“And should sleep, and rise night and day, and the seed should spring and grow up, he knoweth not how.”
Night and day, whether he sleeps or gets up, the seed sprouts and grows, though he does not know how — the farmer's ignorance of the growth mechanism is the parable's central point. He does not know how — the Greek is automatic (automatē), from which we get automatic. The growth happens without the farmer's management, understanding, or effort. Night and day, whether he sleeps or gets up: the farmer's activity (or inactivity) is irrelevant to the seed's growth. The kingdom grows by its own inherent divine power, not by human strategizing, programming, or management. The disciples are called to sow, not to engineer the growth.
Church Fathers on Mark 4:27
The kingdom of God is the Church, which is ruled by God, and herself rules over men, and treads down the powers which are contrary to her, and all wickedness.
Or else Christ sleeps, that is, ascends into heaven, where, though He seem to sleep, yet He rises by night, when through temptations He raises us up to the knowledge of Himself; and in the day time, when on account of our prayers, He sets in order our salvation.
Or else, Man casts seed into the ground, when he places a good intention in his heart; and he sleeps, when he already rests in the hope which attends on a good work. But he rises night and day, because he advances amidst prosperity and adversity, though he knows it not, for he is as yet unable to measure his increase, and yet virtue, once conceived, goes on increasing.