BIOLOGY LAB REPORT

TITLE : THE STRENGTH OF PLANT FIBRES

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Abstract

Plant has the ability to withstand all exerted force due to its high tensile strength , resulted from the cell wall and sclerenchyma cell. In this experiment, we are to demonstrate the tensile strength of plant fibre than was extracted from pumpkin stem after soaking, retting and drying process carried out. The tensile strength of plant fibre of different diameter was calculated by exerting force (load) on the fibre until it is snapped and calculations (refer to results) were carried out to obtain average tensile strength of plant fibre.

Introduction (1)

Since mankind exist on the earth’s surface, they has been strongly dependent on plant fibres for

all kinds of purposes .Fibrous materials such as wood and bamboo have found particular

application in construction. Other important uses have included tools, weapons and energy

generation. A wide variety of fibres have also been used for production of textiles, pulp and

paper and fibre boards. Not to be surprise, the strongest engineering materials are generally

made as fibers (eg:carbon fiber and Ultra-high-molecular-weight polyethylene). With the

appearance of synthetic materials (eg: plastics) at the beginning of this century, which is very

cheap and can be produce in large amount compared to natural fibres, synthetic-based materials

have steadily replaced bio-based products.

Scelerenchyma fibre/ Sclerenchymatous fibre(2)

Sclerenchyma consists of very long, narrow, thick and lignified cells, usually pointed at the both

of the end. Sclerenchyma cell is made up of fibre (elongated cells) and sclereids/stone cells

(roughly spherical). Mature sclerenchyma is composed of dead cells and incapable of elongation

thus do not mature until living cells around them reach maturity. Being hard, elastic and having

thick walls with lignifies walls make sclerenchyma cells important in strengthening and

supporting plant parts that have ceased elongation.

Diagram 1 : The sclerenchyma cross-sectional and components(3)

i. Fibre

Fibres are generally long, slender, in needle-shaped with pointed tips, thick walls ,have rather

small lumen and usually occur in strands or bundles. Their principal cell wall material is

cellulose. Typical fibres contain high proportion of lignin and cellulose cell wall. In the middle of

fibre are the thickening layers of secondary wall that deposited one after another. Growth at

both tips of the cells lead to simultaneous elongation. During development, the layers of

secondary material seem like tubes( the outer one is always longer and older than the next).

After completion of growth the missing parts are supplemented, the wall is evenly thickened up

to the tips of the fibres. Fibres are grouped into xylary fibres(wood fibres) and extraxylery

fibres. Extraxylery fibres are classified as phloem fibres, cortical fibres and perivascular fibres

and lignifications for these fibres are not compulsory.

Fibres usually originate from meristematic tissue; main center of their production are cambium

and procambium which both of them usually associated with the vascular bundles. The xylem

fibres are always lignified while phloem fibres are cellulosic. Fibres are said to be originate from

tracheids , losing its water conductivity ability and reduce in pit’s size. Contrasting hard fibres

are mostly found in monocots. Elasticity in fibres enables the plant body to withstand various

strains.

ii. Sclereids

Sclereids are reduced form of sclerenchyma cells with highly thickened and lignified walls (thus

making the lumen very small) and can be found in small bundles or groups in plants that form

durable layers. The cells are variable in shape (can be isodiametri, prosenchymatic, forked or

more) and much elongated and flexible with tapered ends . Sclereids are relatively short

compared to fibres and existence of branched pits (ramiform pits) are clearly visible on cell wall

. Sclereids are commonly found in fruit wall, seed coat, epidermal scales, and occasionally found

in cortex, pith, mesophyll and petiole of submerged aquatics.

Diagram 2 : Fibre cells(left) and Sclereids(right)(3)

Strength of plant(4)

A plant stems must not only be strong, but they also must be able to bend according to

the wind direction and recoil back to their original shape without any permanent distortion or

damage. Tensile strength is the maximum stress caused by a pulling forces that a material can

withstand without failing as the result of the molecules or atoms hold together or being elastic

when being pulled apart while strength of plant fibre is the maximum stress a material can

withstand without failing. Compression strength is the maximum stress caused by a pushing

forces that a material can withstand without crushing as the result of the molecules or atoms do

not slip pass each other . As long as the molecules or atoms remain in place, then the material

can resist compression without necessarily having tensile strength.

