Thorns
There are various plants that provide thorns. See
foliage for a list of thorn bearing plants. The number of thorns harvested each time is random, and seems to be based on some kind of a discrete version of the exponential probability density function. As such, there is no max number of thorns that can come from a single harvest. The average number of thorns per pick is about 9.
Thorns are required to make the flax comb and to learn foraging.