The poem is filled with metaphorical language throughout. Some of this takes the form of personification, such as the reference to the grandfather's "heart" never speaking. The heart here represents the grandfather's innermost thoughts and emotions, and its inability to speaker represents the grandfather's emotional distance from his family.
Other metaphors abound. For example, the speaker calls the grandfather a "tall tower broken." Towers were originally defensive structures built to withstand sieges during war, so the fact that the grandfather was a "tall tower" emphasizes his physical strength as well as his solitude, cut off like a besieged castle from the surrounding land. Yet now he is "broken," showing how even the mighty eventually lose their strength.
The bagpipes' being able to "set ablaze" the "unchanging cairn" is the next metaphor; bagpipes cannot literally set anything on fire. This metaphor emphasizes the flame-colored "aaronsrod and blossom" growing around the cairn. It also might evoke the mythical phoenix, a bird that dies in a burst of flame and is reborn from the ashes—in turn suggesting the circular relationship between man and nature; when man dies his body nourishes the soil, out which flowers bloom.
Fire and flame generally symbolize strength and vitality in the poem (for more on this, see the Symbols section of this guide). This is reflected by the metaphor of the "lion sun," which is a way for the speaker to describe the intensity and strength of the sun when the grandfather worked outside as a young man.
Later, fire comes into the poem again with the metaphor of "the burning-glass of his mind" in line 23. A burning-glass is a convex lens that focuses sunlight into a single beam, able to set alight certain things like paper. As a description of the old man's mind, this metaphor stresses his passion as well as the sheer degree of concentration he needs to summon up distant memories in his present decrepit state.
Finally, the "green / Boughs of heaven" again relate to the circular vision of humanity and nature. Heaven refers to an afterlife for people, so describing it as a "green / Bough" emphasizes the return of human beings to nature after death. It also evokes the garden of Eden, which in Christianity predated modern humanity, again stressing how death is a form of return.