Stem Growth

1. Primary Growth

Primary growth = increase in length of the plant (shoots and roots)

Occurs at:

  • Apical Meristem

Process:

  1. Cell division (mitosis) at apical meristem

  2. Cell elongation

  3. Cell differentiation into specialized tissues

Result:

  • Plant grows taller / longer

Key Structural Pattern

Monocots

  • Vascular bundles scattered

Dicots

  • Vascular bundles arranged in a ring

Each vascular bundle contains:

  • Xylem (water transport, inner side)

  • Phloem (sugar transport, outer side)

2. Apical Meristem (Shoot Growth Zone)

Functions:

  • Produces new cells

  • Responsible for primary growth

  • Forms:

    • leaves

    • stems

    • buds

Bud Types

  • Apical (terminal) bud → main vertical growth

  • Axillary buds → side branches

Apical Dominance

Apical bud suppresses lateral (axillary) buds

Effect:

  • Plant grows upward, not outward

If apical bud is removed:

  • Axillary buds grow → plant becomes bushier

3. Zones of Growth in Stems

1. Mitotic Zone

  • Rapid cell division

  • Contains:

    • Tunica → outer layers (surface growth)

    • Corpus → inner tissues

2. Elongation Zone

  • Cells increase in size

  • Pushes shoot upward → length increase

3. Differentiation Zone

Cells specialize into:

  • Protoderm → epidermis

  • Procambium → vascular tissue (xylem + phloem)

  • Ground meristem → cortex + pith

4. Secondary Growth

Secondary growth = increase in thickness (girth)

Occurs mainly in:

  • Dicots

  • Woody plants

Driven by:

  • Lateral meristems

Lateral Meristems

1. Vascular Cambium

  • Produces:

    • Secondary xylem (wood) inward

    • Secondary phloem outward

  • Causes stem thickening

2. Cork Cambium

  • Produces cork (protective layer)

  • Replaces epidermis in woody plants

5. Formation of Wood

Wood = secondary xylem

Two Regions:

Heartwood

  • Inner, dark

  • Non-functional (no water transport)

  • Provides structural support

Sapwood

  • Outer, lighter

  • Actively transports water

Growth Rings

Each year produces:

  • Early wood (spring) → light, thin walls

  • Late wood (summer/fall) → dark, thick walls

1 ring = 1 year of growth

6. Bark and Periderm

Bark = all tissues outside vascular cambium

Includes:

  • secondary phloem

  • cork

  • cork cambium

Periderm

Replaces epidermis in woody plants

Components:

  • cork

  • cork cambium

Lenticels

  • Small openings in bark

  • Allow gas exchange

7. Key Tissue Summary

Tissue

Function

Epidermis

protection

Cortex

storage/support

Pith

central storage

Xylem

water transport

Phloem

sugar transport

Vascular cambium

growth in thickness

Cork cambium

protection

8. Key AP Biology Concepts

  • Primary growth → length (apical meristem)

  • Secondary growth → thickness (cambium)

  • Dicots grow in girth; monocots generally do not

  • Xylem = water; phloem = sugars

  • Growth rings indicate age and environmental conditions

9. High-Yield Connections (Exam Focus)

  • Apical meristem → mitosis → elongation → differentiation

  • Cambium → secondary growth → wood + bark

  • Structure supports function:

    • xylem = upward transport

    • phloem = bidirectional sugar movement

No requirement to memorize:

  • detailed molecular pathways

  • specific enzyme names

  • complex intermediate steps

Focus on:

  • function, location, and overall processes