The diameter of the xylem vessels should be small in order for the capillary action to take place.
Explanation:
The vessels through which water is transported to different parts of the plants and trees are the xylem. Water is transported in the trees through a mechanism named as capillary action. This mechanism can occur only if the diameter of the vessels are small.
The water molecules adhere to the walls of xylem and an upward force is generated. If the diameter of the vessels i.e xylem would be small then the surface tension between the xylem and water molecules would be enough to move the water in an upward direction.
