The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
24. (The descending souls enter) into (plants) animated by other (souls), as in the previous cases, on account of scriptural declaration.
In the description of the souls' descent we read, after their coming down in raindrops has been mentioned, 'Then they are born as rice and corn, herbs and trees, sesamum and beans.'--Here a doubt arises whether, at this stage of their descent, the souls to which a remainder of their works continues to cling really pass over into the different species of those immoveable things (plants) and enjoy their pleasures and pains, or if they enter merely into a state of conjunction with the bodies of those plants which are animated by different souls.--The pûrvapakshin maintains that they pass over into those species and enjoy their pleasures and pains, on account of the remainder of works still attaching to them; firstly, because that enables us to take the verb 'to be born' in its literal sense; secondly, because we know from Sruti and Smriti that the condition of a plant may be a place of enjoyment (of the fruits of actions); and thirdly, because sacrifices and similar actions, being connected with harm done to animals, &c., may lead to unpleasant results. We therefore take the 'being born as rice,' &c., of those to whom a remainder of their works attaches, in its literal sense, and consider the case to be analogous to that of a man who is born either as a dog or a hog or a Kândâla, where we have to understand that the man really becomes a dog, and so on, and experiences the pleasures and pains connected with that condition.
To this reasoning we reply as follows:--The souls to which a remainder attaches enter merely into conjunction with rice plants, &c., which are already animated by other souls; and do not enjoy their pleasures and pains; 'as in the previous cases.' As the souls' becoming air, smoke, &c., was decided to mean only that they become connected with them 1, so here too their becoming rice, &c. merely means that they
become connected with those plants--How is this known?--From the fact of the statement here also being of the same nature--Of what nature?--Here, also, as in the case of the souls becoming ether, &c., down to rain, the text does not refer to any operation of the works; hence we conclude that the souls do not enjoy pleasure and pain. Where, on the other hand, the text wants to intimate that the souls undergo pleasure and pain, there it refers to the operation of the former works; so e.g. in the passage which treats of men of good or evil conduct. Moreover, if we should take the souls' being born as rice, &c., in its literal sense, it would follow that when the rice plants are reaped, unhusked, split, cooked and eaten, the souls which have descended into them and are animating them would have to leave them; it being generally known that when a body is destroyed the soul animating it abandons it. And then (if the souls left the plants) the text could not state (as it does state. V, 10, 6) that the souls which had entered into the plants are transmitted by animal generation (on the part of those who eat the plants). Hence it follows that the souls which have descended are merely outwardly connected with the plants animated by other souls. This suffices to refute the assertions that 'to be born' must be taken in its literal sense; and that the state of vegetable existence affords a place for enjoyment. We do not entirely deny that vegetable existence may afford a place for enjoyment; it may do so in the case of other beings which, in consequence of their unholy deeds, have become plants. We only maintain that those souls which descend from the moon with an unrequited remainder of works do not experience the enjoyment connected with plant life.
129:1 This does not agree well with what had been said above about the souls becoming similar to ether, air, &c.