Xylem and Phloem

Xylem

• There are two types of xylem:
  1. Vessels
  2. Tracheids.
• The living part of xylem dies as it matures, so xylem is a dead tissue.
• Xylem is found in the vascular bundles of roots, stems, leaves, and flowers.

Function of Xylem:

1. Transports water and minerals from the roots to the leaves.
2. Provides mechanical support to the plant.

1. Vessels

• Tubes formed by cells joining end to end.
• Wider than tracheids.
• Have pits in their side walls to allow water to move from one vessel to another.
• More efficient at transporting water than tracheids.
• Found mostly in flowering plants.
• Spiral lignin is found in the walls for strength and support.

2. Tracheids

• Long, tapered cells.
• Hollow insides.
• Water passes from tracheid to tracheid or from tracheid to cell through thin wall areas called pits.
• The walls of tracheids are strengthened with lignin for support.
• Tracheids are more primitive than vessels.

Phloem

• Phloem is made up of companion cells and sieve tubes.

• Sieve tubes are long tubular structures formed when sieve tube cells join end to end.

• The end walls of sieve tube cells have pores, called sieve plates, that allow substances to pass between cells.

  

• Sieve tube cells do not contain a nucleus but do contain cytoplasm.

• Their walls are made of cellulose

• Companion cells

  • Each sieve tube element has a companion cell attached.
  • Companion cells contain a nucleus, so each companion cell controls a sieve tube element.
  • Phloem is living tissue because of the companion cells.

• Sieve tubes transport food made by photosynthesis from the leaves to the rest of the plant.

• Phloem is found in the vascular bundles of roots, stems, leaves, and flowers.

Xylem vs Phloem

Xylem	Phloem
Made of tracheids or vessels	Made of sieve tubes and companion cells
Dead tissue	Living tissue
Walls strengthened with lignin	Walls strengthened with cellulose
Transports water and minerals	Transports food
Flow is always upwards	Flow can be upwards or downwards