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To Investigate the Effect of Regular Mowing on the Petiole Height of White Clover Essay Sample

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  • Pages: 8
  • Word count: 2,037
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  • Category: experiment

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Introduction of TOPIC

T Test

A T Test has been conducted to determine whether there is a marked difference in the heights of white clover petioles in mown and unmown areas,

Null hypothesis: there is no significant statistical difference between the heights of petioles of white clover in unmown and mown areas.

Alternative hypothesis: there is a significant statistical difference between the heights of petioles of white clover in unmown and mown areas.

Calculating the degrees of freedom:

(sample size) – (the number of samples) = degrees of freedom

50+50 – 2 = 98

df= 98

Therefore critical t value 1.99

From: http://graphpad.com/quickcalcs/ttest2/

Calculated t value = 6.31 (2dp)

Critical t value = 1.99

Our calculated t value of 6.31 is greater than the critical t value of 1.99. This means there is a significant difference between the two means, and therefore there is a significant difference in the heights of the white clover petioles in the mown verses unmown areas.

Graph showing the mean petiole heights of 50 white clover samples from unmown and mown areas

Comment on Graph:

My bar graph shows that as the heights of clover petioles in unmown areas were greater with a mean of 103 than the petiole heights of clover in mown areas with a mean of 42. The error bars were calculated to show one standard deviation either side of the mean, displaying where 68% of the data lay. My error bars are extremely large.

Conclusion and Evaluation

Conclusion

In my experiment I investigated the effect of mowing on petiole heights of white clover, by sampling petiole heights in unmown and mown areas.

From my results I can infer that mowing had a significant effect on the petiole height of white clove. The mean petiole heights of the unmown areas was considerably higher at 103cm than the mean petiole heights in unmown areas at 43cm. This indicates that mowing does cause a decrease in petiole height.

My data was imprecise as indicated by the large standard deviation/error bars. Because our sample size was 50 our results were reliable and it is more likely that this impreciseness is cause by the fact that height is a continuous measurement and is usually normally distributed in relation to organisms, therefore there will be some plants with petiole heights which vary greatly from the average which will have affected our error bars these outliers though naturally occurring, like the unmown petiole of 330cm, become more frequent with some natural factors which I shall explore in my evaluation. Although this does explain the inconsistency of my original samples, yet the subsequent mean calculations and t test ensures that the integrity of the data is intact. In spite of this there was a difference in the size of error bars between the mown and unmown areas, with the unmown area having a much greater standard deviation of 62. This indicates to me that there must have been some systematic errors within my experiment otherwise I would expect the error bars to be similar.

My results are easily explained by science, in all subterranean clover, like the white clover, petiole length is important due to the plants prostrate growth nature, while still allowing for photosynthesis. Our experiments indicate that petiole length responded rapidly to being cut during mowing and became “more prostrate and smaller leaved under grazing”¹. This is because the shorter petioles keep a greater proportion of leaflets below the grazing/mowing horizon. In this case the mowing has acted as a selection pressure, removing petioles over a certain height while the smaller ones survived. With consistent mowing the clover plant with the stolen which produces shorter petioles is more likely to survive and grow, passing on its genetic material though reproduction, eventually the mown area will become populated with prostrate clover. While the unmown clover also has to compete with other plants for light, which isn’t a problem for the mown clover as the grass is also being mown. In the unmown clover it is favourable to have tall petioles as this allows for greater light access and more effective photosynthesis which directly increases the survival rate

of the plant, therefore tall clover in the unmown areas is more likely to survive and grow, passing

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on its genetic material though reproduction, eventually the mown area will become populated with tall clover. Therefore allowing for the petiole height differences seen in my results between the mown and unmown clover samples.

However my results are reflected by these other experiments such as the one a found online which directly measured the difference of petiole height in hard-grazed and lax-grazed areas. Their results are shown in the table below.

The findings though not as extreme were very similar to my own, as the heavily grazed grass has a shorter petiole than the less heavily grazed grass. Therefore I can conclude that my findings and my explanation is correct as it is confirmed by an outside source.

