Plant fibres: from seed, stem, leaf and fruit

1Why this matters

Long before factories, people in Rwanda made ropes, mats, baskets and bags from the plants around them: sisal, papyrus, grasses and banana plants. Today, sisal baskets and banana-fibre crafts are sold in craft shops in Kigali, and Rwanda's coffee has long travelled to buyers overseas in sacks woven from plant fibre.

Plant fibres can be taken from almost every part of a plant. Where the fibre comes from decides whether it is soft enough for a shirt or stiff enough for a rope. This lesson sorts plant fibres by the part of the plant they come from, and describes how they are extracted and how they behave.

2Four sources in the plant

Bast fibres come from the inner bark around the stem. They are soft and flexible and often very strong, so they suit cloth, yarn and sacks. Leaf fibres are hard and stiff, so they suit ropes, twine, mats and brushes rather than clothing.

3How plant fibres are extracted

• Ginning separates cotton fibres from the seeds.
• Retting frees bast fibres. The stems are soaked in water, or left lying in a damp field, so that bacteria rot away the gum that glues the fibres to the woody core. The fibres are then beaten and combed out.
• Decortication frees leaf fibres. Sisal leaves are crushed and scraped to remove the soft pulp; the fibres are washed, dried in the sun and brushed.

Sisal is a stiff-leaved agave plant that grows best in hot, sunny places. In 2020 the world produced about 210,000 tonnes of sisal fibre; Brazil was the largest producer, followed by Tanzania and Kenya in East Africa. The fibre makes up only about 4% of the plant's weight, and each leaf holds around a thousand fibres.

4Characteristics of plant fibres

All plant fibres are made mainly of cellulose, so they share these characteristics:

• absorbent: they take up water and sweat, so cloth made from them feels cool;
• strong, and cotton and linen are even stronger when wet;
• low elasticity and resilience: they do not stretch much and they crease;
• withstand heat: they take a hot iron, but they burn quickly with a smell like burning paper;
• not eaten by moths, but mould (mildew) grows on them if they are stored damp;
• resist alkalis such as washing soda, but are damaged by strong acids;
• renewable and biodegradable: they grow again every season and rot away when thrown out.

Worked example: choosing a plant fibre for a job

A women's cooperative needs fibres for four products. For each, they match the job to a characteristic.

Worked calculation. If fibre is about 4% of the weight of a sisal harvest, how much fibre comes from 500 kg of plant material? 4% of 500 kg = 0.04 × 500 = 20 kg. The other 480 kg is pulp and water, which is why sisal is processed close to the fields.

Practice

Solution. (a) Stem (bast). (b) Leaf. (c) Fruit husk of the coconut. (d) Seed hairs.

Solution. Retting is soaking stems in water, or leaving them in a damp field, so that bacteria rot away the gum holding the bast fibres to the woody core. Without it the fibres cannot be separated cleanly.

Solution. Sisal is a hard, stiff leaf fibre. That makes it strong and hard-wearing for ropes and mats, but far too rough and stiff to be comfortable against the skin.

Solution. Mould (mildew) will grow on them, leaving spots and a musty smell, and it weakens the fibres. Plant fibres are not eaten by moths, but they are attacked by mould in damp conditions.

Solution. 0.04 × 750 = 30 kg.