Complex Permanent Tissues: Xylem and Phloem in Plants

Complex permanent tissues are made of more than one cell type working as a unit to transport materials. The two complex tissues are xylem and phloem. Xylem carries water and minerals upward, while phloem carries food throughout the plant.

Without these tissues, tall trees could not lift water to top leaves or send food to roots and fruits. Understanding them explains wilting, girdling damage, timber quality, and how crops feed growing grains.

Key Concepts

Xylem has four elements: tracheids, vessels, xylem fibres, and xylem parenchyma. Tracheids and vessels are dead conducting elements; vessels are wide tubes mainly in angiosperms. Only xylem parenchyma is living.

Phloem has sieve tube elements, companion cells, phloem fibres, and phloem parenchyma. Sieve tubes are living but lack a nucleus at maturity and depend on companion cells. In gymnosperms, sieve cells and albuminous cells occur instead.

• Direction: Xylem mostly upward; phloem both upward and downward as needed.
• Material: Xylem — water and minerals; phloem — sugars and amino acids.

How It Works

1. Roots absorb water, which enters xylem vessels.
2. Transpiration pull and root pressure move water up to leaves.
3. Leaves make sugar by photosynthesis and load it into sieve tubes.
4. Phloem translocates food to roots, fruits, seeds, and growing tips.

Worked Example

A plant transports 2 litres of water per hour through xylem. In 5 hours, water moved = 2 × 5 = 10 litres. If phloem delivers 3 grams of sugar per hour to a fruit, in 5 hours sugar delivered = 3 × 5 = 15 grams. Separate calculations show xylem moves bulk water while phloem moves concentrated food.

Common Misconceptions

• Xylem cells are living: Mostly wrong. Tracheids, vessels, and fibres are dead at maturity; only xylem parenchyma lives.
• Phloem moves only downward: Wrong. Food moves from source to sink in either direction.
• Removing a bark ring harms only the surface: Wrong. Girdling removes phloem, starving roots of food and can kill the tree.

Real-World Applications

Wood is secondary xylem used as timber and fuel. Jute and flax fibres include phloem fibres for textiles. Farmers avoid deep bark injury, and scientists study phloem transport to improve grain filling and fruit sweetness in crops.

Xylem lifts water in dead tubes while phloem distributes food through living sieve tubes — two complementary highways that keep the whole plant supplied.