Lichen tiles

So yeah, these are the lichen tiles.
I suppose they're not technically lichen, but they have the same sort of matrix setup... just bigger.
The top layers have a tough, flexible cuticle and lots of tiny vacuoles of water. As they get pushed towards the edge, they desiccate and contract, becoming a soft-ish cushioning layer. Once they have lengthened they go completely dry and become very strong in compression
For the trunk, imagine a massive hardened sponge.

I'll put this into scraps once I make the proper info sheet. Until then, I just had to draw it down.