Support and Transport Systems in Plants

Table of Contents
1. Anatomy of dicotyledonous plants
2. Support and transport tissues in plants
3. Root anatomy
4. Stem anatomy
5. Secondary growth in woody plants
6. Processes in plants
7. Uptake of water and mineral salts
8. Practical investigations
9. Summary
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Plants are sessile organisms that remain fixed in one place. Because they cannot move away from changing environmental conditions, their internal structure must provide mechanical support, protection and systems to transport water, mineral salts and manufactured food between roots, stems and leaves. This chapter explains the main support tissues, the transport tissues (xylem and phloem), root and stem anatomy, mechanisms of water and solute movement, and simple practical investigations used to demonstrate these processes.

Anatomy of dicotyledonous plants

Seed-plant groups

• Gymnosperms - non-flowering seed plants; examples include pines and cycads.
• Angiosperms - flowering plants. Angiosperms are divided into:
  • Dicotyledons (dicots) - usually have two cotyledons in the seed and a characteristic arrangement of vascular tissue in stems and roots.
  • Monocotyledons (monocots) - usually have one cotyledon and different vascular arrangements (not covered in detail here).

Plant tissues - overview

Plant tissues are groups of cells that carry out specific functions. Major tissues referred to in this chapter are:

• Meristematic tissue - actively dividing cells at growing tips (apical meristems) and in the vascular cambium; they give rise to other tissues.
• Epidermal tissue - the outer protective layer of cells; may have root hairs, stomata or a cuticle.
• Parenchyma - general-purpose living cells for storage and short-distance transport; may store starch or other substances.
• Chlorenchyma - parenchyma containing chloroplasts; main site of photosynthesis in leaves.
• Aerenchyma - parenchyma with large air spaces, common in aquatic plants; allows gas diffusion.
• Collenchyma - living cells with unevenly thickened primary walls; provide flexible mechanical support in young stems and leaves.
• Sclerenchyma - cells with thick, lignified secondary walls; dead at maturity and provide rigid support and protection (fibres and sclereids).
• Xylem - vascular tissue that transports water and dissolved mineral salts from roots to shoots; provides mechanical strength.
• Phloem - vascular tissue that transports manufactured organic food (mainly sucrose) from leaves to sinks such as roots, growing points and storage organs.

Support and transport tissues in plants

Collenchyma and sclerenchyma

Collenchyma is found beneath the epidermis of young stems and petioles. Cells are living, elongated and have unevenly thickened cellulose-rich primary walls; they provide flexible support allowing stems to bend without breaking.

Sclerenchyma provides rigid support and protection. Sclerenchyma cells develop thick secondary walls impregnated with lignin and are usually dead at maturity. Sclerenchyma occurs as fibres (long, narrow cells) and sclereids (short, variable-shaped cells). In roots and stems sclerenchyma often forms a protective pericycle or cap around vascular bundles.

Xylem tissue

Xylem transports water and dissolved mineral salts and contributes to the mechanical strength of stems and roots. Xylem is composed of several types of cells: xylem vessels, tracheids, xylem fibres (sclerenchyma) and xylem parenchyma.

Phloem tissue

Phloem transports organic compounds produced by photosynthesis (mainly sucrose) from source tissues (leaves) to sinks (roots, growing tissues, storage organs). Phloem is made up of sieve tubes, companion cells, phloem fibres and phloem parenchyma.

Root anatomy

Functions of root systems

• Anchorage - roots fix the plant in the soil so it resists wind and rain.
• Support - they stabilise stems and leaves.
• Storage - roots can store food reserves (e.g. carrots, sweet potato, radish).
• Nutrient uptake - roots absorb water and dissolved mineral ions from the soil.
• Transport - roots translocate water and minerals to the shoot system via xylem.
• Vegetative reproduction - some roots produce new plants from modified roots.

Origin and types of root systems

In dicotyledonous plants the first root to emerge from the germinating seed is the radicle, which develops into the primary (tap) root and gives rise to lateral roots, forming a tap-root system. In many monocotyledons the radicle is short-lived and is replaced by many adventitious roots forming a fibrous root system.

Support and transport tissues in roots

Roots contain collenchyma, sclerenchyma and vascular tissues (xylem and phloem). Xylem and associated sclerenchyma provide mechanical support and form the stele that conducts water and mineral salts to the shoot. Phloem transports organic food from the leaves to the root for growth and storage.

Typical dicotyledonous root structure (external and internal)

Key tissues and their primary functions in a dicot root:

• Epidermis - outermost layer; root hairs arise from epidermal cells and greatly increase the absorptive surface area for water and ions.
• Exodermis / cortex - outer cortex often contains parenchyma for storage and may include collenchyma for strength.
• Endodermis - inner cell layer surrounding the stele; contains the Casparian strip (waxy band) which forces water and solutes to pass through living cells (symplast) rather than between cells (apoplast) before entering the stele.
• Pericycle - a layer just inside the endodermis that can produce lateral roots and may contain sclerenchyma for protection of the stele.
• Stele (vascular cylinder) - contains xylem and phloem; the central xylem conducts water upward and the phloem conducts organic solutes to the root.
• Root hairs - specialised epidermal outgrowths where most water and mineral absorption occurs.

