The entry of water into the root hair dilutes the cell sap. Thus water molecules in root hair increases as compared to adjacent cortical cells. At this stage diffusion pressure deficit (DPD) of root hairs is lower than that of adjacent cortex. Due to higher DPD in cortex, water enters in these cells. Process continues till the water reaches the passage cells of endodermis. These passage cells lie opposite the protoxylem. They allow water to enter the pericycle. In xylem, turgor pressure is lacking. So, water enters the xylem from pericycle for upward movement of sap.
Water Movement in a Plant From Root Hair to Leaf
The Path of Water from Soil into Root
Water movement from root hair to cortex and to the xylem vessels, showing the apoplast pathway and the cellular pathway through symplast and transmembrane is shown below.
Factors Affecting Water Absorption
- Availability of soil water - capillary water
- Soil temperature - temperature below 4o C reduces the absorption of water. Absorption is maximum between 20o C to 30o soil - temperature
- Soil solutes - high soil solutes inhibit absorption
- Aeration of soil - soil deficient in oxygen inhibits absorption
- Higher the rate of transpiration more will be the rate of absorption
Ascent of Sap or Water Translocation
The upward movement of water from the root towards the top of the plant is called ascent of sap.Experiments to Prove that Water Rises Upwards in the Plant Through Xylem
Movement of Dyes
A leafy green shoot of Balsam plant with a transparent stem is placed in eosine solution. It is noticed that after some time red streaks appear in the stem and veins of leaves become red. When transverse section of stem is taken it is noticed that only walls of xylem vessels have been stained red. This proves that xylem is a true tissue which conducts water upwards in the plant.
Ringing Experiment
In this experiment all tissues external to xylem from a part of branch is girdled. Now this ringed twig is placed in a beaker containing water. It is noticed that upper leaves of shoot remains turgid and indicates water is still moving upwards through xylem.
Ringing Experiment to Show the Path of Ascent of Sap
