Plant growth (triple)

Plants respond to a number of different external stimuli. These include light, gravity, water and even other objects. This allows them to grow in the right direction. For example, all plant shoots will grow towards a light source. This is called phototropism (as in light and tropism which means 'to turn'). Vines and creeping plants send out tendrils that coil around objects when they touch them, this is called Thigmotropism. (Thigmo = to touch). These responses are controlled by a group of different hormones called Auxins. Auxins are a category of hormones, just like 'cars' refers to lots of different types of car.

The practical

This practical investigates whether seedlings respond to a light source. It is most commonly carried out in a shoe box as it is easy to control the light direction.

• The dependent variable is the angle of the seedlings after at least a week of growth. The angle can be measure with a protractor from the surface.
• The independent variable is direction of light. Ideally, at least 3 directions should be used, straight down, directly from the side and at an angle. This can be altered by cutting holes for the light to get through in the shoe box.
• There are several variables that need to be controlled. All factors effecting plant growth should be controlled, such as water, temperature, nutrients etc. External light sources also need to be controlled, as does light intensity and wave length (colour). This might seem obvious, but day light is not the same as a light bulb and plants would respond differently.
• The most common errors are that too few repeats are used. Not all seedlings germinate, so ideally it should be repeated with 3 petri dishes of 10 -100 seeds for EACH angle of light.

Geotropism, although the word Gravitropism may be acceptable. Roots grow down towards gravity.

In response to light, auxins accumulate on the dark side (think of Star wars!)

In response to gravity, auxins accumulate on the lower side of the shoot (imagine gravity pulling them down).

Auxins cause these cells to elongate and divide faster.

Auxins in high doses act as weed killers.

Auxins are also involved in stopping plants becoming more bushy. This is called apical dominance (not needed for GCSE). Picking off the tip of a plant causes it to grow more side shoots because the Auxins are removed from the tip.