2.1.1 - anatomy of dicotyledenous plants  (Page 3/6)

just water usage? - I also wondered this but obtained the investigation elsewhere so don’ t know. I thought perhaps the plant does not put onthat much mass compared to the mass of the water so it becomes insignificant

Dicotyledonous stem

• Leaves develop from the nodes.
• The sections of stem between the nodes are called internodes.
• An axillary bud is often found at the node. These forms lateral branches.
• A terminal bud is found at the tip of the stem and allows the stem to increase in length.

Internal structure of the dicotyledonous stem

This diagram of a cross section shows the internal structure of a young dicot stem

• A waterproof cuticle is found on the outside of the epidermis to prevent water loss.
• The epidermis consists of a single layer of cells to protect the underlying tissue.
• The cortex is made up of parenchyma cells that stores water and food.
• The vascular bundles are arranged in a ring in the medulla and are surrounded by non-living sclerenchyma cells for strengthening and support.
• Each vascular bundle contains the following:
  • Cambium (contains meristematic cells that divide to widen the stem)
  • Phloem (transports food from leaves to the roots)
  • Xylem (transports water from the roots to the stem)

http://bcs.whfreeman.com/thelifewire/content/ch p36/36020.html

This is a link to an online tutorial about phloem, xylem and pressure flow.

Movement of water up the stem

• Water moves up the xylem from the roots to the leaves.
• Adaptations of xylem for transporting water:
  • Long, elongated tubes joined end-to-end without any cross-walls, forming good conducting tubes.
  • The cell walls are thickened with lignin for support (annual or spiral thickening) so that they do not collapse due to the upward pull of water
  • Pitted vessels and tracheids allow for lateral movement of water into neighbouring xylem vessels.
  • Cells are dead, so there is no obstruction to water transport

Diagram of xylem

Three forces are responsible for the movement of water up the xylem – capillarity, root pressure and transpiration suction force.

• Capillarity involves forces of cohesion (forces of attraction between water molecules) and adhesion (forces of attraction between water molecules and thesides of the xylem vessels). Because the xylem’s lumen (opening) is so tiny, water will move up by capillary. However, this force is weak and its rolein moving water up the stem is small.
• Root pressure is a force that pushes water up the xylem. As water enters the root by osmosis, it pushes the water that is already in the xylem of the stemupwards.
• Transpiration suction force is a very important force that pulls water up the xylem of the stem. As water evaporates from the stomata of the leaves duringtranspiration, it creates a sucking force that will pull the water up the xylem.

Investigation: plant tissue anatomy (root and stem)

Aim: To examine the structure of the root and stem

Apparatus

• Scalpel or knife
• Celery stalk (stem)
• Carrot (root)
• Glass slide
• Iodine solution (Stain) or water
• Cover slip
• Dissecting needle or tweezers
• Paper and pencil