2.81 Positive Phototropism.

• Positive phototropism - positive suggests that it is towards light.
•                                    - photo refers to light.
•                                    - tropism refers to growth in response to this light.

•    uniform light coming from all directions means the stem will grow forwards and upwards.

•     however, if the same stem only has light coming from one side (lateral light source) it causes the plant to bend in a grove towards the light.

•  the light on this side causes the opposite side of the stems auxin (plant growth hormone) to grow more which causes the bending.

2.80 Geotropism

• Geotropism= Geo refers to gravity and tropism refers to growth response.
• For example a seed:  
• We find that the embryonic root will grow downwards and this is described as positive geotropism.
• Where as the embryonic shoot will grow upwards towards the soil surface - negative geotropism.

• A different experiment is if you rotate that seed;
• You will see that the embryonic shoot will grow upwards- negative geotropism
• Also that the embryonic root will turn and grow downwards- positive geotropism

2.79 plants respond to stimuli

• Stimuli are changes in the environment. e.g temperature changes or light changes.

• The plant has receptors that detect the stimuli and then turn those into a response.

• The responses often take the form of growth. this type of response to a stimulus is called a tropism.

• Tropisms that involve: light are called phototropism. involve gravity are called geotropism.

• The connection between the receptor and the response usually takes the form of plant hormones sometimes called plant growth regulators.  e.g. Auxin  

2.54 Transpiration

• for evaporation to occur: water (liquid) ---->  water vapour (gas).     and for this to occur you need heat. to get this heat we use sunlight

• water is delivered in from leaf by xylem, and from there it moves through the spongy mesophyll layer and then to just above the stomatal pore. 

• then the gas water vapour diffuses through the stomatal pore through a fairly steep diffusion gradient,  as the water vapour inside the stomatal pore is at 100% humidity and outside the leaf the humidity is less than that and the distance between them is only one cell thick. 

2.53 Uptake of water.

2.53: uptake of water

                                                                

  . . - -  ...Branching pattern: increase surface area for the absorption of water.

The roots will branch out to try to find water source.

....Root hairs – epidermal cells. Root hair extension of cell wall to increase surface area.




                                                                  Epidermal cell
Concentrated minerals.


Active transport of minerals.

....Osmosis- from dilute region (outside root hair) to (water) concentrate region. (Inside root hair)
Concentrated minerals inside root hair. Dilute region water and soil outside.

....Water than moves across context of root to xylem by osmosis.