Transport of water in the xylem

• In plants, water is absorbed by the roots through extensions called root hairs 
• In flowering plants, the vast majority of water is transported through hollow, thick-walled tubes called xylem vessels
• The main force that pulls water through the xylem vessels in the stem of a plant is the evaporation of water from leaves - transpiration
• The energy for this is supplied by the sun and the process is therefore passive
• Water moves through the xylem from the point where water evaporates from the surfaces of cells surrounding the stomatal air space and water vapour diffuses out of the stomatal pore

Movement of water out of the stomata:

• The humidity of the atmosphere is usually less than that of the air spaces next to the stomata
• As a result, there is a water potential gradient from the air spaces through the stomata to the air
• Provided that the stomata open, water vapour molecules diffuse out of the air spaces into the surrounding air
• Water loss by diffusion the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells
• By changing the size of the stomatal pores, plants can control their rate of transpiration

Movement of water across the cells of a leaf:

Water is lost from mesophyll cells by evaporation from their cell walls to the air spaces of the leaf. This is replaced by water reaching the mesophyll cells from the xylem either via cell walls or via the cytoplasm

In the case of the cytoplasmic route, the water movement occurs because -

• Mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun
• These cells now have a lower water potential and so water enters by osmosis from neighbouring cells
• The loss of water…