Most of the water entering a plant does so via the root hairs. It travels across the root cortex to the xylem. It ascends the xylem to the leaves and is lost by evaporation from the surface of the mesophyll cells (the ones sorrounding the air spaces) before diffusing out of the stomata. This is called > transpiration
The flow of water from the roots to the transpiring surfaces is called > the transpirational stream.
It is reckoned that only 1% of the water absorbed by a plant is actually used by the plant in chemical processes.
Water normally leaves a plant as vapour, and it is the heat of the sun that provides the energy to cause this evaporation and maintain the flow of water through a plant.
Transpiration is the loss of water vapour from a plant and occurs from three sites:
- The Stomata: 90%
- The Cuticle: 10%
- Lenticels (what are these? They are responsible for most water loss in deciduous trees after leaf-fall)
Plants can lose large amounts of water through transpiration. A sunflower may lose 1-2 litres in a day, a large oak tree up to 600 litres in a day.
Water arrives in the leaves within xylem vessels in the vascular bundles. Remember the apoplast and symplast pathways you met in the roots? Well water moves from the xylem to the stomata along these same pathways.
Can you identify the apopalstic and symplastic pathways in the diagram below? Mouseover to reveal the answers.
Exit of Water Through the Stomata
Water evaporates from the air spaces and diffuses through the stomata. Immediately next to the leaf is a layer of stationary air. Water vapour must diffuse through this layer before being swept away by moving air, and the thinner the layer of stationary air the faster the rate of transpiration.
Movement up the Xylem
The xylem of a flowering plant is constructed from types of cell, the tracheid and the vessel.
How does water move up the xylem?
The cohesion-tension theory of water movement provides an explanation of xylem transfer of water. According to the theory it is the evaporation of water from the leaf that raises water from the roots. Evaporation from the leaf lowers the water potential of the leaf cells losing water and water flows in from the xylem.
The xylem vessels are full of water and as water leaves them a tension is set up in the columns of water. This is transmitted all the way down the stem by the cohesion of water molecules. Water molecules have a high cohesion as they are polar, and form hydrogen bonds between each other.They also tend to stick to the walls of the xylem vessels- a force termed adhesion. The wallso fxylem vessels are highly lignified in order to give them a great deal of strength to cope with the forces generated in the mass flow of water within them.
Root Pressure
A second force involved in water movement in the xylem is root pressure. When a tree is chopped down, sap exudes out of the cut stump from the severed xylem vessels, and the process is not driven by evaporation. The process is inhibited by cyanide, low temperatures and lack of oxygen. This tells you that the process is active. It is probably caused by the secretion of salts and other solutes into the xylem causing the inward flow of water by osmosis.
Summary
