This hypothesis explains the mechanism of absorption mineral salts in form of nutrient cations from the soil which adsorbed to the surface of the membranes of root cells against the similar ions present adhered to the plasma membrane inner to the cell sap solution. Thus there is exchage of cations takes place through following theories.

i. Contract exchange theory:

The ions adsorbed on the surface of root cells and clay particles are not held tight, but oscillate within small volume of space.

If the root cell and soil particle are in close contract the oscillation volume of ions adsorbed on root. Surface may overlap the oscillation volume of ions adsorbed on clay particle and the ions adsorbed on the clay particle may be exchanged with the ions adsorbed on root. Surface directly-without first being dissolved in soil solution.

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ii. Corbonic acid exchange theory:

The CO2 released during respiration of root cells combines with mater forms carbonic acid which dissociated into H* and anion HCO3 in soil solution. These H* ions may be exchanged for cations adsorbed on the clay particles. The cations thus released into the soil solution from the clay particles may be adsorbed onroot cells in exchange for H* ions.

iii. Indole acetic adid (IAA):

Indole acetic acid (IAA) is the principal natural auxin, which bring about cell elongation and first esolated from the coleoptile of avena sativa, by using chromatographic method.

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Some derivative of IAA have also been discovered, as inhole buteric acid, indole pyruvic acid etc.

Function:

i. It imitates cell elongation.

ii. It helps in apical dominance.

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iii. It helps in root initation.

iv. Prevents abscission formation.

v. It induces parthenocarpy.

vi. Stimulates respiration.

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vii. Activate callus formation and cell division.