stems

1. Definition

Stem:
Plant organ that supports leaves, flowers, and fruits and conducts water, minerals, and sugars throughout the plant.

Primary roles:

• Structural support

• Transport of materials

• Growth and tissue production

• Storage (in some species)

• Photosynthesis (in specialized stems)

Stems may be:

• Above ground (most plants)

• Underground (modified stems such as tubers or rhizomes)

2. External Stem Structure

Nodes

• Points where leaves, buds, or branches originate.

Internodes

• Stem segments between nodes.

Buds

• Undeveloped shoots that can grow into:

  • leaves

  • flowers

  • branches

3. Internal Stem Structure (Tissues)

Three main tissue systems:

Epidermal Tissue

• Outer protective layer

• Made of flattened cells

• Often covered by a cuticle that reduces water loss.

Ground Tissue

Forms the bulk of the stem.

Main regions in dicots:

Hypodermis

• Located directly under epidermis

• Provides mechanical support.

Cortex

• Many layers of parenchyma cells

• Functions:

  • storage

  • support

  • metabolism

Pith

• Central region

• Loosely arranged parenchyma cells

• Storage and transport.

Vascular Tissue

Responsible for transport.

Two major components:

Xylem

• Conducts water and mineral salts upward from roots.

Cell types:

• tracheids

• vessel elements

• xylem fibers

• xylem parenchyma

Water transport mainly occurs through:

• tracheids

• vessel elements

Phloem

• Transports sugars (photosynthesis products) from leaves to the rest of the plant.

Cell types:

• sieve tubes

• companion cells

• phloem fibers

• phloem parenchyma

Main transport cells:

• sieve tubes

• companion cells

4. Monocot vs Dicot Stems

Dicot Stems

Examples:

• sunflower

• pea

• cucumber

• mustard

Characteristics:

• Cuticle present

• Hypodermis under epidermis

• Ground tissue divided into:

  • cortex

  • pith

• Vascular bundles arranged in a ring

• Cambium present

• Vascular bundles are open

Cambium

Meristematic tissue between xylem and phloem responsible for:

• Secondary growth

• Production of wood and bark

• Stem thickening

Dicots can grow into large trees.

Monocot Stems

Examples:

• corn

• rice

• wheat

Characteristics:

• Hypodermis present

• Ground tissue not differentiated

• No distinct pith or cortex

• Vascular bundles scattered

• No cambium

• Vascular bundles are closed

Result:

• Little or no secondary growth

• Most monocots remain herbaceous plants

5. Major Functions of Stems

1. Support

Stems:

• Hold leaves toward sunlight

• Support flowers and fruits

Stem modifications for support:

Runners

• Horizontal stems that spread along the ground

• Form roots at nodes

Examples:

• strawberry

• grasses

Suckers

• Underground lateral stems that emerge above ground.

Example:

• mint

Climbing Modifications

Twiners

• Stems twist around supports.

Example:

• beans

Tendrils

• Coiled structures used for attachment.

Example:

• grapevine

Spines

• Hard, sharp structures for defense.

Example:

• Prunus

2. Conduction (Transport)

Two transport pathways:

Upward Transport

Xylem

• Moves water and mineral salts

• Pathway:
  roots → stem → leaves

Driven partly by:

• transpiration

• capillary action

Downward Transport

Phloem

• Moves sugars and organic compounds

• Produced during photosynthesis

Pathway:
leaves → stems → roots / fruits / growing tissues

3. Storage

Some underground stems store nutrients.

These structures survive unfavorable seasons.

Tubers

Swollen underground stems with buds ("eyes").

Example:

• potato

Rhizomes

Horizontal underground stems with nodes and internodes.

Examples:

• ginger

• water lily

Corms

Short, swollen vertical underground stems.

Examples:

• taro

• crocus

Bulbs

Reduced stem surrounded by fleshy storage leaves.

Examples:

• onion

• tulip

4. Photosynthesis in Stems

In some plants leaves become spines to reduce water loss.

Examples:

• cactus

Adaptations of stems:

• green

• flattened

• thick for water storage

The stem becomes the primary photosynthetic organ.

Example:

• asparagus

5. Production of New Tissue

Stems contain meristems.

Meristems

Regions of actively dividing undifferentiated cells.

Characteristics:

• small cells

• thin cell walls

• no large vacuoles

Apical Meristems

Located at tips of shoots and roots.

Functions:

• produce new stem and leaf tissue

• allow primary growth (length increase)

Floral Meristem

Apical meristem can convert into a floral meristem when triggered by:

• day length

• temperature

• developmental signals

Produces flowers.

AP Biology Core Concepts to Remember

Stem functions

• support

• transport

• storage

• photosynthesis

• growth

Transport tissues

• Xylem → water and minerals

• Phloem → sugars

Stem anatomy difference