Objection 1. It would seem that it was not fitting that the production of plants should take place on the third day. For plants have life, as animals have. But the production of animals belongs to the work, not of distinction, but of adornment. Therefore the production of plants, as also belonging to the work of adornment, ought not to be recorded as taking place on the third day, which is devoted to the work of distinction.
Objection 2. Further, a work by which the earth is accursed should have been recorded apart from the work by which it receives its form. But the words of Genesis 3:17, "Cursed is the earth in thy work, thorns and thistles shall it bring forth to thee," show that by the production of certain plants the earth was accursed. Therefore the production of plants in general should not have been recorded on the third day, which is concerned with the work of formation.
Objection 3. Further, as plants are firmly fixed to the earth, so are stones and metals, which are, nevertheless, not mentioned in the work of formation. Plants, therefore, ought not to have been made on the third day.
On the contrary, It is said (Genesis 1:12): "The earth brought forth the green herb," after which there follows, "The evening and the morning were the third day."
I answer that, On the third day, as said (1), the formless state of the earth comes to an end. But this state is described as twofold. On the one hand, the earth was "invisible" or "void," being covered by the waters; on the other hand, it was "shapeless" or "empty," that is, without that comeliness which it owes to the plants that clothe it, as it were, with a garment. Thus, therefore, in either respect this formless state ends on the third day: first, when "the waters were gathered together into one place and the dry land appeared"; secondly, when "the earth brought forth the green herb." But concerning the production of plants, Augustine's opinion differs from that of others. For other commentators, in accordance with the surface meaning of the text, consider that the plants were produced in act in their various species on this third day; whereas Augustine (Gen. ad lit. v, 5; viii, 3) says that the earth is said to have then produced plants and trees in their causes, that is, it received then the power to produce them. He supports this view by the authority of Scripture, for it is said (Genesis 2:4-5): "These are the generations of the heaven and the earth, when they were created, in the day that... God made the heaven and the earth, and every plant of the field before it sprung up in the earth, and every herb of the ground before it grew." Therefore, the production of plants in their causes, within the earth, took place before they sprang up from the earth's surface. And this is confirmed by reason, as follows. In these first days God created all things in their origin or causes, and from this work He subsequently rested. Yet afterwards, by governing His creatures, in the work of propagation, "He worketh until now."Now the production of plants from out the earth is a work of propagation, and therefore they were not produced in act on the third day, but in their causes only. However, in accordance with other writers, it may be said that the first constitution of species belongs to the work of the six days, but the reproduction among them of like from like, to the government of the universe. And Scripture indicates this in the words, "before it sprung up in the earth," and "before it grew," that is, before like was produced from like; just as now happens in the natural course by the production of seed. Wherefore Scripture says pointedly (Genesis 1:11): "Let the earth bring forth the green herb, and such as may seed," as indicating the production of perfection of perfect species, from which the seed of others should arise. Nor does the question where the seminal power may reside, whether in root, stem, or fruit, affect the argument.
Reply to Objection 1. Life in plants is hidden, since they lack sense and local movement, by which the animate and the inanimate are chiefly discernible. And therefore, since they are firmly fixed in the earth, their production is treated as a part of the earth's formation.
Reply to Objection 2. Even before the earth was accursed, thorns and thistles had been produced, either virtually or actually. But they were not produced in punishment of man; as though the earth, which he tilled to gain his food, produced unfruitful and noxious plants. Hence it is said: "Shall it bring forth TO THEE."
Reply to Objection 3. Moses put before the people such things only as were manifest to their senses, as we have said (67, 4; 68, 3). But minerals are generated in hidden ways within the bowels of the earth. Moreover they seem hardly specifically distinct from earth, and would seem to be species thereof. For this reason, therefore, he makes no mention of them.
Source: Summa Theologica (New Advent)