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• Transport systems in plants

  The structures involved in plant transport, how plant transport is carried out, and the reasons for differences in structure and function between different plant groups
• This Topic should be studied in conjunction with Topics 1, 2 and 3 in Unit 11.2 Nutrition and Topic 2 in this Unit because the three processes are sufficiently linked to make their content part of a whole
• The topics covered in this unit

  • The structures involved in plant transport
  • How plant transport is carried out
  • Reasons for the differences in structure and function between different plant groups
• The structures involved in plant transport

  • May be at cell level (e.g. structure and location of xylem and phloem, root hairs, stomata)
  • May be at organ level (e.g. structure of root, stem, leaves)
• How plant transport is carried out

  1. Osmosis

  2. Diffusion

  3. Transpiration pull
• Reasons for the differences in structure and function between different plant groups

  • Relate to habitat, size, life cycle, way of life
• The two major groups (phyla) that all plants are classified into

  • Bryophyta (non-vascular plants)
  • Tracheophyta (vascular plants having transport tissues called xylem and phloem)
• The transport of materials is largely to meet the needs of the photosynthetic process. Water is also required to keep the cell membranes moist, so that gas exchange can occur.
• Water and mineral transport in plants

  1. Water and minerals enter the plant from the soil into root or root-like structures

  2. Water and minerals need to be transported up the stem to the photosynthetic leaves or fronds

  3. Sugars made in photosynthesis need to be transported down and around the plant
• Vascular tissue


  Xylem and phloem
• Minerals and organic materials travel in separate tissues
• Gases (needed/produced in both photosynthesis and respiration) diffuse into/out of the leaves and stems through pores in the epidermis
• Bryophytes (non-vascular plants)

  • Do not have specialised xylem for the transport of water and minerals or phloem for the transport of glucose
  • Their small size and simple structure allow them to use cell-to-cell diffusion (a slow process) for movement of water, minerals and glucose around the plant, restricting them to damp habitats
• Water transport in Bryophytes

  1. Water enters the root-like rhizoids by osmosis

  2. Water then moves from cell to cell up the plant to the leaf-like photosynthetic structures

  3. This occurs as the concentration gradient of water is always lower higher up the plant, as water is used in photosynthesis or lost in transpiration

  4. If the cells become saturated, the overall movement of water would stop as the concentration would be the same between cells (isotonic)

  5. Water can also move up the outside of the stem by capillary action in wet conditions
• Mineral transport in Bryophytes

  1. Minerals enter the rhizoids by diffusion (or facilitated diffusion)

  2. Minerals then move from cell to cell to where they are needed

  3. The direction of movement is in response to the concentration gradient, though active transport across the cell membrane may also occur
• Bryophytes, the non-vascular plants, live in damp areas and are sufficiently small plants that have no vascular tissue
• The two groups of Bryophytes

  • Mosses
  • Liverworts
• Tracheophyta (vascular plants)


  • Have transport tissue in the form of xylem and phloem
  • Are typically much taller than non-vascular plants
• The three major groups of vascular plants

  • Ferns
  • Conifers
  • Angiosperms
• Ferns


  • Have specific transport tissue in the form of xylem and phloem, allowing them to be much larger plants than mosses
  • In ground ferns, the stem is underground and horizontal, swollen with stored food (a rhizome)
  • Adventitious fibrous roots come from the rhizome and penetrate the soil
  • Fronds (the photosynthetic leaves) rise through the air from the rhizome
  • In tree ferns, the stem is vertical and resembles a trunk, the fronds form at the top
• Water and mineral transport in ferns

  1. Water enters the roots by osmosis

  2. Minerals enter by diffusion (as for mosses)
• Fern rhizomes


  • Have vascular bundles of xylem and phloem, with xylem on the inside and phloem on the outside
  • Xylem tissue is simple pitted, lignified tracheids, not vessels
  • Phloem contains sieve elements
• Transport in ferns


  1. Water and minerals travel in the xylem to and up the fronds

  2. Glucose travels down the fronds to the rhizome and roots in the phloem
• The two major groups of angiosperms

  • Monocotyledons (monocots)
  • Dicotyledons (dicots)
• Monocotyledons (monocots)


