Extraction of fibres from stinging nettles (AS)

The combined effect of these two materials is the strength, which supports the plant stem and is the reason why wood is stronger in tension than compression.

I am going to use water retting to extract my fibres from the stem. I am using this method because it produces more uniformed, higher quality fibres. Also bacteria and fungi break down the soft tissue of the stems leaving the cellulose intact.(8)

Hypothesis

Based on my research I think that all the samples will be surprising strong, however I think that the nettle stem will be the strongest in tension as it is taken from the stem which is strongest part of the plant due to the lignification of the xylem and the cellulose walls

Null hypothesis

All the results will be the same the degree of lignification of the cellulose walls does not play any part in the strength of the plant.

Aim 

My aim is to find out which sample of fibres taken from a stem of: celery, nettle and the nettle its self is the strongest and therefore come to a conclusion about the degree of lignification the fibres.

Independent Variables

My variable will be the type of fibre. I am going to test them in tension and compression to compare the results. This means I can come to a conclusion about the way in which they are made to withstand certain forces and see which fibre is overall strongest.

Constant Variables

The variables which I am going to keep constant are;

1. The height at which I tie the fibres

2. The knot that I use to tie the fibre
3. The amount of weight added each time
4. The length of the fibres

Method

Equipment list for retting

1. Stems of mature stinging nettles
2. Fibres form Celery
3. Fibres from nettle
4. Bowl
5. Paper towels

Procedure for Retting

1. Remove leaves and any flowers from stems
2. Place stems in a bowl of water until completely submerged.
3. Leave for at least a week
4. Remove stems from water
5. Wash to remove softened tissue and dry remaining fibres
6. The outside cuticle and epidermal layers will rub away to leave fibres

Safety Considerations

Eye protection and gloves need to be worn when dealing with unretted nettles to avoid being stung.

(9)

Hands must also be washes after handling soaked fibres.

(10)

When finished retting the water will be nitrogen rich and must be treated before throwing away.

Equipment for investigation

6. Stems of mature stinging nettles
7. Fibres form Celery
8. Fibres from nettle

1. Two clamp stands
2. Weights

Method – Practical

Compression

1. Once fibres are collected set up two clam opposite each other and tie a knot in each end of the fibre
2. Attach to pole, making sure the fibre is taught
3. Add weights 10g at a time ensuring that the weight in the middle of the fibre
4. Record the weight when the fibre breaks

Tension

1. Tie two knots one in each end
2. Attach one end to the pole on the clamp stand the other to the weight holder
3. Add weights 10g at a time ensuring that the weight in the middle of the fibre
4. Record the weight when the fibre breaks

Repeat the experiment three times for each fibre in both tests; compression and tension.

Results

When I had carried out the experiment fully I created a table from my readings

I then worked out the averages of my three reading so that I could plot a graph. The figures have been round to the nearest 10 as I added 10grams at a time.

Discussion

From these tables of results you can see that the nettle stem is the strongest in compression. I believe this is because of the horizontal spirals of lignin that reinforce the cellulose structure. In this experiment they would be perpendicular to the tension cause by the weights therefore acting as ties between cells holding them together. I also think that the reason that the nettle stem came out as the best is because the stem is extendable to a certain degree. It is also strong and can hold a lot of weight as it is designed to hold up the weight of the plant, some of which can grow very tall and weight a lot.  

The Celery fibre is the strongest fibre in compression. I think this is because this is because the lignin still has a strong attraction to the cellulose and so stops splits propagating more than the nettle. Although, celery can hold more weight in compression than in tension. I believe this is because it is lignified but in larger spaces rings, therefore when laid horizontally the coils flex with the fibres gradually getting further apart, whereas when in tension they stretch too far under the force of gravity and snap.

This is shown here to explain my idea more clearly:

I think that the nettle was the weakest because it has not yet had much lignin added to it over the years. It would also be flexible to bend and sway in the wind and if it were lignified it would be stiffer and not be able to do so.

Summary

In this investigated I have found out that the degree of lignification contributes to the strength of the fibres or vascular bundle within a plant. If the lignification is significant then the fibres will be stiff, giving support and strength to hold weight in tension. If the lignification is moderate then they will provide strength by allowing the plant to be flexible this is useful when a plant leans over in the wind and the fact that it is strong in compression is a benefit too, as the plant bends, one side will be in compression. If a plant were only strong in tension it would snap at this point. These parts of the plant will not to take much weight in tension, as they are not required to take a heavy load. If lignification is poor then the fibres will still have the strength of the cellulose and some of the lignin but the main job is to remain flexible and are usually found at the top of the plant, i.e. young leaves and so are not design to take the weight of the plant.

Overall it is the purpose of the particular piece of plant that decides its strength by the amount of lignin that is added. It is how many spirals and the spacing between them around the outside of the tubes that increases the weight of which it can hold.

References

1. Google Images
2. Snab online
3. Google Images
4. Page 156 SNAB AS book
5. Page 158 SNAB AS book
6. Google Images
7. Activity Sheet
8. Activity Sheet
9. Google Images
10. Google Images