Objection 1. It would seem that the parts of the vegetative soul are not fittingly described--namely, the nutritive, augmentative, and generative. For these are called "natural" forces. But the powers of the soul are above the natural forces. Therefore we should not class the above forces as powers of the soul.
Objection 2. Further, we should not assign a particular power of the soul to that which is common to living and non-living things. But generation is common to all things that can be generated and corrupted, whether living or not living. Therefore the generative force should not be classed as a power of the soul.
Objection 3. Further, the soul is more powerful than the body. But the body by the same force gives species and quantity; much more, therefore, does the soul. Therefore the augmentative power of the soul is not distinct from the generative power.
Objection 4. Further, everything is preserved in being by that whereby it exists. But the generative power is that whereby a living thing exists. Therefore by the same power the living thing is preserved. Now the nutritive force is directed to the preservation of the living thing (De Anima ii, 4), being "a power which is capable of preserving whatever receives it." Therefore we should not distinguish the nutritive power from the generative.
On the contrary, The Philosopher says (De Anima ii, 2,4) that the operations of this soul are "generation, the use of food," and (cf. De Anima iii, 9) "growth."
I answer that, The vegetative part has three powers. For the vegetative part, as we have said (1), has for its object the body itself, living by the soul; for which body a triple operation of the soul is required. One is whereby it acquires existence, and to this is directed the "generative" power. Another is whereby the living body acquires its due quantity; to this is directed the "augmentative" power. Another is whereby the body of a living thing is preserved in its existence and in its due quantity; to this is directed the "nutritive" power.
We must, however, observe a difference among these powers. The nutritive and the augmentative have their effect where they exist, since the body itself united to the soul grows and is preserved by the augmentative and nutritive powers which exist in one and the same soul. But the generative power has its effect, not in one and the same body but in another; for a thing cannot generate itself. Therefore the generative power, in a way, approaches to the dignity of the sensitive soul, which has an operation extending to extrinsic things, although in a more excellent and more universal manner; for that which is highest in an inferior nature approaches to that which is lowest in the higher nature, as is made clear by Dionysius (Div. Nom. vii). Therefore, of these three powers, the generative has the greater finality, nobility, and perfection, as the Philosopher says (De Anima ii, 4), for it belongs to a thing which is already perfect to "produce another like unto itself." And the generative power is served by the augmentative and nutritive powers; and the augmentative power by the nutritive.
Reply to Objection 1. Such forces are called natural, both because they produce an effect like that of nature, which also gives existence, quantity and preservation (although the above forces accomplish these things in a more perfect way); and because those forces perform their actions instrumentally, through the active and passive qualities, which are the principles of natural actions.
Reply to Objection 2. Generation of inanimate things is entirely from an extrinsic source; whereas the generation of living things is in a higher way, through something in the living thing itself, which is the semen containing the principle productive of the body. Therefore there must be in the living thing a power that prepares this semen; and this is the generative power.
Reply to Objection 3. Since the generation of living things is from a semen, it is necessary that in the beginning an animal of small size be generated. For this reason it must have a power in the soul, whereby it is brought to its appropriate size. But the inanimate body is generated from determinate matter by an extrinsic agent; therefore it receives at once its nature and its quantity, according to the condition of the matter.
Reply to Objection 4. As we have said above (Article 1), the operation of the vegetative principle is performed by means of heat, the property of which is to consume humidity. Therefore, in order to restore the humidity thus lost, the nutritive power is required, whereby the food is changed into the substance of the body. This is also necessary for the action of the augmentative and generative powers.
Source: Summa Theologica (New Advent)