Compare and contrast xylem tissue and phloem tissue, including their respective structures and functions.
The stems and roots of plants contain two separate transport systems; xylem vessels and phloem tubes, of which neither transport oxygen as it is transported to cells by diffusion. The network of xylem vessels transports water and mineral ions from the roots to all other parts of the plant whereas phloem tubes transport food made in the leaves to all other parts of the plant. In the stems the tissue is collectively known as vascular tissue, within the roots they form a structure called the stele. The movement of water from roots to shoots is conducted via the xylem using mass flow. The force of cohesion - a force produced by the xylem, increases the attraction between the molecules which make up the water in the xylem. The xylem is composed of different kinds of cells; tracheids, vessels, fibres and unthickended xylem parenchyma. Both tracheids and vessels form pipes through which liquid can be moved, conducting water and supporting tissues. Fibres simply play a support role and parenchyma have a storage function. The xylem pipework in all living plants consists entirely of dead cells; composed of lignin rich tissues that once have reached maturity promptly die via a process known as apoptosis. This cell death creates a hollow tube leaving water to move rapidly, free of obstacles. Tracheids are long and thin with tapered end walls and a narrow lumen. Vessel elements however are broad, short and have wider lumen. These vessel elements are arranged in columns forming the vessels. Vessels are vital in rapid water transport from roots to shoots as little resistance to water flow is offered by their broad lumen. Pitted tracheids link with adjacent cells to form a series of pipes along the stem, these pipes increase the resistance of the pipe to the flow of water. When water evaporates from the cells via transpiration, more water is drawn into them to...
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