Question

How does the water get to the leaves, where photosynthesis happens in vascular plants?

Answer 1

Water is absorbed by the root system of plants from the soil water. The root hair zone is the most efficient region of water absorption in roots. This absorbed water enters the xylem tissue of roots through apoplastic and symplastic pathways, from where it is transported to other parts of the plant.

Complete answer:
The ascending movement of water from roots to the tip of branches and their leaves is called the ascent of sap. Sap is water dissolved with minerals and its movement occurs through tracheary elements of xylem. Sap or water is transported from near the root tip to the shoot tip against the gravitational force, sometimes as high as 100 metres. There are several theories to explain the mechanism of ascent of sap. The three major theories are: vital force, root pressure and cohesion tension.

Vital Force Theory (pulsation theory) – According to this theory the inner most cortical cells of the root absorb water from the outer side and pump the same into xylem channels. The living cells of the root are not involved in the ascent of sap as water.

Root Pressure Theory- Root pressure is a positive pressure that develops in the xylem sap of the root of some plants. It is an indicator of active water absorption. Root pressure in certain seasons favour optimum metabolic activity or reduce transpiration. Root pressure is reduced or becomes absent under conditions of low temperature, starvation, drought and reduced availability of oxygen.

Cohesion Tension Theory- According to this theory, there is a continuous column of water in tracheary elements from roots through the stem and into the leaves. Water molecules remain attached to one another by a strong mutual force of attraction known as cohesion force. The leaves are losing water through transpiration which creates a tension, also called transpiration pull. This causes the water column to be pulled up passively from below to top of the plant.

Note: There are various objections by various scientists on the theories of ascent of sap. For example, root pressure is not found in all plants. The gases dissolved in sap shall form air bubbles which may break the continuity of water column as stop ascent of sap due to transpiration pull.