Before we start, a more general discussion of plant fibers can be found in my epic article at Knitty. This is deliberately about the weird stuff. So no leaving "that's weird" comments, I already know that. Of course it's weird. We live for weird around here.
I strongly suspect that the reason a lot of these... odd... fibers were developed/used, is because weaving is a direct offshoot of basket making, and many of these same fibers were probably VERY useful in basketry and someone probably figured, why not weave them, then?
Okeydokey. Quick botany/biology lesson before we go further. Bast fibers are made from the vascular tissues of assorted plants. Think circulatory system. Remember the xylem and phloem in bio class in ninth grade? Those. Check it out:
That's a cross-section of plant stem. (I am so not getting into dicots and monocots and primary and secondary growth and angiosperms. It's 'plant' from here on out. You want gory details, let me know. I will subject you to taxonomy and microbiology and chemistry and you will beg for mercy.) See the solid green bits in a ring, near the outer edge? Those are the phloem. They run the entire length of the stem/branch/root/leaf. You know those ungodly strings in celery? Bingo. Bast fibers.
Linen is the most common bast fiber these days, and the way it is processed is, generally, the way they're all dealt with; the plant is harvested, the stems half-broken (bent to break the outer 'husk' but not pulling the pieces apart). Then they're left to rot, with some kind of moisture, usually running water but sometimes morning dew gets the job done. Once the outer bits of stem are rotted off, the long fibers are collected, cleaned, and spun. You can do this with a kajillion other plants besides flax. Hemp and ramie are two more that are used commercially and most people have heard of. But there are as many others as there are people to process them.
Another of the more common bast fibers in history - though not in commercial production - is nettles. Yes, the stinging weeds. Same deal as linen, above. Let the plant rot, take out the fibers, spin and weave. I could only find one photo of a nettle shirt, and it's from Tibet:
Looks a lot like linen, because it IS a lot like linen, both in processing and in a molecular sense. Nettle shirts were also very popular in Northern Europe, and many shirts labeled as linen in museums are probably nettle. I assume they used nettle instead of linen because, hey, why waste valuable farmland growing something when you can get it from weeds?
Another family of plants that are used widely are the agaves. Yucca, agave, sisal, jute. All related, all used the same way. These fibers are more coarse than those previously mentioned:
These days they're mostly used for floor mats, bags for shipping produce, and shoes. But back in the day they were occasionally used for fabric. I'm unable to find any photos, but I know the Native Americans used Yucca fibers. And Agave fibers are becoming a popular textile for washcloths in high-end bath stores:
If you've ever heard of Azores Lace, they are doilies, knit (yay, knitting!) with fiber from some member of the Agave family:
Trees were also harvested for their fibers, which grew under the bark. The Haida of the Pacific NW and the ancient Japanese were known to use them. This kimono is woven from Japanese elm fiber:
And this one was woven from banana, though the fibers were from the leaves, not the trunk:
Banana fiber was popular all through SE Asia, and is still used, though not commercially. Or at least, not much.
I could go on for days. But basically, ancient people used anything they could get their hands on to weave fabric with, until the trade routes opened up and specific, specialized fibers could be traded across continents. Africans made textiles from raffia, the stripped bark of a palm tree. The Hawaiians used several members of the hibiscus family. The reason we're down to only four or five commonly used bast fibers is because they're the most useful; why kill trees for pitifully short fibers that are hard to process, when you can get nice, long, easily processed fibers from a plant grown specifically for the purpose? It's kind of sad, though, there are a lot of interesting textiles that aren't being made any more, because they're inconvenient.
And while we're discussing freaky fibers, here's the bonus round:
A fisherman's work glove, knit with, get this, the spun-together beards from shellfish. The fibery little bit on the bottom of oysters and clams, that hold them to the rocks? That part. A kajillion of them, gathered and spun and knit. Supposedly, they had to be kept in a bucket of salt water when not in use, to stay flexible. And they were so tough that men left them to their sons and grandsons.
We really will knit anything, won't we?