Organs

Table of Contents
1. Overview
2. Plant organs
3. Leaves
4. Internal structure of a typical dicot leaf
5. Adaptations of leaf structures
6. Movement of substances to and from the leaf
7. Regulation of gas movement by stomata
8. Summary
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Overview

The organs of plants are formed from organised groups of tissues. Each organ performs specific life functions because of its structure and the specialisation of its tissues. This chapter explains the structure and functions of plant organs with a focus on the leaf, its external and internal features, adaptations for gaseous exchange, photosynthesis and water regulation, and the movement of substances to and from the leaf.

Plant organs

Plant organs arise from different combinations of meristematic and permanent tissues. The principal plant organs are:

• Roots - anchor the plant, absorb water and minerals, and often store food.
• Stems - support the plant body and conduct water, minerals and organic food between roots and leaves.
• Leaves - main sites of photosynthesis, gaseous exchange and transpiration.
• Flowers - reproductive organs producing gametes and seeds.
• Fruits - develop from fertilised flowers and protect and disperse seeds.

Leaves

Leaves show wide variation in form and internal structure but share common features that enable photosynthesis, gas exchange and control of water loss. Two major leaf types are recognised:

• Monocotyledonous leaves (isobilateral) - usually have parallel venation, similar upper and lower surfaces, and often lack a distinct petiole.
• Dicotyledonous leaves (dorsiventral) - typically have pinnate or palmate venation and distinct dorsal (upper) and ventral (lower) surfaces; many have a petiole.

External features of a dicot leaf

• Lamina - the flat leaf blade that provides the photosynthetic surface.
• Apex - the tip or terminal point of the leaf.
• Petiole - the stalk that attaches the lamina to the stem.
• Margin - the edge of the leaf (entire, serrated, toothed, lobed, etc.).
• Midrib - the main central vein that supports the lamina and contains vascular tissues.
• Leaflet - one of the segments of a compound leaf.

Functions of leaves

• Photosynthesis - manufacture of organic food (sugars) using light energy.
• Gaseous exchange - uptake of carbon dioxide and release of oxygen for photosynthesis and respiration.
• Transpiration - loss of water vapour that helps in cooling, maintaining sap flow and mineral transport.

Internal structure of a typical dicot leaf

The internal tissues of the leaf are organised into three main regions: the epidermis, the mesophyll and the vascular bundles (veins).

Epidermis

The epidermis forms the outermost single-layered tissue on both upper (adaxial) and lower (abaxial) surfaces. Key features:

• Epidermal cells are generally transparent to allow light to pass to inner photosynthetic tissues.
• A waxy cuticle often covers the epidermis to reduce water loss.
• Specialised epidermal structures include stomata (pores) and their associated guard cells, which regulate gas exchange and transpiration.

Mesophyll

The mesophyll constitutes the bulk of the leaf and is the main photosynthetic tissue. It is differentiated into:

• Palisade parenchyma (palisade mesophyll) - elongated, closely packed cells rich in chloroplasts; primary site of light absorption and photosynthesis.
• Spongy parenchyma (spongy mesophyll) - loosely arranged cells with large intercellular air spaces that facilitate gas diffusion and storage of gases needed for photosynthesis and respiration.

Vascular bundles (veins)

Vascular bundles contain xylem and phloem.

• Xylem transports water and dissolved minerals from roots to leaves.
• Phloem transports organic solutes (mainly sugars) from photosynthetic regions (sources) to other parts of the plant (sinks).

Adaptations of leaf structures

Leaves are adapted structurally to perform gaseous exchange, photosynthesis and regulation of water loss efficiently. The following lists summarise important adaptations.

Adaptations for gaseous exchange

• Stomata - adjustable pores that control entry and exit of gases.
• Thin epidermis and thin leaf lamina to shorten diffusion distances.
• Large surface area to increase gas exchange and light capture.
• Spongy parenchyma with abundant intercellular spaces to provide an internal air reservoir for diffusion.
• Moist internal surfaces to allow gases to dissolve and diffuse across cell membranes.
• Vascular transport system to supply and remove gases and solutes rapidly.

