The transportation of water in plants takes place in following steps :
Absorption of water by roots
In plants, water is absorbed from the soil by root hairs present on roots. Each root hair has a thin permeable cell wall. In soil, the root hairs are directly in contact with water. The water present in soil passes into the root hair cell through its permeable cell wall by the process of osmosis. The water absorbed by root hairs moves by the process of osmosis through epidermis, cortex and endodermis of roots to ultimately reach in root xylem vessels.
Ascent of sap
In plants root xylem vessels directly open in xylem vessels present in their stems. So, the water present in root xylem vessels enter the xylem vessels present in stem. The xylem vessels of stem further opens in xylem vessels of leaves. So, the water present in stem xylem vessels enters the leaves through leaf xylem vessels. In leaves the water is used by the plants along with carbon dioxide gas taken from air to manufacture food by the process of photosynthesis.
The upward movement of water in a plant towards the stem and leaves is called ascent of sap. Various theories have been put forward by different scientists to explain the process of ascent of sap. But, the most widely accepted theory is Dixon’s theory of ascent of sap. According to this theory the water present in the leaves is continuously evaporated by sunlight falling over them. The process of evaporation of water from the leaves of a plant is called transpiration. The loss of water in the form of vapours from the leaves takes place through very small pores called stomata present on the lower surfaces of leaves. This loss of water from the leaves due to transpiration causes a decrease in pressure in the mesophyll cells. Due to the decrease in pressure in the mesophyll cells suction comes into existence which pulls water from xylem vessels in leaves which in turn pulls up water from xylem vessels in stem. The pull that develops in the plant xylem vessels due to the transpiration is called transpiration pull.
Significance of transpiration
Following are the significances of transpiration in plants:
- It is responsible for the production of transpiration pull in xylem vessels of a plant.
- It helps in upward movement of water and minerals from the roots to upper parts of a plant.
Test Your Understanding and Answer These Questions:
- What is ascent of sap?
- What is transpiration?
- Name the various cells through which water move upwards to reach the leaves.
- What do you meant by transpiration pull? Give its significance.