4.1 - Vascular Tissues

The Necessity of Transport in Plants

• Plants are multicellular organisms formed from cells that need water and nutrients.
• Large and tall plants have insufficient total surface area to absorb all their basic needs from the surroundings.
• Vascular tissues transport water, mineral salts and nutrients to all cells.
• Xylem transports water and mineral salts absorbed by the roots to the stems and leaves.
• Phloem transports organic compounds synthesised by the leaves through photosynthesis to the stems and roots.
• Plants with a transport system are vascular plants. Non-vascular plants such as algae and mosses do not have a transport system.

Structural Adaptations of Xylem Vessels and Tracheids

• Xylem transports water and mineral salts from the roots to the stems and leaves and provides mechanical support.

Xylem Vessels

• Consist of dead cells at maturity and do not have cytoplasm.
• Cells are arranged longitudinally from end to end, forming a continuous tube that allows water to flow from the roots to the leaves.
• Walls have uneven lignin thickening that:
  • strengthens the vessels so they do not collapse under tension force and pressure changes as water moves through them;
  • prevents the plant from bending.

Tracheids

• Cell walls have lignin thickening.
• Pits allow water to move to adjacent cells.

Structural Adaptations of Sieve Tubes and Companion Cells

• Phloem transports and distributes dissolved organic compounds such as sucrose, amino acids and plant hormones to all parts of the plant.
• Phloem is living tissue because it has cytoplasm.

Sieve Tubes

• Do not have nuclei, ribosomes or vacuoles, allowing sucrose molecules to pass through easily.
• A sieve plate at both ends has pores that allow organic compounds to flow from one sieve tube to the next.

Companion Cells

• Contain mitochondria that supply energy in the form of ATP.
• The ATP is used to transport sucrose from the leaf to the sieve tube by active transport.