Absorption of water

Fig: Absorption mechanism of water in plants

a) Plant absorb water from soil. The water is utilized for various types of activities in plant.

b) Plant absorb the water through entire surface. i.e. stem, leaves, roots but mainly from the roots.

c) In the aquatic plant and lower plants (algae) absorb water through the entire external surface.

d) Maximum water uptake found in the root hair zone.

e) Water in the soil is found in various forms. i.e. gravitational water, field capacity water, hygroscopic water, chemically combined water , rain water and capillary water.

                                     Absorption of water occurs by two methods.

A) Passive absorption:

                     The absorption of water not due to activity of root cells but due to the other process is called passive absorption. Passive absorption mainly occurs due to transpiration. During transpiration, water evaporates out from the aerial parts of the plant body through the stomata of the leaf. Removal of water through stomata creates a tension into stomatal cavity which reaches to the xylem of leaves. From xylem of leaf , tension is transmitted to xylem of roots through xylem of stem. From Xylem of roots , tension goes into root hair cell through endodermis and cortex. Due to this tension, water can be easily absorbed by root hair cells.

B) Active absorption:

                     According to this theory, water absorb as a result of activity of roots. Here, shoot doesn’t play any role. Active  absorption occurs by two ways:-

1) Osmotic active absorption: This theory was purposed by Atkins(1916) an Priestly (1922). According to this theory , absorption of water occurs due to osmotic concentration of root hair.

                         The absorption of water which occurs due to the osmotic concentration of root hair is called osmotic active absorption. Root hair cells consists of cytoplasm which consider as concentrated solution. In soil , there is presence of water or water along with minerals which behave as weak solution. Water enters into the root hair cells from soil by osmosis because cytoplasm of root hair cells show high osmotic concentration.

2) Non-Osmotic absorption: The absorption of water against the concentration gradient is called non-osmotic absorption. In this case , water enters into the root hair cells from mineral solution by utilizing energy of root hair cells. It takes place during respiration.

Pathway of water in root:

      The entry of water into the root hair dilutes the cell sap. Thus water molecules in root hair increase as compare to adjacent cell. At this stage, DPD of root hair cells is lower than that of adjacent cells an water enters in these cells. Process continues till the water reach the passage cells of enodermis which is present opposite to 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.