Transpiration and xerophytes
Pulling water up a tree
- A tall tree lifts water tens of metres — with no pump.
- The "engine" is water leaving the leaves: transpiration.
- Sticky water molecules let the whole column be pulled up.
Transpiration
- Transpiration is the loss of water vapour from a plant.
- Water evaporates from the wet cell surfaces inside the leaf.
- The water vapour then diffuses out through the stomata into the air.
Practice
Transpiration is:
Transpiration is water evaporating inside the leaf and the vapour diffusing out through the stomata.
Practice
Water vapour leaves the leaf mainly by:
After evaporating from leaf cell surfaces, the vapour diffuses out through the stomata into the atmosphere.
Cohesion and tension
- Water molecules attract each other by hydrogen bonds, so they stick together — this is cohesion.
- Loss at the top puts the column under tension (a pull), and cohesion lets the whole column be drawn up — the cohesion–tension idea.
- Water also sticks to the cellulose walls — adhesion — which helps hold the column in place.
Practice
Cohesion in the cohesion–tension theory means:
Cohesion is water-to-water attraction (hydrogen bonds); it lets the whole column be pulled up under tension.
Xerophytes
- A xerophyte is a plant adapted to live where water is scarce. It cuts transpiration with:
- a thick waxy cuticle,
- stomata sunk in pits,
- hairs that trap moist air,
- leaves that can roll up.
Practice
Which feature helps a xerophyte reduce water loss?
Sunken stomata, a thick cuticle, trapped moist air and rolled leaves all cut transpiration.
You've got it
Key idea
- transpiration = loss of water vapour: evaporates inside the leaf, diffuses out the stomata
- cohesion (hydrogen bonds) holds the water column together; it's pulled up under tension
- adhesion = water sticking to the cell walls
- xerophytes cut water loss: thick cuticle, sunken stomata, hairs, rolled leaves