Xylem vessel(5)

Xylem tissues are produced by meristematic cambium cells that located in a layer just inside the

tree bark. In dicotyledonous stems, the xylem cells can be noted inside the cambium layer while

in monocotyledonous stem, it can be seen to be scattered all over. Xylem tissue conducts water

and mineral ions from root to other parts of plant , against the gravitational force with help of

adhesion and cohesion of water molecule. The xylem tissue is composed of tracheids (elongate

cells with pointed ends) and vessel elements (shorter and wider cells).

Diagram 3 : Xylem tracheids and vessel elements(3)

The xylem walls are heavily lignified with pits(composed of cellulose and pectins) . After the cell

death, tracheids and vessels become hollow, water-conducting pipeline and its protoplasm

disintegerates. In flowering plants, xylem contains numerous fibres (resulting in having harder

and heavier wood than gymnosperms), elongate cells with tapering ends and very thick walls. To

be frank, xylem tissues are actually composed of dead cells that have dried out over the years

and with the presence of lignin in the thickened secondary cell wall, it make the whole structure

hard and dense, providing support for the plant.

Objective

To investigate the tensile strength of plant fibre (pumpkin stem fibre) and compares it to tensile

strength of concrete.

Problem Statement

Are plant fibre’s tensile strength is stronger than the tensile strength of concrete?

Hypothesis

Plant fibre has a higher tensile strength than concrete’s.

Apparatus

Retort stand with clamps, different masses of load (2g and 10g), hook, Rubber gloves, Paper

towels, Measuring cylinder, 100ml beaker, a pair of scissors , tray ,Force Meter

Materials

Stems of mature pumpkin stems, distilled water

Procedure

a) Extracting fibres from mature pumpkin stem

1. The pumpkin stems were prepared by laboratory assistant by removing any leaves or

flowers from the stems of pumpkin using scissors.

2. The stems are then let to dry on a tray covered by tissue papers for three days.

3. The dried pumpkin stems were placed in a measuring cylinder and were fully immersed

in water.

4. The stems were then left soaked (retting process) for 8 days.

5. Pumpkin stems were removed from water and washed gently to remove the softened

tissue and fungus. The stems were rubbed by using gloved hands under running tap

water to remove the softened stem tissue to obtain/extract the fibres needed.

6. The pumpkin stem fibres were separated using gloved hands slowly with care.

7. The fibres were then placed on a tray that was covered by tissue papers to dry and were

left in care of laboratory assistant.

b) Testing Fibre Strength

1. A strand of pumpkin stem fibre was chosen.

2. Loops were made at both ends of the fibre to attach one side to the spring balance and the

other one to the loads hook.

3. The spring balance was hooked onto the clamp of the retort stand (attached to the table)

.the fibre stand was the attached to the spring balance and slowly attached to load’s hook

on the other end (make sure that the fibre can withstand the load’s hook).

4. The spring balance reading was then adjusted until it returns to its zero value.

5. A piece of load with mass 2g was put to the hook.

6. The reading of spring balance was recorded.

7. Loads were continued to put until the fibre strand snap and every load that was slipped

into hook(total load) and the reading of the spring balance was recorded.

8. Light micrometer was set up and stage micrometer was placed on the micrometer stage.

9. The eyepiece graticule and stage micrometer were positioned to be parallel with each

other and scale of eyepiece was calibrated (shown below) and recorded.

10. The fibre strand that was used in experiment was cut horizontally and placed on a slide. It

was then placed in micrometer stage and the diameter of the stem was measured using

the scale on eyepiece graticule.

11. The tensile strength of fibre strand was calculated using formula (shown below) and was

recorded in a table.

12. The experiment was repeated for twice using different diameter of fibre strand and data

obtained were recorded in a table.

