Xylem vessels

• The functions of?xylem?tissue in a plant are:
  • Vascular tissue that?transports?dissolved minerals and water around the plant
  • Structural?support
• Their cell walls contain?lignin, which enables the vessels to withstand the pressure created by the moving column of water
• Xylem vessels form?long, hollow?straw-like structures that are formed by?dead cells?(due to lignification of cell walls)
• This means that they?do not?contain any cytoplasm or organelles that could slow down the flow of water
• There are small regions in the walls that are not lignified, known as?pits, which allows for lateral movement of water and minerals between xylem vessels

Images of xylem vessel elements: A = photomicrograph (drawing) in longitudinal section (lignin is stained red), B = scanning electron micrograph (drawing) in transverse section, C = microscope image (drawing) in transverse section (lignin is stained purple)

Structure and Function of Xylem Vessels Table

Sclerenchyma fibres

• Sclerenchyma fibres are not involved in transport of substances, they?provide support
• Similar to xylem, they consist of bundles of?dead cells
• They form?long, hollow tubes, but they do have?end walls?present
• Lignification?of cell walls occur, but they?do not have pits?like xylem vessels
• They have?more cellulose?in their walls compared to other plant cells

Phloem tissue

• The functions of?phloem?tissue in a plant are:
  • Transport?organic compounds?(assimilates), particularly sucrose, from sources (e.g. leaves) to sinks (e.g. roots). The transport of these compounds can occur?up?and?down?the plant
  • This is known as?translocation
  • Phloem has no support function in a plant
• The organic compounds are dissolved in water to form sap
• Phloem is a complex tissue made up of various cell types; its bulk is made up of?sieve tube elements?which are the main conducting cells and?companion cells
• Other cell types of phloem tissue also include parenchyma for storage and strengthening fibres
• Mature phloem tissue contains living cells, unlike xylem tissue

Structure of phloem tissue: A = microscope slide image (and below drawing) of a sieve tube element and companion cell in transverse section, B = photomicrograph image (and below drawing) of a sieve tube element and companion cell in longitudinal section

Structure of phloem tissue: C = transmission electron micrograph image (and drawing) of a sieve tube element and companion cell in transverse section

Sieve tube elements

• Sieve tube elements line up end to end to form a?continuous tube

Phloem Sieve Tube Elements Structure & Function Table

Companion cells

• Each sieve tube element has a companion cell associated with it as companion cells control the metabolism of their associated sieve tube member
• They also play a role in?loading and unloading of sugars into the phloem sieve tube elements

Phloem Companion Cells Structure & Function Table

Vascular bundles in a stem

• Vascular bundles?consist of xylem vessels which are group together with phloem tissue
• In a stem, the?xylem vessels?are always located towards the middle of the stem, while the?phloem?is located closer towards the outside of the stem
• Sclerenchyma fibres?are associated with the vascular bundles and provide additional support to the stem

Transverse section through a stem, showing the positions of phloem, xylem and sclerenchyma fibres

Longitudinal section through a stem, showing the positions of phloem, xylem and sclerenchyma fibres