Metonymy is a figure of speech where a thing or concept is referred to by another thing closely associated with that thing or concept. And synecdoche is a kind of metonymy to refer to the whole by mentioning a part or vice-versa.
 
When the poet says “The joy that unborn eyes shall see” in the second stanza of the poem, he means our future generations by the expression “unborn eyes”. Here eyes represent babies. Thus this is a metonymy (babies are referred to by eyes) and a synocdoche (a part of the body referring to the whole).
 
All Figures of Speech / Poetic Devices in the poem ‘The Heart of the Tree’