adaptation for transports plants

• Structure of the root


  • Epidermis
  • Exodermis
  • Cortex (parenchyma)
  • Phloem
  • Xylem
  • Endodermis
• Structure of a stem

  • Epidermis
  • Collenchyma
  • Parenchyma cortex
  • Vascular bundle
  • Pith
  • Interfascicular cambium
  • Xylem (vessels, tracheids, fibres, parenchyma)
• Xylem

  Dead cells that transport water and minerals up the plant and provide mechanical strength and support as they are strengthened by waterproof lignin
• Transpiration


  The loss of water as water vapour, by evaporation and diffusion out of the open stomata, from the leaves of plants. It leads to the transpiration stream.
• Transpiration stream


  1. Water moves into the root and enters the xylem (root pressure)

  2. Cohesive forces between water molecules and adhesive forces between water molecules and the hydrophilic lining of the xylem create a transpiration pull as the water leaving the xylem into the leaf cells pulls on molecules below
• Factors increasing transpiration

  • Lower humidity
  • Higher temperature
• Plant adaptations

  • Hydrophyte (water plants, e.g. water lilies)
  • Mesophyte (live with adequate water)
  • Xerophyte (water is scarce, e.g. marram grass)
• Hydrophyte

  • Little/no waxy cuticle as no need to conserve water
  • Stomata on upper surface as lower surface submerged
  • Poorly developed xylem as no need to transport water
  • Large air spaces (aerenchyma) provide buoyancy and act as reservoirs of gas
• Mesophyte

  • Close stomata at night to decrease water loss
  • Shed leaves in unfavourable conditions, e.g. winter
  • Underground organs and dormant seeds survive winter
• Xerophyte


  • Thick waxy cuticle reducing water loss by evaporation from epidermal tissue
  • Sunken stomata increasing humidity in an air chamber above the stomata, reducing diffusion gradient and therefore water loss
  • Rolled leaves - reduces area of leaf exposed directly to air
  • Stiff interlocking hairs trap water vapour inside rolled leaf, reducing water potential gradient and therefore water loss
• Translocation

  The phloem transports the products of photosynthesis from the source (the leaf) to the sink (area of use or storage). This is bidirectional through the phloem.
• Experimental evidence for translocation

  • Ringing experiments (removal of phloem) show accumulation of sucrose products on leaf side of the ring but none on root side
  • Using aphids to sample sap from the phloem
  • Radioactive labelling of carbon dioxide which will become incorporated into sucrose can be used in conjunction with the above technique to determine the rate of transport in the phloem
  • Sources and sinks can be determined by autoradiography using radioactively labelled carbon dioxide
• Theory of mass flow

  Sucrose made at source lowers water potential. Water enters cells and sucrose is forced into phloem (loading). This increases hydrostatic pressure and therefore mass flow occurs along the phloem to the root where sucrose is stored as starch, water potential is less negative and water moves into the xylem.
• Pathways for movement of water across the root cortex

  • Apoplast (from cell wall to cell wall)
  • Symplast (from cytoplasm to cytoplasm through plasmodesmata)
  • Vacuolar (from vacuole to vacuole)
• Endodermis


  Impregnated with areas of suberin called the casparian strip. This blocks the apoplast pathway, forcing water into the symplast pathway. Minerals are selected to move into the symplast by active transport. This sets up a water potential gradient, lower in the xylem so water moves in by osmosis resulting in a force called root pressure.
• Phloem structure

  • Sieve tubes
  • Fibres
  • Parenchyma
  • Sieve plate (containing pores through which cytoplasmic filaments extend linking cells)
  • Companion cells (contain many mitochondria for ATP and the organelles for protein synthesis)