Objection 1. It would seem that the acts of the vegetal soul are subject to the command of reason. For the sensitive powers are of higher rank than the vegetal powers. But the powers of the sensitive soul are subject to the command of reason. Much more, therefore, are the powers of the vegetal soul.
Objection 2. Further, man is called a "little world" [Aristotle, Phys. viii. 2, because the soul is in the body, as God is in the world. But God is in the world in such a way, that everything in the world obeys His command. Therefore all that is in man, even the powers of the vegetal soul, obey the command of reason.
Objection 3. Further, praise and blame are awarded only to such acts as are subject to the command of reason. But in the acts of the nutritive and generative power, there is room for praise and blame, virtue and vice: as in the case of gluttony and lust, and their contrary virtues. Therefore the acts of these powers are subject to the command of reason.
On the contrary, Gregory of Nyssa [Nemesius, De Nat. Hom. xxii.] sats that "the nutritive and generative power is one over which the reason has no control."
I answer that, Some acts proceed from the natural appetite, others from the animal, or from the intellectual appetite: for every agent desires an end in some way. Now the natural appetite does not follow from some apprehension, as to the animal and the intellectual appetite. But the reason commands by way of apprehensive power. Wherefore those acts that proceed from the intellective or the animal appetite, can be commanded by reason: but not those acts that proceed from the natural appetite. And such are the acts of the vegetal soul; wherefore Gregory of Nyssa (Nemesius, De Nat. Hom. xxii) says "that generation and nutrition belong to what are called natural powers." Consequently the acts of the vegetal soul are not subject to the command of reason.
Reply to Objection 1. The more immaterial an act is, the more noble it is, and the more is it subject to the command of reason. Hence the very fact that the acts of the vegetal soul do not obey reason, shows that they rank lowest.
Reply to Objection 2. The comparison holds in a certain respect: because, to wit, as God moves the world, so the soul moves the body. But it does not hold in every respect: for the soul did not create the body out of nothing, as God created the world; for which reason the world is wholly subject to His command.
Reply to Objection 3. Virtue and vice, praise and blame do not affect the acts themselves of the nutritive and generative power, i.e. digestion, and formation of the human body; but they affect the acts of the sensitive part, that are ordained to the acts of generation and nutrition; for example the desire for pleasure in the act of taking food or in the act of generation, and the right or wrong use thereof.
Source: Summa Theologica (New Advent)