28 For the earth bringeth forth fruit of herself; first the blade, then the ear, after that the full corn in the ear.
[AD 407] John Chrysostom on Mark 4:26-29
(non occ.) Again it may be said, that he hath not, who has not truth. But our Lord says that he hath, because he has a lie, for every one whose understanding believes a lie, thinks that he has something.

[AD 604] Gregory the Dialogist on Mark 4:26-29
(in Ezech. 2. Hom. 3) 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. When therefore we conceive good desires, we put seed into the ground; when we begin to work rightly, we are the blade. When we increase to the perfection of good works, we arrive at the ear; when we are firmly fixed in the perfection of the same working, we already put forth the full corn in the ear.

[AD 1107] Theophylact of Ohrid on Mark 4:26-29
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.

For we put forth the blade, when we show a principle of good; then the ear, when we can resist temptations; then comes the fruit, when a man works something perfect. It goes on: and when it has brought forth the fruit, immediately he sendeth the sickle, because the harvest is come.

[AD 1274] Pseudo-Jerome on Mark 4:26-29
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.

For the seed is the word of life, the ground is the human heart, and the sleep of the man means the death of the Saviour. The seed springs up night and day, because after the sleep of Christ, the number of Christians, through calamity and prosperity, continued to flourish more and more in faith, and to wax greater in deed.

But when He says, He knoweth not how, He is speaking in a figure; that is, He does not make known to us, who amongst us will produce fruit unto the end.

That is, fear. For the fear of God is the beginning of wisdom. Then the full corn in the ear; (Ps. 111:10. Rom. 13:8) that is, charity, for charity is the fulfilling of the Law.

The sickle is death or the judgment, which cuts down all things; the harvest is the end of the world.

[AD 1274] Pseudo-Chrysostom on Mark 4:26-29
(Vict. Ant. e Cat. in Marc.) A parable occurred, a little above, about the three seeds which perished in various ways, and the one which was saved; in which last He also shows three differences, according to the proportion of faith and practice. Here, however, He puts forth a parable concerning those only who are saved. Wherefore it is said, And he said, So is the kingdom of God, as if a man should cast seed into the ground, &c.

(Vict. Ant. e Cat. in Marc.) Or else He calls by the name of kingdom of God, faith in Him, and in the economy of His Incarnation; which kingdom indeed is as if a man should throw seed. For He Himself being God and the Son of God, having without change been made man, has cast seed upon the earth, that is, He has enlightened the whole world by the word of divine knowledge.

(Vict. Ant. e Cat. in Marc.) Or Christ Himself is the man who rises, for He sat waiting with patience, that they who received seed should bear fruit. He rises, that is, by the word of His love, He makes us grow to the bringing forth fruit, by the armour of righteousness on the right hand, by which is meant the day, and on the left, by which is meant the night of persecution; for by these the seed springs up, and does not wither. (2 Cor. 6:7)

(Vict. Ant. e Cat. in Marc.) Or else He says, He knoweth not, that He may show the free-will of those who receive the word, for He commits a work to our will, and does not work the whole Himself alone, lest the good should seem involuntary. For the earth brings forth fruits of its own accord, that is, she is brought to bear fruit without being compelled by a necessity contrary to her will. First the blade.

(Vict. Ant. e Cat. in Marc.) Or, first it produces the blade, in the law of nature, by degrees growing up to advancement; afterwards it brings forth the ears, which are to be collected into a bundle, and to be offered on an altar to the Lord, that is, in the law of Moses; afterwards the full-fruit, in the Gospel. Or because we must not only put forth leaves by obedience, but also learn prudence, and, like the stalk of corn, remain upright without minding the winds which blow us about. We must also take heed to our soul by a diligent recollection, that, like the ears, we may bear fruit, that is, show forth the perfect operation of virtue.

[AD 202] Irenaeus on Mark 4:28
And the cup His blood, if they do not call Himself the Son of the Creator of the world, that is, His Word, through whom the wood fructifies, and the fountains gush forth, and the earth gives "first the blade, then the ear, then the full corn in the ear."
[AD 220] Tertullian on Mark 4:28
Observe how the created order has advanced little by little toward fruitfulness. First comes the grain, and from the grain arises the shoot, and from the shoot emerges the shrub. From there the boughs and leaves gather strength, and the whole that we call a tree expands. Then follows the swelling of the germen, and from the germen bursts the flower, and from the flower the fruit opens. The fruit itself, primitive for a while, and unshapely, keeping the straight course of its development, is matured, little by little, to the full mellowness of its flavor. In just this way has righteousness grown in history. The proximate righteousness found in the created order is grounded in the holy God whose righteousness first emerged in a rudimentary stage as an undeveloped natural apprehension in the presence of the holy One. Then it advanced through the law and prophets to childhood. At long last through the gospel, God’s righteousness has been personally manifested with the vital energies of youth. Now through the paraclete, righteousness is being manifested in its mature stage.

[AD 220] Tertullian on Mark 4:28
First comes the grain, and from the grain arises the shoot, and from the shoot struggles out the shrub: thereafter boughs and leaves gather strength, and the whole that we call a tree expands: then follows the swelling of the germen, and from the germen bursts the flower, and from the flower the fruit opens: that fruit itself, rude for a while, and unshapely, little by little, keeping the straight course of its development, is trained to the mellowness of its flavour. So, too, righteousness-for the God of righteousness and of creation is the same-was first in a rudimentary state, having a natural fear of God: from that stage it advanced, through the Law and the Prophets, to infancy; from that stage it passed, through the Gospel, to the fervour of youth: now, through the Paraclete, it is settling into maturity.

[AD 397] Ambrose of Milan on Mark 4:28
While you are asleep, O man, and without your being aware of it, the earth of itself is producing its fruits.

[AD 604] Gregory the Dialogist on Mark 4:28
To produce “the blade” is to hold the first tender beginning of good. The “blade” arrives at maturity when virtue conceived in the mind leads to advancement in good works. The “full corn” fructifies in “the ear” when virtue makes such great progress that it comes into its fullest possible expression.