Download Dynamic Plant (Lecture 6

5/16/2012
Dynamic Plant
Focus on Stems
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.
Carnegiea gigantea - Saguaro cactus
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
Stipules - Paired, often
leaflike appendages at base
of a leaf
Growth
Apical meristem – increases length
Ground meristem – makes cortex & pith
Procambium – produces primary Xylem & Phloem
Vascular cambium – produces secondary Xylem & Phloem
Cork cambium/Phellogen – produces bark to reduce water
loss & protects stem (in woody plants only).
Bundle scars mark food
and water conducting
tissue within leaf scars.
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Origin and Development of Stems
Narrow band of cells between the primary xylem
and primary phloem may become vascular
cambium.
Internal Vascular
System
Similar to interconnected
pipes in a house.
Cells produced by the vascular cambium become
components of secondary xylem toward center and
secondary phloem toward surface.
Tissue Patterns in Stems
Cotyledons - Seed leaves attached to
embryonic stems
Function: Store food needed by young seedling
Tissue Patterns in Stems
Herbaceous Dicot Stems
Have discrete vascular bundles arranged in a cylinder.
Vascular cambium arises between primary xylem and
primary phloem.
- Adds secondary xylem and secondary phloem
Dicotyledons (Dicots) - Flowering plants that
develop from seeds having two cotyledons
Monocotyledons (Monocots) - Flowering plants
that develop from seeds with a single cotyledon
Dicot
stem
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Tissue Patterns in Stems – Wood
Black locust tree (longitudinal section)
Tissue Patterns in Stems - Monocots
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.
Cross
section
– Parenchyma (ground
of
tissue) surrounds
monocot
stem
vascular bundles.
Tissue Patterns in Stems - Monocots
In a typical monocot vascular bundle:
Transpiration and
cohesion tension theory
Method by which water moves from the roots to the
shoot system through the xylem.
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.
Loss of water out of stomata by evaporation.
Hydrogen bonds link water molecules together.
Water moves “up” and the xylem in a long chain.
Water molecules pull each other up one molecule at
a time from previous location below.
Column of water is under “tension.” - Cohesion
Monocot vascular bundle
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How Materials move in the Xylem
Water Stress
Occurs when a break in the water chain contained in
the xylem vessel elements breaks.
May occur:
When transpiration rates increase
During very hot, dry weather
Note- Extreme wilting can kill the plant.
Transport of Sugars
Sugars flow from:
Source (leaf) Sink
Sink = any structure that uses up sugars or
stores them e.g. fruits, roots, stems.
Pressure-flow theory relies on differential
hydrostatic pressure to move fluid through
the phloem cells.
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
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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
Prickly pear cactus
Formation of Vascular Tissues in
Woody Plants
Secondary meristem – Increase Girth
1)
2)
Vascular cambium: generates vascular tissues.
Cork cambium: generates protective outer covering,
including bark.
Elements:
Fusiform initials > Generates secondary Ph & Xy
Ray initials > Vascular rays (for lateral transport)
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Dendrochronology
Putting trees to work to see into the
past. “Tree time keeping.”
> Early Wood (less dense – rainy season)
> Late Wood (more dense – arid season)
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