Flax
One of the oldest cultivated plant fibers, a bast fiber from the plant of the same name. Flax is the fiber linen is woven of. There are two basic types of flax, "long flax" is the variety that fiber and much of the oil crop is gotten from. Flax oil, linseed oil, is harvested from the fully mature seed, while the fiber is gotten from the slightly green stem. Since the seed is fully mature when the plant is completely dry and brown, you may only efficently harvest one or the other from a single plant.
Flax is the strongest of the common plant fibers. It is two to three times stronger than cotton. Because of the natural waxes in the fiber it is very smooth and lint free. When fabric is made of flax it may be stiff for the first several years of its life, but will soften with repeated laundering. Boiling or repeated laundering are often used to soften new linen, as this removes some of the natural waxes. However this stiffness is usually maintained in threads for specialty applications as the waxes will add years to the durabilty of it. Traditionally flax is the thread of choice for lacemaking, leatherworking, and a number of other skilled crafts.
In addition to linen and thread, flax fiber is made into a variety of products. It is used in rope, paper, packing materials, sculptural composites, and etc. Some of these products are made from the straw left over after the seed is harvested, however there is actually quite a demand for the superior quality fibers in many of these applications. Paper money in many countries for instance, contains quite a bit of cloth quality flax fiber.
The unspun fiber makes excellent doll hair, and has the advantage of being completely washable with no special steps. Since flax dyes extremely well and is _very_ colorfast after dying, the fiber both spun and unspun is useful in a number of crafts.