2.81 Phototropism

Positive phototropism of stems.
Light - growth - towards light.

Phototropism = "The way a plant grows or bends in response to light"

A. With light coming from all directions on the stem, it grows forwards and upwards.
B. The light is on one side of the stem, the plant bends in the growth towards the light source.

Auxin - Is a plant hormone. It is made at the top, & diffuses down the stem, & this stimulates growth. Auxin  diffuses away from the light. So in image "B", the side of the stem in more shade, is where more auxin is found, so grows faster. This causes the stem to bend towards the light.

Why do you think sunflowers were given their name?

2.80 Geotropism

Geotropic response of roots & stems.
"Geotropic" = gravity to growth response.

Sum up: Tropisms occur when plants respond to external stimuli. Tropisms are movements are movements caused by a change in a plant's growth pattern.

Negative or Positive?
Tropisms can be negative or positive. If the plant moves toward stimulus, the tropism is defined as positive. If the plant moves away from the stimulus, the tropism is considered negative. 


In this image, the trunk exhibits a positive tropism, and the roots exhibit negative tropism.

Example in a seed:

2.79 Plants & Stimuli

How plants respond to stimuli:

1. Stimuli are changes in the environment. Eg: Temp, light.
2. The plant has "Receptors" which can recognize the stimuli, & then change it into a "Response."
3. The response is often "Growth", which is called a "Tropism."

There are three types of stimuli that include tropisms that change the shape of the body.
These are:

1. Light-phototropism, = the growth of a plant shoot towards or away from the light.
2. Touch-thigmotropism, = the direction growth that is a response to touch.
3. Gravity-gravitropism, = is a response to gravity.

2.54 Transpiration

http://www.youtube.com/watch?v=At1BJJDcXhk

Transpiration: is the evaporation of water from the surface of the plant.
Light energy is turned to heat energy as the structure of the leaf absorbs the light.
Evaporation: takes place through the stomatal pores.

-1. Sunlight warms the leaf.
-2. Water phase to gas phase
-3. The diffusion of water vapor through the stomatal pores.

2.53 - Roots

Features in a root structure: branching pattern. This increases the surface area for absorption of water.
Process of "absorbtion": active transport of minerals from roots.

- 1. Active transport takes up minerals into their cytoplasm making it concentrated.

-2. Water is transported from roots to the leaf by the xylem tissue. Water enters the cell because it is concentrated inside the cells and its is dilute in the soil water.

-3. Osmosis takes place, changes water from dilute to concentrated.
Roots branch out to find a water source. They have hairs, caused by cell wall extended outwards "(this increases the surface area absorption of H20)"