Ah, that. I think that's because of the displacement. I've repaired the ferns anyway. Quite a job, as the objects I imported in ST came out broken (too small geometry), so I made them 10x as big. That was okay, but the I noticed that there were no normals

So I imported the leaves in Lightwave, added the good fiddleheads, saved as lwo, imported in Poseray, exported as obj, imported in TG and saved as tgo. Pffff. They look good now, but I have to re-render the image, see what comes out.