Water movement is active or passive? When the water moves against the ground in the stem, it needs some energy for moving up.
The ions in the soil move towards the vascular tissues of the roots in active transport. Due to the change in the potential gradient, water also moves and enhances the xylem pressure. This positive pressure in the xylem is called as root pressure, which is responsible for movement of water to a certain height in the stem.
Let us see how the root pressure functions. A small soft stem is chosen for the test when there is lots of moisture in the atmosphere. The stem is cut at its base during the early part of the day which ends up in the release of a few drops of solution oozed out from the stem. The water drops coming out of the stem is due to the root pressure. If any rubber tube is fixed under the stem then the exudates can be gathered and rate of exudates can be measured. The ingredients of the exudates also can be evaluated.
The root pressure can be observed during the nights and even in the morning when the evaporation is less. The edges of the grass blades and leaves exudes water droplets from the vein openings of many herbs. This type of water loss is known as guttation. The water transport process can at best be stimulated by root pressure. Root pressure itself is not solely responsible for the movement of water to the top of the tall trees. Root pressure aids in establishing the continuation of chain of water molecules in the xylem which might frequently be broken due to intensive tension formed by the transpiration pull. Most of the plants have the water movement aided by transpiration pull rather than by root pressure.
Though there is no specific circulatory system in the plants, water movement through the xylem can be faster and can reach even upto 15 meters of height in an hour. There was a big question regarding this aspect for many years. People were wondering whether the water is reaching the plant top by a ‘push’ or ‘pull’. Many research studies have proved that water was ‘pulled’ towards top and it was due to the transpiration process in the leaves. This model was known as cohesion-tension transpiration pull of water. The force that is responsible for this ‘pull’ is called transpiration. It is observed that only about one percent of water that is absorbed into the plant leaves are used for plant growth and photosynthesis, and the rest is evaporated through the stomata by a process called transpiration.