Formula’s used :

Calibration

Under x4 Magnification

100 division of stage micrometer = 40 division of eyepiece graticule

100 division of stage micrometer = 1 millimeter

1mm of division of stage micrometer = 40 division of eyepiece graticule

1 division of eyepiece graticule = 1mm/40 = 0.025 mm or 2.5 x10-5m

Tensile strength

=

Safety precaution

In order to avoid any accident or injury during the experiment in laboratory, the precautionary

steps should be taken and applied. Wearing lab coat and a pair of suitable shoes are

compulsory when conducting an experiment in the lab at all times to protect the skin and

clothing from spillage of any chemical substance. Washing hands thoroughly with soap and

water before and after conducting experiment is vital to avoid contamination. Cover hands

with rubber gloves during washing process of the stem is important to avoid getting contact

with microorganism or prevent smelly hand which will be unpleasant as the smell will stay on

hand for quite a while. Furthermore, the glassware such as measuring cylinder should be

handled with full care because they are fragile. The apparatus such as forceps and scissors are

also sterilized to prevent infection of microorganism. After using all samples and apparatus at

the end of experiment, they should be discarded properly and returned back to their places to

avoid injuries and unnecessary accidents that may result fatal results.

Risk Assessment

The pumpkin plant should have to be fully immersed in a big measuring cylinder of water for at

least one week, or in this case for 8 days. This is to soften the tissues of pumpkin stem and it will

be easier to extract fibres out from the stem later. The extracted fibres need to be thinner as

much as possible to make sure only one strand fibre will be used in this experiment. The plant

fibre is handled with care while transferring onto tissue paper to let them dry as they are very

fragile. Once apparatus (measuring cylinder) was used, they were washed and placed in their

place. Other than that, when the horizontal sectional of fibre is transferred to a coverslip and is

being covered, the coverslip must not be pressed too firmly that may cause breakage of the cover

slip or alter the radius of the stem. Tying the fibre using thread also should be given care so that

the knot done is able to prevent the fibre from sliding off. It is also to prevent the fibre from

detached when load is added to the hook. But, the knot made shouldn’t be too tight as it will

break the fibre. During placing the load into the hook, extra care was taken to prevent any

exertion of sudden force onto the fibre which will lead to breakage of plant fibre.

Results

Fibre Radius Mean Radius

1 2 3

1 6.5 12.5 11.0 10.0

2 20.0 19.0 20.0 19.67

3 21.0 22.5 22.5 22.0

Table 1 : Mean Radius of each Fibre

Fibre 1 Fibre 2 Fibre 3

Force

= 0.006kg x 9.8ms-2

= 0.069 N

Radius

= 10 x 2.5 x10-5m

= 2.5 x 10-4m

Cross-sectional Area

= 3.142 x (2.5 x 10-4m)2

= 1.96375 x 10-7m2

Tensile Strength

= 0.069 N / 1.96375 x 10-7m2

= 3.51 x 105Nm-2

Force

= 0.036kg x 9.8ms-2

= 0.353 N

Radius

= 19.67 x 2.5 x10-5m

= 4.92 x 10-4m

Cross-sectional Area

= 3.142 x (4.92 x 10-4m)2

= 7.60565 x 10-7m2

Tensile Strength

= 0.353 N / 7.60565 x 10-9m2

= 4.64 x 105Nm-2

Force

= 0.086kg x 9.8ms-2

= 0.843 N

Radius

= 22.0 x 2.5 x10-5m

= 5.50 x 10-4m

Cross-sectional Area

= 3.142 x (5.50 x 10-4m)2

= 9.50455 x 10-7m2

Tensile Strength

= 0.843 N / 9.50455 x 10-9m2

= 8.87 x 105Nm-2

Table 2 : Calculations for the fibres

Fibre Mass of loads

(m)/kg

Force (mg)/N Cross-sectional

Area ( )/m2

Tensile

Strength (Nm-2)

1 0.006 0.069 1.96375 x 10-7 3.51 x 105

2 0.036 0.353 7.60565 x 10-7 4.64 x 105

3 0.086 0.843 9.50455 x 10-7 8.87 x 105

Table 3 : Tensile Strength of Different Fibres

Average tensile Strength of Different Fibres

= ( 3.51 x 105Nm-2 + 4.64 x 105Nm-2 + 8.87 x 105Nm-2)

