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Saturday, 12 January 2019

Transport of Water in the Xylem Tissue

 We can easily show that xylem conducts water by placing a herbaceous plant in a beaker containing Eosin solution and examining sections of the root, stem and leaf a few hours later. The xylem vessels will be stained red by the dye showing that water has moved through them. Transport of water in xylem tissue is due to • Root pressure

• Capillary action, and

 • Transpiration pull.

 Root pressure and capillary action water

Conducted through the root cells to the xylem is under pressure. This is due to the accumulation of water in the roots as a result of continuous absorption by the root hairs. This root pressure causes water to move across the root cells and up xylem vessels to a certain height.

     The xylem vessels that extend from the roots to the leaves from very fine capillary tubes. Water rises up such tubes ad a result of capillary action. This is due to the attraction between the water molecules and the walls of the xylem vessels.

      Activities 4 and 5 demonstrate that root pressure and capillary action cause water to rise in xylem vessels. However, root pressure and capillary action are not powerful enough to cause water to move up the xylem vessels to the leaves.

 Transpiration pull

 Most of the water that is absorbed by the toots of the plant, move up the xylem vessels vessels to the cells of the leaf. Here , it evaporates into the intercellular spaces and diffuses out of the plant as water vapour through the stomata. energy for the the evaporation of water comes from the surrounding cells and the sun.

     The loss of water from the leaf surface of pal ts by evaporation is called transpiration. The continuous flow of water evaporates from the leaf cells and as photosynthesis produces more sugar molecules, the osmotic pressure in the leaf cells increases (with respect to that in the cortical cells and the xylem). This causes more water to flow into the leaf cells from the xylem vessels. There is a 'pull' on the water columns in the xylem vessels, and water is drawn up. These water column have great tensile strength and do not break because of the attractive forces

 • Which hold the water molecules together (cohesion), and

 • Between the molecules of the xylem vessels and water (adhesion).

 The forces which maintain the transpiration stream are very strong. These forces constitute the transpiration pull which plays an important role in the ascent of water in the xylem vessels of stems.  
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