Plant Responses and Hormones Plants respond to internal and external stimuli which cause them to change their growth and development.

Plant Hormones A group of chemicals called hormones can control plants growth and development. Plant hormones are chemical substances that control patterns of development as well as plant responses to the environment Most hormones promote growth.

Auxins Auxins stimulate cell elongation Responsible for plant responses to external stimuli (tropisms)

Cytokinins Cytokinins stimulate cell division and cause dormant seeds to grow.

Gibberellins Responsible for stem elongation and seed germination Gibberellins cause dramatic increases in size of the plant, especially stems and fruit. Dwarf plants have very little gibberellins

Which Banana Ripens Faster? Bananas in a bunch, single banana, or it makes no difference? Class tally: Answer: Single Bunch No difference IIIIIIIIIIIIIIIIIIIIIII IIIIII IIII Bananas in a bunch ripen fastest!

Ethylene Ethylene stimulates fruits to ripen. It is a gas - “One bad apple spoils the bunch”

Tropisms Plants grow, reproduce, and shift the position of their roots, stems, and leaves in response to environmental conditions such as light, gravity, and touch. The responses of plants to external stimuli are called tropisms.

Phototropism Plants grow toward the light

Gravitropism Roots move down and shoots grow up in the dark in response to gravity.

Photoperiodism Some plants respond to the amount of daylight available to them Example 1: Poinsettia turn red and flower in response to shorter days Example 2: Some plants flower in response to longer days such as the spinach plant