The tallest living tree known is a coast redwood (Sequoia sempervirens) named Hyperion, about 116 m tall (taller than a 35-storey building). Giant sequoias are bulkier but not as tall. Why do trees not grow ever taller?

Part of the answer is the water pull of lesson 2. Every extra 10 m of height costs another 0.098 MPa of pull just to balance gravity, and the longer path adds friction. So:

• the leaves at the top must work at a more negative water potential than leaves lower down;
• the water columns are under more tension, which raises the risk of cavitation;
• the top leaves have a harder time keeping their cells turgid, which they need to expand and to keep stomata open for photosynthesis.