The Vascular Structure of Plants (Edexcel International AS Biology)

• Plant stems have two main functions in a plant
  • They provide support
  • They transport various materials around the plant

Support

• Leaves are attached to stems and are responsible for carrying out photosynthesis
• Stems ensure that leaves are positioned in order to receive maximum levels of sunlight to perform this task
• It also supports flowers in a way that will attract the attention of pollinators
• Plant stems need to be strong, but also flexible, in order to withstand the forces of nature (wind, rain etc.)

Transport

• Leaves require water and mineral ions in order to photosynthesise and carry out other important functions
• The transport tissue in stems provide a route for these substances to travel from the roots to the leaves
• The products of photosynthesis also need to be transported from the tissue where photosynthesis happens to where these substances are needed
• Stems provide a transport route for this to occur

Vascular structure of plants

• Plant stems are made up of different types of plant tissue
• The focus will be on the following tissue types:
  • Xylem
  • Phloem
  • Sclerenchyma
• Xylem and phloem are collectively known as vascular tissue
• They are responsible for transporting various substances around the plant
• Sclerenchyma fibres are associated with the vascular bundles and provide additional support to a stem
• Xylem and phloem are arranged in structures known as vascular bundles
• The arrangement of xylem and phloem differs in the different plant organs
• In a stem, the xylem vessels are always located towards the middle of the stem, while the phloem can be found closer towards the outside of the stem

The arrangement of vascular tissue (xylem and phloem) in roots, stems and leaves

• Sclerenchyma fibres are not involved in the transport of substances, they provide support
• They consist of bundles of dead cells which form long, hollow tubes, but they do have end walls present
• Lignification of cell walls occur, which provides structural support to allow the plant to bear the load of its own mass
• They have more cellulose in their walls compared to other plant cells

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

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