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When our second hard frost hit this year, our Coyote tomatoes were still going ... but the water inside the tissues froze solid, bursting the main stalks (and, obviously, killing the plant). You can see the stem structures separately in this neat photo. 
See how the ice crystals are curling away from a central "thread"? That's what remains of the pith of the plant. In this rugged main stalk of a large, months-old tomato plant, most of the pith has been replaced by additional xylem fibers, which draw up water from the roots ...
... and thus have frozen solid on a 28-degree morning, bursting the tissues as the ice crystals expanded.
Those stringy tan fibers lying on top of the ice are phloem, the vascular tissues that transport the nutrients of photosynthesis throughout the plant.
Those stringy tan fibers lying on top of the ice are phloem, the vascular tissues that transport the nutrients of photosynthesis throughout the plant.
(The mnemonic I was taught for this in fifth-grade plant biology was "xylem xucks stuff up, phloem phlows it around the plant.") Here's a Wikimedia image by Curtis_Clark of those vascular tissues in a healthy flax stem. Pi=pith, X=xylem, P=phloem.
The green/brown flakes on top of the stringy phloem are epidermis (skin) and cambium (the bit that makes more vascular tissue so the stem can continue to thicken). Like the phloem layer, they've been torn open by the expanding frozen xylem.
Here's the fun bit of ethnobotany: Those stringy phloem fibers are also called "bast." They're relatively flexible in dicots (the plants that have branching vascular systems), which makes them awfully handy for creating things like paper, fabric, and rope.
What's happened to this unfortunate tomato stalk is a rapid form of "field retting," or "dew retting," the most "natural" way of getting the bast separated from the xylem, pith, and epidermis (skin) of the plant so that you can use it to make stuff.
I'm now in the process of pulling off those bast fibers in long clumps and drying them. Then, I'll boil them in a mildly caustic solution to break down the pectin that "glues" the bast tissue together.
After a certain amount of mechanical disintegration (i.e., beating it with a mallet or chucking it in a sharp-bladed blender), that fiber will become a pulp I can use to make sheets of paper.
All plants have that bast fiber (though it's arranged a bit differently in monocotyledons [the ones with stiff parallel veins in their leaves), but not all have enough to be worth gathering. Tomato is hopefully worthwhile for small-batch hand papermaking (we'll see how it goes!).
If I wanted to make fabric, I'd need a plant that has a larger amount of strong phloem fiber ... like, say, a flax plant, which lets me make linen. (That's actually why linen creases so easily. The cellulose in phloem likes to re-bond with itself whenever moisture changes.)
Jute also has very useful bast; its coarse phloem fibers can be beaten into submission for paper or left long to make robe or a rough fabric, which you may know as burlap or hessian depending on where you live.
I'm looking forward to seeing whether leaving the tomatoes standing until hard frost will reliably ret them this way. I've been scraping the stems as they lose their leaves, but the "ice retting" would certainly get me a larger amount more easily!
I'll post my results when I've got enough tomato phloem fiber prepared to pull some tomato paper. It may not work; not all fibers do. But given how hard it is to find information on artisan papermaking, there's only one way to find out!
