Chapter 25 Hormones and Plant Responses

Plant Growth

• Unlike animals, plant have no true pattern of growth

- no pre-determined number of branches and leaves

• Also unlike animals in adulthood, who stop growing, plants continue to grow by:

- adding new needles- adding new wood- produce cones or flowers

Plant Hormones

• Plant Hormones - chemical substances that control a plant’s division, growth, maturation, and development

- can also control plant’s response to the environment

• Auxins – plant hormone found in the seedling tip or apical meristem that stimulate cell elongation

- Stem is stimulated to grow toward the light- Roots cells are inhibited and forced to grow

downward- Also responsible for roots growing around objects or obstructions

Plant Hormones

• Cytokinins – plant hormone that stimulates cell division and growth of lateral buds

- may also cause dormant seeds to bud- delays aging of leaves* counteracts effects of auxins- Rate of cell growth in plants is

determined by the ratio of auxins to cytokinins

Plant Hormones

• Gibberellins – hormone known to produce dramatic increases in size of stems and fruit

- also responsible for rapid early growth

• Ethylene – hormone that stimulates the ripening of fruit in the plant

Ex: lemons and tomatoes

Plant Responses

• Tropism – responses of plants to external stimuli

- can change patterns and directions of growth- Plants can demonstrate positive

(toward stimulus) or negative (away from stimulus) tropisms

Plant Responses

• 3 kinds of plant tropisms:1. Phototropism – response or growth

toward light2. Gravitropism – response to the force

of gravity – plant roots 3. Thigmotropism – response to touch

Ex: growth of vines and climbing plants

Plant Responses

• Photoperiodism – responsible for timing of seasonal activities such as flowering and growth

- Phytochrome plant pigment that causes photoperiodism

Ex: - short day plants like poinsettas- long day plants like spinach