Figure 1
Water is absorbed from the soil by the plant root and transported to all parts of the plant. This passage of water is called the transpiration stream. Several factors play a role in pushing water up the transpiration stream against the force of gravity: Root Pressure and Osmosis – Water penetrates root cells through osmosis, a force created by unequal concentrations of water inside the root cells and the surrounding soil. Osmosis continues to push water upward due to unequal osmotic pressure across the plant. Capillary action or capillarity is the ability of narrow tubes to draw liquid upwards against the force of gravity. The flow of water in a plant’s thin xylem tubes is influenced by two opposite forces: adhesive forces of water molecules to the surface of the tube walls and cohesive forces that attract water molecules to each other. When the adhesive forces between the water and the tube walls are stronger than the cohesive forces between the water molecules, a capillary action is generated. Transpiration Pull is the major force driving water circulation in plants and trees. Water evaporates through the stomata located in the leaves: This process is called Transpiration. As water evaporates from leaves, more water is pulled up from the root system by osmosis – a process that causes water to flow from an area where it is plentiful to an area where it is scarce. The gas carbon dioxide or CO2 is needed for photosynthesis. During the day, the stomata open allowing the plant to “breathe in” CO2. This also greatly increases the amount of water lost through evaporation. Ultimately, 90% of water taken in by plants is lost through transpiration.
In this experiment water rising through the xylem is observed in a celery stalk (Apium graveolens( as it absorbs water colored with methylene blue dye.