Plants defend themselves from herbivores and survive extreme environments.

• 19.1 Plants actively resist being eaten.
• 19.2 This is how we do it: Do thorns really protect plants from being eaten?
• 19.3 Special adaptations help some plants thrive in extreme habitats.

Hormones regulate growth and development.

• 19.4 Hormones help plants respond to their environments.
• 19.5 Seed germination and stem elongation are stimulated by gibberellins.
• 19.6 Seedlings grow and properly orient themselves under the direction of auxins.
• 19.7 Other plant hormones regulate flowering, fruit ripening, and responses to stress.

External cues trigger internal responses.

• 19.8 Tropisms influence plants’ direction of growth.
• 19.9 Plants have internal biological clocks.
• 19.10 With photoperiodism and dormancy, plants detect and prepare for winter.