3

= 5.67 x 105Nm-2

Data Interpretation

Table 1 shows the mean radius of all three fibres that were used in the experiment. The radius

for each fibre was taken for three times to obtain the mean .The mean radius for each fibre (in

metre) was calculated . Mean radius for one fibre is obtained by adding all three readings and

divided it with three (number of readings taken). Then, it was multipled with the calibration,

which is 2.5 x10-5m to obtain radius of the fibre in metre. This is clearly shown in table 2

(calculations). From there it can be concluded that the third fibre has the biggest radius (5.50 x

10-4m) while the first fibre has the smallest radius (2.5 x10-4m) among the three.

In Table 2, all the calculations were made clearly to obtain the force exerted on the fibre (N),

radius of the fibre (as discussed earlier), cross-sectional area of the fibre and the tensile strength

of the pumpkin fibre. According to the Table 3, third fibre withstands the maximum load

(0.086kg) , followed by the second fibre (0.036kg) and the least maximum load withstand by the

first fibre (0.006kg). This results was obtained by adding loads (in gram) into the hook that was

attached to the fibre , (starting from 2g) until the fibre is snapped. By multiplying the mass (final

mass before the fibre snap) with gravitational force (approximately 9.81ms-2 ), the maximum

force (N) withstand by the all three fibres were calculated and recorded. From both (table 2

and 3), it can be seen that the first fibre had withstand the least load (0.069N), followed by the

second fibre (0.353N) and the third fibre had withstand the most highest load (0.843N) . For the

calculations of cross-sectional area of each fibre, it is shown on Table 2. Basically, radius that

obtained earlier were squared and multiplied with pi (3.142). Using formula, it can be seen that

cross-sectional area of the third fibre is the highest (9.50455 x 10-7 m2), which results in the

highest tensile strength among all three fibres (8.87 x 105 Nm-2).

This is followed by the second fibre which has cross-sectional area of 7.60565 x 10-7 m2,

resulting in 4.64 x 105 Nm-2 tensile strength. The lowest one in cross-sectional area among all

three fibres (first fibre), which is 1.96375 x 10-7 m2 resulting in the lowest tensile strength

among the three fibres (3.51 x 105 Nm-2).

The results shows the tensile strength of fibre with the highest radius (the third fibre) has the

highest cross-sectional area, giving rise to highest tensile strength. The vice versa happened to

the first fibre(lowest radius results in lowest tensile strength). Average tensile strength of fibres

that were used in this experiment is 5.67 x 105Nm-2.

DISCUSSION

From the experiment that carried out, we know that the average strength of the plant fibre or

more precisely, pumpkin fibre is 5.67 x 105Nm-2. This plant fibre is concluded to be strong

because of some reason. The strength and rigidity from these fibres are provided by cellulose

cell wall that contains interlocking fibre arrangements .These cell walls are heavily thickened

with sclerenchyma cells that are designed to support the plant body. As the plant grow in size

and height, more support is required to ensure the survival of the plant and various types of

sclerenchyma tissues (most entirely of fibres) are formed to meet the need. As discussed earlier

in the introduction, fibre plays important role in determining the tensile strength of plant

because of their interlace arrangement and their capability in stretching and contracting.

Presence of lignin further increases the strength of the cell wall without affecting its water

conducting ability.

It’s a well known fact that concrete is stronger than a mere plant fiber. But in reality, concrete

has lower tensile strength but higher compressive strength. Instead of one plant fibre, a bundle

of fibres will result in stronger tensile strength than concrete. Assumption were made that a

concrete sample's tensile strength is about 10 percent to 15 percent of its compressive strength.

As a result, without compensating, concrete would almost always fail from tensile stresses even

when loaded in compression. The practical implication of this is that concrete elements

subjected to tensile stresses must be reinforced with materials that are strong in tension and

this is influenced by the water-cementitious ratio (w/cm), the design constituents, and the

mixing, placement and curing methods employed. All things being equal, concrete with a lower

water-cement ratio makes a stronger concrete than that with a higher ratio.

