Complex Permanent Tissues:
These are found in the vascular bundles of plants. They are of two types; Xylem and Phloem. Xylem and phloem form a continuous system inside the plants that is from the roots through the stem and leaves. They are known as vascular tissues and form vascular bundles in roots and stems.
Xylem is made of several kinds of cells. This tissue conducts water and mineral sap in plants. The important ones are thick-walled, tubular cells, often dead at maturity. Xylem performs the function of transportation of water and mineral salts from root to leaves in plants. The tissues of xylem are derived from primary meristem. The xylem tissue is found in all parts, such as the root, stem and leaves of the higher plants. It is made up of four elements; Tracheids, Vessels, xylem Fibres and xylem parenchyma.
They are elongated dead cells with both ends tapering. Tracheids are tubular, thick, lignified cells containing cell cavity. Their cell wall shows different types of thickenings. Have pores on the walls. They are found in xylem of gymnosperms, pteridophytes and angiosperms.
Vessels are rows of cells placed one above another it. Vessels are tube-like cellular structures formed of rows of dead cells placed end to end with different types of thickenings; annular, spiral, pitted and scalariform. Walls thick with lignin and have pores. End walls open and the cells join to form a long tube. They conduct water and minerals from roots to leaves and give mechanical support to xylem.
Xylem fibres are dead sclerenchymatous fibres. They are usually found in primary and secondary xylem. The walls of these cell are very thick and no pores on the walls. They provide strength to the plant organs.
Xylem parenchyma is parenchymatous. These cells are living and thin walled with cellulose walls. They stored food materials.
Phloem is a conducting tissue concerned with the conduction of food material in plants. It is made of four elements; sieve tube, comparison cell, Phloem Parenchyma and Phloem fibres. Phloem is mainly responsible for the conduction or transport of organic food prepared by plant.
Sieve tube is slender, tubular structures with perforated cross walls in the form of sieve plates. Through these plates, the food material passes from cell to cell. End walls of the cells have holes on them, which give them the name sieve. Cell wall is cellulosic.
Companion cells are long, living and rectangular cells associated with sieve tubes. They control flow of food through the sieve tubes. Cell wall made up of cellulose.
These are living parenchymatous cell concerned with food. Elongated cells and cell wall thin and made of cellulose.
These are sclerenchymatous cells associated with phloem. They are more frequency in secondary phloem and very long cells with thick lignified walls.