the cambium

Wood Products
• About half of U.S. and Canadian wood production is
used as lumber, primarily for construction.
–
–
Sawdust and waste used for particle board and pulp.
Veneer - Thin sheet of desirable wood glued to cheaper
lumber
• Pulp is second most widespread use of wood.
– Paper, synthetic fibers, plastics, linoleum
• In developing countries, about half of cut timber is
used for fuel.
–
Less than 10% in US and Canada.
Review

External Form of a Woody Twig

Origin and Development of Stems

Tissue Patterns in Stems
• Herbaceous Dicotyledonous Stems
• Woody Dicotyledonous Stems
• Monocotyledonous Stems

Specialized Stems

Wood and Its Uses