Transpiration = main force that pulls water up stem of a plant = evaporation of water from the leaves.
Movement of water out through stomata
Humidity of atomsphere = less than that of air spaces next to stomata usually.
When stomata are open water vapour molecules diffuse into surrounding air.
Water lost form air spaces = replaced by water evapourating from cell walls of surrounding mesophyll cells.
Plants can control rate of transpiration by changing the size of stomatal pores.
Movement of water across cells of a leaf
Water = lost from mesophyll cells by evaporation from their surfaces to air spaces next to the leaf.
Replaced by water reaching mesophyll cells from xylem - either by symplastic or apoplastic pathways.
Water movement in case of symplastic pathway occurs because:
- Mesophyll cells lost water to air spaces.
- Cells have lower w.p. than neighbouring cells - water enters by osmosis.
- Neighbouring cells now have lower w.p. than their neighbours - osmosis continues in turn.
Water potential gradient is established - pulls water from xylem, across leaf mesophyll and into the atmosphere.