Chapter 4:26-29 Parable of the Seed

4:26 And He said, So is the kingdom of God, as if a man should cast seed into the ground;

So is the kingdom of God - This parable is mentioned only by Mark, a proof that Mark did not abridge Matthew.
The doctrine of the kingdom, received in a good and honest heart, is like seed sown by a man in his ground, properly prepared to receive it; for when he hath sown it, he sleeps and wakes day after day, and, looking on it, he sees it spring and grow up through the virtue of the earth in which it is sown, though he knows not how it doth so; and when he finds it ripe, he reaps it, and so receives the benefit of the sown seed. 

So is it here: the seed sown in the good and honest heart brings forth fruit with patience; and this fruit daily increaseth, though we know not how the Word and Spirit work that increase; and then Christ the husbandman, at the time of the harvest, gathers in this good seed into the kingdom of heaven.

4:27 And should sleep, and rise night and day, and the seed should spring and grow up, he knoweth not how.

And should sleep and rise night and day - That is, he should sleep by night, and rise by day; for so the words are obviously to be understood.
He knoweth not how - How a plant grows is a mystery in nature, which the wisest philosopher in the universe cannot fully explain.

4:28 For the earth bringeth forth fruit of herself; first the blade, then the ear, after that the full corn in the ear.

Bringeth forth - of herself - Αυτοματη. By its own energy, without either the influence or industry of man.
first the blade, then the ear, after that the full corn in the ear; which, as it very aptly describes the progress of the seed from first to last; so it very beautifully represents the gradual increase

--of the work of grace, 

--under the instrumentality of the word, 

--accompanied with the Spirit and power of God.

4:29 But when the fruit is brought forth, immediately he putteth in the sickle, because the harvest is come.

But when the fruit is brought forth - Unto perfection, and is fully ripe; signifying that when grace is brought to maturity, and faith is performed with power, and the good work begun is perfected; then, as the husbandman, immediately he putteth the sickle; and cuts it down, and gathers it in;
because the harvest is come; at death or at the end of the world, which the harvest represents: when all the elect of God are called by grace, and grace in them is brought to its perfection, and they have brought forth all the fruit they were ordained to bear, they will then be all gathered in; 

--either by Christ Himself who comes into His garden, 

and gathers His lilies by death; 

--or by the angels, the reapers, at the close of time, who will gather the elect from the four winds;