Evaluation:

Limitation of the method

How significantly could this have impacted on your results and why

Improvement

The method wasn’t clear from where on the petiole we should measure from, or if we were to include the leaves in this measurement.

This wouldn’t have been very significant as we had many samples yet it would have increased the size of our error bars and therefore would slightly reduce the reliability of our results yet as the limitation would have been repeated for both the unmowed and mown samples it wouldn’t have effected validity. If we were including part of the stolon in our measurements this would have effected both the reliability and validity of our results.

Next time I would be extra careful to measure from where the petiole joins the stolon to the base of the leaves. To ensure accuracy I would cut the clover petiole by cutting the stolon 1cm either side of the petiole, then place the petiole on a flat surface to conduct my measurements.

The area we were measuring was not consistent, and there were areas of increased shade from trees.

This would have effected how much light some clover plants would have had access too. Light is requited for photosynthesis and subsequently is necessary for the growth of petioles. Therefore if there is decreased light in an area there will also be a decrease in growth. This would have effected out experiment as we are not sure if we are measuring the effects of decreased light access or mowing. Because of this our validity and reliability would have been significantly affected.

Next time we could take our samples from the middle of a large field which would receive sunlight during the day and would never be shaded by trees.

Our sampling of the area was not truly random, due to the limited number of clovers.

This would have affected the reliability and validity of our results as because we were not completely random it becomes more likely that we selected clovers that were easily visible as we were scanning the field, This would have made it more likely for us to select the taller clovers as there were immediately more visible. This would have increased our mean.

Next time we would use 30 by 30 quadrats and count all of the samples in a quadrat.

The area we were measuring was not consistent, and there were areas of increased water access at the bottom of ditches.

This would have effected how much water some clover plants would have had access too. Water is requited for photolysis and subsequently is necessary for the growth of petioles. Therefore if there is increased water in an area there will also be an increase in growth. This would have effected out experiment as we are not sure if we are measuring the effects of increased water access or mowing. Because of this our validity and reliability would have been significantly affected.

Next time we could take our samples from the middle of a large flat field which would receive an equal amount of water when it rained.

We measured clovers without removing them from the ground therefore the millimetres at the bottom of ruler would have been included in my results.

This wouldn’t have been very significant as we had many samples and it wouldn’t have affected the size of our error bars. Also as the limitation would have been repeated for both the unmowed and mown samples it wouldn’t have significantly affected validity. Yet we were still making incorrect measurements by including part of the ruler in our measurements, therefore partially effecting both the reliability and validity of our results.

Next time I would be extra careful to measure from where the petiole joins the stolon to the base of the leaves. To ensure accuracy I would cut the clover petiole by cutting the stolon 1cm either side of the petiole, then place the petiole on a flat surface to conduct my measurements. And I would only measure from the start of the measuring part of the ruler.

Due to the limited number of clovers we tended to measure clusters of clover petioles.

This would have affected the reliability and validity of our results as because we often measured multiple petioles from one cluster it becomes more likely that we measured petioles which originate from the same stolon and therefore are more likely to be similar in height. This would have caused and increase in modes, and our mean to have been effected.

We could have measured a bigger area, of 4m, using 30 by 30 quadrats.

In the areas we were measuring there were many other grass and plant species.

Because there were other grass and plant species there would have been competition for resources, this was more common in the unmown areas as some species can’t survive mowing. Because of this competition some clover plants wouldn’t have as much access to light and water, causing a decrease in growth rate. This would have effected out experiment as we are not sure if we are measuring the effects of decreased water and light access or mowing. Because of this our validity and reliability would have been significantly affected.

We would take some clover plant sample which we would then grow until we have many different similar clover plants which we could then plant into two, 4 by 4m area which had no other plants we could then begin to mow one of the areas while consistently weeding to remove any other grass species.

Sources

1) http://www.grassland.org.nz/publications/nzgrassland_publication_1214.pdf

2) Competition and Succession in Pastures edited by P. G. Tow, Alec Lazenby

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