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Dynamic Plant
Focus on Stems
Carnegiea gigantea - Saguaro cactus
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.
Growth
Apical meristem – increases length
Ground meristem – makes cortex & pithProcambium – 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).
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Origin and Development of StemsNarrow 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.
Internal Vascular System
Similar to interconnected pipes in a house.
Tissue Patterns in Stems
Cotyledons - Seed leaves attached to
embryonic stems
Function: 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 StemsHerbaceous Dicot StemsHave discrete vascular bundles arranged in a cylinder.
Vascular cambium arises between primary xylem and primary phloem.
- Adds secondary xylem and secondary phloem
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.
– Parenchyma (ground tissue) surrounds vascular bundles.
Cross section
of monocot
stem
Tissue Patterns in Stems - Monocots
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
Transpiration and
cohesion tension theory
Method by which water moves from the roots to the
shoot system through the xylem.
� 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
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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 StemsRhizomes - 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 PlantsSecondary meristem – Increase Girth
1) Vascular cambium: generates vascular tissues.
2) Cork cambium: generates protective outer covering, including bark.
Elements:
Fusiform initials > Generates secondary Ph & Xy
Ray initials > Vascular rays (for lateral transport)