The plant referred to here is generally considered to be black mustard, a large annual plant up to tall, but growing from a proverbially small seed. According to rabbinical sources, Jews did not grow the plant in gardens, and this is consistent with Matthew's description of it growing in a field. Luke tells the parable with the plant in a garden instead; this is presumably recasting the story for an audience outside the Levant. I. Howard Marshall writes that the parable "suggests the growth of the kingdom of God from tiny beginnings to worldwide size." The Parable of the Leaven shares this theme of large growth from small beginnings. As with the Parable of the Sower, which in Matthew and Mark occurs earlier in the same chapter, the man sowing the seed represents Jesus, and the plant is the Kingdom of God. New Testament scholar, Adolf Jülicher, viewed the parable of the mustard seed as a similitude, or an extended simile/metaphor, that has three parts: a picture part, a reality part, and a point of comparison. The picture part is the mustard seed that grows into a large plant, the reality part is the kingdom of God, and the point of comparison is the growth of the kingdom from small beginnings. The nesting birds may refer toOld Testament texts which emphasize the universal reach of God's empire, such as Daniel 4:12. However, a real mustard plant is unlikely to attract nesting birds, so that "Jesus seems deliberately to emphasize the notion of astonishing extravagance in his analogy." Other commentators have suggested that the birds represent Gentiles seeking refuge with Israel or the "sinners" and tax collectors with whom Jesus was criticised for associating. A few commentators view the birds negatively, as representing false teachers invading the church. Some have identified a "subversive and scandalous" element to this parable, in that the fast-growing nature of the mustard plant makes it a "malignant weed" with "dangerous takeover properties". Pliny the Elder, in his Natural History writes that "mustard... is extremely beneficial for the health. It grows entirely wild, though it is improved by being transplanted: but on the other hand when it has once been sown it is scarcely possible to get the place free of it, as the seed when it falls germinates at once." Ben Witherington notes that Jesus could have chosen a genuine tree for the parable, and that the mustard plant demonstrates that "Though the dominion appeared small like a seed during Jesus' ministry, it would inexorably grow into something large and firmly rooted, which some would find shelter in and others would find obnoxious and try to root out."
