Hello. You did not name the poem to which this question refers, which makes it difficult for it to be answered accurately. However, I will try to help you in the best possible way.
The metaphor presents a stage of development and growth. We can see this when the speaker asks an element, how it acted when it was a plant. In this case, we can see that this element was already part of the composition of a plant, but it went through a process of growth and development that transformed it into other things, but it may be able to remember, or not, if it heard the things that trees spoke when he was part of one.
It is necessary to remember that the metaphor is the figure of speech that presents an implicit or explicit comparison between two related elements and that have certain similarities.