Mustard

a plant of the genus sinapis, a pod-bearing, shrub-like plant,
growing wild, and also cultivated in gardens. The little round
seeds were an emblem of any small insignificant object. It is
not mentioned in the Old Testament; and in each of the three
instances of its occurrence in the New Testament (Matt. 13:31,
32; Mark 4:31, 32; Luke 13:18, 19) it is spoken of only with
reference to the smallness of its seed. The common mustard of
Israel is the Sinapis nigra. This garden herb sometimes grows
to a considerable height, so as to be spoken of as "a tree" as
compared with garden herbs.