If the cut end of a tree is put in eosin solution then, A. Leaves remain fresh but ascent of sap stops. B. Phloem gets coloured because of the ascent of sap. C. Xylem elements get stained showing ascent of sap. D. Ascent of sap stops.
Hint: Eosin is a red or pink coloured water soluble stain, which can be used to track the movement of water inside the plants. As phloem does not take part in transportation of water, the phloem cells will not be stained during this experiment.
Complete answer: The water that root absorbs from soil is distributed to all parts of the plant. In order to reach the aerial parts of the plant, the water has to move upward through the stem (xylem). This upward movement of water from roots to the topmost part of the plant is called the Ascent of Sap. To observe the path of ascent of sap, one experiment can be done. Take a leafy twig of any plant (usually balsam twigs are taken as it has a semi transparent stem) and cut it under water to avoid entry of air-bubbles through the cut end. Now, place it in a beaker containing water with some eosin (a dye) dissolved in it. After some time coloured lines can be observed moving upward in the stem. If transverse sections of stem are cut at this time, only the xylem elements will appear to be filled with coloured water.
So, the correct answer is option C.
Additional information: The ascent of sap occurs through non-living xylem tracheids and vessels. Hence, only xylem elements will be coloured due to stain in this experiment. The force entirely responsible for the pull of water to great heights of plants is the transpiration pull. The properties of adhesion, cohesion and capillarity help in the ascent of sap through non-living tracheids and vessels.
Note: Eosin is a water-soluble stain. When the cut end of the plant is immersed in an eosin solution, the colored solution enters the xylem vessels and they only appear red indicating that water uptake in plants is through xylem only.