Who are the two parts in "Naming of Parts"?

The setting may be visualized as a lecture room in which
recruits are attending a weapons lecture as a part of their basic training. A group
leader is standing in front of a drawing which lays out the parts of a rifle, which he
is explaining. He does not, however, have an actual gun as his example, and things are
dull. It is a lovely spring day, there are nearby gardens, and one thoroughly bored
recruit’s mind keeps slipping away from the lecture to consider the burgeoning fertility
of spring. The two voices that we hear in the poem are the instructor’s lecture and the
recruit’s musings about nature and the garden. It is fairly easy to be able to establish
when (or where) one voice stops and the other begins in each
stanza.