Question

A column of water within xylem vessels of tall trees does not break under its weight because of
A. Positive root pressure
B. Dissolved sugars in water
C. Tensile strength of water
D. Lignification of xylem vessels

Answer 1

Hint: The upward movement of water from the root to the aerial part of a plant is called the ascent of sap. Water molecules form a continuous column with the help of two forces that act, i.e. cohesion and adhesion.

Complete step by step answer: There is a continuous column of water from the root through the stem and to the leaves. Water molecules are attached with each other by the cohesion force, and by walls of the tracheary by the adhesion force. This high tensile property depends on cohesion, adhesion and also the surface natural phenomenon property of water. The strength of water has the ability to resist the effect of external stress and maintain flow. Dissolved sugar increases the solute potential, while root pressure provides a push to water only up to a particular extent. Lignin provides mechanical support to the xylem vessels. The cohesion between water molecules prevents the column from breaking. The latter doesn't dissolve in water because it is not polar. Xylem vessels: incorporates dead hollow cells because the walls are lignified and also the cell contents disintegrate. The lignin makes the tissue layer impermeable. It also helps to make the vessels extremely strong and prevents them from collapsing. The root pressure is the positive pressure that develops in the roots of plants by the active absorption of the nutrients from the soil. This creates the negative pressure or the tension in the xylem vessels, from the surfaces of the leaves to the tips of the roots, through the stem.
Hence, the correct answer is option D.

Note: Unlike xylem (which consists primarily of dead cells), the phloem consists of still-living cells that transport sap. The sap is also a water-based solution, but rich in sugars made by photosynthesis.