Explain Cohesion-transpiration theory.

The Cohesion-transpiration theory was given by Dixon and Jolly, in the year 1894. This hypothesis is based on three main facts. 
(a) Cohesion between water molecules keeps the xylem vessel fully filled with water. The molecular force between water molecules is strong enough to hold together a column of water as high as 400 meters. The capillary attraction between water and walls of vessels also helps to maintain the column of water.
(b) Xylem vessels are so arranged that they are in continuity from root to leaves. So columnar continuity of water is maintained. 
(c) Plants transpire water. It creates a reduced pressure in leaves. So, a suction pressure is exerted on the water column and the water column is drawn up because of suction, due to transpiration. The root pressure which draws in water from the soil also helps to keep the xylem filled with water. The suction due to transpiration is transmitted through a continuous column of water, from root tip to leaves and whole water column rises up.