Extracting fibres from plants
Class practical
As well as taking a closer look at the vascular tissue of plants, this procedure gives an opportunity for students to plan an investigation as practice for coursework. Making biomechanical measurements of the plant tissue is relatively simple, but still requires care and consistency to deliver reliable results. Plant fibres are still of economic importance even though many synthetic polymer alternatives are now available.
Lesson organisation
Some stems need to be prepared in advance. Students need time to try out techniques before devising a hypothesis and planning their investigation in detail.
Apparatus and Chemicals
For each group of students:
Rubber gloves
Paper towels
Clamp stand, boss and clamp, 2
Mass carrier, 1
Masses, slotted, several
Ruler
Force meter, 1
Bulldog clip, 1
For the class – set up by technician/ teacher:
Stinging nettles, mature stems, leaves and flowers removed, soaked in water for a week (Note 1)
Bucket or bowl of water
Other stems for comparison (Note 2)
Health & Safety and Technical notes
Wear eye protection and gauntlet gloves when harvesting nettles. Wear rubber gloves when immersing them to avoid stings.
1 Soaking in water like this is called retting. Retting plant material smells strongly! Consider where to set it up. Removing the leaves and flowers reduces the smell as they make a slimy mass when they rot. After soaking for a week (exact time depends on temperature and may take longer in cooler weather) all the soft tissue, both outside and inside the vascular bundles will wash away in running water. The ring of vascular bundles may need to be opened to wash out the pith within.
2 Other stems can be used for comparison. Celery vascular bundles are easy to extract and test and do not need retting. New Zealand flax (Phormium) is also suggested for this procedure.
Ethical issues
There are no ethical issues associated with this procedure.
Procedure
SAFETY: Wear eye protection and gauntlet gloves when harvesting nettles. Wear rubber gloves when immersing them to avoid stings.
Set out shallow boxes with crumpled paper for masses to fall into: these will absorb the force of the falling mass.
Preparation
a Remove the leaves and flowers from stems of mature stinging nettles. Place the stems in a bucket/ bowl of water so that they are completely submerged. Leave them soaking for at least a week. (See Note 1.)
Investigation
a Remove the stems from the water.
b Wash the stems to remove the softened tissue and then dry the remaining fibres. The outside cuticle and epidermal layer will rub away and the central pith will be left when you peel away the fibres. These fibres are made up of vascular tissue; they contain both the xylem vessels and sclerenchyma fibres.
Teaching notes
The aim of this practical is for students to design and carry out an experiment to test the strength of extracted fibres. This can be a very straightforward experiment – many biomechanical experiments are. The aim is to get students thinking carefully about planning – to highlight experimental skills and practise for advanced level coursework investigations.
Retting is very smelly. Students could extract the fibres themselves, or it could be done for them in advance.
Producing good results is fiddly and requires a certain skill.
Suggested methods
Refer to the SAPS newsletter Osmosis 22 – see link below – for details and diagrams of ideas on testing strain and stiffness of plant material.
Tensile strength Clamp the fibres at each end. Apply a force in the centre using suspended masses or a force meter. The mass (or force) required to break the fibres gives a measure of tensile strength.
Strain Measure how much a length of fibre bends when supported at each end (but not firmly clamped) and loaded in the middle of the length. With fine fibres this will need very small masses – perhaps single paperclips or shorter lengths of wire.
Stiffness Cut measured lengths of fibre. Support them horizontally at a fixed point on their length. Measure the distance of the free end of the fibre from a mark at the same horizontal level as the supported end.
Health and safety checked, September 2008
Downloads
Download the student sheet 
Extracting fibres from plants (61 KB) which has questions and answers.
Thanks to Salters-Nuffield Advanced Biology (SNAB) for permission to use this procedure
Download the original SNAB PDFs 
Extraction of ‘fibres’ from stinging nettles teacher & technician notes (52 KB) Extraction of ‘fibres’ from stinging nettles student sheets (46 KB)
Web links
http://www.saps.org.uk/library/555
See Science and Plants for Schools (SAPS) Osmosis newsletter 22, with ideas for testing strain and stiffness of plant material.
(Website accessed October 2011)