XylemStructureXylem's structure serves a duel purpose; support and transport. Cells that will become xylem add more material to the primary cell walls that most plant cells lay down. The thicker walls can be either disconnected rings or extensive secondary cell walls that cover the cell almost completely. Secondary thickenings are made up of cellulose and lignin. Lignin is a tough organic compound that makes wood strong and dense. When the cell walls of xylem conducting cells are complete they die. The contents within the cell disintegrate and leave a strong hollow cylinder filled with water. Water can travel in kind of a straight line due to the fact that these cells are stacked on top ...view middle of the document...
Water can't just defy the laws of gravity. This is why root pressure is a major part of root transport. The pressure depends on active transport of ions from the soil to the xylem in the center of the root. Osmosis is responsible for the water following, in result this builds up pressure in the xylem vessels. Since the endodermis keeps water from escaping back to the soil the water climbs up. Active transport in roots uses a considerable amount of energy so then; they need a nice supply of oxygen. When the soil is full of moisture and oxygen and it's rather humid out, guttation occurs. Guttation is when the leaves are close to the roots and the pressure is high in the roots; because of the humidity in the air, the water can't evaporate fast enough and water pokes out the tips of the leaves. (Aww, cute, it pokes out) Root pressure does not account for all the transport of water; it can only push water up about a meter, not much further. That means there must be something pulling from above, such as transpiration pull. Transpiration is the evaporation of water from the leaves. The way in which water is moved upward depends on the properties of water. (1) Transpiration occurs. (2) Water loss creates deficit which must be quickly remedied. The neighboring cell walls replace the water and this process goes on for a bit. After there aren't any neighbors to draw water from the water lost is replaced by water from a conducting cell in the xylem at the tip of a veinlet in the leaf. (3) Water molecules attract each other, in other words they're coherent. (4) Xylem vessels and tracheids are narrow tubes. Transpiration is responsible for getting the process going. Cohesion allows it to keep going. That's why it's called ...
