The process involved in the absorption of water and dissolved mineral salts, the movements with in the bodies of plant are following:

1. Diffusion
2. Osmosis
3. Active Transprot
4. Imbition
5. Plasmolysis

Energy is not required for the process of diffusion. The process of diffusion discontinued until the concentration of diffusing particles become uniform throughout the system.

A gas can diffuse into another gas; solids and gases dissolve in liquids by diffusion. One liquid can diffuse into another liquid in the same way.

If a crystal of CuSO₄ is placed in a beaker filled with water. At constant temperature copper and sulphate ions will spread out among the water molecules and every part of beaker will contain same copper and sulphate ions.

Take a thistle funnel cover its mouth with semipermeable membrane e.g sheep’s bladder, and firmly tie this membrane with the mouth of this funnel by means of thread. Fill the bulb and portion of tube at lever A with strong aqueous solution of sugar suspend the funnel vertically into the beaker containing distill water and clamp the funnel to a stand.

The semipermeable membrane is freely permeable to water molecules so the level of the sugar solution in the tube of this funnel will rise slowly and reach to lever B.

There are two types of osmosis:

1. Exosmosis
2. Endosmosis

Due to presence of semi-permeable plasma membrane osmosis plays an important role in the absorption of water by plant cells. The cell sap contained in the vacuoles is an aqueous solution of salts, sugars and organic acids and possesses a fairly high osmotic pressure.

If the cell is in contact with pure water or with hypotonic solution, water will pass into the cell i.e. endosmosis will occur. If it is in contact with isotonic solution, the cell will neither lose nor absorb water and if it is in contact with a hypertonic solution, water will be lost from the cells i.e. exosmosis will occur.

The osmotic quantities are inter-related to each other. If we represent osmotic pressure of cell by P, its turgor pressure by T and suction pressure by S, the relation between various forces will be as under:

In a plasmolysed cell, the value of T being zero, suction equals its osmotic pressure i.e.

As the absorption of water takes place, the value of P falls due to dilution of cell sap and the value of T increased until P = T.

At this stage, the value of S is zero I.

The swelling up of seeds and wooden frame work during rainy season are the examples of Imbition.

There are two aspects of the process, namely:

1. The path of ascending of sap
2. The forces responsible for raising sap to the top of tall trees i.e. 100 – 150 meters.

the water absorbed by the root of a plant is conducted upwards to the leaves through xylem. This can be proved by an experiment described below:

Take a potted plant. Cut its branch under water and place it in a coloured solution. After sometime the midrib veins and veins let of the leaves become coloured due the presence of coloured water conducted upward. If a transverse section of stem is out at any point, it will be found that only tracheas and vessels are coloured. This proves that the tracheas and vessels of xylem.

Many theories have been put forwarded to account for the ascent of sap. These thories have been placed into two categories:

1. Vital Theories
2. Physical Force Theory

In a884, Godlewski proposed that:

The named these periodic changes in osmotic pressure as Pumping Activity.

In 1893, this theory was critisized by strans berger due to the fact that water remains continue to rise up even when living cells are killed by high temperature or some poisonous substance.

This theory was postulated by Sir J.C. Bose in 1923. According to this theory:

He proved this activity by help of an electrical probe connected with a galvanometer.

This theory has similar objection that if a plant is placed in a poisonous solution even then upward movement remains continue.

Sachs proposed that:

Darwin rejected this theory as he found that the large quantity of water move through lamina of xylem and if lumen is blocked by gelatin, the supply is checked severely and plant shows wilting.

This force explains that:

In plants xylem vessels acts as capillary tubes extending from root to leaf and responsibilities for ascent of sap.

According to this theory:

Root pressure is generated by the osmotic pressure of the cortical cells of the root. This theory explains that root pressure is responsible for forcing up water from root to leaves.

This theory was proposed by a plant physiologist Dixon and physicist Joly.

In plants, water moves in the form of column due to transpiration pull the water column remains unbroken due to cohesive force of water molecule, which is actually due to formation of hydrogen bonds between molecules. Further studies of Dixon reveals that cohesive force of water molecule is about 300 atm to 350 atm and the total force needed to raise water column about 350 atom to 350 atm and the total force needed to raise water column about 350 feet’s is 12 atm but due to cross-walls and other resistance’s it may reach to 30 atm.

This theory was criticized that due to such a high tension air bubbles may enter in water column and break up the column. But studies reveals that even in extreme dry conditions air bubbles could not enter in water column, but if they entered they do not block the whole system.