Definition of "acicular"

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.”

1992, Oliver Sacks, Migraine, Berkeley: University of California Press, revised and expanded edition, Part 5, Chapter 17, p. 279

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

1860, John Ruskin, chapter 3, in Modern Painters, volumes V (parts VI–IX), London: Smith, Elder, and Co., […], page 189