Transport Systems in Dicotyledonous PLants
Need for Transport Systems:
- Although some parts of the cell photosynthesise to create their own energy, the other parts of the plant need glucose for respiration transported to them, and metabolic waste products transported away. Hormones and mineral ions that are required for different metabolic processes need to me moved from their source to their sink.
- Multicellular plants are too large to receive everything they need via diffusion because their SA:V ratio is too large, and also are too large to move required substances throughout the plant by diffusion efficiently.
Key Terms:
Dicotyledonous: A plant which has seeds made of two parts.
Herbaceous: Leafy, soft tissued plants which have a short life cycle.
Aborescent: Woody, hard tissued plants with a long life cycle.
Vascular System: A complex of vessels which transports substances.
Vascular Bundles: The transport system of plants made of xylem and phloem vessels.
Xylem:
- Non-living tissue which transports water and mineral ions from roots to the leaves.
- Xylem vessels are long, non living, hollow vessels which are fused end to end.
- Parenchyma packs around xylem, which stores food and tannin (a foul tasting chemical to prevent attacks from animals).
- Lignin is a waterproof substance which stops water leaving xylem and gives the xylem vessels mechanical strength.
- Lignin can form rings, spirals or solid tubes with pits around xylem vessels to control water exit.
Observing Xylem Vessels in Living Plant Stems:
- Place plant such as celery in strongly dyed water for at least 24 hours so xylem vessels fill with coloured water.
- Remove the plant from the dye and rinse it.
- Make transverse cuts across a dicotyledonous stem.
- Xylem vessels should show up as coloured spots.
- Make longitudinal cuts across a stem where you expect xylem vessels to be.
- Xylem should show up as coloured lines.
Phloem:
- Living tissue which transports food (as solutes) from the leaves around the plant, supplying respiring cells with required sugars and amino acids.
- Sieve tube elements are fused end to end, with perforated sieve plates separating each element, forming a long hollow structure.
- Companion cells are linked to sieve tube elements by many plasmodesmata, and act as a 'life support system' for the sieve tube elements which have lost most of their normal cell functioning.
- Plasma tissue is supported by sclereid cells, which have extremely thick walls.