Question

Which of the following is a correct statement about the cohesion–tension theory for the ascent of water in the xylem?

• A. The evaporation of water from leaf mesophyll cells generates tension in the water column
• B. Water moves from a source region with high hydrostatic pressure to a sink region with low hydrostatic pressure
• C. The cohesion of water molecules is responsible for the formation of embolisms (air bubbles) in the water column
• D. Living cells of the xylem provide the main conduit for an ascent of water

Answer 1

The correct option is A The evaporation of water from leaf mesophyll cells generates tension in the water column

According to cohesion–tension theory, transpiration is the main driving force of water movement in xylem.
During this, water from the surface of mesophyll cells is drawn into nearby internal air space and then finally to atmosphere. This creates a negative water tension in mesophyll cells.

The most distinctive xylem cells, for water transporation / conduit for ascent of water are the tubular dead cells called xylem tracheids and vessels not living xylem parenchyma.
As opposed to all other tissues of plant body, xylem cells are having negative turgor pressure, caused by transpiration, this negative turgor pressure ensure’s ascent of sap. The cells of xylem are dead at maturity except of parenchymatous cells. They lose their end walls become a hollow tube, they are strengthered by lignin. Lignin gives strength and support to xylem cells.