5 “Let the earth bring forth grass.” What spontaneous provision is included in these words—that which is present in the root, in the plant itself, and in the fruit, as well as that which our labour and husbandry add! God did not command the earth immediately to give forth seed and fruit, but to produce germs, to grow green, and to arrive at maturity in the seed; so that this first command teaches nature what she has to do in the course of ages. But, they ask, is it true that the earth produces seed after his kind, when often, after having sown wheat, we gather black grain?
This is not a change of kind, but an alteration, a disease of the grain. It has not ceased to be wheat; it is on account of having been burnt that it is black, as one can learn from its name. If a severe frost had burnt it, it would have had another colour and a different flavour. They even pretend that, if it could find suitable earth and moderate temperature, it might return to its first form. Thus, you find nothing in nature contrary to the divine command. As to the darnel and all those bastard grains which mix themselves with the harvest, the tares of Scripture, far from being a variety of grain, have their own origin and their own kind; image of those who alter the doctrine of the Lord and, not being rightly instructed in the word, but, corrupted by the teaching of the evil one, mix themselves with the sound body of the Church to spread their pernicious errors secretly among purer souls.
The Lord thus compares the perfection of those who believe in Him to the growth of seed, “as if a man should cast seed into the ground; and should sleep and rise, night and day, and the seed should spring and grow up, he knows not how. For the earth brings forth fruit of herself; first the blade, then the ear, after that the full grain in the ear.” “Let the earth bring forth grass.” In a moment earth began by germination to obey the laws of the Creator, completed every stage of growth, and brought germs to perfection.
The meadows were covered with deep grass, the fertile plains quivered with harvests, and the movement of the grain was like the waving of the sea. Every plant, every herb, the smallest shrub, the least vegetable, arose from the earth in all its luxuriance. There was no failure in this first vegetation: no husbandman's inexperience, no inclemency of the weather, nothing could injure it; then the sentence of condemnation was not fettering the earth's fertility. All this was before the sin which condemned us to eat our bread by the sweat of our brow.
Source: Nine Homilies of Hexaemeron (New Advent)