acicular

Needle-shaped; slender like a needle or bristle.

Near-synonyms: needlelike, pinlike

1992, Oliver Sacks, Migraine, Berkeley: University of California Press, revised and expanded edition, Part 5, Chapter 17, p. 279, Sometimes these networks have an acicular or crystalline appearance, and may grow visibly, sometimes with sudden jerks, “like frost on a windowpane,” or “primitive plants.”

Having sharp points like needles.

Of a leaf, slender and pointed, needle-like.

the acicular foliage of coniferous trees

[…] though fond of foliage, their trees always had a tendency to congeal into little acicular thorn-hedges, and never tossed free.