Centuries ago, the Inca developed
ingenuous suits of armor 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 featuresâ strength and flexibility,
softness and durabilityâ have their roots in the intricate biology
of the nearly invisible cotton fiber. These fibers begin life
deep within a cotton flower, on the surface of a seed. As many as 16,000 fibers
will festoon a single seed, bulging from the seedâs surface
like miniature water balloons. Each cotton fiber,
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 fiber to elongate. The fiber 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 fiber
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 wonât be sturdy enoughâ producing fibers that are too weak
to hold fabrics together well. In ideal growing conditionsâ with the right temperature, water,
fertilizer, pest control, and lightâ a cotton fiber can grow
up to 3.6 centimeters long with only a 25 micrometer width. Long, fine fibers
can wrap around one another better than shorter,
less fine fibers, which means those long, fine fibers
make stronger threads that hang together better as fabric. Cotton with these qualities
has diverse usesâ from soft textiles
to the U.S. dollar bill, which is 75% cotton. The next crucial stage
of the cotton fiberâs 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 fiberâs weight. The more cellulose that gets deposited, the denser that secondary layer becomesâ and this determines
the strength of the final fiber. This stage is essential
for developing long-lasting material for the likes of, say, a t-shirt. The garmentâs 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 fiber, established with the remodeling
of the primary wall layer. Finally, after about 50 days,
the fiber 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 fibers cracks open, unveiling a burst of several thousand
fiber cells in a fluffy mass. The thread-like fibers we seeâ
thinner than a human hairâ are the remains of those dense,
dried out walls of cellulose. Tens of thousands of these fibers
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 optimize its growth based on nutrients, weather conditions,
and genetics.
That's my cousin!