Q. Transpiration and root pressure cause water to rise in plants by :
(a) Pushing and pulling it respectively
(b) Pulling it upward
(c) Pulling and pushing it respectively
(d) Pushing it upward
Solution :
Correct option : C (i) The physiology of water uptake and transport is not so complex. The main driving force of water uptake and transport into a plant is transpiration of water from stomata present in the leaves. (ii) Transpiration is the process of water evaporation through specialized openings in the leaves, called stomata. The evaporation creates a negative water vapor pressure, developed in the surrounding cells of the leaf. (iii) Once this happens, water is pulled into the leaf from the vascular tissue, the xylem, to replace the water that has transpired from the leaf. (iv) This pulling of water, or tension, that occurs in the xylem of the leaf, will extend all the way down through the rest of the xylem column of the tree and into the xylem of the roots due to the cohesive forces holding together the water molecules along the sides of the xylem tubing. (Remember, the xylem is a continuous water column that extends from the leaf to the roots.) Finally, the negative water pressure that occurs in the roots will result in an increase of water uptake from the soil. (v)Now if transpiration from the leaf decreases, as usually occurs at night or during cloudy weather, the drop in water pressure in the leaf will not be as great, and so there will be a lower demand for water (less tension) placed on the xylem.