Jamieson, Fausset and Brown on Mark 4:26-27

26-27. So is the kingdom of God, as if a man should cast seed into the ground; and should sleep, and rise night and day--go about his other ordinary occupations, leaving it to the well-known laws of vegetation under the genial influences of heaven. This is the sense of "the earth bringing forth fruit of herself," in Mr 4:27.