Differentiated between Xylem and Phloem

Aug 09, 2022, 16:45 IST

About Xylem

It is a tissue that transports water and dissolved minerals nutrients from the roots to all other parts of the vascular plant.

Xylem consists of four kinds of elements

(a) Vessels: Vessels are cylindrical in shape with their ends open and are placed one above the other.

(b) Tracheids: They are elongated, thin, spindle-shaped dead cells with pits in their thick cell walls.

(d) Xylem Fiber

Xylem vessels and tracheids are both non-living conducting tissues.

Mechanism of Transport of Water and Minerals in a Plant

The vessels and tracheids of roots, stems and leaves in xylem tissue are interconnected to form a continuous system of water conducting channels reaching all parts of the plant.

Movement of water from the soil into the Plant.Due to transpiration, the aerial parts of the plant are constantly losing water to the external environment.

This loss of water tends to lower the concentration of water in the cell sap. Thus, the root hair, with its semipermeable membrane and hypertonic cell sap establishes an osmotic system with the water available in the soil surrounding the root hair. Hence, water from the soil passes through the semipermeable membrane of the root hair into the root hair.As the water accumulates in the root hair, an osmotic system develops between the cells of the root hair and the adjacent cell of the root cortex which has a comparatively lower concentration of water molecules. Hence water from the root air diffuses into the cortical cell, and the root hair becomes flaccid. Once again it absorbs water from the soil. Meanwhile the cortical cell establishes an osmotic system with its adjacent cell and water passes inwards into the inner cortical cell.

Ascent of Sap (Upward movement sap)

The water, that is absorbed from the soil by the root hairs, gradually accumulates in the tissue of the cortex. Some part of this water is transpired by the aerial surfaces of the plant. However, before the water can be transpired, it must reach the aerial surfaces against the downward pull of gravity. This upward movement of water and dissolved minerals from the roots to the leaves is called ascent of sap.

About Phloem

The food synthesized in the leaves and substances like hormones synthesized at the tips of roots and stems are transported to other parts of the plant through a conducting tissue called phloem.

Phloem consists of sieve tube companion cells, phloem parenchyma and phloem fiber, which are placed one above the other forming continuous column with the ends covered with sieve plate.The movement of food materials and other substances through phloem depends on the action of living cells called sieve tubes.

Mechanism of Transport of food and other Substances in a plant

Food materials are being translocated from the region of their manufacture or storage to the region of their utilization. The region of supply of food is called source while the area of utilization is called sink. The direction of translocation can be downward upward or both. The channels of transport are sieve tubes (sieve cells in non-flowering plants). Sieve tubes are specialized for this purpose. They are devoid of nuclei and internal membranes. The cytoplasm of one tube cell is continuous with that of adjacent sieve tube cell through sieve plates. The force required for translocation is produced by companion cells which lie adjacent to sieve tube cells. With the help of energy food, materials pass into the phloem from the region of manufacture or storage (source end). After entering the sieve tubes the nutrients being in high concentration exert an osmotic pressure which causes entry of water into this region. A high turgor pressure develops. It forces the nutrients to pass towards the region which has low turgor pressure. The movement is like a mass flow (Munch 1930). Low turgor pressure is maintained in the area where soluble food is being withdrawn for consumption or storage by an active process.