ASSERTION: According to the theory proposed by Godlewski, water rises in the stem due to living tissues.

REASON: Water rises due to pulsatory activity of xylem parenchyma and medullary rays.

Which is true : Assertion or Reason? Plese explain it.

Both the assertion and reason are true. 

According to Godlewski, ascent of sap is due to the changes in osmotic pressure of living cells of xylem parenchyma and medullary rays. These living cells absorb water due to osmosis from bordering cells (water reservoirs). The water is then pumped into xylem vessel due to lowering of pressure in living cells. The ultimate result is the upward movement of water.