Stem anatomy

Plant growth forms based on stems

• Herbs - non-woody stems.
• Shrubs - woody, several stems from the base, usually shorter than trees.
• Trees - woody, typically one main stem and tall height.
• Vines - climbing or twining stems, may be herbaceous or woody.

Functions of stems

• Transport - move water, dissolved minerals and sugars between roots and leaves (xylem and phloem).
• Support and positioning - hold leaves, flowers and fruits in favourable positions for light capture and reproduction.
• Storage - store nutrients and water in stems or specialised stem structures.
• Asexual reproduction - vegetative propagation from stems (runners, tubers, bulbs).
• Protection - thorns or thick bark protect the plant.

Origin of stems

In dicotyledons the shoot develops from the embryonic plumule; the stem arises from the epicotyl and grows by activity at apical meristems. In monocotyledons protective structures such as coleoptile/coleorhiza surround the growing shoot and roots during germination.

Stem tissues for strength and transport

Support and strength are provided by collenchyma, sclerenchyma and xylem in both woody and herbaceous stems. Transport of water and organic solutes takes place in vascular bundles that contain xylem and phloem. In dicot stems vascular bundles are usually arranged in a ring; a vascular cambium between xylem and phloem produces secondary vascular tissues during secondary growth.

Typical dicotyledonous stem structure (external and internal)

Principal tissues and features:

• Epidermis with a cuticle - outer protective layer reducing water loss; may contain stomata and lenticels in woody stems.
• Cortex - composed mainly of parenchyma for storage and may include collenchyma near the surface for support.
• Endodermis / starch sheath - a layer that can regulate movement of substances into vascular tissues.
• Pericycle / pericyclic cap - sclerenchyma fibres that protect the vascular bundle and strengthen the stem.
• Vascular bundles - arranged in a ring in dicot stems; each bundle has xylem (inner side) and phloem (outer side) with vascular cambium between them.
• Vascular cambium - a ring of meristematic cells between xylem and phloem that produces secondary xylem (wood) and secondary phloem, causing stems to thicken.
• Pith (medulla) - central parenchyma region for storage and transport within the stem.

Secondary growth in woody plants

Perennial plants (those that live for more than two years) increase in girth by secondary growth. Secondary growth is due to activity of lateral meristems: the vascular cambium produces secondary xylem (wood) and secondary phloem, while the cork cambium (phellogen) produces cork (phellem) that replaces the epidermis and forms part of the bark.

Cork cambium and bark

The cork cambium arises from parenchyma cells in the outer cortex. It produces cork cells with suberised walls that form a protective layer (bark). Bark protects the stem from desiccation, pathogens and mechanical damage while the plant continues to grow in diameter.

Annual rings, heartwood and sapwood

In temperate woody stems, alternating growth seasons produce visible annual rings in cross-section. Each ring usually contains lighter, less dense spring/summer wood and darker, denser late-season (winter) wood. The central older xylem becomes heartwood (often resin-filled and darker) and provides structural support; the outer, functional xylem conducting water is called sapwood.

Lenticels

Lenticels are small spongy openings that arise in the cork and facilitate gaseous exchange between internal living tissues and the atmosphere in woody stems.

Processes in plants

Transpiration

Transpiration is the loss of water vapour from plant aerial parts, mainly leaves. Water is lost through:

• Stomata - stomatal (or stomatal) transpiration through stomatal pores controlled by guard cells.
• Cuticle - cuticular transpiration through the waxy cuticle covering the epidermis.

Evaporation of water from mesophyll cell walls creates a tension (suction) known as transpiration pull, which helps draw water up the xylem from roots to leaves. Large-scale transpiration also plays an important role in the water cycle.

Relationship between leaf structure and water loss

• Cuticle - a thicker waxy cuticle reduces cuticular transpiration.
• Guard cells - control opening and closing of stomata and therefore control stomatal transpiration.
• Number and location of stomata - many plants have more stomata on the shaded lower surface of leaves, reducing direct water loss.
• Leaf position and shading - overlapping leaves or leaf orientation can reduce evaporation.
• Leaf size - larger leaves have more surface area and tend to lose more water than small leaves.
• Leaf hairs (trichomes) - hairs trap a layer of still air and reduce transpiration by diffusion.

Stomatal mechanism

The opening and closing of stomata depends on changes in guard-cell turgidity driven by osmotic movement of water related to photosynthesis and metabolism in the guard cells. A simple day-night comparison is:

Effect of environmental factors on transpiration

Transpiration rate changes with environmental conditions:

Wilting and guttation

Wilting occurs when water loss by transpiration exceeds water uptake by roots; cells lose turgor and the plant droops. Prolonged wilting can lead to plant death by dehydration.

