In case of passive absorption, the root system does not play a significant role in absorption of water. It merely provides the surface for absorption of water. The main driving forces responsible for absorption of water originate in the aerial portion of the plant.

The water at the top of the plant develops a large tension or a negative hydrostatic pressure due to loss of water by transpiration from leaves. This tension pulls water through the xylem.

Water in the plant can be considered a continuous hydraulic system connecting the water in the soil with the water vapour in the atmosphere.

The upper terminus of water in the plant system is represented by the meniscus of water in the substomatal cavities. The lower terminus of water is represented by epiblemal cells and root hairs.

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The mesophyll cells within the leaves are in direct contact with the atmosphere through an extensive system of air spaces. Initially water evaporates from a thin film lining (of water) air spaces. Subsequently water evaporates from the wall of mesophyll cells during transpiration. The water potential of the cell wall decreases.

Thus water is absorbed by the cell wall from the protoplasm which in turn takes water from the vacuole of the cell. Due to decrease in water potential of the mesophyll cell which loses water, the above said mesophyll cell absorbs water from the adjoining mesophyll cells and ultimately from the xylem or the veins of leaf.

The tension developed due to transpiration acts as a pull from above on the whole water column of the plant. The tension is transmitted all the way down the unbroken water column through the stem to the absorbing part of the root. Due to the rapid rate of transpiration, transport of water through the whole system reflects a progressive drop in water potential from soil to the atmosphere. A water potential difference develops between the root and soil.

Thus soil water enters into root hairs and cells of epiblema and ultimately to xylem of roots via cortical cells.

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Water moves through soils predominantly by bulk flow driven by pressure gradient. As plants absorb water from the soil, they deplete the soil water near the surface of the roots.

This depletion reduces in the water near the root surface. Also it establishes a pressure gradient with respect to neighbouring regions of soil that have higher \j/p values. Because the water filled pore spaces in the soil are interconnected, water moves to the root surface by bulk flow through these channels down the pressure gradient.

In the passive mechanism, transpiration rate is very high and large quantities of water are absorbed at a relatively faster rate.

The best evidence in support of passive type of absorption comes from the experimental finding that the rate of absorption of water by a plant is approximately equal to the rate of transpiration, provided water is available in optimum quantity in the soil.