  • Leaves are typically long and thin, with parallel veins (the transporting xylem and phloem) along their length
  • Vascular bundles (of xylem and phloem) are scattered throughout the width of the stem, and there is rarely secondary growth of the stem
  • Flowers typically have parts arranged in threes (or multiples of three)
  • Seed has only one seed leaf or cotyledon
• Dicotyledons (dicots)


  • Leaves are typically broad and have a network of veins
  • Vascular tissue is arranged in a circle around the stem, and secondary growth of the stem occurs
  • Flowers typically have their parts in fours or fives (or multiples thereof)
  • Seed has two seed leaves or cotyledons
• The angiosperms continue the trend shown by the ferns to terrestrial living, and are completely adapted to a terrestrial existence (though some forms have become adapted again to an aquatic existence, eg water lilies)
• Water and organic compound transport in angiosperms

  1. Vascular tissue is continuous from roots to leaves

  2. Water is transported from roots upwards to the leaves in the xylem vessels

  3. Organic compounds are transported in the phloem both downwards (leaves to roots) and upwards (roots to areas of growth, e.g. buds)
• Dicotyledon roots


  • Have a wide area of tissue in the cortex made of general packing cells (parenchyma cells), where starch is stored
  • In the centre of the root is an area called the stele, which contains the xylem and phloem
  • The xylem begins as a central, star-shaped bundle, with clusters of phloem in the arms of the star
  • Thickening of the stem leads to a complete solid core of xylem, surrounded by a complete circle of phloem
  • The outer layer of the stele is the endodermis, a ring of cells which functions to control the entry of minerals from the root into the xylem
• Monocotyledon roots


  • Have a wide cortex for starch storage
  • The stele is centrally placed, with clusters of phloem and xylem alternating in a circle around the root
  • The endodermis is more thickened and prominent
  • There is a central band of packing cells present (the pith) inside the vascular tissue
• Water and mineral transport in roots

  1. Water and minerals enter the root from the soil via the root hairs (located just behind the growing root tip)

  2. Water and minerals travel across the cells of the cortex to the xylem

  3. Sugars move from the leaf to the cortex of the root in the phloem
• Dicotyledon stems


  • In the young plant, the xylem and phloem form vascular bundles, with xylem on the inside and phloem on the outside, often with a cap of fibres (for support)
  • Sandwiched between the xylem and the phloem is a cell layer called the cambium
  • Between the vascular bundles and the epidermis, is a layer of packing cells (the cortex)
  • In the centre of the stem is another layer of packing cells (the pith)
• Secondary growth in dicotyledon stems

  1. As the young plant grows, the cambium cells divide to form a complete ring around the stem

  2. Cells made on the inside of the ring develop into xylem, the cells on the outside develop into phloem

  3. Potentially complete rings of both xylem and phloem form

  4. As the xylem is laid down on the inside, the cambium and tissues outside it are pushed outwards and the stem gets wider - this is secondary growth

  5. The old xylem fills with deposits and becomes wood, providing support

  6. The old phloem and cortex get crushed as the stem widens
• The only functional xylem and phloem in the older dicot stem are those on either side of the cambium, so all the living tissue, including the transport tissue, is on the outside of the stem. This is why large, old trees can become hollow and still survive, and ring barking a tree will kill it.
• Secondary growth in dicot stems

  1. Xylem is laid down on the inside

  2. Cambium and tissues outside it are pushed outwards

  3. Stem gets wider
• Wood


  Old xylem that fills with deposits of resins and other materials, and is no longer functional for the transport of water and minerals
• Changes in older dicot stems

  1. Old phloem and cortex get crushed as the stem widens

  2. Only functional xylem and phloem are on either side of the cambium

  3. Living tissue, including transport tissue, is on the outside of the stem
• Large, old trees can become hollow and still survive
• Ring barking a tree will kill it
• Vascular bundles in monocot stems


  Arranged differently than in dicot stems, do not form a circle, scattered throughout the cortex and pith