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Push/pull theory (Click to select text)
Plants need much more water to survive than animals do, a sunflower must take in 17 times more water than a human of comparable weight. At least 500 pounds of water must be lifted from the roots to the leaves to make a single pound of organic tissue. This tremendous amount of water is required to undergo photosynthesis each and every day of a plant\'s life. But how does a tree lift hundreds of gallons of water hundreds of feet in the air against the force of gravity? Scientists have developed two theories to explain this \"impossible\" phenomenon. In the following, I will discuss the theories and show how the pull theory is more viable. The push theory states that positive pressure pushes up from the roots continuously and that force maintains a constant flow of water circulating throughout the plant. This constant pressure that exerts force from the water it takes from the ground, supposedly should provide enough power to continue the circulation and distribution of water efficiently throughout the entire plant. Proof of root pressure is exhibited through a process called guttation. Through guttation, tiny droplets of water are sometimes forced out of some plants. Guttation only works when soil is almost completely saturated with water and leaves are not losing much water through evaporation. When the xylem is full of water, the molecules will not move back down the structure, but rather need to be expelled from the plant. They will move, by the process of osmosis, to an areas where there is the least amount of pressure. These areas are usually openings located on plant leaves. Once the water molecules reach the leaves, the excess water will be forced out of the plant by root pressure. This push theory presents two major inconsistencies 1)many plants exhibit no root pressure and 2)root pressure is not strong enough. In many plants and trees, there is a substantial amount of root pressure that could possibly explain the movement of so much water throughout a tree, defying gravity. However, in many plants, root pressure is not nearly enough to exert the amount of force needed to circulate such large quantities of water throughout the plant and against gravity. There is also absolutely no root pressure in many trees and shrubs. When a plant stem is punctured, water does not spew out, but rather, air is sucked in. So how can this theory be correct? The pull theory was developed in 1895 by Irish plant physiologists H. H. Dixon and J. Joly. They proposed that water was being pulled up the plant by tension, or negative pressure, from above. The above force pulling the remaining water inside the plants was due to the water evaporating from the leaves on the surface. The water being evaporated is bonded to the other water molecules inside the plant because of the hydrogen bonds that exist between the two H2O molecules. The intermolecular forces between these molecules pull each other, starting from the top layer of molecules being evaporated all the way to the initial molecules being pulled in from the ground through the roots. Water molecules tend to cohere very strongly to each other. This essentially forms a chain of molecules within the xylem pipeline. Water evaporates from the leaf surface, this, in turn, causes the next water molecule in line to move up and take its place. Each molecule lost moves the chain further up and maintains a constant flow of water in the xylem column. As the chain moves up, the force causes more water to be taken in from the roots to continue the sequence. This process will continue and maintain a constant tension as long as evaporation continues and as long as the water column remains unbroken. The plant expends no energy during this process. There are two main problems with this theory however. 1)The exhibit of cavitation and 2)the need for constant negative pressure. Cavitation is the formation of air bubbles in the xylem column. These bubbles disrupt the flow of the water molecules, breaking the chain of succession. This explains why even the tallest trees don\'t grow beyond 350 feet. The need for constant pressure also makes the theory inconclusive. If there is not a constant negative pressure exhibited throughout, the plants\' insides would boil. So what keeps the pressure constantly negative? Scientists have not been able to easily measure the pressure inside plants to accurately answer this question. Though neither one of these theories has been proven, studies show that the viability of the pull theory is much more accurate than that of the push theory. I believe it could be a combination of both push and pull, as do many scientists. Both theories have much evidence to show the possibility of their happenings, however, both theories also posses reasonable arguments that implore doubt on their concepts. There is still no legitimate argument found to answer the question that has puzzled scientists for so long- how does a tree lift hundreds of gallons of water hundreds of feet in the air against the force of gravity? -erica http://www.employees.csbsju.edu/ssaupe/biol116/plant_transport.htm http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/X/Xylem.html http://www.k-hahn.de/index.html Wright,K (2002). Antigravity Plumbing. From Discover, Vol. 23, No 9, pps: 20-21 Webster\'s New World Dictionary (1995). Xylem, p:685. Guttation, p:264. Cavitation, p:96. Root Pressure, p:512 . Transpiration, p: 627 Encarta (1999).computer version-- . Xylem, Transpiration
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