Question

Can someone help me to describe the theory of roots (plants).

Answer 1

It was proposed by Priestley (1916). According to this theory the water, which is absorbed by the root-hairs from the soil collects in the cells of the cortex. The cortical cells become fully turgid. In such circumstances the elastic walls of the cortical cells, exert pressure on their fluid-contents and force them towards the xylem vessels. Due to this loss of water these cortical cells become flaccid, again absorb water, become turgid and thus again force out their fluid contents. Thus the cortical cells of the root carry on intermittent pumping action, as a result of which considerable pressure is set up in the root. This pressure forces water up the xylem vessels. Thus the pressure, which is set up in the cortical cells of the roots due to osmotic action, is known as the root pressure. This term was used by Stephan Hales. According to Style, root pressure may be defined as "the pressure under which water passes from the living cells of the root in the xylem".
Objections
(i) Taller plants like Eucalyptus need higher pressure to raise the water up. While the value of root pressure ranges from 2-5 atmospheres, a pressure of about 20 atm. is required to raise the water to tops of tall trees.
(ii) The absence of root pressure, ascent of sap continues.
(iii) Plants growing in cold, drought or less aerated soil, root pressure fails to appear and transport of water is normal.

Priestley was the first to explain the process of upward flow of water in bleeding to be due to hydrostatic pressure which is developed in root system.

S. Hales coined the term root pressure and describe it as "The hydrostatic pressure developed in roots due to accumulation of water absorbed by the roots”.

Root pressure can be shown experimentally. A potted plant is watered profusely. The stem is cut near the base, below the lowest leaf. A mercury manometer is attached to the cut end of the stump with the help of rubber tubing . After sometime the level in the manometer rises indicating that water is being forced upward from the cut end of the stump.

Root pressure, though largely an osmotic phenomenon, is maintained by the activity of the living cells. Hence, Stocking described root pressure as a pressure developing in the tracheary elements of the xylem as a result of the metabolic activity of the root. It is a positive pressure, which is developed in roots and pumps the sap up in the xylem ducts.

Root pressure can be observed in many plants but it alone does not seem to be a force that can translocate water to the top of the tallest plants, i.e., heights of 125 meters. In this connection following objections are put forward.

(1) Magnitude of root pressure is very low (about 2 atm).

(2) It is not sufficient to push up water to more than a few meters (about 64 feet).

(3) This pressure is able to push up water only in small herbaceous plants.

(4) Even in the absence of root pressure ascent of sap continues e.g., When a leafy branch is cut under water and placed in a beaker full of water it remains fresh and green for sufficient long time.

(5) Root pressure is not observed in plants growing in cold drought or less aerated soil e.g., conifers.

(6) The rapidly transpiring plants do not show any root pressure, instead a negative pressure is observed in most of the plants. In gymnosperms root pressure has rarely been observed.


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