Absorption of water and mineral salts takes place through the much branched root system. A large part of water and mineral nutrients is absorbed by root. Most active absorption however, is known to occur at the root-hair zone, located a little behind the root tip. Root hairs increase the total absorptive area of root.

Dittmer (1937) has attempted to measure length of roots and root hairs of rye plant. Grown in a box of soil 12 inches square and 22 inches deep, during 4 months, it produced roots about 387.17 miles and root hairs 6603.86 miles. Number of root hairs on this plant was calculated by him to be 14 billion.

Root Structure and Water Uptake

Water enters the root mainly through walls of root hairs, which are simply outgrowths on the outer walls of the cells of piliferous layer. From the cells of piliferous layer water passes through successive rows of thin walled cortical cells and then through the cells of endodermis. The cells of endodermis may be thin walled or thick walled. In the latter case, the radial and tangential walls may be suberized and thickened and strips called ‘casparian strips’ may be present on the inner surface of radial and transverse walls. Opposite and outside xylem, certain cells of endodermis are thin walled. They are called passage cells. Water from the cortical cells passes into the pericycle through these cells. From these, it is exuded into the xylem vessels through which it ascends up into the stem and leaves.

Root Structure

Entry of Water into Cells

There are two views about the entry of water into the cells:

1. Passive Uptake.
2. Active Uptake.

Passive Uptake of Water

It is generally agreed that water enters the cells by osmosis. It moves from one cell to the other down an osmotic gradient, that is from a cell having high water potential to a cell having low water potential.

Active Uptake of Water

According to this view water is forced through the cell membrane against potential gradient. This requires energy provided by respiration. Certain, experiments confirm the idea that energy may be needed for water uptake by the cells. But it has been pointed out that respiratory energy is needed for water uptake indirectly; energy is actually used to transport solutes from cell to cell to produce osmotic potential gradient which results in movement of water into the cells.

Related posts:

1. Mechanism of Water Absorption and Antagonism Mechanism of Water Absorption American physiologists, A. S. Crafts and...
2. Entry of Water into Roots Water diffuses directly from the soil into the free space...
3. Mechanism of Ionic Uptake – Active Absorption of Ions (Part One) Active Absorption of Ions The active transport of ions into...
4. Factors Affecting Rate of Water Absorption The following factors affect the rate of absorption of water...
5. Active Absorption of Ions (Part Three) and Ionic Uptake by Roots Active Absorption of Ions (Part Three) ATPase Mediated Transport ATPase...