Mark 4: 26-34

He also said the kingdom of God is as if someone would scatter seed on the ground and would sleep and rise night and day and the seed would sprout and grow, he does not know how.  The earth produces of itself first the stalk then the head, then the full grain in the head.  But when the grain is ripe at once he goes in with his sickle because the harvest has come.

He also said with what can we compare the Kingdom of God or what parable will we use for it?  It is like a mustard seed which when sown upon the ground is the smallest of all the seeds on earth.  Yet when it is sown it grows up and becomes the largest of all shrubs and puts forth large branches so that the birds of the air can make nests in its shade.

With many such parables He spoke the Word to them as they were able to hear it.  He did not speak to them except in parables but He explained everything in private to His disciples.