Adaptations for photosynthesis

• Transparent epidermal cells and a thin cuticle to permit maximum light penetration.
• Large surface area and broad lamina to intercept light.
• Palisade parenchyma located beneath the upper epidermis with numerous chloroplasts for efficient light capture and carbon fixation.
• Spongy parenchyma that facilitates CO₂ diffusion to photosynthetic cells.
• Extensive vascular bundles to supply water and remove synthesized sugars.
• Many chloroplasts in mesophyll cells for high photosynthetic capacity.

Adaptations for water regulation (transpiration control)

• Thick cuticle in xerophytic species to reduce transpiration.
• Distribution of stomata (often fewer on the upper surface and more on the lower surface in many dicots) to reduce direct water loss.
• Small or rolled leaves and sunken stomata in dry environments.
• Regulatory mechanisms in guard cells that control stomatal opening and closing according to environmental conditions.
• Intercellular spaces that moderate internal humidity.

Movement of substances to and from the leaf

The main substances moving through and across the leaf include water, carbon dioxide, oxygen and sugars. Different transport processes operate depending on the substance.

Movement of carbon dioxide (CO₂)

Carbon dioxide required for photosynthesis enters the leaf from the atmosphere primarily through open stomata. Movements involved:

• Diffusion through the stomatal pore into the intercellular air spaces of the mesophyll.
• Diffusion across cell walls and plasma membranes into mesophyll cells where CO₂ is fixed in the Calvin cycle.
• Diffusion through the epidermis is possible but slower; stomata provide the rapid pathway.

Movement of water

Water reaches the leaf in the xylem of the vascular bundles and moves to photosynthetic cells and to the atmosphere as vapour. Pathways and processes:

• Water is carried to the leaf by the transpiration stream within xylem vessels and tracheids.
• Once water leaves the xylem it moves in the apoplast (cell walls and intercellular spaces) and symplast (through cell cytoplasm via plasmodesmata) toward sites of evaporation.
• Water enters cells across the plasma membrane by osmosis when the cell sap has lower water potential.
• Water evaporates from cell walls into intercellular spaces and diffuses out through stomata as water vapour - this process is called transpiration.

Movement of sugars

Sugars (mainly sucrose and other transportable forms of glucose) are produced in chloroplasts and distributed by the phloem. Key points:

• Sugars formed in mesophyll cells are loaded into phloem sieve tubes either by active transport or by facilitated diffusion depending on species and tissue.
• Phloem translocation distributes sugars from source regions (e.g., mature leaves) to sink regions (e.g., roots, growing shoots, developing seeds).
• Excess sugars are often transiently stored as starch in chloroplasts or other storage tissues and can be mobilised later for respiration and growth.

Movement of oxygen

Oxygen produced during photosynthesis diffuses from the chloroplasts into intercellular spaces and then to the atmosphere through stomata. Oxygen required for respiration can enter the leaf by diffusion through stomata and epidermis. Diffusion through stomata is faster than diffusion through intact epidermis.

Regulation of gas movement by stomata

Stomata are pores formed by a pair of specialised guard cells. The opening and closing of stomata regulate gaseous exchange (CO₂ in, O₂ out) and transpiration (water vapour loss). Guard cells change shape as their water and solute content change; this alters the stomatal pore size.

Mechanism (generalised): when guard cells accumulate solutes (for example, K⁺ ions and sugars), their water potential becomes lower, water enters by osmosis and the guard cells become turgid. The turgid guard cells curve apart because of their specialised cell wall thickness differences, and the pore opens. When solutes are removed, water leaves, the cells become flaccid and the pore closes. Guard-cell metabolism (including photosynthesis and ion transport) and environmental cues (light, CO₂ concentration, humidity, water availability) control these changes.

Figure 5.1 How do the stomata open and close?

Summary

Leaves are complex organs adapted structurally and physiologically to capture light energy, exchange gases and regulate water loss. The epidermis, mesophyll and vascular tissues each contribute to these functions. Stomata and guard cells provide dynamic control of gas exchange and transpiration, while xylem and phloem enable transport of water, minerals and organic nutrients between the leaf and the rest of the plant.