From: Mark 4:26-34
Parables of the Seed and of the Mustard Seed
 And He (Jesus) said, "The Kingdom of God is as if a man should scatter seed upon the ground,  and should sleep and rise night and day, and the seed should sprout and grow, he knows not how.  The earth produces of itself, first the blade, then the ear, then the full grain in the ear.  But when the grain is ripe, at once he puts in the sickle, because the harvest is come."  And He said, "With what can we compare the Kingdom of God, or what parable shall we use for it?  It is like a grain of 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 greatest of all shrubs, and puts forth large branches, so that the birds of the air can make nests in its shade."
The End of the Parables Discourse
 With many such parables He spoke the word to them, as they were able to hear it;  He did not speak to them without a parable, but privately to His own disciples He explained everything.
26-29. Farmers spare no effort to prepare the ground for the sowing; but once the grain is sown there is nothing more they can do until the harvest; the grain develops by itself. Our Lord uses this comparison to describe the inner strength that causes the Kingdom of God on earth to grow up to the day of harvest (cf. Joel 3:13 and Revelation 14:15), that is, the day of the Last Judgment.
Jesus is telling His disciples about the Church: the preaching of the Gospel, the generously sown seed, will unfailingly yield its fruit, independently of who sows or who reaps: it is God who gives the growth (cf. 1 Corinthians 3:5-9). It will all happen "he knows not how", without men being fully aware of it.
The Kingdom of God also refers to the action of grace in each soul: God silently works a transformation in us, whether we sleep or watch, causing resolutions to take shape in our soul--resolutions to be faithful, to surrender ourselves, to respond to grace--until we reach "mature manhood" (cf. Ephesians 4:13). Even though it is necessary for man to make this effort, the real initiative lies with God, "because it is the Holy Spirit who, with His inspirations, gives a supernatural tone to our thoughts, desires and actions. It is He who leads us to receive Christ's teaching and to assimilate it in a profound way. It is He who gives us the light by which we perceive our personal calling and the strength to carry out all that God expects of us. If we are docile to the Holy Spirit, the image of Christ will be found more and more fully in us, and we will be brought closer every day to God the Father. `For whoever are led by the Spirit of God, they are the children of God' (Romans 8:14)" ([St] J. Escriva, "Christ Is Passing By", 135).
30-32. The main meaning of this parable has to do with the contrast between the great and the small. The seed of the Kingdom of God on earth is something very tiny to begin with (Luke 12:32; Acts 1:15); but it will grow to be a big tree. Thus we see how the small initial group of disciples grows in the early years of the Church (cf Acts 2:47; 6:7; 12:24), and spreads down the centuries and becomes a great multitude "which no man could number" (Revelation 7:9). This mysterious growth which our Lord refers to also occurs in each soul: "the Kingdom of God is in the midst of you" (Luke 17:21); we can see a prediction of this in the words of Psalm 92:12: "The righteous grow like a cedar in Lebanon." To allow the mercy of God to exalt us, to make us grow, we must make ourselves small, humble (Ezekiel 17:22-24; Luke 18:9-14).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.