The uptake of water by plants is called water absorption. Water is absorbed from the soil mainly through root hairs. The absorbed water crosses a variety of cells such as cortical cells, endodermis(passage cells),  pericycle cells and xylem tubes to reach the leaves.
In 1949, Kramer proposed that water is absorbed by two mechanisms. They are
a) Active absorption
b) Passive absorption
water absorption

Active absorption: When the roots absorb water by their own efforts, it is known as active absorption. It takes place when transpiration is low and the quantity of water in the soil is high.In this process the root cells play active role in the absorption of water.
Passive absorption: According to passive absorption , the root hair cells do not play any active role in the absorption of water. The root hair cells remain passive during absorption of water. It takes place, when rate of transpiration is usually high.
Difference between Active and Passive water absorption
Active absorption
1. It occurs due to the activity of roots  and particularly  root hairs is due to osmotic and non osmotic mechanisms
3. In active absorption the osmotic process involves diffusion pressure deficit(DPD) of the cells. The root hairs have more DPD as compared to soil solution and therefore, water is taken in. Water is still absorbed if soil solution has greater DPD, But under expenditure of metabolic energy.
4.Active absorption involves symplast movement of water in root hairs. The water first enters the cell sap and then passes from one cell to another. Such types of movement where type of movement where living protoplasm involved, is called symplast.
5. Evidences in support of active water absorption are root pressure, bleeding and guttation.
Passive absorption
1. The passive absorption occurs due to the activity of upper part of plant, such as shoot and leaves.
2. Passive absorption is due to the process of active transpiration in the upper part.
3. The passive absorption occurs due to the tension created in the xylem sap by transpiration pull.
4. In passive absorption water moves probably through the free spaces or apoplast of root. The apoplast path of water movement includes cell wall and intercellular spaces. Which are fully permeable. The water can reach up to endodermis through apoplast but it moves through the endodermis by symplast.
5. Evidences in support of passive water absorption can be given by cutting the roots under water. The absorption of water still occurs if all the roots are removed.
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