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

Internal Stimuli: A group of chemicals called hormones can control plants growth and development. A hormone is a chemical that is produced in one part but can cause a reaction in another part. Most types of plant hormones promote growth. Examples include:

Auxins which stimulate cell elongation. Gibberellins increase size of stems and fruit. Cytokinins which stimulate cell division. Ethylene speeds up the ripening of fruits.

External Stimuli: Plants grow, reproduce and shift the position of their roots, stems and leaves in response to the environmental conditions such as gravity, touch, light and cold temperatures.

A tropism is a plant’s response to external stimulus. Positive tropism: grow toward the stimulus. Negative tropism: grow away from stimulus.

Types of tropisms: Phototropism- growth of plant toward the light.

Gravitropism- direction of plant growth in response to gravity.

Photoperiodism: responsible for timing of seasonal activities such as flowering by responding to the length of light. Short day plants only flower when exposed to extended periods of dark. Long day plants flower only when exposed to a short period of darkness.