His disciples approached him and said, "Explain to us the parable of the weeds in the field." He said in reply, "He who sows good seed is the Son of Man, the field is the world, the good seed the children of the kingdom."
I'm so used to thinking of the "seed" as the Word of God, due to the Parable of the Sower, that I haven't registered Jesus' other usage - that you and I are the seed He sows in the world. We are the manifestation of the Kingdom of God prior to His Second Coming. That is why the Kingdom can permeate this world like leaven. If the seed is damaged through sin however, the Kingdom can shrink back in size from one generation to the next.