XYLEM: TISSUE RESPONSIBLE FOR WATER TRANSPORT

The xylem is a complex tissue found in vascular plants that plays a crucial role in the transportation of water and minerals from the roots to the other parts of the plant, including the leaves and stems. It is composed of various specialized cells that work together to create a continuous system of tubes for water movement.

Structure of Xylem
The xylem tissue is made up of four main types of cells: tracheids, vessel elements, fibers, and parenchyma cells. These cells are arranged in a specific manner to form different structures within the xylem.

1. Tracheids: Tracheids are elongated cells with tapered ends and overlapping secondary cell walls. They are the main conducting cells in gymnosperms and some vascular plants. Tracheids transport water through pits, small openings in their walls, which allow for lateral movement of water.

2. Vessel Elements: Vessel elements are wider and shorter than tracheids and have perforated end walls called sieve plates. These sieve plates allow for efficient water movement between adjacent vessel elements, forming continuous tubes for water transport.

3. Fibers: Fibers are long, slender cells that provide structural support to the xylem tissue. They have thick secondary cell walls composed of lignin, making them rigid and strong.

4. Parenchyma Cells: Parenchyma cells are thin-walled cells that surround the tracheids, vessel elements, and fibers. They function in storage and metabolic activities.

Transport Mechanism
Water and minerals are transported through the xylem tissue in a process called transpiration. Transpiration occurs when water is lost from the plant through small openings called stomata on the surface of leaves. This creates a water potential gradient that pulls water up from the roots through the xylem vessels.

The cohesion-tension theory explains the mechanism of water transport in xylem. According to this theory, water molecules are cohesive, meaning they stick together due to hydrogen bonding. As water is lost from the leaves, it creates a negative pressure or tension that pulls water molecules upward through the xylem. This tension is transmitted through the continuous water column in the xylem, allowing water to be pulled up from the roots.

PHLOEM: TISSUE RESPONSIBLE FOR FOOD TRANSPORT

Phloem is another complex tissue found in vascular plants that is responsible for the transportation of food, primarily sugars, from the leaves to other parts of the plant. It is a living tissue composed of four main types of cells: sieve tube elements, companion cells, phloem fibers, and phloem parenchyma cells.

Structure of Phloem
1. Sieve Tube Elements: Sieve tube elements are elongated cells that form the main conducting cells in the phloem. They are connected end-to-end through sieve plates, which have small pores that allow for the movement of sugars and other organic compounds.

2. Companion Cells: Companion cells are closely associated with sieve tube elements and provide metabolic support to these cells. They are responsible for loading sugars into the sieve tubes and maintaining their functions.

3. Phloem

Phloem transport soluble food in it whereas xylem transport water along with mineral