Xylem is a complex permanent tissue consisting of three kinds of elements, xylem vessels, tracheids and xylem parenchyma which help in conducting water and minerals dissolved in water from roots to upper parts of the plant. Xylem vessels and tracheids are non-living plant tissues that are thick-walled while xylem parenchyma is a living tissue. In flowering plants, both xylem vessels and tracheids, or only vessels are concerned with conduction of water and minerals from roots to the aerial parts of the plant. However, in non-flowering plants, only tracheids are the conducting tissues. Do you know that in some flowering plants, water rises very rapidly at the rate of 10 to 100 cm per minute during the daytime?
Absorption of Water by Roots:
Water and minerals are absorbed from the soil by roots and then transported to various parts of the plant like stem, leaves and flowers, etc. Soil particles, which are coated with water and dissolved minerals, adhere to the root hairs.
The soil solution flows into the epidermal cells of the root hairs and reach the root cortex. As the soil solution moves along cell walls, some of the water and mineral solutes are taken up by cells of the epidermis and cortex. This water containing solutes passes into the stem and then flows up the xylem vessels to the shoot system. About 1-2 per cent of total water absorbed is used up in photosynthesis and other metabolic activities.
Absorption of Water:
The absorption of water occurs through root hair. Root hair are thin-walled extensions from the cells of the outer layer of a root. They grow out pushing between the soil particles. There is a film of water that surrounds the soil particles and in turn root hair also.
i. The root hair contains cell sap which has a higher concentration of salts than the outside soil water. This causes osmosis and the water from outside diffuses into the cells of root hairs (let us take it as cell A). This is due to root pressure.
ii. As water enters the vacuole of cell A, it dilutes the concentration of sugar and salts in its cell sap. iii. Another cell (say cell B) next to cell A has a higher concentration of cell sap (salts and sugar). As a result water from cell A moves to cell B. iv. The water entering cell B makes its cell sap dilute and then moves to cell C. This way water moves from one cell to another by cell to cell osmosis.
v. The water ultimately passes into the xylem vessels at the centre of root and is conducted up the root and stem into the leaves. You will study more about the absorption of water by roots and the ascent of sap in the next chapter.
Absorption of Minerals:
Absorption of mineral elements by the root from the soil takes place by active transport. The water film around the soil particles also contains a low concentration of mineral elements. These mineral elements move from soil into the root cells against the concentration gradient. Energy in the form of ATP is required by the cell for the absorption of minerals.