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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)

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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)