Guttation is the exudation of liquid water droplets from special pores (hydathodes) on leaf margins or tips. It occurs when root pressure forces water up the plant and transpiration is low (for example at night), so water is expelled as drops rather than vapour.

Uptake of water and mineral salts

Absorption from soil into roots

Water is absorbed mainly by root hairs where the root surface area is greatly increased. Water enters root hair cells by osmosis - the passive movement of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential.

Mineral ions are absorbed in solution. Some ions enter passively with the mass flow of water, but many essential ions (e.g. nitrate, potassium) are taken up by active transport across cell membranes using metabolic energy.

Pathways of water movement through the root into the stele

Water may reach the central vascular cylinder (stele) by three pathways:

• Apoplast pathway - through cell walls and intercellular spaces without crossing membranes; rapid but blocked at the Casparian strip in the endodermis.
• Symplast pathway - through the living cytoplasm of cells connected by plasmodesmata; water must cross membranes to enter the cells initially and then flows cell-to-cell via cytoplasm.
• Transmembrane (transcellular) pathway - water repeatedly crosses cell membranes and vacuolar membranes (tonoplast) as it moves from cell to cell.

The Casparian strip in endodermal cell walls contains suberin and forces water and dissolved ions to move through living cells (symplast) and pass through membrane-mediated transport before entering the xylem; this allows the plant to control ion uptake.

Transport from the stele to the leaves

Water and dissolved mineral salts move from root xylem up the stem to leaves by a combination of mechanisms:

• Root pressure - osmotic entry of water into the root xylem can generate a positive pressure that pushes water upward a short distance; visible in small plants as guttation.
• Capillarity - adhesive attraction of water to xylem walls and the narrowness of vessels and tracheids can assist upward movement of water over short distances.
• Cohesion-tension (transpiration pull) - cohesion between water molecules and adhesion to xylem walls transmits the tension (negative pressure) created by evaporation at the leaf surface down the continuous water column in xylem, effectively drawing water up from the roots to the leaves; this is the dominant mechanism in tall plants.

Translocation of manufactured food (phloem transport)

Phloem translocation moves sucrose and other organic solutes from sources (leaves and storage organs) to sinks (growing tissues, roots, fruits, storage tissues). Key points:

• Loading - companion cells actively load sucrose into sieve-tube elements in source regions; this requires chemical energy.
• Osmotic influx of water into loaded sieve tubes increases turgor pressure at the source; this creates a pressure gradient along the sieve tube toward sinks where sucrose is removed.
• Mass flow (pressure-flow) hypothesis - describes bulk movement of phloem sap driven by pressure differences between source and sink; water follows solute movement and sap flows from high-pressure (source) to low-pressure (sink) regions.

Practical investigations

Aim: Investigate root pressure

Apparatus: pot plant, clamp, rubber tube, ruler, water reservoir, stem segment or cut shoot.

Procedure (outline): attach a cut stem or rubber tubing to the severed stem and connect to a water reservoir or capillary tube. Observe water emerging as droplets or movement of an air bubble up the tube.

Result and interpretation: root pressure, generated by osmotic uptake of water into the root xylem, can push water up a short distance in the stem and out of a cut surface; root pressure is most noticeable when transpiration is low.

Aim: Investigate capillarity

Apparatus: narrow glass capillary tubes of different diameters, water.

Procedure: place capillary tubes vertically in water and measure the height of the water column in each tube.

Result: water rises higher in narrower capillaries due to the balance of adhesive and cohesive forces; this demonstrates capillary action that assists water movement in xylem vessels and tracheids.

Aim: Measure rate of transpiration

Apparatus: leafy shoot, cut stem in a capillary tube connected to a water reservoir and containing an air bubble, ruler to measure bubble movement, stopwatch.

Measurement: the speed of movement of the air bubble (mm s^-1) multiplied by cross-sectional area of the capillary tube (mm²) gives the volumetric rate of water uptake (mm³ s^-1).

Typical observations: the rate of transpiration increases under high temperature, wind and low humidity; it decreases under low temperature, still air and high humidity.

Aim: Investigate movement of water into the root

Apparatus: plant tissue or stem segment, sugar solution, water, glass tube, oil layer to prevent evaporation, rubber stopper sealed with petroleum jelly, beaker.

Procedure (outline): set up a system to observe movement of water into the tissue and the relative position of solutions; use control and experimental preparations to compare osmotic movements.

Result: endosmotic movement of water into tissue causes measurable levels or pressure changes. Controls without living tissue show no similar movement; living cells with active transport and selective membranes can cause net water uptake leading to root pressure or swelling.

Summary

Plant support and transport systems combine specialised tissues (collenchyma, sclerenchyma, xylem and phloem) and physiological mechanisms (osmosis, active transport, capillarity, root pressure and transpiration pull) to move water, mineral salts and manufactured food between roots and shoots and to provide mechanical strength. Understanding tissue structure and these transport processes is essential to explain how plants grow, survive environmental change and distribute resources within the organism.