Table of Contents
Overview: Plant Body as a Transport System
For understanding transport in plants, it is useful to see the whole plant body as a network of connected compartments. The main organs (roots, stems, leaves, flowers, fruits) are built from tissues that either:
- Absorb substances (e.g. water and mineral salts),
- Transport substances (e.g. water, ions, sugars),
- Store or use substances (e.g. starch, structural materials).
This chapter introduces those basic components that are especially important for transport: organs, tissues, and the main transport pathways.
Main Plant Organs Relevant to Transport
Roots
Key roles in transport:
- Uptake
- Absorb water and dissolved mineral salts from the soil.
- Root hairs greatly enlarge the surface area, improving uptake.
- Entry into the transport system
- Water and ions move from the soil into the root cortex and then into the central cylinder, where the xylem begins.
- At the endodermis (a special cell layer) a selective barrier (Casparian strip) controls which substances can enter the vascular tissue.
- Storage and anchoring
- Many roots (e.g. carrots, beets) store carbohydrates that can later be transported to other organs.
- Firm anchorage stabilizes the whole transport system against mechanical forces like wind.
Stems and Shoots
Key roles in transport:
- Conduction pathways
- Contain the main vascular bundles with xylem and phloem.
- Xylem conducts water and mineral salts upward.
- Phloem distributes organic substances, especially sugars, throughout the plant.
- Mechanical support
- Hold leaves in favorable positions for photosynthesis.
- Support the vertical water column in tall plants.
- Connection between organs
- Link roots (source of water and ions) with leaves (source of sugars) and storage/consuming organs (fruits, seeds, growing tissues).
Leaves
Key roles in transport:
- Sites of photosynthesis
- Produce sugars and many organic substances that then enter the phloem.
- Gas and water exchange
- Contain stomata (pores) that regulate CO₂ uptake and water vapor loss (transpiration).
- Transpiration creates a “pull” that helps draw water upward through the xylem.
- Veins (leaf vascular bundles)
- Fine network of xylem and phloem.
- Xylem brings water and minerals to photosynthetic cells.
- Phloem collects sugars and transports them away to other parts of the plant.
Flowers, Fruits, and Seeds (Brief Transport Relevance)
- Flowers
- Short-lived, but supplied by xylem and phloem from the stem.
- Fruits and seeds
- Act as strong sinks for sugar and other nutrients during development.
- Phloem delivers building materials and energy reserves (e.g. starch, oils, proteins).
Basic Plant Tissues and Their Roles in Transport
Plant organs are built from three broad tissue systems, each contributing to transport in different ways.
Dermal Tissue System
This forms the outer covering of the plant.
- Epidermis
- Single outer cell layer on young roots, stems, and leaves.
- In leaves, the epidermis contains stomata (guard cells) that control gas exchange and water loss, indirectly affecting water transport.
- Periderm (bark)
- Replaces epidermis in older stems and roots.
- Less directly involved in transport, but protects inner tissues and limits uncontrolled water loss.
- Root hairs
- Extensions of epidermal cells in roots.
- Greatly increase the contact area with soil water, enhancing uptake into the transport system.
Ground Tissue System
This includes tissue filling the interior between dermal and vascular systems.
- Cortex (in roots and stems)
- Often used for storage (e.g. starch).
- Pathway through which water and ions pass on their way from epidermis to xylem.
- In roots, includes the endodermis, a specialized inner layer that controls entry into the vascular cylinder.
- Pith (in stems)
- Central region inside vascular tissue in many stems.
- Mainly storage and mechanical support; limited direct long-distance transport.
- Mesophyll (in leaves)
- Main photosynthetic tissue.
- Produces sugars that diffuse into nearby phloem cells for long-distance transport.
Vascular Tissue System (Transport Tissues)
The vascular tissue system is central to transport processes and consists of two main types: xylem and phloem.
Xylem: Water and Mineral Transport
Functions:
- Conducts water and dissolved mineral salts mainly upward from roots to shoots and leaves.
- Provides significant mechanical support due to thick, lignified cell walls.
Key cell types:
- Tracheids
- Long, narrow cells with tapered ends.
- Overlapping ends and pits in the walls allow water to move from cell to cell.
- Vessel elements (common in flowering plants)
- Shorter, wider cells stacked end-to-end to form vessels.
- End walls are partially or completely open, allowing more efficient water flow.
General features:
- Mature conducting cells are typically dead, forming hollow tubes.
- Lignin in cell walls strengthens the tubes and helps resist collapse under the tension generated by transpiration.
Phloem: Transport of Organic Substances
Functions:
- Transports mainly sucrose and other organic substances from sources (e.g. mature leaves) to sinks (e.g. roots, developing fruits, growing shoots).
- Transport can be upward or downward, depending on where sources and sinks are located.
Key cell types:
- Sieve tube elements
- Main conducting cells in many vascular plants.
- Arranged end-to-end to form sieve tubes.
- End walls are sieve plates with pores that allow cytoplasm continuity between cells.
- Lack a nucleus at maturity, relying on companion cells.
- Companion cells
- Living cells closely associated with sieve tube elements.
- Contain nucleus and many organelles.
- Help load and unload sugars into sieve tubes and maintain sieve tube metabolism.
- Phloem parenchyma
- Storage and lateral transport within the phloem region.
Arrangement of Vascular Tissue in Different Organs
Roots
- Vascular tissues are grouped in a central cylinder (stele).
- Xylem often forms a star-shaped core, with phloem between the arms of the star (in many herbaceous dicot roots).
- The endodermis with its Casparian strip surrounds the stele, forcing water and solutes to cross cell membranes before entering xylem and phloem.
Stems
- Herbaceous dicots
- Vascular bundles arranged in a ring near the periphery.
- Each bundle contains xylem on the inside, phloem on the outside; between them may lie cambium (a meristem that can produce new xylem and phloem in woody plants).
- Monocots
- Vascular bundles scattered throughout the stem cross-section.
- Organization within each bundle still typically has xylem and phloem adjacent.
- This arrangement defines the main vertical “pipes” that connect roots and leaves.
Leaves
- Vascular tissues form leaf veins, visible as the network of lines.
- Each vein:
- Contains xylem (typically towards the upper side of the leaf) and phloem (towards the lower side).
- Is surrounded by support and sometimes photosynthetic tissues.
- Fine veins ensure that every photosynthetic cell is close to both water supply (xylem) and sugar export routes (phloem).
Source and Sink Regions in the Plant Body
Although details of transport mechanisms belong to later chapters, it is useful to label the plant’s basic components as:
- Sources (net exporters of organic substances)
- Mature, photosynthesizing leaves.
- Storage organs during periods of mobilization (e.g. tubers in spring).
- Sinks (net importers of organic substances)
- Growing roots and shoots.
- Developing flowers, fruits, and seeds.
- Storage organs during periods of accumulation.
These sources and sinks are connected by the xylem–phloem network that runs through all plant organs.
Summary of Key Components for Transport
- Organs:
- Roots: uptake and initial entry of water and minerals.
- Stems: main long-distance conduits.
- Leaves: main production sites for sugars and main sites of transpiration.
- Tissues:
- Dermal tissue: exchange with environment (e.g. root hairs, stomata).
- Ground tissue: passageways, storage, and sites of photosynthesis.
- Vascular tissue: xylem (water and mineral transport) and phloem (organic transport).
Together, these basic components form a continuous, integrated system that allows substances to be moved throughout the plant, enabling growth, metabolism, and adaptation to changing environmental conditions.