The vascular (transport) system in plants is made of vascular bundles. The vascular bundles aremade of xylem and phloem which are separated by a wall called vascular cambium, often simplyshortened as cambium (see diagrams discussed in the previous section).
1. Xylem: It is the vascular tissue that transports water across the plant body. Xylem is made up of fourdifferent types of cells. They are tracheids, vessels, xylem fibres and xylem parenchyma. Ofthese only tracheids and vessels are involved in the transport of water and minerals.
2. Tracheids: Tracheids are elongated dead cells that have sloping end walls. The cavity is empty as the cellsare dead. The walls are thickened with a material called lignin to prevent them from collapsingas water is transported up the plant. These thickenings are in different patterns. The cells arearranged end to end.
3. Vessels: Vessels are also dead cells that have variously patterned thickened walls. These thickenings aredue to lignin. The vessels are arranged end to end. The end walls of the vessels are eitherpartially or fully dissolved. This results in the formation of long tubes that carry water.The xylem vessels and tracheids together form long tubes that have a narrow diameter. Thus theyfunction as capillaries (narrow tubes) to transport water.
4. Phloem: It is the vascular tissue that transports organic substances like sucrose across the plant body. It ismade up of four types of cells namely sieve tubes, companion cells, phloem fibres and phloemparenchyma. Except for phloem fibres, all the other three types of cells are living. Sieve tubesand companion cells are mainly involved in the transport of the materials.
5. Sieve tubes: They are tubes formed by cells that are joined end to end. The end walls of these cells haveperforations. The mature sieve tube cells are enucleated. The cytoplasm of the sieve tube cells iscontinuous through the perforations of the end walls. This helps in the transport of materials.
6. Companion cells: They are smaller cells associated with the sieve tubes. They have dense cytoplasm and elongatednucleus. They are in contact with the sieve tube cell through pores in the wall.