This investigation was designed to compare the height of Pteridium aquilinum in a light area and a dark area of Rushy Plains, Epping Forest and to establish if light intensity does have an effect on the growth of Pteridium aquilinum, commonly known as Bracken. From my research it was clear that the height of Bracken is affected by abiotic factors, other than light intensity, such as: soil moisture, soil temperature, air temperature and soil pH so I had to control these factors. I carried out preliminary experiments to find a suitable site where all these factors where constant. For my actual investigation I measure twelve random Brackens from both light and dark areas. I then carried out a Mann Whitney U test and concluded, with 95% significance level, that there is in fact a difference between height of Brackens from a light area and a dark area.
Alternative hypothesis (H1): There is a significant difference in height of Brackens from a light area and the height of Brackens from a dark area. Null hypothesis (H0): There is no significant difference in height of Brackens from a light area and the height of Brackens from a dark area. Prediction: The height of Brackens from a light area will be greater than the height of Brackens from a dark area.
During my investigation, I aimed to look at whether there is a difference in height of Brackens from a light area and a dark area due to light intensity. In order to carry out my investigation I needed to research into the different factors that affect the growth of the plant. I also planned and carried out preliminary investigations to make sure all the control variables are constant before I did the main investigation. I recorded my findings and did a statistical test on my data to see whether the data show a significant result.
Research and Rationale
Bracken (Pteridium aquilinum) is a native British fern commonly found in woodland and heathland. It is tolerant of a wide range of soils and climates. Bracken is typically fern-like, producing triangular fronds, divided into three, that can reach up to 2m in height. In autumn the fronds turn reddish-brown and die back to ground level, with new fronds unfurling from the base in spring. (Jan Toman and Kvetoslav Hisek, 1998) Picture 1
Investigations such as this widen our knowledge of how living things interact with non living factors, and this knowledge becoming increasingly important as ecologists are trying to find new methods of cultivating plants in conditions that might not be particularly favourable and also new methods of controlling the growth of plants. I chose to carry out my investigation on this particular plant as it is known to have a toxin called Ptaquiloside, which is highly carcinogenic. Knowing more about what affects the growth of the plant can help limit the growth of it therefore avert any harm to any animals or even humans.
There are many factors that affect the growth of a plant with light intensity being one of the major ones. Other factors include: Air temperature, Soil temperature, Soil moisture and Soil pH  How does light intensity affect the plant?
Light is an absolute requirement for plant growth and development, without light there is no energy to drive essential reactions. There are three processes in a plant that requires light: Photosynthesis, Photoperiodism and Phototropism. All of these processes affect the growth ergo the height of the plant.
Photosynthesis is the process by which plants convert carbon dioxide and water into simple carbohydrates and oxygen in the presence of chlorophyll using sunlight. Photosynthesis is a process that is gravely important to not only plants but animals too, as it starts off the food chain; animals which are unable to make their own food (hetrotrophs) feed on plants that are able make their own food through photosynthesis (autotrophs). Photosynthesis is summarised by the equation :
However, this equation is somewhat misleading, as photosynthesis is a two-stage process: Light-dependent stage and light-independent.
The diagram above shows the two stages of photosynthesis. As you can see, light is a key component as it start the light-dependent reactions which produces ATP and NADPH, which is needed in the light-independent reactions to make glucose. So without light, there is no ATP and NADPH produced and thus no glucose produced, therefore the food chain cannot be started. Glucose is also needed to make DNA and hormones for plant growth, it is also require during plant respiration. So it can be established the light is vital for plant growth and it affects the height of plants indefinitely. Photoperiodism:
Photoperiodism is the effect of the relative lengths of light and dark periods on plant growth. This occurs when photoreceptor proteins e.g phytochromes are produced by plants in response to light intensity or the absence of light. This in effect affects the height of plants.
Phototropism is the growth of a plant in the direction of its light source. Phototropism occurs due to a hormone called auxin which is produced at shoot tips. Auxin influences cell division, if there is light on one side then the dark side will receive more auxin and therefore more cell division will occur on that side, which will force the plant to bend towards the light therefore affecting the height of the plant 
How other factors affect the plant?
Air temperature affects the growth of the plant as the rate at which photosynthesis occurs varies with temperature. At a higher temperature (in warmer conditions), the enzymes and substrates involved in photosynthetic reactions have more energy to collide and cause more successful collisions. As a result, the rate of photosynthesis increases. At lower temperatures (cooler conditions), the enzyme substrates have less energy to collide so less successful collisions. Therefore, the temperature affects the growth of plant as the more photosynthesis occurs, the more food produced for the plant which would result in an increased height of the plant.
But it is also to be noted that photosynthesis increases with temperature only up to a certain point. This point is usually at which the enzymes become denatured.  Temperature also affects respiration and transpiration. Respiration rapidly increases with temperature, this is because the gases can diffuse more easily through the stomata and give the mitochondria a greater supply of gases for respiration to occur. Transpiration also increases with temperature, this is because the stomata are usually open more at higher temperature which allows more water to evaporate through leaves.
 Soil pH:
The pH of the soil affects the height of the plant as most plant usually prefer slightly acidic soil and Bracken is one of them. Studies have shown that “Bracken litter reduced soil pH to 4 – 4.5.” This suggests that the preferred soil pH of a Bracken plant is between 4 – 4.5. Due to this, if the pH is too acidic (lower than pH 4) or too neutral or alkali (higher than 4.5), then Bracken won’t be able to grow to its greatest height because process such as photosynthesis, active transport and simple diffusion will occur at a slower rate at these conditions.  Bracken plants maintain their ideal pH by having leave litter of a certain pH, which will control the pH of the soil.
 Soil moisture:
Plants need water for cell metabolism, cell expansion, turgor support and transpiration. Too little water would cause plant growth to be stunted or plants will die. For example, if there is insufficient water in the soil, stomata will close and a plant will be unable to photosynthesise because it cannot get carbon dioxide gas. This is why water is vital for plants to survive. The moisture of the soil has an impact on the growth of bracken because too little water may cause the plant to dehydrate, whereas too much water may cause the plant to become waterlogged. This is why soil moisture is an important factor and has to be kept constant. 
Summary of Research and Rationale:
Bracken is a plant whose toxic nature makes it a plant that is worth researching about. Knowing more about what influences its growth will help increase our knowledge base, also it is a plant of high ecological value as it is a primary producer therefore starts off the food chain. There are several factors that affect the growth of Bracken: Light intensity, temperature, soil pH and soil moisture. In light of my research, these factors have an effect on the growth of Bracken and need to be controlled to ensure that the only factor that influences height is light intensity. Therefore, my research has influenced my planning of the investigation as I knew what factors affected the growth of Bracken and needed to be controlled to ensure that my results were accurate in determining whether there is a difference in height of Brackens from a light area and a dark area.
Dependant variable: Height of Brackens
Independent variable: Light intensity
* Air temperature
* Soil temperature
* Soil pH
* Time of day
* Soil moisture
As my investigation involves comparing two different sites with different light intensities, my sampling technique involved either stratified or random sampling. I decided to use random sampling, as this method supplies less bias results, as each of the co-ordinate of the site has an equal chance of being used. I have used a random number table to generate my co-ordinates. To further reduce any bias, I chose the co-ordinates while my eyes closed to eliminate any human impact.
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