TOPIC 06-TRANSPORTATION OF LIVING MATERIALS -02
TRANSPORT IN PLANT
Introduction:
The transport system in plants is not as complex as that of animals. Materials are transported by vascular bundles made up of xylem and phloem tissues. Xylem tissue transports water and mineral salts from the soil to all parts of the plant. Phloem tissue transports manufactured food from the sites of photosynthesis to all parts of the plant. In between the xylem and phloem is cambium. The cambium divides to form newxylem and phloem
Vascular bundle in a stem showing the position of cambium
Components of the vascular system
Xylem
Xylem tissue is made up of the xylem vessels and the tracheids. Mature xylem vessels and tracheids are made up of hollow and dead cells. Their walls are made of cellulose and lignin. Lignin strengthens the cell walls and makes them rigid. Therefore, xylem has an additional function of giving support to the plant.
Xylem vessels
The movement of substances in the xylem is always upward and is by conduction. A xylem vessel is made of hollow cells without end walls. These cells are joined end to end to form a pipe-like structure. See above Xylem vessels begin in the roots, go up through the stem and branch into every leaf of the plant.
Xylem vessels have no cytoplasm and nuclei. This enables them to transport a larger volume of water and mineral salts.
Tracheid elements are elongated with pointed (tapering) ends (Figure. below) they are also laid end to end to enable continuous flow of water. Their end walls have perforations (pits) unlike in xylem where end walls are missing. This makes them less efficient in conduction of water.