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is the pressure created in xylem enough to conduct water in tall tree ? give reasons.
Almost every vascular plant consists of tracheids of large diameters; particularly angiosperms contain additional structures called tracheae.  Such structures are present in large numbers and they are always filled with water where the water is continuously transported upwards as a column of water day and night.
The upward movement of water from the root to aerial parts of the plant body is called ascent of sap or often called translocation of water. Xylem elements are mainly responsible for the movement water upwards.  Among the four xylem elements, tracheids and trachea from a kind of continuous tubular system running from the root system to aerial branches.  The end to end association of trachea with their disintegrated transverse walls provides tubular and network of channels for the movements of water.  The trachea and tracheids with their large lumen and end to end association act as excellent pipelines for the movement of water, minerals and some organic compounds as well.  Living cells, by rhythmic pulsatory activity are believed to pump water upwards, just like the heart pumping blood. It is not just root pressure but the  forces  called cohesive forces are actively taking part.; they are nothing but hydrogen bonds between the water molecules.  The overall strength of water column in such narrow xylem elements has been estimated to be many folds higher than the transpiration pull and the gravitational pull put together.transpiration pull, the water column found in xylem elements is virtually pulled upwards and outwards from one end, but the same water column is also subjected to another opposing force called gravitational pull.  Because of this, the water column becomes narrow because of tension. As the outer region of the water column is still adhered strongly to the hydrophilic xylem wall materials, the xylem cells are also subjected to tension, hence they become narrow. As the water column is strong enough to withstand the opposing pulls, the direction of the movement of water depends upon the strength of the opposing forces.  It has been found that the transpiration pull is greater than its counter force.  So water is pulled upwards.  It has been estimated that the transpiration pull of one atmospheric pressure can pull the water up to 15-20 feet in height.  With the cell walls, and perforated transverse septa acting as resistant forces, still one atmospheric pressure can pull the water up at least to the height of 10 feet.  The tallest plants known to mankind are about 400 ft. in height.  And the forces required are about 40 atms or 40 bars.  Even an ordinary plant like tomato or helianthus is known to develop a transpiration pull to the extent of 100-200 atmospheric pressures; which is more than enough to pull the water to the height of 1000-2000 ft.While transpiration pull is responsible for the upward movement of water, any mineral salts that are absorbed will be loaded into the water column, thus even mineral nutrients move upwards and also laterally for the nonliving xylem elements are in association with living parenchymatous cells, thus mineral nutrients  are also transported.


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