One molecule of water joins with four other water molecule by hydrogen bonds. In a water molecule, one end has positive charge and the other end has negative charge. This difference in charge creates polarity in the molecule.
The cohesion of water molecules helps transport water from roots to the leaves in plants. Plants absorb water from the soil by osmosis. They absorb mineral ions by active transport, against the concentration gradient. Root hair cells are adapted for taking up water and mineral ions by having a large surface area to increase the rate of absorption.
Cohesion is a measure of how well molecules stick to each other. Water molecules is a typical example of Cohesion.
Hydrogen bonds are formed between each water molecule, enabling them stick to each other strongly. Due to this sticky nature of water molecules, they form droplets on surfaces (e.g dew drops) and a dome like-shape when filling a container just before it overflows.
Cohesion also produces Surface Tension; a phenomenon that makes it possible for light objects like needle to float when placed gently and insects to walk on water.
Also aided by capillary action (the movement of a liquid across the surface of a solid caused by adhesion between the two), Cohesion makes it possible for water to be taken as single huge molecule from Xylem in the roots of plant to its leaves.
