Why is cotton
Centuries ago, the Inca developed
ingenuous suits of armour that could flex with the blows of sharp spears and
maces, protecting warriors from even the fiercest physical attacks. These hardy
structures were made not from iron or steel, but rather something unexpectedly
soft – cotton.
These thickly woven, layered quilts
of cotton could distribute the energy from a blow across a large surface area, shielding
warriors without restricting their mobility. These seemingly contradictory
features – strength and flexibility, softness and durability – have their roots
in the intricate biology of the nearly invisible cotton fibre.
These fibres begin life deep within
a cotton flower, on the surface of a seed. As many as 16,000 fibres will
festoon a single seed, bulging from the seed’s surface like miniature water
Each cotton fibre, no matter how
large it grows, is made of just one cell. That cell has multiple layers of cell
wall. After a few days, the sides of the first layer, called the primary cell
wall, stiffen, pushing cell growth in one direction and causing the fibre to
elongate. The fibre elongates quickly for about 16 days.
Then it begins the next stage;
strengthening the cell wall. It does this by making more of the carbohydrate
cellulose. Cellulose will make up 34% of the cell wall at this stage and
swiftly increases. This new growth also reinforces the cell wall by going
against the grain of the existing wall. The strengthened wall is more rigid,
restricting further growth. That means if the fibre remodels its walls too
early, it will be short, and ultimately make rough, weak fabrics. But if cell
wall strengthening begins too late, the wall won’t be sturdy enough – producing
fibres that are too weak to hold fabrics together well.
In ideal growing conditions – with
the right temperature, water, fertilizer, pest control, and light – a cotton
fibre can grow up to 3.6 centimetres long with only a 25 micrometre width. Long,
fine fibres can wrap around one another better than shorter, less fine fibres, which
means those long, fine fibres make stronger threads that hang together better
as fabric. Cotton with these qualities has diverse uses, from soft textiles to
the U.S. dollar bill, which is 75% cotton.
The next crucial stage of the cotton
fibre’s growth begins as it thickens its secondary cell wall by depositing large
quantities of cellulose into the secondary layer. Cellulose goes on to make up
over 90% of the fibre’s weight. The more cellulose that gets deposited, the
denser that secondary layer becomes and this determines the strength of the
final fibre. This stage is essential for developing long-lasting material for
the likes of, say, a t-shirt. The garment’s capacity to withstand years of
washing and wear is largely determined by the density of that secondary cell
wall. On the other hand, its softness is strongly influenced by the length of
the fibre, established with the remodelling of the primary wall layer.
Finally, after about 50 days, the
fibre is fully grown. The living matter within the cell dies off, leaving
behind only the cellulose. The dried cotton seed pod, or boll, that surrounds
the fibres cracks open, unveiling a burst of several thousand fibre cells in a
fluffy mass. The thread-like fibres we see, thinner than a human hair, are the
remains of those dense, dried out walls of cellulose.
Tens of thousands of these fibres
spun into yarn will go on to make everything from fabric, to coffee filters,
diapers, and fishing nets. And with the help of modern science, cotton might
soon be softer, stronger, and more resilient than ever as researchers investigate
how to optimise its growth based on nutrients, weather conditions, and
Source: TED Ed – Created by Michael R. Stiff