I wonder which of these birch was referring to?
noun 1. a very fine thread or threadlike structure; a fiber or fibril: filaments of gold.
2. a single fibril of natural or synthetic textile fiber, of indefinite length, sometimes several miles long.
3. a long slender cell or series of attached cells, as in some algae and fungi.
4. Botany. the stalklike portion of a stamen, supporting the anther.
5. Ornithology. the barb of a down feather.
6. (in a light bulb or other incandescent lamp) the threadlike conductor, often of tungsten, in the bulb that is heated to incandescence by the passage of current.
7. Electronics. the heating element (sometimes also acting as a cathode) of a vacuum tube, resembling the filament in an incandescent bulb.
8. Astronomy. a solar prominence, as viewed within the sun's limb.