19  •  Plants Respond to Their Environments 763

Defending and regulating while rooted in the ground

Plants defend themselves from herbivores and survive extreme environments. 764

19.1

Plants actively resist being eaten. 764

19.2

This is how we do it: Do thorns really protect plants from being eaten? 767

19.3

Special adaptations help some plants thrive in extreme habitats. 768

Hormones regulate growth and development. 770

19.4

Hormones help plants respond to their environments. 770

19.5

Seed germination and stem elongation are stimulated by gibberellins. 771

19.6

Seedlings grow and properly orient themselves under the direction of auxins. 773

19.7

Other plant hormones regulate flowering, fruit ripening, and responses to stress. 775

External cues trigger internal responses. 778

19.8

Tropisms influence plants’ direction of growth. 778

19.9

Plants have internal biological clocks. 780

19.10

With photoperiodism and dormancy, plants detect and prepare for winter. 782

StreetBIO: KNOWLEDGE YOU CAN USE

Practical botany and Aloe vera: can Aloe vera lessen the pain or duration of burns? 784