On the "soul" Plutarch was something of a dualist. The soul, he decided was "eternal after death." When it was part of a body it was like a "caged bird." A restlessness. But when the soul remained in a body for a long time, it became tame. And after the release that follows death, such a tame soul would develop a hankering, or perhaps a nosiness, to once again involve itself in the affairs of the living and it would return to inhabit another being. And probably for Plutarch, that other being was a person in the form of a new born.
He did have wonderful things to say about what he called "other living things".
It was his opinion that when "other living things" became old and "useless" they
weren't to be thrown away as one might an old shoe. Whether he thought plants
"other living things" I just don't know. Nor can I grasp what he might have
meant by "useless."