The total quantity of cementitious materials can affect strength, water demand, shrinkage,

abrasion resistance and density.

In conclusion, tensile strength of plant fibre is stronger than concrete and all the credits goes to

the plant fibres. This high tensile strength enables the plant to keep in structure and face all

natural forces without getting damage.

Limitations

There are several limitations that have been identified throughout this experiment.

Fibre strands that were extracted from pumpkin stem have different level of maturity.

Although the stem is taken from pumpkin of same species, but the stem may extracted

from different part of plant and this give rise to variation which may lead to inaccurate

result.

Besides that, the radius of the stem may differ along the strand and this will influence the

reading of radius depending on the part of stem that they were taken, making the

calculations inaccurate.

Other than that, drying process of the fibres also may not fulfill. The fibres were too over-

dried during the experiment which results in oversensitivity and more brittleness in the

fibre. This will cause the fibre to snap even for the lowest load thus giving wrong

information on their real tensile strength.

Assumptions were made that the cross-sectional of the strand is perfectly round when in

reality it’s the vice versa. Thus taking radius reading using this might not be perfectly

accurate.

Sources of errors

Several sources of error in this experiment were identified and steps were taken to

minimize these errors to make the result more accurate.

Stem was soaked in water for more than one week and this might led to unwanted

moisture absorption and production off bacteria of breakdown of cellulose cell wall may

cause the fibre to lose its strength. This was overcome by carrying retting process right in

the early morning on the eight day.

Since the stem provided is long, thus it was bended when soaking process carried out. This

bending damages the fibre of the pumpkin stem and weakens its strength ability, leading to

the same inaccurate tensile strength. Thus, to avoid this, the stems were placed were

slowly and exert slight pressure to bend it when soaking process was carried out.

During extraction of fibre from the stem, the fibres may accidently experience some

damage at some part which results in less strength, thus causes less accurate in tensile

strength calculation. Thus, hands were covered by gloves to reduce friction formation

between the stem and fingertips. Care was taken by applying less pressure on the fibre

during peeling process to avoid the fibre from tear.

During measurement of plant stem fibre radius when viewing under light microscope, the

sales on eyepiece graticule might interpret wrongly due to parallax error. This was avoided

by using different people positioning their eyes perpendicularly to the scales to read the

scales and take three readings for each fibre.

Putting loads without care onto the hook may cause the fibre to break. Thus,extra care

were taken during adding load on the hook that was tied to the fibre. The loads should be

place one by one, from the smallest weight, gently and softly to avoid exert unintentional

forces that may affect the stress tension of the fibre and lead to unreliable and invalid data.

Conclusion

Theoretically, plant fibre are stronger than concrete (2x106Nm-2). But, in this experiment , due to

some limitations and error, the tensile strength of plant fibre is lower (5.67 x 105Nm-2) than the

concrete’s tensile strength. The sole purpose of have high tensile strength in plant is to ensure its

survival by overcoming the external force while tensile strength of concrete is more used in

construction. Thus, the hypothesis is accepted.

Further Investigation

Another experiment can be carried out using hair as substitution for the plant fibre to find out

whether hair is stronger than the plant fibre or not. This is because hair is also made up of strong

fibre structure and keratin.

References

1. http://www.ienica.net/fibresseminar/olesen.pdf . Accessed on 17th February 2012

2. Wikipedia Foundation. Last modified on 2012. Ground Tissue. Available from http://en.wikipedia.org/wiki/Ground_tissue. Accessed on 17th February 2012.

3. http://preuniversity.grkraj.org/html/3_PLANT_ANATOMY.htm. Accessed on 17th February 2012.

4. Gan W.Y . 2007. Biology SPM Success. Edition 4. 135.p.Shah Alam : Oxford Fajar Sdn.Bhd.

5. Wikipedia Foundation. Last modified on 2012. Xylem. Available from http://en.wikipedia.org/wiki/Xylem. Accessed on 17th February 2012.