9.1.5 Skills: Drawing Xylem Vessels

• The internal structure of plants can be examined by taking transverse sections of different parts of a plant, including the leaves, stem and roots
  • Note that cross sections can be either transverse (TS), across the root or stem, or longitudinal (LS), along the length of the root or stem.
• From these transverse sections, tissue maps can be drawn
• These tissue maps show the relative positions of structures within the different parts of a plant but are not detailed enough to see individual cells
• Tissue maps can be used to show the distribution of tissues types in plant leaves, stems, and roots

Tissue maps can be used to show the distribution of tissues in the leaf, stem and root 

The arrangement of vascular tissue in the vascular bundles can also be seen using a tissue map

Primary xylem vessels

• The lengthening growth of a plant is known as primary growth and takes place in certain plant meristems
• The primary xylem vessels form from meristem tissue called cambium
  • The xylem vessels form on the inside of the cambium tissue, towards the centre of plant stems and roots
• The detailed structure of xylem vessels can be seen when sections of plant stem are viewed under a microscope
• If stain is added to the plant tissue on a prepared slide, the cell walls that have been strengthened with lignin stand out clearly
  • The lignin in primary xylem forms rings or spirals in the xylem walls
    • The ring structure is known as annular, while the spiral structure is helical
  • The structure of the lignin in primary xylem allow the vessels to continue to grow in length as the plant grows taller; the xylem rings or spirals can stretch apart from each other as the xylem vessels grow
• Primary xylem vessels have thinner walls than the secondary xylem tissue that forms later in plant growth 
  • Once the stem has stopped growing longer there is less need for the lignin to stretch, so the lignin strengthening becomes much thicker
  • Secondary xylem vessels are therefore stronger than primary xylem but less flexible

When viewed in longitudinal section the annular and helical structure of lignin in primary xylem vessels can be seen