Hormones are organic compounds that are effective at very low concentrations. These chemicals are usually synthesized in one part of the plant and transported to another location. They interact with specific tissues to causes changes such as growth and fruit ripening. Because hormones stimulate or inhibit plant growth, they are referred to as growth regulators.

Five groups of plant hormones:

Auxins - hormones that promote plant-cell elongation, apical dominance, and rooting.
Gibberellins - a group of hormones that primarily stimulate elongation growth.
Ethylene - the hormone responsible for the ripening of fruit.
Cytokinins - a group of hormones that promote cell division.
Abscisic acid - a hormone that generally inhibits other hormones.

There is a group of synthetic chemicals that does not fit into any of the five classes of hormones. Growth retardants are chemicals that prevent plant growth. Most of the developed chemicals slow the growth of woody plants.

Tropisms:

plant movement toward or away from an environmental stimulus.

 

• Phototropism - a growth response to light. Solar tracking is the phototropism of leaves or flowers as they follow the sun's movement across the sky.
• Thigmotropism - a growth response to contact with a solid object. Thigmotropism allows vines to climb. It is thought that an auxin or ethylene are involved in this response.
• Gravitropism - a growth response to gravity. Roots are positively gravitropic, usually growing downward and stems are negatively gravitropic, usually growing upward. Auxins are probably responsible for this growth.
• Chemotropism - a response to chemicals. The growth of a pollen tube is in response to chemicals produced produced by the plant ovary.
• Hydrotropism - a response to water. Most plants have a positive response to water.

Photoperiodism:

plant response to changes in the length of days and nights

 

• Critical length is the length of daylight above or below which a species of plant will flower.
• Long-day plants flower only when exposed to day lengths longer than their critical length. These are usually late spring and early summer flowers.
• Short-day plants flower only when exposed to day lengths shorter than their critical length. These are usually early spring and fall flowers.
• Day-neutral plants are not affected by the length of days and nights.

EOI Question # 6.

Which question is this apparatus most likely designed to answer?

A . Does atmospheric pressure affect transpiration?
B. Does gravity affect the direction of plant growth?
C. Does turgor pressure affect the absorption of water?
D. Does absorbed light increase the environmental temperature?

 

Answer the following questions. Head your paper Unit 13 Day 4 Plant Tissues

1. Why don't all species of plants produce flowers at the same time?
2. Why do tree leaves change color in the fall?

Assignment # 16

Assignment # 17

Assignment # 18

With your Lab partner:

1. Place a thick fold of paper towel in the bottom of a plastic petri dish.
2. Liberly wet the towel.
3. Lay several seeds on top of the wet towel and close the dish.
4. Keep the towel moist for the length of the activity.
5. After the seeds begin to sprout, turn the dish on edge and observe how the roots and stems begin to grow.