1) Explain the formation of complex permanent tissue in plants. Mention the two types of complex tissues and write their functions.
Xylem: It is the principal water-conducting tissue in vascular plants. Its main functions are transportation of water and solutes from root to the leaves, to provide mechanical strength to the plant and food storage. It forms a continuous vascular system throughout the plant body along with the principal food-conducting tissue or the phloem. The first xylem differentiates in the embryo or young seedling. New xylem along with phloem develops continuously, as the plant grows, from the apical meristem. The vascular tissues differentiating in the primary plant body are called primary xylem and primary phloem, respectively. The meristematic tissue that leads to the development of vascular tissues and is their immediate precursor in the procambium. In some plants, secondary growth is observed in which additional growth takes place leading to the thickening of stems and roots. This growth is called secondary growth. This growth is partly due to the activity of vascular cambium (lateral meristem) that leads to the production of secondary xylem and secondary phloem.
Phloem: It is the principal food-conducting tissue of vascular plant. It transports food from the leaves to other parts of the plant body. A wide range of substances such as sugars, amino acids, micronutrients, lipids, hormones, floral stimulus are transported in the phloem. Phloem also transports water to fruits, young leaves and storage organs such as tubers. As already discussed, phloem is associated with xylem in the vascular tissue. The primary phloem differentiates in the embryo or young seedling while the secondary phloem differentiates from vascular cambium.
Complex permanent tissueA complex permanent tissue may be classified as a group of more than one type of tissue having a common origin and working together as a unit to perform a function. These tissues are concerned with transportation of water, mineral, nutrients and organic substances. The important complex tissues in vascular plants are xylem, phloem. Complex permanent tissue is heterogeneous tissue which is formed of two or more than two types of mature cells of common origin which coordinate their activities to form a specific function. Conducting tissues of plants are complex permanent tissues. They are of two types, xylem and phloem.Xylem Xylem is a chief, conducting tissue of vascular plants. It is responsible for conduction of water and mineral ions.Xylem is a very important plant tissue as it is part of the plumbing of a plant. It carries water and dissolved substances throughout and consists of a combination of parenchyma cells, fibers, vessels, tracheids and ray cells. Long tubes made up of individual cells are the vessels, while vessel members are open at each end. Internally, there may be bars of wall material extending across the open space. These cells are joined end to end to form long tubes. Vessel members and tracheids are dead at maturity. The pit pairs allow water to pass from cell to cell. While most conduction in the xylem is up and down, there is some side-to-side or lateral conduction via rays. Rays are horizontal rows of long-living parenchyma cells that arise out of the vascular cambium. In trees, and other woody plants, ray will radiate out from the center of stems and roots and in cross-section will look like the spokes of a wheel.Xylem caries water and disolved nutrients from the roots throughout the plant.Phloem carries/ distributes the products of photosynthesis (mainly from the leaves) to the rest of the plants.A good way to remember their function is:Xylem is the upward conduction of water while phloem is the upward and downward conduction of food and minerals.